SEMICONDUCTORS AND SEMIMETALS VOLUME 23 Pulsed Laser Processing of Semiconductors
Volume Editors R . F. WOOD and C . W...
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SEMICONDUCTORS AND SEMIMETALS VOLUME 23 Pulsed Laser Processing of Semiconductors
Volume Editors R . F. WOOD and C . W. WHITE SOLID STATE DIVISION OAK RIDGE NATIONAL LABORATORY OAK RIDGE, TENNESSEE
R . T. YOUNG ENERGY CONVERSION DEVICES, INC. TROY, MICHIGAN
1984
ACADEMIC PRESS, INC. (Harcourt Brace Jovanovich, Publishers)
Orlando San Diego New York London Toronto Montreal Sydney Tokyo
Academic Press Rapid Manuscript Reproduction
COPYRIGHT @ 1984, BY ACADEMIC PRESS,INC.
ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.
ACADEMIC PRESS, INC.
Orlando, Flonda 32887
United Kingdom Edition published by
ACADEMIC PRESS,
INC. (LONDON) 24/28 Oval Road, London N W l 7DX
LTD.
Library of Congress Cataloging in Publication Data
I S B N 0-12-752123-2 PRINTED IN THE UNITED STATES OF AMERICA
04 85 86 87
9 8 7 6 5 4 3 2 1
65-26058
Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin
R. B. JAMES, SANDIA, Division 8341, Livermore, California 94550 (555) G. E. JELLISON, JR., Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (95, 165, 313) D.H . LOWNDES,Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (313, 471) C . W. WHITE, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ( I , 43) R. E WOOD,Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ( I , 165, 251, 625) R. T. YOUNG, Energy Conversion Devices, Inc.. Troy, Michigan 48084 (1, 625) E W. YOUNG,JR., Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (251) D. M . ZEHNER, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (405)
ix
Foreword All of the contributors to this volume received their primary support during the writing of the book and for their own research from the Division of Materials Science of the United States Department of Energy under contract number DEAC05-840R21400 with Martin Marietta Energy Systems, Inc. Valuable additional support for research on the development of laser-processed high-efficiency solar cells was received from the Solar Energy Research Institute under contract number DB-2-02076- 1. The support of these two agencies is gratefully acknowledged. Invaluable assistance was rendered by members of the secretarial staff of the Solid State Division of Oak Ridge National Laboratory not only in taming the frequently recalcitrant word processors and authors, but also in all other aspects of preparing the camera-ready copy. J. T. Luck and V. G. Hendrix bore the heaviest burdens, but the contributions of A. M. Keesee, T. K. Miller, and S. E. Thomas were also indispensable and greatly appreciated. Ms. Hendrix coordinated the entire preparation of the manuscript and a special thanks is her due. Finally, the contributors wish to thank all of their colleagues who allowed illustrative material from published papers to be included for discussion in the book.
xi
Preface This book is concerned with the pulsed laser processing of semiconductors, a field that has emerged as a well-defined area of condensed matter physics and materials science over approximately the last ten years. It is hardly an exaggeration to characterize developments during this period, and particularly during the last five years, as explosive. Moreover, there seems little doubt that the interest and excitement generated by new results of both fundamental and applied significance will continue at a high level for some time. We may also expect laser-related techniques that are continuing to evolve to have a significant impact in a number of areas of semiconductor materials preparation and device applications. Nevertheless, it is apparent that the field has now matured to the point where many of the early misconceptions and controversies, that inevitably arise during a period of rapid growth of a new area of science have been largely resolved. Therefore, although it may still be too early to discern clearly the direction the field will take in the coming years, it does seem particularly appropriate for a book such as this to appear at this time. The authors of the various chapters in the book have in common the fact that they were members of the Solid State Division at the Oak Ridge National Laboratory during the period of very rapid growth of the field of pulsed laser processing of semiconductors. Each of them made significant contributions that led to the recognition of ORNL as a pioneering center for development of the field. All of the chapters were essentially completed while the authors were at ORNL, although R. T. Young and R. B. James have now moved on to other research establishments. In spite of the close interaction of many of the authors, the editors did not insist on extensive cross referencing of the material in the various chapters, so that the individual contributions can generally be read independently of one another. As a consequence, there is some overlap of material in different chapters; on the whole, however, we feel that this overlap has been kept to an acceptable level. We trust that readers will find the book interesting and informative and that it will serve as a useful reference for much of the original work in the field.
...
Xlll
CHAPTER 1 LASER PROCESSING
OF SEMICONDUCTORS:
AN
OVERVIEW
R. F. Wood C. W. White R. T. Young
. . ..
.
I. INTRODUCTION * 11. LASER MACHINING AND LASER PROCESSING * 111. DEVELOPMENT OF LASER ANNEALING 1. Pulsed Laser Annealing 2. T h e o r e t i c a l Modeling o f Pulsed L a s e r Annealing. 3. CW Laser Annealing. IV. OTHER FORMS OF LASER PROCESSI~G 4. Background 5. Laser-Induced D i f f u s i o n of Dopants 6, S i l i c i d e Formation. 7. Ohmic Contacts t o GaAs 8. Laser-Induced E p i t a x i a l Growth o f Deposited S i Films. 9. Laser R e c r y s t a l l i z a t i o n of S i F i l m s on I n s u l a t i n g Substrates. 10. Pulsed Laser Photochemical Processing 11. Excimer Laser L i t h o g r a p h y V. TYPES OF LASERS FOR PULSED LASER PROCESSING 12. Pulsed S o l i d - s t a t e Lasers 13. Pulsed Gas Lasers VI. OTHER SOURCES FOR ENERGY BEAM PROCESSING VII. LASER PROCESSING OF COMPOUND SEMICONDUCTORS, METALS, AND INSULATORS VIII. PLAN OF BOOK REFERENCES
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2
R. F. WOOD E T A L .
I.
Introduction
T h i s i n i t i a l c h a p t e r p r o v i d e s a combined i n t r o d u c t o r y overview and h i s t o r i c a l survey o f t h e development of l a s e r p r o c e s s i n g o f semiconductors.
The h i s t o r i c a l o r c h r o n o l o g i c a l aspects o f t h e
development, i n a d d i t i o n t o t h e i r i n t r i n s i c i n t e r e s t , should serve t o g i v e t h e reader t h e f l a v o r o f t h e e v o l u t i o n o f l a s e r t e c h n i q u e s f o r machining and p r o c e s s i n g m a t e r i a l s , and t o i n d i c a t e t h e e x p l o s i v e growth which t h e f i e l d o f l a s e r p r o c e s s i n g o f semiconductors has undergone.
The overview serves i n p a r t t o i n t r o d u c e and t o
f a m i l i a r i z e t h e reader w i t h some o f t h e t o p i c s which w i l l be d i s cussed i n much g r e a t e r d e t a i l i n l a t e r chapters o f t h e book.
More
i m p o r t a n t l y , however, i t a l l o w s us t o c o n s i d e r several o t h e r t o p i c s which w i l l n o t be covered anywhere e l s e i n t h e book, and t h u s t o g i v e a b e t t e r rounded o v e r a l l view o f t h e s u b j e c t o f l a s e r p r o c e s s i n g o f semiconductors.
F o r example, a b b r e v i a t e d d i s c u s s i o n s
o f s e v e r a l aspects o f cw l a s e r p r o c e s s i n g o f semiconductors were i n c l u d e d i n t h i s c h a p t e r when i t was f e l t t h e y would complement t h e d i s c u s s i o n s o f p u l s e d l a s e r processing.
D e t a i l e d reviews o f t h e
development and c u r r e n t s t a t u s o f cw l a s e r p r o c e s s i n g are c o n t a i n e d i n a companion volume t o t h i s one i n t h e Semiconductor and Semimetals S e r i e s (Vol. 17, e d i t e d by Gibbons).
A d d i t i o n a l examples i n c l u d e
b r i e f s e c t i o n s on v a r i o u s types o f l a s e r s used f o r l a s e r processing, o t h e r energy beam sources f o r processing, and l a s e r p r o c e s s i n g o f m a t e r i a l s o t h e r t h a n semiconductors. 11.
Laser Machining and Laser Processing
A f t e r t h e i n v e n t i o n o f t h e l a s e r i n 1960 i t r a p i d l y came i n t o widespread use f o r a h o s t o f a p p l i c a t i o n s .
Because t h e l a s e r can
s u p p l y monochromatic, coherent 1 i g h t a t extremely h i g h power densities,
i t s p o t e n t i a l as a unique t o o l f o r m a t e r i a l s p r o c e s s i n g
was immediately recognized by m a t e r i a l s s c i e n t i s t s , m e t a l l u r g i s t s , and engineers.
As e a r l y as 1963,
p u b l i s h e d accounts o f l a s e r
w e l d i n g and d r i l l i n g began t o appear and t h e s e were soon f o l l o w e d
1 . LASER PROCESSING OF SEMICONDUCTORS by r e p o r t s of l a s e r c u t t i n g , s c r i b i n g , f r a c t u r e , e t c .
3 These t e c h -
niques were a p p l i e d t o v i r t u a l l y every c l a s s o f m a t e r i a l s i n c l u d i n g many metals, semiconductors, and ceramics. was done w i t h CO,,
ruby, Nd:YAG,
Most o f t h e e a r l y work
and Nd:glass l a s e r s t h a t were of
r e l a t i v e l y low average power by today I s standards.
These a p p l i c a -
t i o n s were o f t e n c h a r a c t e r i z e d by a r a t h e r u n s o p h i s t i c a t e d approach i n which t h e extremely h i g h power d e n s i t i e s o f t i g h t l y focussed beams were used t o m e l t , vaporize, and "explode" t h e m a t e r i a l .
A
review o f t h i s work up t o about 1972 i s g i v e n i n t h e book Lasers i n I n d u s t r y e d i t e d by Charschan (1972).
T h i s book i s s t i l l an
e x c e l l e n t source o f i n f o r m a t i o n on many aspects o f l a s e r physics and l a s e r technology, e s p e c i a l l y as t h e y a r e r e l a t e d t o m a t e r i a l s processing.
Another q u i t e u s e f u l general
r e f e r e n c e on v a r i o u s
aspects o f l a s e r p r o c e s s i n g i s t h e volume by Ready (1971).
Recently
a volume, e d i t e d by Poate and Mayer (1982), on t h e e a r l y phases o f t h e c u r r e n t development o f l a s e r p r o c e s s i n g o f semiconductors has appeared. It was noted i n t h e s e e a r l y a p p l i c a t i o n s t h a t m a t e r i a l s , espec i a l l y metals,
m e l t e d by l a s e r s o f t e n e x h i b i t e d r a t h e r unusual
m e t a l l u r g i c a l c h a r a c t e r i s t i c s and t h i s q u i c k l y l e d t o s t u d i e s which used l a s e r s f o r heat t r e a t i n g , annealing, zone r e f i n i n g , r e c r y s t a l lization,
g r a i n growth, and a v a r i e t y o f o t h e r such a p p l i c a t i o n s .
It was recognized t h a t i r r a d i a t i o n o f m a t e r i a l s w i t h high-powered l a s e r s c o u l d l e a d t o h e a t i n g and c o o l i n g r a t e s s e v e r a l o r d e r s o f magnitude g r e a t e r than those o b t a i n e d by any o t h e r means.
However,
t h e u t i l i z a t i o n o f t h i s aspect o f l a s e r s was i n h i b i t e d by t h e small areas over which h i g h l y u n i f o r m energy d e n s i t i e s c o u l d be o b t a i n e d w i t h l a s e r s a v a i l a b l e a t t h e time.
W i t h t h e gradual improvement i n
l a s e r technology and t h e development o f techniques f o r r a p i d l y and r e p r o d u c i b l y scanning beams over l a r g e areas, t h e i n t e r e s t i n l a s e r s f o r heat t r e a t i n g t o o b t a i n m e t a l l u r g i c a l m o d i f i c a t i o n s o f t h e m i c r o s t r u c t u r e o f m a t e r i a l s has grown r a p i d l y .
A survey o f e a r l y
developments i n t h i s area i s a l s o i n Charschan (1972), and Breinan e t al.
(1976) have given a b r i e f review o f more r e c e n t developments
4
R. F. WOOD ET AL
up t o about 1975 i n t h e area o f what t h e y r e f e r t o as " l a s e r g l a z i n g " . Laser g l a z i n g ,
which u t i l i z e s t h e e x t r e m e l y r a p i d quench r a t e s
c h a r a c t e r i s t i c o f l a s e r processing, has been a p p l i e d p r i m a r i l y t o m e t a l s t o produce a v a r i e t y o f unusual m e t a l l u r g i c a l m i c r o s t r u c t u r e s . T h i s book i s about p u l s e d l a s e r p r o c e s s i n g o f semiconductors. I t i s concerned w i t h t h e remarkably r a p i d e v o l u t i o n and progress o f t h e f i e l d which has t a k e n p l a c e s i n c e about 1976.
An overview
o f t h e developments i n l a s e r a n n e a l i n g o f semiconductors w i l l be given i n t h e next section, but here i t i s useful, i n t h e context o f t h e f o r e g o i n g d i s c u s s i o n , t o d e l i n e a t e what we mean by " l a s e r p r o c e s s i n g " o f semiconductors. any way w i t h micromachining, semiconductors.
The book w i l l n o t be concerned i n scribing,
welding,
or drilling of
These a r e a p p l i c a t i o n s which a r e a l r e a d y we1 1-
developed and i n use; we w i l l c o n s i d e r them t o f a l l i n a category which we can c a l l ' ' l a s e r machining".
The m a t e r i a l s science, metal-
l u r g y , and c r y s t a l l o g r a p h y (and t h e i r a p p l i c a t i o n s ) d e s c r i b e d i n t h i s volume a r e r e l a t e d t o t h e " l a s e r g l a z i n g " phenomena discussed by B r e i n a n e t a l .
(1976).
The i n t e n s e i n t e r e s t and a c t i v i t y i n
t h e area o f science t o be d e s c r i b e d here, has a l r e a d y pushed t h e f r o n t i e r s o f l a s e r p r o c e s s i n g o f semiconductors w e l l beyond t h o s e o f l a s e r processing o f other materials.
Nevertheless, i t must be
emphasized t h a t l a s e r p r o c e s s i n g i s s t i l l i n t h e research and
and t h e i n s t a n c e s o f i t s a d a p t a t i o n t o and i n t e g r a t i o n i n commercial p r o d u c t i o n f a c i l i t i e s a r e s t i l l few.
development stage,
F o r t h i s reason, t h i s book must o f n e c e s s i t y be p r i m a r i l y about t h e fundamentals o f l a s e r p r o c e s s i n g r a t h e r t h a n about i t s demons t r a t e d applications. 111.
1.
Development o f Laser Annealing o f Semiconductors
PULSED LASER ANNEALING
The r e c e n t developments i n l a s e r p r o c e s s i n g o f semiconductors were i n i t i a l l y t i e d c l o s e l y t o t h e problems o f e l e c t r i c a l l y a c t i v a t i n g t h e dopants and removing t h e l a t t i c e damage caused by i o n
5
1. LASER PROCESSING OF SEMICONDUCTORS i m p l a n t a t i o n o f those dopants.
The c o n v e n t i o n a l methods f o r s o l v i n g
t h e s e problems i n v o l v e t h e use o f furnaces t o heat t h e samples t o h i g h temperatures (-lOOO°C)
f o r times s u f f i c i e n t l y long t h a t t h e
l a t t i c e damage i s r e p a i r e d and t h e dopants e l e c t r i c a l l y a c t i v a t e d . U n f o r t u n a t e l y , t h i s high-temperature furnace h e a t i n g o f t h e e n t i r e sample has u n d e s i r a b l e s i d e e f f e c t s f o r d e v i c e f a b r i c a t i o n t h a t
w i l l be discussed i n l a t e r chapters.
One g r e a t advantage o f l a s e r
a n n e a l i n g i s t h a t t h e l a s e r r a d i a t i o n i s h e a v i l y absorbed i n a t h i n s u r f a c e l a y e r a few hundred t o s e v e r a l thousand angstroms deep. T h i s produces t h e very h i g h temperatures (and even m e l t i n g ) i n t h e i m p l a n t e d r e g i o n which a r e necessary f o r
annealing t h e l a t t i c e
damage; y e t t h e absorbed photon energy i s i n s u f f i c i e n t t o r a i s e t h e temperature o f t h e undamaged s u b s t r a t e s i g n i f i c a n t l y above ambient, and hence t h e d e l e t e r i o u s e f f e c t s of h i g h temperatures i n t h i s r e g i o n a r e circumvented.
Although preceded by e a r l i e r e f f o r t s
a t l a s e r p r o c e s s i n g (see Sec. IV.4), t h e r a p i d growth o f i n t e r e s t i n l a s e r p r o c e s s i n g o f semiconductors can be t r a c e d t o t h e work o f S o v i e t s c i e n t i s t s i n t h e p e r i o d 1974-76 on t h e l a s e r i r r a d i a t i o n o f i o n - i m p l a n t e d S i and GaAs.
F o r example, Shtyrkov e t a l . (1976)
observed t h a t p u l s e s from a Nd:YAG l a s e r produced changes i n t h e optical
and e l e c t r i c a l p r o p e r t i e s o f i o n - i m p l a n t e d S i samples.
They r e p o r t e d t h a t t h e l a t t i c e damage caused by t h e i m p l a n t a t i o n process c o u l d be removed and t h e i m p l a n t e d dopants made e l e c t r i c a l l y active.
The S o v i e t s c i e n t i s t s
used t h e t e r m i n o l o g y "laser
annealing" t o d e s c r i b e t h e process and e s t a b l i s h e d many o f i t s most interesting characteristics,
several of which we w i l l now discuss.
The a n n e a l i n g o f l a t t i c e damage by p u l s e d l a s e r i r r a d i a t i o n o f semiconductors S o v i e t work.
has been e x t e n s i v e l y s t u d i e d s i n c e t h e o r i g i n a l Transmission e l e c t r o n microscopy
(TEM) r e v e a l s t h a t ,
a f t e r a s i n g l e p u l s e o f l a s e r r a d i a t i o n of an a p p r o p r i a t e wavel e n g t h and power d e n s i t y , no extended damage remains i n annealed s i l i c o n specimens down t o t h e r e s o l u t i o n of t h e microscopes used, which has been b e t t e r t h a n 10 A (Young e t a1
., 1978).
I n contrast,
a f t e r thermal a n n e a l i n g s i g n i f i c a n t damage u s u a l l y remains i n t h e
6
R. F. WOOD ET AL
f o r m o f d i s l o c a t i o n loops.
This i s i l l u s t r a t e d i n Fig.
1 which
shows a s e r i e s o f micrographs f o r l a s e r - and t h e r m a l l y annealed, i o n - i m p l a n t e d samples,
as d e s c r i b e d i n t h e f i g u r e c a p t i o n .
The
t o t a l lack o f i r r e g u l a r i t i e s i n e l e c t r o n d i f f r a c t i o n patterns from l a s e r - a n n e a l e d samples shows t h a t t h e i m p l a n t e d r e g i o n anneals w i t h t h e same l a t t i c e o r i e n t a t i o n as t h e s u b s t r a t e .
Measurements on
ion-implanted s i l i c o n c r y s t a l s w i t h Rutherford i o n backscatteri n g (RBS) and i o n - c h a n n e l i n g techniques show t h a t t h e long-range c r y s t a l l i n e o r d e r i s r e s t o r e d t o t h e i m p l a n t e d r e g i o n by p u l s e d l a s e r i r r a d i a t i o n , t h u s v e r i f y i n g t h e TEM r e s u l t s .
Such measure-
ments c l e a r l y e s t a b l i s h t h e e f f e c t i v e n e s s o f p u l s e d l a s e r a n n e a l i n g i n removing l a t t i c e damage and r e s t o r i n g c r y s t a l l i n e order. However, t h e r e i s evidence (Mooney e t al.,
1978; K a c h u r i n e t al.,
Benton e t al.,
1983) t h a t small complexes o f
1980; Young e t al.,
vacancies w i t h dimensions l e s s t h a n
- 10 A
1980;
remain o r a r e formed
i n t h e m a t e r i a l a f t e r c e r t a i n t y p e s o f l a s e r annealing.
The e x t e n t
t o which these p o i n t d e f e c t s can be e l i m i n a t e d d u r i n g o r a f t e r l a s e r a n n e a l i n g and t h e i r e f f e c t s on t h e performance o f v a r i o u s devices i s n o t y e t c l e a r (see t h e d i s c u s s i o n s i n Chapters 3 and 10 o f t h i s book ) The e f f e c t i v e n e s s o f p u l s e d l a s e r a n n e a l i n g i n e l e c t r i c a l l y a c t i v a t i n g t h e i m p l a n t e d dopants has been e s t a b l i s h e d by measurements o f t h e sheet c a r r i e r c o n c e n t r a t i o n a f t e r l a s e r a n n e a l i n g o f samples i m p l a n t e d w i t h v a r i o u s dopants o v e r a wide range o f doses. W i t h c o n v e n t i o n a l t h e r m a l a n n e a l i n g i t i s d i f f i c u l t , i f n o t imposs i b l e , t o dope a sample t o c o n c e n t r a t i o n s s u b s t a n t i a l l y above t h e e q u i l i b r i u m s o l u b i l i t y l i m i t ; i t i s remarkable t h a t i n l a s e r anneali n g t h i s l i m i t can be g r e a t l y exceeded.
A s a consequence o f t h i s
d i f f e r e n c e between t h e two t y p e s o f annealing, t h e c a r r i e r concent r a t i o n as a f u n c t i o n o f i m p l a n t e d dose s a t u r a t e s f o r thermal a n n e a l i n g , whereas i n laser-annealed s i l i c o n (Wood and Young, 1980) i t c o n t i n u e s t o i n c r e a s e l i n e a r l y up t o doses t h a t g i v e concentra-
t i o n s w e l l above t h e e q u i l i b r i u m s o l u b i l i t y l i m i t .
This i s i l l u s -
t r a t e d i n F i g . 2, which shows t h e c a r r i e r d e n s i t y as a f u n c t i o n o f
7
1 . LASER PROCESSING OF SEMICONDUCTORS
Fig.
1.
Transmission electron micrographs comparing ( a to c ) laser- and
( d to f ) thermally annealed ion-implanted silicon o f (001 ) orientation.
Implanted
species, energy, dose, projected range, and range straggling were: ( a and d ) I l B (35 keV, 3x1015 cm-2,
1100 A, 420
A ) ; (b and e ) 3 1 P ( 8 0 keV, lx1015
l O O O A , 4 0 0 4 ) ; ( c a n d f ) 75As (lOOkeV, 1x1016cm-*, 560A, 2 0 0 A ) . The boron and phosphorus samples were thermally annealed at llOO°C for 30 cm-2,
minutes and the arsenic sample at 900°C for 30 minutes. Micrographs ( a ) through ( d ) were taken in bright field, and ( e ) and ( f ) in dark field. i s the d i f f r a c t i o n vector.
The symbol g
8
R. F. WOOD ETAL.
i m p l a n t e d dose f o r boron i m p l a n t e d i n t o s i l i c o n a t an energy o f
A dose o f 1 . 5 ~ 1 0 1 6 corresponds t o a c o n c e n t r a t i o n o f
35 keV.
-6x1020/cm3
under t h e l a s e r - a n n e a l i n g c o n d i t i o n s used.
Since t h e
e q u i l i b r i u m s o l u b i l i t y l i m i t o f boron i n s i l i c o n i s -6x10*0/cm3, t h e f i g u r e gives c l e a r evidence t h a t e l e c t r i c a l a c t i v a t i o n can occur w e l l above t h e s o l u b i l i t y l i m i t a t doses o f ~ 3 x 1 0 1 6and h i g h e r . The f o r m a t i o n o f s u p e r s a t u r a t e d s u b s t i t u t i o n a l a l l o y s by l a s e r p r o c e s s i n g techniques
has been demonstrated and s t u d i e d u s i n g
R u t h e r f o r d b a c k s c a t t e r i n g and i o n - c h a n n e l i n g a n a l y s i s (White e t a1 1980, Stuck e t al.,
1980).
.,
A comprehensive i o n - c h a n n e l i n g a n a l y s i s
by White e t a l . (1980) showed t h a t l a s e r a n n e a l i n g o f As-, Ga-, I n - , Sb-, and B i - i m p l a n t e d S i r e s u l t e d i n t h e s u b s t i t u t i o n a l i n c o r p o r a t i o n o f t h e dopants a t c o n c e n t r a t i o n s f a r i n excess o f t h e e q u i l i brium s o l i d s o l u b i l i t y .
T h i s phenomenon, and o t h e r s a s s o c i a t e d w i t h
i t , w i l l be discussed i n d e t a i l i n Chapters 2 and 4.
and co-workers
As K h a i b u l l i n
(1978) recognized, t h e f a c t t h a t equi 1ib r i u m s o l u-
b i l i t y l i m i t s can be exceeded makes i t apparent t h a t t h e phys ica 1
'0l4
Fig.
2.
A
LASER ANNEALING
0
900 "C/30 min
1015 1016 I M P L A N T E D DOSE (crn-')
10'7
C a r r i e r concentration as a function of implanted dose for laser-
and thermally annealed silicon B-implanted
a t an energy o f 35 keV.
9
1. LASER PROCESSING OF SEMICONDUCTORS
processes which t a k e p l a c e d u r i n g p u l s e d l a s e r a n n e a l i n g occur w e l l . away from thermodynamic e q u i l i b r i u m .
The s i g n i f i c a n c e o f these
e f f e c t s f o r device a p p l i c a t i o n s has n o t y e t been e x p l o r e d i n d e t a i l , b u t t h e i r importance f o r i m p r o v i n g our knowledge o f t h e physics of nonequilibrium s o l i d i f i c a t i o n
processes cannot be exaggerated.
Obviously, t h e c a p a b i l i t y o f o b t a i n i n g s u b s t i t u t i o n a l doping conc e n t r a t i o n s which exceed t h e s o l u b i l i t y l i m i t , w h i l e a l s o r e a l i z i n g v i r t u a l l y one hundred p e r c e n t e l e c t r i c a l a c t i v a t i o n ,
provides a
unique t o o l f o r s t u d y i n g heavy-doping e f f e c t s i n semiconductors (Miyao e t a l . ,
1981).
I n t h e e a r l i e s t S o v i e t l i t e r a t u r e on t h e s u b j e c t , i t was noted t h a t l a s e r a n n e a l i n g d i d n o t s i g n i f i c a n t l y reduce t h e m i n o r i t y c a r r i e r l i f e t i m e (MCL) i n t h e s u b s t r a t e .
E x t e n s i v e measurements
by s e v e r a l groups have c o n f i r m e d t h a t values o f t h e MCL i n t h e base r e g i o n b e f o r e and a f t e r l a s e r a n n e a l i n g a r e v e r y n e a r l y equal,
whereas thermal a n n e a l i n g a t 1100°C f o r t h i r t y minutes
reduces t h e MCL by a f a c t o r o f about t e n (Young e t al.,
1978).
On t h e o t h e r hand, t h e r e have been some i n d i c a t i o n s t h a t t h e p-n j u n c t i o n leakage c u r r e n t s i n t h e laser-annealed samples a r e somewhat h i g h and, i f so, t h i s may be r e l a t e d t o r e s i d u a l d e f e c t s l e f t i n t h e laser-annealed l a y e r . An i m p o r t a n t c o n s i d e r a t i o n i n t h e a p p l i c a t i o n o f l a s e r processi n g o f semiconductors
i s t h e f a c t t h a t pulsed l a s e r annealing
u s u a l l y r e s u l t s i n a s u b s t a n t i a l spreading o f t h e c o n c e n t r a t i o n p r o f i l e s o f implanted dopants (Kachurin e t a l . 1978; C e l l e r e t al.,
1978; White e t al.,
, 1976a;
1978).
Young e t a l .
,
This i s i l l u s t r a t e d
i n F i g . 3a, which shows how t h e dopant p r o f i l e s i n B-implanted S i vary w i t h t h e energy d e n s i t y o f i n d i v i d u a l p u l s e s from t h e ruby l a s e r used f o r t h e annealing.
Dopant r e d i s t r i b u t i o n o f t h e magni-
t u d e shown i n Fig. 3a cannot be e x p l a i n e d by any known mechanism o f d i f f u s i o n i n t h e s o l i d f o r t h e t i m e s i n v o l v e d , and i t s t r o n g l y suggests t h a t t h e near-surface annealing.
region melts during pulsed l a s e r
R e d i s t r i b u t i o n o f i m p l a n t e d dopants may be e i t h e r an
advantage o r a disadvantage depending on t h e a p p l i c a t i o n o f l a s e r
Fig. 3. Concentration profiles o f 6 in Si before and a f t e r laser annealing.
Panel ( a )
illustrates :he profile spreading that accompanies annealing w i t h pulses of various energy densities.
Panel ( b ) illustrates the effects of up to three successive pulses of 1 . 1 j / c m 2 .
11
1 . LASER PROCESSING OF SEMICONDUCTORS p r o c e s s i n g t h a t i s b e i n g considered.
F i g u r e 3b shows t h e gradual
f l a t t e n i n g o f dopant p r o f i l e s as a r e s u l t o f t h r e e successive l a s e r pulses.
A f t e r t h e n a t u r e o f t h e p u l s e d l a s e r a n n e a l i n g process i s
d e s c r i b e d i n more d e t a i l below, i t w i l l be apparent t h a t repeated l a s e r pulses can l e a d t o very n e a r l y f l a t p r o f i l e s which t e r m i n a t e a b r u p t l y a t t h e maximum depth o f m e l t i n g o b t a i n e d f o r a g i v e n s e t
o f l a s e r a n n e a l i n g parameters.
T h i s i s another i n d i r e c t i n d i c a t i o n
t h a t m e l t i n g occurs t o a depth determined by t h e l a s e r i r r a d i a t i o n parameters.
2.
THEORETICAL MODELING OF PULSED LASER ANNEALING The r e s u l t s o f mathematical modeling o f t h e p u l s e d l a s e r anneal-
i n g process have been i n v a l u a b l e i n e s t a b l i s h i n g t h e p h y s i c a l mechanisms i n v o l v e d . (Baeri e t a l . , e t al.,
C a l c u l a t i o n s w i t h thermal m e l t i n g models
1978, Wang e t al.,
1979, Wood e t al.,
1978, B a e r i e t al.,
1979a, Surko
1980, Wood and G i l e s , 1981) were c a r r i e d
o u t s h o r t l y a f t e r t h e experimental d a t a began t o accumulate.
The
r e s u l t s gave c o n v i n c i n g evidence t h a t t h e near-surface r e g i o n o f a sample m e l t s d u r i n g p u l s e d l a s e r annealing.
The c a l c u l a t i o n s a l s o
e s t a b l i s h e d t h a t , because d i f f u s i o n c o e f f i c i e n t s i n molten s i l i c o n a r e many o r d e r s o f magnitude h i g h e r t h a n i n t h e s o l i d , t h e spreading o f dopant p r o f i l e s d u r i n g l a s e r a n n e a l i n g was r e a d i l y e x p l a i n e d by t h e m e l t i n g model.
The most s i g n i f i c a n t r e s u l t s o f thermal
transport calculations
(Wood and G i l e s , 1981) a r e i l l u s t r a t e d i n
Fig. 4.
The l e f t - h a n d panel shows c a l c u l a t e d temperature p r o f i l e s
a t v a r i o u s times a f t e r i n i t i a t i o n o f t h e l a s e r pulse.
As discussed
i n Chapter 4, t h e s e and s i m i l a r curves a r e o b t a i n e d from numerical s o l u t i o n s o f t h e one-dimensional heat c o n d u c t i o n equation, generali z e d t o a l l o w f o r t h e p o s s i b i l i t y o f phase changes ( m e l t i n g and v a p o r i z a t i o n ) and f o r temperature-dependent properties.
thermal and o p t i c a l
The break i n each curve a t t h e m e l t i n g temperature
indicates t h e p o s i t i o n o f t h e melt f r o n t a t t h e time a f t e r t h e b e g i n n i n g o f t h e l a s e r p u l s e f o r which t h e c u r v e i s shown.
From
0.8 2400
.-~ I
---- TMnx
2000
-p W
LT
$
\
I
I
= 2220 "C
I
I
'2
Ed= 1.75 J/cm
0.7
I
-
0.6 E i
z 0.5 2 k
1600 'M
v)
0 a 0.4
1200
[L
c
a
0
z
w
0.3
800
5 W I
400
0.2
0 0
Fig. 4. laser pulse.
0
100
200 TIME (nsec)
300
400
L e f t panel: Temperature as a function o f depth at several times t a f t e r beginning o f the Right panel: Melt front position as a function o f time and laser energy density,
i s the pulse duration.
EQ;
13
1. LASER PROCESSING OF SEMICONDUCTORS
a s e r i e s of curves such as these, t h e p o s i t i o n o f t h e m e l t f r o n t as a f u n c t i o n o f t i m e can be determined; t y p i c a l r e s u l t s a r e shown i n t h e r i g h t hand panel o f Fig. 4.
F o r Ell
= 1.75 J/cm*,
t h e melt
f r o n t very r a p i d l y p e n e t r a t e s t o a depth o f about 0.7 urn i n t h e s o l i d , b e f o r e r e c e d i n g back t o t h e s u r f a c e w i t h an average v e l o c i t y o f approximately 3-4 m/sec.
While t h i s occurs, a r e g i o n a p p r o x i -
mately 0.4-pm t h i c k remains i n t h e m o l t e n s t a t e f o r t i m e s o f t h e o r d e r of a hundred nanoseconds, d u r i n g which t h e dopants d i f f u s e i n t h e l i q u i d where d i f f u s i o n c o e f f i c i e n t s a r e so much h i g h e r t h a n i n the solid.
Dopant p r o f i l e s f o r v a r i o u s dopants i n s i l i c o n , c a l -
c u l a t e d by assuming t h a t t h e i m p l a n t e d i o n s d i f f u s e i n t h e l i q u i d , a r e i n good agreement w i t h experimental p r o f i l e s ( B a e r i e t al.,
1978;
Wang e t al.,
1980;
Wood e t a1
1978; B a e r i e t al.,
., 1981a).
1979a; K i r k p a t r i c k e t al.,
Based on t h e experimental and t h e o r e t i c a l r e s u l t s d i s c u s s e d thus far,
t h e p u l s e d l a s e r - a n n e a l i n g process i n t h e nanosecond
regime can be p i c t u r e d as f o l l o w s .
The i n c i d e n t l a s e r energy i s
absorbed through e l e c t r o n i c e x c i t a t i o n s and q u i c k l y t r a n s f e r r e d t o t h e l a t t i c e , m e l t i n g t h e c r y s t a l t o a depth g r e a t e r t h a n t h a t o f t h e implanted p r o f i l e and accompanying l a t t i c e damage.
The m e l t e d
r e g i o n t h e n r e c r y s t a l l i z e s from t h e u n d e r l y i n g undamaged s u b s t r a t e by means o f l i q u i d phase e p i t a x i a l regrowth, p e r f e c t s i ngl e-crystal
resulting i n nearly
m a t e r i a l w i t h dopants i n s u b s t i t u t i o n a l
s i t e s i n t h e l a t t i c e . T h i s u l t r a r a p i d m e l t i n g and r e s o l i d i f i c a t i o n sequence has been e x t e n s i v e l y s t u d i e d w i t h a v a r i e t y o f t i m e resolved o p t i c a l
(Auston e t al.,
1978a;
Lowndes, 1982), e l e c t r i c a l ( G a l v i n e t al., e t al.,
Lowndes e t al.,
1981;
1982), and x-ray (Larson
1982) t e c h n i q u e s which w i l l be discussed i n d e t a i l i n
Chapter 6.
D u r i n g t h e t i m e t h e i m p l a n t e d r e g i o n i s molten, dopants
d i f f u s e r a p i d l y i n t h e l i q u i d l a y e r , and hence s u b s t a n t i a l spreading o f dopant p r o f i l e s i s observed.
However, t h e observed dopant d i s t r i -
b u t i o n s are n o t those c h a r a c t e r i s t i c o f r e c r y s t a l l i z a t i o n processes o c c u r r i n g near thermodynamic e q u i l i b r i u m , as we s h a l l now e x p l a i n .
14
R. F. WOOD ETAL.
I n t h e t h e o r y o f c r y s t a l growth o f a d i l u t e b i n a r y a l l o y (see e.g.,
Smith e t a1
., 1955), t h e
i n t e r f a c e segregation c o e f f i c i e n t
k i o f t h e s o l u t e i s d e f i n e d as t h e r a t i o of t h e s o l u t e concentrat i o n i n t h e s o l i d t o t h e solute concentration i n t h e l i q u i d a t t h e l i q u i d - s o l i d interface.
I f k i = 1, t h e s o l u t e i s e n t i r e l y i n c o r -
p o r a t e d i n t o t h e b u l k o f t h e s o l i d and no s e g r e g a t i o n t o t h e s u r f a c e occurs.
When k i d e p a r t s s i g n i f i c a n t l y f r o m u n i t y ,
segregation
e f f e c t s b e g i n t o m a n i f e s t themselves by an accumulation of i m p u r i t i e s i n f r o n t o f t h e advancing l i q u i d - s o l i d i n t e r f a c e ; t h i s w e l l known e f f e c t i s t h e b a s i s f o r f l o a t - z o n e r e f i n i n g .
For c r y s t a l
growth near thermodynamic e q u i l i b r i u m , kq has t h e values ky = 0.80,
0.35, and 0.30 f o r 8, P , and As i n S i , r e s p e c t i v e l y .
I n t h e case
o f p u l s e d l a s e r annealing, we would expect s e g r e g a t i o n t o produce pronounced s p i k e s i n t h e dopant c o n c e n t r a t i o n j u s t a t t h e surface, p r o v i d e d t h e r e i s no l o s s o f dopant.
No such s p i k e s appear i n t h e
p r o f i l e s o f B y P, and As i n l a s e r annealed s i l i c o n and y e t no s i g n i f i c a n t loss o f dopant occurs.
Moreover, when c a l c u l a t i n g t h e
p r o f i l e s o f these dopants, o n l y a value o f k i = 1 g i v e s s a t i s f a c t o r y f i t s f o r t h e l a s e r a n n e a l i n g c o n d i t i o n s used t h u s f a r .
I f recrystal-
l i z a t i o n o c c u r r e d near e q u i l i b r i u m , s e g r e g a t i o n e f f e c t s should have been observed f o r P and As, and hence we a r e f o r c e d t o conclude t h a t t h e c r y s t a l regrowth d u r i n g p u l s e d l a s e r a n n e a l i n g i s a n o n e q u i l i b r i u m process.
Much more d r a m a t i c e f f e c t s have been observed f o r
1979b; White 1980), and these
i m p u r i t i e s w i t h very small values o f k q ( B a e r i e t al., e t al.,
1979; C u l l i s e t a l . ,
1980; White e t a l . ,
w i l l be discussed i n d e t a i l i n Chapter 2.
F u r t h e r evidence f o r
t h e n o n e q u i l i b r i u m n a t u r e of p u l s e d l a s e r a n n e a l i n g comes from t h e c e l l u l a r s t r u c t u r e t h a t i s observed i n t h e d i s t r i b u t i o n o f some dopants a f t e r l a s e r a n n e a l i n g (van Gurp e t al.,
1980; Narayan,
1980).
1979; C u l l i s e t a l . ,
This s t r u c t u r e i s c h a r a c t e r i s t i c o f t h e
breakdown o f a p l a n a r m e l t f r o n t due t o c o n s t i t u t i o n a l s u p e r c o o l i n g and t h e c o n d i t i o n s under which i t appears have been t r e a t e d t h e o r e t i c a l l y by a number o f authors, b u t i n a p a r t i c u l a r l y e l e g a n t manner by M u l l i n s and Sekerka (1964).
The c e l l u l a r f o r m a t i o n which occurs
15
1 . LASER PROCESSING OF SEMICONDUCTORS
d u r i n g p u l s e d l a s e r a n n e a l i n g can be understood w i t h t h e M u l l i n s and Sekerka t h e o r y o n l y i f n o n e q u i l i b r i u m s e g r e g a t i o n e f f e c t s a r e i n c l u d e d (Narayan, 1981; Wood, 1982).
Another remarkable i l l u s -
t r a t i o n o f t h e occurrence o f n o n e q u i l i b r i u m e f f e c t s d u r i n g p u l s e d l a s e r a n n e a l i n g i s t h e o b s e r v a t i o n by s e v e r a l groups o f t h e conv e r s i o n o f molten s i l i c o n t o amorphous s i l i c o n a t very h i g h (15-20 m/sec) regrowth v e l o c i t i e s ( L i u e t al., C u l l i s e t al.,
1982).
1979; Tsu e t al.,
1979a;
T h i s aspect o f l a s e r a n n e a l i n g i s d i s c u s s e d
i n several chapters o f . t h i s book.
3.
CW LASER ANNEALING S h o r t l y a f t e r t h e i n i t i a l work o f S h t y r k o v e t a l .
a p u l s e d Nd:YAG l a s e r , Kachurin e t a l .
(1976) w i t h
(1976b) and Klimenko e t a l .
(1976) e s t a b l i s h e d t h a t cw l a s e r s c o u l d a l s o produce annealing. Annealing w i t h cw l a s e r s d i f f e r s f r o m p u l s e d l a s e r a n n e a l i n g i n t h a t t h e c h a r a c t e r i s t i c t i m e s i n v o l v e d a r e much l o n g e r and m e l t i n g i s u s u a l l y n o t a1 lowed t o occur (Kachurin e t a l .
, 1976b;
1976; Gat and Gibbons, 1978; W i l l i a m s e t al., 1978b).
K1 imenko e t a1
.,
1978; Auston e t al.,
The t y p i c a l d w e l l t i m e o f t h e beam on a g i v e n p o i n t o f t h e
sample d u r i n g cw l a s e r a n n e a l i n g i s o f t h e o r d e r o f msec and t h e s u r f a c e temperature i s h e l d below t h e m e l t i n g p o i n t so t h a t s o l i d phase e p i t a x i a l regrowth occurs.
S i g n i f i c a n t dopant r e d i s t r i b u t i o n
i n t h e regrown l a y e r i s n o t observed, s i n c e regrowth t a k e s p l a c e i n t h e near-surface r e g i o n i n t i m e s t o o s h o r t f o r s o l i d s t a t e d i f f u s i o n . As w i t h p u l s e d l a s e r annealing, complete e l e c t r i c a l a c t i v a t i o n o f
dopants can be achieved and s o l u b i l i t y l i m i t s exceeded ( L i e t o i l a , e t al.,
1979).
However, i n c o n t r a s t t o p u l s e d laser-annealed samples
i n which a d i s l o c a t i o n - f r e e e p i t a x i a l l a y e r can u s u a l l y be obtained, t h e cw laser-annealed
samples n o r m a l l y c o n t a i n some s t r u c t u r a l
d e f e c t s such as m i s f i t d i s l o c a t i o n s , s t a c k i n g f a u l t s , and d i s l o c a t i o n loops.
However, t h e i r d e n s i t y has been shown t o be l e s s t h a n
t h a t i n t h e r m a l l y annealed samples (Gat e t al.,
1978a).
d e f e c t s have been found by b o t h DLTS (Johnson e t al.,
Several 1979) and
16
R. F. WOOD ET AL.
luminescence ( S t r e e t e t al.,
1979; Mizuta, e t a l . ,
1981) s t u d i e s ,
b u t most o f them can be removed by p o s t - i r r a d i a t i o n thermal a n n e a l i n g above 700°C. I n t h e commonly used cw l a s e r a n n e a l i n g systems,
t h e beam i s
focused t o a d e s i r e d s p o t s i z e t h r o u g h a l e n s and t h e a n n e a l i n g can be accomplished e i t h e r by scanning t h e sample under t h e beam on a microprocesser c o n t r o l l e d X-Y t a b l e o r by d e f l e c t i n g t h e beam across t h e sample w i t h an automated X-Y m i r r o r system. s u b s t r a t e h e a t i n g (300-350°C)
Supplemental
i s e s s e n t i a l i n most a p p l i c a t i o n s o f
cw l a s e r a n n e a l i n g t o reduce t h e thermal g r a d i e n t s d u r i n g l o c a l i z e d l a s e r i r r a d i a t i o n so t h a t s u r f a c e s l i p and c r a c k i n g can be prevented and a b e t t e r q u a l i t y o f regrown l a y e r can be o b t a i n e d (Rozgonyi e t al.,
1979).
F o r a p a r t i c u l a r d w e l l t i m e o f t h e l a s e r beam on
an area o f t h e i m p l a n t e d l a y e r , t h e r e i s a minimum s u r f a c e temp e r a t u r e t h a t must be reached f o r f u l l annealing.
Therefore, t h e
c o n t r o l o f s u r f a c e temperature t h r o u g h t h e l a s e r energy d e n s i t y and t h e spot d w e l l t i m e must be p r e c i s e l y m a i n t a i n e d i n o r d e r t o ensure good e p i t a x i a l growth ( H i l l
,
1981).
I n v e s t i g a t i o n s o f cw
l a s e r - i n d u c e d r e c r y s t a l 1i z a t i o n o f i o n - i m p l a n t e d S i by R u t h e r f o r d b a c k s c a t t e r i n g ( W i l l i a m s e t al.,
1978;
C h r i s t o d o n l i d e s e t al.,
1978) show t h a t t h e p h y s i c a l mechanisms o f regrowth are, i n many respects, s i m i l a r t o those o f f u r n a c e annealing.
Several f e a t u r e s
c h a r a c t e r i s t i c o f furnace-annealed samples a r e p r e s e n t i n cw l a s e r annealed samples,
b u t n o t i n p u l s e d l a s e r - a n n e a l e d samples.
For
example, i n s o l i d phase e p i t a x i a l regrowth i n a furnace, t h e growth r a t e and t h e q u a l i t y o f t h e regrown l a y e r a r e dependent on t h e i m p l a n t e d dose,
substrate
orientation,
and i m p l a n t e d species.
Furthermore, t h e p e r f e c t i o n o f t h e regrown l a y e r i s extremely sens i t i v e t o t h e m i c r o s t r u c t u r e a t t h e i n t e r f a c e between t h e damaged r e g i o n and t h e u n d e r l y i n g c r y s t a l 1 i n e s u b s t r a t e .
High-dose o r high-
c u r r e n t i o n i m p l a n t a t i o n may be accompanied by s e l f annealing, which
w i l l p a r t i a l l y d e s t r o y t h e amorphous l a y e r t h a t i s o f t e n produced and cause s e r i o u s problems i n s o l i d phase e p i t a x i a l regrowth.
For
1.
17
LASER PROCESSING OF SEMICONDUCTORS
s i m i a r reasons, o v e r l a p p i n g l a s e r scans can cause p a r t i a l r e c r y s t a l l z a t i o n and e f f e c t t h e q u a l i t y o f t h e e p i t a x i a l regrowth.
All
these phenomena have been observed by W i l l i a m s (1980) i n cw l a s e r annealed S i and c o n f i r m t h e n a t u r e o f solid-phase r e c r y s t a l l i z a t i o n by l a s e r s .
CW l a s e r a n n e a l i n g o f i o n - i m p l a n t e d GaAs has n o t been successful. The problems o f s u r f a c e s l i p and c r a c k i n g d u r i n g l a s e r scanning a r e more s e r i o u s i n GaAs t h a n i n S i . 1980; Olson e t al.,
Several s t u d i e s (Anderson e t al.,
1980a; W i l l i a m s and H a r r i s o n , 1981) have i n d i -
c a t e d t h a t a cw l a s e r power "window" f o r a n n e a l i n g GaAs may n o t e x i s t , i.e.,
a t l a s e r powers j u s t below t h e t h r e s h o l d f o r s u r f a c e
damage, t h e s u r f a c e temperature and t i m e s (< 100 msec) a r e n o t s u f f i c i e n t t o remove t h e l a t t i c e d i s o r d e r i n i o n - i m p l a n t e d GaAs.
IV. 4.
Other Forms o f Laser Processing
BACKGROUND
Even b e f o r e t h e S o v i e t work on l a s e r a n n e a l i n g o f i o n - i m p l a n t e d samples appeared, t h e r e s u l t s o f s e v e r a l a t t e m p t s a t v a r i o u s t y p e s
o f l a s e r p r o c e s s i n g o f semiconductors had been reported.
Rao (1968)
r e p o r t e d t h a t r e s i s t i v i t y changes i n s i l i c o n c o u l d be induced by i r r a d i a t i o n w i t h a ruby l a s e r .
Solomon and M u e l l e r (1968) o b t a i n e d
a p a t e n t on a l a s e r - r e l a t e d method f o r f o r m i n g p-n j u n c t i o n s i n s i l i c o n and GaAs immersed i n a doping atmosphere o f a r s e n i c o r antimony.
F a i r f i e l d and Schwuttky (1968) showed t h a t p-n j u n c t i o n s
c o u l d be formed by d e p o s i t i n g a t h i n f i l m o f phosphorus on s i l i c o n and i r r a d i a t i n g t h e sample w i t h a p u l s e d ruby l a s e r .
Probably
because o f t h e s t a t e o f l a s e r t e c h n o l o g y a t t h a t time, t h e q u a l i t y
o f t h e j u n c t i o n s was n o t high, and t h i s may have caused t h e t e c h n i q u e t o have been overlooked. Pounds e t a l .
(1974) demonstrated t h a t l a s e r s can be used t o
form ohmic c o n t a c t s i n III-V compound semiconductors. L a f f and Hutchings (1974) r e p o r t e d t h a t t h e r a d i a t i o n from a scanned A r - i o n
18
R. F. WOOD ET AL.
l a s e r can induce r e c r y s t a l l i z a t i o n o f f i n e - g r a i n e d p o l y c r y s t a l l i n e s i l i c o n f i l m s d e p o s i t e d on f u s e d s i l i c a s u b s t r a t e s ; c r y s t a l l i t e s as l a r g e as 5 pm were observed.
I n t h i s s e c t i o n , some o f t h e s e
o t h e r forms o f l a s e r p r o c e s s i n g o f semiconductors w i l l be discussed briefly. 5.
LASER-INDUCED DIFFUSION OF DOPANTS
a.
S o l i d Sources
I t has been shown t h a t p-n j u n c t i o n s can be formed i n S i by means o f l a s e r - i n d u c e d d i f f u s i o n o f s u r f a c e - d e p o s i t e d dopant f i l m s ( F a i r f i e l d and Schwuttke, e t al., 1975;
1968; Harper and Cohen,
1978; A f f o l t e r e t al., Young e t a l .
,
1970; Narayan
1978) and GaAs ( P i l i p o v i c h e t al.,
1979b) , w i t h o u t any i o n - i m p l a n t a t i o n and/or
t h e r m a l - d i f f u s i o n steps.
I n t h i s approach, a t h i n dopant f i l m i s
d e p o s i t e d on t h e sample by e-beam e v a p o r a t i o n ,
o r by any o t h e r
technique
which y i e l d s
(painting,
spraying,
reasonably u n i f o r m f i l m .
spin-on,
etc.)
a
A f t e r i r r a d i a t i o n o f the f i l m s with a
p u l s e d l a s e r , t h e dopants a r e i n c o r p o r a t e d i n t o t h e sample s u b s t i t u t i o n a l l y and e l e c t r i c a l l y a c t i v a t e d as a consequence o f l i q u i d phase d i f f u s i o n d u r i n g l a s e r - i n d u c e d s u r f a c e m e l t i n g .
In sili-
con, t h e doped l a y e r s u s u a l l y have about t h e same q u a l i t y as i o n implanted,
laser-annealed layers,
a r e u s u a l l y q u i t e poor.
b u t i n GaAs t h e p-n j u n c t i o n s
Dopant c o n c e n t r a t i o n s may exceed t h e
s o l i d s o l u b i l i t y l i m i t i f h i g h l y c o n c e n t r a t e d dopant sources a r e used (Narayan e t al.,
1978).
p-n j u n c t i o n s i l i c o n s o l a r c e l l s w i t h
e f f i c i e n c i e s approaching t h o s e o f i on-imp1 anted,
1aser-annealed
c e l l s have been f a b r i c a t e d u s i n g t h i s t e c h n i q u e (Young e t a1 Fogarrasy e t al.,
1981).
., 1980;
Laser-induced d i f f u s i o n , e s p e c i a l l y w i t h
a s u i t a b l e l o w - c o s t f i l m d e p o s i t i o n technique, c o u l d be q u i t e u s e f u l f o r t h e large-volume p r o d u c t i o n o f s o l a r c e l l s o r o t h e r b a s i c e l e c t r o n i c s t r u c t u r e s such as p-n j u n c t i o n diodes, ohmic contacts, back surface f i e l d s , etc. a r e needed.
,
s i n c e n e i t h e r masking n o r vacuum t e c h n o l o g y
1. b.
19
LASER PROCESSING OF SEMICONDUCTORS
L i q u i d and Gaseous Sources An obvious e x t e n s i o n o f t h e s t u d i e s o f l a s e r doping from s o l i d
sources i s work on doping from l i q u i d and gaseous sources.
Stuck
e t a l . (1981) have shown t h a t h i g h doping c o n c e n t r a t i o n s and s a t i s f a c t o r y p-n j u n c t i o n s can be o b t a i n e d u s i n g one o r two pulses o f l a s e r r a d i a t i o n i n c i d e n t on a s i l i c o n s u r f a c e i n c o n t a c t w i t h a l i q u i d c o n t a i n i n g t h e d e s i r e d dopant.
Doping d i r e c t l y from t h e
gaseous s t a t e has been demonstrated by Turner e t a l .
(1981).
The
low d e n s i t y o f dopant i o n s i n t h e gaseous s t a t e , even a f t e r phot o l y s i s , would seem t o make t h i s method c o n s i d e r a b l y l e s s a t t r a c t i v e t h a n l a s e r - i n d u c e d d i f f u s i o n from s o l i d and l i q u i d sources. Indeed, Deutsch e t a l . (1979,1981)
found t h e y had t o i r r a d i a t e t h e
same spot on t h e sample w i t h 25 pulses from t h e l a s e r b e f o r e s a t i s f a c t o r y doping l e v e l s c o u l d be obtained.
Increasing t h e pressure
of t h e gas and o t h e r developments may make t h i s method o f doping u s e f u l i n some instances,
b u t c o n s i d e r a b l e research i s necessary
b e f o r e t h e f u t u r e of l a s e r - i n d u c e d gaseous doping can be p r o p e r l y evaluated.
6.
SILICIDE FORMATION Because o f c e r t a i n l i m i t a t i o n s t o s i l i c i d e f o r m a t i o n by conven-
t i o n a l p r o c e s s i n g (see, e t al.,
f o r example,
t h e volume e d i t e d by Poate
1978a), t h e use o f l a s e r r a d i a t i o n t o promote t h e r e a c t i o n
of metal f i l m s w i t h s i l i c o n s u b s t r a t e s i s another p r o m i s i n g area o f l a s e r processing.
Potential applications include the formation
o f gate m a t e r i a l i n MOS t r a n s i s t o r s , device i n t e r c o n n e c t s and ohmic contacts, etc.
As w i t h l a s e r a n n e a l i n g o f i o n - i m p l a n t e d s i l i c o n ,
b o t h pulsed and cw l a s e r s have been used i n t h i s t y p e of process. The mechanism o f s i l i c i d e f o r m a t i o n i n t h e case o f p u l s e d i r r a d i a t i o n i n v o l v e s m e l t i n g and i n t e r d i f f u s i o n o f t h e c o n s t i t u e n t s i n t h e molten phase, f o l l o w e d by r a p i d s o l i d i f i c a t i o n (van Gurp e t al., 1979; Poate e t al., and von Allmen,
1978b; von Allmen and Wittmer,
1979).
1979; Wittmer
S i l i c i d e s w i t h m u l t i p l e phases, many o f
20
R. F. WOOD ET AL.
which a r e thermodynamically metastable,
a r e observed and , as a
consequence o f c o n s t i t u t i o n a l s u p e r c o o l i n g , morphologies o f t h e n e a r - s u r f a c e r e g i o n s e x h i b i t c e l l u l a r s t r u c t u r e s (van Gurp e t a l . 1979; Poate e t al.,
1978b).
,
On t h e o t h e r hand, s i l i c i d e f o r m a t i o n
by cw l a s e r i r r a d i a t i o n i s very s i m i l a r t o t h a t observed w i t h f u r nace h e a t i n g , i n which s o l i d - s t a t e d i f f u s i o n dominates t h e process (Shibata e t al.,
1980; Shibata e t al.,
1981).
With b o t h types o f
l a s e r i r r a d i a t i o n , new m e t a s t a b l e s i l i c i d e phases u n a t t a i n a b l e by thermal a n n e a l i n g can be formed.
Research on t h e l a s e r f o r m a t i o n
o f new s i l i c i d e s w i t h low enough sheet r e s i s t i v i t i e s t o s a t i s f y a new g e n e r a t i o n o f V L S I t e c h n o l o g i e s and f o r o t h e r a p p l i c a t i o n s such as superconducting t h i n f i l m s has been pursued i n s e v e r a l 1a b o r a t o r i e s . 7.
OHMIC CONTACTS TO GaAs The major problems encountered w i t h t h e conventional f a b r i c a t i o n
o f e u t e c t i c c o n t a c t s t o GaAs devices stem from t h e high-temperature t r e a t m e n t o f t h e e n t i r e sample f o r l o n g t i m e s and from f o r m a t i o n of t h e l i q u i d phase.
These problems can be g r e a t l y d i m i n i s h e d when
l o c a l i z e d t r a n s i e n t h e a t i n g by l a s e r s i s u t i l i z e d . The f i r s t s t u d i e s o f t h e use o f p u l s e d l a s e r r a d i a t i o n f o r t h e f o r m a t i o n o f e u t e c t i c c o n t a c t s i n GaAs a t t a i n e d o n l y l i m i t e d success (Pounds e t a l . Margalit e t al.
,
1978).
, 1974;
Subsequently Eckhardt (1980) s t u d i e d i n
more d e t a i l t h e f o r m a t i o n o f AuGe- and InAuGe-based ohmic c o n t a c t s i n n-type GaAs u s i n g p u l s e d CO,, cw A r - i o n l a s e r .
and ruby l a s e r s , and a
The best r e s u l t s were o b t a i n e d by i r r a d i a t i o n w i t h
t h e cw A r - i o n l a s e r . periods,
Nd:YAG,
Because o f t h e l o c a l i z e d h e a t i n g f o r b r i e f
t h e s u r f a c e morphology,
compositional u n i f o r m i t y ,
and
dimensional c o n t r o l were f a r s u p e r i o r t o furnace-annealed contacts. S p e c i f i c c o n t a c t r e s i s t a n c e s as low as 1 x 10’6 ohm-cm* were o b t a i n e d (Eckhardt e t al.,
1980).
refractory metal/epitaxial
The use o f a p u l s e d ruby l a s e r t o form Ge ohmic c o n t a c t s t o n-GaAs has been
s t u d i e d by Anderson e t a l . (1981).
Ta/Ge c o n t a c t s t o 2
x
1017 cm-3
21
1. LASER PROCESSING OF SEMICONDUCTORS doped GaAs w i t h s p e c i f i c c o n t a c t r e s i s t a n c e s as low as 1 ohm-cm2 were obtained;
x
t h i s i s more t h a n an o r d e r o f magnitude
lower t h a n t h e s p e c i f i c r e s i s t a n c e o f t h e same t y p e o f c o n t a c t s formed by t h e thermal a n n e a l i n g process ( 65OoC/5 min).
A1 though
t h e experimental d a t a r e p o r t e d so f a r make i t c l e a r t h a t l a s e r p r o c e s s i n g can be used t o produce ohmic c o n t a c t s w i t h p r o p e r t i e s i n many respects s u p e r i o r t o f u r n a c e annealing, f u r t h e r experiments t o e v a l u a t e c o n t a c t s on completed devices,
especially tests f o r
r e l i a b i l i t y and l i f e t i m e , a r e r e q u i r e d . 8.
LASER-INDUCED EPITAXIAL GROWTH OF DEPOSITED S i FILMS Techniques f o r t h e growth o f h i g h q u a l i t y t h i n e p i t a x i a l f i l m s
on s i n g l e - c r y s t a l
s u b s t r a t e s w i t h l i t t l e o r no dopant r e d i s t r i b u -
t i o n a t t h e i n t e r f a c e have been sought f o r years.
Many e f f o r t s i n
t h e past have been concentrated on t h e study o f solid-phase c r y s t a l l i z a t i o n o f an evaporated amorphous S i f i l m on S i by c o n v e n t i o n a l h e a t i n g a t t h e c r y s t a l 1 i z a t i o n temperature o f 500-600°C e t al.,
1974; Canali e t al.,
Anderson,
1977).
1975; C h r i s t o u e t al.,
(Canali
1977; Roth and
The advantages o f t h i s t e c h n i q u e compared t o
e p i t a x i a l growth by chemical vapor d e p o s i t i o n a r e t h e easy c o n t r o l o f f i l m t h i c k n e s s and low p r o c e s s i n g temperatures t h a t a r e required. However,
t h e growth o f good q u a l i t y s i l i c o n f i l m s by solid-phase
e p i t a x y (SPE) n o r m a l l y r e q u i r e s an u l t r a - h i g h vacuum (UHV) ( < 1 0 - l 0 t o r r ) system because SPE growth i s extremely s e n s i t i v e t o contamina n t s a t t h e growth i n t e r f a c e and t o i m p u r i t i e s t r a p p e d i n t h e f i l m . I n any case, i t seems l i k e l y t h a t t h e combination o f low temperat u r e f i l m d e p o s i t i o n technology w i t h l o c a l i z e d and t r a n s i e n t heat t r e a t m e n t by l a s e r i r r a d i a t i o n can broaden t h e range o f a t t a i n a b l e f i l m p r o p e r t i e s and add f l e x i b i l i t y t o semiconductor device design. CW l a s e r s have been used t o c r y s t a l l i z e e-beam d e p o s i t e d S i
films. that,
Olson e t al.,
(1980b) and Roth e t a l .
as w i t h c o n v e n t i o n a l SPE,
(1981) have found
good q u a l i t y e p i t a x i a l f i l m s can
22
R. F. WOOD ET AL..
be o b t a i n e d o n l y i f t h e e n t i r e process, which i n c l u d e s s u r f a c e cleaning,
f i l m d e p o s i t i o n , and l a s e r c r y s t a l l i z a t i o n , i s c a r r i e d
o u t under UHV c o n d i t i o n s and w i t h o u t b r e a k i n g t h e vacuum between steps.
The presence o f n a t i v e oxides a t t h e i n t e r f a c e o r t h e
a b s o r p t i o n o f i m p u r i t i e s d u r i n g exposure t o t h e a i r w i l l u s u a l l y lead t o t h e formation o f p o l y c r y s t a l l i n e layers.
Because o f t h e
porous n a t u r e o f evaporated f i l m s , t h e y can e a s i l y absorb i m p u r i t i e s from t h e a i r ,
and u n l e s s t h e q u a l i t y o f as-deposited f i l m s
can be improved, l a s e r - i n d u c e d SPE w i l l have t o be performed i n UHV.
Saitoh e t a l .
(1981) r e p o r t e d t h a t i n - s i t u thermal a n n e a l i n g
o f e-beam d e p o s i t e d f i l m s a t temperatures h i g h e r t h a n 200°C can s u b s t a n t i a l l y improve t h e f i l m q u a l i t y .
Whether good q u a l i t y f i l m s
o f t h i s t y p e w i l l improve cw l a s e r induced SPE regrowth i n a i r s t i l l remains t o be e s t a b l i s h e d . The s t r i n g e n t
requirements on t h e vacuum and on i n t e r f a c e
c l e a n i n g procedures a r e n o t so c r i t i c a l f o r f i l m s c r y s t a l l i z e d by p u l s e d l a s e r induced l i q u i d phase e p i t a x y (LPE).
Good q u a l i t y
e p i t a x i a l l a y e r s can be o b t a i n e d s i m p l y by p e r f o r m i n g t h e LPE i n a i r immediately a f t e r f i l m e v a p o r a t i o n i n a vacuum of
torr
and w i t h o u t i n i t i a l l y s p u t t e r c l e a n i n g t h e s u b s t r a t e (Lau e t al., 1978; Revesz, e t a l . ,
1978; Young e t al.,
1979a).
S i n c e t h e den-
s i t y o f t h e evaporated f i l m s i s l e s s t h a n t h a t o f s i n g l e c r y s t a l s i l i c o n , t h e f o r m a t i o n o f s p h e r i c a l v o i d s o r microbubbles i n t h e e p i t a x i a l l a y e r i s o f t e n observed ( C e l l e r e t a l .
, 1981).
However,
t h e s e can be removed by repeated p u l s e s o r by h i g h e r energy pulses. On t h e o t h e r hand, t h i s repeated m e l t i n g o r l o n g e r m e l t d u r a t i o n s o f t h e d e p o s i t e d l a y e r may cause severe dopant r e d i s t r i b u t i o n a t t h e i n t e r f a c e , which may be unacceptable i f a sharp dopant p r o f i l e a t t h e i n t e r f a c e i s desired.
The advantage o f f i l m d e p o s i t i o n by
e-beam e v a p o r a t i o n i s t h a t t h e s u b s t r a t e can be h e l d a t room temperature.
However, as we have seen, t h e p o r o s i t y o f t h e evaporated
f i l m i s t h e major problem i n f i l m c r y s t a l l i z a t i o n by e i t h e r s o l i d
o r l i q u i d phase e p i t a x y .
Methods f o r i n c r e a s i n g t h e evaporated
23
1. LASER PROCESSING OF SEMICONDUCTORS f i l m d e n s i t y ( S a i t o h e t al.,
1981) and a l t e r n a t i v e low temperature
f i l m d e p o s i t i o n techniques, such as low temperature chemical vapor
d e p o s i t i o n (Minagawa e t al.,
1981; van d e r Leeden e t al.,
1982)
and m o l e c u l a r beam e p i t a x y , a r e c u r r e n t l y under i n v e s t i g a t i o n . 9.
LASER RECRYSTALLIZATION
OF S i FILMS ON INSULATING SUBSTRATES
The problems a s s o c i a t e d w i t h t h e c u r r e n t technology o f SOS (silicon-on-sapphire),
dielectric-isolation,
integrated c i r c u i t s
and t h e need f o r h i g h e r p a c k i n g d e n s i t i e s and o p e r a t i n g speeds i n t h e development o f three-dimensional m i c r o e l e c t r o n i c c i r c u i t s make l a s e r p r o c e s s i n g o f p o l y c r y s t a l l i n e S i f i l m s on i n s u l a t i n g substrates quite attractive.
Laser-induced r e c r y s t a l l i z a t i o n of f i n e -
g r a i n e d (300-600 A ) p o l y c r y s t a l l i n e S i f i l m s deposited on a t h i n amorphous d i e l e c t r i c (Si02 o r S i 3 N 4 ) l a y e r on a S i o r g l a s s subs t r a t e o r on glass has been s t u d i e d i n s e v e r a l l a b o r a t o r i e s .
These
f i l m s may be i n t h e f o r m o f u n i f o r m continuous sheets o r t h e y may have t h e form o f i s o l a t e d i s l a n d s t r u c t u r e s .
Laser i r r a d i a t i o n i s
used t o promote g r a i n growth o r t o grow s i n g l e c r y s t a l i s l a n d s , thus improving t h e e l e c t r i c a l properties o f the films.
Both p u l s e d
( C e l l e r e t al.,
1981; Wu and Magee, 1979; Young e t al.,
and Crosthwait,
1981) and cw l a s e r s (Fan and Zeiger, 1975; Gat e t
al.
1978b; Roulet e t a l .
1980; Fastow e t al., i n these studies.
1980; Yaron e t al.,
1980; Gibbons e t al.,
1981; B i e g e l s e n e t al., G e n e r a l l y speaking,
1980; Shah
1981) have been used
t h e f i l m s annealed by cw
l a s e r s have l a r g e r g r a i n s i z e s and t h e e l e c t r i c a l p r o p e r t i e s a r e l e s s s e n s i t i v e t o subsequent thermal treatment.
Fan and Z e i g e r
(1975) demonstrated t h a t a cw Nd:YAG l a s e r can be used t o c r y s t a l l i z e amorphous S i f i l m s up t o 10 pm t h i c k on A1203 substrates. C r y s t a l l i t e s as l a r g e as 25
pm
were observed,
measurements on t h e m a t e r i a l were reported.
b u t no e l e c t r i c a l Gat e t a l .
r e p o r t e d t h a t , w i t h cw A r - i o n l a s e r i r r a d i a t i o n ,
(1978b)
a 0.4 urn t h i c k
boron-implanted f i n e - g r a i n e d p o l y c r y s t a l l i n e S i f i l m c o u l d be conv e r t e d i n t o a f i l m w i t h chevron-shaped g r a i n s -2
pm
wide and -25
pm
24
R. F. WOOD ET AL.
long.
The e l e c t r i c a l p r o p e r t i e s o f t h e s e f i l m s i n terms o f t h e
r e c o v e r y o f c a r r i e r c o n c e n t r a t i o n s and m o b i l i t i e s were e x c e l l e n t . Several a u t h o r s have concluded t h a t t h e improvement o f t h e sheet r e s i s t i v i t y observed i n such f i l m s i s due n o t o n l y t o t h e i n c r e a s e i n grain size,
but a l s o t o t h e laser-induced reduction o f g r a i n
boundary t r a p p i n g s t a t e s (Roulet e t a l .
, 1980;
Yaron e t al.,
1980).
O p t i c a l s t u d i e s i n d i c a t e d t h a t t h e optimum g r a i n growth occurs when t h e l a s e r power i s j u s t h i g h enough t o m e l t t h e e n t i r e f i l m .
Due
t o t h e low thermal c o n d u c t i v i t y o f t h e d i e l e c t r i c f i l m , t h e p o l y c r y s t a l l i n e s i l i c o n f i l m can be m e l t e d w i t h r e l a t i v e l y l o w l a s e r power w i t h o u t m e l t i n g t h e u n d e r l y i n g s u b s t r a t e , t h u s a v o i d i n g f i l m damage and d e v i a t i o n s from s u r f a c e p l a n a r i t y .
To grow o r i e n t e d f i l m s on amorphous s u b s t r a t e s , Geis and coworkers (1979) have used a t e c h n i q u e c a l l e d graphoepitaxy. d e p o s i t e d t h i n S i f i l m s (0.5-2
They
urn) on f u s e d s i l i c a s u b s t r a t e s i n
which a square wave s u r f a c e r e l i e f p a t t e r n had been produced by p h o t o l i t h o g r a p h y and r e a c t i v e i o n e t c h i n g .
A cw l a s e r o r g r a p h i t e
s t r i p h e a t e r was used as t h e heat source f o r f i l m r e c r y s t a l 1i z a t i o n . Large < l o o > - o r i e n t e d g r a i n s (-100 pin) w i t h o n l y small m i s f i t angles were obtained.
C o n t i n u i n g research on t h e improvement o f f i l m
q u a l i t y and t o achieve b e t t e r u n d e r s t a n d i n g of t h e mechanism o f n u c l e a t i o n f o r t h e o r i e n t e d growth i s b e i n g pursued. 10.
PULSED LASER PHOTOCHEMICAL PROCESSING Laser induced photochemical p r o c e s s i n g i s another r a p i d l y grow-
i n g area o f research t h a t may p r o v i d e many a p p l i c a t i o n s i n t h e microelectronics industry.
Deutsch e t a l . (1979) and E h r l i c h e t a l .
(1982) have demonstrated t h a t by u s i n g a focused UV excimer l a s e r , i t i s now p o s s i b l e t o w r i t e submicron metal l i n e s on v a r i o u s semi-
conductors and q u a r t z s u b s t r a t e s .
T h i s t y p e o f processing, which
does n o t r e l y on p h o t o l i t h o g r a p h y b u t i s c u r r e n t l y l i m i t e d by i t s low throughput, may be used i n t h e r e p a i r o f p h o t o l i t h o g r a p h i c masks and f o r f a b r i c a t i o n o f i n t e r c o n n e c t s i n customized programmable
25
1. LASER PROCESSING OF SEMICONDUCTORS l o g i c arrays.
I n addition,
chemical r e a c t i o n s induced by l a s e r
r a d i a t i o n have been used by B i l e n c h i e t a l . e t al.
(1982) and A n d r e a t t a
(1982) t o d e p o s i t semiconductor f i l m s and by Boyer and co-
workers (1982) t o d e p o s i t i n s u l a t o r (Si02, S i 3 N 4 ) f i l m s on subs t r a t e s a t low temperatures. ( E h r l i c h e t al.,
A l s o e t c h i n g (Chuang, 1982) and doping
1981) o f m a t e r i a l s i n h i g h l y l o c a l i z e d r e g i o n s
have been demonstrated.
In t h e s e p r o c e s s i n g steps, t h e chemical
r e a c t i o n s may be d r i v e n by s e l e c t i v e bond breakage i n t h e molecules v i a t h e a b s o r p t i o n o f t h e i n t e n s e UV o r i n f r a r e d l i g h t , by t r a n s i e n t s u r f a c e h e a t i n g , o r even by l a s e r induced plasma formation. The main advantages o f l a s e r chemical p r o c e s s i n g a r e t h e low temp e r a t u r e a t which t h e s u b s t r a t e can be maintained, t h e s u p e r i o r c o n t r o l o f t h e environment which can be r e a l i z e d , and t h e c a p a b i l i t y
o f d i r e c t , maskless e t c h i n g , doping, and w r i t i n g .
We a n t i c i p a t e
t h a t one o r more o f these processes w i l l e v e n t u a l l y be i n t e g r a t e d i n t o m i c r o e l e c t r o n i c f a b r i c a t i o n technology.
11.
EXCIMER LASER LITHOGRAPHY
Laser r a d i a t i o n has l o n g been t h o u g h t t o be i m p r a c t i c a l f o r h i g h r e s o l u t i o n l i t h o g r a p h y because t h e coherent n a t u r e o f t h e 1 i g h t gives r i s e t o c o n s t r u c t i v e and d e s t r u c t i v e i n t e r f e r e n c e a t t h e sample s u r f a c e t h a t produces a random p a t t e r n o f f l u c t u a t i n g i n t e n s i t y c a l l e d "speckle."
Very r e c e n t l y , J a i n and co-workers
(1982) demonstrated t h a t h i g h - r e s o l u t i o n , f i n e - l i n e (0.5 wn) photol i t h o g r a p h i c p a t t e r n s can be d e f i n e d w i t h mask exposure by UV excimer l a s e r r a d i a t i o n o f 248 and 308 nm wavelengths. were o f h i g h q u a l i t y and t o t a l l y speckle f r e e .
The images
These f i n d i n g s a r e
g e n e r a l l y regarded as a major advancement i n deep UV l i t h o g r a p h y ; t h e y w i l l be discussed i n some d e t a i l i n Chapter 10.
In p a r a l l e l graphy,
w i t h J a i n ' s work
on UV excimer l a s e r p h o t o l i t h o -
S r i n i v a s a n and Mayne-Banton
(1982) r e c e n t l y developed a
new process f o r t h e c o n t r o l l e d e t c h i n g o f o r g a n i c polymer f i l m s u s i n g an ArF (193 nm) excimer l a s e r .
They demonstrated t h a t t h e
26
R. F. WOOD E T A L .
193 nm r a d i a t i o n can e t c h o r g a n i c m a t e r i a l i n a p a t t e r n whose r e s o l u t i o n i s determined e n t i r e l y by t h e d i a m e t e r o f t h e l a s e r beam.
The mechanism o f t h i s process, which S r i n i v a s a n r e f e r s t o
as " a b l a t i v e photodecomposition,"
i s b e l i e v e d t o be a b s o r p t i o n o f
UV 1ig h t a t wave1 engths c o r r e s p o n d i n g t o a1 1owed e l e c t r o n i c t ran-
s i t i o n s from bonding t o a n t i - b o n d i n g s t a t e s (>6 eV f o r most o r g a n i c polymers), f o l l o w e d by breakup o f t h e polymer c h a i n s i n t o s m a l l e r fragments and e j e c t i o n o f t h e fragments c o m p l e t e l y o u t o f t h e f i l m , l e a v i n g a v e r t i c a l w a l l d e f i n e d by t h e l i g h t source.
The impor-
t a n t p o i n t i s t h a t t h e e x c i t a t i o n (bond-breaking) energy r e s i d e s e n t i r e l y w i t h i n t h e e j e c t e d fragments, w i t h no evidence o f f l o w o r h e a t i n g o f t h e s u r r o u n d i n g polymer; hence t h e t e r m " a b l a t i v e photodecomposition."
T h i s process appears t o be very a t t r a c t i v e f o r
p h o t o l i t h o g r a p h y s i n c e i t p r o v i d e s b o t h exposure and e t c h i n g i n a s i n g l e step, and t h e c o n v e n t i o n a l wet c h e m i s t r y development process can be e l i m i n a t e d t o t a l l y .
T h i s work w i l l a l s o be d i s c u s s e d f u r t h e r
i n Chapter 10.
V.
Types o f Lasers f o r Pulsed Laser Processing
A v a r i e t y o f l a s e r s can be used f o r l a s e r p r o c e s s i n g and t h e advantages and disadvantages of d i f f e r e n t t y p e s w i l l be d i s c u s s e d here.
F i r s t , however, i t s h o u l d be r e c o g n i z e d t h a t t h e r e a r e essen-
t i a l l y two d i f f e r e n t ways i n which p u l s e d l a s e r s can be used.
In
a scanning mode a l a s e r beam o f small diameter and h i g h p u l s e r e p e t i t i o n r a t e i s r a s t e r scanned o v e r t h e sample and t h e scanning parameters a r e chosen so t h a t s a t i s f a c t o r y a n n e a l i n g i s obtained. The r a s t e r scanning can be arranged e i t h e r by d e f l e c t i n g t h e l a s e r p u l s e s o v e r t h e sample w i t h m i r r o r s o r by t r a n s l a t i n g t h e sample under t h e f i x e d l a s e r beam.
Automated,
microprocesser-control l e d
systems s u i t a b l e f o r e i t h e r t y p e o f scanning a r e now a v a i l a b l e . I n t h e o t h e r method o f o p e r a t i o n , t h e l a s e r system i s designed s o
t h a t one o r two p u l s e s o f t h e r e q u i r e d energy d e n s i t y over l a r g e areas can be used f o r annealing.
27
1. LASER PROCESSING OF SEMICONDUCTORS 12.
PULSED SOLID-STATE LASERS The work r e p o r t e d t o date on p u l s e d l a s e r p r o c e s s i n g has gen-
e r a l l y been c a r r i e d o u t w i t h ruby, Nd:YAG,
and Nd:glass l a s e r s .
The ruby l a s e r operates a t a wavelength X o f 694 nm o r 1.79 eV and t h e Nd:YAG l a s e r has X = 1064 nm o r 1.17 eV i n t h e fundamental i t can be frequency doubled, t r i p l e d ,
and quadrupled.
, but
Since t h e
i n d i r e c t band gap o f s i l i c o n i s -1.1 eV a t room temperature, t h e absorption c o e f f i c i e n t a t
)i
= 1064 nm i s q u i t e small
, and
YAG l a s e r s
a r e o f t e n operated a t t h e frequency-doubled wavelength o f 532 nm, o r i n modes which combine v a r i o u s r a t i o s o f t h e 1064 and 532 nrn radiation.
O f course, frequency d o u b l i n g , t r i p l i n g (353 nm), and
q u a d r u p l i n g (265 nm) can be o b t a i n e d o n l y a t t h e s a c r i f i c e o f efficiency,
and t h e 353 and 265 nm wavelengths a r e l i k e l y t o be
u s e f u l p r i m a r i l y f o r b a s i c s t u d i e s and s p e c i a l i z e d a p p l i c a t i o n s where o n l y very small areas are i n v o l v e d .
The a l e x a n d r i t e l a s e r
(which r e c e n t l y appeared on t h e market), w i t h wavelength t u n a b i l i t y i n t h e range from 680 t o 800 nm, seems s u i t a b l e f o r semiconductor processing, b u t very few r e s u l t s w i t h t h i s l a s e r have been r e p o r t e d
a t t h i s time. A t t h e present time, s o l i d - s t a t e l a s e r s have c e r t a i n l i m i t a t i o n s which make them l e s s t h a n i d e a l f o r l a r g e area l a s e r processing. Foremost among t h e s e l i m i t a t i o n s a r e t h e s p a t i a l inhomogeneities c h a r a c t e r i s t i c o f t h e energy d i s t r i b u t i o n i n t h e pulses and a p u l s e r e p e t i t i o n r a t e l i m i t e d by t h e heat d i s s i p a t i o n o f t h e i n s u l a t i n g crystals.
I f a l a s e r c a v i t y i s c a r e f u l l y tuned and operated i n t h e
TEMoo mode, a n e a r l y gaussian energy p r o f i l e can be obtained, b u t Fraunhofer d i f f r a c t i o n f r o m t h e circumference o f t h e p i n h o l e used t o s e l e c t t h e TEMoo mode and from t h e l a s e r r o d i t s e l f w i l l superimpose i n t e n s i t y modulations on t h i s p r o f i l e i n near and i n t e r mediate f i e l d s .
Under f a r - f i e l d c o n d i t i o n s t h e p r o f i l e assumes an
Airy p a t t e r n i n which over 90% o f t h e i n t e n s i t y i s i n t h e gaussian-
l i k e c e n t r a l peak.
The d i f f i c u l t y w i t h f a r f i e l d c o n d i t i o n s , f o r
l a s e r s o f i n t e r e s t i n l a s e r processing, i s t h a t t h e y a r e g e n e r a l l y
28
R. F. WOOD ET AL
a t t a i n e d o n l y a t very l a r g e distances.
There a r e ways around t h i s
d i f f i c u l t y by u s i n g lenses, s p a t i a l f i l t e r i n g , etc.,
b u t t h e con-
sequences a r e almost always a decrease i n t h e a v a i l a b l e energy d e n s i t y and an i n c r e a s e i n t h e c o m p l e x i t y o f t h e system.
The a t t a i n -
ment o f good beam homogeneity i n r e a l l y l a r g e s o l i d - s t a t e systems, such as some o f t h o s e r e c e n t l y used f o r l a s e r processing, r e q u i r e devices f o r homogenizing t h e beam even when t h e l a s e r i s o p e r a t i n g i n t h e TEMoo mode.
It i s a l s o q u i t e p o s s i b l e o f course t o operate
t h e l a s e r s i n multimode c o n d i t i o n s and t o use beam homogenizers t o smooth o u t t h e s p a t i a l f l u c t u a t i o n s i n t h e beam p r o f i l e s . tunately,
Unfor-
beam homogenization which w i l l be discussed i n g r e a t e r
d e t a i l i n Chapter 10 o f t h i s book, always increases t h e complexity o f t h e p r o c e s s i n g system and i s seldom e n t i r e l y s a t i s f a c t o r y . PULSED GAS LASERS
13.
There are now c l e a r i n d i c a t i o n s t h a t gas l a s e r s a r e i n p r i n c i p l e i n h e r e n t l y s u p e r i o r t o s o l i d - s t a t e l a s e r s f o r l a s e r processing. The e f f i c i e n c i e s o f gas l a s e r s a r e g e n e r a l l y g r e a t e r t h a n t h o s e o f s o l i d - s t a t e l a s e r s , and t h e e l i m i n a t i o n o f l a r g e o p t i c a l l y p e r f e c t c r y s t a l and g l a s s rods which a r e d i f f i c u l t and expensive t o grow, and which are e a s i l y damaged i s an i m p o r t a n t c o n s i d e r a t i o n . power CO,
High-
l a s e r s w i t h n e a r l y 30% e f f i c i e n c y have been designed and
c o n s t r u c t e d , b u t s i n c e t h e coup1 i n g o f t h e long-wavelength r a d i a t i o n (10.6
pm)
t o semiconductors by way o f f r e e c a r r i e r s and phonons i s
n o t very s t r o n g t h e o v e r a l l e f f i c i e n c y o f energy usage i s n o t high. However, because t h e CO,
l a s e r s a r e so e f f i c i e n t ,
it i s clearly
w o r t h w h i l e t o e x p l o r e techniques which w i l l r e s u l t i n b e t t e r coup l i n g between t h e 10.6 urn r a d i a t i o n and t h e more common semiconductors.
Moreover, t h e l a r g e p e n e t r a t f o n depth of t h e CO,
radiation
may be advantageous i n a p p l i c a t i o n s where very deep m e l t i n g i s desirable.
Annealing o f i o n - i m p l a n t e d s i l i c o n w i t h a p u l s e d CO,
l a s e r was r e p o r t e d by Miyao (1979), b u t t h e q u a l i t y o f t h e a n n e a l i n g was n o t t h o r o u g h l y s t u d i e d . More r e c e n t l y , Naukkarinen e t a l . (1982)
29
1. LASER PROCESSING OF SEMICONDUCTORS
demonstrated t h a t Cop l a s e r a n n e a l i n g o f h e a v i l y doped s i l i c o n s u b s t r a t e s i m p l a n t e d w i t h antimony c o u l d y i e l d almost complete r e c r y s t a l l i z a t i o n and a c t i v a t i o n of t h e i m p l a n t e d ions.
Good r e -
c r y s t a l l i z a t i o n has a l s o been achieved f o r l i g h t l y doped samples w i t h a dopant c o n c e n t r a t i o n o f 7x1015 CO,
(Blomberg e t al.,
1983).
l a s e r s have been on t h e market f o r a r e l a t i v e l y l o n g time,
b u t r a r e gas h a l i d e excimer l a s e r s o p e r a t i n g i n t h e u l t r a v i o l e t have o n l y r e c e n t l y appeared commercially and a r e s t i 11 undergoing r a p i d development.
An e a r l y r e p o r t o f l a s e r a n n e a l i n g w i t h excimer
l a s e r s was made by Anderson e t a1
., (1980).
Recently, more thorough
s t u d i e s o f t h e q u a l i t y o f t h e a n n e a l i n g o b t a i n e d w i t h XeCl l a s e r s have been p u b l i s h e d by Young e t a l . (1982b) and Lowndes e t a l . (1982), and Young and co-workers
(1983) have demonstrated t h a t s i 1i c o n
s o l a r c e l l s w i t h remarkably h i g h e f f i c i e n c i e s can be f a b r i c a t e d u s i n g low-cost
i o n i m p l a n t a t i o n and XeCl l a s e r annealing.
This
work w i l l be discussed i n d e t a i l i n Chapter 10, and we w i l l o n l y remark here t h a t t h e same c h a r a c t e r i s t i c s o f U V excimer l a s e r s t h a t make them so e f f e c t i v e f o r UV l i t h o g r a p h y discussed above a l s o make them e x c e l l e n t sources f o r a n n e a l i n g r a d i a t i o n . specifically,
More
t h e reduced coherence o f t h e r a d i a t i o n v i r t u a l l y
e l i m i n a t e s d i f f r a c t i o n and i n t e r f e r e n c e e f f e c t s and g i v e s a very u n i f o r m beam t h a t does n o t r e q u i r e t h e use o f beam homogenizers. Recent developments i n excimer l a s e r t e c h n o l o g y ( L i n and L e v a t t e r , 1979; L e v a t t e r and L i n , 1980) suggest t h a t very high-powered excimer l a s e r s w i t h e x c e l l e n t homogeneity o f t h e energy d e n s i t y over l a r g e areas can be constructed.
Such l a s e r s would undoubtedly be o f
great u t i l i t y i n t h e l a s e r processing o f a l l types o f materials.
VI.
Other Sources for Energy Beam Processing
I t should be obvious from t h e d i s c u s s i o n s i n t h e p r e c e d i n g sections o f t h i s chapter t h a t t h e effectiveness o f pulsed l a s e r p r o c e s s i n g depends t o a g r e a t e x t e n t on t h e c a p a b i l i t y o f d e p o s i t i n g r e l a t i v e l y small amounts of energy i n t o r e g i o n s o f small volume
30
R. F. WOOD ETAL.
i n very s h o r t times; t o a l e s s e r e x t e n t cw l a s e r a n n e a l i n g u t i l i z e s t h e same p r i n c i p l e s .
Lasers are used because o f t h e power d e n s i -
t i e s t h e y can d e l i v e r by p o p u l a t i o n i n v e r s i o n , s t i m u l a t e d emission, and Q-switching.
The coherent n a t u r e o f t h e l a s e r r a d i a t i o n i s
n o t o n l y unnecessary,
it is,
because of d i f f r a c t i o n
e f f e c t s caused by a p e r t u r e s , l a s e r rods,
more o f t e n t h a n n o t ,
lenses, d i r t and dust p a r t i c l e s , e t c .
a nuisance
An i n c o h e r e n t l i g h t source
w i t h s u f f i c i e n t power d e n s i t y should be q u i t e e f f e c t i v e f o r energy beam annealing.
S h o r t l y a f t e r t h e advent o f l a s e r annealing,
s e v e r a l r e p o r t e d and u n r e p o r t e d attempts t o use i n c o h e r e n t l i g h t sources such as a r c lamps (Cohen e t al., i n t e n s i t y halogen lamps (Nishiyama e t a l .
1978; Gat 1981), h i g h -
, 1981) , etc.,
f o r anneal-
i n g o f i o n - i m p l a n t a t i o n damage i n semiconductors were made.
The
r e s u l t s were s i m i l a r t o those o b t a i n e d by cw l a s e r a n n e a l i n g because t h e u l t r a r a p i d m e l t i n g and c o o l i n g c h a r a c t e r i s t i c o f p u l s e d l a s e r a n n e a l i n g was n o t achieved.
Moreover, t o o b t a i n a n n e a l i n g i t was
necessary t o h o l d t h e e n t i r e sample a t h i g h temperatures t o prevent wafer d i s t o r t i o n .
T h i s h i g h temperature i s l i k e l y t o degrade t h e
m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h and make t h e samples u n s u i t a b l e f o r some a p p l i c a t i o n s . Another energy source which i s c o m p e t i t i v e even now w i t h l a s e r s i n many areas o f m a t e r i a l s p r o c e s s i n g i s a p u l s e d e l e c t r o n beam generator.
There i s an e x t e n s i v e body o f l i t e r a t u r e on e-beam
p r o c e s s i n g o f m a t e r i a l s ( f o r a d i s c u s s i o n o f e-beam a n n e a l i n g o f s i l i c o n see Greenwald e t al., t o summarize i t here.
1979) and we w i l l n o t even attempt
However, f o r t h e purposes o f t h i s book, i t
i s w o r t h w h i l e emphasizing t w o o f t h e main d i f f e r e n c e s l a s e r s and e l e c t r o n beams as energy sources.
between
The d e p o s i t i o n o f
energy i n t h e sample by a l a s e r i s s t r o n g l y dependent on t h e o p t i c a l p r o p e r t i e s ( r e f l e c t i v i t y and a b s o r p t i o n c o e f f i c i e n t ) o f t h e m a t e r i a l a t t h e wavelength o f t h e l a s e r r a d i a t i o n .
I n contrast,
t h e energy d e p o s i t i o n by e-beams depends on t h e e l e c t r o n energy and t h e s t o p p i n g power o f t h e m a t e r i a l f o r e l e c t r o n s o f t h a t energy,
1. LASER PROCESSING OF SEMICONDUCTORS
31
and t h i s i s p r i m a r i l y a f u n c t i o n o f t h e d e n s i t y o f t h e m a t e r i a l . G e n e r a l l y speaking,
100 keV e-beams from commercial e-beam p r o -
cessors d e p o s i t energy i n t h e sample a t s i g n i f i c a n t l y deeper depths t h a n do ruby and YAG l a s e r s , and t h i s may o f f e r advantages i n some a p p l i c a t i o n s and disadvantages i n others.
For example, i t i s n o t
l i k e l y t o be an advantage i n t h e f a b r i c a t i o n o f s h a l l o w - j u n c t i o n s o l a r c e l l s , and indeed e-beam processed s o l a r c e l l s show a r a t h e r poor response i n t h e l o n g wavelength p o r t i o n o f t h e s o l a r spectrum ( K i r k p a t r i c k and Minnucci , 1979).
The o t h e r major d i f f e r e n c e
between e-beam and l a s e r a n n e a l i n g i s t h a t t h e former must be done i n a f a i r l y good vacuum whereas t h e l a t t e r can be done i n a i r .
It
a l s o appears t o be d i f f i c u l t t o achieve u n i f o r m beams over l a r g e areas w i t h present day e-beam sources and t h i s makes i t d i f f i c u l t t o o b t a i n p r e c i s e c o n t r o l o f j u n c t i o n depths,
especially i n t h e
f o r m a t i o n o f abrupt s h a l l o w j u n c t i o n s . Other obvious forms of energy-beam a n n e a l i n g a r e t h o s e which u t i l i z e p a r t i c l e s heavier than electrons.
Reports o f a n n e a l i n g o f
i o n - i m p l a n t e d samples w i t h p r o t o n beams have appeared (Hodgson e t a1
., 1980).
A p u l s e d 200 keV p r o t o n beam can d e p o s i t energy
u n i f o r m l y t o a depth o f 2 um and t h i s should be u s e f u l i n a n n e a l i n g samples w i t h deeply i m p l a n t e d dopants.
I n materials processing o f
semiconductors t h a t i n v o l v e i o n i m p l a n t a t i o n and annealing, t h e q u e s t i o n n a t u r a l l y a r i s e s as t o whether o r n o t i t i s p o s s i b l e t o o b t a i n i m p l a n t a t i o n c o n d i t i o n s which w i l l r e s u l t i n s a t i s f a c t o r y self-annealing.
The experience t o d a t e seems t o i n d i c a t e t h a t t h e
s e l f - a n n e a l i n g t h a t i s known t o occur d u r i n g c e r t a i n i m p l a n t a t i o n c o n d i t i o n s may induce t h e growth o f c l u s t e r t y p e d e f e c t s which a r e subsequently v e r y d i f f i c u l t t o anneal out.
I n s p i t e of t h i s , it
seems t h a t i t may be p o s s i b l e e v e n t u a l l y t o i o n i m p l a n t under energy and c u r r e n t c o n d i t i o n s which r e s u l t i n a power d e n s i t y h i g h enough t o g i v e solid-phase and,
perhaps even l i q u i d - p h a s e regrowth, of
t h e implanted and damaged region.
32
R. F. WOOD ET AL.
VII.
Laser Processing of Compound Semiconductors, Metals, and Insulators
The success o f l a s e r p r o c e s s i n g o f t h e elemental semiconductors s i 1 con and germanium has n a t u r a l l y l e d t o e x t e n s i v e research on t h e a p p l i c a t i o n o f s i m i l a r techniques t o o t h e r m a t e r i a l s . t h e compound semiconductors, bec use industry.
of
i t s potential
Among
GaAs has been o f p a r t i c u l a r i n t e r e s t importance f o r
t h e microelectronics
The thermal p r o p e r t i e s o f GaAs, S i , and Ge a r e roughly
comparable, and i n t h e wavelength range used i n many l a s e r a n n e a l i n g experiments t h e o p t i c a l p r o p e r t i e s o f GaAs and amorphous s i l i c o n are also not grossly d i f f e r e n t .
It i s n o t s u r p r i s i n g t h e n t h a t a
number o f t h e f e a t u r e s o f p u l s e d l a s e r a n n e a l i n g o f GaAs and s i l i con appear t o be q u i t e s i m i l a r (Golovchenko and Venkatesan, Barnes e t al.,
1978).
1978;
The l a t e n t heat o f f u s i o n o f GaAs i s approx-
i m a t e l y one t h i r d o f t h a t of s i l i c o n , which i n d i c a t e s t h a t GaAs s h o u l d r e q u i r e l e s s e n e r g e t i c l a s e r p u l s e s t o o b t a i n comparable melt f r o n t penetration.
That t h i s i s indeed t h e case i s borne o u t
by b o t h experiment and t h e o r y (Auston e t al., 1981b).
1978b; Wood e t a l . ,
Dopant p r o f i l e spreading i s o f about t h e same magnitude
i n GaAs and s i l i c o n ,
and n o n e q u i l i b r i u m s e g r e g a t i o n e f f e c t s have
been observed i n GaAs as i n s i l i c o n (Eisen, 1980; Lowndes e t al., 1981). Important d i f f e r e n c e s between t h e two m a t e r i a l s become apparent when t h e e l e c t r i c a l p r o p e r t i e s are s t u d i e d , as d e s c r i b e d i n Chapter 8.
Pulsed l a s e r a n n e a l i n g has been s u c c e s s f u l i n
a c t i v a t i n g o n l y t h e h i g h e s t f l u e n c e i m p l a n t s i n GaAs, and c a r r i e r m o b i l i t i e s i n t h e c o n d u c t i n g l a y e r s formed i n t h i s way a r e much lower t h a n would be expected i n h i g h - q u a l i t y GaAs, d e s p i t e evidence o f good c r y s t a l l i n i t y i n t h e laser-annealed regions.
Furthermore,
when u n i f o r m l y doped c r y s t a l l i n e GaAs i s p u l s e annealed, h i g h conc e n t r a t i o n s o f compensating d e f e c t s a r e produced near t h e surface. There a r e several apparent problems i n l a s e r a n n e a l i n g compound semiconductors (and p r o b a b l y compounds o f a l l t y p e s ) .
The most
obvious problem concerns t h e v o l a t i l e n a t u r e o f many o f t h e elements
1. LASER PROCESSING OF SEMICONDUCTORS i n these materials.
33
F o r example, i n GaAs a t e l e v a t e d temperatures
(even f a r s h o r t o f t h e m e l t i n g p o i n t ) t h e r e i s r a p i d d e p l e t i o n of a r s e n i c i n t h e n e a r - s u r f a c e r e g i o n (Tsu e t a l e , 1979b); p u l s e d l a s e r m e l t i n g can r e s u l t i n an a g g r e g a t i o n o f g a l l i u m "puddles". A second problem i s t h a t when compound m a t e r i a l s a r e i o n i m p l a n t e d
w i t h o n l y one t y p e o f i o n , a n o n s t o i c h i o m e t r y i s c r e a t e d which i s d i f f i c u l t , though perhaps n o t impossible, t o prevent. C o i m p l a n t a t i o n o f more t h a n one species i s a p o s s i b l e way around t h i s problem, b u t i t complicates t h e i m p l a n t a t i o n process and has n o t been s t u d i e d extensively yet.
However,
recent s t u d i e s i n d i c a t e t h a t t h e most
fundamental d i f f i c u l t i e s i n a p p l y i n g p u l s e d l a s e r p r o c e s s i n g t o compound semiconductors a r e i n h e r e n t consequences of imposing a r a p i d s o l i d i f i c a t i o n process upon t h e more complex p h y s i c s and chemi s t ry o f c r y s t a 11 ine compounds.
U 1t r a r a p i d r e c r y s t a1 1i z a t ion
f r o m t h e l i q u i d phase may n o t g i v e s u f f i c i e n t t i m e f o r t h e v a r i o u s atoms t o f i n d t h e i r p r o p e r l o c a l chemical c o n f i g u r a t i o n s , arrange themselves on t h e c o r r e c t s u b l a t t i c e .
or t o
Thus, d i f f i c u l t i e s
w i t h p u l s e d a n n e a l i n g of GaAs a r e n o t a s s o c i a t e d s i m p l y w i t h t h e ion -i mpl a n t a t ion process
.
Several groups have r e c e n t l y observed h i g h d e n s i t i e s o f compens a t i n g defects
(perhaps
"quenched-in"
concentrations o f mobile
vacancies) i n pulse-annealed c r y s t a l l i n e GaAs.
Unlike the behavior
of s i l i c o n , t h e r e i s now a l s o d i r e c t evidence f o r oxygen i n p u l s e annealed GaAs, b o t h from t h e n a t i v e o x i d e l a y e r and from ambient air.
F i n a l l y , u l t r a r a p i d s o l i d i f i c a t i o n would be expected t o r e s u l t
i n a h i g h c o n c e n t r a t i o n o f a n t i - s i t e d e f e c t s i n compound semiconductors, though t h e y have a p p a r e n t l y n o t been p o s i t i v e l y i d e n t i f i e d yet.
As a r e s u l t , t h e problem of a p p l y i n g pulsed-annealing t e c h -
niques t o compound semiconductors i s now viewed n o t s i m p l y as a problem of removing i m p l a n t a t i o n damage, o r a c t i v a t i n g i m p l a n t e d i o n s (which would be d i f f i c u l t enough), b u t o f l e a r n i n g how t o a v o i d i n t r o d u c i n g new defects t h a t a r e i n h e r e n t t o r a p i d s o l i d i f i c a t i o n . P a r t i c u l a r l y i n t e r e s t i n g i n t h i s r e g a r d a r e t h e use o f s u b s t r a t e
34
R. F. WOOD E T A L .
h e a t i n g t o reduce t h e regrowth v e l o c i t y and t h e use o f a h i g h p r e s s u r e ambient atmosphere d u r i n g p u l s e d a n n e a l i n g t o c o n t r o l s t o i c h i o m e t r y d u r i n g regrowth.
The d i f f i c u l t i e s encountered and
new techniques developed are discussed i n Chapter 8. I n t e r e s t i n t h e l a s e r p r o c e s s i n g o f m e t a l s , i n s u l a t o r s , ceramics, and glasses i s i n c r e a s i n g r a p i d l y and many s t u d i e s a r e c u r r e n t l y under way.
P r e l i m i n a r y r e s u l t s o f some o f these s t u d i e s have been Here we w i l l o n l y make a few general
reported i n the l i t e r a t u r e .
comments about t h e l a s e r p r o c e s s i n g o f such m a t e r i a l s .
First, it
s h o u l d be recognized t h a t t h e goals o f l a s e r p r o c e s s i n g a r e d i f f e r e n t i n d i f f e r e n t materials.
I n semiconductors, one i s almost
always t r y i n g t o modify t h e e l e c t r i c a l p r o p e r t i e s f o r v a r i o u s device applications.
The e l e c t r i c a l p r o p e r t i e s o f i n t e r e s t a r e u s u a l l y
e x t r e m e l y s e n s i t i v e t o p o i n t and l i n e d e f e c t s and t o v a r i o u s impurities.
I n contrast, w i t h the possible exception o f the modification
o f superconducting f i l m s ,
magnetic bubble devices,
etc.,
laser
p r o c e s s i n g o f metals appears t o be d i r e c t e d toward m o d i f y i n g s u r face properties (usually i n conjunction w i t h ion implantation) for g r e a t e r wear r e s i s t a n c e , l e s s f r i c t i o n , g r e a t e r hardness, s u p e r i o r corrosion resistances, etc. amics,
Laser p r o c e s s i n g o f i n s u l a t o r s , c e r -
and glasses o f t h e t y p e discussed h e r e f o r semiconductors
i s s t i l l i n i t s i n f a n c y and i t i s d i f f i c u l t t o p r e d i c t t h e d i r e c t i o n i t w i l l take.
Obviously by l a s e r p r o c e s s i n g o f t h e s e m a t e r i a l s
we do n o t mean t o i n c l u d e t h e many i n t e r e s t i n g o p t i c a l e f f e c t s such as t h e r u l i n g o f h o l o g r a p h i c g r a t i n g s ,
information storage
by c r e a t i o n o f p o i n t d e f e c t s , e t c .
Plan of Book
VIII.
The p l a n o f t h e book i s f a i r l y obvious from t h e Table o f Contents and from t h e d i s c u s s i o n i n t h i s i n t r o d u c t o r y chapter. 3
However, t h e
i d e a behind t h e arrangement o f t h e chapters i n t h e o r d e r i n which t h e y appear i s t h e f o l l o w i n g .
Chapters 2 and 3 cover mostly e x p e r i -
mental r e s u l t s which a r e obtained a f t e r a sample has been s u b j e c t e d
35
1. LASER PROCESSING OF SEMICONDUCTORS
t o v a r i o u s t y p e s of l a s e r p r o c e s s i n g techniques. I n Chapters 4 and 5, v a r i o u s t h e o r e t i c a l developments, p a r t i c u l a r l y i n t h e areas o f
heat flow c a l c u l a t i o n s , dopant r e d i s t r i b u t i o n , and nonequi 1ib r i u m segregation, a r e presented t o r e i n f o r c e t h e v a l i d i t y o f t h e i n t e r p r e t a t i o n s of v a r i o u s experimental r e s u l t s g i v e n i n o t h e r chapters of t h e book.
The r e s u l t s of t i m e - r e s o l v e d measurements and t h e
agreement of t h e s e r e s u l t s w i t h d e t a i l e d c a l c u l a t i o n s based on t h e m e l t i n g model o f p u l s e d l a s e r annealing a r e discussed i n Chapter 6. The i n t e r n a l
c o n s i s t e n c y and remarkable agreement
between t h e
experimental and t h e o r e t i c a l r e s u l t s serve t o e s t a b l i s h t h e b a s i c v a l i d i t y o f t h e m e l t i n g model and t o g i v e c o n f i d e n c e t h a t t h e r e s u l t s o b t a i n e d from i t can be used i n a v a r i e t y o f a p p l i c a t i o n s . and p r o b a b l y more i m p o r t a n t l y i n t h e l o n g run,
Moreover,
the results of
Chapters 2-6 taken t o g e t h e r i n d i c a t e t h a t t o o l s a r e now a v a i l a b l e t o a i d i n t h e development o f our fundamental understanding o f r a p i d m e l t i n g and s o l i d i f i c a t i o n phenomena.
The m a t e r i a l i n Chapter 7
on s u r f a c e s t u d i e s o f p u l s e d l a s e r i r r a d i a t e d m a t e r i a l s i s a l s o of b o t h fundamental and a p p l i e d s i g n i f i c a n c e ,
p a r t i c u l a r l y because
o f t h e prominent r o l e p r e s e n t l y p l a y e d by s u r f a c e sciences i n t h e s o l i d s t a t e and m a t e r i a l s sciences.
Chapter 8 i s devoted t o a
review o f p u l s e d l a s e r p r o c e s s i n g o f GaAs and i n d i c a t e s t h e problems and successes accompanying t h e a p p l i c a t i o n o f l a s e r techniques t o compound semiconductors.
Work on CO,
l a s e r a n n e a l i n g has been
i n c l u d e d as a separate c h a p t e r (Chapter 9 ) because r e c e n t s t u d i e s have i n d i c a t e d t h a t p u l s e d CO,
l a s e r s may have g r e a t e r p o t e n t i a l
f o r semiconductor p r o c e s s i n g t h a n was f o r m e r l y thought. The l a s t c h a p t e r o f t h e book i s devoted t o a p p l i c a t i o n s . Although i t i s s t i l l t o o e a r l y t o p r e d i c t t h e u l t i m a t e impact o f l a s e r p r o c e s s i n g on t h e semiconductor i n d u s t r y , i t was f e l t t h a t a survey o f t h e p r e s e n t s i t u a t i o n i n t h i s r e g a r d would be u s e f u l t o t h e reader.
Single crystal solar c e l l s o f quite high efficiencies
have been f a b r i c a t e d by l a s e r - p r o c e s s i n g techniques; t h e s e techniques and t h e performance o f t h e s o l a r c e l l s r e s u l t i n g from them
36
R. F. WOOD ET AL
a r e d e s c r i b e d i n t h e t h i r d s e c t i o n o f Chapter 10.
Discussions i n
o t h e r s e c t i o n s o f t h e c h a p t e r g i v e b r i e f reviews o f t h e c u r r e n t status o f t h e applications o f l a s e r processing t o t h e f a b r i c a t i o n o f a number of semiconductor devices and t o o t h e r aspects o f d e v i c e re1 a t e d work.
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1.
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.
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CHAPTER 2 SEGREGATION,
SOLUTE TRAPPING^ A N D SUPERSATURATED ALLOYS
C. W. White
. . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .. .
I. INTRODUCTION EXPERIMENTAL APPROACH AND MEASUREMENT TECHNIQUES 1. I o n I m p l a n t a t i o n and Laser Annealing 2. R u t h e r f o r d B a c k s c a t t e r i n g and I o n Channeling A n a l y s i s 3. L a t t i c e L o c a t i o n o f I m p u r i t i e s 4. D e t e r m i n a t i o n o f t h e I n t e r f a c i a l Distribution Coefficient 111. DOPANT INCORPORATION DURING R A P I D SOLIDIFICATION 5. S e g r e g a t i o n Behavior of B y P, and As i n S i l i c o n 6 . S e g r e g a t i o n Behavior o f Other Group 111-V Dopants i n S i l i c o n 7. E f f e c t s o f Regrowth V e l o c i t y and S u b s t r a t e O r i e n t a t i o n on k ' 8. Maximum S u b s t i t u t i o n a l Sol u b i 1 it i es 9. Measurements o f E q u i l i b r i u m Solubility Limits 10. Zone R e f i n i n g o f I n t e r s t i t i a l Impu r i t i e s 11. E f f e c t s a t F a s t e r Regrowth Velocities I V . MECHANISMS LIMITING SUBSTITUTIONAL SOLUBILITIES 12. L a t t i c e S t r a i n 13. I n t e r f a c i a l I n s t a b i l i t y 14. Dopant P r e c i p i t a t i o n i n t h e L i q u i d Phase 15. Fundamental Thermodynamic L i m i t s V. SUMMARY AND CONCLUSIONS REFERENCES
11.
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Copyright 0 1984 by Academic Press, Inc All nghts of reproduction in any form reserved ISBN 0-12-752123-2
44
C. W. WHITE
I.
Introduction
As discussed b r i e f l y i n Chapter 1, t h e r a p i d d e s o r p t i o n o f energy from a Q-switched l a s e r i n t o t h e n e a r - s u r f a c e r e g i o n o f a semiconductor can l e a d t o m e l t i n g o f t h e s u b s t r a t e t o a depth o f s e v e r a l thousand angstroms.
Regrowth o f t h e melted r e g i o n occurs
b y l i q u i d - p h a s e e p i t a x y ( F e r r i s e t al.,
1979; White and Peercy,
., 1981; Appleton and C e l l e r , 1982; Narayan e t al., 1983). I n s i l i c o n the v e l o c i t y o f s o l i d i f i c a t i o n i s calcul a t e d (Wang e t a1 ., 1978; B a e r i e t a1 ., 1978; Wood e t a1 ., 1981; 1980; Gibbons e t a1
B a e r i , 1982; B a e r i and Campisano, 1982) t o be several meters/sec.
These p r e d i c t i o n s a r e i n good agreement w i t h v e l o c i t i e s i n f e r r e d from measurements o f t i m e - r e s o l v e d e l e c t r i c a l c o n d u c t i v i t y o f t h e m o l t e n l a y e r ( G a l v i n e t al.,
1982).
C a l c u l a t i o n s o f t h e melted
depth and v e l o c i t y o f r e c r y s t a l l i z a t i o n achieved d u r i n g pulsed l a s e r a n n e a l i n g are discussed i n d e t a i l i n Chapter 4.
The annealed
r e g i o n i s observed t o be f r e e o f any extended d e f e c t s i f proper a n n e a l i n g c o n d i t i o n s a r e used (Narayan e t al., 1979), and Group 111, I V , tional
i n the
1978; White e t al.,
and V i m p u r i t i e s a r e h i g h l y s u b s t i t u -
l a t t i c e even when t h e i r c o n c e n t r a t i o n s g r e a t l y
exceed e q u i l i b r i u m s o l u b i l i t y l i m i t s (White e t a1
., 1980).
The growth v e l o c i t i e s achieved d u r i n g pulsed l a s e r annealing a r e so h i g h t h a t r e c r y s t a l l i z a t i o n o f t h e melted r e g i o n takes p l a c e under c o n d i t i o n s t h a t moving
liquid-solid
interface.
are
far
from e q u i l i b r i u m a t the
The v e l o c i t i e s
which
can be
achieved d u r i n g pulsed l a s e r annealing, and t h e a b i l i t y t o change t h e v e l o c i t y i n a p r e d i c t a b l e manner p r o v i d e unique o p p o r t u n i t i e s t o perform systematic s t u d i e s o f h i g h speed, n o n e q u i l i b r i u m c r y s t a l growth phenomena under we1 1 d e f i n e d experimental c o n d i t i o n s .
In
t h i s c h a p t e r we d i s c u s s t h e s t u d i e s o f dopant i n c o r p o r a t i o n i n s i l i c o n d u r i n g t h e h i g h speed l i q u i d - p h a s e
epitaxial
process induced by pulsed l a s e r annealing.
These s t u d i e s have
shown t h a t s u b s t i t u t i o n a l lattice
regrowth
species can be i n c o r p o r a t e d i n t o t h e
b y s o l Ute t r a p p i n g a t c o n c e n t r a t i o n s
that
f a r exceed
2.
45
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
e q u i l i b r i u m s o l u b i l i t y l i m i t s (White e t al., 1980; Stuck e t al.,
1979a; White e t al.,
1980; White, 1982; White e t al.,
1983).
Values
f o r the (nonequilibrium) i n t e r f a c i a l d i s t r i b u t i o n c o e f f i c i e n t ( k l ) from t h e l i q u i d can be determined by comparing measured dopant c o n c e n t r a t i o n p r o f i l e s a f t e r annealing t o model c a l c u l a t i o n s f o r dopant d i f f u s i o n i n t h e l i q u i d d u r i n g s o l i d i f i c a t i o n (White e t al.,
1980).
These comparisons show t h a t values f o r k ' are much
g r e a t e r than corresponding equi 1 ib r i u m Val ues ko f o r a1 1 Group
111, I V ,
The values f o r k ' a r e func-
and V species i n s i l i c o n .
t i o n s o f both growth v e l o c i t y ( C u l l i s e t al., 1981) and c r y s t a l o r i e n t a t i o n ( B a e r i e t al.,
1980; B a e r i e t al., 1981a).
c o n c e n t r a t i o n s , t h e r e i s a maximum c o n c e n t r a t i o n (C!ax) Group 111,
I V o r V species
t i o n a l l y i n t o the s i l i c o n (White e t al.,
which can
A t high for
each
be i n c o r p o r a t e d s u b s t i t u -
l a t t i c e d u r i n g pulsed l a s e r annealing
1980; White e t al.,
1983).
The values f o r
Cax
a r e f u n c t i o n s o f growth v e l o c i t y , t h e y can be f u n c t i o n s o f c r y s t a l orientation,
and t h e y approach
predicted l i m i t s t o solute trap-
p i n g even a t i n f i n i t e growth v e l o c i t y (White e t al., mechanisms have
1983).
l i m i t t h e values f o r
been i d e n t i f i e d which
which can be achieved d u r i n g pulsed l a s e r annealing.
Four
Cax
These l i m i -
t a t i o n s t o s u b s t i t u t i o n a l s o l u b i l i t y w i l l be discussed and r e s u l t s f o r Group 111, I V and V species w i l l be compared w i t h p r e d i c t e d l i m i t s t o solute trapping.
Finally,
t h e behavior e x h i b i t e d by
Group 111, I V and V species w i l l be c o n t r a s t e d w i t h t h a t e x h i b i t e d by n o n s u b s t i t u t i o n a l species (such as Cu, Fey Zn, C r y W, Yb, etc.) where
complete
zone
r e f i n i n g t o t h e s u r f a c e can be achieved i f
t h e i m p u r i t y c o n c e n t r a t i o n i s l o w enough (White e t a l . ,
11. 1.
1982).
Experimental Approach and Measurement Techniques
I O N IMPLANTATION AND LASER ANNEALING
Systematic s t u d i e s o f dopant i n c o r p o r a t i o n d u r i n g h i g h speed crystal
growth have been c a r r i e d o u t f o r t h e most p a r t u s i n g
silicon
single
crystals
ion
implanted
by various
impurities.
46
C. W. WHITE
A l t h o u g h t h e g r e a t m a j o r i t y o f t h e work r e p o r t e d t o date has been c a r r i e d out u s i n g s i l i c o n as a s u b s t r a t e , a few s t u d i e s have been r e p o r t e d i n Ge ( H o l l a n d e t al.,
1983; C l a r k and Poate,
GaAs (Golovchenko and Venkatessan,
1983) and
1978; Lowndes e t a l .
,
1981).
I n most o f t h e s t u d i e s t o date, i o n i m p l a n t a t i o n has been used t o introduce the i m p u r i t y i n t o the near-surface region. advantage o f
The g r e a t
i o n i m p l a n t a t i o n i s t h a t i t a l l o w s t h e species,
t o t a l dose and c o n c e n t r a t i o n p r o f i l e o f t h e i m p u r i t y t o be v a r i e d i n a r e l i a b l e and r e p r o d u c i b l e manner.
For t h e work d e s c r i b e d
i m p l a n t a t i o n e n e r g i e s i n t h e range 35 keV t o 250 keV and
here,
doses i n t h e range 1014 t o 1017/cm2 were used.
Ion implantation
i s now r o u t i n e l y used i n d e v i c e p r o c e s s i n g as a method f o r i n t r o d u c i n g dopants i n t o s i l i c o n a t known c o n c e n t r a t i o n s and d e p t h distributions.
I n t h e process
of
implantation,
however,
the
i m p l a n t e d i o n s cause massive damage i n t h e n e a r - s u r f a c e r e g i o n o f the
semiconductor,
and t h e
s i n g l e c r y s t a l may even be t u r n e d
amorphous t o a d e p t h o f a few thousand angstroms.
I n order t o
remove t h e damage and a c t i v a t e t h e dopants e l e c t r i c a l l y , necessary t o anneal t h e imp1 anted c r y s t a l .
it i s
Conventionally t h i s
has been done by t h e r m a l l y a n n e a l i n g t h e i m p l a n t e d samples i n an oven a t temperatures o f s e v e r a l hundred degrees f o r whatever t i m e is
required t o
Recently,
remove t h e damage ( s e v e r a l
minutes t o hours).
as discussed i n Chapter 1, i t has been shown t h a t h i g h
power p u l s e s from Q-switched l a s e r s can be used t o c o m p l e t e l y anneal
the
extended
defects
in
ion
implanted
semiconductors
(pulsed l a s e r annealing). Following implantation, a pulsed r u b y l a s e r (0.6943
l a s e r a n n e a l i n g was c a r r i e d o u t u s i n g
um wavelength, 15 x
s p u l s e dura-
t i o n t i m e ) o r a pulsed XeCl l a s e r (0.3080 um wavelength -35 x sec p u l s e d u r a t i o n t i m e ) . were used.
Energy d e n s i t i e s i n t h e range o f 1-2 J/cm2
These c o n d i t i o n s g i v e r i s e t o l i q u i d phase e p i t a x i a l
r e g r o w t h v e l o c i t i e s o f several meters/sec.
Regrowth v e l o c i t y can
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
be v a r i e d by changing t h e wavelength, t h e energy d e n s i t y , d u r i n g annealing.
47
t h e p u l s e d u r a t i o n time,
o r b y changing t h e s u b s t r a t e temperature
C a l c u l a t i o n s r e l a t i n g t o depth o f m e l t i n g and
v e l o c i t y o f r e c r y s t a l l i z a t i o n d u r i n g pulsed l a s e r annealing a r e summarized i n Chapter 4.
The range o f regrowth v e l o c i t i e s which
has been achieved thus f a r u s i n g r a d i a t i o n from Q-switched l a s e r s i s approximately 2-20 m/sec.
2.
RUTHERFORD BACKSCATTERING AND I O N CHANNELING ANALYSIS The samples
were analyzed
i n t h e as i m p l a n t e d and l a s e r
an eal ed c o n d i t i o n s u s i n g R u t h e r f o r d b a c k s c a t t e r i n g spectroscopy (RBS),
and
i o n channeling techniques.
Measurements by these
techniques can be used t o determine t h e c o n c e n t r a t i o n p r o f i l e s o f t h e dopant, t h e l a t t i c e l o c a t i o n o f t h e dopant ions, t h e s u b s t i t u t i o n a l c o n c e n t r a t i o n s as a f u n c t i o n o f depth and t h e damage d i s t r i b u t i o n as a f u n c t i o n o f depth i n t h e c r y s t a l . h i g h speed c r y s t a l growth,
I n studies o f
knowledge o f t h e l a t t i c e l o c a t i o n o f
t h e dopant and t h e s u b s t i t u t i o n a l c o n c e n t r a t i o n as a f u n c t i o n o f depth a r e e s s e n t i a l because o n l y t h e s u b s t i t u t i o n a l component i s i n s o l u t i o n i n t h e regrown l a t t i c e .
Selected c r y s t a l s were exam-
i n e d subsequently b y t r a n s m i s s i o n e l e c t r o n microscopy (TEM) t o determine t h e n a t u r e o f t h e remaining d e f e c t s ( i f any) and t h e m i c r o s t r u c t u r e i n t h e near s u r f a c e region.
These measurements
a r e p a r t i c u l a r l y v a l u a b l e i n t h e case o f very h i g h dose i m p l a n t s where c o n s t i t u t i o n a l s u p e r c o o l i n g d u r i n g regrowth leads t o l a t e r a l segregation
of
the
dopant
and t h e f o r m a t i o n o f a w e l l d e f i n e d
c e l l s t r u c t u r e i n t h e near-surface region.
3.
LATTICE LOCATION OF I M P U R I T I E S R u t h e r f o r d b a c k s c a t t e r i n g and i o n channeling measurements a r e
powerful methods f o r d e t e r m i n i n g t h e p o s i t i o n o f i m p u r i t y atoms i n c r y s t a l s (Picraux, 1975).
They have been used t o show t h a t Group
48
C . W. WHITE
I 1 1 and V dopants a r e s u b s t i t u t i o n a l o r n e a r l y s u b s t i t u t i o n a l i n t h e s i l i c o n l a t t i c e a f t e r pulsed l a s e r annealing (White e t a1 1979a; White e t al.,
1980).
.,
The l a t t i c e l o c a t i o n o f t h e dopant
can be determined most e a s i l y from d e t a i l e d angular scans across t h e major a x i a l d i r e c t i o n s .
An i l l u s t r a t i v e r e s u l t f o r t h e case
o f lZ1Sb i n (100) S i f o l l o w i n g i m p l a n t a t i o n and l a s e r a n n e a l i n g
1.
i s shown i n Fig. He'
For these measurements,
a beam o f 2.5
MeV
i o n s was used and p a r t i c l e s s c a t t e r e d from both S i and Sb
atoms i n t h e same depth i n t e r v a l were d e t e c t e d and p l o t t e d as a f u n c t i o n o f t i l t angle across t h e <110> and tions.
From Fig.
<111> a x i a l d i r e c -
1 i t can be seen t h a t t h e y i e l d o f He i o n s
s c a t t e r e d from Sb has t h e same shape and angular w i d t h as t h e y i e l d o f s c a t t e r i n g from S i ,
thus demonstrating t h a t t h e dopant
atoms occupy s u b s t i t u t i o n a l s i t e s i n t h e r e c r y s t a l l i z e d l a t t i c e . The s u b s t i t u t i o n a l f r a c t i o n Fs can be determined from t h e angular 4.4
I
I
I
(440)
I
I
I
I
4.2
2
w>
1.0
.a
0.8
W
8
3
$ 0.6 z IT 0
8
.
0
0.4
0
0
0.2 0
I 4.5
I 4.0
I 0.5
0
I 0.5
I 1.0 4.5 4.5
I
4.0
I 0.5
I
0
0.5
I 4.0
1 4.5
TILT ANGLE (deg)
Fig. 1 . Angular scans for 2 . 5 MeV He ions across the (110, and <111> channels of lZ1Sb (100 keV, 1 . 6 x 1 0 1 6 / c m 2 ) implanted ( 1 0 0 ) S i after ruby laser annealing. Solid circles refer to scattering from Sb, open circles refer to scattering from Si, and Ax refers to the depth interval from which scattered particles were detected. From White et a l . , 1979a.
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
49
r e s u l t s as
Fs = 100 x ( l - ~ ~ ~ " ( S b ) ) / ( l - x ~ ~ ~ ( S3 i ) ) where +n,i
(1)
i s t h e minimum y i e l d d e f i n e d i n t h e usual manner as
the r a t i o o f the scattered p a r t i c l e y i e l d i n the aligned direct i o n t o t h a t i n t h e random d i r e c t i o n . Fig.
For t h e r e s u l t s shown i n
1, t h e s u b s t i t u t i o n a l f r a c t i o n i s 99%.
Corresponding e l e c -
t r i c a l measurements show t h a t a l l t h e implanted Sb i s e l e c t r i c a l l y active
after
laser
annealing
thus
confirming
the
channeling
results. R e s u l t s s i m i l a r t o those i n F i g .
1 have been o b t a i n e d a l s o
f o r t h e case o f implanted As (White e t al., t h a t , even f o r q u i t e h i g h c o n c e n t r a t i o n s ,
1979a), demonstrating
As i s s u b s t i t u t i o n a l i n
t h e l a t t i c e a f t e r l a s e r annealing w i t h a s i m i l a r h i g h degree o f s u b s t i t u t i o n a l i t y (99%).
For dopants such as Ga and B i , however,
t h e i o n channeling r e s u l t s show t h a t t h e dopants a r e d i s p l a c e d s l i g h t l y from s u b s t i t u t i o n a l l a t t i c e s i t e s (White e t a1
., 1980).
T h i s i s demonstrated i n t h e angular scan r e s u l t s o f Fig. t h e case o f Ga i n S i a f t e r l a s e r annealing.
2 for
As shown i n Fig. 2,
t h e y i e l d curves of s c a t t e r i n g from Ga a r e n o t as wide as those f o r Si.
T h i s i m p l i e s t h a t a t l e a s t a p a r t o f t h e implanted Ga i s
d i s p l a c e d s l i g h t l y from a normal
substitutional
l a t t i c e site.
S u b s t i t u t i o n a l f r a c t i o n s , obtained u s i n g Eq. 1 show Ga t o be -98% substitutional
after
l a s e r annealing.
Therefore,
t h e angular
scan r e s u l t s , show t h a t w h i l e Ga i s r e g u l a r l y placed i n t h e l a t t i c e , i t may be d i s p l a c e d s l i g h t l y from a s u b s t i t u t i o n a l s i t e . S i m i l a r r e s u l t s have been obtained a l s o f o r t h e case o f B i i n s i 1icon. I n summary,
i o n channeling (White e t a1
nuclear reaction analysis
., 1979a,
r e s u l t s (Swanson e t al.,
1980) and 1981) show
t h a t Group I 1 1 and V dopants a r e r e g u l a r l y placed i n t h e s i l i c o n l a t t i c e a f t e r pulsed l a s e r annealing.
Dopants such as As, Sb, B,
and P occupy s u b s t i t u t i o n a l l a t t i c e s i t e s , w h i l e Ga, B i and prob a b l y I n are d i s p l a c e d s l i g h t l y from s u b s t i t u t i o n a l l a t t i c e s i t e s .
50
C. W. WHITE
1.6
I
I
-0-
t.4
.
1
I
I
t
Si
Ga
1.2
sw 1.0 >
n A
0.8
a
5
0.6
0.4
0.2
0
0.5
1.0
Fig. 2 .
0.0
0.5
1.0 1.0 TILT ANGLE (deg)
0.5
0.0
0.5
1.0
Angular scans across the <110> and till> axial directions for 69Ga
(100 keV, 3 . 2 x 1 0 1 5 / c m 2 ) implanted (100) Si after ruby laser annealing. From White et a l . , 1980.
I n a1 1 cases, t h e s u b s t i t u t i o n a l f r a c t i o n s a r e c o n s i d e r a b l y b e t t e r t h a n those o b t a i n e d by thermal levels of substitutionality
annealing,
and these very h i g h
can be achieved even when t h e dopant
c o n c e n t r a t i o n s g r e a t l y exceed e q u i l i b r i u m s o l u b i l i t y l i m i t s . 4.
DETERMINATION OF THE INTERFACIAL DISTRIBUTION COEFFICIENT
I n o r d e r t o t r e a t segregation a t t h e l i q u i d - s o l i d i n t e r f a c e , it
is
necessary t o know t h e i n t e r f a c i a l segregation c o e f f i c i e n t
k ' d e f i n e d by k ' = Cs/CL
,
(2 1
2. SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS 51 where Cs and CL a r e c o n c e n t r a t i o n s i n t h e s o l i d and l i q u i d phases a t the interface.
During s o l i d i f i c a t i o n , t h e i n t e r f a c i a l d i s t r i -
b u t i o n c o e f f i c i e n t determines t h e p a r t i t i o n o f dopant between t h e s o l i d and l i q u i d phases a t t h e i n t e r f a c e .
Since d i f f u s i o n coef-
f i c i e n t s i n t h e l i q u i d are several o r d e r s o f magnitude g r e a t e r t h a n those i n t h e s o l i d phase, assume t h a t
t h e dopant
it i s
a good approximation t o
r e d i s t r i b u t i o n i n t h e s o l i d phase i s
n e g l i g i b l e compared t o t h a t i n t h e l i q u i d phase d u r i n g t h e s h o r t T h i s means t h a t Cs i n
t i m e s i n v o l v e d i n pulsed l a s e r annealing. Eq.
2 i s e a s i l y determined from t h e dopant p r o f i l e s measured by A d i r e c t measurement o f CL on t h e o t h e r hand i s g e n e r a l l y d i f f i c u l t i f n o t i m p o s s i b l e t o c a r r y o u t and i t is
RBS or SIMS.
necessary t o r e l y on t h e t h e o r y o f s o l i d i f i c a t i o n processes t o c a l c u l a t e CL i n o r d e r t o determine k'. e q u i l i b r i u m value o f
k',
which
The d e t e r m i n a t i o n o f t h e
we w i l l
denote b y ko,
=4i/
i.e.,
n
L'
(3)
equilibrium
i s s i m p l i f i e d because ko can be r e l a t e d d i r e c t l y t o t h e e q u i l i b r i u m phase diagram.
T h i s i s i l l u s t r a t e d i n Fig. 3 which shows a por-
t i o n o f t h e phase diagram f o r system.
a typical
two-component
alloy
Although n o t s t r i c t l y r e q u i r e d by s o l i d i f i c a t i o n theory,
i t i s customary t o r e q u i r e t h a t Cs and
t i o n s i n solution a t the interface.
CL r e f e r t o t h e concentraWith t h i s r e s t r i c t i o n i n
mind, any d e p a r t u r e s from homogeneous s o l u t i o n s due t o p r e c i p i t a t i o n , c e l l u l a r f o r m a t i o n , etc.
are t o be excluded i n t h e d e f i n i -
t i o n o f k' and ko. A t very l o w growth v e l o c i t i e s ,
s o l i d i f i c a t i o n occurs under
c o n d i t i o n s o f l o c a l e q u i l i b r i u m a t t h e i n t e r f a c e and thus k' = ko. The i n f l u e n c e o f t h e d i s t r i b u t i o n c o e f f i c i e n t on s o l u t e p r o f i l e s i n t h e l i q u i d and s o l i d phases a t several stages d u r i n g s o l i d i f i c a t i o n i s i l l u s t r a t e d i n Fig.
3 f o r t h e case o f ko
f i r s t s o l i d t o f r e e z e w i l l have a s o l u t e c o n c e n t r a t i o n
<
1.
keno
The
i f one
52
C. W. WHITE
SOLlDlFl CATION (ko
z
:“j2
LIQUID
r
PHASE DIAGRAM
0
0
DISTANCE
“OL w
a
3 I-
a a
w
a I w
I-
cs C L COMPOSITION CS =---
k O
3.
Fig.
Relationship between phase diagrams, distribution coefficients ( k )
and solidification. right
k,
hand
CL
side
Solute profiles in the liquid and solid are shown on the at
several
stages
during
solidification,
assuming
that
< 1.
s t a r t s w i t h an i n i t i a l l y u n i f o r m c o n c e n t r a t i o n no i n t h e l i q u i d if
and
solidification
occurs
e q u i l i b r i u m a t t h e i n t e r f a c e (i.e., proceeds,
rejected
solute
under
conditions
k ’ = ko).
accummulates
of
local
As s o l i d i f i c a t i o n
i n the
liquid at
the
i n t e r f a c e u n t i l a steady s t a t e c o n c e n t r a t i o n no/ko i s reached i n the l i q u i d a t the interface.
Rejected dopant w i l l be t r a n s p o r t e d
t o t h e s u r f a c e where i t w i l l appear as a t e r m i n a l spike. When n o n e q u i l i b r i u m c r y s t a l
growth occurs,
i t i s necessary
t o , i n e f f e c t , determine CL by c a l c u l a t i o n b e f o r e k ’ can be d e t e r mined.
In those cases i n which t h e i n i t i a l dopant d i s t r i b u t i o n
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
53
i s u n i f o r m and a p l a n a r l i q u i d - s o l i d i n t e r f a c e moves with u n i f o r m v e l o c i t y v t h e problem o f d e t e r m i n i n g CL can be solved a n a l y t i c a l l y (see e.g.,
T i l l e r e t al.,
1953, Smith e t al.,
1958) from t h e d i f -
f e r e n t i a1 equat ion ,t )
a2C,(x' 3x12
D~
where
XI
aC,(x',t)
-
ax'
+
ac,(x't)
at
i s measured r e l a t i v e t o t h e l i q u i d - s o l i d i n t e r f a c e , v i s
t h e growth v e l o c i t y and D,
is
t h e l i q u i d phase d i f f u s i v i t y .
At
steady s t a t e and w i t h t h e customary boundary c o n d i t i o n s
co
C,(X')
=
C,(x')
= Co/k'
at
XI
(4)
=
and
a t x' = 0
(5)
t h e s o l u t i o n t o Eq. 8 i s Co( 1-k ' ) C,(X')
= co
+
k'
exp((-v/Dg)x')
From t h i s e q u a t i o n we see t h a t under steady s t a t e c o n d i t i o n s t h e c o n c e n t r a t i o n o f t h e dopant i n t h e l i q u i d decreases e x p o n e n t i a l l y w i t h d i s t a n c e from t h e i n t e r f a c e . n o t uniform,
I f the i n i t i a l d i s t r i b u t i o n i s
as f o r i o n i m p l a n t e d p r o f i l e s ,
t h e problem becomes
more d i f f i c u l t and i t i s necessary t o use f i n i t e d i f f e r e n c e techn i ques. The general problem o f dopant d i f f u s i o n and segregation i s discussed i n some d e t a i l i n Chapter 4, b u t here i t may be u s e f u l t o i l l u s t r a t e some of t h e general ideas i n v o l v e d s c h e m a t i c a l l y by r e f e r r i n g t o Fig.
4 which i l l u s t r a t e s t h e mass d i f f u s i o n model
used t o c a l c u l a t e t h e dopant p r o f i l e i n t h e l i q u i d and s o l i d phases d u r i n g s o l i d i f i c a t i o n (White e t al., 4a,
1980).
As i n d i c a t e d i n Fig.
t h e l a s e r l i g h t m e l t s t h e c r y s t a l t o a c e r t a i n depth; t h i s
depth i s dependent on t h e energy d e n s i t y , and depth of t h e amorphous region.
p u l s e d u r a t i o n time,
The m e l t f r o n t then begins t o
54
C. W. WHITE
recede toward t h e surface.
While t h i s occurs, t h e dopant (whose
p r o f i l e was i n i t i a l l y a p p r o x i m a t e l y Gaussian i n shape) d i f f u s e s i n the l i q u i d u n t i l front
(Fig. 4a).
i t begins t o encounter t h e r e c e d i n g m e l t
The dopant p r o f i l e i n t h e l i q u i d a t t h i s t i m e
( t o ) i s determined using t h e e q u a t i o n MELT FRONT
F
I
D A t [c,,, (Ax)z
C,[t')=CM[t)+
(t) tC,-((t)-2CY(t)]
I
I
X-
Profile Calculations Fig. 4. Schematic diagram for calculating solute profiles for k' < 1 b y solving the mass diffusion equation numerically using the finite difference method.
td'
-
-E g
n
n I
z to2O
5 4O2O
a
f
G
5 W
W
8
z V
8
Id8 DEPTH (p) 34 P (80 keV
Fig.
x
5.
s).
L\6xfOt6/cm2) in Si
0
0.4
0.2 0.3 DEPTH ( p )
0.4
0.5
7 5 A ~(100keV, 1.4 xt0'%m2) in si
Profiles for I l B , 31P, and 75As implanted into (100) Si and ruby laser annealed ( 1 . 5 J / c m 2 , From White e t a \ . , 1978. Profiles a r e shown for the as-implanted and laser annealed conditions.
50
56
C . W. WHITE
c(x,to)
=
exp-[=]
4n(a2+Dto)' /2
+
4 (a2+Dto)
f i c i e n t f o r t h e dopant i n l i q u i d s i l i c o n , maximum of
(7)
x ' i s t h e peak o f t h e
and a i s r e l a t e d t o t h e f u l l w i d t h a t h a l f
A t t i m e to t h e
t h e implanted p r o f i l e .
l i q u i d i s divided
4 (a 2+Dt )
D i s t h e d i f f u s i o n coef-
where Co i s t w i c e t h e implanted dose, implanted p r o f i l e
exp-[
i n t o equal
segments o f w i d t h
25-40 A ) c o n t a i n i n g a dopant c o n c e n t r a t i o n C;(xm,to).
AX
remaining (typically The f i r s t
segment i s s o l i d i f i e d (Fig. 4b) i n s t a n t a n e o u s l y w i t h an amount o f being incorporated i n t o the s o l i d a t L m o x , and j l - k ' ) C ' ( x ,t )Ax b e i n g r e j e c t e d i n t o t h e nearest l i q u i d m L m o I n t h e above expressions, C ' i s t h e dopant c o n c e n t r a t i o n 'm-1' L i n the l i q u i d a t the interface p r i o r t o s o l i d i f i c a t i o n . The
dopant
C'AX
= k ' C ' ( x ,t )Ax
r e j e c t e d i m p u r i t y i s t h e n a l l o w e d t o d i f f u s e i n t h e remaining l i q u i d w i t h t h e i n t e r f a c e h e l d s t a t i o n a r y as shown i n Fig. 4c f o r a t i m e A t = Ax/v where v i s t h e growth v e l o c i t y . solution
By numerical
o f t h e mass d i f f u s i o n equation expressed i n f i n i t e d i f -
ferences, we f i n d f o r t h e m-th l i q u i d l a y e r
where D i s t h e l i q u i d phase d i f f u s i o n c o e f f i c i e n t . was solved n times f o r t h e t i m e i n t e r v a l n was chosen t o be > 10 so t h a t nAx
>>
AT =
At/n.
Equation 8 The value o f
m t and D A T / ( A x ) ~was
r e q u i r e d t o be < 0.5 t o i n s u r e t h e convergence o f t h i s numerical method.
Then a t t h e t i m e t = (to+ A t ) t h e m e l t f r o n t advances
t o p o s i t i o n xMel
(Fig.
4d) w i t h t h e corresponding p a r t i t i o n o f
dopant between t h e s o l i d and l i q u i d phase g i v e n by t h e i n t e r f a c i a l d i s t r i b u t i o n c o e f f i c i e n t as discussed above.
These calcu-
l a t i o n s are continued u n t i l t h e i n t e r f a c e reaches t o w i t h i n 200 A o f t h e s u r f a c e and t h e dopant remaining i n t h e l i q u i d i s considered t o be segregated t o t h e surface. diffusion coefficient, 1963).
D,
Values f o r t h e l i q u i d phase
a r e taken from t h e l i t e r a t u r e (Kodera,
T h i s model assumes t h a t t h e o n l y mechanism f o r mass t r a n s -
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
p o r t i s l i q u i d phase d i f f u s i o n ,
57
and regrowth v e l o c i t y i s assumed
t o be constant d u r i n g s o l i d i f i c a t i o n .
The dopant f l u x out o f t h e
s u r f a c e d u r i n g regrowth i s r e q u i r e d t o be zero unless t h e r e was a n e t l o s s o f dopant d u r i n g t h e a n n e a l i n g process as determined by
I f dopant loss occurs, t h i s i s taken
i o n backscattering analysis.
i n t o account by r e q u i r i n g t h a t l o s s occurred from t h e s u r f a c e a t a r a t e p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n a t t h e surface.
I n the
model k l i s assumed t o be a c o n s t a n t , independent o f t h e dopant c o n c e n t r a t i o n , and i s t r e a t e d as a f i t t i n g parameter.
A value of
k ' f o r each dopant was determined by f i t t i n g t h e c a l c u l a t e d profile
in
the solid
t o the
measured p r o f i l e u s i n g l e a s t squares
anal'ysi s.
111. 5.
Dopant Incorporation During Rapid S o l i d i f i c a t i o n
SEGREGATION BEHAVIOR OF B, P, AND As I N SILICON Boron, phosphorus,
used
dopants
in
the
device applications.
and a r s e n i c a r e t h e t h r e e most commonly p r o c e s s i n g of
silicon
for
semiconductor
F i g u r e 5 shows t h e e f f e c t s o f l a s e r annealing
of c r y s t a l s implanted with By P, and As, each a t a dose o f -1016/cm2 (White e t al.,
1978).
The l a s e r a n n e a l i n g c o n d i t i o n s were such
t h a t t h e r e c r y s t a l l i z a t i o n v e l o c i t y was -3 m/sec. and P were measured by SIMS, measured by RBS.
P r o f i l e s for B
w h i l e those f o r As p r o f i l e s were
Concentrations determined by SIMS were estimated
by comparison w i t h r e s u l t s obtained from samples implanted i n t h e dose range lo1'+ t o 1016/cm2.
For each dopant i n F i g . 5, t h e as-
implanted p r o f i l e i s v e r y n e a r l y Gaussian, b u t i n each case l a s e r a n n e a l i n g causes a s i g n i f i c a n t r e d i s t r i b u t i o n o f t h e dopant, b o t h toward t h e s u r f a c e as w e l l
as deeper i n t o t h e c r y s t a l t o t h e
e x t e n t t h a t t h e p r o f i l e i s n e a r l y u n i f o r m i n t h e depth range 1000-2000
k a f t e r l a s e r annealing.
These r e s u l t s demonstrate
t h e r a p i d r e d i s t r i b u t i o n o f t h e dopants which can occur i n t h e l i q u i d phase, due t o t h e very h i g h d i f f u s i v i t i e s i n t h e l i q u i d (DL
- lo4
cm2/sec);
r e d i s t r i b u t i o n over these extended d i s t a n c e s
58
C . W. WHITE
would be i m p o s s i b l e b y s o l i d phase d i f f u s i o n because s o l i d phase d i f f u s i v i t i e s are almost e i g h t o r d e r s o f magnitude lower, and t h e t i m e a v a i l a b l e f o r d i f f u s i o n (a few hundred nanoseconds) i s t o o short. Values o f t h e e q u i l i b r i u m d i s t r i b u t i o n c o e f f i c i e n t ko f o r P, and As i n S i a r e 0.80,
1960).
0.35,
and 0.30
B,
r e s p e c t i v e l y (Trumbore,
With these values o f ko pronounced s u r f a c e s e g r e g a t i o n
should have been observed f o r P and As but, as Fig. 5 i n d i c a t e s were not.
The l a c k o f a s u r f a c e s e g r e g a t i o n s p i k e i n t h e t h r e e
p r o f i l e s a f t e r l a s e r a n n e a l i n g i s good evidence t h a t k ' has grown from ko t o n e a r l y u n i t y f o r v
- 3 m/sec.
F i g u r e 6 shows how a comparison o f experimental and c a l c u l a t e d p r o f i l e s f o r As i n s i l i c o n (White e t al., v a l u e o f k ' t o be determined.
1980) a l l o w s t h e
F o l l o w i n g l a s e r annealing,
c h a n n e l i n g r e s u l t s show t h a t As i s
>
95% s u b s t i t u t i o n a l
ion
i n the
l a t t i c e and i s e l e c t r i c a l l y a c t i v e as determined from H a l l e f f e c t measurements.
T h i s h i g h degree of s u b s t i t u t i o n a l i t y i s achieved
even though t h e As c o n c e n t r a t i o n exceeds t h e e q u i l i b r i u m s o l u b i l i t y l i m i t by a f a c t o r o f -4 i n t h e n e a r - s u r f a c e region.
This
demonstrates t h e f o r m a t i o n o f a s u p e r s a t u r a t e d a l l o y as a consequence o f t h e h i g h speed, 1 iquid-phase e p i t a x i a l regrowth process. value
The s o l i d l i n e i n Fig. 6 i s a p r o f i l e c a l c u l a t e d u s i n g a for
the d i s t r i b u t i o n
coefficient
of
k'
=
1.0
and t h e
agreement w i t h t h e experimental p r o f i l e r e s u l t s ( s o l i d c i r c l e s ) i s excellent.
The value determined f o r k ' i s c o n s i d e r a b l y h i g h e r
t h a n t h e e q u i l i b r i u m value distribution coefficient
(ko = 0.3).
The i n c r e a s e i n t h e
r e l a t i v e t o t h e e q u i l i b r i u m value i s a
consequence o f t h e h i g h regrowth v e l o c i t y which causes a depart u r e from c o n d i t i o n s o f l o c a l e q u i l i b r i u m a t t h e i n t e r f a c e d u r i n g sol i d i f ication.
6.
SEGREGATION BEHAVIOR OF OTHER GROUP 111-V DOPANTS I N SILICON As we have j u s t seen,
for
B y P, and As i n
values o f t h e s e g r e g a t i o n c o e f f i c i e n t
S i have a l r e a d y grown from t h e i r ko values t o
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
59
1022
5
2
102' I
m
1020
5
2
1049
F i g . 6 . Profiles for 75As (100 keV, 6 . 4 ~1 0 l 6 / c m 2 ) in ( 1 0 0 ) S i c o m p a r e d to model calculations. The equilibrium solubility limit is indicated by the horizontal line. From White e t a t . . 1980.
nearly unity for v
- 3 m/sec.
Other Group 111-V dopants have
s u b s t a n t i a l l y s m a l l e r values o f ko than do B, P, and As and more pronounced changes i n t h e s e g r e g a t i o n b e h a v i o r w i t h r e c r y s t a l l i z a t i o n v e l o c i t y can be expected. B i (ko = 0.0007)
and I n (0.0004)
F i g u r e s 7 and 8 show p r o f i l e s f o r i n S i o b t a i n e d w i t h v = 4.5 m/sec.
I n F i g . 7, as a consequence o f l a s e r annealing, a p p r o x i m a t e l y 15% of t h e B i segregates t o t h e s u r f a c e but t h e c o n c e n t r a t i o n remaining
60
C. W. WHITE 102'
5
2 1020
L
z
$
5
z
8 2 1018
5 2 10'7
Fig.
7.
Profiles for
2096i ( 2 5 0 keV,
1.2 x 1015/cm2)
in ( 1 0 0 ) S i
compared to model calculations. The horizontal line indicates the equilibrium From solubility limit and the dashed p r o f i l e i s calculated assuming k' = k,. White et a l . ,
1980.
i n the bulk i s though
this
>
95% s u b s t i t u t i o n a l ( i o n channeling r e s u l t s ) even
concentration
exceeds
the
equilibrium
l i m i t by a p p r o x i m a t e l y two orders o f magnitude. i n Fig.
solubility
The s o l i d l i n e
7 is a p r o f i l e c a l c u l a t e d u s i n g a value f o r k ' = 0.4 and
assuming t h a t t h e l i q u i d phase d i f f u s i v i t y f o r B i i n Si i s
DL
=
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
61
102’
5
2 1020
2 10’8
5
2
0
0.1 DEPTH ( p m )
0.2
0.3
Fig. 8. Profiles for l i 5 I n ( 1 2 5 keV, 1.2 x lOi5/Cm2) i n Si compared to model calculations. The horizontal line indicates the equilibrium solubility limit and the dashed p r o f i l e i s calculated assuming k l = ko. From White e t a l . , 1980.
1.5 x lo-‘+. Values for DL ( B i i n S i ) have not been reported in the l i t e r a t u r e , b u t the value of 1.5 x gives a satisfactory f i t t o the experimental results and i s in reasonable agreement with the extrapolation o f measured liquid phase diffusivities for
62
C. W. WHITE
lower mass impurities i n liquid s i l i c o n . In Fig. 7 , the calcul a t e d p r o f i l e ( s o l i d l i n e ) i s in good agreement with the experimental p r o f i l e s ( f i l l e d c i r c l e s ) measured a f t e r l a s e r annealing. By c o n t r a s t , a p r o f i l e calculated using the equilibrium value f o r the d i s t r i b u t i o n c o e f f i c i e n t of Bi in Si (ko = 7 x l o m 4 ) , i s shown by the dashed curve in Fig. 7. If s o l i d i f i c a t i o n occurred under conditions of local equilibrium a t t h e i n t e r f a c e , very l i t t l e Bi would remain in t h e b u l k of t h e crystal and almost a l l of the Bi would have zone refined t o t h e surface. Clearly t h i s does not f i t t h e experimental r e s u l t s . Similar r e s u l t s a r e obtained f o r t h e case of In in Si as shown in Fig. 8. As a r e s u l t of l a s e r annealing, approximately 60% of the In i s zone refined t o the surface, b u t the remainder i n the bulk i s highly substitutional and the p r o f i l e can be f i t with reasonable accuracy by using a value f o r k ' = 0.15. This value i s f a r greater than the equilibrium value f o r In in Si If local equilibrium conditions prevailed during (ko = 4 x the s o l i d i f i c a t i o n , very l i t t l e In would have remained in t h e bulk a s indicated by the dotted p r o f i l e in Fig. 8. I t i s i n t e r e s t i n g t o note t h a t t h e experimental p r o f i l e r e s u l t s in Figs. 6, 7 and 8 can be f i t by a s i n g l e value of k ' over t h e e n t i r e range of concentrations. This indicates t h a t the value f o r k ' i s not a strong function of concentration, and is determined, t o f i r s t order, by t h e regrowth velocity. For t h e case of B i i n S i , experiments s i m i l a r t o those i l l u s t r a t e d in Fig. 7 have been c a r r i e d out a t both higher and lower implanted doses (concentrations). In each case the value determined f o r k ' l i e s in t h e range 0.35 t o 0.40 even though the implanted dose was varied by over an order of magnitude. This f u r t h e r reinforces the conclusion t h a t the value f o r k ' i s not a strong function of concentration. Using similar methods, values f o r k ' have been determined f o r a wide variety of Group I11 and V dopants in (100) Si a t t h e very high growth v e l o c i t i e s which can be achieved by pulsed l a s e r
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
63
Table I Comparison of Distribution Coefficients Under Equilibrium (k,) and Laser Annealed ( k ' ) Regrowth Conditions Dopant B P As Sb Ga In Bi
(a)
(b) k'
0.80 0.35 0.30 0.023 0.008 0.0004
1.0 1.0 1.0
ko
0.0007
0.7 0.2
0.15 0.4
( a ) From Trumbore, 1960. (b) Values f o r k ' were determined a t a growth velocity of 2.7 m/sec f o r B, P, Sb, and a t 4.5 m/sec f o r As, Ga, In, and Bi.
.
anneal i ng (White et a1 , 1980). The values determi ned f o r k ' during pulsed l a s e r annealing a r e summarized i n Table I and compared with the corresponding equi 1ibri um values , ko (Trumbore, 1960). These values f o r k ' were determined a t a growth velocity of 4.5 m/sec except f o r the cases of B, P and Sb. For these t h r e e dopants, a somewhat longer pulse duration time (- 50 x sec) was used f o r annealing, r e s u l t i n g i n a growth velocity of -2.7 m/sec. Values f o r the l i q u i d phase d i f f u s i v i t i e s used t o f i t the experimental p r o f i l e s were taken from the l i t e r a t u r e (Kodera, 1963) except f o r t h e case of Bi where a value of DL = 1.5 x cm2/sec was assumed (see previous discussion). The r e s u l t s presented in Table I show t h a t in every case k ' i s s i g n i f i c a n t l y greater than ko by f a c t o r s t h a t extend up t o -600. The values reported in Table I were the f i r s t determination o f i n t e r f a c i a l d i s t r i b u t i o n c o e f f i c i e n t s under conditions of high speed nonequilibrium crystal growth f o r any system. The l a r g e increase in k ' r e l a t i v e t o ko r e f l e c t s the nonequilibrium nature of the l a s e r annealing induced liquid-phase e p i t a x i a l regrowth process. The departure from conditions o f local equilibrium a t
64
C . W. WHITE
the i n t e r f a c e i s brought about by the very high growth v e l o c i t i e s (several meters/sec) which can be achieved by l a s e r annealing. In crystal growth a t low v e l o c i t i e s where local equilibrium cond i t i o n s prevail, s o l u t e atoms exchange many times across the i n t e r f a c e in order t o e s t a b l i s h t h e i r equilibrium concentrations i n the s o l i d and l i q u i d before being permanently incorporated i nto the sol id. During pul sed l a s e r anneal i ng , regrowth vel oci t i e s a r e so high t h a t a new plane of atoms i s being added t o the growing crystal every sec. On t h i s time s c a l e , s o l u t e atoms cannot be exchanged across the i n t e r f a c e a s u f f i c i e n t number of times t o e s t a b l i s h t h e i r equilibrium concentrations before being incorporated i n t o the sol id. Consequently, s o l u t e atoms a r e trapped i n t o the s o l i d a t concentrations t h a t can f a r exceed equilibrium s o l u b i l i t y l i m i t s , a process referred t o as s o l u t e trapping
.
7.
EFFECTS OF REGROWTH VELOCITY AND SUBSTRATE ORIENTATION ON k '
Experiments have shown t h a t the i n t e r f a c i a l d i s t r i b u t i o n coeff i c i e n t is a function of both growth velocity (Cullis e t al., 1980; Baeri et al., 1981) and crystal orientation (Baeri e t al., 1981a). The velocity dependence i s e n t i r e l y expected because as t h e velocity decreases, k ' must approach the equi 1 i bri um Val ue , ko. C u l l i s et a l . (1980) reported t h e f i r s t observations of t h i s expected velocity dependence f o r several d i f f e r e n t Val ues of v f o r the case o f P t in S i where i t was observed t h a t increasing t h e growth velocity resulted in more implanted P t being incorporated i n t o the l a t t i c e during l a s e r annealing. Similar r e s u l t s on velocity dependence a r e shown in Fig. 9 f o r t h e case of Bi in Si (White et a l . , 1981). Substrate temperatures of 650 K, 300 K and 100 K give r i s e t o regrowth velocit i e s of -1.5, 4.5 and 6.0 m/sec f o r the l a s e r conditions used f o r sec, 1.4 J/cm2). A t the low annealing (X = 6943 A , 15 x growth velocity (1.5 m/sec), almost 55% of the implanted B i
d
0
rd C C
tu U 0
L
rd
*-
- +I 2
oa
66
C . W. WHITE
segregates t o the surface as a r e s u l t of l a s e r annealing, while a t the highest growth velocity only 5% i s segregated t o the surface. In each case, the Bi remaining in the bulk of the crystal i s >95% substitutional in the l a t t i c e . Dotted l i n e s in Fig. 9 a r e calculated p r o f i l e s using values f o r k ' = 0.1, 0.35 and 0.45 a t growth v e l o c i t i e s of 1.5, 4.5 and 6.0 m/sec. The agreement between the calculated and experimental p r o f i l e s in Fig. 9 i s excellent and these r e s u l t s demonstrate t h a t k ' and the amount of B i segregated t o the surface a r e strong functions of growth veloci t y , as expected. A similar dependence of k' on regrowth velocity has been reported also f o r the case of In in Si (Baeri et al., 1981), and similar dependencies should be observable f o r a l l Group 111, V species in s i l i c o n . These experiments, i f carried out over a wider velocity range can be expected t o provide fundament a l insight into d e t a i l e d mechanisms of importance t o high speed nonequi 1 i b r i u m crystal growth processes. Baeri e t a l . (1981a) f i r s t demonstrated t h a t in c e r t a i n ranges, t h e value f o r k ' i s a l s o a strong function of crystal o r i e n t a t i o n . An example of t h i s e f f e c t i s shown in Fig. 10 f o r (100) and (111) c r y s t a l s implanted by l151n (125 keV, 1.2 x 1015/cm2) and l a s e r annealed under identical conditions (XeC1 l a s e r , -35 x sec, 1.3 J/cm2). Considerably more In i s trapped i n the b u l k of the (111) crystal implying t h a t the value f o r k ' i s systematically l a r g e r f o r the (111) case. Figure 11 shows the velocity dependence of k ' f o r In in (100) and (111) Si (Poate 1982). For v e l o c i t i e s below -4 m/sec the value f o r k ' in (111) Si i s systematically higher than t h a t f o r (100) Si For identical 1a s e r anneal i ng conditions, t h e regrowth velocity normal t o the surface should be the same since velocity i s determined by heat flow i n t o the underlying substrate. Consequently the anisotropic dependence of k ' on growth velocity must be related t o differences in d e t a i l e d mechanisms of crystal growth f o r (100) and (111) c r y s t a l s . In p a r t i c u l a r , i t has been suggested
.
2. SEGREGATION,SOLUTE TRAPPING,AND SUPERSATURATED ALLOYS 67
I
0
I
I
0.4 DEPTH ( p m )
I
0.2
0
O.!
0.2
DEPTH (,urn 1
Fig. 10. Dopant profiles for 1151n ( 1 2 5 keV, 1.2 x 1 0 1 5 / ~ m 2i)n (100) and ( 1 1 1 ) Si. From White e t al. , 1983.
that a larger interfacial undercooling on the (111) face compared t o the (100) face (Baeri e t al., 1981a; Jackson, 1981) m i g h t explain the differences in dopant incorporation for these two cases. Alternatively the greater ledge velocity which i s expected on the (111) face may be responsible for the increased value for k' (Spaepen and Turnbull , 1982). A dependence of k ' on orientation has been observed for B i , I n , Gay Sn, and Pb i n s i l i con a t velocities of 2-4 m/sec. I n each case the value f o r k' i s greater f o r the (111) case. Impurities for which k ' i s very near t o unity do not show this effect. These include B, P, As, Ge, and Sb.
68
C. W. WHITE
I .o
I
I
I
I
I
I
I
*-----
-,-
A
Fig. 1 1 . i n silicon.
8.
Dependence o f kl on growth velocity and crystal orientation for In From Poate ( 1 9 8 2 ) .
MAXIMUM SUBSTITUTIONAL SOLUBILITIES
White e t a l . (1980) have shown t h a t as the implanted dose of each of the Group 111, V species i s increased, there i s a maximum concentration t h a t can be incorporated s u b s t i t u t i o n a l l y i n t o the Si l a t t i c e as a r e s u l t o f pulsed l a s e r annealing. (See a l s o Stuck et a l . , 1980). This i s shown in Fig. 12 f o r four d i f f e r e n t doses of In (125 keV) in (100) S i , where both the t o t a l dopant concentration and t h e substitutional dopant concentration a f t e r l a s e r annealing are plotted as a function o f depth. These r e s u l t s
Cyx
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
69
r-7 DOSE = 7.9 x 10'5/cm2
*--+SUBSTITUTIONAL
0
0.! 0.2 DEPTH ( p m )
t ,
0
0.1 0.2 DEPTH ( p m )
Fig. 12. Dose dependence o f solute trapping for I n i n ( 1 0 0 ) Si. Total and substitutional concentration p r o f i l e s a r e shown for each dose a f t e r ruby laser From White e t a l . , 1983. annealing.
were obtained by using Rutherford backscattering and i o n channeling measurements to determine the total dopant concentration and the substitutional concentration as a function of depth. At the two
70
C. W. WHITE
Table I1 S u b s t i t u t i o n a l S o l u b i l t i e s i n S i l i c o n Achieved by R e c r y s t a l l i z a t i o n a t 4.5 m/sec
(100) S i
(111) S i
As
1.5 x 1021
6.0 x 1021
6.0 x 1021
Sb Bi
7.0 x 1019 8.0 x lOl7
2.0 x 1O2l 4.0 x 102O
2.0 x 1O2l 8.6 x 102O
Ge
5.0 x 1022
6.0 x 1021
>1.2 x 1022
Sn Pb
5.5 x loL9
9.8 x 1020 1.0 x 1020
1.4 x 1O2l 3.0 x 102O
B
6.0 x 1020
2.0 x 1021
2.0 x 1021
Ga In T1
4.5 x 1019 8.0 x 1017
4.5 x 1020 1.5 x 1020
7.2 x 1020 4.5 x 1020
---
---
l o w e r doses, tional
---
Thermodynamic Limit C e l l Formation Precipitation C e l l Formation C e l l Formation on (100) C e l l Formation Precipitation C e l l Formation Mechanical Strain C e l l Formation C e l l Formation Coherent Prec ip i t a t ion
---
i n t h e b u l k o f t h e c r y s t a l t h e t o t a l and s u b s t i t u -
c o n c e n t r a t i o n s a r e v i r t u a l l y i d e n t i c a l and t h e p r o f i l e s
s c a l e w i t h implanted dose. For t h e two h i g h e r doses, t h e t o t a l and s u b s t i t u t i o n a l c o n c e n t r a t i o n s a r e n e a r l y t h e same up t o a c o n c e n t r a t i o n o f 1.5-2.0 x 102°/cm3. As t h e t o t a l c o n c e n t r a t i o n increases
above
this
value,
the
substitutional
remains t h e same o r decreases somewhat. d i t i o n s (v = 4.5 m/sec)
t h i s value
maximum c o n c e n t r a t i o n (Cyax)
concentration
For these regrowth con-
o f 1.5-2.0 x 1020/cm3 i s t h e
o f I n which can be i n c o r p o r a t e d i n t o
substitutional l a t t i c e sites. Using
similar
techniques,
values f o r Ca:x
have been d e t e r -
mined f o r n i n e Group 111, I V and V species i n (100) and (111) S i a t a growth v e l o c i t y o f -4.5
m/sec.
These values are l i s t e d i n
Tab1 e I1 and compared t o correspondi ng e q u i l ib r i um s o l u b i l it y
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
LIQUID
71
+ SOLID
c:
CONC E NT RAT I0N
Fig.
13.
Schematic phase diagram for retrograde alloys.
l i m i t s Ci. With t h e exception of Ge, values f o r C:ax are l a r g e r than those f o r C: by f a c t o r s t h a t range from 4 in the case of As, t o -500 f o r the case of B i . On an equilibrium phase diagram (shown schematically in Fig. 13) most of these dopants exhibit retrograde s o l u b i l i t y in s i l i c o n . T h i s means t h a t the dopant has i t s maximum s o l u b i l i t y C z a t a temperature which i s not simply As shown by Baker and Cahn r e l a t e d t o a e u t e c t i c temperature. (1969), t h e retrograde maximum concentration cannot be exceeded by s o l i d i f i c a t i o n from the l i q u i d unless t h e r e i s a departure from equilibrium a t t h e i n t e r f a c e during s o l i d i f i c a t i o n . In Table 11, t h e large values f o r r e l a t i v e t o C! convincingly demonstrate the nonequil ibrium nature o f the l a s e r anneal ing induced 1 iquid-phase e p i t a x i a l regrowth process.
Cyx
72
C. W. WHITE
Dopant i n c o r p o r a t i o n i n t o t h e l a t t i c e a t these h i g h concent r a t i o n s i s a r e s u l t o f "solute trapping" during s o l i d i f i c a t i o n .
I n t h e s i m p l e s t terms t h i s means t h a t i f t h e t i m e r e q u i r e d t o regrow one o r more monolayers o f atoms d u r i n g s o l i d i f i c a t i o n i s s i g n i f i c a n t l y s h o r t e r t h a n t h e residence t i m e o f t h e i m p u r i t y a t t h e i n t e r f a c e then t h e i m p u r i t y has a h i g h p r o b a b i l i t y o f b e i n g i n c o r p o r a t e d i n t o t h e growing s o l i d .
Theoretical treatments o f
s o l u t e t r a p p i n g are g i v e n i n Baker and Cahn (1969), Cahn e t a l .
(1980), Wood (19801, Jackson e t a l . F o r several Ca:x
of
are l a r g e r i n
the
(1980),
and A z i z (1982).
dopants l i s t e d i n Table 11,
(111) S i compared t o t h e
values f o r
(100) case.
These
species i n c l u d e B i , Ge, Sn, Pb, Ga and I n and f o r these i m p u r i ties
k'
<
1,
and
I n each o f these
the
value
f o r k'
cases t h e values f o r
is
l a r g e r i n (111) S i .
Cyxare l i m i t e d by i n t e r -
f a c e i n s t a b i l i t y which develops d u r i n g regrowth and l e a d s t o c e l l f o r m a t i o n i n t h e near s u r f a c e region.
The l a r g e r v a l u e f o r k ' i n
t h e (111) c r y s t a l means t h a t h i g h e r dopant c o n c e n t r a t i o n s can accummulate a t t h e i n t e r f a c e b e f o r e i n s t a b i l i t y develops.
This
w i l l happen o n l y when t h e v a l u e f o r k' i s l e s s t h a n u n i t y .
Mech-
anisms l i m i t i n g s u b s t i t u t i o n a l
s o l u b i l i t y w i l l be discussed i n
more d e t a i l i n S e c t i o n I V . I t i s i n t e r e s t i n g t o note t h a t e q u i l i b r i u m s o l u b i l i t y l i m i t s
a l s o can be g r e a t l y exceeded d u r i n g low temperature thermal anneali n g ( s o l i d phase e p i t a x i a l
regrowth)
o f i o n implanted s i l i c o n
( W i l l i a m s and Elliman, 1981; Campisano e t al., al.,
1980b).
1980; Campisano e t
Published r e s u l t s i n d i c a t e t h a t maximum s u b s t i t u -
t i o n a l s o l u b i l i t i e s obtained a f t e r thermal annealing (55OoC, 30 mins) a r e o n l y a f a c t o r o f 2 t o 3 lower than those achieved by p u l s e d l a s e r annealing.
I n t h e thermal annealing case,
dopant
i n c o r p o r a t i o n appears t o be t h e r e s u l t o f ' ' s o l u t e t r a p p i n g " a t t h e s l o w l y moving amorphous/crystall i n e i n t e r f a c e . p e r a t u r e s used f o r annealing (-550°C),
A t t h e tem-
t h e v e l o c i t y o f t h e amor-
p h o u s / c r y s t a l l i n e i n t e r f a c e i s o n l y -10-lo m/sec, b u t t h e i m p u r i t y
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
73
residence t i m e a t t h e i n t e r f a c e i s very l o n g s i n c e t h i s t i m e i s i n v e r s e l y p r o p o r t i o n a l t o t h e s o l i d phase d i f f u s i v i t y which i s v e r y low.
Consequently, even i n s o l i d phase e p i t a x y , a tempera-
t u r e range can be s e l e c t e d such t h a t t h e i m p u r i t y residence t i m e a t t h e i n t e r f a c e i s l o n g e r t h a n t h e monolayer regrowth time, and t r a p p i n g can occur. annealing,
However, i f h i g h e r temperatures a r e used f o r
then p r e c i p i t a t i o n o f
excess o f t h e sol u b i 1 it y 1i m i t
t h e dopant c o n c e n t r a t i o n
in
a t t h e anneal i n g temperature w i 11
occur.
9.
MEASUREMENTS OF EQUILIBRIUM SOLUBILITY LIMITS Equi 1ib r i urn sol u b i 1i t y 1i m i t s a r e usual l y e s t a b l ished by t h e
same methods used f o r experimental d e t e r m i n a t i o n o f phase diagrams. Ion
i m p l a n t a t i o n and l a s e r annealing
highly scribed
accurate
alternative
i n the following.
method
p r o v i d e an i n t e r e s t i n g and f o r d e t e r m i n i n g ,C:
as de-
Supersaturated s o l i d s o l u t i o n s o f
Group 111, I V and V species i n S i can be r e a d i l y formed by pulsed
1aser anneal i n g o f i o n implanted s i l i c o n . a l l o y s a r e completely
These supersaturated
s t a b l e a t room temperature,
c r y s t a l s subsequently a r e t h e r m a l l y annealed,
but i f the
precipitation of
t h a t p a r t o f t h e dopant c o n c e n t r a t i o n i n excess o f t h e e q u i l i b r i u m s o l u b i l i t y l i m i t a t t h e a n n e a l i n g temperature w i l l occur a t a r a t e determined p r i m a r i l y by t h e sol i d phase d i f f u s i v i t y .
After ther-
mal annealing, a n a l y s i s by RBS-ion channeling techniques can be used t o e s t a b l i s h e x p e r i m e n t a l l y t h e e q u i l i b r i u m s o l u b i l i t y a t t h e annealing temperature.
An example o f t h i s method o f d e t e r m i n i n g s o l u b i l i t y l i m i t s i s shown i n Fig. 14 f o r t h e case o f Sb i n S i (White e t al.,
1980a).
F o l l o w i n g i m p l a n t a t i o n and l a s e r
annealing, t h e near-surface r e g i o n was d e f e c t f r e e and t h e Sb was measured t o be very n e a r l y 100% i n s u b s t i t u t i o n a l l a t t i c e s i t e s even though t h e c o n c e n t r a t i o n exceeded t h e r e p o r t e d e q u i l i b r i u m s o l u b i l i t y l i m i t by more than a f a c t o r o f two.
The sample was
t h e n t h e r m a l l y annealed a t 1150°C (a temperature which i s very
74
C. W. WHITE
I
-
I
I
I
I
-
0 TOTAL
0 SUBSTITUTIONAL
-
2c
0
E‘u
g- 1020 -
Y
I 0.
I-
0
5 0
a,
c- smRO.(3
5 z w
00 0
.
- -- - - --
.-oO--. EQUILIBRIUM SOLUBILITY LIMIT 0
0.0
0
- 0
Q
5 -
-
e
n m
1 -
-
8@
2-
0
Id9
I
I
I
c l o s e t o t h e r e t r o g r a d e temperature)
d
0
f o r 30 min.
I
Rutherford
b a c k s c a t t e r i ng and i o n channel ing measurements were t h e n used t o measure t h e t o t a l
dopant c o n c e n t r a t i o n and t h e s u b s t i t u t i o n a l
c o n c e n t r a t i o n as a f u n c t i o n o f depth. show t h a t a f t e r thermal
processing,
The r e s u l t s i n Fig.
14
t h e maximum s u b s t i t u t i o n a l
dopant c o n c e n t r a t i o n was 8.2 x 1019/cm3, which i s t a k e n t o be t h e e q u i l i b r i u m s o l u b i l i t y l i m i t f o r Sb i n S i a t l l O O ° C .
This r e s u l t
i s i n reasonably good agreement w i t h t h e value (7.0 x 1019/cm3) r e p o r t e d p r e v i o u s l y f o r Sb i n S i (Trumbore, 1960).
I n principle,
t h i s method c o u l d be extended t o determine t h e s o l i d u s l i n e on t h e equi 1 ib r i urn phase diagram.
2. 10.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
75
ZONE REFINING OF INTERSTITIAL IMPURITIES The atoms i n Groups 111, I V Y and V o f t h e p e r i o d i c c h a r t can
be r e a d i l y incorporated i n t o s u b s t i t u t i o n a l s i t e s i n t h e s i l i c o n l a t t i c e a t concentrations t h a t f a r exceed t h e e q u i l i b r i u m s o l u b i l i t y l i m i t s as a r e s u l t o f pulsed l a s e r annealing.
By c o n t r a s t
t h e r e are many i m p u r i t i e s outside Group 111, I V or V which cannot be incorporated i n t o l a t t i c e s i t e s by pulsed l a s e r annealing. These i m p u r i t i e s i n c l u d e Fe, C r , W,
Yb, Zn, Cu, Ag, Mn, Mg and T i
and they are i n t h e general category o f those i m p u r i t i e s which do not form covalent bonds w i t h s i l i c o n .
A l l o f these i m p u r i t i e s
segregate toward t h e surface as a r e s u l t o f pulsed l a s e r annealing (Baeri e t al.,
1979; White e t al.,
1979b; White e t al.,
1982).
The degree o f segregation, however, i s a f u n c t i o n o f t h e concent r a t i o n o f the impurity.
A t low concentrations, a complete zone
r e f i n i n g o f t h e i m p u r i t y t o t h e surface can be achieved,
with
no measurable (by RBS) concentrations remaining i n t h e bulk.
At
h i g h concentrations, t h e i n t e r f a c e becomes unstable d u r i n g regrowth l e a d i n g t o t h e formation o f a c e l l s t r u c t u r e i n t h e near surface region, w i t h t h e i m p u r i t y l o c a t e d i n t h e c e l l walls. F i g u r e 15 i l l u s t r a t e s these c h a r a c t e r i s t i c s f o r t h e case o f t h r e e d i f f e r e n t doses o f 56Fe (150 keV) i n (111) S i .
I n t h e low
dose case, a s i n g l e l a s e r pulse causes complete zone r e f i n i n g o f t h e implanted Fe t o t h e surface w i t h none (as measured by RBS) remaining i n t h e bulk. A t t h e intermediate dose o f 6 x lO15/cm* t h e Fe can be segregated t o t h e near surface r e g i o n but several pulses (two o r more) are required.
I n t h e h i g h dose case, Fe i s
c l e a r l y segregated toward t h e surface, b u t s u b s t a n t i a l q u a n t i t i e s remain i n t h e b u l k o f t h e c r y s t a l even a f t e r t e n pulses.
The Fe
remaining i n t h e b u l k o f t h e c r y s t a l shows no channeling e f f e c t , i n d i c a t i n g no p r e f e r r e d l a t t i c e s i t e . The i m p u r i t y i s l o c a l i z e d i n the w a l l s o f a well defined c e l l s t r u c t u r e i n the near surface region.
This c e l l s t r u c t u r e r e s u l t s from i n t e r f a c i a l i n s t a b i l i t y
76
C. W. WHITE
0
0
0.1
0.2
0
AS IMPLANTED LASER ANNEALE
0.1
0.2 DEPTH ( p )
0
0.1
0.2
0.3
Fig. 15. Redistribution o f 56Fe ( 1 5 0 keV) i n ( 1 1 1 ) Si as a result of ruby laser annealing. Results presented i n ( a ) , ( b ) , and ( c ) were achieved using one pulse ( a ) , five pulses ( b ) , and lopulses ( c ) , respectively. From White e t a l . , 1982.
which develops d u r i n g regrowth, cooling a t the interface. c e l l u l a r structure,
caused by c o n s t i t u t i o n a l super-
Because o f t h e occurrence o f t h e
c a l c u l a t i o n s s i m i l a r t o those described i n
subsection 11-4 can o n l y be used t o determine an upper l i m i t f o r Such c a l c u l a t i o n s i n d i c a t e t h a t k ' ( F e ) < 10-2 f o r regrowth v e l o c i t i e s o f -4 m/sec. This value i s much lower than those
k'.
determined f o r Group I I 1 , V
i m p u r i t i e s (see Table I)and suggests
t h a t f o r these i m p u r i t i e s segregation a t t h e l i q u i d - s o l i d i n t e r f a c e takes place under c o n d i t i o n s t h a t are c l o s e r t o e q u i l i b r i u m t h a n f o r t h e case o f Group I I 1 , V i m p u r i t i e s . A behavior s i m i l a r t o t h a t shown i n Fig. 15 has been observed f o r a wide v a r i e t y o f n o n s u b s t i t u t i o n a l i m p u r i t i e s (Cu,Fe,Zn,Mn, W ,Mg,Cr,Yb,Ag,Mg
and T i ) . A t low concentrations these species can be zone r e f i n e d t o t h e surface w i t h a s i n g l e l a s e r pulse. A t h i g h concentrations, c o n s t i t u t i o n a l supercooling causes t h e i n t e r -
f a c e t o become unstable d u r i n g regrowth,
resulting i n lateral
2. SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS 77 segregation and t h e formation o f a c e l l s t r u c t u r e i n t h e near
As w i t h t h e Group I11 and V dopants, f o r a given
surface region.
regrowth v e l o c i t y t h e concentration a t which t h e i n t e r f a c e becomes unstable depends on t h e impurity.
Studies o f t h e type described
here can be expected t o provide i n f o r m a t i o n on t h e c r y s t a l growth parameters which govern t h e development o f t h e i n t e r f a c i a l i n s t a b i l ity. The
fact
that
substitutional
species
can be e f f i c i e n t l y
trapped d u r i n g l a s e r annealing w h i l e n o n s u b s t i t u t i o n a l
species
cannot must be r e l a t e d t o d i f f e r e n c e s i n t h e residence time a t t h e i n t e r f a c e o r t o d i f f e r e n c e s i n t h e bonding p r o p e r t i e s o f these i m p u r i t i e s i n t h e s o l i d . impurity
solid
phase
It has been suggested t h a t t h e
diffusivity
plays
an
important
role
., 1980a).
The i m p u r i t y residence time a t t h e
i n t e r f a c e i s given by A2/Di
where A i s t h e i n t e r f a c e thickness
(Campisano e t a1 and Di
i s the d i f f u s i o n c o e f f i c i e n t i n the
i n t e r f a c i a l region.
I f t h e regrowth time o f a r e g i o n o f thickness A i s s h o r t e r than t h e residence time i n t h e i n t e r f a c e region, then i n c o r p o r a t i o n o f t h e dopant i n t o the s o l i d becomes l i k e l y .
The i n t e r f a c e r e g i o n
separates two phases ( s o l i d and l i q u i d ) i n which d i f f u s i o n coeff i c i e n t s are d i f f e r e n t by a t l e a s t several orders o f magnitude. V i r t u a l l y a l l i m p u r i t i e s i n l i q u i d s i l i c o n have l i q u i d phase d i f f u s i o n c o e f f i c i e n t s which are i n t h e range 10-5-10-3 cm2/sec. However, sol i d phase d i f f u s i v i t i e s
DS f o r s u b s t i t u t i o n a l species
( t y p i c a l l y l e s s than cm*/sec a t t h e m e l t i n g p o i n t ) are much smaller than s o l i d phase d i f f u s i v i t i e s f o r i n t e r s t i t i a l species which are i n t h e range o f point.
to
cm2/sec a t t h e m e l t i n g
It has been suggested (Campisano e t a1
., 1980a)
t h a t the
d i f f u s i o n c o e f f i c i e n t i n t h e " i n t e r f a c i a l region" i s i n t e r m e d i a t e between t h a t o f t h e l i q u i d and t h e s o l i d and can be approximated by Di = (DsDL)l/*. Under t h i s approximation, t h e d i f f u s i o n coeff i c i e n t f o r i n t e r s t i t i a l species i n t h e i n t e r f a c i a l r e g i o n w i l l be much higher than s u b s t i t u t i o n a l
species,
and t h e i m p u r i t y
residence time f o r i n t e r s t i t i a l species i s ( t y p i c a l l y ) l e s s than
78
C. W. WHITE
t h e regrowth t i m e f o r a monolayer.
T h i s may account f o r t h e f a c t
t h a t Group I I 1 , V species can be e f f i c i e n t l y i n c o r p o r a t e d i n t o t h e sol i d while nonsubstitutional
species a r e not.
Significantly
h i g h e r growth v e l o c i t i e s w i l l be r e q u i r e d i n o r d e r t o t r a p these interstitial
species
i n the
l a t t i c e during
laser
annealing.
These v e l o c i t i e s can be o b t a i n e d o n l y by u s i n g picosecond l a s e r p u l s e s p o s s i b l y combined w i t h s u b s t r a t e c o o l i n g d u r i n g i r r a d i a t i o n . C a l c u l a t i o n s (see Chapter 3 ) i n d i c a t e t h a t increases i n v e l o c i t y by a f a c t o r o f 3-4 may be obtained. see whether
these v e l o c i t y
It w i l l be i n t e r e s t i n g t o
i n c r e a s e s w i l l be enough t o t r a p t h e
n o n s u b s t i t u t i o n a l species.
11.
EFFECTS AT FASTER REGROWTH VELOCITIES S i l i c o n l i q u i d phase e p i t a x y a t v e l o c i t i e s i n t h e range o f
1-6 m/sec
gives r i s e t o single c r y s t a l material
f r e e o f any
extended d e f e c t s i f t h e maximum m e l t depth p e n e t r a t e s i n t o t h e undamaged s u b s t r a t e .
A t even f a s t e r v e l o c i t i e s ,
it i s possible
t o produce t h e amorphous phase d u r i n g s o l i d i f i c a t i o n .
T h i s was
f i r s t demonstrated u s i n g 30 picosecond pulses ( L i u e t a1 and -10 nsec pulses (Tsu e t al., single crystal silicon.
., 1979)
1979) o f uv r a d i a t i o n i n c i d e n t on
More r e c e n t l y ,
( C u l l i s e t al.,
1982)
l a r g e area, u n i f o r m amorphous l a y e r s have been formed on s i l i c o n c r y s t a l s u s i n g 2.5 nsec pulses o f 347 nm r a d i a t i o n . Velocities o f up t o 20 m/sec have been achieved u s i n g these s h o r t uv pulses. F i g u r e 16 summarizes t h e observed s i l i c o n r e c r y s t a l l i z a t i o n phenomena i n t h e v e l o c i t y range 1-20 m/sec (Poate, a v e l o c i t y o f -10
m/sec on (100) S i and -5
1982).
Up t o
m/sec on (111) S i ,
e p i t a x i a l regrowth i s observed i n which dopant i n c o r p o r a t i o n i s a s t r o n g f u n c t i o n o f v e l o c i t y f o r Group 111, I V and V i m p u r i t i e s . I n t h i s v e l o c i t y range values f o r k ' can be g r e a t e r on t h e (111) s u r f a c e compared w i t h t h e (100) s u r f a c e f o r t h e same growth velocity.
Above -18
(111) case,
m/sec f o r t h e (100) case and -15
m/sec f o r t h e
r e g r o w t h v e l o c i t y i s so h i g h t h a t t h e atoms a t t h e
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
79
Si SOLIDIFICATION VS LIQUID- SOLID VELOCITY
k'
5
20
15
10
v(m/sec)
Fig. 16. Schematic representation o f phenomena occurring when ( 1 0 0 ) and ( 1 1 1 ) Si i s solidified in the velocity range 1-20 m/s. From Poate, 1982.
i n t e r f a c e do not have t i m e t o undergo t h e s t r u c t u r a l rearrangements necessary f o r e p i t a x y ,
and an amorphous l a y e r i s formed.
The lower v e l o c i t y r e q u i r e d f o r t h e amorphous phase f o r m a t i o n on
(111) c r y s t a l s presumably i s due t o t h e g r e a t e r u n d e r c o o l i n g o r increased ledge v e l o c i t y on t h e (111) surface. velocities, case
epitaxial
defects
in
the
r e c r y s t a l 1ized 1ayer.
regrowth i s observed, form of
twins
are
A t intermediate
but f o r the
(111)
found throughout t h e
80
C. W. WHITE
V.
Mechanisms Limiting Substitutional Sol ubil i ties
As we have seen above, t h e e q u i l i b r i u m s u b s t i t u t i o n a l s o l u b i l i t y l i m i t s f o r some dopants i n s i l i c o n can be exceeded by 2-3
orders o f magnitude d u r i n g pul sed 1aser anneal ing.
The question
n a t u r a l l y a r i s e s as t o whether o r not t h e r e are l i m i t s t o t h e c o n c e n t r a t i o n o f dopants which can be incorporated s u b s t i t u t i o n -
i f so, what are t h e mechanisms t h a t
a l l y i n t o t h e l a t t i c e and, determine these l i m i t s .
Results r e p o r t e d t o date i n d i c a t e t h a t
t h e r e are a t l e a s t f o u r mechanisms (White e t al., must be considered, i.e.,
1983) which
l a t t i c e s t r a i n , c e l l u l a r formation due
t o t h e breakdown o f t h e s t a b i l i t y o f t h e planar m e l t f r o n t , f o r mation o f p r e c i p i t a t e s i n t h e l i q u i d d u r i n g s o l i d i f i c a t i o n , and a fundamental thermodynamic l i m i t t o dopant i n c o r p o r a t i o n which i s expected even a t i n f i n i t e growth v e l o c i t i e s .
These mechanisms
a r e dominant i n t h e v e l o c i t y range l e s s than t h a t r e q u i r e d f o r amorphous phase formation where t h e d e f i n i t i o n o f s u b s t i t u t i o n a l s o l u b i l i t y has no meaning.
Table I1 summarizes measured maximum
s u b s t i t u t i o n a l s o l u b i l i t i e s f o r t e n Group 111, I V o r V dopants i n b o t h (100) and (111) S i measured a t a regrowth v e l o c i t y o f -4.5 m/sec.
The l a s t column i n Table I 1 i n d i c a t e s which mechanism i s
dominant
for
each i m p u r i t y a t t h i s v e l o c i t y .
I n the following,
experimental evidence f o r occurence o f each mechanims i s summarized.
12.
LATTICE STRAIN This mechanism provides a p r a c t i c a l l i m i t t o t h e i n c o r p o r a t i o n
o f boron i n t o t h e s i l i c o n l a t t i c e .
Larson e t al.,
(1978) have
found t h a t s u b s t i t u t i o n a l i n c o r p o r a t i o n o f boron i n t o t h e s i l i c o n l a t t i c e d u r i n g pulsed l a s e r annealing causes t h e l a t t i c e t o undergo a one dimensional c o n t r a c t i o n i n t h e implanted r e g i o n i n a d i r e c t i o n normal t o t h e surface.
The l a t t i c e c o n t r a c t s because the
c o v a l e n t bonding r a d i u s o f t h e boron atom i s s i g n i f i c a n t l y smaller t h a n t h a t o f t h e s i l i c o n atom i t replaces i n t h e l a t t i c e .
The
c o n t r a c t i o n occurs i n one dimension o n l y because c o n t r a c t i o n i n
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
Fig. 17. Cracks produced in the surface region 6 x 1 0 l 6 / c r n 2 ) implanted (100) Si by laser annealing. 1981.
81
of I l l 3 ( 3 5 keV, From White et a l . ,
t h e l a t e r a l d i r e c t i o n i s l i m i t e d by adherence t o t h e underlying c r y s t a l planes.
The magnitude o f t h e c o n t r a c t i o n i s p r o p o r t i o n a l
t o the l o c a l boron contraction.
This c o n t r a c t i o n gives r i s e t o
s t r a i n i n t h e implanted region and when t h e s t r a i n exceeds t h e f r a c t u r e s t r e n g t h o f s i l i c o n , cracks w i l l develop i n t h e near surface region.
For t h e case o f boron i n s i l i c o n , t h i s occurs
when t h e l o c a l boron c o n c e n t r a t i o n exceeds -4 atomic percent. This i s shown by t h e SEM micrographs i n Fig. 17 f o r t h e case o f a B-implanted (35 keV, 6 x 10l6/cm2) l a s e r annealed sample. annealing,
Following
cracks approximately 1 pm wide were present i n t h e
near surface region.
The cracks penetrate t o a depth o f -1 pm,
and extend over t h e e n t i r e l e n g t h o f t h e sample (-1 cm).
For
implanted doses lower than 2.5 x 1016/cm2, t h e near surface r e g i o n
82
C. W. WHITE
i s h i g h l y s t r a i n e d a f t e r l a s e r annealing, b u t cracks do not devel-
op.
If a dopant such as Sb, which has a l a r g e r covalent r a d i u s
than S i ,
i s incorporated i n t o t h e l a t t i c e by l a s e r annealing,
a
one dimensional expansion i n s t e a d o f a c o n t r a c t i o n can be produced (Appleton e t al.,
1979).
The l a t t i c e s t r a i n mechanism can be
circumvented by simultaneously i n c o r p o r a t i n g compensating types o f dopants.
For example, i n order t o i n c o r p o r a t e more B s u b s t i t u -
t i o n a l l y i n t h e s i l i c o n l a t t i c e by l a s e r annealing,
i t would be
necessary t o simulataneously i n c o r p o r a t e a dopant w i t h l a r g e r c o v a l e n t bonding r a d i u s along
with
t h e B.
Possible candidates
i n c l u d e Ga, In, B i o r Sb.
13.
INTERFACIAL INSTABILITY The second mechanism which l i m i t s s u b s t i t u t i o n a l s o l u b i l i t i e s
achieved by l a s e r annealing i s an i n t e r f a c i a l i n s t a b i l i t y which develops d u r i n g regrowth and leads t o l a t e r a l segregation o f t h e r e j e c t e d dopant and t h e formation o f a w e l l defined c e l l s t r u c t u r e i n t h e near-surface region.
F i g u r e 18 shows examples o f t h e
c e l l s t r u c t u r e s formed i n t h e near surface r e g i o n as a r e s u l t o f l a s e r annealing s i l i c o n c r y s t a l s c o n t a i n i n g i o n implanted In, Ga, and Fe.
The i n t e r i o r o f each c e l l i s an e p i t a x i a l column o f
s i l i c o n extending t o t h e surface. Surrounding each column i s a t h i n w a l l c o n t a i n i n g massive concentrations o f the r e j e c t e d impurity.
F i g u r e 19 shows c o n c e n t r a t i o n p r o f i l e s ( t o t a l
and
s u b s t i t u t i o n a l ) and t h e c e l l u l a r m i c r o s t r u c t u r e i n the near surf a c e region f o r t h e c r y s t a l implanted by l 1 5 1 n (125 keV, 1.3 x 10L6/cm2) a f t e r l a s e r annealing.
From the concentration p r o f i l e s ,
up t o t h e maximum s u b s t i t u t i o n a l s o l u b i l i t y (1.5-2.0 almost a l l o f t h e I n i s s u b s t i t u t i o n a l .
x 1020/cm3)
I n t h e near surface
region, down t o a depth o f -1200 A, t h e t o t a l and s u b s t i t u t i o n a l concentrations d i f f e r considerably. shown i n the micrograph o f Fig.
The i n t e r i o r o f t h e c e l l s
19 contains t h e near surface
s u b s t i t u t i o n a l In, w h i l e t h e n o n s u b s t i t u t i o n a l I n i s l o c a t e d i n
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
83
10.2p m I
'% (1.3~10 j6/cm
1
6 9 G ~(1.2 x 10'6/cm2)
Fe (1.8x 10' 6/ crn2 1
Fig. 18. C e l l structure in the near surface region o f silicon a f t e r laser annealing high dose implants o f In, Ga, and Fe. Results for In and Ga were obtained on ( 1 00) Si. Those for Fe were obtained on ( 1 1 1 ) Si. From White e t a l . , 1981.
the c e l l walls.
These c e l l w a l l s p e n e t r a t e t o a depth o f -1200 A,
as determined by plan-view microscopy, i n good agreement w i t h t h e c o n c e n t r a t i o n p r o f i1e r e s u l t s . The c e l l s t r u c t u r e d e p i c t e d i n Figs.
18 and 19 a r i s e s from
l a t e r a l segregation o f t h e r e j e c t e d dopant which i s due t o i n t e r f a c i a l i n s t a b i l i t y which develops d u r i n g regrowth. l i t y i s caused by c o n s t i t u t i o n a l
f r o n t o f the interface,
The i n s t a b i -
supercooling i n t h e l i q u i d i n
a phenomena which i s w e l l recognized i n
c r y s t a l growth a t c o n v e n t i o n a l growth v e l o c i t i e s (Jackson 1975). I n t e r f a c e i n s t a b i l i t y w i l l occur o n l y when k ' i s l e s s t h a n u n i t y and when t h e c o n c e n t r a t i o n o f t h e r e j e c t e d i m p u r i t y a t t h e i n t e r face i s large.
T h i s mechanism l i m i t s t h e i n c o r p o r a t i o n o f Sb, B i ,
Ge, Sn, Pb, Ga and I n i n s i l i c o n as a r e s u l t o f l a s e r annealing. The l i m i t i n g c o n c e n t r a t i o n which can be i n c o r p o r a t e d i n t h e
1a t t i c e b e f o r e t h e i n t e r f a c e becomes u n s t a b l e depends on t h e growth v e l o c i t y .
I f t h e v a l u e f o r k l increases w i t h v e l o c i t y
t h e n h i g h e r c o n c e n t r a t i o n s can be i n c o r p o r a t e d s u b s t i t u t i o n a l l y ,
84
C. W. WHITE
-
I
1
I
-
o
zt
I
I
TOTAL In SUBSTITUTIONAL I n
-
-
1
402'
i,o:l 5
2 0
4049
I 0
I
I 0.1
I 0.2
I
DEPTH [pin)
Fig. 19. Profiles ( l e f t ) and microstructure ( r i g h t ) for ' l 5 I n ( 1 2 5 keV, 1 . 3 x 1 0 1 6 / c m 2 ) in ( 1 0 0 ) Si a f t e r ruby laser annealing. From White e t a l . , 1981.
a t faster growth velocities. This i s consistent with measured a t two growth velocities presented i n Table I 1 1 values f o r Cax! (see Section IV-15). I n addition, when k ' is larger for the (111) case then higher concentrations can be incorporated in (111) crystals compared t o results in (100) crystals a t the same growth velocity. This i s shown in Fig. 20 for the case o f l151n (125 keV, 8.9 x 1015/cm2) in (100) and (111) S i . The value f o r in the (111) case ( 5 x 1020/cm3) is a factor o f 2-3 greater than t h a t in the (100) case, due t o t h e difference i n k ' for In i n (111) and (100) crystals (see Fig. 12). Similar results have been obtained f o r B i , Ge, Sn, Pb and Ga (see Table 11). Both the concentration a t which instability develops as well a s the resulting cell size can be predicted with remarkable accuracy
Cyx
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
85
3 Si (it11
0
2
TOTAL SUBSTITUTIONAL
5
2 40'~
0
0.1 DEPTH (pm)
0
0.2
0.4 DEPTH (pin)
0.2
Fig. 20. Limitations to the incorporation of In in (100) and ( 1 1 1 ) Si a t a regrowth velocity of - 4 . 5 m / s . From White et a l . , 1983.
Table I 1 1
(Ctax)
Maximum Substitutional Dopant Concentrations in (100) Si Obtained a t Growth V e l o c i t i e s o f 4.5 and 6.0 rn/s.,- Compared t o Predicted Limits t o Solute Trapping (Cs).
Dopant AS
Sb Ga
In
Bi
Cmax ( v = 4.5 m/s) cm-3 6.0 x 1021 2.0 x 1021 4.5 x 1020 1.5 x 1020 4.0 x 102O
Cy
(v = 6.0 m/s) cm-3
6.0 x 1021
---
8.8 x 1020 2.8 x 102O 1.1 x 102'
Ck
cm-3 5.0 x 102l 3.0 x 1021 6.0 x 1021 2.0 x 1021 1.0 x 1021
86
C. W. WHITE
u s i n g t h e M u l l i n s and Sekerka (1964) p e r t u r b a t i o n theory o f i n t e r face s t a b i l i t y , m o d i f i e d t o account f o r t h e l a r g e departures from e q u i l i b r i u m d u r i n g regrowth.
The o r i g i n a l theory was developed f o r
c r y s t a l growth a t low v e l o c i t i e s and thermodynamic e q u i l i b r i u m i n t h e l i q u i d and t h e s o l i d was assumed.
However, i t has been shown
t h a t one can make allowances f o r t h e l a r g e departures from e q u i l i b r i u m d u r i n g l a s e r annealing by using i n t h e theory t h e values f o r k ' appropriate t o t h i s h i g h speed growth process (Narayan, C u l l i s e t al.,
1981).
1981;
With t h i s m o d i f i c a t i o n , p r e d i c t i o n s o f t h e
c e l l s i z e and t h e c o n c e n t r a t i o n a t which i n s t a b i l i t y develops are i n excel 1e n t agreement w i t h experimental measurements. 14.
DOPANT PRECIPITATION I N THE LIQUID PHASE The t h i r d mechanism which l i m i t s s u b s t i t u t i o n a l s o l u b i l i t y i s
t h e f o r m a t i o n o f p r e c i p i t a t e s i n t h e l i q u i d phase d u r i n g regrowth. This mechanism i s important a t lower v e l o c i t i e s f o r i m p u r i t i e s such as B i ,
Pb, and T1 which are immiscible i n l i q u i d s i l i c o n .
These i m p u r i t i e s w i l l p r e c i p i t a t e i n t h e l i q u i d i f t h e concentrat i o n i s h i g h enough and i f t h e time a v a i l a b l e f o r p r e c i p i t a t e n u c l e a t i o n and growth i s l o n g enough.
P r e c i p i t a t e s formed i n t h e
l i q u i d w i l l be incorporated i n t o the s o l i d as t h e melted r e g i o n solidifies. An example i s shown i n Fig. 21 f o r t h e case o f B i (250 keV, 1.4 x 1016/cm2) in (100) S i and l a s e r annealed under c o n d i t i o n s t o produce regrowth v e l o c i t i e s o f 1.5 0.7
m/sec (b).
m/sec (a) and
A t t h e lower growth v e l o c i t y , a random d i s t r i b u -
t i o n o f p r e c i p i t a t e s i s observed i n t h e implanted r e g i o n f o l l o w i n g l a s e r annealing.
I f t h e growth v e l o c i t y i s increased t o 1.5 m/sec
(a), t h e c e l l s t r u c t u r e r e s u l t i n g from i n t e r f a c e i n s t a b i l i t y i s observed, tates.
but t h e c e l l w a l l s are h e a v i l y decorated w i t h p r e c i p i
-
A decoration o f c e l l w a l l s by p r e c i p i t a t e s i s expected
because t h e i m p u r i t y concentration i s t h e highest near the w a l l s. I f t h e growth v e l o c i t y i s increased t o more than 3 m/sec,
the
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
87
Microstructure i n the near surface region o f ( 1 0 0 ) Si implanted b y Fig. 21. 209Bi ( 2 5 0 keV, 1.4 x 1016/crn2) and ruby laser annealed to give regrowth From White et a l . , velocities o f 1 . 5 m / s ( a ) and 0.7 m / s ( b ) . 1982.
usual c e l l s t r u c t u r e i s observed w i t h l i t t l e o r no evidence o f precipitates. Dopant p r e c i p i t a t i o n i n t h e l i q u i d phase ( a t low s o l i d i f i c a t i o n velocities)
l i m i t s the substitutional
Pb, and T1 i n s i l i c o n .
solubilities of Bi ,
I n t h e case o f T1, p r e c i p i t a t e s formed i n
t h e l i q u i d and f r o z e n i n t o t h e s o l i d a r e observed t o be coherent w i t h t h e s i l i c o n l a t t i c e (Appleton e t al.,
1983).
A t h i g h e r growth
88
C. W. WHITE
velocities,
interface i n s t a b i l i t y w i l l l i m i t the substitutional
i n c o r p o r a t i o n o f these dopants. 15.
FUNDAMENTAL Cahn e t al.
THERMODYNAMIC LIMITS (1980) have p r e d i c t e d t h a t t h e r e are fundamental
thermodynamic l i m i t s t o s o l u t e trapping, and s u b s t i t u t i o n a l solub i l i t i e s , i n s i l i c o n even a t i n f i n i t e growth v e l o c i t y .
The basic
ideas u n d e r l y i n g these p r e d i c t i o n s are i l l u s t r a t e d schematically i n Fig. 22.
On a p l o t o f t h e Gibbs f r e e energy versus composition
a t f i x e d temperature,
t h e s o l i d u s and l i q u i d i u s l i n e s i n t e r s e c t
a t one p o i n t , which determines t h e upper l i m i t f o r t h e s o l i d comp o s i t i o n which can be formed from t h e l i q u i d a t any composition. P l o t t i n g t h e locus o f these p o i n t s o f i n t e r s e c t i o n a t d i f f e r e n t temperatures on t h e e q u i l i b r i u m phase diagram defines t h e To curve. This curve gives t h e maximum s o l i d composition which can be formed from t h e l i q u i d a t any temperature, even a t i n f i n i t e growth velocities.
The To curve thus d e f i n e s t h e l i m i t t o d i f f u s i o n l e s s s o l i d i -
fication.
For r e t r o g r a d e systems, thermodynamic arguments can be
used t o o b t a i n a simple estimate f o r t h e maximum c o n c e n t r a t i o n L ( C s ) on t h e To curve. This maximum c o n c e n t r a t i o n on t h e To curve is t h e 1iqui d i us c o n c e n t r a t i o n on t h e equi 1 ib r i um phase d i agram a t t h e r e t r o g r a d e temperature (Cahn e t a1
., 1980).
PHASE DIAGRAM
I
Fig. 22. Schematic representation o f the method used t o determine the thermodynamic l i m i t t o solute trapping.
2.
SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS
84
L Values of Cs p r e d i c t e d by Cahn e t a l . (1980) f o r f i v e dopants i n (100) s i l i c o n
are l i s t e d i n Table I 1 1 and
compared w i t h mea-
Cyxobtained a t
surements of maximum s u b s t i t u t i o n a l s o l u b i l i t i e s two d i f f e r e n t growth v e l o c i t i e s annealing a t temperatures o f 300 v e l o c i t i e s o f -4.5 v b l ues
f o r s o l u t e trapping.
1981).
Laser
K and 77 K r e s u l t s i n regrowth
and -6.0 m/sec.
Cpxare
for
(White e t al.,
A t e i t h e r growth v e l o c i t y ,
approaching p r e d i c t e d thermodynamic 1i m i t s Dopants such as As and Sb f o r which k ' i s
very n e a r l y u n i t y (see Table I ) have measured s o l u b i l i t i e s which are q u i t e close t o t h e p r e d i c t e d thermodynamic l i m i t s .
Measured
s o l u b i l i t i e s f o r dopants w i t h r e l a t i v e l y lower values f o r k' (Ga, L I n ) are somewhat lower than t h e p r e d i c t e d values o f Cs but are still
within
an order o f magnitude o f t h e thermodynamic l i m i t s .
Values o f
Cyxf o r Ga,
-6 m/sec
are l a r g e r by f a c t o r s o f two t o t h r e e than r e s u l t s
In, and B i i n S i obtained a t a v e l o c i t y o f
obtained a t a regrowth v e l o c i t y o f -4.5
m/sec.
These r e s u l t s
a r e s t i l l l i m i t e d by i n t e r f a c i a l i n s t a b i l i t y d u r i n g regrowth, but they demonstrate t h a t a t higher growth v e l o c i t i e s t h e onset o f i n s t a b i 1i t y can be delayed u n t i l higher concentrations accummul a t e a t t h e i n t e r f a c e , as expected from t h e discussion o f i n t e r f a c e instability. Although s u b s t i t u t i o n a l s o l u b i l i t i e s achieved by pulsed l a s e r annealing are approaching Cahn's p r e d i c t e d l i m i t s , case do we
appear t o have reached t h i s l i m i t .
I 1 1 t h e value o f m/sec.
This value
i n only one
As shown i n Table
CyX for As i n S i i s t h e same a t 4.5 and 6.0 f o r yxi s a l s o independent o f c r y s t a l o r i e n -
t a t i o n as demonstrated i n Fig. 23.
The r e s u l t s o f Fig. 23 were
obtained using c r y s t a l s implanted by 75As (100 keV) t o a dose of 1.2 x 10i7/cm2. For both (100) and (111) S i t h e As i s measured t o be s u b s t i t u t i o n a l up t o a c o n c e n t r a t i o n o f 6 x lO2l/crn3 f o l lowing l a s e r annealing.
When t h e t o t a l concentration reaches
t h i s value, e p i t a x i a l growth stops. surface appears t o be amorphous.
To i o n channeling, t h e near However, TEM r e s u l t s reveal
90
C. W. WHITE
0
0.i
0.2 0.3 DEPTH (pm)
0.4
0
0.4
0.2 0.3 DEPTH ( p m )
0.4
Limitations t o the incorporation o f As i n (100) and ( 1 11 ) S i . Total Fig. 23. and substitutional concentrations a r e plotted as a function o f depth a f t e r XeCl From White e t a l . , 1983. laser annealing (regrowth velocity -6 m / s ) .
t h a t the near surface contains p o l y c r y s t a l l i t e s , As p r e c i p i t a t e s , and even small regions of amorphous material. The l i n e of demarcation between the epitaxial l y recrystal 1 ized region and the I t appears as i f disordered near surface i s r e l a t i v e l y sharp. e p i t a x i a l regrowth proceeded normally until a concentration o f 6 x lO2I/crn3 was reached, a t which point the advancing i n t e r f a c e slowed considerably and t h e r e a f t e r polycrystal 1i t e s nucleated and As p r e c i p i t a t e s formed. Based on the f a c t t h a t the value f o r Cmax does not increase with velocity and i s independent of crysS t a l o r i e n t a t i o n we conclude t h a t the thermodynamic l i m i t has been reached (White et a l . , 1983). The r e s u l t i s in reasonable agreement with Cahn's predicted value of 5 x 1021/cm3 f o r t h i s alloy system, and the difference i s probably due t o uncertainties on the equilibrium phase diagram which was used in making the theoretical predictions.
2. SEGREGATION, SOLUTE TRAPPING, AND SUPERSATURATED ALLOYS 91 V.
Sumnary and Conclusions
Laser anneal ing o f i o n imp1 anted s i 1i c o n has p r o v i d e d v e r y fundamental
information
growth processes.
on h i g h
The l e v e l
speed
nonequil i b r i u m c r y s t a l
o f understanding which has been
brought t o t h i s new regime of c r y s t a l growth i s impressive cons i d e r i n g t h e s h o r t p e r i o d o f t i m e these s t u d i e s have been conducted.
T h i s r a p i d advance has been made p o s s i b l e because one
can use two compl ementary nonequi 1ib r i um processing techniques , i o n i m p l a n t a t i o n and pulsed l a s e r annealing,
i n order t o c a r r y
o u t experiments under c a r e f u l l y c o n t r o l l e d c o n d i t i o n s . D u r i n g t h e r a p i d l i q u i d - p h a s e e p i t a x i a l regrowth process, i m p l a n t e d Group 111,
I V Y and V i m p u r i t i e s can be i n c o r p o r a t e d
i n t o t h e l a t t i c e a t c o n c e n t r a t i o n s t h a t exceed e q u i l i b r i u m s o l u b i l i t y l i m i t s by o r d e r s o f rnagnitude. coefficients,
Interfacial distribution
i n many cases as a f u n c t i o n o f v e l o c i t y ,
have been
determined f o r a wide v a r i e t y o f i m p u r i t i e s i n s i l i c o n . Group 111, I V Y and V i m p u r i t i e s ,
For
values f o r k' are f a r g r e a t e r
t h a n t h e e q u i l i b r i u m values, and can be f u n c t i o n s o f b o t h growth v e l o c i t y and c r y s t a l o r i e n t a t i o n .
T h e o r e t i c a l models have been
developed which may e x p l a i n q u a n t i t a t i v e l y t h e s o l u t e t r a p p i n g mechanism.
Limits t o substitutional
s o l u b i l i t y which can be
achieved by l a s e r annealing have been measured, and i n s i g h t has been gained i n t o t h e mechanisms t h a t l i m i t s u b s t i t u t i o n a l s o l u b i l Measured s u b s t i t u t i o n a l s o l u b i l i t i e s a r e approaching pred i c t e d thermodynamic l i m i t s t o d i f f u s i o n l e s s s o l i d i f i c a t i o n , and ity.
i n one case t h e l i m i t appears t o have been reached. I n t h e f u t u r e , experiments w i l l be c a r r i e d o u t a t f a s t e r and slower growth v e l o c i t i e s .
Q u e s t i o n s t h a t need f u r t h e r i n v e s t i g a -
t i o n include the s a t u r a t i o n value f o r d i s t r i b u t i o n c o e f f i c i e n t s a t h i g h growth v e l o c i t i e s , f u r t h e r t e s t s o f thermodynamic l i m i t s t o dopant i n c o r p o r a t i o n a t h i g h e r v e l o c i t i e s , o r i e n t a t i o n e f f e c t s i n solute trapping,
i n c o r p o r a t i o n o f n o n s u b s t i t u t i o n a l species,
and f u r t h e r s t u d i e s o f t h e t r a n s i t i o n t o t h e amorphous s t a t e .
92
C. W. WHITE
The r e s u l t s of these experiments should p r o v i d e a sound basis f o r theoretical
understanding
of
h i g h speed nonequi librium c r y s t a l
growth phenomena. References Appleton, B. R., Larson, B. C., White, C. W., Narayan, J., Wilson, S. R. and Pronko, P. P. (1979). I n "Laser-Solid I n t e r a c t i o n s and Laser Processing-1978" (S. D. F e r r i s , H. J. Leamy and J. M. Poate eds.), p. 291. Am. I n s t . Phys., New York. Appleton, B. R., and C e l l e r , G. K., eds. (1982). Mat. Res. SOC. Symp. Proc. 4. Appleton, B. R., Narayan, J . , Holland, 0. W., and Pennycook, S. J. (1983). Mat. Res. SOC. Symp. Proc. 13, 281. Aziz, \I. J. (1982). J. Appl. Phys. 53, 1158. Baeri, P., Campisano, S. U., F o t i , G., and R i m i n i , E. (1978). J. Appl. Phys. 50, 788. B a e r i , P., Campisano, S. U., F o t i , G., Rimini, E. (1979). Phys. Rev. L e t t . 41, 1246. Baeri, P., Poate, J. M., Campisano, S. U., F o t i , G., Rimini, E., and C u l l i s , A. G. (1981). Appl. Phys. L e t t . 37, 912. B a e r i , P., F o t i , G., Poate, J. M., Campisano, S. U., and C u l l i s , A. G. (1981a). Appl. Phys. L e t t . 38, 800. Baeri, P., and Campisano, S. U. (1982). I n "Laser Annealing o f Semiconductors," (J. W. Mayer and J. M. Poate, eds.), Chapter 4. Academic Press, New York. Baeri, P. (1982). Mat. Res. SOC. Symp. Proc. 4 , 151. Baker, J . C., and Cahn, J. W. (1969). Acta. M e t a l l . 17, 575. Cahn, J. W., C o r i e l l , S. R., and Boettinger, W. J. (1980). In "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 89. Academic Press, New York. Campisano, S . U., Rimini, E., Baeri, P., and F o t i , G. (1980). Appl. Phys. L e t t . 37, 170. Campisano, S. U., F o t i , G., Baeri, P., Grimaldi, M. G. , and R i m i n i , E. (1980a). Appl. Phys. L e t t . 37, 719. Clark, G. J., C u l l i s , A. G., Jacobson, D. C., Poate, J. M., and Thompson, M. 0. (1983). Mat. Res. SOC. Symp. Proc. 13, 303. C u l l i s , A. G., Webber, H. C., Poate, J. M., and Simons, A. L. (1980). Appl. Phys. L e t t . 36, 320. C u l l i s , A. G., Hurle, D.T.J., Webber, H. C., Chew, N. G., Poate, J. M., Baeri, P., and F o t i , G. (1981). Appl. Phys. L e t t . 38, 642. Cul l i s , A. G., Webber, H. C., Chew, N. G., Poate, J. M., and Baeri , P. (1982). Phys. Rev. L e t t . 49, 219. F e r r i s , S. D., Leamy, H. J . , and Poate, J. M., eds. (1979). In "Laser S o l i d I n t e r a c t i o n s and Laser Processing-1978," Am. I n s t . Phys. New York.
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Galvin, G. J. , Thompson, M. 0. , Mayer, J. W. , Hammond, R. B. , Paulter, N., and Peercy, P. S. (1982). Phys. Rev. Lett. 48, 33. Gibbons, J. F., Hess, L. D. and Sigmon, T. W., eds. (1981). Mat. Res. SOC. Symp. Proc. 1. Golovchenko, J. A. , Venkatessan, T.N.C. (1978). Appl. Phys. Lett. 32, 148. Holland, 0. W., Narayan, J., White, C. W., and Appleton, B. R. (1983). Mat. Res. SOC. Symp. Proc. 13, 297. Jackson, K. A. (1975). In "Treatise on S o l i d State Chemistry" (N. B. Hanay, ed.), Vol. 5, Chapter 5. Plenum Press, New York. Jackson, K. A., Gilmer, G. H., and Leamy, H. J. (1980). I n "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 104. Academic Press, New York. Jackson, K. A. (1981). P r i v a t e comnunication. Kodera, H. (1963). Jpn. J. Appl. Phys. 2, 212. Larson, B. C., White, C. W., and Appleton, B. R. (1978). Appl. Phys. Lett. 32, 801. L i u , P. L., Yen, R., Bloembergen, N., and Hodgson, R. T. (1979). Appl. Phys. L e t t . 34, 864. LOwndes, 0. H., Cleland, J. W., C h r i s t i e , W. H., and Eby, R. E. (1981). Mat. Res. SOC. Symp. Proc. 1, 223. M u l l i n s , W. W., and Sekerka, R. F. (1964). J. Appl. Phys. 35, 444. Narayan, J., Young, R. T., and White, C. W. (1978). J. Appl. Phys. 49, 3127. Narayan, J. (1981). J. Appl. Phys. 52, 1289. Narayan, J., Brown, W. L., and Lemons, R. A., eds. (1983). Mat. Res. SOC. Symp. Proc. 13. Picraux, S. T. (1975). I n "New Uses o f I o n Accelerators" (J. Ziegler, ed.), p. 244. Plenum Press, New York. Poate, J. M. (1982). Mat. Res. SOC. Symp. Proc. 4, 121. Smith, V. G., T i l l e r , W. A., Rutter, J. W. (1955). Can. J. Phys. 33, 723. Spaepen, F. and Turnbull, D. (1982). I n "Laser Annealing o f Semiconductorsn (J. M. Poate and J. W. Mayer, eds.), Chapter 2. Academic Press, New York. Stuck, R., Fogarassy, E., Grob, J. J., and S i f f e r t , P. (1980). Appl. Phys. Lett. 23, 15. Swanson, M. L., Howe, L. M., Saris, F. W., and Quenneville, A. F. (1981). Mat. Res. SOC. Symp. Proc. 2, 71. T i l l e r , W. A., Jackson, K. A., Rutter, J. W., and Chalmers, B. (1953). Acta Metall. 1, 428. Trumbore, F. (1960). B e l l Syst. Tech. Jour. 39, 205. TSU, R., Hodgson, R. T., Tan, T. Y., and Baglin, J. E. (1979). Phys. Rev. L e t t . 42, 1356. Wang, J. C., Wood, R. F., and Pronko, P. P. (1978). Appl. Phys. L e t t . 33, 455. C h r i s t i e , W. H., Appleton, B. R., Wilson, S. R., White, C. W., Pronko, P. P., and Magee, C. W. (1978). Appl. Phys. L e t t . 33, 455.
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White, C. W., Narayan, J., and Young, R. T. (1979). Science 204, 461. White, C. W., Pronko, P. P. , Wilson, S. R., Appleton, B. R . , Narayan, J., and Young, R. T. ( 1 9 7 9 ~ ) . J. Appl. Phys. 50, 3261. White, C. W., Narayan, J., Appleton, B. R., and Wilson, S. R. (1979b). J. Appl. Phys. 50, 2967. White, C. W. , Wilson, S. R. , Appleton, B. R., and Young, F. W. , Jr., (1980). J. Appl. Phys. 51, 738. White, C. W., Wilson, S . R., Appleton, B. R., Young, F. W., Jr., and Narayan, J., (1980a). I n "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s (C. W. White and P. S. Peercy, eds.) p. 124. Academic Press, New York. White, C. W. and Peercy, P. S., eds. (1980). I n "Laser and E l e c t r o n Beam Processing o f Materials," Academic Press, New Y ork White, C. W., Appleton, B. R., S t r i t z k e r , B., Zehner, D. M., and Wilson, S. R. (1981). Mat. Res. SOC. Symp. Proc. 1. 59. White, C. W. (1982). Mat. Res. SOC. Symp. Proc. 4, 109. White, C. W., Naramoto, H., Williams, J. M., Narayan, J., Appleton, B. R., and Wilson, S. R. (1982). Mat. Res. SOC. Symp. Proc. 4, 241. White, C. W., Zehner, D. M., Narayan, J. , Holland, 0. W., Appleton, 8. R., and Wilson, S. R. (1983). Mat. Res. SOC. Symp. Proc. 13, 287. Williams, J. S., and Elliman, R. G. (1981). Nucl. Instrum. Meth. 182/183, 389. Wood, R. F. (1980). Appl. Phys. L e tt. 37, 302. Wood, R. F., K i r k p a t r i c k , J. R., and G i l e s , G. E. (1981). Phys. Rev. B 23, 5555.
.
CHAPTER 3
OPTICAL A N D ELECTRICALPROPERTIES OF PULSED LAs E R -ANNEAL E D S I L I c o N G. E. J e l l i s o n , Jr.
I.
11.
. . . .. .. .. .. .. .. .. .. .. .. .. .............
INTRODUCTION. OPTICAL PROPERTIES. 1. Background and O p t i c a l Measurement Techniques 2. O p t i c a l P r o p e r t i e s o f S i l i c o n as a Function o f Temperature. 3. O p t i c a l P r o p e r t i e s o f Ion-Implanted Amorphous S i l i c o n . 4. Laser-Annealed, H e a v i l y Doped Silicon. ELECTRICAL PROPERTIES 5. E l e c t r i c a l Measurement Techniques. 6. Sheet P r o p e r t i e s o f Ion-Implanted Laser-Anneal ed Layers. 7. P r o p e r t i e s o f Pul sed Laser-Anneal ed Junctions. DEFECTS 8. Background 9. Photo1 umi nescence. 10. Deep Level Transient Spectroscopy
...... ......... . . . . .. .. .. .. .. .. .. .. .. .. 111. . ....... IV . . . . . ... ... ... ... ... ... ... ... ... ... ... ... ... ......... (DLTS) . . . . . . . . . . . . . . . 11. Other Defect-Related Experiments . . REFERENCES . . . . . . . . . . . . . . . I. INTRODUCTION I n t h i s chapter we discuss t h e r e l a t i o n s h i p o f pulsed l a s e r processing to, and i t s impact on, t h r e e areas which have t r a d i tionally
played a c e n t r a l
r o l e i n semiconductor research and
95
Copyright Q1984 by Academic Press, Inc. All rights of reproduction In any form reserved. ISBN 0-12.752123-2
96
G . E. JELLISON, JR.
applications,
namely,
optical properties,
and d e f e c t s t r u c t u r e s .
e l e c t r i c a l properties,
Each o f these areas has r e c e i v e d a t t e n t i o n
e i t h e r because i t d i r e c t l y i n f l u e n c e s t h e l a s e r annealing process i t s e l f or because l a s e r a n n e a l i n g under a p p r o p r i a t e c o n d i t i o n s can l e a d t o e f f e c t s which have n o t been observed p r e v i o u s l y . Perhaps t h e most extreme example o f t h e new e f f e c t s which have been observed as a r e s u l t o f pulsed l a s e r annealing i s t h e convers i o n o f a c r y s t a l l i n e semiconductor i n t o an amorphous form by t h e e x t r e m e l y r a p i d regrowth v e l o c i t i e s t h a t can be o b t a i n e d under c e r t a i n conditions.
Another example i s t h e r e v e r s e t r a n s f o r m a t i o n
i n which a s u i t a b l y prepared amorphous l a y e r i s r e c r y s t a l l i z e d by pulsed l a s e r i r r a d i a t i o n .
Both o f these t r a n s f o r m a t i o n s r e s u l t
i n more o r l e s s r a d i c a l a l t e r a t i o n s i n t h e o p t i c a l and e l e c t r i c a l p r o p e r t i e s o f t h e semiconductors, and o b v i o u s l y t h e " d e f e c t s t r u c t u r e " o f t h e m a t e r i a l i s c o m p l e t e l y changed.
When t h e l a s e r pulses
a r e i n c i d e n t on samples which have been implanted w i t h v a r i o u s dopants, o t h e r new e f f e c t s may be e x h i b i t e d ; f o r example, e q u i l i b r i u m s o l u b i l i t y l i m i t s can be g r e a t l y exceeded w h i l e m a i n t a i n i n g f u l l e l e c t r i c a l activation.
To b r i n g t h e scope o f t h i s chapter i n t o accord with t h e gene r a l c o n t e n t and purpose o f t h e book, we w i l l l i m i t our c o n s i d e r a t i o n s o f o p t i c a l p r o p e r t i e s t o those e f f e c t s which i n f l u e n c e t h e interaction
of
the
incident
radiation
w i t h t h e semiconductor
sample and t o t h e i n f l u e n c e o f heavy doping l e v e l s on t h e o p t i c a l parameters.
The f i r s t o f these c o n s i d e r a t i o n s i s b a s i c t o t h e
fundamentals o f l a s e r annealing, w h i l e t h e second i s i m p o r t a n t f o r light-sensitive
devices
such as s o l a r
cells.
Similarly,
the
e l e c t r i c a l p r o p e r t i e s w i l l be discussed b r i e f l y i n terms o f t h e p o s s i b i l i t i e s t h a t l a s e r annealing o f f e r s f o r s t u d y i n g t h e e f f e c t s o f heavy doping on t h e e l e c t r i c a l p r o p e r t i e s o f semiconductors, b u t more e x t e n s i v e l y i n terms o f t h e p e r f e c t i o n o f e l e c t r i c a l j u n c t i o n s formed f o r d e v i c e a p p l i c a t i o n s ,
Finally,
t h e r e s u l t s o f deep
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
level transient
spectroscopy
(DLTS),
which
97
i n d i c a t e t h a t some
p r e v i o u s l y unobserved e l e c t r i c a l l y a c t i v e d e f e c t s may be formed d u r i n g l a s e r annealing, w i l l be discussed. A l l o f t h e s t u d i e s considered here f a l l i n t o t h e c a t e g o r i e s o f
pre-irradiation
and post-anneal i n g s t u d i e s ;
measurements d u r i n g
t h e i r r a d i a t i o n process i t s e l f are covered i n Chapter 6.
From a
p u r e l y t e c h n o l o g i c a l p o i n t o f view, post-anneal i n g experiments a r e c r u c i a l because t h e y p r o v i d e r e s u l t s t h a t d i r e c t l y determine t h e s u i t a b i l i t y o f p u l s e d laser-annealed m a t e r i a l f o r d e v i c e a p p l i c a tions.
From a more b a s i c p o i n t o f view,
post-annealing e x p e r i -
ments are i m p o r t a n t i n t h a t i n f o r m a t i o n about t h e fundamental p h y s i c a l mechanisms o f t h e a n n e a l i n g process can be deduced from them, as a l r e a d y demonstrated i n Chapter 2. I n t h i s chapter, we w i l l devote our a t t e n t i o n almost e n t i r e l y t o s i l i c o n because t h e t e c h n o l o g i c a l importance o f t h i s m a t e r i a l has made i t by f a r t h e most w i d e l y s t u d i e d laser-processed semiconductor.
11. 1.
Optical Properties
BACKGROUND AND OPTICAL MEASUREMENT TECHNIQUES
I n o r d e r t o understand t h e phenomena a s s o c i a t e d w i t h l a s e r annealing, i t i s necessary t o know a c c u r a t e l y t h e o p t i c a l propert i e s of
the material
b e i n g studied,
because these p r o p e r t i e s
determine how t h e i n t e n s e l a s e r r a d i a t i o n couples t o t h e e l e c t r o n i c and/or v i b r a t i o n a i s t a t e s o f t h e system.
This i s t r u e f o r a l l
m a t e r i a l s , b u t it i s p a r t i c u l a r l y s i g n i f i c a n t f o r an i n d i rect-gap semiconductor such as s i l i c o n ,
i n which t h e o p t i c a l p r o p e r t i e s
change markedly w i t h temperature over range.
an extended wavelength
I n t h i s s e c t i o n , t h e o p t i c a l p r o p e r t i e s o f c r y s t a l l i n e (c),
amorphous (a), and l i q u i d (I) s i l i c o n a r e presented as a f u n c t i o n o f wavelength and temperature.
It i s shown t h a t several c r i t i c a l
p o i n t s i n t h e j o i n t d e n s i t y o f e l e c t r o n i c s t a t e s o f c-Si move t o
98
G . E. JELLISON, JR.
lower energy with increasing temperature and t h a t the optical absorption c o e f f i c i e n t increases exponentially w i t h increasing temperature f o r photon energies well below the d i r e c t band gap. The optical properties of ion-implanted a-Si layers are also discussed here, since many practical applications of pulsed l a s e r annealing involve the r e c r y s t a l l i z a t i o n o f these layers. I t i s shown t h a t i t i s d i f f i c u l t t o assign a unique s e t of optical prope r t i e s t o ion-implanted material , since the properties depend on the implanted species, t h e dose, the s u b s t r a t e temperature, and any heat treatment t o which t h e sample i s subjected a f t e r implantation. Because of the primary r o l e played by l a t t i c e damage, which i s l i k e l y t o be nonuniform, t h e absorption c o e f f i c i e n t and other optical parameters a r e expected t o be depth dependent, espec i a l l y i f , as i s usually t h e case, t h e sample i s implanted a t only one imp1 antation energy. The process of ion implantation followed by pulsed l a s e r anneal i ng i s ideal f o r prepari ng sampl es f o r optical properties measurements, since, under proper annealing conditions, i t leaves the surface o p t i c a l l y f l a t . In addition, higher substitutional doping d e n s i t i e s (even exceeding t h e s o l i d s o l u b i l i t y l i m i t s ) can be achieved with t h i s technique than with conventional diffusion techniques. Also, r e l a t i v e l y f l a t dopant p r o f i l e s a t high doping concentrations can readily be obtained by annealing the material with several pulses. In t h i s section, a variety of optical s t u d i e s , including ellipsometry, i n f r a r e d , and Raman, t h a t have been reported on ion-implanted, laser-annealed s i l i c o n will be reviewed. The optical properties o f a material are most frequently given i n terms of e i t h e r the complex r e f r a c t i v e index (il= n + ik, where n i s t h e r e f r a c t i v e index, and k i s the extinction c o e f f i c i e n t ) + E ~ ) . The two repreo r t h e complex d i e l e c t r i c function (z = s e n t a t i o n s a r e related by
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
99
and E~
= 2nk.
I n many p r a c t i c a l a p p l i c a t i o n s , o t h e r parameters r e l a t e d t o these, such as t h e o p t i c a l a b s o r p t i o n c o e f f i c i e n t a and t h e normal i n c i dence r e f l e c t a n c e R, are more u s e f u l q u a n t i t i e s .
The a b s o r p t i o n
c o e f f i c i e n t i s a measure o f t h e a b s o r p t i v e power o f a medium, and i s n o r m a l l y expressed i n u n i t s o f cm-1.
The energy d e n s i t y I a t
a d i s t a n c e d i n t o t h e medium i s given by I = Ioe-ad,
where 1, i s t h e l i g h t energy i n c i d e n t on t h e surface.
The absorp-
t i o n c o e f f i c i e n t can be expressed as a = 4nk/h,
(3)
w i t h A t h e wavelength o f t h e l i g h t .
The normal i n c i d e n c e r e f l e c -
tance i s t h e f r a c t i o n o f energy d e n s i t y r e f l e c t e d from an o p t i c a l boundary when t h e l i g h t i s i n c i d e n t normal t o t h e boundary. a simple a i r - m a t e r i a l
For
boundary (n = 1, k = 0 f o r a i r ) ,
O p t i c a l t r a n s m i s s i o n i s t h e most common method f o r t h e measurement o f a f o r pure s i l i c o n (Dash and Newman, 1955; Macfarlane e t al.,
1958; Weakliem and R e d f i e l d , 1979; J e l l i s o n and Lowndes, 1982).
T h i s i s c e r t a i n l y t h e best method a v a i l a b l e when a
<
lo4, but f o r
l a r g e r a, extremely t h i n samples a r e r e q u i r e d and a c c u r a t e measurements become d i f f i c u l t .
For l a r g e r values o f a, r e f l e c t a n c e mea-
surements ( P h i l l i p and T a f t ,
1960; P h i l l i p and Ehrenreich, 1963;
P h i l l i p , 1972) can be used, t o g e t h e r w i t h a Kramers-Kronig analys i s t o e x t r a c t t h e o p t i c a l parameters from t h e r e s u l t s o f normal i n c i d e n c e r e f l e c t a n c e measurements.
The major problem a s s o c i a t e d
100
G . E. JELLISON, JR.
50 40 30 20 €1
I0
0
- 10 -20
50 -
I ..........
---40 - --
I
10 K 297 K
I
I
I
I
-
-.-
€2
30 20 10
0
-
-
2
3
ENERGY (eV)
4
5
Fig. 1. The complex dielectric function ( Z = €1 + € 2 ) o f silicon vs photon energy for several temperatures (Jellison and Modine, 1 9 8 3 ) .
3.
101
PROPERTIES OF PULSED LASER-ANNEALED SILICON
w i t h t h e determination o f o p t i c a l parameters from r e f l e c t a n c e measurements i s t h a t u n c e r t a i n e x t r a p o l a t i o n s o r approximations must be made i n order t o perform t h e Kramers-Kronig analysis. Recently,
scanning e l l i p s o m e t r y techniques have been used t o
determine the o p t i c a l parameters o f s i l i c o n as a f u n c t i o n o f wave1ength (Aspnes and Theeten , 1980; 1982b).
J e l l i s o n and Modine,
1982a,
Because e l 1 ipsometry measurements y i e l d two parameters a t
each wavelength, t h e o p t i c a l p r o p e r t i e s can be determined uniquely throughout t h e spectrum, subject o n l y t o t h e assumptions of the I n t h e i r work, Aspnes and Theeten used a
surface model employed.
r o t a t i n g analyzer e l l i p s o m e t e r
(RAE),
w h i l e J e l l i s o n and Modine The PME i s
used a p o l a r i z a t i o n modulation e l l i p s o m e t e r (PME).
more s e n s i t i v e than t h e RAE t o small values o f k ( o r a ) , and y i e l d s more accurate data below -3.5
eV i n S i ; here t h e data o f J e l l i s o n
and Modine w i l l be used i n discussing t h e o p t i c a l p r o p e r t i e s of s i 1icon.
2.
OPTICAL PROPERTIES. OF SILICON AS A FUNCTION OF TEMPERATURE
a.
Results o f E l l i p s o m e t r y The complex d i e l e c t r i c f u n c t i o n o f s i l i c o n has been measured
on t h e (100) face from 1.6 t o 4.7
eV and from 10 t o 1000
J e l l i s o n and Modine (1982a, 1982c, and 1983) using PME. ( E ~ and )
imaginary
K by
The r e a l
parts of the d i e l e c t r i c function of s i l i -
( E ~ )
con a t several selected temperatures are shown i n Fig.
1.
Since
t h e complex d i e l e c t r i c f u n c t i o n i s a l i n e a r response f u n c t i o n , E~ and E~ (as w e l l as p a i r e d q u a n t i t i e s such as n and k) are r e l a t e d by t h e Kramers-Kronig r e l a t i o n s .
I n o t h e r words, i f one q u a n t i t y
i s known f o r a l l photon energies, t h e o t h e r q u a n t i t y can be calculated.
A general c h a r a c t e r i s t i c o f l i n e a r response f u n c t i o n s i s
t h a t when the r e a l p a r t i s a t a maximum, t h e imaginary p a r t w i l l have a l a r g e slope w i t h respect t o photon energy.
Conversely, i n
102
G . E. JELLISON, JR
107
Fig. 2. The absorption coefficient o f silicon a t several selected temperatures plotted vs photon energy (Jellison and Modine ( 1 9 8 2 ~ ) .
t h e present case, when cl i s a t a maximum, .sl w i l l have slope o f l a r g e magnitude. Alternatively,
T h i s f e a t u r e i s apparent i n t h e data o f Fig.
t h e data can be present i n terms o f a and
Figs. 2 and 3 , r e s p e c t i v e l y .
1.
R, as i n
3.
103
PROPERTIES OF PULSED LASER-ANNEALED SILICON
0.8
0.7 W 0
z 0.6 a I0 W
0.4
0.3
2
3
5
4
ENERGY (eV)
Fig. 3. The normal incidence reflectance o f silicon a t several selected temperatures plotted vs photon energy. The reflectance was calculated from the optical functions using Eq. ( 4 ) (Jellison and Modine, 1983).
A q u a l i t a t i v e understanding o f these o p t i c a l f u n c t i o n s can be o b t a i n e d by comparing t h e data o f Figs. energy band diagram o f s i l i c o n ,
1-3 w i t h t h e e l e c t r o n
shown i n Fig.
4.
Photon wave
v e c t o r s a r e a p p r o x i m a t e l y zero on t h e wave v e c t o r s c a l e o f t h e B r i l l o u i n zone ( t h e abscissa o f Fig. electron-hole
4) and,
therefore,
direct
p a i r c r e a t i o n processes r e s u l t i n g from t h e i n t e r -
a c t i o n o f l i g h t w i t h t h e c r y s t a l w i l l be represented by v e r t i c a l l i n e s on t h e band diagram.
D i r e c t o p t i c a l absorption takes place
when l i g h t o f s u f f i c i e n t energy i n t e r a c t s w i t h t h e l a t t i c e , t a k i n g an e l e c t r o n from a p o i n t i n t h e f i l l e d valence band t o t h e p o i n t d i r e c t l y above i n t h e conduction band. c o e f f i c i e n t then can be represented as
The o p t i c a l a b s o r p t i o n
104
G . E. JELLISON, JR.
Reduced wave vector
c
= k/(27r/a)
Fig. 4. The band structure o f silicon calculated by Mostoller ( 1 9 8 3 ) from the pseudopotential parameters o f Chelikowsky and Cohen (1 9 7 6 ) .
where K i s a constant,
f ( E ) gives t h e o s c i l l a t o r s t r e n g t h as a
f u n c t i o n o f E, and Nd(E) i s t h e j o i n t d e n s i t y o f s t a t e s , which i s t h e p r o b a b i l i t y o f f i n d i n g two s t a t e s ,
one i n t h e valence band
and t h e o t h e r i n t h e conduction band a t t h e same p o i n t in t h e B r i l l o u i n zone,
separated by energy E.
g r e a t e r than t h e d i r e c t band gap (3.4
For photons w i t h energy eV a t room temperature),
N d ( E ) w i l l be l a r g e , and t h e r e f o r e t h e o p t i c a l a b s o r p t i o n c o e f f i c i e n t w i l l be l a r g e (-lo6 crn-l,
see Fig. 2).
For photons o f energy
l e s s than t h e d i r e c t band gap b u t g r e a t e r than t h e i n d i r e c t band gap, ' o p t i c a l a b s o r p t i o n i s s t i l l p o s s i b l e through t h e simultaneous emission o r a b s o r p t i o n o f a phonon t o conserve c r y s t a l momentum. These i n d i r e c t o r phonon-assi s t e d t r a n s i t i o n s processes and are represented by n o n - v e r t i c a l energy-band diagram (see E i o f Fig. 4).
a r e second-order t r a n s i t i o n s on t h e
3.
105
PROPERTIES OF PULSED LASER-ANNEALED SILICON
There are several s p e c i f i c f e a t u r e s o f t h e o p t i c a l f u n c t i o n s which can be understood by a c a r e f u l comparison o f t h e data (Figs. 1-3) w i t h t h e energy-band diagram o f Fig. 4.
Most o f t h e symmetry
assignments are made from e l e c t r o r e f l e c t a n c e studies (see Daunois and Aspnes, 1978 and references t h e r e i n ) and cannot be determined d i r e c t l y from t h i s e l l i p s o m e t r y data.
1) The peak i n E~ near 4.4 eV a t 10 K decreases i n magnitude and moves t o lower energy as t h e temperature increases. The corresponding cl spectrum changes sign a t n e a r l y t h e same energy, and t h e slope decreases as t h e zero c r o s s i n g m v e s t o lower energy w i t h i n c r e a s i n g temperature. i n Fig.
points,
including that
and
2) nitude,
E2
a r i s i n g from t h e t r a n s i t i o n s
+ 1; (Kondo and M o r i t a n i , 1977) (see
in
E ~ labeled ,
1, i s not c l e a r , b u t i t i s thought t o be due t o several
critical
cl
The o r i g i n o f t h i s peak i n
E~
Fig. 4).
These features
are not as w e l l resolved a t higher temperatures.
The peak i n
E~
near 3.4 eV a t 10 K a l s o decreases i n mag-
moves t o lower energy and broadens as t h e temperature
increases; i t i s manifested i n t h e tz2 spectra as a low-energy cuto f f shoulder.
The
peak i n
E~
is
labeled
thought t o a r i s e p r i m a r i l y from an M, density
of
states f o r the
r;,,
-+
r;,
Ei
in
Fig. 1 and i s
c r i t i c a l point i n the j o i n t transition.
Daunois and
Aspnes (1978) found t h a t t h e c r i t i c a l p o i n t energy f o r t h i s t r a n s i t i o n d i d not occur e x a c t l y a t t h e t o p o f t h e peak but a t t h e
1ow-energy side. 3) upon t h e
The peak i n
E~
near 3.4 eV a t 10 K, which i s superimposed
shoulder due t o
EA decreases i n magnitude and moves t o
lower energy as t h e temperature i s increased u n t i l i t i s no longer observable a t -500°C.
A high-energy c u t o f f shoulder i n t h e
spectrum a t the same energy o f t h e 10 K data. corresponds t o transition.
observable upon close examination
This peak i n
E~
i s l a b e l e d El
i n Fig.
1 and
o r M, c r i t i c a l p o i n t f o r t h e A: -+A: EA, Oaunois and Aspnes (1978) found t h a t t h e
e i t h e r an M,
As w i t h
is
E~
106
G. E. JELLISON. JR
c r i t i c a l p o i n t energy a t 10 and 300
K i s a t s l i g h t l y lower energy
t h a n t h e peak p o s i t i o n . The the
El
disappearance o f
t h i s peak c o u l d
gap moving t o lower energy
p o s s i b l y be caused by
f a s t e r than t h e EL gap, making
i t unobservable once t h e energy o f El
i s l e s s than t h e energy o f
EL. Another p o s s i b l e e x p l a n a t i o n f o r t h i s disappearance i s t h a t t h e valence and conduction band branches, A: and A: r e s p e c t i v e l y , which a r e n e a r l y e q u i d i s t a n t i n energy a t low temperature, become non-parallel
as t h e temperature increases; t h i s would r e s u l t i n
t h e peak i n t h e j o i n t d e n s i t y o f s t a t e s a t El
a t low temperatures
broadening c o n s i d e r a b l y as t h e temperature i s increased, making t h e peak i n t h e El
1980).
unobservable.
E~
peak i n
E~
Recently, i t has been thought t h a t
i s due t o e x c i t o n f o r m a t i o n (Hanke and Sham,
I f t h i s i s t r u e , then t h e disappearance o f t h i s peak w i t h
i n c r e a s i n g temperature c o u l d be understood,
since the p r o b a b i l i t y
o f e x c i t o n f o r m a t i o n decreases w i t h i n c r e a s i n g temperature. Whatever t h e o r i g i n s o f t h e s h i f t s i n
E*,
i t i s apparent t h a t
above 500°C c2 i s o n l y weakly dependent on temperature i n t h e range f r o m 3.8 eV t o 3.2 eV
.
An expanded p l o t o f a vs hv f o r several
temperatures (not shown h e r e ) i n d i c a t e s t h a t a(hv,T) independent o f
a1 so becomes
T above a c r i t i c a l photon energy hvc(T). An e m p i r i -
c a l f i t t o t h i s s a t u r a t i o n value o f a i n t h e v i c i n i t y o f 3.4 eV yields a =
where a.
(6 1
a0 exp (hv/Eo), = 4.1 x lo4 cm-l and Eo = 1.09 eV.
From an examination o f
a(hv) a t 5 O O 0 C , 600"C, and 7OO0C, it appears t h a t hvc moves monot o n i c a l l y t o a lower energy w i t h i n c r e a s i n g temperature.
Therefore,
i t i s reasonable t o expect t h e above expression t o y i e l d t h e asymp-
t o t i c value f o r a a t temperatures h i g h e r t h a n 700°C.
(4) Below t h e d i r e c t gap o f s i l i c o n , represented by t h e El and t h e EA f e a t u r e s i n E~ and E ~ c2 , increases monotonically w i t h temperature. For photon energies w e l l below t h e d i r e c t gap and
3.
107
PROPERTIES OF PULSED LASER-ANNEALED SILICON
f o r temperatures between 300 K and 1000 K, t h e a b s o r p t i o n c o e f f i c i e n t , which i s r e l a t e d t o
E~
[see Eqs.
( l b ) and (3)],
obeys t h e
empi r i c a l re1a t i on
where To = 43OOC f o r a l l photon energies.
For photon e n e r g i e s near
t h e d i r e c t gap, a more complicated behavior i s observed. a increases e x p o n e n t i a l l y as i n Eq.
Initially,
(7), b u t as t h e photon energy
o f i n t e r e s t approaches t h e d i r e c t gap, a approaches a s y m p t o t i c a l l y t h e l i m i t given i n Eq. (6). The temperature dependence o f t h e normal-incidence r e f l e c t a n c e was determined from t h e o p t i c a l f u n c t i o n s and i s shown i n Fig. 3. As can be seen, t h e peak i n R near 3.4 eV a t 10 K moves t o lower e n e r g i e s w i t h i n c r e a s i n g temperature , and disappears around 500OC. Above 500°C,
R i s a m o n o t o n i c a l l y i n c r e a s i n g f u n c t i o n o f energy
f r o m 2 t o 4 eV.
From 2 t o 3 eV, R increases l i n e a r l y w i t h tempera-
t u r e , and i s given by R(hv,T)
= Ro(hv,
300 K )
+ 5 ~ 1 0 - ~ ( -T 300 K).
(8)
S i m i l a r l y , values o f n f o r photon energies between 2 and 3 eV can be e m p i r i c a l l y expressed as n(hv,T)
= n(hv,300
K ) + 5xlO-'+(T
These r e s u l t s f o r R and n agree,
-
300
K).
19 1
w i t h i n t h e experimental e r r o r
l i m i t s , w i t h t h e e m i s s i v i t y r e s u l t s o f Sat0 (1967). The values o f t h e o p t i c a l f u n c t i o n s o f s i l i c o n a t 300
K are
t a b u l a t f t d i n Table I f o r several s e l e c t e d l a s e r wavelengths; a l s o l i s t e d a r e t h e values o f a. and To [see Eq. f i t t o the absorption c o e f f i c i e n t .
( 7 ) l of the empirical
Equations (8) and (9) can be
used t o determine R and n f o r photon energies l e s s than -3 eV and T
<
1000 K.
An extended t a b l e o f t h e o p t i c a l f u n c t i o n s
(1 nm
i n t e r v a l s ) a t 300 K and a t 10 K i s given by J e l l i s o n and Modine
Table I The o p t i c a l f u n c t i o n s o f s i l i c o n ( n and R; q and E ~ a) t room temperature p l u s t h e o p t i c a l absorption c o e f f i c i e n t (a) and t h e c a l c u l a t e d normal incidence r e f l e c t a n c e a t several l a s e r wave1 engths ( J e l l ison and Modi ne, 1982a). A1 so presented are t h e parameters t o t h e empirical fit o f t h e absorption c o e f f i c i e n t as a f u n c t i o n o f temperature given by Eq. (4) J e l l i s o n and Modine, 1 9 8 2 ~ ) .
T =
A
Laser
( nm)
n
k
€1
E~
a(l/cm)
R
cro
TO
(l/cm)
(K 1
694
Ruby
3.763
0.013
14.16
0.10
2 . 4 ~ 1 0 ~ .336
1.3420.29~10
42 7 282
633
HeNe
3.866
0.018
14.95
0.14
3.6~103
.347
2.08+0.32~103
447262
532
Nd:YAG (doubled)
4.153
0.038
17.24
0.32
9.0~103
.374
5.0220.49~103
430239
514
Argon i o n
4.241
0.046
17.98
0.39
1.12~104 .382
6.28t0.55~103
433k39
488
Argon i o n
4.356
0.064
18.97
0.56
1 . 5 6 ~ 1 0 ~ .392
9.0720. 66x103
438233
485
N i trogen-pumped dye
4.375
0.066
19.14
0.58
1 . 7 1 ~ 1 0 ~.394
9.3120.67~103
434k31
458
Argon i o n
4.633
0.096
21.45
0.89
2 . 6 4 ~ 1 0 ~ .416
1.45k0.08~ 10
429k34
405
N i trogen-pumped dye
5.493
0.290
30.08
3.19
9.01~104 .479
5.5120.15~10~
420269
355
Nd:YAG ( t r i p l e d )
5.683
3.027
23.13
34.41
1 . 0 7 ~ 1 0 ~ .575
1.0920.01x106
-870025300
337
Nitrogen
5.185
3.039
17.65
31.51
1. 13x106
.560
1.1320.01 x10
25000+25000
4.945
3.616
11.37
35.76
1 . 4 8 ~ 1 0 ~.587
1.43+O.01x1O6
470021300
Exci mer 308 -
3.
109
PROPERTIES OF PULSED LASER-ANNEALED SILICON
(1982b), and e x t e n s i v e tab1 es a t h i g h e r temperatures a r e avai l a b 1 e f r o m t h e same a u t h o r s on request. b.
Temperature Dependence o f a a t A = 1.152 and 1.064 pm Because o f t h e importance o f t h e n e a r - i n f r a r e d l i n e o f t h e HeNe
l a s e r (A = 1.152 pm) as a probe and t h e Nd:YAG l a s e r ( A = 1.064 mm) f o r h e a t i n g i n p u l s e d l a s e r - a n n e a l i n g experiments, i t i s u s e f u l t o know t h e o p t i c a l a b s o r p t i o n c o e f f i c i e n t f o r s i l i c o n a t these wavel e n g t h s as a f u n c t i o n o f temperature. measured a f o r t h e 1.152-pm Fig. 5. al.
J e l l i s o n and Lowndes (1982)
l i n e and o b t a i n e d t h e r e s u l t s shown i n
A l s o i n c l u d e d i n Fig. 5 a r e data taken from Macfarlane e t
(1958) and Weakliem and R e d f i e l d (1979) f o r both t h e 1.152 pm
and t h e 1.064 pm l i n e s .
The s o l i d ( d o t t e d ) l i n e shown i n Fig.
5
i s t h e value o f a a t 1.152 pm (1.064 pm) as a f u n c t i o n o f temperat u r e , c a l c u l a t e d i n t h e manner d e s c r i b e d below. I n f i t t i n g t h e temperature dependence o f a(hv,T) Macfarlane e t a l .
of silicon,
(1958) found t h a t t h e o p t i c a l a b s o r p t i o n f o r
e n e r g i e s near 1.1 eV was due p r i m a r i l y t o phonon-assisted i n d i r e c t transitions
from t h e
V r25, point
in
the
valence band t o a p o i n t
a l o n g t h e A branch o f t h e conduction band ( t r a n s i t i o n E i i n Fig. 4). Several phonons have been found t o p a r t i c i p a t e i n t h i s a b s o r p t i o n process, b u t those p r i m a r i l y i n v o l v e d a r e t h e t r a n s v e r s e and l o n g i t u d i n a l a c o u s t i c a l phonons w i t h k v e c t o r s i n t h e The expression given by Macfarlane e t a l .
direction.
(1958) f o r t h e absorp-
t i on c o e f f i c i e n t i s
a(hv,T)
2
=
c
2
c
i = l R=l
(-1)'
[ai(hv-E
1
-
(T)
-
(-1)'koi)l
exp((-l)'ei/T)
(10)
where t h e c o n t r i b u t i o n a r i s i n g from t h e i n t e r a c t i o n w i t h t h e t r a n s verse a c o u s t i c a l phonon ( i = l ) i s given by
110
G . E. JELLISON, JR.
104
,
-
--
I
I
I
I
0 WEAKLIEM 8 REDFIELD A MACFARLANE et 01.
0
0
--
j' N
00N
0'
102
5
/
= -
--
Y r
a 10'
/
:' A
t
-
B '
0'
0 '
0
7
5
-
2' 0'
---
w
PI
a'
-
-
\
0
/
..: -
--
0 THISWORK
-
103
d -
I
r'
-
--
0 '
?'
0
8'
d /
-
-
--
-- hh.1152
--
nm = 1064 nm
1 200
I
400
I 600
I
I
I
800
-
I
1000
1200
Fig. 5. The absorption coefficient o f silicon a t h = 1 . 1 5 2 pn (the neari n f r a r e d line o f the HeNe l a s e r ) and a t h = 1 . 0 6 4 p (Nd:YAG) as a function o f temperature. The lines represent the best f i t to the data assuming phononassisted indirect transitions (see t e x t ; from Jellison and Lowndes, 1 9 8 2 ) .
and t h e c o n t r i b u t i o n a r i s i n g from t h e i n t e r a c t i o n w i t h t h e l o n g i t u d i n a l a c o u s t i c a l phonon ( i = 2 ) i s given by a 2 ( E ) = 18.08 J r
+ 5760 (E-0.0055)2; o2
= 670 K
.
(12)
3.
111
PROPERTIES OF PULSED LASER-ANNEALED SlLICON
The expressions f o r al
and a2 have been s i m p l i f i e d from those of
Macfarlane e t a l . by approximating one o f t h e i r numerical funct i o n s by a quadratic, i n agreement w i t h t h e t h e o r e t i c a l c a l c u l a t i o n o f phonon-assisted o p t i c a l t r a n s i t i o n s ( E l l i o t , 1957).
The
o p t i c a l band gap i s a l s o a f u n c t i o n o f temperature, and i s given bY
Eg(T) = Eo 9
-
ATZ/(p+T).
This semi-empirical expression has been d e r i v e d by Varshni (1967) and more recent l y , u s i n g a thermodynamic argument, by Thurmond (1975). Thurmond used a more e l a b o ra te f i t t i n g procedure than d i d Varshni t o a r r i v e a t t h e parameters A = 4.73 x
and p = 635 K.
The value o f t h e o p t i c a l band gap a t 0 K was determined by Bludau e t a1
. (1974)
and by Macfarlane e t a1 .(1958)
t o be 1.17 eV, b u t
1.155 eV was used by J e l l i s o n and Lowndes (1982) a f t e r s u b t r a c t i n g o u t t h e e x c i t o n energy o f 15 meV.
As can be seen from Fig. 5,
t h e f i t t o t he observed data a t 1.152 ptn i s e x c e l l e n t .
Though
r e l i a b l e data i s not a v a i l a b l e f o r t h e 1.064 pm l i n e above 473 K, t h e calculat ed f i t i s good f o r t h e data p o i n t s below t h i s temperatu r e.
This good fit a t both 1.152 and 1.064 pm i n d i c a t e s t h a t t h e
f o r m u l a t i o n o f Macfarlane e t a l .
(1958), using t h e band gap tem-
p e rat ure dependence o f Thurmond (1975), i s accurate we1 1 beyond t h e previously measured data and can be used r e l i a b l y t o determine a(hv,T)
f o r photon energies near t h e i n d i r e c t band gap.
One would
n o t necessarily expect t h i s e x t r a p o l a t i o n t o be v a l i d , since t h e d e r i v a t i o n o f Eqs. (10)-(12) re q u i re s t h a t t h e bands be p a r a b o l i c , which may not be t r u e a t h i g h e r temperatures and/or h i g h e r photon energies. The index o f r e f r a c t i o n o f s i l i c o n a l s o increases s l i g h t l y w i t h temperature a t these wavelengths [see L i (1980) f o r a complete l i t e r a t u r e com p i l a ti o n o f t h e measured values o f t h e index o f r e f r a c t i o n o f s i l i c o n as a f u n c t i o n of wavelength and temperature i n t h e near infrared].
E x t r a p o l a t i n g and i n t e r p o l a t i n g t h e data
112
G . E. JELLISON. JR
o f Lukes (1959), t h e approximate values o f t h e r e f r a c t i v e index can be expressed as n(A = 1.152 pin, T ) = 3.51
+ 2.2xlO-+T,
(14a 1
n(A = 1.064 pm, T) = 3.53
+ 2.4xlO-+T,
(14b)
and
where t h e temperature i s i n
OC.
Note t h a t these expressions a r e
o n l y approximate, because t h e avai 1a b l e data f o r f i t t i n g o n l y goes t o 650°C, and a l i n e a r approximation has been used. c.
Liquid Silicon The o p t i c a l f u n c t i o n s o f l i q u i d s i l i c o n have been measured by
Shvarev e t a l .
(1975) and more r e c e n t l y by Shvarev e t a l .
u s i n g an e l l i p s o m e t r i c method.
(1977)
Since t h e l a t t e r work was more
accurate, o n l y t h e r e s u l t s i n i t w i l l be presented.
The data f o r
n and k were taken a t 3 wavelengths (400 nm, 700 nm, and 1000 nm), and c o u l d be f i t t o s t r a i g h t l i n e s o f t h e form: n = -0.2
+ 4.8 A (pm),
k = 2.3 + 4.7 A (pm)
U s i n g Eqs.
(15a)
.
(15b)
(15a) and (15b) t h e a b s o r p t i o n c o e f f i c i e n t a and t h e
normal i n c i d e n c e r e f l e c t a n c e can be c a l c u l a t e d , and are shown i n Table I 1 f o r several s e l e c t e d l a s e r wavelengths.
Shvarev e t a l .
(1975) found t h a t t h e o p t i c a l p r o p e r t i e s o f l i q u i d S i a r e n o t strongly
dependent
melting point. e t al.
(1981),
on temperature
f o r temperatures above t h e
These r e s u l t s agree w i t h t h e r e s u l t s o f Lampert who measured R o f l i q u i d s i l i c o n a t A = 632.8 nm
and found R = 0.72
a t t h e m e l t i n g p o i n t w i t h a temperature coef-
f i c i e n t o f -0.0002/°C
up t o -16OOOC.
It should be p o i n t e d out
t h a t some o f t h e r e s u l t s quoted i n Table I1 ( a t A = 1152 and 308 nm) r e p r e s e n t e x t r a p o l a t i o n s from t h e experimental data and may be i n e r r o r .
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
113
Table I 1 The o p t i c a l constants o f l i q u i d S i obtained using Eqs. (15a) and (15b) a t several selected 1aser wave1 engths. A
3.
a(x106) (l/cm)
R
(nm)
n
k
1152
5.33
7.71
0.84
.786
1064
4.91
7.30
0.86
.777
694
3.13
5.56
2.82
5.28
1.01 1.05
.739
633 532
2.35
4.80
1.13
.730
485
2.13
4.58
1.19
.723
308
1.28
3.75
1.53
.734
OPTICAL PROPERTIES
.734
OF ION-IMPLANTED AMORPHOUS SILICON
The o p t i c a l p r o p e r t i e s o f amorphous s i l i c o n have been studied f o r many years,
and t h e l a r g e body o f data t h a t has accumulated
can be discussed only b r i e f l y here.
I n p a r t i c u l a r , t h e work on
a-Si formed by i o n i m p l a n t a t i o n w i l l be emphasized, since t h i s i s t h e form t h a t i s o f most i n t e r e s t f o r l a s e r processing o f semiconductors. As w i l l be documented l a t e r , t h e study o f t h e o p t i c a l propert i e s o f ion-implanted amorphous m a t e r i a l s i s complicated by t h e f a c t t h a t those p r o p e r t i e s are o f t e n a f u n c t i o n o f sample prepar a t i o n variables,
i n c l u d i n g (1) t h e i o n used,
( 2 ) t h e dose,
(3)
t h e sample s u b s t r a t e temperature,
and (4) any post-imp1 a n t a t i o n
annealing t h a t may have occurred.
S t r i c t l y speaking, t h e o p t i c a l
p r o p e r t i e s o f ion-imp1 anted s i l i c o n and o t h e r semiconductors cannot be s p e c i f i e d a c c u r a t e l y w i t h o u t i n c l u d i n g t h e p a r t i c u l a r s o f sample preparation. The study o f t h e o p t i c a l p r o p e r t i e s o f ion-implanted m a t e r i a l s i s f u r t h e r complicated by t h e f a c t t h a t , i n general, t h e o p t i c a l p r o p e r t i e s may vary s i g n i f i c a n t l y w i t h t h e depth from t h e surface o f t h e sample. That i s , t h e surface i s o f t e n o n l y l i g h t l y damaged,
114
G . E. JELLISON, JR.
and may therefore have optical properties d i f f e r e n t from those near the damage peak, which in t u r n may be q u i t e d i f f e r e n t from those
of the substrate. The minimum number of layers t h a t must be considered t o obtain accurate values of t h e optical functions from ellipsometry in a case l i k e t h i s i s s i x ; namely, a i r , t h e nativeoxide l a y e r , a l i g h t l y damaged layer, t h e heavily damaged layer, another l i g h t l y damaged l a y e r , and f i n a l l y , t h e semi-infinite subs t r a t e . Fewer layers can be used if precautions are taken t o (1) damage t h e f r o n t layer uniformly (by implanting a t several d i f f e r ent energies) so t h a t t h e optical properties are roughly the same over t h e f i r s t -2000 A and/or ( 2 ) work with wavelengths short enough t h a t the l i g h t does not penetrate t o t h e underlying layers. Short of t h i s , one can speak only of "effective" optical functions, which may or may not approximate s u f f i c i e n t l y accurately t h e actual optical functions of the ion-implanted amorphous s i l i c o n . For l a s e r annealing applications using a ruby l a s e r (694 nm) or a Nd:YAG l a s e r (1064 nm) and a single ion implantation energy, the optical prope r t i e s of i n t e r e s t (absorption c o e f f i c i e n t and surface reflectance) of the near-surface region a r e l i k e l y t o be q u i t e complicated, because the l i g h t will penetrate the e n t i r e damaged region. The d i e l e c t r i c functions of c-Si, a-Si implanted with 10l6 Si+/cm2 a t several implantation energies (a-Si : S i + ) , and c r y s t a l l i n e s i l i c o n ion implanted w i t h 10l6 B+/cm* (c-Si:B+) are shown i n Fig. 6. Ion implantation of Si+ atoms creates an amorphous surface l a y e r , r e s u l t i n g in q u i t e d i f f e r e n t optical properties f o r the a-Si and the c-Si samples. On t h e other hand, ion implantation of B+ atoms damages the front-surface region, b u t does not make i t amorphous and therefore t h e optical properties of t h e c-Si :B+ and the c-Si samples are very similar. The d i e l e c t r i c function of a-Si:Si+ i s very s i m i l a r t o t h a t of other f u l l y amorphous materials formed by chemical vapor deposition (see f o r example Aspnes, 1981). T h e peak in E~ occurs a t -3.31 eV with a value of -24.
50 40
30 20 €1
10 0
- 10 -20
50 40
€2
I
------
I
I
c-Si a - S i Si IMPLANT c - S i B IMPLANT
I
I
I
-
30 -
-
-
-
20 10
-
0
Fig. 6. + € 2 ) plotted vs photon The complex dielectric functions ( a = energy o f ( 1 ) crystalline silicon (c-Si, Jeilison and Modine, 1 9 8 2 a ) , ( 2 ) ionimplanted, amorphous silicon, implanted with 10l6 Si+/cm2 (a-Si:Si+, Jellisonet a l . , 1 9 8 3 ) , and ( 3 ) silicon implanted with 10l6 B+/cm2 (c-Si:B+, Jellison e t al., 1980).
116
G . E. JELLISON, JR.
ENERGY (eV) Fig. 7. The absorption coefficient plotted vs photon energy for the three samples o f Fig. 6.
The a b s o r p t i o n c o e f f i c i e n t i s a more i m p o r t a n t parameter f o r l a s e r annealing c a l c u l a t i o n s ; t h i s i s shown i n Fig. 7 f o r t h e same t h r e e samples as Fig. 6. c i e n t o f a-Si:Si+ o f c-Si
As can be seen, t h e a b s o r p t i o n c o e f f i -
i s as much as a f a c t o r o f 50 l a r g e r than t h a t
w e l l below t h e d i r e c t band gap,
above t h i s energy.
b u t i s n e a r l y t h e same
The values shown here f o r a o f a-Si
a t low
3.
117
PROPERTIES OF PULSED LASER-ANNEALED SILICON
photon energies compare w e l l w i t h t h e values given by Brodsky e t al.
(1970) f o r rf s p u t t e r e d , unannealed amorphous s i l i c o n f i l m s .
The a f o r c-Si:Bt
i s n e a r l y t h e same as t h a t o f c-Si
above t h e
d i r e c t band gap, b u t s t a r t s t o d e v i a t e s i g n i f i c a n t l y f o r photon energies w e l l below 3.4 eV. t h e f a c t that the
T h i s i n c r e a s e i n a i s e x p l a i n e d by
B+ i o n i m p l a n t a t i o n process c r e a t e s a s i g n i f i -
c a n t number o f d e f e c t s i n t h e m a t e r i a l
(see Q i n e t a1
., 1982).
The d e f e c t s can enhance t h e o p t i c a l a b s o r p t i o n process by c r e a t i n g s t a t e s within t h e band gap, a l l o w i n g some d i r e c t t r a n s i t i o n s even f o r photon energies much l e s s t h a n 3.4
eV, o r by d i s t u r b i n g t h e
phonon p o p u l a t i o n and t h e r e b y changing t h e phonon-assi s t e d i n d i r e c t absorption
mechanism r e s p o n s i b l e
for
absorption a t
less than
3.4 eV. Several authors have s t u d i e d t h e o p t i c a l p r o p e r t i e s o f i o n i m p l a n t e d amorphous
s i l i c o n under v a r i o u s c o n d i t i o n s ;
we w i l l
o n l y mention a few r e s u l t s . 1) Watanabe e t a l .
(1979) used single-wavelength e l l i p s o m e t r y
(A = 546 nm) t o s t u d y t h e " e f f e c t i v e " index o f r e f r a c t i o n 5 and e x t i n c t i o n c o e f f i c i e n t b ( t h a t i s , n and k assuming a one boundary model) as a f u n c t i o n o f low dose Bf implants. even
doses i n t h e
range o f 1012/cm2
increased
remained c o n s t a n t t o a dose o f 3 x 1013/cm2.
They found t h a t
1and
x, w h i l e n
Although boron-
imp1 anted s i 1i c o n has o f t e n been r e f e r r e d t o as c r y s t a l 1 i n e s i 1i-
con, t h i s i s n o t a s t r i c t l y c o r r e c t c h a r a c t e r i z a t i o n because even boron i m p l a n t a t i o n i n t r o d u c e s a s i z a b l e number of d e f e c t s t h a t a l t e r the optical properties. 2)
The p r o f i l e o f t h e complex r e f r a c t i v e index o f P+-implanted
s i l i c o n was s t u d i e d by Adams and Bashara (1975) u s i n g anodic o x i d a t i o n and s t r i p p i n g t o bare successive l a y e r s , f o l l o w e d by e l l i p sometry measurements a t A = 632.8
nm.
They found t h a t n and k
peaked very c l o s e t o t h e depth of maximum damage, w h i l e t h e s u r f a c e r e g i o n and t h e r e g i o n deeper i n t o t h e damaged l a y e r had lower values o f n and k, b u t s t i l l d i f f e r e n t from those o f t h e s u b s t r a t e .
There-
fore, t h e o p t i c a l p r o p e r t i e s of i o n - i m p l a n t e d S i u s i n g o n l y a s i n g l e
118
G . E. JELLISON, JR.
i m p l a n t a t i o n energy can be expected t o be depth dependent.
Tech-
n o l o g i c a l l y , s i n g l e i m p l a n t a t i o n energies represent t h e most import a n t case; however, f o r basic studies i t i s p o s s i b l e t o implant samples a t several d i f f e r e n t energies, making t h e degree o f damage, and hence t h e o p t i c a l p r o p e r t i e s , more uniform. 3) The o p t i c a l p r o p e r t i e s o f S i near t h e d i r e c t band gap have been observed t o change w i t h P+ o r As+ i m p l a n t a t i o n using scanning e l l i p s o m e t r y (Cortot and Ged,
1982), w i t h
P+ i m p l a n t a t i o n using
wavelength modulated r e f l e c t i v i t y (Lue and Shaw, 1982), and w i t h Sb+ i m p l a n t a t i o n using normal-incidence r e f l e c t i v i t y (McGill al.,
1970).
I n a l l cases,
, et
t h e main f e a t u r e i n t h e spectrum a t
3.4 eV, due t o t h e onset o f d i r e c t band-gap absorption, g r a d u a l l y disappeared w i t h i n c r e a s i n g dosage.
The disappearance o f t h i s
f e a t u r e i s a t t r i b u t e d t o t h e i n c r e a s i n g l a t t i c e d i s o r d e r produced by i o n implantation,
and provides a t l e a s t a crude experimental
probe o f t h e c-Si t o a-Si t r a n s i t i o n .
Very l i t t l e change i n t h e
B+ i s observed near 3.4 eV (see Figs. 6 and 7), i n d i c a t i n g t h a t t h e d e f e c t s t h a t are introduced by Bt
d i e l e c t r i c spectra o f c-Si
i m p l a n t a t i o n do not g r e a t l y d i s t u r b t h e l o n g range order. 4)
The e f f e c t s o f annealing on t h e complex r e f r a c t i v e index
o f P+-imp1 anted S i has been s t u d i e d using s i n g l e wavelength e l 1 ipsometry a t 546 nm by Nakamura e t al. (1980).
(1979) and Watanabe e t a l .
They found t h a t t h e complex index o f r e f r a c t i o n does not
undergo a discontinuous jump from t h e amorphous values t o c r y s t a l -
1i n e values, but r a t h e r changes gradually.
Nakamura a1 so studied
n and k as a f u n c t i o n o f s u b s t r a t e temperature d u r i n g i m p l a n t a t i o n and found l a r g e d i f f e r e n c e s i n t h e o p t i c a l p r o p e r t i e s , p a r t i c u l a r l y k, o f m a t e r i a l implanted a t 250°C compared t o m a t e r i a l implanted a t 77 K.
F i n a l l y , Fredrickson e t a l .
(1982) monitored t h e near-
i n f r a r e d o p t i c a l p r o p e r t i e s of S i + - and P+-implanted S i as a funct i o n o f anneal i n g c o n d i t i o n s and concluded t h a t two we1 1-defined o p t i c a l s t a t e s o f a-Si produced by i o n i m p l a n t a t i o n e x i s t :
one,
c h a r a c t e r i z e d by a h i g h value o f n (-4.0 a t A = 1.0 p), i s produced by a h i g h fluence of S i + o r P+ ions a t room temperature,
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
w h i l e t h e second,
119
c h a r a c t e r i z e d by an i n t e r m e d i a t e value o f n
a t h = 1.0 pm), i s observed a f t e r thermal annealing o f t h e
(-3.8
same samples.
C l e a r l y , t h e previous thermal h i s t o r y o f t h e i o n -
implanted m a t e r i a l i s a l s o important. 4.
LASER-ANNEALED,
HEAVILY DOPED SILICON
The use o f i o n i m p l a n t a t i o n f o l l o w e d by l a s e r annealing f o r p r e p a r a t i o n o f very h e a v i l y doped samples has many advantages:
(1) t h e s o l u b i l i t y l i m i t can be exceeded t o g i v e higher s u b s t i t u t i o n a l dopant concentrations than are a v a i l a b l e using conventional doping techniques (White e t a1
., 1980a,
1980b), (2) t h e r e s u l t i n g
near-surface r e g i o n i s f r e e o f extended defects and p r e c i p i t a t e s (down t o -10 A ) ,
(3) t h e surface i s o p t i c a l l y f l a t (Aspnes e t al.,
1980), and (4) t h e p r o f i l e o f t h e dopant concentration i s more n e a r l y f l a t than t h e p r o f i l e obtained by d i f f u s i o n doping (Zehner e t a1 a.
., 1980).
E l l i p s o m e t r y Studies J e l l i s o n et al.
(1981a) have performed PME experiments on S i
implanted w i t h B, As, and P and pulsed l a s e r annealed w i t h t e n 1.3 J/cm2 pulses from a Q-switched ruby l a s e r .
The values o f n and k
were c a l c u l a t e d using a 3 - l a y e r model ( a i r - n a t i v e oxide-substrate), where it was assumed t h a t t h e n a t i v e oxide had an n o f 1.46 and a k o f 0. The value of t h e oxide thickness f o r most samples was taken t o be 24 A from e l l i p s o m e t r y measurements made on s i m i l a r l y t r e a t e d , undoped s i l i c o n .
The oxide thicknesses o f t h e two most
h e a v i l y doped samples were determined from t h e surface oxygen conc e n t r a t i o n measured by Rutherford backscattering,
assuming t h e
surface l a y e r was Si02. F i g u r e 8 shows t h e index of r e f r a c t i o n and t h e e x t i n c t i o n coefficient
obtained from samples h e a v i l y doped w i t h arsenic.
Several p o i n t s can be made from t h i s f i g u r e .
(1)
As t h e doping
120
G . E. JELLISON. JR.
-4'
I
I
1
Y
k-
z L+'
0
2 3 ' LL W
0 0
2 2 '-
0
0
PHlLlPP AND TAFT
I-
u
z F 1 X
W
0
1
2
3
4
ENERGY ( e V ) Fig. 8. ( a ) Index of refraction and ( b ) extinction coefficient vs photon energy for several As concentrations ( / c m 3 ) in silicon. The error bars show typical confidence limits a t the specified energy (Jellison e t al., 1 9 8 1 a ) .
3.
121
PROPERTIES OF PULSED LASER-ANNEALED SILICON
concentration o f As i s increased, t h e peak i n n decreases i n magn i t u d e and moves t o lower energies.
This corresponds t o t h e
general increase i n k f o r a l l photon energies below t h e d i r e c t gap (since n and k are Kramers-Kronig p a i r s , they are not independent; i n f a c t , a peak i n n should correspond roughly t o a maxi-
mum i n dk/dE, as i s observed). a small peak i n k near 3.4 exceeded (>5 x 1 0 l 8 As/cm3), s i m i l a r rounding o f t h e
E*
(2)
The undoped sample e x h i b i t s When t h e degenerate l i m i t i s
eV.
t h i s peak becomes rounded o f f .
peak near 3.4
A
eV has been. observed by
Vina and Cardona (1983) and Aspnes (1983) f o r samples h e a v i l y doped by conventional methods.
(3)
The most h e a v i l y doped sam-
p l e e x h i b i t s a rounding o f t h e shoulder i n k a t 3.4
eV and a
broadening o f t h e peak i n n; t h i s i s probably because t h i s sample c o n t ains 6% As, and can no longer be considered a doped S i c r y s t a l b u t r a t h e r an a l l o y [ r e c a l l t h a t t h e doping concentration o f t h i s sample exceeds the s o l i d s o l u b i l i t y l i m i t o f As i n S i (-1021/cm3 see Chapter 2)]. The most pronounced e f f e c t on t h e o p t i c a l p r o p e r t i e s o f samples prepared by i o n imp1a n t a t i on fa1 1owed by pul sed l a s e r annealing (PLA), however, i s shown i n Figs. 9 and 10 (taken from J e l l i s o n e t al.,
1981a), where t h e absorption c o e f f i c i e n t i s p l o t t e d versus
photon energy.
As can be seen, t h e r e i s a l a r g e increase i n a w i t h
doping f o r As doped samples (Fig. 9).
I n contrast, the effects
observed f o r samples doped w i t h 3 x 1020 B o r P/cm3 are small (Fig. 10). The B- and P-doped samples a l s o show t h e rounding o f t h e shoulder i n a o r k a t t h e d i r e c t gap edge (3.4 eV) i n d i c a t i n g t h a t t h e rounding i s a heavy doping e f f e c t , and not dependent upon t h e dopant atom.
However, t h e increase i n a below t h e d i r e c t gap
depends s i g n i f i c a n t l y on dopant species. Aspnes e t a1
. (1980) have a1 so performed scanning e l l i p s o m e t r y
measurements o f pulsed laser-anneal ed, frequency-doubled
Nd:YAG
l a s e r,
As-imp1 anted S i using a
and v a r y i n g t h e i n c i d e n t l a s e r
122
G . E. JELLISON, JR.
40’
,-
I
I
p
A 4
1
1
I
o DASH AND NEWMAN
I
I
I
2
3
4
I
ENERGY (eV) Fig. 9.
Absorption coefficient vs photon energy f o r several As concentrations
i n silicon (Jellison e t a l . ,
1981a).
observed f o r samples doped w i t h 3 x 10*O 10).
B
o r P/cm3 a r e small (Fig.
The B- and P-doped samples a l s o show t h e rounding o f t h e
shoulder i n a o r k a t t h e d i r e c t gap edge (3.4 eV) i n d i c a t i n g t h a t t h e rounding i s a heavy doping e f f e c t , and n o t dependent upon t h e dopant atom.
However,
t h e i n c r e a s e i n a below t h e d i r e c t gap
depends s i g n i f i c a n t 1y on dopant species.
3. PROPERTIES OF PULSED LASER-ANNEALED SILICON
123
c
z
w
0
L
t o4
lo3
2
3
4
ENERGY ( e V )
Fig. 10. Absorption coefficient for Si heavily doped with B ( 3 x 1020 B / c m 3 ) and P (3.2 x 1 0 2 0 / c m 3 ) , compared t o undoped Si (Jellison e t a l . , 1981a).
Aspnes e t a l .
(1980) have a l s o performed scanning e l l i p s o m e t r y
measurements o f pulsed laser-annealed, frequency-doubled energy d e n s i t y .
Nd:YAG
laser,
As-implanted
S i using a
and v a r y i n g t h e i n c i d e n t l a s e r
F i g u r e 11 shows t h e d i e l e c t r i c f u n c t i o n s p e c t r a
o b t a i n e d by Aspnes e t a1
. (1980) f o r t h e
l a s e r annealed and r e f -
erence samples, where Ll-L4 r e f e r t o t h e energy d e n s i t y ( L 1 = 0.6, L 2 = 1.0, L3 = 1.5, L4 = 2.3 J/cm2), t h e a-Si r e f e r s t o t h e o r i g i n a l
124
G . E. JELLISON, JR
50
40
30 20 N
W
w' 10 0
-10
-20
5
4
3
E (eV) Fig. 11. Dielectric function spectra for laser-annealed and reference samples. The symbols L1-L4 r e f e r t o the pulse energy density (L1 = 0.6 J / c m 2 , 12 = 1.0 J / c m 2 , L3 = 1.5 J / c m 2 , and L4 = 2.3 J / c m 2 ) , a-Si r e f e r s t o the ionimplanted, unannealed sample, and c-Si refers t o an undoped crystalline Si sample (Aspnes e t al., 1980).
As-implanted (1 x 1 0 l 6 As/cm', sample,
30 keV,
surface),
and c-Si r e f e r s t o an undoped S i sample.
unannealed
The r e s u l t i n g
o v e r l a y e r f o r t h e L1 sample was p o l y c r y s t a l l i n e , so t h e f a c t t h a t t h e o p t i c a l spectra f o r t h e
L1 sample are i n t e r m e d i a t e between t h e
a-Si sample and t h e f u l l y annealed sample i s not s u r p r i s i n g . lower values o f
E~
near t h e peaks a t 3.4 and 4.25
o f i o n i m p l a n t a t i o n f o l l o w e d by PLA.
The
eV are a r e s u l t
Below t h e d i r e c t band gap a t
eV, E~ i s much l a r g e r than i t i s i n t h e undoped sample, a r e s u l t which was also observed by J e l l i s o n e t a l . (1981a) and was
3.4
discussed above.
Recently, Aspnes e t a l . (1984) have f i t these
3.
125
PROPERTIES OF PULSED LASER-ANNEALED SILICON
s p e c t r a by assuming a 16-8, rough l a y e r w i t h 50% voids over pure silicon. features
Aspnes e t a l . (near 3.4
(1980) a l s o noted t h a t t h e El
eV) and t h e
and E 1 + ~
E, f e a t u r e (near 4.25 eV) move
l i n e a r l y t o lower energy as t h e doping c o n c e n t r a t i o n i s increased and concluded t h a t t h i s e f f e c t arose p u r e l y from doping e f f e c t s . S i n g l e wavelength (A = 546.1 nm) e l l i p s o m e t r y measurements o f p u l se
laser-annealed
,
Si
As-implanted
have been performed b y
Nakamura and Kamoshida (1979) as a f u n c t i o n o f i m p l a n t a t i o n dose (1013-1016 /cm2)
and l a s e r energy.
They concluded t h a t l a s e r
a n n e a l i n g w i t h s u f f i c i e n t energy b 2 . 3
J/cm2
f o r a Q-switched
Nd:YAG l a s e r ( A = 1.064 pin) w i t h a 50-ns p u l s e w i d t h ] r e s u l t s i n t h e recovery o f o p t i c a l parameters t o p r e - i m p l a n t a t i o n values.
A
c l o s e examination o f Figs. 3 and 4 o f Nakamura and Kamoshida (1979) r e v e a l s t h a t 4 and A a r e c l o s e t o t h e unimplanted values, f o r h i g h l a s e r energy d e n s i t i e s , b u t n o t c o i n c i d e n t , which may be s u f f i c i e n t t o e x p l a i n t h e d i f f e r i n g c o n c l u s i o n s o f Nakamura and Kamoshida (1979) on t h e one hand and J e l l i s o n e t a l . al.
(1981a) and Aspnes e t
(1980) on t h e o t h e r . Several o b s e r v a t i o n s can now be made concerning t h e o p t i c a l
p r o p e r t i e s of i o n - i m p l a n t e d PLA s i l i c o n :
( 1 ) The i n c r e a s e i n a
i s r o u g h l y l i n e a r w i t h t h e doping c o n c e n t r a t i o n f o r t h e As-doped samples doped t o c o n c e n t r a t i o n s l e s s t h a n 4 . 6 ~ 1 0 ~ ~ / c(m J e~l l i s o n e t al.,
1981a).
(2) The doping e f f e c t observed i n As-doped samples
i s n o t s i m p l y a f u n c t i o n o f doping type,
s i n c e i t i s n o t seen
f o r P-doped (n-type) o r B-doped (p-type) samples ( J e l l i s o n e t al., 1981a).
( 3 ) The quantum e f f i c i e n c y i n t h e b l u e r e g i o n o f t h e s o l a r
spectrum o f s o l a r c e l l s f a b r i c a t e d with As-doped e m i t t e r s does n o t v a r y s i g n i f i c a n t l y f r o m s o l a r c e l l s f a b r i c a t e d w i t h P- o r B-doped e m i t t e r s ( J e l l i s o n e t a1
., 1981b)
i n d i c a t i n g t h a t t h e increased
a b s o r p t i o n may be an e l e c t r o n - h o l e c r e a t i n g e f f e c t . recently
been observed
by Lowndes
(1983)
(4)
It has
u s i n g t i m e - r e s o l ved
r e f l e c t i v i t y measurements d u r i n g PLA w i t h a ruby l a s e r t h a t t h e t h r e s h o l d f o r s u r f a c e m e l t i n g i s c o n s i d e r a b l y lower f o r As-doped
126
G . E. JELLISON, JR
s i l i c o n (-0.55 J/cm2) t h a n f o r undoped c r y s t a l l i n e s i l i c o n ( 4 . 8 J/cm2).
T h i s d i f f e r e n c e can be e x p l a i n e d by t h e i n c r e a s e d value
of a f o r As-doped S i compared t o undoped S i . s t r a i n s are n o t expected f o r As-doped
( 5 ) Large l a t t i c e
S i , even a t h i g h doping
.
Therel e v e l s , s i n c e As has n e a r l y t h e same c o v a l e n t r a d i u s as S i fore, any change i n t h e o p t i c a l p r o p e r t i e s o f As-doped S i p r o b a b l y does n o t come from s t r a i n s i n t r o d u c e d by t h e dopant atom. The c o r r e c t i n t e r p r e t a t i o n o f these r e s u l t s i s not obvious and w i l l r e q u i r e a s i g n i f i c a n t t h e o r e t i c a l and experimental e f f o r t t o completely explain t h e results. g i v e n by J e l l i s o n e t a l .
One p l a u s i b l e e x p l a n a t i o n has been
(1981a), where t h e enhanced a b s o r p t i o n
was a t t r i b u t e d t o t h e d - e l e c t r o n s i n t r o d u c e d by t h e As dopant. a l t e r n a t i v e e x p l a n a t i o n has been g i v e n by Aspnes e t a l .
An
(1984),
who invoked s u r f a c e roughness t o e x p l a i n t h e observed n and k o f F i g . 8.
Although t h i s e x p l a n a t i o n y i e l d s a good e m p i r i c a l f i t t o
t h e e l l i p s o m e t r y data, it appears t o be i n c o n s i s t e n t w i t h observat i o n s ( 3 ) and ( 4 ) above.
A rough s u r f a c e would most l i k e l y r e s u l t
i n a v e r y l a r g e s u r f a c e r e c o m b i n a t i o n v e l o c i t y , which would decrease t h e b l u e response o f s o l a r c e l l s f a b r i c a t e d from As-implanted PLA S i ; t h i s i s n o t observed.
Also,
a 20-40 A t h i c k rough l a y e r on
t h e s u r f a c e would n o t i n c r e a s e t h e l i g h t a b s o r p t i o n i n t h e nears u r f a c e r e g i o n f o r ruby
PLA n e a r l y enough t o e x p l a i n o b s e r v a t i o n
( 4 ) above.
I n r e c e n t r o t a t i n g analyzer e l l ipsometry (RAE) measurements o f h e a v i l y doped S i u s i n g c o n v e n t i o n a l doping techniques, Vina and Cardona (1983) observed no d i f f e r e n c e i n t h e o p t i c a l s p e c t r a among samples doped w i t h B, P, o r As, up t o As doping c o n c e n t r a t i o n s o f 5 x 1019/cm3.
They a t t r i b u t e d t h e d i f f e r e n c e s between t h e i r r e s u l t s
and those o f J e l l i s o n and co-workers t o p e c u l i a r i t i e s o f t h e i o n i m p l a n t a t i o n process. As i n t o S i
One p o s s i b i l i t y i s t h a t i o n i m p l a n t a t i o n o f
f o l 1owed by p u l sed 1aser anneal ing r e s u l t s i n As-defect
s i t e s i n t h e c r y s t a l which a r e s u b s t i t u t i o n a l and e l e c t r i c a l l y active,
but d i f f e r e n t o p t i c a l l y from conventional s u b s t i t u t i o n a l
3.
127
PROPERTIES OF PULSED LASER-ANNEALED SILICON
As donor s i t e s . This i s supported by recent Mossbauer r e s u l t s ( P f e i f f e r e t al., 1982) f o r samples prepared by laser-induced d i f f u s i o n (see Chapter l ) , which show t h a t most As atoms are associated w i t h one o r m r e defects. b.
I n f r a r e d Measurements Engstrom (1980) has made i n f r a r e d r e f l e c t i v i t y and transmis-
s i v i t y measurements on boron-implanted, laser-annealed s i l i c o n from
2.5 t o 20 pm.
The samples were implanted w i t h 35-keV boron i o n s
t o doses ranging from 1014 t o 10L6/cm2, and were l a s e r annealed u s i n g a Q-switched ruby l a s e r w i t h an energy d e n s i t y o f -1.7 and pulse d u r a t i o n o f -40 ns.
J/crn2
Taking i n t o account t h e boron con-
c e n t r a t i o n p r o f i l e i n t h e a n a l y s i s o f t h e data,
and using t h e
Drude theory o f e l e c t r i c a l c o n d u c t i v i t y , Engstrom (1980) was able t o o b t a i n very good f i t s t o h i s data even a t t h e highest doping levels.
He concluded t h a t t h e hole s c a t t e r i n g r e l a x a t i o n time was
independent o f implant dose and was -7 x 10-15 sec. f r a r e d measurements were performed by Miyao e t a1
Similar in-
. (1981)
for
h e a v i l y doped n-type s i l i c o n using i o n i m p l a n t a t i o n f o l l o w e d by pulsed l a s e r annealing w i t h a Q-switched ruby l a s e r (pulse d u r a t i o n 25 ns, energy d e n s i t y :
0.5-1.5
J/cm2).
They observed t h a t t h e
e f f e c t i v e mass increased w i t h doping c o n c e n t r a t i o n and t h a t t h e electron
scattering
r e l a x a t i o n time decreased
i n i t i a l l y with
i n c r e a s i n g c a r r i e r c o n c e n t r a t i o n before l e v e l i n g o f f a t a value -3 x sec. These r e s u l t s were i n t e r p r e t e d by i n v o k i n g a minimum i n t h e conduction band along t h e [ l l O ] d i r e c t i o n t h a t was p r e d i c t e d by Chelikowski and Cohen (1976); t h e e f f e c t i v e mass associated w i t h t h i s minimum i s supposed t o be l a r g e r than t h a t of t h e normal v a l l e y along t h e A branch.
As t h e c a r r i e r concentra-
t i o n is increased above -5 x 1020/cm3, t h i s v a l l e y s t a r t s t o f i l l , i n c r e a s i n g t h e observed e f f e c t i v e mass.
G . E. JELLISON, JR
c.
Other O p t i c a l Measurements Raman s t u d i e s o f boron-imp1 anted , pul se l a s e r annealed s i l i c o n
have been performed by Engstrom and Bates (1979).
They observed
two f e a t u r e s i n t h e Raman spectrum:
a very s t r o n g peak a t 523
r25, o p t i c
mode, and a much weaker peak
cm-',
which i s due t o t h e
a t 620 cm-1 , due t o t h e boron l o c a l mode. o f t h e 523 cm'l shoulder,
peak,
Upon c l o s e examination
Engstrom and Bates observed a d i s t i n c t
which was explained by a Fano-type i n t e r a c t i o n (Fano,
1961) between a l o c a l i z e d o p t i c mode and t h e continuous band states. T h i s i n t e r a c t i o n occurs because t h e presence o f t h e boron acceptor l e v e l s allows e l e c t r o n i c Raman s c a t t e r i n g by way o f t r a n s i t i o n s from t h e l i g h t - h o l e band t o t h e heavy-hole band.
Since, i n s i l i c o n ,
t h e energy o f t h e l o c a l mode occurs w i t h i n t h e range o f energies a1 lowed f o r e l e c t r o n i c Raman s c a t t e r i n g , an i n t e r f e r e n c e between t h i s mode and t h e s t a t e s o f t h e heavy-hole band occurs, g i v i n g r i s e t o t h e low energy shoulder on t h e Raman o p t i c mode peak. Engstrom and Bates performed a d e t a i l e d computer f i t t o t h e Raman o p t i c mode peak, using dopant p r o f i l e s obtained by SIMS, and concluded t h a t 89
k
9% o f t h e boron atoms occupied s u b s t i t u t i o n a l
s i t e s , f o r a sample doped w i t h 3.82
x 102O B/cm3 a t t h e surface.
A s i m i l a r attempt t o f i t t h e data f o r a t h e r m a l l y annealed sample r e s u l t e d i n a poorer fit and a much lower f r a c t i o n o f boron atoms It can be concluded from t h i s Raman i n substitutional sites. study t h a t pulsed l a s e r annealing i s more e f f e c t i v e than thermal annealing i n naking boron atoms s u b s t i t u t i o n a l ; t h i s same conclus i o n can be drawn from a v a r i e t y o f experiments,
including the
e l e c t r i c a l measurements discussed i n t h e next section. Ion-damaged, laser-annealed s i l i c o n has been s t u d i e d by P o l l a k e t a1
. (1980) using e l e c t r o l y t e e l e c t r o r e f l e c t a n c e (EER).
Silicon
samples t h a t had been implanted w i t h 1 0 l 6 Si/cm2 were l a s e r annealed a t various energy d e n s i t i e s from 0.1 t o 0.75 J / c d u s i n g a 15-ns p u l s e from frequency-doubled Nd:YAG l a s e r .
P o l l a k and co-workers
concluded from t h e nature o f t h e EER lineshape narrowing t h a t
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
129
short-range t e t r a h e d r a l o r d e r i n g o f t h e Si atoms occurs even f o r v e r y low energy pulses (-0.13 densities
(-0.55
J/cm2)
J/cm2),
and t h a t a t h i g h e r energy
t h e l o n g e r range S i atom o r d e r i n g i s
t a k i n g place.
111.
E l e c t r i c a l Properties
5. ELECTRICAL MEASUREMENT TECHNIQUES The e l e c t r i c a l p r o p e r t i e s o f most i n t e r e s t i n laser-annealed m a t e r i a l s f a l l b a s i c a l l y i n t o two c a t e g o r i e s : ties,
(1) sheet proper-
which a r e determined by t h e e l e c t r i c a l c h a r a c t e r i s t i c s o f
t h e t h i n l a y e r o f ion-implanted,
laser-annealed m a t e r i a l i t s e l f
and ( 2 ) j u n c t i o n p r o p e r t i e s , which a r e concerned w i t h t h e e l e c t r i c a l c h a r a c t e r i s t i c s across t h e t r a n s i t i o n r e g i o n from t h e l a s e r annealed,
front-surface
l a y e r t o the underlying substrate.
The
sheet p r o p e r t i e s a r e g e n e r a l l y measured u s i n g H a l l e f f e c t and r e s i s t i v i t y techniques, w h i l e measurements o f t h e j u n c t i o n prope r t i e s a r e made by capacitance-vol t a g e and c u r r e n t - v o l t a g e techniques.
The reader i s r e f e r r e d t o any one o f t h e several e x c e l l e n t
books f o r d e t a i l s o f t h e measurement o f e l e c t r i c a l p r o p e r t i e s (see, f o r example, Sze, 1980, o r McKelvey, 1966). a.
Sheet P r o p e r t y Measurements The H a l l e f f e c t (see Beer, 1963, f o r a d e t a i l e d t r e a t m e n t ) i n
a t h i n sample i s n o r m a l l y measured by passing a c u r r e n t t h r o u g h a sample i n a d i r e c t i o n p e r p e n d i c u l a r t o an a p p l i e d magnetic f i e l d . The H a l l v o l t a g e i s s e t up p e r p e n d i c u l a r t o b o t h t h e c u r r e n t and t h e a p p l i e d magnetic f i e l d , and i s given by VH = IHRHKHB/d
,
where RH i s t h e H a l l c o e f f i c i e n t , B i s t h e magnetic f i e l d s t r e n g t h ,
I H i s t h e f o r c e d c u r r e n t , d i s t h e t h i c k n e s s o f t h e sample, and KH i s a c o n s t a n t o f p r o p o r t i o n a l i t y depending upon geometrical f a c t o r s .
WO
G . E. JELLISON, JR.
F o r a sample where t h e d e n s i t y o f m a j o r i t y c a r r i e r s i s much l a r g e r than t h e density o f m i n o r i t y c a r r i e r s , the Hall c o e f f i c i e n t can be expressed as R H = -rn /en
f o r n-type m a t e r i a l ,
(17a)
RH = rp/ep
f o r p-type m a t e r i a l .
(17b)
and
The q u a n t i t y r i s a dimensionless f a c t o r (-1) which depends on t h e m a j o r i t y c a r r i e r s c a t t e r i n g mechanism, e i s t h e e l e c t r o n i c charge, and n ( o r p ) i s t h e c a r r i e r c o n c e n t r a t i o n .
I n zero magnetic f i e l d ,
t h e r e s i s t i v i t y o f a sample i s p = Kpd V p / I p
= (enp)-l,
where Kp i s a c o n s t a n t o f p r o p o r t i o n a l i t y dependent upon geometry and p i s t h e d r i f t m o b i l i t y .
From t h e H a l l c o e f f i c i e n t and t h e
r e s i s t i v i t y , one can a l s o d e f i n e t h e H a l l m o b i l i t y as
I n l a s e r - a n n e a l i n g a p p l i c a t i o n s , t h e f r o n t l a y e r can o f t e n be t r e a t e d as a t h i n sheet o f t h i c k n e s s d i f (1) t h e t h i n l a s e r annealed s u r f a c e l a y e r i s on a s u b s t r a t e o f o p p o s i t e t y p e and ( 2 ) t h e t r a n s i t i o n between t h e laser-annealed p o r t i o n o f t h e sample and t h e s u b s t r a t e i s a b r u p t ;
i n t h i s way Eqs.
(16)-(19)
used t o determine p, pH, and c a r r i e r c o n c e n t r a t i o n . d i t i o n (1) i s easy t o s a t i s f y ,
(2) i s not.
can be
Though con-
Therefore,
i f the
a b s o l u t e c a r r i e r c o n c e n t r a t i o n i s t o be determined one o f two t h i n g s must be done: laser-annealed
(1) t h e j u n c t i o n between t h e h e a v i l y doped,
r e g i o n and t h e l i g h t l y doped s u b s t r a t e must be
assumed t o be abrupt, and t h e t h i c k n e s s d taken t o be t h e t h i c k n e s s o f t h e h e a v i l y doped r e g i o n o r ( 2 ) successive measurements must be made o f t h e sample a f t e r several t h i n u n i f o r m l a y e r s have been
3.
131
PROPERTIES OF PULSED LASER-ANNEALED SILICON
removed ( u s i n g a technique such as anodic o x i d a t i o n ) .
Many authors
choose t o i g n o r e t h i s problem e n t i r e l y and t o r e p o r t j u s t t h e sheet r e s i s t i v i t y , where Eq.
(18) i s r e p l a c e d by
which i s n o r m a l l y r e p o r t e d i n t h e u n i t s 62/13.
It may be noted
t h a t t h e H a l l m o b i l i t y i s independent o f t h i c k n e s s , and so can be determined even i f d i s n o t known. b.
J u n c t i o n Measurements The two measurements t h a t a r e commonly used t o c h a r a c t e r i z e a
p-n j u n c t i o n a r e Capacitance-Vol t a g e (C-V) and Current-Vol t a g e ( I - V ) measurements.
I f t h e j u n c t i o n i s abrupt, and one s i d e i s much more
h e a v i l y doped than t h e o t h e r ,
t h e capacitance per u n i t area i s
g i v e n by C = C~E~N,/~(V,~+V)]~/~
where
,
i s t h e low-frequency d i e l e c t r i c constant o f t h e s u b s t r a t e
E~
material,
NB i s t h e doping d e n s i t y o f t h e s u b s t r a t e ,
b u i l t - i n p o t e n t i a l of t h e j u n c t i o n , bias.
Vbi
i s the
and V i s t h e j u n c t i o n reverse
Equation (21) w i l l also be v a l i d i f t h e j u n c t i o n i s n o t
s t r i c t l y abrupt, i f t h e doping d e n s i t y a t t h e edge of t h e deplet i o n r e g i o n a t zero b i a s i s c o n s t a n t w i t h depth. The I - V measurement i s a l s o u s e f u l i n c h a r a c t e r i z i n g p-n j u n c tions. An i d e a l abrupt p-n j u n c t i o n w i l l pass c u r r e n t d e n s i t y J g i v e n by
J = Jo[exp(eV/AkT)
-
11,
i n which Jo i s t h e s a t u r a t i o n c u r r e n t d e n s i t y ,
k i s Boltzrnann's
c o n s t a n t , T i s t h e temperature, A i s t h e diode q u a l i t y f a c t o r (A = 1 f o r an i d e a l diode), and V i s t h e a p p l i e d b i a s ( p o s i t i v e o r
132
G . E. JELLISON. JR.
negative).
The c u r r e n t - v o l t a g e r e l a t i o n s h i p i n r e a l diodes, how-
ever, can d e v i a t e s u b s t a n t i a l l y from t h a t o f Eq. (22).
In partic-
u l a r , t h e s a t u r a t i o n c u r r e n t d e n s i t y Jo can be several o r d e r s o f magnitude g r e a t e r than t h a t of an i d e a l j u n c t i o n (and a l s o depend weakly on r e v e r s e b i a s ) , i n d i c a t i n g l a r g e leakage c u r r e n t s o r i g i n a t i n g from defect-induced recombination e f f e c t s i n t h e d e p l e t i o n region.
A t h i g h i n j e c t i o n l e v e l s ( f o r w a r d b i a s ) , Eq. (20) i s s t i l l
v a l i d , b u t w i t h A = 2 i n s t e a d o f 1.
The forward v o l t a g e a t which
t h e h i g h - i n j e c t i o n c o n d i t i o n becomes Val i d depends on t h e diode p e r f e c t i o n , b e i n g l a r g e f o r diodes w i t h r e l a t i v e l y few d e f e c t s i n t h e h e a v i l y doped region, and small f o r those w i t h a l a r g e number o f d e f e c t s i n t h e h e a v i l y doped region.
For most p r a c t i c a l diodes,
t h e t r a n s i t i o n s from A = 1 t o A = 2 i s n o t r e a d i l y apparent, b u t i t can be determined q u a n t i t a t i v e l y f r o m t h e I - V data.
Since A i n d i -
c a t e s whether a diode i s "good" o r "bad" t h e reason f o r r e f e r r i n g t o i t as t h e q u a l i t y o r diode p e r f e c t i o n f a c t o r i s apparent.
6.
SHEET PROPERTIES OF ION-IMPLANTED, LASER-ANNEALED LAYERS I n some o f
K h a i b u l l i n e t al.
the
earliest
experiments on
laser
annealing,
(1977) noted t h a t t h e process o f l a s e r a n n e a l i n g
r e s u l t s i n a higher u t i l i z a t i o n c o e f f i c i e n t (the f r a c t i o n o f imp l a n t e d atoms t h a t are e l e c t r i c a l l y a c t i v e ) f o r many dopant atoms i n s i l i c o n than do c o n v e n t i o n a l thermal-anneal i n g techniques.
In
t h e i r experiments, ruby l a s e r p u l s e s o f 20-30 ns d u r a t i o n and energy d e n s i t i e s from 1.0-1.3
J/cm2 were used.
From f o u r - p o i n t
probe
measurements and i n f r a r e d r e f l e c t i o n spectra, t h e y determined t h e c a r r i e r c o n c e n t r a t i o n and Hal 1 m o b i l i t y f o r several S i samples, i m p l a n t e d with P,
As, Sb, and B i o n s t h a t had been e i t h e r l a s e r
annealed o r t h e r m a l l y annealed.
K h a i b u l l i n e t a l . found t h a t i n
most cases t h e c a r r i e r c o n c e n t r a t i o n o f t h e laser-annealed samples was h i g h e r t h a n i n s i m i l a r samples which had been t h e r m a l l y annealed; t h e u t i l i z a t i o n c o e f f i c i e n t f o r laser-annealed samples was f r e q u e n t l y c l o s e t o 1, b u t was c o n s i d e r a b l y l e s s t h a n 1 f o r s i m i l a r
3.
133
PROPERTIES OF PULSED LASER-ANNEALED SILICON
t h e r m a l l y annealed samples.
Moreover, t h e y noted t h a t t h e h i g h e s t
c a r r i e r d e n s i t i e s i n t h e 1aser-anneal ed sampl es were above t h e e q u i l i b r i u m s o l i d s o l u b i l i t y l i m i t s of t h e i n v e s t i g a t e d i m p u r i t y atoms i n s i l i c o n .
When these very h e a v i l y doped samples were
subsequently heat t r e a t e d under c o n d i t i o n s t y p i c a l
of thermal
anneal ing , t h e c a r r i e r c o n c e n t r a t i o n s decreased t o those obtained by thermal annealing, i n d i c a t i n g t h a t t h e laser-annealed m a t e r i a l was i n a m e t a s t a b l e s t a t e .
I n l a t e r work of t h i s t y p e by Young e t al. Narayan (1978),
(1978), Young and
(1982), B-implanted S i l a s e r
and Young e t a l .
annealed with r u b y l a s e r pulses was s t u d i e d e x t e n s i v e l y .
-5
NI
z
e
0
-
-
I- ,016 z w 0 z 0 V a a
Li!
% 1015 V a w a
l
It A
I I
1111
I
I
Using
I I11111
B+ (35 keV) IN S i
LASER ANNEALING
v 1 100OC /30 min 0
-
90OoC/30 min
0
-
-
-
0
-
5,
v)
a W
E I
I
I I II111
loi4
I
1016
IMPLANTED DOSE
I 1 I1I l l
10~
(~t-n-~)
Fig. 12. Comparison o f measured c a r r i e r concentrationasa function o f implant dose i n thermally and laser-annealed samples o f B-implanted Si (Young e t a l . , 1982).
134
G . E. JELLISON, JR
Hal 1 e f f e c t measurements, Young e t a1
. (1982)
found t h a t t h e sur-
fa c e c a r r i e r co n c e n tra ti o n o f laser-annealed samples was proport i o n a l t o t h e implanted dose up t o -2.5 Fi g ure 12.
x 1016/cm2, as shown i n
I n o rd e r t o e x t r a c t from t h e data a surface c a r r i e r
c o n cent rat ion equal t o t h e implanted dose, they had t o choose r = 0.70
[see Eq.
(17)],
i n good agreement w i t h t h e value o f 0.73
r e p ort ed by W o l f s t i r n (1960) f o r h e a v i l y doped p-type m a t e r i a l . It can a l s o be seen from Fig.
12 t h a t c a r r i e r concentrations f o r
t h e r m a l l y annealed samples are f a r below those o f t h e laser-annealed samples.
I n t h e th e rma l l y annealed samples,
s a turat es a t -1.5
t h e concentration
x 1016/cm2; t h i s corresponds t o volume concentra-
t i o n s o f -4 x lO20/cm3, which i s t h e normal s o l i d s o l u b i l i t y l i m i t f o r B i n S i a t ll0O’C
(Trumbore, 1960).
S i m i l a r measurements on pulsed laser-annealed S i have been performed by Tamara e t a l . samples, by White e t a l . Wilson e t al.
(1980) f o r P-,
As-,
and B-implanted
(1979) f o r Sb-implanted samples, and by
(1979) f o r As-implanted m a t e r i a l .
A l l results for
these dopants show t h a t t h e r e a l c a r r i e r d e n s i t y i s equal t o t h e implanted dose f o r doses l e s s than 1016/cm2.
Above t h i s implant
dose, a s a t u r a t i o n of c a r r i e r c o n c e n tra t i o n was observed i n Asdoped m a t e r i a l a t -3.5
x 1016/cm2, and i n P-doped m a t e r i a l a t -6
.
x 1016/cm2 (Tamura e t a1 , 1980). Since t h e process o f i o n i m p l a n t a t i o n f o l l o w e d by pulsed l a s e r annealing y i e l d s doping d e n s i t i e s g re a t e r than t h e s o l u b i l i t y l i m i t , curves o f r e s i s t i v i t y ( I r v i n , 1962) and m o b i l i t y as a funct i o n o f c a r r i e r concentration can be extended t o very h i g h concentrations.
R e s i s t i v i t y measurements have been c a r r i e d out by Tsu
e t a l . (1978) f o r As-implanted samples, and by Tamura e t a l . (1980) f o r P-implanted m a t e r i a l , w h i l e both r e s i s t i v i t y and m o b i l i t y measurements have been made by F i n e t t i e t a l .
(1981) on P-implanted
specimens; t h e r e s u l t s o f F i n e t t i e t a l . are shown i n Fi g u r e 13. Note t h a t t h e e l e c t r o n m o b i l i t y decreases
monotonically w i t h
i n creasing dopant concentration and t h a t s i l i c o n r e s i s t i v i t i e s as
3.
80
PROPERTIES OF PULSED LASER-ANNEALED SILICON
b -
1
I
I
I
I Ill1
0 LASER
'y
0
I
0 LASER
I
ANNEALING
+ THERMAL
135
I 1 l t 1
ANNEALING
'
-
20 -
-
. ' \ q
0
0
Fig.
13.
I
I
I
I
I
I 1 1 1 1
Electrical characteristics
of
I
I
I 1
Ill-
silicon very heavily doped with
phosphorous using ion implantation followed by pulsed laser annealing ( F i n e t t i e t al.,
1 9 8 1 ) : a ) resistivity and b ) mobility vs c a r r i e r concentration.
The
dashed lines o f a ) are the results o f conventionally doped material from Masetti and Solmi ( 1 9 7 9 ) and Esaki and Miyahara ( 1 9 6 0 ) .
136
G . E. JELLISON, JR
low as 110 pQ-cm can be o b t a i n e d u s i n g these m a t e r i a l s .
Finetti
e t a l . (1981) and B e n t i n i (1980) observed t h a t t h e m o b i l i t y values f o r laser-annealed samples were 10-20% lower than t h e m o b i l i t y f o r d i f f u s e d samples, which was a t t r i b u t e d t o a h i g h c o n c e n t r a t i o n o f e l e c t r i c a l l y a c t i v e p o i n t defects. a l . (1978,
Young e t
1979, and 1982) noted no s i g n i f i c a n t d i f f e r e n c e between
mobi 1it y o f
the
On t h e o t h e r hand,
1 aser-anneal ed samples compared t o thermal l y
annealed samples o f t h e same c a r r i e r c o n c e n t r a t i o n . imp1 anted , thermal l y annealed
samples
m o b i l i t y t h a n t h e ion-implanted,
(The i o n -
general l y had a h i gher
laser-annealed samples, b u t t h i s
was a t t r i b u t e d t o t h e h i g h e r degree o f i o n i z a t i o n o f t h e i m p u r i t y c o n c e n t r a t i o n o b t a i n e d w i t h l a s e r annealing.) workers used r = 1 [see data.
Eqs.
(17) and (19)]
Note t h a t most i n analyzing t h e i r
There i s c o n s i d e r a b l e c o n t r o v e r s y concerning t h e p r e c i s e
v a l u e o f t h i s f a c t o r i n h e a v i l y doped m a t e r i a l s , b u t i t i s c l e a r t h a t i t need n o t remain c o n s t a n t over a wide range o f dopant concentrations
(see M o t t and Davis,
1979).
Clearly,
t h i s must be
t a k e n i n t o account i n any a c c u r a t e d e t e r m i n a t i o n o f c a r r i e r conc e n t r a t i o n from H a l l e f f e c t measurements. The recovery o f e l e c t r i c a l a c t i v i t y (sheet r e s i s t i v i t y and m o b i l i t y ) as a f u n c t i o n o f l a s e r energy d e n s i t y i n a 6'-implanted
( 5 keV, 6 x 1015/cm2) sample i s shown i n F i g u r e 14.
These r e s u l t s
p r o v i d e a rough i n d i c a t i o n o f t h e annealing t h r e s h o l d (-1.0 i n t h i s case),
J/cm*
b u t cannot be e x t r a p o l a t e d t o s i t u a t i o n s i n which
d i f f e r e n t l a s e r s o p e r a t i n g under d i f f e r e n t c o n d i t i o n s a r e used, s i n c e t h e s u r f a c e dopant c o n c e n t r a t i o n changes w i t h l a s e r energy d e n s i t y and p u l s e d u r a t i o n (see Chapters 2 and 4), and t h i s w i l l cause changes i n t h e sheet r e s i s t i v i t y and t h e m o b i l i t y . t e c h n i q u e was employed by Hamer e t a l .
This
(1980) t o examine t h e
b e h a v i o r of S i implanted w i t h t h e p-type dopants A l , Ga, and In a t doses from 3 x t h a t ps
>
electrical
loi3
t o 3 x 1016/cm2.
I n a l l cases, t h e y noted
100 Q / U , which was e x p l a i n e d as i n d i c a t i n g incomplete a c t i v a t i o n of
t h e dopant atoms;
t h e y a l s o noted an
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
137
II4
B+ (5 keV 6 X d ' ern-') IMPLANTED Si 0 0
SHEET RESISTIVITY MOBILITY
Fig. 14. Recovery o f sheet reslstivity and c a r r i e r mobility as a function o f laser energy density i n B-implanted Si (Young e t a l . , 1982).
-4
0
4
2 3 4 5 6 7 8 REVERSE BIAS VOLTAGE (V)
Fig. 15. Reverse biased C-V annealed junctions (Young e t a l . ,
9
40
measurements o f boron-implanted 1982).
laser-
138
G . E. JELLISON, JR.
i n c r e a s e o f ps with i n c r e a s i n g energy d e n s i t y , which was i n t e r p r e t e d as i n d i c a t i n g dopant s e g r e g a t i o n t o t h e s u r f a c e o f t h e samples.
However, c e l l f o r m a t i o n f o r Ga and I n i m p l a n t s i s known t o
occur a t h i g h doses,
which may a l t e r t h e i n t e r p r e t a t i o n o f t h e
r e s u l t s o f e l e c t r i c a l measurements.
7.
PROPERTIES OF
PULSED LASER-ANNEALED JUNCTIONS
F i g u r e 15 shows t h e r e v e r s e b i a s e d capaci tance-vol t a g e (C-V) measurements o f a p-n j u n c t i o n f a b r i c a t e d from 6-imp1 anted, l a s e r annealed S i (Young e t al.,
C l e a r l y , t h e C-V c h a r a c t e r i s t i c
1982).
f o l l o w s Eq. (21), which shows t h a t a good j u n c t i o n was formed and t h a t t h e doping p r o f i l e r e s u l t i n g from t h e i o n - i m p l a n t a t i o n , l a s e r a n n e a l i n g process d i d n o t extend beyond t h e zero b i a s d e p l e t i o n width.
The b u i l t - i n p o t e n t i a l , Vbi,
was determined t o be 0.85 V,
which agreed w i t h t h e t h e o r e t i c a l l y c a l c u l a t e d value f o r t h e dopant profile.
S i m i l a r C-V r e s u l t s were o b t a i n e d from measurements on a
d i o d e f a b r i c a t e d by l a s e r r e c r y s t a l l i z i n g an As-doped amorphous
f i l m deposited on
Si
(Young e t a1
y i e l d e d a s t r a i g h t l i n e and Vbi w i t h t h e t h e o r e t i c a l value,
= 0.82,
.,
1979).
The P 2 - V
plot
again i n good agreement
which i n d i c a t e s t h a t good j u n c t i o n s
can a l s o be formed u s i n g t h i s technique. A b e t t e r i n d i c a t i o n o f t h e q u a l i t y o f t h e d i o d e can be o b t a i n e d
from forward- and r e v e r s e - b i a s I - V measurements. t h e forward-
and reverse-biased
I-V
F i g u r e 16 shows
c h a r a c t e r i s t i c s o f a diode
f a b r i c a t e d from 6-implanted, laser-annealed S i (Young e t a1
., 1982;
t h e C-V measurements o f Fig. 15 were made on t h e same diode). f o r w a r d c h a r a c t e r i s t i c f o l l o w s Eq.
The
(22) c l o s e l y , w i t h A = 1.20.
However , measurements o f t h e r e v e r s e leakage c u r r e n t gave c u r r e n t d e n s i t i e s i n t h e range o f
t o lom8 A/cm2,
which i s somewhat
h i g h e r than t h e range r e p o r t e d f o r t h e best d i f f u s e d j u n c t i o n s . I n c r e a s i n g t h e l a s e r energy d e n s i t y from 1.2 t o 1.6 J/cm2 d i d not l o w e r t h e values o f t h e s a t u r a t i o n c u r r e n t , b u t improvements
were
made by u s i n g s u b s t r a t e temperatures o f 4OOOC d u r i n g t h e l a s e r
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
139
40-
B+ (5 keV 6 x d5 A n- 4 "
-8
IMPLANTED
cm-*)
L t
t
.
I=
/
I ASFI
_....I
I
I=I0
-a
AT 4.2 J/cm2 - --(,O"'AkT 4)
-
A=4.20
Y
REVERSE (A/cm2)
I-
,
i Ti=25OC\
Ti = 4OOOC
0.2
0
0.4 0.6 0.8 BIAS VOLTAGE ( V )
4.2
1.0
Fig. 16. Dark I-V characteristics of boron-implanted junction in a mesa diode (Young e t al., 1982).
annealing process,
as i s shown i n Fig.
laser-annealed
16 (Young e t al.,
1982).
The r e d u c t i o n o f t h i s s a t u r a t i o n c u r r e n t i s a t t r i b u t e d p r i m a r i l y t o a r e d u c t i o n o f t h e recombination-generation e m i t t e r region.
c u r r e n t from t h e
Very good p-n diodes ( u s i n g t h e I - V c h a r a c t e r i s t i c
as a d e t e r m i n a t i o n o f q u a l i t y ) have a l s o been made u s i n g t h e technique mentioned above o f vacuum d e p o s i t i o n o f As-doped a-Si on c-Si s u b s t r a t e s , f o l l o w e d by l a s e r annealing (Young e t a l . t h i s technique gave diodes w i t h f o r a reverse b i a s
<
1.2 V.
A
= 1.15
and Jo
<
3 x
1979); A/cm2
140
G. E. JELLISON, JR
Using a Q-switched Nd:glass l a s e r , Wang e t a1
. (1979)
fabri-
cated p-n diodes from ion-implanted (8 x 1015 As/cm2 a t 90 keV), 1aser-anneal ed s i 1 icon.
They found t h a t t h e leakage c u r r e n t
Jo = 3 x A/cm2), and a t t r i b u t e d t h i s t o t h e increased melt depth obtained w i t h t h e h i g h e r energy l a s e r pulses. For samples imp1anted w i t h 170 keV As, t h e y found t h a t t h e leakage c u r r e n t was a f a c t o r o f 100 l a r g e r than f o r samples implanted a t 90 keV. This can be qua1 i t a t i v e l y understood by r e a l i z i n g t h a t t h e d e f e c t c o n c e n t r a t i o n a t depths g r e a t e r than -1 pm w i l l be considerably l a r g e r f o r t h e sample implanted a t 170 keV than f o r one implanted a t 90 keV. Since l a s e r annealing i s expected t o anneal o n l y those defects l e s s than -0.5 pm from t h e surface, t h e r e w i l l be many more defects, and hence a l a r g e r leakage c u r r e n t , remaining f o r a sample o r i g i n a l l y implanted a t 170 keV than f o r one implanted a t 90 keV. decreases w i t h i n c r e a s i n g energy d e n s i t y (lowest value:
IV.
a.
Defects
BACKGROUND
As mentioned i n t h e l a s t section, t h e reverse leakage c u r r e n t o f diodes f a b r i c a t e d using pulsed l a s e r annealing i s f r e q u e n t l y g r e a t e r than t h a t o f diodes made by conventional thermal annealing techniques. This d i f f e r e n c e can u s u a l l y be a t t r i b u t e d t o t h e existence
of recombination centers i n t h e d e p l e t i o n r e g i o n o f t h e diodes. However, TEM studies show t h a t t h e r e are no observable defects l a r g e r than -10 A i n t h e near-surface region.
Therefore, these
d e f e c t s must be of t h e p o i n t o r c l u s t e r type.
The existence o f
such defects i n s i l i c o n a f t e r high-energy (-1 MeV) e l e c t r o n , proton, or neutron i r r a d i a t i o n i s w e l l documented (see, f o r example, Corbett e t a1
., 1981),
and presumably s i m i l a r defects may e x i s t
a f t e r pulsed l a s e r annealing.
However, t h e exact nature o f t h e
d e f e c t s i n laser-annealed samples has not y e t been determined and many questions remain t o be answered.
I n t h i s section, we w i l l
3.
141
PROPERTIES OF PULSED LASER-ANNEALED SILICON
discuss t he r e s u l t s o f luminescence and deep l e v e l t r a n s i e n t spectroscopy detail, results
(DLTS) studies o f d e fe c ts i n laser-melted s i l i c o n i n and giv e somewhat more abbreviated c o n s i d e r a t i o n t o t h e of
electron
paramagnetic
resonance
and
Hall
effect
measurements. There i s a fundamental d i f f e r e n c e between t h e annealing chara c t e r i s t i c s o f a m a t e r i a l t h a t has been melted and one t h a t has been heated t o a h i g h temperature below th e m e l t i n g point.
I f the
m a t e r i a l i s heated b u t not melted ( s o l i d phase e p i t a x y ) , t h e t i m e r e q u i r e d f o r t h e annealing o f d e fe c ts can be expressed, t o a f i r s t approximation, as t = t o exp (-AE/kT)
(23)
where AE i s t he a c t i v a t i o n energy o f t h e annealing process, k i s Bol tzmann's constant, and T i s t h e absol Ute temperature; t h a t i s , t h e higher t h e temperature, t h e s h o rte r t h e amount o f t i m e r e q u i r e d f o r annealing.
I f t h e m a t e r i a l i s melted ( l i q u i d phase e p i t a x y ) ,
a l l t h e defects are, i n a sense, a n n i h i l a t e d i n t h e l i q u i d phase, and any defects t h a t remain a f t e r r e s o l i d i f i c a t i o n must come from t h e regrowth process.
A prime example o f t h i s , a p p l i e d t o pulsed
l a s e r annealing, i s seen by examining t h e e f f e c t s o f annealing on electron-irradiated silicon.
K i me rl i n g and Benton (1980), using
DLTS, examined t h e annealing e f f e c t s o f 1.06
pm (Nd:YAG
laser)
i r r a d i a t i o n o f P-doped, float-zoned s i l i c o n t h a t had been i r r a d i a t e d w i t h 1 MeV elect r o n s t o a dose o f 5 x 1014/cm2. The defects i n t h i s ty pe o f materi a1 have been we1 1 c h a ra c te r i z e d (Kimerl ing , 1977), w i t h t h e t w o p r i n c i p a l defects being t h e oxygen-vacancy p a i r (0-V, o r A-center)
and t h e phosphorus-vacancy p a i r (P-V,
o r E-center)
K i merling and Benton found t h a t a 40 ns l a s e r pulse d i d
.
not a f f e c t
t h e DLTS response o f e i t h e r o f these defects, even f o r energy d e n s i t i e s up t o t h e m e l t i n g th re s h o l d (-7 J/cm2). However, when a cw Nd:YAG l a s e r (-35 W/cm2) was used, a l l t h e P-V defects were e l i m i n a t e d a f t e r a l-sec i l l u m i n a t i o n , w h i l e most o f t h e 0-V defects were eliminat ed a f t e r a 30-sec
illumination.
Therefore,
for
142
G . E. JELLISON, JR
anneal ing temperatures below t h e m e l t i n g p o i n t , must be a lowed t o ann h i l a t e t h e defects. was drawn by Kachurin
s u f f i c i e n t time
A s i m i l a r conclusion
nd Nidaev (1980), who found t h a t complete
a n n e a l i n g of d e f e c t s c r e a t e d by e l e c t r o n bombardment was i m p o s s i b l e w i t h nanosecond l a s e r pulses; o n l y when t h e d u r a t i o n o f a heating, but
non-melting,
p u l s e exceeded
1 ps
could
any s o l i d
phase
anneal i n g occur. I n a d d i t i o n t o the annealing o f defects already present i n the m a t e r i a l ( f r o m i o n i m p l a n t a t i o n o r e l e c t r o n bombardment), i n c e r t a i n cases, t h e process o f p u l s e d - l a s e r i r r a d i a t i o n w i l l i n t r o d u c e defects
o r m o d i f y t h e s t r u c t u r e o f a l r e a d y e x i s t i n g defects.
D e f e c t s c r e a t e d by i o n o r e l e c t r o n bombardment depend i n a complic a t e d way on t h e i m p u r i t i e s p r e s e n t i n t h e s t a r t i n g m a t e r i a l , t h e i o n - i m p l a n t a t i o n dose o r e l e c t r o n f l u e n c e , and on t h e i r r a d i a t i o n energies.
C l e a r l y , much more work needs t o be done f o r a complete
understanding o f d e f e c t s i n l a s e r - i r r a d i a t e d semiconductors. 9.
PHOTOLUMINESCENCE Photo1 uminescence
(PL) measurements have been used f o r y e a r s
t o s t u d y d e f e c t s i n semiconductors.
I n modern photo1 uminescence
experiments, t h e e x c i t a t i o n l i g h t source i s u s u a l l y a cw l a s e r ( f o r example, an Ar-ion),
and t h e e m i t t e d l i g h t i s wavelength d i s c r i m -
i n a t e d u s i n g a monochromator.
Photoluminescence p r o v i d e s informa-
t i o n about e l e c t r o n - h o l e recombination mechanisms i n t h e m a t e r i a l b e i n g studied.
E l e c t r o n s and holes can combine e i t h e r r a d i a t i v e l y
o r non-radiatively.
I n non-radiative transitions, the e x c i t a t i o n
energy i s d i s s i p a t e d e n t i r e l y by t h e emission o f a cascade o f phonons, w h i l e i n a r a d i a t i v e t r a n s i t i o n , energy i s d i s s i p a t e d by t h e emission o f a photon and one o r more phonons; i t i s t h e l a t t e r process t h a t i s o f i n t e r e s t i n s t u d y i n g photoluminescence. Since most o f t h e recombination t r a n s i t i o n s i n amorphous s i l i c o n a r e n o n r a d i a t i v e , l i t t l e i f any photoluminescence can be seen from t h i s material.
A t y p i c a l photoluminescence spectrum, taken a t 4.2
K
3.
143
PROPERTIES OF PULSED LASER-ANNEALED SILICON
on float-zoned s i l i c o n t h a t had been implanted w i t h 2*Sit 80 keV and l a s e r annealed a t 2.5 l a s e r (Skolnick e t al.,
J/cm2 using a Q-switched ruby
1981a), i s shown i n Figure 17.
t h e observed l i n e s are t y p i c a l o f high-grade,
Several o f
undoped S i t h a t has
PNp, PTA, and PTo
n o t been l a s e r i r r a d i a t e d :
ions a t
are r e s p e c t i v e l y
l i n e s due t o t h e no-phonon t r a n s i t i o n o f t h e phosphorous-bound exciton,
t h e transverse-acoustical,
and t h e t r a n s v e r s e - o p t i c a l
phonon r e p l i c a s o f t h e same t r a n s i t i o n ( t h e energy of t h e phonon r e p l i c a i s j u s t t h e no-phonon energy minus t h e e m i t t e d phonon energy).
The FETO and EHLTO l i n e s are from t h e f r e e e x c i t o n and
29Si, 80 keV
W
Ix 1044 cm-2 2.54 J TEMP. 4 K
[+1.5) pTO
G
FETO
PTA
I
I
0.95
1.00
II
1.05 ENERGY (eV)
I
1.10
I
1.15
Fig. 17. Photoluminescence spectrum a t 4.2 K for S i sample implanted a t 80 keV with "Si a t 1014 cm-2 and laser annealed a t 2.5 J/cm2. PNP and PTA, PTO are no-phonon phosphorus-bound excitons and TA- and TO- phonon replicas, respectively. The damage related peaks Wand G are present with comparable intensity t o PTO. W1 i s a phonon replica of W ( a f t e r Skolnick et a l . , 1 9 8 1 a ) .
144
G . E. JELLISON, JR.
from electron-hole l i q u i d transverse-optical phonon r e p l i c a s , respectively. The peaks labeled G and W are defect r e l a t e d (W1 i s t h e phonon replica of W) and will be discussed l a t e r . Nakashima e t a1 (1979) examined the photoluminescence a t room temperature a f t e r phosphorous ions were implanted ( a t 50 keV) i n t o B-doped (20-30 62-cm) S i and a f t e r l a s e r annealing with 25 ns ruby l a s e r pulses. In one experiment, they measured the integrated 1umi nescence a t room temperature , without energy di scrimi nation , as a function of position across t h e wafer f o r a sample implanted t o 8 x 1015/cm2. From areas on the wafer which had not been l a s e r annealed, no PL was observed because of the amorphous nature of the near-surface region. For an energy density l e s s than the melting threshold (0.45 J/cm2) no annealing, and therefore no PL, was observed. For an energy density well above the melting threshold (1.4 J/cm2), when the e n t i r e implanted region was annealed, t h e PL i n t e n s i t y was high. On similar samples t h a t were thermally annealed instead of l a s e r annealed, they did not observe PL, indic a t i n g t h a t t h e r e was s t i l l a high concentration of non-radiative c e n t e r s remaining i n t h e near-surface region. A t energy d e n s i t i e s near the melt threshold (0.8 J/cm2), they observed some P L Y b u t i t was not nearly as intense as t h a t observed a f t e r annealing a t 1.4 J/cm2. Although they obtained complete e l e c t r i c a l activation with t h i s sample, the melt front may not have penetrated e n t i r e l y through the damaged region of the sample. If t h i s were the case, then PL would have been observed only from t h e near-surface annealed region, since r a d i a t i v e t r a n s i t i o n s from t h e damaged region beyond t h e deepest melt-front penetration would be very unlikely. A much lower PL integrated i n t e n s i t y was observed by Nakashima et a l . (1979) with a low-dose sample (-lo1* P/cm2) than with the high-dose sample (8 x 1015 P/cm2), even a t a l a s e r energy density a s high as 1.5 J/cm2. There a r e a t l e a s t two possible explanat i o n s f o r t h i s : (1) The optical properties of the near-surface region may be quite d i f f e r e n t f o r t h e 10l2 P/cm2 sample (low a )
.
3,
145
PROPERTIES OF PULSED LASER-ANNEALED SILICON
compared t o those o f t h e 8 x 1015 P/cm2 ( h i g h a; see S e c t i o n 11.3, t h i s chapter).
T h i s c o u l d r e s u l t i n q u i t e d i f f e r e n t regrowth
k i n e t i c s (see Chapter 5) f o r t h e two samples,
and p o s s i b l y more
d e f e c t s t r a p p e d f o r t h e low-dose sample t h a n t h e high-dose sample. ( 2 ) Another p o s s i b l e e x p l a n a t i o n i n t h i s case i s t h a t t h e lower
i m p l a n t dose r e s u l t s i n a s m a l l e r c o n c e n t r a t i o n o f phosphorous atoms, and t h e r e f o r e t h e r e a r e fewer r a d i a t i v e paths (such as PNp, PTo,
and PTA i n Fig. 17) t h r o u g h which recombination can occur. A d d i t i o n a l photoluminescence s t u d i e s by S k o l n i c k e t a1
e x t e n d i n g those a1 ready d e s c r i b e d ( S k o l n i c k e t a1
.,
.
(1981b)
1981a), were
c a r r i e d o u t on samples implanted w i t h Si t o several doses (1013,
1014, l O l 5 , and 3 x
loi5
Si/cm2); a r e p r e s e n t a t i v e spectrum, taken
a t 4.2 K, i s shown i n Fig. 17.
The peaks due t o d e f e c t s a r e l a b e l e d
W and G [two a d d i t i o n a l f e a t u r e s , I 3 and X a r e discussed by S k o l n i c k The peak W i s much s t r o n g e r t h a n any o t h e r
e t al.,
(1981b)l.
feature,
and has been t e n t a t i v e l y assigned t o recombination a t a
five-vacancy
complex w h i l e t h e I 3 peak has been a t t r i b u t e d t o a
m u l t i v a c a n c y complex [ K i r k p a t r i c k e t a1
., (1976)l.
Recent O p t i c a l l y
Detected Magnetic Resonance (ODMR) work of O'Donnell e t a l . (1983) has i d e n t i f i e d t h e G - l i n e w i t h t h e carbon " s p l i t i n t e r s t i t i a l " ( S i
Gll c e n t e r ) , i n which a near-neighbor s u b s t i t u t i o n a l carbon has p a i r e d w i t h a s i l i c o n atom d i s p l a c e d t o a nearby i n t e r s t i t i a l position.
The X-band observed by S k o l n i c k e t a l . was very broad
and no i d e n t i f i c a t i o n was attempted. Skolnick e t al.
(1981a,
1981b) s t u d i e d t h e h e i g h t o f t h e PL
i n t e n s i t y o f t h e W, G, and PTo l i n e s as a f u n c t i o n o f l a s e r energy density.
They found t h a t t h e i n t e n s i t y o f t h e W peak was very
small b e f o r e l a s e r annealing and increased w i t h energy d e n s i t i e s up t o -1.2-1.5
J/cm2 ( i t was s l i g h t l y dependent upon i o n implan-
t a t i o n dose);
for
energy d e n s i t i e s g r e a t e r t h a n 1.5
J/cm2,
the
i n t e n s i t y o f t h e W peak decreased w i t h i n c r e a s i n g energy d e n s i t y . The h e i g h t o f G and PTo peaks, however, were observed t o g e n e r a l l y i n c r e a s e w i t h i n c r e a s i n g energy d e n s i t y .
S t r i p p i n g experiments
146
G. E. JELLISON, JR.
were a l s o performed,
which i n d i c a t e d t h a t t h e W,
as w e l l as I 3
and X c e n t e r s , were m o s t l y l o c a t e d between 4000 A and 5000 A from t h e sample surface, w h i l e t h e G c e n t e r s were found m o s t l y i n t h e 6000 A t o 7000 A r e g i o n , though s i g n i f i c a n t numbers o f G c e n t e r s were found c l o s e r t o t h e surface. The observed behavior o f peak h e i g h t s w i t h energy d e n s i t y now becomes c l e a r :
Before l a s e r annealing, t h e f r o n t - s u r f a c e r e g i o n i s
amorphous and t h e r e f o r e t h e probe-laser r a d i a t i o n (488 nm) used t o e x c i t e t h e luminescence w i l l n o t be a b l e t o p e n e t r a t e t o t h e r e g i o n where t h e r a d i a t i v e d e f e c t s a r e located.
For low energy d e n s i t y
l a s e r pulses, t h e m e l t f r o n t w i l l n o t p e n e t r a t e through a l l t h e damaged r e g i o n ,
so good e p i t a x i a l growth cannot occur.
Conse-
q u e n t i a l l y , some, b u t n o t a l l , o f t h e probe-laser l i g h t w i l l penet r a t e t h e damaged region, r e s u l t i n g i n some observed luminescence. A t pulsed l a s e r energy d e n s i t i e s which g i v e a m e l t - f r o n t penetra-
t i o n o f -4000 A , none o f t h e W c e n t e r s w i l l be annealed, but t h e maximum PL s i g n a l w i l l be seen from these centers.
As t h e energy
d e n s i t y i s increased, c r e a t i n g deeper m e l t - f r o n t p e n e t r a t i o n s , some W c e n t e r s w i l l become annealed, and t h e PL s i g n a l w i l l decrease.
Since t h e PTo peak r e s u l t s from t h e c r y s t a l l i n e m a t e r i a l
,
one
would expect t o f i n d a l e v e l i n g o f f o f t h e PL i n t e n s i t y above -1.5 J/cm2,
as i s observed.
higher densities either,
No
decrease i n t h e G peak i s observed a t s i n c e most o f these c e n t e r s l i e w e l l
beyond t h e deepest m e l t - f r o n t p e n e t r a t i o n . e t a1
.,
It i s b e l i e v e d ( S k o l n i c k
1981a, 1981b) t h a t t h e W c e n t e r s a r e produced by t h e s o l i d
phase annealing o f t h e i o n - i m p l a n t a t i o n damage i n t h e t a i l o f t h e i m p l a n t e d S i caused by channeling beyond t h e amorphous region, w h i l e t h e G c e n t e r was produced d u r i n g t h e i m p l a n t a t i o n step, again i n t h e channeled t a i l . It i s p o s s i b l e t o e l i m i n a t e t h e W,
G,
13, and X luminescence
l i n e s by thermal a n n e a l i n g a t 800 K f o r 30 min ( S k o l n i c k e t al., 1981b).
However,
i t i s found t h a t these luminescence f e a t u r e s
a r e rep1 aced by o t h e r s ,
w h i l e t h e i n t e g r a t e d i n t e n s i t y remains
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
147
approximately c o n s t a n t ; even a f t e r thermal t r e a t m e n t a s u b s t a n t i a l
A f u r t h e r exam-
number o f r a d i a t i v e recombination c e n t e r s remain.
i n a t i o n o f t h e PNp l i n e s g i v e s a d d i t i o n a l evidence about t h e q u a l i t y o f t h e near-surface r e g i o n a f t e r pulsed l a s e r anneal ing. e t al.
Nakashima
(1979), by s t u d y i n g t h e energy-resolved PL spectrum a t 4.2
K from a very low-dose Pt implanted, laser-annealed sample, found
t h a t a sharp no-phonon l i n e a s s o c i a t e d w i t h a phosphorous-bound e x c i t o n was c l e a r l y observable beside t h e s i m i l a r no-phonon l i n e o f t h e e x c i t o n bound t o a boron atom.
This indicates t h a t the P
atoms occupy s u b s t i t u t i o n a l l a t t i c e s i t e s a f t e r pulsed l a s e r ann e a l i n g and t h a t t h e r e i s very l i t t l e s t r a i n a s s o c i a t e d w i t h t h e i n c o r p o r a t i o n o f t h e P atom i n t o t h e l a t t i c e . Up u n t i l now, a d e t a i l e d assignment o f luminescence f e a t u r e s t o s p e c i f i c d e f e c t s t r u c t u r e s has n o t been p o s s i b l e .
However, as
has been r e c e n t l y reviewed by Sauer and Weber (1983),
detailed
a n a l y s i s o f luminescence s p e c t r a has now become p o s s i b l e u s i n g Zeeman e f f e c t s , Hopefully,
u n i a x i a l s t r e s s s p l i t t i n g s , and i s o t o p e e f f e c t s .
some o f these new techniques can be brought t o bear on
d e f e c t s c r e a t e d by l a s e r annealing.
10.
DEEP LEVEL TRANSIENT SPECTROSCOPY (DLTS) DLTS i s a t r a n s i e n t capacitance technique t h a t has proved t o
be a powerful t o o l f o r t h e d e t e c t i o n and c l a s s i f i c a t i o n o f deepl y i n g recombination c e n t e r s i n semiconductors [see Lang (1974) o r M i l l e r e t a l . (1977) f o r an i n t r o d u c t i o n t o t h e subject]. Majority c a r r i e r d e f e c t s can be s t u d i e d w i t h t h i s technique u s i n g e i t h e r p-n j u n c t i o n s o r S c h o t t k y b a r r i e r j u n c t i o n s ,
but p-n j u n c t i o n s
must be employed t o study m i n o r i t y c a r r i e r defects. d e f e c t parameters can be determined by DLTS: ( f r o m t h e valence o r conduction band),
The f o l l o w i n g
a c t i v a t i o n energies
m a j o r i t y c a r r i e r capture
c r o s s s e c t i o n s , m i n o r i t y c a p t u r e c r o s s s e c t i o n s ( i n p-n j u n c t i o n s ) , and t r a p c o n c e n t r a t i o n p r o f i l e s .
DLTS can be used t o determine
o n l y t h e e l e c t r i c a l c h a r a c t e r i s t i c s o f a defect;
o t h e r techniques
148
G. E. JELLISON, JR
(such as EPR o r i n f r a r e d a b s o r p t i o n ) must be employed i n o r d e r t o make s t r u c t u r a l and symmetry statements concerning t h e d e f e c t . However, because DLTS i s r e l a t i v e l y f a s t , and i s s e n s i t i v e t o comp a r a t i v e l y small numbers o f d e f e c t s , i t has proved t o be e x t r e m e l y useful. One i m p o r t a n t c h a r a c t e r i s t i c o f DLTS i s t h a t t h e s i g n a l from t h e spectrometer i s p r o p o r t i o n a l t o NT/ND,
where NT i s t h e d e f e c t
d e n s i t y and ND i s t h e dopant c o n c e n t r a t i o n ( f o r most spectrometers t h e l i m i t o f d e t e c t i v i t y i s NT/ND
lo-'+).
This unfortunately
l i m i t s t h e usefulness o f DLTS as a t o o l f o r t h e examination o f d e f e c t s i n t h e e m i t t e r o f a p-n j u n c t i o n , where t h e dopant concent r a t i o n i s o f t e n g r e a t e r t h a n 1020/cm3. a.
D e f e c t s Found i n Pulsed Laser-Annealed S i l i c o n f r o m DLTS Measurements Table I11 l i s t s t h e DLTS parameters f o r d e f e c t s d e t e c t e d by
s e v e r a l authors i n pulsed laser-annealed s i l i c o n samples, b o t h i o n i m p l a n t e d and unimplanted.
It should be understood t h a t t h e r e s u l t s
f r o m DLTS experiments on laser-annealed s i l i c o n are s t i l l few i n number, so an e f f o r t was made here t o i n c l u d e a l l t h e a v a i l a b l e experimental data i n t h e t a b l e .
I n c o m p i l i n g Table 111, an attempt
was made t o group t h e r e s u l t s f r o m v a r i o u s r e s e a r c h groups a c c o r d i n g t o t h e l i k e l y o r i g i n o f t h e DLTS response. T h i s i s n o t an e n t i r e l y s a t i s f a c t o r y procedure because i t i s o f t e n d i f f i c u l t t o e s t i m a t e t h e e r r o r s i n v o l v e d i n d e t e r m i n i n g t h e v a r i o u s parameters used f o r defect identification.
Moreover, i n most cases, a l l t h e parameters
(such as t h e c a p t u r e cross s e c t i o n and t h e anneal-out temperature) a r e n o t r e p o r t e d by a l l workers. Normally, d e f e c t i d e n t i f i c a t i o n has been made f r o m e l e c t r o n i r r a d i a t e d samples (see K i m e r l i n g , 1977), b u t these i d e n t i f i c a t i o n s have f r e q u e n t l y become t h e s u b j e c t o f c o n s i d e r a b l e controversy.
I n l i g h t o f t h e foregoing,
any
i d e n t i f i c a t i o n o f d e f e c t s as w e l l as groupings made here must be regarded as t e n t a t i v e .
Nevertheless , t h e d i s c u s s i o n be1 ow w i 11
be organized accordi ng t o these groupi ngs.
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
149
Table 111 The observed parameters f o r t r a p s detected i n pul sed-1 aser-anneal ed s i 1i c o n using DLTS techniques. Table I I I a : AE (ev)
Cross
Section (cm2)
E l e c t r o n traps.
Anneal Out Temp (C)
Implant Species
Refs.
Notes
El
0.18 0.18 0.18 0.19 0.19
LPE LPE LPE,M UA SPE 1
E2 A
0.19
LPE SPE 294
E3
0.23 0.23 0.23 0.23 0.24
UA LPE LPE LPE,3 LPE * 4
E4
0.23
LPE ,M
E5
0.25 0.26 0.27
LPE ,4 LPE,3 LPE,4
E6
0.30 0.28 0.30 0.30
SPE 1 SPE, 1 LPE ,M
0.32
SPE * 1
E7 E8
B
E9
0.31 0.31 0.32 0.33-0.36
LPE .M
0.34
3x10-l6
LPE LPE LPE,4 LPE/SPE 2 94 UA
0.37 0.37 0.40
3x10-l6 8x10-l5 3x10-16
LPE,M LPE,M UA
5x10-l6
150
G . E. JELLISON, JR.
Table I I I a (contd.) AE (eV) El0
0.42 0.42 0.44 0.42 0.44 0.45
Cross Section (cm2) 4~10-l~ 9x10-15
---
<500
---
As
--
--
m
UA
1pe,4
LPE
1pe,4 LPE ,M
As P
e,k
LPE,M
--
650
--
d
1pe,4
<500 500 600 600
As
--
k
1
--
d f
UA LPE ,M
--
P
f k
LPE ,4 LPE
0.45 0.47 0.48 0.49
2x10-16 10-15
0.52 0.53
6~10-l~ 4~10-l~
--
7x10-1
500
Cross
AE (ev)
Section (cm2)
H1
0.15
--
H2
0.22 0.23
------
--
8x10-1 4
PF5
PF5
j
1pe,4 1pe,4
Hole traps.
Table IIIb.
0.31 0.33
1
f j
Notes
--
El2
H3
P Si
Refs.
500
--
0.43
0.23 0.24 0.25
Implant Species
8x10-l5
Ell
El3
Anneal O u t Temp (C)
Anneal O u t Temp (C)
------
---
300
Implant Species
Refs.
Notes LPE
Si
j
---
a
Si
--
j a b
1pe,4
Si
j i
LPE LPE
As
PF5
h
LPE ,4 LPE, 2,4 LPE LPE
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
151
Table I I I b (contd.)
H4
AE (eV)
Cross Section (cm2 )
Anneal Out Temp (C)
Implant Speci es
0.36
--
--
-,AS
b
0.37 0.38
--
--
---
a h
0.38 0.39 0.41
--
Si Si
i j
---
3x10-l5
----
<200
Refs : a. Mooney e t a1 (1978) b. Benton e t a l . (1979) c. Benton e t a l . (1980b) d. Kimerling and Benton (1980) e. Mesli e t a l . (1981) f. Fan e t a1 (1982) g. Benton e t a1 (1980a)
.
. .
Refs.
Notes
PF5
k
LPE SPE LPE,4 LPE 294 LPE,6 LPE LPE
Si
3
LPE
h.
Lawson and Pearton (1982)
1. m. n.
Blosse and Bourgoin (1983) Kachurin e t a1 (1982b) Kachurin e t a1 (1982a)
i. Young e t al. (1983) j. Wang e t a l . (1983) k. Mesl i e t a1 (1982)
.
..
Notes : UA Unanneal ed sampl e LPE L i q u i d phase e p i t a x i a l regrowth SPE S o l i d phase e p i t a x i a l regrowth M Minority carrier trap 1 Defect produced by t h e l a s e r (SPE) 2 Defect passivated by atomic hydrogen 3 Defect d i f f u s e d i n from t h e surface 4 Defect quenched i n (LPE) 5 Found up t o 10 p deep 6 Sample annealed w i t h excimer l a s e r El:
As mentioned above, t h i s d e f e c t designated as t h e (0-V,
oxygen-vacancy)
o r t h e A-center (Kimerling,
electron-irradiated silicon.
1977), i s common i n
I n laser-annealed samples, i t i s due
t o a r e s i d u a l defect from t h e i o n i m p l a n t a t i o n o r a quenched-in d e f e c t r e s u l t i n g from r e c r y s t a l l i z a t i o n from t h e l i q u i d phase. Mesli e t a l .
(1981) measured t h i s center as a m i n o r i t y - c a r r i e r
t r a p and, t h e r e f o r e , t h e capture cross s e c t i o n i s not expected t o
152
G . E. JELLISON, JR
be t h e same as t h a t o b t a i n e d from measurements made when t h e c e n t e r i s a m a j o r i t y - c a r r i e r trap. E2: (1980a,
T h i s d e f e c t [designated
as d e f e c t A by Benton e t a l .
1980b) and K i m e r l i n g and Benton ( 1 9 8 0 ) l i s observed i n
b o t h n- and p-type s i l i c o n , and i n Cz- and Fz-grown m a t e r i a l , though t h e a c t i v a t i o n energy i s somewhat d i f f e r e n t f o r Fz samples (0.15 eV) t h a n i t i s f o r Cz samples (0.19 eV).
It i s found w i t h i n t h e s u r f a c e
l a y e r a p p r o x i m a t e l y equal t o t h e depth o f m e l t i n g , and i s thought t o be due t o i n - d i f f u s i o n o f an i m p u r i t y from t h e surface.
A
p o s s i b l e reason t h a t d e f e c t A has n o t been observed by o t h e r workers i s because t h e c a p t u r e cross s e c t i o n i s so small t h a t a l a r g e p u l s e must be used t o observe t h e d e f e c t i n DLTS experiments.
A similar
d e f e c t has a l s o been observed i n cw laser-annealed
s i l i c o n by
Johnson e t a1 E3:
. (1979).
A d e f e c t w i t h energy 0.23-0.24
eV below t h e c o n d u c t i o n
band has been observed by s e v e r a l authors.
The p h y s i c a l charac-
t e r i s t i c s correspond c l o s e l y t o those o f t h e divacancy (V-V) o r t h e C-center,
commonly observed i n e l e c t r o n - i r r a d i a t e d s i l i c o n .
It i s found i n ion-imp1 anted annealed and unanneal ed samples, and
i n unimplanted, laser-annealed samples. E4:
A d e f e c t w i t h a c t i v a t i o n energy 0.23 eV from t h e conduc-
t i o n band has been observed by Wang e t a l . c a r r i e r trap.
(1983) as a m i n o r i t y
A t e n t a t i v e assignment o f t h i s d e f e c t t o e i t h e r t h e
i n t e r s t i t i a l oxygen-boron complex o r t h e boron-boron p a i r has been made by Wang e t a1
E5, E6, E7:
. (1983).
These d e f e c t s have been observed 0.25-0.32
below t h e conduction band and are u n i d e n t i f i e d ; g r o u p i n g presented here i s a r b i t r a r y . two m i n o r i t y c a r r i e r t r a p s r e p o r t e d by M e s l i e t a l . s o l i d phase e p i t a x i a l Table 111), a t 0.28,
0.30
(1981,
therefore,
eV the
I n c l u d e d i n t h i s group a r e
eV below t h e c o n d u c t i o n band
1982), and 3 t r a p s r e s u l t i n g from
growth ( i n d i c a t e d by t h e n o t a t i o n SPE i n
0.30,
and 0.32 eV below t h e c o n d u c t i o n band.
From t h e work o f Blosse and Bourgoin (1983), t h e SPE t r a p s a t 0.28 and 0.32 eV were c r e a t e d i n As-implanted s i l i c o n (1-4 x 10L2/cm2)
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
153
when t h e i n c i d e n t ruby l a s e r i r r a d i a t i o n (pulse width = 15 ns) exceeded 0.5 J/cm2, b u t was not s u f f i c i e n t l y e n e r g e t i c t o produce
LPE.
A l l t h e E5 t r a p s are created by l a s e r i r r a d i a t i o n , although
regrowth o f t h e near-surface re g i o n occurs i n t h e liquid-phase regime.
E8:
A def e c t between 0.31 and 0.36
eV has been observed i n
1aser-anneal ed n- and p-type Fz and Cz-grown s i 1i c o n by many workers and has been lab e l e d l e v e l B by Ki m e rl i n g and Benton.
The concen-
t r a t i o n i s found t o be a f u n c t i o n o f l a s e r power, s a t u r a t i n g f o r l a s e r powers (or e q u i v a l e n t l y m e l t depths) g r e a t e r than t h a t wq u i r e d t o m e l t through t h e re g i o n behind t h e j u n c t i o n probed by t h e DLTS experiments (Ki m e rl i n g and Benton, 1980). It i s found i n both implanted and unimplanted ma te ri a l a f t e r l a s e r annealing (Fan e t a l .
1982); a r e l a t e d d e fe c t i s found i n implanted, unannealed m a t e r i a l a t 0.34 eV (Blosse and Bourgoin, 1983). I n t h e laser-annealed mater i a l s , it has been suggested by K i me rl i n g and Benton t h a t E8 i s a m e l t - r e l a t e d defe c t produced by t h e f a s t quench r a t e o f pulsed l a s e r anneal ing , and may be c o r r e l a t e d w i t h a mu1t ivacancy center commonly observed i n e l e c t r o n - i r r a d i a t e d s i l i c o n (Kimerl ing, 1977).
E10:
A t r a p w i t h energy 0.42 t o 0.45 eV below t h e conduction
band has been reported fre q u e n tl y ; it i s c o r r e l a t e d w i t h t h e Asvacancy o r t h e P-vacancy (€-center) i n e l e c t r o n - i r r a d i a t e d s i 1i c o n (Kimerling, 1977). divacancy (V-V)
It i s a l s o known t h a t one charge s t a t e o f t h e
d e fe c t l i e s near t h i s energy (0.42 eV), and so it
i s o f t e n d i f f i c u l t t o d i s t i n g u i s h between t h e E-center and t h i s deep divacancy l e v e l . It has been observed i n ion-implanted, unannealed samples, as well as i n pulsed laser-annealed samples w i t h and w i t h out i o n implantation.
E9, E l l , E12, E13:
Several d e fe c ts l y i n g deep i n t h e band gap
o f s i 1i c o n have been observed under many d i f f e r e n t c i rcumstances
.
A s i m i l a r defect t o El2 has a l s o been observed by Johnson e t a l .
(1979) i n cw laser-annealed m a te ri a l . H 1 : This shallow h o l e t r a p has been reported o n l y by Wang e t a l . (1983) , and no i d e n t i f i c a t i o n was attempted.
154
G . E. JELLISON, JR
Several workers have observed a hole t r a p 0.22 t o 0.25 eV above the valence band. I t has commonly been detected i n both implanted and unimplanted, pulsed laser-annealed s i l i c o n , and can be correlated with the lowest energy charge s t a t e of the divacancy (Wang e t a1 1983). The t r a p a t 0.24 eV observed by Mooney e t a1 (1978) had an asymmetric DLTS spectrum (possibly indicating t h a t more than one t r a p is present); i t was reported t h a t concentrations of t h i s t r a p extended up t o 10 pm into the sample. H3: A t r a p 0.31 t o 0.33 eV above the valence band has been detected by Wang et a1 (1983) f o r laser-annealed, self-implanted s i l i c o n , and by Mesli et a l . (1982) f o r PF;-implanted material. Wang et a l . (1983) assigned t h i s defect t o t h e vacancy-oxygenboron cornpl ex. H4: A defect 0.36 t o 0.41 eV above the valence band has frequently been reported i n p-type s i l i c o n i n both ion-implanted and unimplanted materials, following pulsed l a s e r annealing i n both the l i q u i d and solid phase. I t has been assigned t o the vacancyoxygen-carbon complex (Wang et a1 , 1983). H5: A hole t r a p of small concentration, a t 0.47 eV was assigned by Wang et a1 (1983), t o the vacancy-boron complex. H2:
.
.
.
.
.
b.
Hydrogen Passivation
Benton et a l . (1980ay 1980b) have found t h a t t h e electron t r a p s E2 and E8 can be neutralized by reaction w i t h atomic hydrogen. They found t h a t a four-hour anneal a t 300°C i n a molecular hydrogen atmosphere had no effect on these traps; however, a four-hour, 200°C anneal i n a hydrogen plasma atmosphere resulted i n no observable e l e c t r i c a l l y a c t i v e defects. If t h e sample i s t h e n vacuum annealed a t 400°C f o r 1 hour a t a pressure o f 0.002 Torr t o evolve t h e hydrogen, the DLTS spectrum shows t h a t E8 returns, b u t t h a t E,Z does not. Similar experiments have been performed by Lawson and Pearton (1982) i n p-type s i l i c o n . They found t h a t two t r a p s a t 0.23 and 0.38 eV were created a f t e r pulsed laser annealing (Traps H2 and
3.
155
PROPERTIES OF PULSED LASER-ANNEALED SILICON
H4); a f t e r a one-hour, 200°C anneal i n molecular hydrogen, no change i n t h e DLTS spectrum was observed, w h i l e no DLTS s i g n a l was detected from these two t r a p s f o l l o w i n g a 10-min, 150°C anneal i n a hydrogen plasma.
A vacuum anneal o f 1 hour a t 250°C r e s u l t e d i n t h e reap-
pearance o f t h e h o l e t r a p s H2 and H4. These experiments open t h e e x c i t i n g p o s s i b i l i t y o f p a s s i v a t i n g e l e c t r i c a l l y a c t i v e d e f e c t s i n devices f a b r i c a t e d using pulsed l a s e r annealing.
S i m i l a r experiments have been performed i n p o l y -
crystal1ine silicon,
where Seager e t a1
. (1979)
found t h a t t h e
hydrogen plasma passivated some o f t h e g r a i n boundaries , and Young e t a l . (1981) found t h e same e f f e c t by using l i t h i u m .
c.
Defect Concentration P r o f i l e s
It i s p o s s i b l e t o determine t h e concentration o f defects as a f u n c t i o n o f d i s t a n c e from t h e edge o f t h e zero-bias d e p l e t i o n r e g i o n u s i n g DLTS.
This type o f measurement has been performed by many
workers f o r laser-annealed s i l i c o n . Working w i t h samples t h a t had been i o n implanted p r i o r t o pulsed l a s e r annealing,
Wang e t a l .
(1983) found t h a t several
d e f e c t s ( a t 0.39 and 0.47 eV above t h e valence band) had a charact e r i s t i c p r o f i l e i n which t h e d e f e c t concentration f i r s t increased and then decreased w i t h depth i n t o t h e sample.
I f t h e energy den-
s i t y o f t h e annealing Nd:glass l a s e r was increased, t h e d e f e c t concent r a t i o n p r o f i1e mai n t a i ned t h e same shape , but t h e magni tude was decreased. This behavior was explained on t h e basis o f t h e m e l t i n g model o f pulsed l a s e r annealing, assuming t h a t t h e two d e f e c t s were a r e s u l t o f t h e t a i l o f t h e ion-damage p r o f i l e .
For
h i g h e r energy d e n s i t y annealing pulses, t h e melt f r o n t goes deeper i n t o t h e sample,
annealing more o f t h e ion-damage t a i l ,
leaving
fewer unanneal ed defects. The observed increase i n t h e concentration o f t h e d e f e c t prof i l e near t h e f r o n t surface o f t h e sample was explained by Wang e t al.
(1983) by assuming a dense e l e c t r o n - h o l e plasma.
By t h i s
mechanism, heat can extend beyond t h e molten region, and " t h e dense
156
G . E. JELLISON, JR.
electron-hole
plasma can screen o u t t h e Coulombic t r a p p i n g o f
vacancies and promote i n t e r s t i t i a l m i g r a t i o n , which r e s u l t s i n t h e a n n i h i l a t i o n o f p o i n t d e f e c t s " (Wang e t al.,
1983).
A sharp decrease i n t h e d e f e c t c o n c e n t r a t i o n w i t h i n c r e a s i n g d e p t h has been observed by Wang e t a1 f o r ion-implanted,
. (1983) and M e s l i e t a l .
laser-annealed s i l i c o n .
(1981)
T h i s b e h a v i o r i s due t o
t h e f a c t t h a t t h e m e l t f r o n t from t h e l a s e r w i l l extend o n l y <0.5 pm i n t o t h e sample,
w h i l e t h e t a i l o f t h e d e f e c t p r o f i l e produced
b y t h e i o n i m p l a n t a t i o n extends much deeper.
Since t h e l a s e r p u l s e
w i l l a n n i h i l a t e o n l y t h o s e d e f e c t s i n t h e m a t e r i a l t h a t has been m e l t e d ( w i t h t h e e x c e p t i o n o f t h e r e g i o n very c l o s e t o t h e i n n e r most p e n e t r a t i o n o f t h e m e l t f r o n t ,
see above),
t h e remaining
d e f e c t s o u t s i d e o f t h e r e g i o n t h a t has been melted, w i l l have t h e same p r o f i l e a f t e r l a s e r annealing as before. The experiments o f Kachurin e t a l .
(1982b) were performed on
unimplanted, p u l s e d laser-annealed samples i n which t h e dopant conc e n t r a t i o n was h i g h (-loL7 P/cm3).
I n t h i s m a t e r i a l , t h e y were
a b l e t o probe d e f e c t s very c l o s e t o t h e sample surface. laser irradiation,
t h e y found 3 t r a p s a t 0.23,
below t h e conduction band, as t h e depth increased. tration
0.26,
After
and 0.42 eV
a l l o f which decreased e x p o n e n t i a l l y
They a l s o found t h a t t h e d e f e c t concen-
c o u l d be d r a m a t i c a l l y
reduced upon s u r f a c e
cleaning,
i n d i c a t i n g t h a t these d e f e c t s o r i g i n a t e d from i m p u r i t i e s d i f f u s e d i n from t h e s u r f a c e d u r i n g t h e t i m e t h a t t h e f r o n t s u r f a c e r e g i o n o f t h e sample was molten. d.
Laser-Induced Defects There appear t o be two main sources o f d e f e c t s i n ion-implanted,
p u l s e d laser-annealed s i l i c o n :
(1) d e f e c t s i n t h e t a i l o f t h e i o n -
i m p l a n t a t i o n damage p r o f i l e , which cannot be reached by t h e m e l t f r o n t and t h e r e f o r e remain a f t e r t h e laser-anneal i n g process and
( 2 ) d e f e c t s t h a t come from t h e i n t e r a c t i o n o f t h e l a s e r r a d i a t i o n w i t h the solid.
The d e f e c t s t h a t o r i g i n a t e from t h e l a s e r r a d i a t i o n
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
157
i t s e l f can come s i m p l y f r o m t h e f a c t t h a t t h e l a t t i c e has been heated, b u t n o t melted ( l i s t e d as SPE i n Table 111), o r t h e d e f e c t s can be quenched i n even though t h e l a t t i c e melted. Examples of d e f e c t s produced by l a s e r pulses not s u f f i c i e n t t o m e l t t h e m a t e r i a l were found by Blosse and Bourgoin (1983). t h e i r experiments,
t h e y i r r a d i a t e d As-implanted
In
(1-4 x 10L2/cm2;
100 o r 300 keV) s i l i c o n w i t h 15-ns ruby l a s e r pulses o f from 0.3 t o 0.6
J/cm2,
t h e sample.
which i s i n s u f f i c i e n t t o m e l t t h e f r o n t r e g i o n o f They found several o f t h e d e f e c t s decreased i n concen-
t r a t i o n f o r even t h e lowest energy pulses used, b u t t h a t u n i d e n t i f i e d d e f e c t s would o f t e n be c r e a t e d i n t h e i r place. however,
They were able,
t o observe t h a t some annealing had taken p l a c e a t 0.5
J / c d (below t h e m e l t i n g t h r e s h o l d f o r these samples), i n c o n t r a d i c t i o n w i t h t h e r e s u l t s o f K i m e r l i n g and Benton (1980), u s i n g a 40-ns Nd:YAG l a s e r .
Blosse and Bourgoin i n t e r p r e t e d t h i s as i n d i -
c a t i n g t h a t t h e i r a n n e a l i n g d i d not occur v i a a simple thermal process, b u t was enhanced by t h e e l e c t r o n - h o l e plasma c r e a t e d by t h e l a s e r pulse.
As can be seen from Table 111, many o f t h e d e f e c t s seen i n p u l s e d laser-annealed s i l i c o n a r e thought t o be quenched i n by t h e v e r y r a p i d r e s o l i d i f i c a t i o n a f t e r t h e l a s e r pulse. Fan e t a l .
Using DLTS,
(1982) looked a t samples t h a t had been unimplanted and
p u l s e d l a s e r annealed , impl anted and unannealed , and impl anted f o l l o w e d by pulsed l a s e r annealing.
They compared t h e DLTS s p e c t r a
from these samples w i t h samples t h a t had been quenched v e r y r a p i d l y (1000 C/s) from 1100 C, and found very good agreement w i t h d e f e c t s E3 and El0 o f Table 111.
These d e f e c t s can a l l be e l i m i n a t e d by
a n n e a l i n g t h e samples t o 6OOOC f o r twenty minutes.
11.
OTHER DEFECT-RELATED EXPERIMENTS Traditionally,
e l e c t r o n paramagnetic resonance (EPR) e x p e r i -
ments have been very u s e f u l i n d e t e r m i n i n g s t r u c t u r a l and symmetry p r o p e r t i e s o f p o i n t d e f e c t s i n s i l i c o n (see, f o r example, C o r b e t t
158
G. E. JELLISON, JR.
e t al.,
1981).
EPR d e t e c t s t h e a b s o r p t i o n o f microwave energy by
u n p a i r e d e l e c t r o n s whose energy l e v e l s have been s p l i t by a magnetic field. spins,
Generally, t h e l i m i t o f d e t e c t a b i l i t y o f EPR i s 1 0 l 2
a l t h o u g h t h i s l e v e l may i n c r e a s e ( o r decrease) o r d e r s o f
magnitude w i t h t h e decrease ( o r i n c r e a s e ) o f t h e EPR l i n e w i d t h . One can immediately see t h e fundamental l i m i t a t i o n o f t h i s techn i q u e t o t h e s t u d y o f d e f e c t s c r e a t e d by l a s e r annealing:
i f one
has a 1 x 1 cm square sample (which i s l a r g e by EPR standards),
a
d e f e c t d e n s i t y o f 1014/cm3 i n t h e t o p 1 pm o f t h e sample p r o v i d e s
- l o l o spins, which i s p r o b a b l y n o t d e t e c t a b l e w i t h EPR. However,
t h e l a s e r - i n d u c e d decrease o f d e f e c t s produced by
i o n i m p l a n t a t i o n can be s t u d i e d u s i n g EPR (Brower and Peercy, 1980; Murakami e t al.,
1979).
Brower and Peercy (1980) s t u d i e d s i l i c o n
t h a t had been i o n i m p l a n t e d w i t h S i and P (200 keV), evidence f o r d i s t o r t e d four-vacancy a f t e r i o n implantation.
d e f e c t s (Si-P3)
and found immediately
I n examining t h e EPR s p e c t r a as a f u n c t i o n
o f l a s e r energy, t h e y found t h a t t h e EPR Si-P3 s i g n a l decreased m o n o t o n i c a l l y w i t h i n c r e a s i n g l a s e r energy; above 1.8 J/cm2, t h e Si-P3 s i g n a l was very weak, b u t t h e EPR s i g n a l from f r e e conduct i o n resonance was apparent i n samples implanted w i t h o n l y P (3 x 1013/cm2).
The magnitude o f t h e f r e e e l e c t r o n conduction reso-
nance o f a P-implanted, laser-annealed sample was 80-100% o f t h a t observed i n a s i m i l a r sample s u b j e c t e d t o a 90OoC/20 m i n thermal anneal.
For samples i m p l a n t e d w i t h b o t h S i
P (3 x 1013/cm2),
(2 x 1015/cm2) and
t h e r e s u l t s o f l a s e r annealing were somewhat
d i f f e r e n t i n t h a t no f r e e e l e c t r o n c o n d u c t i o n resonance was observed. R u t h e r f o r d b a c k s c a t t e r i n g experiments i n d i c a t e f u l l c r y s t a l l i n i t y f o r t h e s e samples, but a p p a r e n t l y t h e r e a r e s u f f i c i e n t p o i n t d e f e c t s r e s u l t i n g from t h e high-dose S i i m p l a n t t o reduce t h e e l e c t r o n l i f e t i m e t o t h e e x t e n t t h a t EPR from conduction e l e c t r o n s i s n o t observable. Murakami e t a l .
(1979) have a l s o used EPR as a c r i t e r i o n f o r
p r e d i c t i n g t h e annealing e f f e c t s o f v a r i o u s pulsed l a s e r s used i n
3.
PROPERTIES OF PULSED LASER-ANNEALED SILICON
159
l a s e r annealing and found a p o s i t i v e c o r r e l a t i o n between t h e i n t e g r a t e d EPR s i g n a l ( p r o p o r t i o n a l t o t h e t o t a l number o f u n p a i r e d spins, Ns) and t h e c r y s t a l l i n e recovery r a t i o .
Since t h e o p t i c a l
a b s o r p t i o n c o e f f i c i e n t o f amorphous s i l i c o n i s d i r e c t l y c o r r e l a t e d w i t h Ns (Mott and Davis, 1979), t h e y reasoned t h a t an i n c r e a s e i n
Ns ( o r a) would r e s u l t i n more l a s e r energy b e i n g absorbed i n t h e n e a r - s u r f a c e region, enhancing t h e annealing e f f e c t . Kachurin e t a l .
(1980) have used e l e c t r i c a l c o n d u c t i v i t y i n
c o n j u n c t i o n w i t h l a y e r - b y - l a y e r e t c h i n g t o examine S i P-implanted
with d i f f e r e n t doses.
They found t h a t t h e u t i l i z a t i o n c o e f f i c i e n t
K increased w i t h i n c r e a s i n g doses f o r laser-annealed m a t e r i a l ; furthermore,
t h e y found t h a t t h e c o n d u c t i v i t y o f t h e f r o n t l a y e r
increased w i t h a low temperature thermal anneal, r e a c h i n g a maximum f o r a 200°/15
min anneal.
These two r e s u l t s i n d i c a t e t h e
e x i s t e n c e of P-V d e f e c t s (E-centers) i n t h e f r o n t s u r f a c e l a y e r s , which c o u l d r e s u l t i n compensation e f f e c t s f o r t h e low-dose samp l e s r e s u l t i n g i n a s m a l l e r K. i s preceded with a
If t h e pulsed l a s e r annealing s t e p
500°C/15 min thermal anneal , K's approaching
1 a r e a l s o obtained, even f o r very low i m p l a n t a t i o n doses. Acknowledgments It i s a p l e a s u r e f o r me t o thank R. F. Wood, R.
T. Young, F. A.
Modine, and G. Watkins f o r r e a d i n g and commenting on t h i s manuscript.
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M a s e t t i , G., and Solmi, S. (1979). IEEE S o l i d S t a t e E l e c t r o n Devices ED-3, 65. M c G i l l , T. C., K u r t i n , S. L., and S h i f r i n , G. A. (1970). J. Appl. Phys. 41, 246. McKelvey, J. P. (1966). " S o l i d - S t a t e and Semiconductor Physics." H a r p e r and ROW, New York. M e s l i , A., M u l l e r , J. C., S a l l e s , D., and S i f f e r t , P. (1981). Appl. Phys. L e t t . 39, 159. M e s l i , A. , Goltzene, A. , M u l l e r , J. C. , Meyer, B. , Schwab, C. , and S i f f e r t , P. (1982). Mat. Res. SOC. Symp. Proc. 4 , 349. M i l l e r , G. L., Lang, D. V. , and K i m e r l i n g , L. C. (1977). Ann. Rev. Mat. Sci. 7, 377. Miyao, M., Motooka, T., Natsuaki, N., Tokuyama, T. (1981). Mat. Res. SOC. Symp. Proc. 1, 163. Mooney, P. M., Young, R. T., K a r i n s , J., Lee, Y. H., and C o r b e t t , J. W. (1978). Phys. Stat. Sol. A 48, K31. M o s t o l l e r , M. (1983). P r i v a t e communication. M o t t , N. F., and Davis, E. A. (1979). " E l e c t r o n i c Processes i n Non-Crystal 1 ine M a t e r i a1s" C1arendon, Oxford. Murakami, K., Ikawa, E., Gamo, K., Namba, S., Akasaka, Y., and Musuda, Y. (1979). Appl. Phys. L e t t . 35, 413. Nakamura, K. Gotoh, T., and Kamoshida, M. (1979). J. Appl. Phys.
50, 3985.
Nakamura, K. , and Kamoshida, M. (1979). R a d i a t i o n E f f e c t s 42, 29. Nakashima, H., S h i r a k i , Y., and Miyao, M. (1979). J. Appl. Phys.
50, 5966.
O'Donnell, K. P., Les, K. M., and Watkins, G. (1983). Physica 1168, 258. P f e i f f e r , L., Kovacs, C e l l e r , G. K., Trimble, and Jacobson, D. C. (1982). Mat. Res. SOC. Symp. Proc. 4 , 275. P h i l i p p , H. R., and T a f t , E. A. (1960). Phys. Rev. 120, 37. P h i l i p p , H. R., and Ehrenreich, H. (1963). Phys. Rev. 129, 1550. P h i l i p p , H. R. (1972). J. Appl. Phys. 43, 2835. P o l l a k , F. H., Tsu, R., and Mendez, E. (1980). I n "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White, and P. s. Peercy, eds.), p. 195. Academic Press, New York. Qin, G. G., L i , M. F., and Sah, C. T. (1982). J. Appl. Phys. 53,
4800.
Sato, T. (1967). Jpn. J. Appl. Phys. 6 , 339. Sauer, R., and Weber, J. (1983). Physica 1168, 195. Seager, C. H., G i n l e y , D. S., and Zook, J. D. (1979). Appl. Phys. L e t t . 36, 831. Shvarev, K. M., Baum, B. A., and Gel'd, P. V. (1975). Sov. Phys. S o l i d S t a t e 16, 2111. Shvarev, K. M., Baum, B. A., and Gel'd, P. V. (1977). High Temperature 15, 548. S k o l n i c k , M. S., C u l l i s , A. G., and Webber, H. C. (1981a). Appl. Phys. L e t t . 38, 464. S k o l n i c k , M. S., C u l l i s , A. G. , and Webber, H. C. (1981b). Mat. Res. SOC. Symp. Proc. 1, 185.
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163
Sze, S . M. (1980). "Physics o f Semiconductor Devices." J. Wiley and Sons , New Y ork. I n "Laser and Tamura, M. , Natsuaki, N., and Tokuyama, T. (1980). E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 247. Academic Press, New York. Thurmond, C. D. (1975). J. Electrochem. SOC. 122, 1133. Trumbore, F. A. (1960). B e l l System Tech. J. 39, 205. Tsu, R. , Baglin, J. E., Tan, T. Y., Tsai, M. Y., Park, K. C., and Hodgson, R. (1979). I n "Laser-Solid I n t e r a c t i o n s and Laser Processing--1978" (S. D. F e r r i s , H. J. Leanly, and J. M. Poate, eds.), p. 344. American I n s t i t u t e of Physics, New York. Varshni, Y. P. (1967). Physica 34, 149. Vina, L., and Cardona, M. (1983). Physica 117B, 1188, 356. Wang, K. L., Liu, Y. S., and Burman, C. (1979). Appl. Phys. L e t t . 35, 263. Wang, K. L., L i u , Y. S., Possin, G. E., Karins, J . , and Corbett, J. (1983). J. Appl. Phys. 54, 3839. Watanabe, K., Miyao, M., Takemoto, I., and Hashimoto, N. (1979). Appl. Phys. L e t t . 34, 518. Watanabe, K. , Motooka, T. , Hashimoto, N., and Tokuyama, T. (1980). Appl. Phys. L e t t . 36, 451. Weakliem, H. A., and R e d f i e l d , D. (1979). J. Appl. Phys. 50, 1491. White, C. W., Narayan, J . , and Young, R. T. (1979). I n "LaserS o l i d I n t e r a c t i o n s and Laser Processing--1978" (S. D. F e r r i s , H. J . Leamy, and J. M. Poate, eds.), p. 275. American I n s t i t u t e o f Physics, New York. White, C. W., Wilson, S. R. , Appleton, B. R. , and Young, F. W., Jr. (1980a). J. Appl. Phys. 51, 738. White, C. W., Pronko, P. P., Wilson, S. R., Appleton, B. R., Narayan, J., and Young, R. T. (1980b). J. Appl. Phys. 50, 3261. Wilson, S. R., White, C. W., Pronko. P. P., Young, R. T., and Appleton, B. R. (1979). I n "Laser-Solid I n t e r a c t i o n s and Laser Processing--1978" (S. D. F e r r i s , H. J. Leamy, and J. M. Poate, eds.), p. 351. American I n s t i t u t e o f Physics, New York. W o l f s t i r n , K. B. (1960). Phys. Chem. S o l i d s 16, 279. Young, R. T., White, C. W., Clark, G. J . , Narayan, J., C h r i s t i e , W. H., Murakami, M., King, P . W., and Kramer, S. D. (1978). Appl. Phys. L e t t . 32, 139. Young, R. T., and Narayan, J. (1978). Appl. Phys. L e t t . 33, 14. Young, R. T., Narayan, J . , and Wood, R. F. (1979). Appl. Phys. L e t t . 35, 447. Young, R. T., Lu, M. C., Westbrook, R. D., and J e l l i s o n , G. E. (1981). Appl. Phys. L e t t . 38, 628. Young, R. T., Wood, R. F., and C h r i s t i e , W. H. (1982). J. Appl. Phys. 53, 1178. Young, R. T., Narayan, J., C h r i s t i e , W. H., van der Leeden, G. A., Rothe, 0. E., and Cheng, L. J. (1983). S o l i d S t a t e Tech. 26, 183.
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Zehner, D. M., White, C. W., Ownby, G. W., and C h r i s t i e , W. H. (1980). I n "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.) p. 201. Academic Press, New York.
CHAPTER 4
M E L T I N G MODEL
OF
PULSED LASER PROCESSING
K. F. Wood
G. E. J e l l i s o n , Jr.
. . . . . . . . . . . .. .. .. .
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I. INTRODUCTION. 11. INTERACTION OF LASER RADIATION WITH SEMICONDUCTORS 1. A b s o r p t i o n Mechanisms 2. C a r r i e r - L a t t i c e I n t e r a c t i o n 3. C a r r i e r D i f f u s i o n and C a r r i e r Confinement 4. Summary. 111. FORMULATION OF THE MELTING MODEL 5. Heat Conduction Equations and Boundary Conditions. 6. Phase Changes. 7. Temperature-Dependent Thermal and Optical Properties 8. I n p u t Data. I V . RESULTS OF MELTING MODEL CALCULATIONS 9. C r y s t a l l i n e and Amorphous Models 10. C a l c u l a t i o n s f o r Ruby and Frequency-Doubled Nd Lasers 11. C a l c u l a t i o n s f o r U l t r a v i o l e t Excimer Lasers 12. E f f e c t s o f Amorphous Layers 13. Miscellaneous I l l u s t r a t i v e Calculations 14. A d d i t i o n a l Discussion o f R e s u l t s V. DOPANT REDISTRIBUTION I N THE MELTING MODEL. 15. I n t r o d u c t i o n 16. Segregation C o e f f i c i e n t s 17. One-Dimensional S o l i d i f i c a t i o n
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Copyright 0 1984 by Academic Press, Inc. All nghts of reproduction cn any form reserved. ISBN 0-12-752123-2
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18.
VI.
. .. ... ... . . . . . . . . . ..
Models and Approximations f o r Dopant T r a n s p o r t C a l c u l a t i o n s 19. C a l c u l a t i o n s and R e s u l t s 20. D i s c u s s i o n and Conclusions SUMMARY AND DIRECTIONS FOR FUTURE WORK REFERENCES
I.
Introduction
I n p u l s e d l a s e r p r o c e s s i n g o f s i l i c o n , t h e l a s e r may t y p i c a l l y d e l i v e r t o t h e sample s u r f a c e an energy d e n s i t y , E,
o f 1.5 J/cm2 i n
a s i n g l e p u l s e o f d u r a t i o n T, = 15 nsec (FWHM), o r a power d e n s i t y
o f r o u g h l y 50 megawatts/cm2 averaged over t h e pulse.
For photon
e n e r g i e s g r e a t e r t h a n t h e i n d i r e c t band gap i n s i l i c o n (1.16 300 K),
eV a t
as much as 70% o f t h i s power w i l l be r e f l e c t e d f r o m t h e
s u r f a c e f o r some wavelengths, w h i l e t h e r e s t o f t h e l i g h t w i l l be absorbed by e l e c t r o n i c e x c i t a t i o n s i n t h e sample.
Calculations
discussed below show t h a t i f t h e energy absorbed by t h e c a r r i e r system i s t r a n s f e r r e d t o t h e l a t t i c e i n a t i m e l e s s t h a n o r comp a r a b l e t o t h e p u l s e d u r a t i o n , a 1.5-J/cm2,
15-nsec p u l s e f r o m a
ruby l a s e r can m e l t t h e near-surface r e g i o n o f c r y s t a l l i n e s i l i c o n t o a depth o f about 0.3 wn.
When an amorphous l a y e r i s p r e s e n t a t
t h e surface, t h e m e l t depth may be somewhat g r e a t e r .
I f such m e l t -
i n g does occur, t h e n a l l o f t h e observed phenomena a s s o c i a t e d w i t h l a s e r a n n e a l i n g and w i t h o t h e r forms o f p u l s e d l a s e r p r o c e s s i n g can be expl a i ned, p r o v i d e d t h e nonequi 1 ib r i um thermodynamic aspects o f t h e p h y s i c a l processes i n v o l v e d a r e t a k e n i n t o account.
The
m a t e r i a l t h r o u g h o u t t h i s volume demonstrates how w e l l t h i s " m e l t i n g model" o f p u l s e d l a s e r p r o c e s s i n g e x p l a i n s t h e experimental observations.
I n t h i s and t h e n e x t chapter, t h e t h e o r e t i c a l f o u n d a t i o n
o f t h e m e l t i n g model w i l l be developed, w i t h p a r t i c u l a r emphasis on t h o s e aspects o f i t l i k e l y t o be i m p o r t a n t f o r l a s e r - p r o c e s s i n g applications. C a l c u l a t i o n s o f heat conduction i n s o l i d s . s u b j e c t e d t o i n t e n s e l a s e r r a d i a t i o n have been c a r r i e d out s i n c e t h e e a r l y days o f l a s e r r e l a t e d science and technology (see, e.g.,
Ready, 1971).
Most o f
4.
167
MELTING MODEL OF PULSED LASER PROCESSING
these c a l c u l a t i o n s were concerned w i t h t h e e f f e c t s o f l a s e r r a d i a t i o n on metals, b u t some work on semiconductors was a l s o reported. When S i ,
Ge,
InP,
and o t h e r semiconductors a r e i r r a d i a t e d w i t h
s u f f i c i e n t l y i n t e n s e l a s e r pulses,
a h i g h - r e f l e c t i v i t y phase i s
observed (see, f o r example, Sooy e t al., Auston e t al.,
1978),
1964; B l i n o v e t al.,
1967;
s t r o n g l y suggesting t h a t t h e near-surface
The e x t e n t o f t h e dopant d i f f u s i o n that occurs d u r i n g
r e g i o n melts.
t h e l a s e r a n n e a l i n g o f i o n - i m p l a n t e d S i and GaAs gave a d d i t i o n a l s t r o n g evidence t h a t m e l t i n g occurs.
S h o r t l y a f t e r t h e experimental
d a t a on p u l s e d l a s e r a n n e a l i n g became a v a i l a b l e ,
t h e r e s u l t s of
m e l t i n g model c a l c u l a t i o n s were r e p o r t e d by several groups (Baeri e t al.,
1978,
1979; Wang e t al.,
e t al.,
1979; Wood e t al.,
1978,
1979; B e l l , 1979; Surko
1980; K i r k p a t r i c k e t al.,
and G i l e s , 1981; von Allmen e t al.,
1981).
1980; Wood
The modeling i n v o l v e d
f i n i t e - d i f f e r e n c e or f i n i t e - e l e m e n t s o l u t i o n s o f t h e heat d i f f u s i o n equations and f r e q u e n t l y took i n t o account e x p l i c i t l y b o t h t h e temperature dependence o f t h e thermal p r o p e r t i e s and t h e p o s s i b i l i t y o f melting. The success o f t h e m e l t i n g t h e o r y o f p u l s e d l a s e r a n n e a l i n g leaves l i t t l e doubt t h a t t h e conceptual f o u n d a t i o n o f t h e modeling i s b a s i c a l l y c o r r e c t i n t h e regime o f nanosecond, and probably even picosecond, l a s e r pulses.
Nevertheless, a p p a r e n t l y some proponents
o f a "plasma model" o f p u l s e d l a s e r a n n e a l i n g a r e r e l u c t a n t t o concede t h i s .
Although t h e plasma model has n o t been developed t o t h e
e x t e n t t h a t q u a n t i t a t i v e comparisons between i t s p r e d i c t i o n s and experiment can be made, i t i s i n t e r e s t i n g t o c o n s i d e r b r i e f l y t h e general idea on which i t i s based.
I n t h e plasma model (van Vechten,
1980, 1983; Lo and Compaan, 1980, 1981; Compaan e t al.,
1983) i t i s
assumed t h a t a t t h e power l e v e l s i n v o l v e d i n p u l s e d l a s e r annealing,
a dense e l e c t r o n - h o l e plasma i s c r e a t e d i n t h e sample and
remains decoupled from t h e l a t t i c e f o r p e r i o d s o f t h e o r d e r o f 100 nsec.
It i s t h i s plasma,
and n o t t h e l i q u i d s t a t e ,
i s responsible f o r t h e h i g h - r e f l e c t i v i t y t r a n s i e n t - r e f l e c t i v i t y experiments.
which
phase observed d u r i n g
The d i s r u p t i o n o f t h e covalent
168
R. E WOOD ET AL.
bonding i m p l i e d by t h e f o r m a t i o n o f t h e plasma i s supposed t o l e a d t o a v a r i e t y o f e f f e c t s normally associated w i t h t h e l i q u i d state, e.g.,
very r a p i d d i f f u s i o n o f dopant atoms.
I n o u r view, such a
g r e a t body o f evidence i s so overwhelmingly i n f a v o r o f t h e thermal m e l t i n g model t h a t t h e plasma model as such w i l l n o t be considered f u r t h e r here.
However, some o f t h e i d e a s i n v o l v e d i n t h e model
become v a l i d i n t h e low picosecond and femtosecond p u l s e range and
w i l l be considered b r i e f l y below. M a t e r i a l i n t h i s chapter f o l l o w s c l o s e l y work i n t h e f i r s t two papers i n a s e r i e s o f t h r e e by Wood and co-workers (Wood and G i l e s , 1981; Wood e t al.,
1981a; and Wood, 1982) on t h e macroscopic t h e o r y
o f p u l s e d l a s e r annealing.
However, t h e m a t e r i a l has been updated
e x t e n s i v e l y w i t h m o d i f i e d c a l c u l a t i o n s (many o f which have n o t been p u b l i s h e d elsewhere) t h a t t a k e i n t o account new i n f o r m a t i o n on t h e temperature-dependent o p t i c a l p r o p e r t i e s o f s i l i c o n d e s c r i b e d i n Chapter 3 and t h e r e s u l t s o f t i m e - r e s o l v e d o p t i c a l measurements d e s c r i b e d i n Chapter 6.
Although i t was n o t o u r i n t e n t i o n t o g i v e
a d e t a i l e d review o f t h e development o f t h e m e l t i n g model, a reasonably comprehensive l i s t o f r e f e r e n c e s i s provided. The r e s t o f t h e c h a p t e r i s d i v i d e d i n t o f i v e s e c t i o n s .
In
t h e n e x t s e c t i o n , an a b b r e v i a t e d d i s c u s s i o n o f t h e i n t e r a c t i o n of i n t e n s e l a s e r r a d i a t i o n w i t h a semiconducting m a t e r i a l i s given. T h i s d i s c u s s i o n covers p r i m a r i l y t h o s e t o p i c s w i t h i m p l i c a t i o n s f o r t h e remainder o f t h e chapter.
The s p e c i f i c f o r m u l a t i o n o f t h e
m e l t i n g model developed a t our l a b o r a t o r y , and some o f t h e d e t a i l s o f i t s a p p l i c a t i o n , a r e d e s c r i b e d i n S e c t i o n 111.
I n Section I V ,
a v a r i e t y o f c a l c u l a t e d r e s u l t s f o r t h e more common p u l s e d l a s e r s i s given.
Dopant d i f f u s i o n and s e g r e g a t i o n e f f e c t s d u r i n g p u l s e d
l a s e r m e l t i n g a r e reviewed i n S e c t i o n V.
The i n t e r f a c e s e g r e g a t i o n
c o e f f i c i e n t i s t r e a t e d as a f i t t i n g parameter i n t h i s s e c t i o n , w i t h a d e t a i l e d d i s c u s s i o n o f i t l e f t u n t i l Chapter 5.
The c h a p t e r
concludes w i t h a b r i e f summary and a few c o n c l u d i n g remarks about l i k e l y f u t u r e developments i n t h e modeling.
4. 11. 1.
MELTING MODEL OF PULSED LASER PROCESSING
169
Interaction o f Laser Radiation with Semiconductors
ABSORPTION MECHANISMS Discussions o f t h e a b s o r p t i o n o f i n t e n s e l a s e r r a d i a t i o n by
semiconductors have appeared f r e q u e n t l y i n t h e l i t e r a t u r e (see, f o r example, G r i n b e r g e t al.,
1967; E l c i e t al.,
1977).
Recent develop-
ments a s s o c i a t e d w i t h p u l s e d l a s e r a n n e a l i n g have c l a r i f i e d some aspects o f t h e a b s o r p t i o n process, complex i t can become.
b u t t h e y have a l s o shown how
For example, experimental and t h e o r e t i c a l
r e s u l t s have p r o v i d e d a c l e a r e r p i c t u r e o f t h e importance o f t h e temperature dependence o f t h e o p t i c a l p r o p e r t i e s o f an i n d i r e c t band gap m a t e r i a l such as s i l i c o n ,
b u t t h e r o l e o f heavy-doping
e f f e c t s and i n t e n s i t y - d e p e n d e n t a b s o r p t i o n i s s t i l l unclear. F i v e d i s t i n c t mechanisms o r processes f o r t h e a b s o r p t i o n o f l i g h t by semiconductors can be i d e n t i f i e d :
[a]
Photons w i t h energy (hv) much l e s s than t h e band-gap energy (Eg) can e x c i t e l a t t i c e v b r a t i o n s d i r e c t l y .
[b]
Free o r n e a r l y f r e e c a r r i e r s can be e x c i t e d by absorpt i o n o f l i g h t w i t h hv
<
Eg; such c a r r i e r s w i l l always
be p r e s e n t as a r e s u l t o f f i n i t e temperatures and/or doping. [c]
An induced m e t a l l i c - l i k e a b s o r p t i o n due t o f r e e c a r r i e r s generated by t h e l a s e r r a d i a t i o n i t s e l f can occur.
[d]
For photon e n e r g i e s
>
Eg, a b s o r p t i o n w i l l t a k e p l a c e by
d i r e c t and/or i n d i r e c t (phonon-assisted) e x c i t a t i o n o f e l e c t r o n - h o l e p a i rs. [el
A b s o r p t i o n induced by broken symmetry o f t h e c r y s t a l l i n e l a t t i c e i s possible.
Mechanism [a] w i l l p l a y a s i g n i f i c a n t r o l e f o r l a s e r r a d i a t i o n i n t h e wavelength range corresponding t o l a t t i c e v i b r a t i o n s ; f o r example, C02 l a s e r r a d i a t i o n a t X = 10.6 vm couples very e f f i c i e n t l y t o l a t t i c e v i b r a t i o n s of many m a t e r i a l s .
However, t h i s a b s o r p t i o n
mechanism has n o t proved p a r t i c u l a r l y u s e f u l f o r l a s e r p r o c e s s i n g
o f semiconductors and w i l l n o t be considered f u r t h e r here.
170
R. F. WOOD E T A L .
Mechanism [b] i s i n v o l v e d i n t h e a b s o r p t i o n o f l a s e r r a d i a t i o n o f a l l wavelengths b u t i s p a r t i c u l a r l y c r u c i a l f o r l a s e r photon energies l e s s than
Eg i n h e a v i l y doped i n d i r e c t band gap semicon-
d u c t o r s such as s i l i c o n .
The i n f l u e n c e o f doping e f f e c t s on t h e
o p t i c a l p r o p e r t i e s o f s i l i c o n a t v a r i o u s wavelengths i s discussed i n Chapter 3.
The d i s c u s s i o n t h e r e suggests t h a t t h e e f f e c t s o f
doping may n o t be a t t r i b u t a b l e s o l e l y t o f r e e c a r r i e r s b u t may a l s o i n v o l v e some symmetry-breaking aspects o f t h e presence o f t h e dopant atoms (mechanism [ e l ) .
A l s o i n Chapter 3, i t i s shown t h a t t h e
e f f e c t s o f temperature on t h e a b s o r p t i o n c o e f f i c i e n t , a, f o r hv near
E g f o r s i l i c o n i n v o l v e s p r i m a r i l y temperature-dependent band s t r u c t u r e e f f e c t s and has l i t t l e t o do w i t h t h e i n c r e a s e d d e n s i t y o f f r e e c a r r i e r s as t h e temperature i s r a i s e d .
For C02 l a s e r s , on t h e
o t h e r hand, f r e e c a r r i e r s i n t r o d u c e d b o t h by doping and e l e v a t e d temperatures a r e i m p o r t a n t f o r t h e c o u p l i n g o f t h e r a d i a t i o n t o s i l i c o n (see Chapter 9). Mechanism [c],
which w i l l produce an i n t e n s i t y - d e p e n d e n t a, may
be o f c o n s i d e r a b l e p o t e n t i a l importance a t t h e photon d e n s i t i e s used i n p u l s e d l a s e r processing, e s p e c i a l l y f o r wavelengths near t h e i n d i r e c t band gap i n s i l i c o n and/or f o r very s h o r t d u r a t i o n pulses.
I t s a c t u a l importance
will be c o n s i d e r e d b r i e f l y below.
For t h e p r e s e n t , we note o n l y t h a t t h e e q u i l i b r i u m d e n s i t y o f f r e e c a r r i e r s generated by t h e l a s e r r a d i a t i o n , and hence t h e importance o f mechanism [ c l ,
depends on t h e c a r r i e r r e c o m b i n a t i o n r a t e d i s -
cussed i n t h e n e x t subsection.
I t appears t h a t t h i s recombination
r a t e i s so r a p i d t h a t i n t h e nanosecond regime o f p u l s e d u r a t i o n s , mechanism [c] i s r e l a t i v e l y unimportant compared t o [d] i n determini n g a f o r l a s e r r a d i a t i o n w e l l above t h e i n d i r e c t band gap i n s i1icon.
The l a r g e s t c o n t r i b u t i o n t o a ( f o r nanosecond p u l s e s ) f o r r a d i a t i o n w i t h hu
>
Eg i n i n d i r e c t band gap semiconductors which
have n o t been h e a v i l y damaged o r made amorphous by, i m p l a n t a t i o n a r i s e s f r o m mechanism [d].
e.g.,
ion
The s t r o n g dependence
o f t h e a b s o r p t i o n c o e f f i c i e n t on temperature comes about t h r o u g h
171
4. MELTING MODEL OF PULSED LASER PROCESSING
t h i s mechanism (because o f t h e temperature dependence o f Eg and i n creased phonon p o p u l a t i o n s a t e l e v a t e d temperatures) and n o t t h r o u g h mechanism [b]. Mechanism [ e l i s q u i t e i m p o r t a n t i n l a s e r p r o c e s s i n g o f semiconductors because i o n i m p l a n t a t i o n i s f r e q u e n t l y used.
I n those
near-surface r e g i o n s where t h e i o n i m p l a n t a t i o n c r e a t e s amorphous material,
a may e a s i l y be i n c r e a s e d by an o r d e r o f magnitude o r
more over t h e c r y s t a l l i n e value (Brodsky e t al.,
1970).
I t i s obvious from t h e f o r e g o i n g d i s c u s s i o n t h a t t h e absorpt i o n o f r a d i a t i o n by a semiconductor,
e s p e c i a l l y one l i k e s i l i c o n
w i t h an i n d i r e c t gap, d u r i n g p u l s e d l a s e r i r r a d i a t i o n i s a complex process:
It i s made even more complex because t h e c o n t r i b u t i o n s
o f t h e v a r i o u s mechanisms given above w i l l change d u r i n g t h e l a s e r pulse.
Measurements o f a as a f u n c t i o n o f temperature,
such as
t h o s e d e s c r i b e d i n Chapter 3, are i n v a l u a b l e i n p r o v i d i n g i n f o r m a t i o n f o r use i n m e l t i n g model c a l c u l a t i o n s .
However, s i n c e t h e s e
h i g h l y a c c u r a t e measurements a r e u s u a l l y made o n l y a t low l i g h t intensities,
t h e q u e s t i o n a r i s e s as t o whether o r n o t n o n l i n e a r
a b s o r p t i o n i n v a l i d a t e s t h e r e s u l t s a t t h e very h i g h i n t e n s i t i e s used i n p u l s e d l a s e r processing. (mechanism [c])
The i n d u c e d - m e t a l l i c a b s o r p t i o n
l e a d i n g t o an i n t e n s i t y - d e p e n d e n t a b s o r p t i o n co-
e f f i c i e n t i n s i l i c o n f o r t h e 1.06 pm l i g h t o f t h e fundamental o f t h e Nd:YAG l a s e r has been e x t e n s i v e l y s t u d i e d ( G r i n b e r g e t al., 1967; Gauster and Bushnell , 1970; Svantesson and Nilsson,
1978),
and i t i s c l e a r t h a t i t p l a y s an i m p o r t a n t r o l e in t h i s case. A d e f i n i t i v e answer t o t h i s q u e s t i o n when ruby, frequency-doubled Nd, and some u l t r a v i o l e t l a s e r s a r e used i s g r e a t l y c o m p l i c a t e d by t h e s t r o n g temperature dependence o f mechanism [d],
which can
cause an apparent n o n l i n e a r i t y resembling so c l o s e l y t h e b e h a v i o r expected from mechanism [c] t h e two.
t h a t it i s d i f f i c u l t t o disentangle
( E a r l y c a l c u l a t i o n s by L i e t o i l a and Gibbons (1979) f o r a
ruby l a s e r overestimated t h e r o l e o f induced f r e e - c a r r i e r a b s o r p t i o n because t h e c o r r e c t temperature dependence o f a was n o t t a k e n i n t o I f i n t e n s e p u l s e d l a s e r r a d i a t i o n i s used i n a t t e m p t s account.)
172
R. F. WOOD ET AL.
t o measure a, f u r t h e r c o m p l i c a t i o n s may be caused by t h e l a r g e temperature g r a d i e n t s s e t up o v e r t h e p e n e t r a t i o n depth of t h e radiation.
Although a c o m p l e t e l y s a t i s f a c t o r y s o l u t i o n t o t h e
problem o f d e t e r m i n i n g a under t h e extreme c o n d i t i o n s o f l a s e r a n n e a l i n g i s n o t y e t a v a i l a b l e , t h e r e a r e good reasons t o be o p t i m i s t i c t h a t t h i s d i f f i c u l t y does n o t g r e a t l y e f f e c t t h e modeling, as d i s c u s s e d next. I n Chapter 6 i t i s shown t h a t t h e r e i s good agreement between t h e r e s u l t s o f t r a n s i e n t r e f l e c t i v i t y and t r a n s m i s s i v i t y e x p e r i ments on s i l i c o n and m e l t i n g model c a l c u l a t i o n s employing t h e temperature-dependent o p t i c a l data o f J e l l i s o n and Modine (1982, 1983) and J e l l i s o n and Lowndes (1982).
Therefore, i t appears t h a t
c o n t r i b u t i o n s t o a from mechanism [ c ] a r e n o t l a r g e i n t h e nanosecond p u l s e range,
a l t h o u g h t h e d i s c r e p a n c i e s t h a t do e x i s t may
be due t o t h i s mechanism (Lowndes e t al.,
1983).
Furthermore,
F i g . 2 o f Chapter 3 shows t h a t a f o r photon e n e r g i e s g r e a t e r t h a n t h e d i r e c t band. gap has n e a r l y s a t u r a t e d a t c h a r a c t e r i s t i c o f metals.
-
lo6 cm-l,
a value
The c o n t r i b u t i o n s t o a o f n o n l i n e a r
f r e e - c a r r i e r a b s o r p t i o n i s almost c e r t a i n l y i n s i g n i f i c a n t f o r t h e s e s h o r t wavelengths,
o r more p r e c i s e l y
f u r t h e r i n c r e a s e s a r e unimportant. apparent t h a t t h e use o f u l t r a v i o l e t and KrF (249 nm),
a i s a l r e a d y so l a r g e t h a t I n t h i s connection,
it i s
asers, such as XeCl (308 nm)
i s advantagous i n s i m p l i f y i n g t h e t h e o r e t i c a l
treatment o f laser-induced melting.
l i t h these lasers, a i s v i r -
t u a l l y independent o f b o t h t h e temperature and t h e s t a t e o f t h e m a t e r i a l , i.e., and l i q u i d (9.) 2.
i t i s t h e same f o r c r y s t a l l i n e ( c ) , amorphous ( a ) ,
silicon.
CARRIER-LATTICE INTERACTION The t r a n s i e n t r e f l e c t i v i t y change e x h i b i t e d by semiconductors
d u r i n g i n t e n s e l a s e r i r r a d i a t i o n has been s t u d i e d s i n c e about 1964 (Carmichael and Simpson, 1964; Sooy e t al.,
1964; Birnbaum, 1965).
4.
173
MELTING MODEL OF PULSED LASER PROCESSING
I n some o f these e a r l i e r papers, t h i s r e f l e c t i v i t y change was a t t r i b u t e d t o t h e h i g h d e n s i t y o f photogenerated c a r r i e r s , which p e r s i s t e d a f t e r t h e t e r m i n a t i o n o f t h e l a s e r pulse.
However, B l i n o v
e t a l . (1967) concluded t h a t t h i s e x p l a n a t i o n d i d n o t f i t t h e i r d a t a on t h e a b s o r p t i o n o f long-wavelength r a d i a t i o n d u r i n g i r r a d i a t i o n o f S i and GaAs w i t h p u l s e s f r o m a Q-switched ruby l a s e r .
They
surmized i n s t e a d t h a t t h e r e f l e c t i v i t y change was due t o t h e m e l t i n g o f a t h i n surface layer.
A
c r u c i a l question f o r the a p p l i c a b i l i t y
o f t h e m e l t i n g model, a t l e a s t i n t h e form used here, concerns t h e l i f e t i m e o f electron-hole p a i r s during intense laser i r r a d i a t i o n and t h e t r a n s f e r o f energy from t h e c a r r i e r system t o t h e l a t t i c e . There i s a f a i r l y s u b s t a n t i a l body o f l i t e r a t u r e on t h i s t o p i c and v i r t u a l l y a l l o f t h e experimental data i n d i c a t e t h a t t h i s t r a n s f e r occurs i n t i m e s o f t h e o r d e r o f 1 0 - l 0 sec,
o r less.
I n fact,
Svantesson e t a l . (1971) found t h a t i n s i l i c o n t h e p u l s e w i d t h and shape o f t h e recombination r a d i a t i o n i n t h e r e g i o n around 1.1 eV ( i n d i r e c t band gap o f s i l i c o n ) t r a c k e d t h e 30-nsec e x c i t a t i o n p u l s e almost i d e n t i c a l l y , except f o r a very l o w - i n t e n s i t y (<<1% o f primary) component which l a s t e d f o r times o f t h e o r d e r o f a few microseconds. From t h e decay c h a r a c t e r i s t i c s o f t h e r a d i a t i o n , t h e a u t h o r s conc l u d e d t h a t t h e f a s t component o f t h e recombination was due t o Auger processes. Bloembergen (1979),
von Allmen (1980,
1982) and Brown (1980)
have p r o v i d e d i n f o r m a t i v e overviews o f v a r i o u s aspects o f c a r r i e r l a t t i c e energy r e l a x a t i o n processes i n semiconductors i n connection w i t h l a s e r annealing.
Y o f f a (1980a,
1980b) has g i v e n a d e t a i l e d
treatment o f t h e c a r r i e r - l a t t i c e i n t e r a c t i o n during pulsed l a s e r i r r a d i a t i o n and we w i l l f o l l o w c l o s e l y her work i n t h e d i s c u s s i o n g i v e n here.
A f t e r t h e l a s e r energy has been absorbed by t h e e l e c -
t r o n i c system, t h e f o l l o w i n g processes may occur t o r e d i s t r i b u t e t h e energy:
1) c a r r i e r c o l l i s i o n s ; 2) 3)
plasmon p r o d u c t i o n ; e l e c t r o n - h o l e recombination by an Auger process;
174
R. F. WOOD ET AL. 4)
e l e c t r o n - h o l e c r e a t i o n by impact i o n i z a t i o n ( t h e i n v e r s e of 3 ) ) ; and
5)
phonon emission.
The f i r s t f o u r processes r e s u l t o n l y i n t h e r e d i s t r i b u t i o n o f energy among t h e c a r r i e r s , w h i l e t h e l a s t process r e s u l t s i n t r a n s f e r o f energy t o t h e l a t t i c e , t h u s r a i s i n g i t s temperature. Y o f f a considered a model laser-anneal i n g experiment i n which a p u l s e o f 0.532-pm (2.3 eV) r a d i a t i o n w i t h energy d e n s i t y o f 1 J/cm2 and d u r a t i o n o f 10 nsec i s i n c i d e n t on a sample i n which t h e d e n s i t y o f c a r r i e r s due t o doping i s n e g l i g i b l e compared t o t h e d e n s i t y due t o photoexcitation.
l o 5 cm-l,
Assuming a r e f l e c t i v i t y o f 0.50 and an a o f
a p p r o x i m a t e l y 1031 cm-3 sec-1 c a r r i e r s a r e e x c i t e d by
the laser pulse i n a layer
cm t h i c k .
t h e generation o f c a r r i e r densities, t h e 10-nsec pulse.
-
T h i s corresponds t o
Ne, o f
1019
during
For Ne > 1 0 l 8 c ~ n - ~ Yoffa , found t h a t energy
r e l a x a t i o n by c a r r i e r c o l l i s i o n s and plasmon p r o d u c t i o n dominates r e l a x a t i o n by phonon emission.
Therefore, t h e c a r r i e r system w i l l
reach i n t e r n a l thermal e q u i l i b r i u m b e f o r e any s i g n i f i c a n t amount o f energy i s t r a n s f e r r e d t o t h e l a t t i c e . 10-14 sec,
I n times o f t h e order o f
an e q u i l i b r i u m thermal d i s t r i b u t i o n o f c a r r i e r s ( b o t h
e l e c t r o n s and h o l e s ) c h a r a c t e r i s t i c o f a temperature much h i g h e r t h a n t h a t o f t h e l a t t i c e i s achieved. Auger and impact i o n i z a t i o n a c t t o change Ne b u t do n o t e x t r a c t energy f r o m t h e c a r r i e r system.
An Auger process (see, f o r example,
B e a t t i e and Landsberg, 1958) i s t h e d e s t r u c t i o n o f an e l e c t r o n - h o l e p a i r w i t h t h e simultaneous t r a n s f e r e n c e o f t h e energy i n v o l v e d t o a n o t h e r e l e c t r o n i n t h e c o n d u c t i o n band.
Impact i o n i z a t i o n i s t h e
i n v e r s e process i n which a s i n g l e e l e c t r o n c r e a t e s an e l e c t r o n hole pair.
Both processes a r e t h i r d o r d e r i n t h e c a r r i e r d e n s i t y
and hence t h e i r importance i n c r e a s e s r a p i d l y w i t h i n c r e a s i n g Ne. However, energy c o n s e r v a t i o n f a v o r s Auger processes because t h e minimum energy r e q u i r e d f o r e l e c t r o n - h o l e t h e band-gap energy,
recombination i s j u s t
whereas an e l e c t r o n i n t h e c o n d u c t i o n band
175
4. MELTING MODEL OF PULSED LASER PROCESSING
must have an energy o f a t l e a s t t w i c e Eg i n o r d e r t o c r e a t e an additional electron-hole pair. Y o f f a c a l c u l a t e d t h e phonon emission r a t e t o be
-
sec a t
t h e c a r r i e r d e n s i t i e s r e a l i z e d i n h e r "standard" pulse.
However,
she c a u t i o n e d t h a t r e l i a b l e e s t i m a t e s of t h e phonon emission r a t e
w i l l depend c r i t i c a l l y on t h e e l e c t r o n i c band s t r u c t u r e .
Factors
such as t h e degree o f amorphization of t h e l a t t i c e , r a p i d atomic rearrangements, etc. can have l a r g e e f f e c t s which a r e d i f f i c u l t t o determine.
Also, l a r g e temperature-dependent band s t r u c t u r e e f f e c t s
become i m p o r t a n t i f s i g n i f i c a n t l a t t i c e h e a t i n g occurs d u r i n g t h e pulse.
By e q u a t i n g t h e r a t e a t which energy i s g i v e n t o plasmons
t o t h e r a t e a t which energy leaves by phonon emission and c a r r i e r d i f f u s i o n (discussed below), Yoffa e s t i m a t e d t h a t t h e steady s t a t e e l e c t r o n d e n s i t y d u r i n g her standard p u l s e would be
- 1020 c ~ n - ~ .
Y o f f a ' s t r e a t m e n t gives v a l u a b l e i n s i g h t s i n t o t h e processes occurr i n g as a r e s u l t o f t h e a b s o r p t i o n o f i n t e n s e l a s e r pulses; however, t h e c o m p l e x i t y o f t h e problem i s such t h a t t h e d e t a i l s o f h e r conc l u s i o n s s h o u l d be t r e a t e d w i t h c a u t i o n .
Nevertheless,
those
c o n c l u s i o n s a r e c o n s i s t e n t w i t h experimental o b s e r v a t i o n s b e f o r e and a f t e r t h e advent o f p u l s e d l a s e r annealing, and c o n f i r m t h a t i n t h e nanosecond regime,
t h e energy i s t r a n s f e r r e d f r o m t h e
c a r r i e r system t o t h e l a t t i c e i n t i m e s s h o r t compared t o t h e l a s e r p u l s e d u r a t i o n , as has been s o f r e q u e n t l y assumed. Bloembergen e t a1 ation rates o f
-
.
(1982) have argued t h a t a t t h e c a r r i e r gener~
m s- e c~- l expected f o r picosecond pulses,
t h e t h e r m a l i z a t i o n t i m e f o r t h e c a r r i e r system should be even l e s s than
sec.
Since t h e Auger recombination r a t e i n c r e a s e s as
t h e t h i r d power i n t h e c a r r i e r d e n s i t y , rapidly,
i t a l s o should i n c r e a s e
a l t h o u g h s c r e e n i n g e f f e c t s (Haug,
1978) are expected t o
s e t i n a t c a r r i e r d e n s i t i e s o f approximately 1021 cm-3.
Bloembergen
e t a l . e s t i m a t e d t h a t d u r i n g i r r a d i a t i o n w i t h a 0.2-J/cm2,
20-psec
p u l s e t h e c a r r i e r d e n s i t y should n o t exceed a p p r o x i m a t e l y 2 x 1021 CI~'~,
corresponding t o r o u g h l y 4% o f t h e s i l i c o n atoms b e i n g i n an
e x c i t e d s t a t e before m e l t i n g s e t s i n .
From t h e r e s u l t s o f a number
176
R. F. WOOD E T A L .
o f experiments i n t h e picosecond regime, Bloembergen e t a l . conc l u d e d t h a t t h e phonon-emission r a t e i n t h i s regime i s a l s o o f t h e s e c - l , so t h a t thermal e q u i l i b r i u m between e l e c t r o n s
order o f
and phonons i s e s t a b l i s h e d i n l e s s than
-
10 psec.
R e s u l t s o f pump-and-probe experiments i n t h e p i c o - and femtosecond regime r e p o r t e d i n t h e l a t t e r p a r t o f 1982 and e a r l y i n 1983 have g r e a t l y c l a r i f i e d questions concerning t h e r o l e o f t h e e l e c t r o n h o l e plasma and t h e t i m e f o r energy t r a n s f e r from t h e c a r r i e r system t o the l a t t i c e i n silicon.
(1982) used -20-psec
L i u e t al.
pulses
of b o t h 532-nm and 266-nm l i g h t t o demonstrate t h a t t h e r e i s a small drop i n r e f l e c t i v i t y a t f l u e n c e s below t h e m e l t i n g t h r e s h o l d . T h i s decrease i n t h e r e f l e c t i v i t y i s t h e expected consequence o f t h e f o r m a t i o n o f a dense e l e c t r o n - h o l e plasma.
They concluded t h a t
a t h i g h e r energy d e n s i t i e s m e l t i n g occurs w i t h i n t h e d u r a t i o n o f t h e 20-psec pulse.
F o r a 0.5 wn t h i c k s i l i c o n l a y e r on a s a p p h i r e
s u b s t r a t e (SUS), t h e y e s t i m a t e d t h a t a t a pump f l u e n c e o f 0.1 J/cm2 more t h a n 80% o f t h e absorbed energy i s t r a n s f e r r e d t o t h e l a t t i c e
-
d u r i n g t h e pulse, w i t h t h e remainder s t o r e d i n an e l e c t r o n - h o l e plasma o f d e n s i t y
2-5 x
a t i m e s c a l e o f 10 psec.
lo2*
cm-3; m e l t i n g e v i d e n t l y occurred on
Using 25-psec p u l s e s o f 532-nm l i g h t ,
von der L i n d e and F a b r i c i u s (1982) were a l s o a b l e t o r e s o l v e t h e i n i t i a l plasma-related r e f l e c t i v i t y dip,
which f o r a 0.35
J/cm2
p u l s e reached a minimum about 20 psec b e f o r e t h e peak o f t h e pump pulse.
The f a s t recovery o f t h e h i g h - r e f l e c t i v i t y phase a f t e r t h i s
i n i t i a l minimum i n d i c a t e s t h a t t h e plasma decays i n t i m e s s h o r t e r t h a n 25 psec.
Shank e t a l . (1983) extended t h i s t y p e o f experiment
i n t o t h e femtosecond range u s i n g 90-fsec e x c i t a t i o n p u l s e s o f 620-nm wavelength l i g h t and were a b l e t o study i n some d e t a i l t h e r e f l e c t i v i t y decrease due t o t h e f o r m a t i o n o f t h e e l e c t r o n - h o l e plasma, f o l l o w e d by t h e r i s e due t o m e l t i n g .
A t and above t h e t h r e s h o l d
energy d e n s i t y f o r m e l t i n g , t h e r e f l e c t i v i t y r i s e s t o i t s maximum v a l u e i n a t i m e o f approximately 5 psec even f o r a probe p u l s e o f
-
1-wn wavelength. Apparently, m e l t i n g occurs a f t e r t h e h e a t i n g p u l s e i s over, b u t w i t h i n a t i m e o f 5 psec a f t e r i n i t i a t i o n o f
177
4. MELTING MODEL OF PULSED LASER PROCESSING t h e pulse.
There i s some evidence f o r t h e s c r e e n i n g o f Auger recom-
b i n a t i o n and f o r c a r r i e r d i f f u s i o n on t h e low picosecond t i m e scale. Although t h e r e a r e s t i l l some m i n o r d i s c r e p a n c i e s between t h e work o f v a r i o u s groups, i t would appear t h a t t h e sequence o f events, on a subnanosecond t i m e scale, i n i t i a t e d by an i n t e n s e l a s e r p u l s e o f e n e r g e t i c photons i s reasonably c l e a r .
These events a r e i l l u s -
t r a t e d s c h e m a t i c a l l y on Fig. 1, which i s adapted f r o m von Allmen The i n i t i a l events i n t h e a b s o r p t i o n process a r e band-to-
(1980).
band e l e c t r o n i c t r a n s i t i o n s i n d i c a t e d by event a on t h e f i g u r e .
A small f r a c t i o n o f t h e absorbed energy may be t r a n s f e r r e d t o t h e l a t t i c e by phonon emission, b u t t h e more l i k e l y events a r e f r e e c a r r i e r a b s o r p t i o n by t h e e x c i t e d e l e c t r o n s ( b and c ) f o l l o w e d by electron-electron collisions.
These c o l l i s i o n s produce a thermal
e q u i l i b r i u m d i s t r i b u t i o n about t h e energy kTe (cross-hatched area
on diagram), w i t h Te much g r e a t e r t h a n t h e l a t t i c e temperature,
-
a BAND-TO-BAND ABSORPTION
b,c FREE CARRIER ABSORPTION
---)
-.-t
Te
Fig.
1.
ELECTRON-PHONON ( T = ELECTRON-ELECTRON ( T =
sec)
AUGER AND IMPACT IONIZATION CARRIER TEMPERATURE
Schematic illustration o f the elementary photoexcitation processes,
c a r r i e r equilibration mechanisms, and electron-phonon conductor exposed to laser radiation with h u
(1980).
sec)
> Eg.
interactions in a semi-
Adapted from von Allmen
178
R. F. WOOD ET A L
i n t i m e s o f t h e o r d e r o f 10-14 sec.
The e l e c t r o n and h o l e popula-
t i o n s a r e brought i n t o thermal e q u i l i b r i u m w i t h one another a t about t h e same t i m e t h r o u g h impact i o n i z a t i o n and Auger recombination.
The schematic i l l u s t r a t i o n o f t h e s e events on t h e f i g u r e
i s meant t o i n d i c a t e t h a t t h e r e i s t r a n s f e r o f energy between t h e e l e c t r o n s and h o l e s w i t h o u t any d i s s i p a t i o n o f t h e energy o f t h e c a r r i e r system.
Phonon emission on t h e t i m e s c a l e o f 10-12 sec
t r a n s f e r s energy t o t h e l a t t i c e and heats i t t o t h e m e l t i n g p o i n t i n a few picoseconds.
Simultaneously,
the density o f carriers
decays (and t h e r e f o r e t h e plasmon frequency decreases) by way o f Auger recombination.
On a much l o n g e r t i m e scale,
a very low
d e n s i t y o f e l e c t r o n s a t t h e bottom o f t h e conduction band can r a d i a t i v e l y recombine w i t h t h e r e m a i n i n g h o l e s i n t h e valence band, p r o d u c i n g a weak, l o n g - l i v e d emission.
This p i c t u r e c l e a r l y
i m p l i e s t h a t melting-model c a l c u l a t i o n s i n which i t i s assumed t h a t t r a n s f e r o f t h e l a s e r energy from t h e e l e c t r o n i c system t o t h e l a t t i c e occurs i n t i m e s comparable t o , o r l e s s than, t h e p u l s e d u r a t i o n s h o u l d remain v a l i d even i n t h e picosecond regime.
However, t h e
e f f e c t i v e a b s o r p t i o n c o e f f i c i e n t may r e q u i r e some m o d i f i c a t i o n i f c a r r i e r d i f f u s i o n o r confinement e f f e c t s become s i g n i f i c a n t , as we now discuss. 3.
CARRIER DIFFUSION AND CARRIER CONFINEMENT
I n c o n n e c t i o n w i t h t h e general problem o f t h e c a r r i e r - l a t t i c e interaction,
Yoffa
(1980a,
1980b) a l s o considered t h e r o l e o f
c a r r i e r d i f f u s i o n i n determining t h e heating o f t h e l a t t i c e . concluded t h a t t h i s r o l e was i m p o r t a n t ,
and even dominant,
d e t e r m i n i n g t h e temperature r i s e o f t h e l a t t i c e . Y o f f a ' s t r e a t m e n t cons id e r a b l y overestimates
carrier diffusion.
She in
It i s l i k e l y t h a t
t h e importance o f
As we have seen, t h e r e i s good evidence t h a t
t h e l a s e r energy i s t r a n s f e r r e d t o t h e l a t t i c e i n times o f t h e o r d e r o f 10-'2-10-'1
sec f o r both nanosecond and picosecond l a s e r pulses,
i n which case l a r g e temperature g r a d i e n t s w i l l be s e t up i n t h e l a t t i c e d u r i n g t h e l a s e r pulse.
The e f f e c t o f these g r a d i e n t s on
4.
179
MELTING MODEL OF PULSED LASER PROCESSING
t h e band s t r u c t u r e a r e such t h a t confinement, r a t h e r than expansion, o f t h e gas o f c a r r i e r s i s expected.
Brown (1980) has drawn a t t e n -
t i o n t o t h i s problem and p o i n t e d o u t t h a t ,
i n t h e l i m i t when t h e
c a r r i e r s and t h e l a t t i c e a r e i n e q u i l i b r i u m , carrier diffusion.
t h e r e i s no e x t r a
The work o f Combescot (1981) and van D r i e l e t a l .
(1982) supports t h i s argument, a t l e a s t a t c a r r i e r d e n s i t i e s l i k e l y t o be encountered i n t h e nanosecond and h i g h picosecond p u l s e durat i o n ranges.
Since t h e experiments o f Shank e t a l . (1983), i n d i c a t e
t h a t Auger recombination i s suppressed and t h a t c a r r i e r d i f f u s i o n becomes i m p o r t a n t on t h e femtosecond t i m e scale,
it i s d i f f i c u l t
t o see how t h e s e e f f e c t s c o u l d have any s i g n i f i c a n t i n f l u e n c e f o r t h e nanosecond 1aser p u l ses g e n e r a l l y employed d u r i n g 1aser p r o cessing.
Moreover, as discussed l a t e r , Wood and G i l e s (1981) have
shown t h a t even a s i g n i f i c a n t amount o f c a r r i e r d i f f u s i o n w i l l n o t g r e a t l y change t h e r e s u l t s o f m e l t i n g model c a l c u l a t i o n s i f t h e a b s o r p t i o n c o e f f i c i e n t i s above values o f
4.
-
3 x
lo4
cm-l.
SUMMARY The p r i n c i p a l a b s o r p t i o n mechanisms f o r i n t e n s e l a s e r r a d i a t i o n
i n semiconductors a r e induced a b s o r p t i o n by photogenerated f r e e c a r r i e r s ( m e t a l l i c mechanism), e l e c t r o n - h o l e e x c i t a t i o n by photons w i t h energies above t h e band gap, breakdown i n c r y s t a l l i n e symmetry.
and processes induced by t h e Once t h e l i g h t has been absorbed
by e l e c t r o n i c e x c i t a t i o n s , t h e c a r r i e r s come t o thermal e q u i l i b r i u m among themselves i n t i m e s o f t h e o r d e r o f densities less than
-
lOZ1
sec.
For c a r r i e r
cm-3, Auger recombination r a p i d l y reduces
t h e d e n s i t y o f e x c i t e d c a r r i e r s w h i l e l e a v i n g t h e energy o f t h e c a r r i e r system v i r t u a l l y unchanged.
The energy i n t h e c a r r i e r
system i s t r a n s f e r r e d t o t h e l a t t i c e by way o f phonon emission p r o cesses i n t i m e s o f t h e o r d e r o f
sec.
On a nanosecond t i m e
s c a l e , normal c a r r i e r d i f f u s i o n i s p r o b a b l y suppressed by t h e l a r g e temperature g r a d i e n t s s e t up i n t h e sample d u r i n g t h e l a s e r pulse, and i n f a c t c a r r i e r confinement may even occur.
R. F. WOOD ET AL.
111. 5.
Formulation o f the Melting Model
HEAT CONDUCTION EQUATIONS AND BOUNDARY CONDITIONS I n a l l o f i t s complexity, t h e general problem o f heat f l o w and
phase change i n a m a t e r i a l i r r a d i a t e d w i t h i n t e n s e l a s e r pulses i s intractable.
F o r t u n a t e l y , however, experience has shown t h a t i t
i s u s u a l l y a good approximation t o t r e a t t h e heat conduction problems encountered i n t h e l a s e r p r o c e s s i n g o f semiconductors as one dimensional.
The diameter o f t h e l a s e r beam i s seldom l e s s t h a n
100 pm, w h i l e i n a m a t e r i a l such as s i l i c o n t h e depth i n which s i g n i f i c a n t temperature g r a d i e n t s occur i s l e s s t h a n 10 pm and m e l t i n g w i l l g e n e r a l l y be l i m i t e d t o a p p r o x i m a t e l y 1 pm.
Spatial
inhomogeneities o f t h e energy d e n s i t y i n t h e l a s e r pulse, t o g e t h e r w i t h i n t e r f e r e n c e and d i f f r a c t i o n e f f e c t s a s s o c i a t e d w i t h t h e co-
h e r e n t n a t u r e o f t h e l i g h t , present troublesome problems which, however, can be overcome with r e f i n e d experimental techniques. Also, t h e t i m e s c a l e a s s o c i a t e d w i t h t h e experiments i s so b r i e f t h a t c o n v e c t i o n i n t h e l i q u i d , which c o u l d d e s t r o y t h e o n e - d i m e n s i o n a l i t y o f t h e problem, i s unimportant.
It i s o n l y when l a s e r i r r a d i a t i o n
o f h e a v i l y doped m a t e r i a l s w i t h low dopant s e g r e g a t i o n c o e f f i c i e n t s a r e s t u d i e d t h a t c l e a r evidence f o r t h e breakdown i n t h e onedimensional c h a r a c t e r o f t h e problem i s found,
as discussed i n
Chapter 5. Although t h e one-dimensional
nature o f t h e heat f l o w provides
a major s i m p l i f i c a t i o n , t h e problem we must deal w i t h remains i n h e r e n t l y n o n l i n e a r and i n v o l v e s a moving boundary between t h e s o l i d and l i q u i d m a t e r i a l .
T h i s problem, f i r s t s t u d i e d e x t e n s i v e l y by
S t e f a n and Newman (Carslaw and Jaeger, 1959), has r e c e i v e d a g r e a t deal o f a t t e n t i o n from p h y s i c i s t s and mathematicians (see t h e volume e d i t e d by Wilson e t al., 1978). The q u a n t i t i e s t o be determined a r e t h e temperature d i s t r i b u t i o n and t h e l o c a t i o n o f t h e p l a n a r l i q u i d - s o l i d i n t e r f a c e (assumed t o be a s u r f a c e ) as a f u n c t i o n of time.
The equations which must be s o l v e d a r e t h e heat c o n d u c t i o n
e q u a t i o n s i n t h e l i q u i d and s o l i d , an e q u a t i o n p r o v i d i n g f o r energy
4.
MELTING MODEL OF PULSED LASER PROCESSING
181
c o n s e r v a t i o n across t h e i n t e r f a c e , and a p p r o p r i a t e space and t i m e boundary c o n d i t i o n s .
F o r our p r e s e n t d i s c u s s i o n , t h e d i f f e r e n t i a l
e q u a t i o n f o r heat conduction can be w r i t t e n i n terms o f t h e temperature d i s t r i b u t i o n T(x,t)
as
The heat g e n e r a t i o n f u n c t i o n P ( x , t ) i s determined by t h e i n t e r a c t i o n o f t h e l a s e r r a d i a t i o n w i t h t h e sample and t h e subsequent t r a n s f e r o f t h e energy t o t h e l a t t i c e ,
as discussed above.
diffusion coefficient, or d i f f u s i v i t y
The thermal
D, i s r e l a t e d t o t h e thermal
c o n d u c t i v i t y K , t h e s p e c i f i c heat c, and t h e d e n s i t y p o f t h e sample m a t e r i a l by t h e e q u a t i o n D = K/cp.
D u r i n g p u l s e d l a s e r annealing,
t h e temperature o f t h e sample may be r a i s e d i n a few nanoseconds ( o r even picoseconds) from ambient t o t h e m e l t i n g p o i n t and even through t h e vaporization point, i f t h e l a s e r pulse i s s u f f i c i e n t l y powerful.
Over t h e s e temperature ranges, t h e thermal c o n d u c t i v i t y
and s p e c i f i c heat a r e n o t constant, as can be seen i n Fig. 2; t h e d a t a on Fig. 2 w i l l be discussed i n more d e t a i l below.
Equation (1)
cannot be used when K and c a r e s t r o n g l y temperature dependent and/or when phase changes occur; t h e n a f o r m u l a t i o n of t h e problem based on f i n i t e d i f f e r e n c e s i s r e q u i r e d .
(1) i s r e p l a c e d by a system o f
I n f i n i t e - d i f f e r e n c e form, Eq.
equations d e r i v e d f r o m a heat balance c o n d i t i o n a t each o f a s e t o f p o i n t s o r nodes a l o n g t h e x axis.
F o r t h e j - t h node ( n o t a t a
f r o n t o r back s u r f a c e ) t h i s c o n d i t i o n i s expressed by
T3m .
i s t h e temperature a t t i m e t n o f t h e j+m node immediately
a d j a c e n t t o t h e j - t h node,
jKj+m i s t h e thermal conductance be-
tween p o i n t s j and j+m, C j i s t h e heat c a p a c i t a n c e o f t h e m a t e r i a l associated
w i t h p o i n t j, and P!
t h e l a t t e r material a t time tn.
is
t h e heat g e n e r a t i o n r a t e i n
182
R. F. WOOD ET AL.
0 0 Q
E
1.6
y
1.2
g
1.0
5 v
2 c
a
2 0
0 1
-----
Si Ge GaAs
i.4
0.8 0.6
a E a 0.4 w
I
c J
0.2 0
Fig.
2.
I
0
I 400
I
I
I
I
I
!200 T, TEMPERATURE ("C)
Temperature-dependent
liquid S i , G e , and GaAs.
I
800
I
1600
2000
thermal conductivity o f crystalline and
A quasi-discontinuity in
K
occurs on melting when
the materials properties transform from those o f a semiconductor to those o f a metal.
S t a t i o n a r y boundary c o n d i t i o n s imposed on Eq. ( 2 ) a r e t h e f i n i t e d i f f e r e n c e e q u i v a l e n t s o f t h e expressions
a p p r o p r i a t e t o Eq.
(1).
The f i r s t e q u a t i o n i m p l i e s t h a t no heat
i s l o s t from t h e f r o n t s u r f a c e w h i l e t h e second r e f l e c t s t h e f a c t t h a t t h e sample i s t h i c k enough t o a c t as a good heat sink. p r a c t i c a l reasons,
t h e f i n i t e - d i f f e r e n c e approach w i l l
For
usually
r e q u i r e t h e second o f these c o n d i t i o n s t o be s a t i s f i e d a t a v a l u e o f x l e s s t h a n t h e t h i c k n e s s of t h e sample.
T r i a l c a l c u l a t on s
i n v a r i a b l y showed both r a d i a t i v e and c o n v e c t i v e heat t r a n s f e r
rom
t h e f r o n t s u r f a c e t o be n e g l i g i b l e because o f t h e s h o r t t i m e s i n volved, t h u s v a l i d a t i n g t h e f i r s t boundary c o n d i t i o n .
C a l c u l a t ons
f o r a wide range o f l a s e r p u l s e energy d e n s i t i e s and d u r a t ons
4. MELTING MODEL OF PULSED LASER PROCESSING
183
a l s o e s t a b l i s h e d t h a t an e f f e c t i v e sample t h i c k n e s s o f 25-50 vm i s s u f f i c i e n t f o r most cases l i k e l y t o be encountered i n p r a c t i c e . Boundary c o n d i t i o n s a t t h e moving l i q u i d - s o l i d i n t e r f a c e a r e u s u a l l y chosen t o make t h e temperature o f t h e i n t e r f a c e T i equal t o t h e phase change temperature Tc and t o s a t i s f y t h e e q u a t i o n Lvp = K,GQi
(4)
- KsGsi
I n t h i s equation, L i s t h e l a t e n t heat of m e l t i n g , KQ and Ks are r e s p e c t i v e l y t h e thermal c o n d u c t i v i t i e s i n t h e l i q u i d and s o l i d , G Q i and Gsi a r e t h e corresponding temperature g r a d i e n t s a t t h e
Equation ( 4 ) i s a
i n t e r f a c e , and v i s t h e i n t e r f a c e v e l o c i t y .
statement o f t h e requirement t h a t when t h e i n t e r f a c e moves a d i s t a n c e dx i n a t i m e i n t e r v a l d t , t h e l a t e n t heat t h a t i s l i b e r a t e d must be removed by conduction.
Equations (1) o r ( 2 ) and t h e bound-
a r y c o n d i t i o n s discussed above d e f i n e a moving boundary problem f r e q u e n t l y r e f e r r e d t o as t h e S t e f a n problem.
Although t h e r e q u i r e -
ments t h a t T i = Tc and t h a t Eq. ( 4 ) holds a r e q u i t e reasonable under most circumstances,
i t should be recognized t h a t c o n d i t i o n s can
occur where t h e y a r e n o t v a l i d . a t a regrowth v e l o c i t y o f
F o r example,
i t i s thought t h a t
- 15 m/sec i n a d i r e c t i o n o f s i l i c o n ,
t h e i n t e r f a c i a l u n d e r c o o l i n g i s so g r e a t t h a t t h e temperature may be s e v e r a l hundred degrees below Tc;
t h i s w i l l be discussed i n
Chapter 5. To apply t h e f i n i t e - d i f f e r e n c e f o r m u l a t i o n , t h e p h y s i c a l problem i s approximated by i n t r o d u c i n g t h e s e t o f nodes a l o n g t h e x a x i s and a s s o c i a t i n g w i t h each node a small volume o f m a t e r i a l w i t h p r e s c r i b e d thermal and o p t i c a l p r o p e r t i e s . between a d j a c e n t nodes.
Heat flow occurs o n l y
By choosing t h e increments between t h e
nodal p o i n t s and t h e t i m e steps small enough, t h e s o l u t i o n t o t h e system o f equations y i e l d s an a c c u r a t e approximation t o t h e approp r i a t e d i f f e r e n t i a l equation.
S u i t a b l e space and t i m e increments
can be determined f o r each a p p l i c a t i o n by successive r e d u c t i o n i n t h e increments u n t i l t h e r e i s an a c c e p t a b l y small change i n t h e solution.
184
R. F. WOOD ET AL.
Increments on t h e space and t i m e g r i d s can be chosen t o g i v e s a t i s f a c t o r y r e s u l t s f o r a v a r i e t y o f c l o s e l y r e l a t e d problems; however,
major e x t e n s i o n s o f a model t o r a d i c a l l y d i f f e r e n t con-
d i t i o n s may r e q u i r e a r e d e t e r m i n a t i o n of t h e optimum space and t i m e increments.
These choices u s u a l l y i n v o l v e a compromise be-
tween accuracy and computer time.
Typically,
the calculations
r e p o r t e d l a t e r i n t h i s s e c t i o n used a minimum s p a t i a l increment a t t h e s u r f a c e o f 1 x l o m 6 cm, b u t t h i s increment was reduced t o 2 x
cm f o r problems i n which t h e l a s e r r a d i a t i o n was e n t i r e l y
absorbed w i t h i n t h e f i r s t few hundred angstroms o f t h e surface. Deeper i n t h e m a t e r i a l , where t h e temperature i s s l o w l y v a r y i n g , t h e s p a t i a l increment can be i n c r e a s e d s i g n i f i c a n t l y .
The t i m e
increment was v a r i e d w i t h t h e l a s e r power and/or p u l s e d u r a t i o n and ranged between 2 x
and
seconds,
subject t o t h e
convergence c r i t e r i a d i c t a t e d by t h e numerical techniques used.
6.
PHASE
CHANGES
Since t h e temperature a t each node i s c o n s t a n t l y m o n i t o r e d i n finite-difference
calculations,
t h e programs can be designed t o
determine when t h e m a t e r i a l o f a node i s ready t o undergo a phase change.
D u r i n g a phase change, t h e node's temperature can be main-
t a i n e d a t Tc u n t i l t h e n e t heat c o n t e n t a s s o c i a t e d w i t h t h e phase change equals o r exceeds t h e l a t e n t heat o f t h e nodal m a t e r i a l . A f t e r t h e phase change, t h e node's temperature i s again determined by t h e c o n d u c t i v e heat t r a n s f e r equation. The r a t i o o f t h e node's heat c o n t e n t above t h a t r e q u i r e d t o j u s t reach t h e t r a n s i t i o n temperature t o t h e l a t e n t heat r e q u i r e d f o r t h e phase change i s sometimes c a l l e d t h e t r a n s i t i o n r a t i o o r t h e liquid-solid fraction.
The t r a n s i t i o n r a t i o can be i n t e r p r e t e d
e i t h e r as a measure o f t h e f r a c t i o n o f t h e node's m a t e r i a l t h a t has completed t h e phase change o r as t h e e x t e n t t o which a l l o f t h e node's m a t e r i a l has completed t h e phase change. case, t h e
I n t h e former
nodal volume i s comprised o f r e g i o n s o f s o l i d and l i q u i d
185
4. MELTING MODEL OF PULSED LASER PROCESSING
m a t e r i a l separated by t h e phase i n t e r f a c e , w h i l e i n t h e l a t t e r case, t h e nodal volume c o n t a i n s m a t e r i a l i n a two-phase m i x t u r e r e f e r r e d t o as " s l u s h " o r "mush."
I n a h e a t - t r a n s f e r problem dominated by
conduction from a small r e g i o n i n which heat i s generated o r l a t e n t heat released, t h e former case w i l l e x i s t and t h e t r a n s i t i o n r a t i o can be used as an i n t e r p o l a t i n g parameter t o determine t h e l o c a t i o n o f t h e phase i n t e r f a c e w i t h i n t h e n o d e ' s volume.
This i n t e r p o l a t i o n
t e c h n i q u e i s e a s i l y a p p l i e d t o one-dimensional problems s i n c e t h e t r a n s i t i o n r a t i o i s j u s t equal t o a f r a c t i o n o f t h e node's length. When heat i s generated n e a r l y u n i f o r m l y throughout an extended r e g i o n o f a sample (as w i t h l a s e r r a d i a t i o n o f a low a b s o r p t i o n c o e f f i c i e n t ) i t i s p o s s i b l e f o r t h e m a t e r i a l a t a l l t h e nodes i n t h a t r e g i o n t o reach t h e m e l t i n g temperature a t approximately t h e same t i m e and t o undergo m e l t i n g a t a p p r o x i m a t e l y t h e same r a t e . A d e f i n i t e phase i n t e r f a c e may be d i f f i c u l t t o l o c a t e i n such cases
and t h e e n t i r e r e g i o n can c o n s t i t u t e a t r a n s i t i o n zone. zones are o f t e n observed i n c a l c u l a t i o n s ,
Transition
but w i t h the conditions
u s u a l l y emphasized here t h e y disappear very q u i c k l y a f t e r t h e l a s e r p u l s e has t e r m i n a t e d (see, however, S e c t i o n 1'4.12). 7.
TEMPERATURE-DEPENDENT THERMAL AND OPTICAL PROPERTIES I n l a s e r annealing, t h e heat g e n e r a t i o n r a t e a t each p o i n t i n
t h e sample i s l a r g e l y determined by t h e r e f l e c t i v i t y and t h e e f f e c t i v e optical absorption c o e f f i c i e n t o f t h e material,
t h e energy
t r a n s f e r r a t e t o t h e l a t t i c e , and t h e energy d e n s i t y and p u l s e d u r a t i o n t i m e o f t h e l a s e r pulse.
The f u n c t i o n
Pi
i n Eq. 2 can
be w r i t t e n as n n Py = (1 - R.)F
J
j '
i n which RY and F Y a r e r e f l e c t i v i t y and a b s o r p t i o n f u n c t i o n s r e s p e c t i v e l y f o r t h e j - t h l a y e r a t t i m e tn.
Both
RY and FY
can be
complicated f u n c t i o n s o f t h o s e m a t e r i a l and p h y s i c a l parameters
R . F. WOOD ET AL.
describing t h e j - t h l a y e r a t time tn.
A number o f t h e s e param-
e t e r s show l a r g e changes when t h e m a t e r i a l undergoes a change o f phase.
Thus,
RJ and F!
can change c o n t i n u o u s l y
w i t h t i m e and
d i s t a n c e as t h e m e l t f r o n t advances i n t o t h e sample d u r i n g t h e It w i l l be assumed t h a t c a r r i e r d i f f u s i o n and con-
l a s e r pulse.
finement a r e n o t i m p o r t a n t f a c t o r s ( S e c t i o n 11.3),
and t h e r e f o r e
we w i l l p u t k ( T ) = a(T), where k ( T ) w i l l be r e f e r r e d t o as a h e a t absorption
coefficient
and
a(T)
i s the
temperature-dependent
o p t i c a l absorption coefficient. The i t e r a t i v e procedures used i n most f i n i t e - d i f f e r e n c e programs a l l o w t h e temperature-dependent
o p t i c a l (and t h e r m a l ) p r o p e r t i e s
t o be i n c l u d e d i n a r e l a t i v e l y s t r a i g h t f o r w a r d manner.
A f t e r each
i t e r a t i o n ( o r t i m e s t e p ) an approximate temperature d i s t r i b u t i o n T(x,t)
i s known t h r o u g h o u t t h e sample.
Therefore, u s i n g expressions
such as Eqs. ( 7 ) and (8) o f Chapter 3 , values of k ( T ) and R ( T ) a t any p o i n t i n t h e sample can be determined and used i n t h e next iteration.
To i l l u s t r a t e t h e approach, c o n s i d e r a heat c o n d u c t i o n problem f o r m u l a t e d i n t h e more f a m i l i a r d i f f e r e n t i a l e q u a t i o n form g i v e n by Eq.
(1).
D u r i n g p u l s e d l a s e r annealing,
f u n c t i o n P(x,t)
t h e heat g e n e r a t i o n
d e s c r i b e s how t h e energy o f t h e l a s e r p u l s e i s
d e p o s i t e d i n t h e sample; i t has t h e u n i t s o f W/cm3. t o Eq.
To correspond
( 5 ) , we can w r i t e
P(x,t)
= [l-R(t)]F(~,t)
I [l-R(t)]fl(x,t)f~(t)
.
The r e f l e c t i v i t y R i s a f u n c t i o n o f t i m e because t h e temperature o f t h e s u r f a c e changes w i t h time; i n p r i n c i p l e , i t should a l s o be a f u n c t i o n o f x because t h e temperature g r a d i e n t s may produce i n t e r f e r e n c e e f f e c t s from d i f f e r e n t depths o f t h e n e a r - s u r f a c e r e g i o n . I f i t i s assumed t h a t t h e energy i n c i d e n t on t h e sample a t any
i n s t a n t d u r i n g t h e l a s e r p u l s e i s t r a n s f e r r e d from t h e e l e c t r o n i c system t o t h e l a t t i c e i n t i m e s s h o r t compared t o t h e p u l s e d u r a t i o n , t h e f u n c t i o n f 2 ( t ) should be approximated very w e l l by t h e f u n c t i o n which d e s c r i b e s t h e temporal e v o l u t i o n o f t h e l a s e r pulse.
187
4. MELTING MODEL OF PULSED LASER PROCESSING The f u n c t i o n fl(x,t)
describes t h e s p a t i a l a b s o r p t i o n o f t h e p u l s e
energy; i t may be q u i t e complicated i f l a t t i c e damage, h i g h l y nonuniform concentrations o f impurities, carrier diffusion i s significant.
etc.
a r e present,
or i f
It i s a f u n c t i o n o f t i m e because
o f t h e t i m e dependence o f t h e temperature and phase changes.
Here,
we w i l l t a k e
and w r i t e f o r I ( x , t ) , I ( x , t ) = I, exp(-
t h e l i g h t i n t e n s i t y i n t h e sample, X
1 k(xo,t)dxo) 0
.
The n o r m a l i z a t i o n constant co i s determined from t h e i n t e g r a l F(x,t)dxdt
= co
”
tP
1 [ 1 fl(x,t)dx]f2(t)dt
0
=
0
E,
.
(9)
tp i s t h e t o t a l d u r a t i o n o f t h e p u l s e ( n o t t h e f u l l w i d t h a t h a l f
maximum), a l s o r e c a l l t h a t
E, i s t h e t o t a l p u l s e energy d e n s i t y .
We o b t a i n t
co
=
P
E 11 {I 1 o o
Equations (6-8,
”
1 k(x,t)exp[-
X
1 k ( ~ ~ , t ) d x ~ l d x f ~ ( t ) d t } ~ ~ (10 . 0
10) determine P(x,t)
i n terms o f t h e temperature
dependent h e a t - a b s o r p t i o n c o e f f i c i e n t k ( T ( x,t ) ) , t h e r e f l e c t i v i t y R(T(x=O,t))
and t h e pulse-shape f u n c t i o n f z ( t ) .
I n the f i n i t e -
d i f f e r e n c e f o r m u l a t i o n , t h e choice o f t h e space and t i m e increments
w i l l determine t h e accuracy w i t h which t h e i n t e g r a l s i n Eqs. (8) and (10) can be evaluated.
I n view of t h e l i m i t e d accuracy o f t h e
experimental values o f such q u a n t i t i e s as t h e thermal c o n d u c t i v i t y , s p e c i f i c heat, a b s o r p t i o n c o e f f i c i e n t , etc.,
i t seems l i k e l y t h a t
increments chosen f o r a c c u r a t e s o l u t i o n s o f t h e heat conduction problem w i l l a l s o g i v e e n t i r e l y a c c e p t a b l e approximations t o P(x,t).
188 8.
R. F. WOOD E T A L . INPUT DATA The data on t h e thermal p r o p e r t i e s of t h e sample r e q u i r e d f o r
t h e c a l c u l a t i o n s i n c l u d e l a t e n t heats o f f u s i o n and v a p o r i z a t i o n and t h e corresponding temperatures a t which t h e phase changes occur, thermal c o n d u c t i v i t y , s p e c i f i c heat, d e n s i t y , and t h e i n i t i a l temperature o f the substrate.
R a d i a t i v e and c o n v e c t i v e heat t r a n s f e r
f r o m t h e f r o n t s u r f a c e can be c a l c u l a t e d b u t , as a l r e a d y mentioned, t e s t s i n t h e p a s t have shown t h e s e e f f e c t s t o be e n t i r e l y n e g l i g i b l e f o r pulsed l a s e r heating.
The o p t i c a l d a t a r e q u i r e d c o n s i s t s o f
t h e r e f l e c t i v i t y and a b s o r p t i o n c o e f f i c i e n t o f t h e sample and t h e p u l s e shape, d u r a t i o n , energy d e n s i t y , and wavelength o f t h e l a s e r radiation;
t h e y w i l l be discussed s h o r t l y .
Table I l i s t s t h e
thermal p r o p e r t i e s , t h e symbols and u n i t s used f o r them, and e i t h e r t h e values o r r e f e r e n c e s t o d i s c u s s i o n s i n t h e t e x t .
Data a r e g i v e n
i n t h e t a b l e f o r Si, Ge, and GaAs, a l t h o u g h a l l o f t h e i l l u s t r a t i v e c a l c u l a t i o n s i n t h i s chapter are f o r s i l i c o n ( r e s u l t s o f calculat i o n s on GaAs a r e g i v e n i n Chapter 8).
The p r i m a r y purposes f o r
l i s t i n g and d i s c u s s i n g t h e data on Ge and GaAs a r e t o i n d i c a t e t h e a v a i l a b i l i t y o f t h e data t h a t i s needed f o r c a l c u l a t i o n s on t h e t h r e e semiconductors most o f t e n s t u d i e d i n c o n n e c t i o n w i t h l a s e r p r o c e s s i n g and t o p r o v i d e a comparison of t h e magnitude o f t h e q u a n t i t i e s involved.
G e n e r a l l y speaking, t h e body of d a t a on t h e
thermal p r o p e r t i e s o f Ge i s more e x t e n s i v e t h a n t h a t o f any o t h e r semiconductor, w h i l e i t i s t h e o p t i c a l p r o p e r t i e s o f S i t h a t have been most t h o r o u g h l y i n v e s t i g a t e d .
Although GaAs i s undoubtedly
t h e most f r e q u e n t l y s t u d i e d compound semiconductor, data on i t o f t h e t y p e needed f o r m e l t i n g model c a l c u l a t i o n s i s sparse compared t o t h a t a v a i l a b l e f o r t h e elemental semiconductors (Wood e t al., 1981b). a.
Phase Change Temperatures and L a t e n t Heats The m e l t i n g temperatures o f c r y s t a l l i n e S i , Ge, and GaAs a r e
w e l l e s t a b l i s h e d , b u t v a r i a t i o n s i n t h e r e p o r t e d values o f Lc, Tv, and L v a r e sometimes q u i t e l a r g e .
We b e l i e v e t h a t t h e thermal data
4. MELTING MODEL OF PULSED LASER PROCESSING
189
Table I Thermodynamic Data f o r S i , Ge, and GaAS Quantity
Symbol
Phase change temperature c rysta1 TC amorphous Ta vaporization TV
Units
Value and Comments Si Ge GaAs
"C
L a t e n t heats c r y s t a1 amorphous v a p o r i z a t i on
L LC La LV
Dens it y
P
g/cm3
Thermal c o n d u c t i v i t y c r y st a1 a m r p h ou s 1i q u i d
K
W/cm deg
KC
Speci f ic heat
c
Ka KQ
1410 1150 3267
937 695 2834
1238
1800 1320 16207
508 355 5116
548 2230
2.33
5.24
5.3
Figure 2 -0.13 Figure 2
---
-0.02 J / g deg
---
see d i s c u s s i o n i n t e x t
f o r s i l i c o n and germanium g i v e n i n H u l t g r e n e t a l . sonably r e l i a b l e .
---
1800
(1973) i s rea-
Since t h e c o m p i l a t i o n by t h e s e authors i s t h e
most r e c e n t l y published, t h e r e i s reason t o b e l i e v e t h a t our knowledge o f t h e thermal p r o p e r t i e s o f semiconductors i s g r a d u a l l y improving.
We a r e c o n f i d e n t t h a t t h e r e s u l t s o f p u l s e d l a s e r -
a n n e a l i n g experiments and c a l c u l a t i o n s w i l l h e l p t o f u r t h e r r e f i n e t h e thermal data, e s p e c i a l l y i n t h e molten phase. The n a t u r e o f t h e phase t r a n s i t i o n and t h e m e l t i n g temperatures and l a t e n t heats o f a-Si and a-Ge c o n t i n u e t o be s u b j e c t s o f l i v e l y debate.
Estimates by Bagley and Chen (1978) and Spaepen and T u r n b u l l
(1978), u s i n g data o f Chen and T u r n b u l l (1969), have i n d i c a t e d t h a t t h e m e l t i n g temperature of a-Ge should be
- 240 K l e s s than t h a t o f
c-Ge and t h a t t h e l a t e n t heat of m e l t i n g s h o u l d be f o r c-Ge.
-
70% o f t h e value
S c a l i n g o f t h e a-Ge r e s u l t s by t h e r a t i o Tc(Si)/Tc(Ge)
suggested t h a t T a ( S i ) should be approximately 300 K l e s s t h a n Tc(Si).
R. F. WOOD ET AL.
Some c o r r o b o r a t i o n o f these r e s u l t s i s given by t h e work o f Fan and Anderson (1981) and Baeri e t a l .
(1980),
although both o f these
s t u d i e s used methods t h a t are subject t o l a r g e e r r o r s .
I n contrast
t o these r e s u l t s , Kokorowski e t a l . (1982) and Olson e t a1
. (1983)
have concluded from t h e i r r e s u l t s on cw l a s e r h e a t i n g o f amorphous l a y e r s t h a t t h e d i f f e r e n c e between Ta and Tc i s no more than about
50 K; Knapp and Picraux (1981) a r r i v e d a t a s i m i l a r conclusion from t h e i r experiments on electron-beam regrowth o f amorphous s i 1 icon. Donovan e t a l . (1983) c a r r i e d o u t d i f f e r e n t i a l scanning c a l o r i m e t r y measurements on a-Si l a y e r s produced by i o n i m p l a n t a t i o n o f argon and xenon.
They concluded t h a t Ta = 1147°C and La = 1319 J/g, which
we have rounded o f f t o 1150°C and 1320 J/g i n Table I. We emphasize t h a t t h e values o f Ta and La shown i n Table I should be considered o n l y as reasonable estimates a t t h i s time.
Complicating t h e prob-
lem o f determining accurate values o f these q u a n t i t i e s a r e u n c e r t a i nt i e s about t h e nature o f t h e amorphous s t a t e produced by various techniques.
It i s c l e a r from many experiments t h a t t h e d e n s i t y
and s t r u c t u r e o f amorphous l a y e r s formed by i o n i m p l a n t a t i o n , glow
discharge,electron-beamevaporation,sputtering,andchemical vapor d e p o s i t i o n are not t h e same (see Chapter 3, Section 111.3 f o r a d i s c u s s i o n o f t h e o p t i c a l p r o p e r t i e s o f ion-implanted s i l i c o n ) . Moreover, t h e r e i s some evidence t h a t w i t h some o f t h e measurement techniques t h e s t r u c t u r e o f t h e amorphous l a y e r s may change d u r i n g t h e measurements, thus f u r t h e r c o m p l i c a t i n g t h e i n t e r p r e t a t i o n o f t h e experiments.
O f p a r t i c u l a r i n t e r e s t i n t h i s connection i s t h e
observation o f Fredrickson e t al.
(1982) t h a t two w e l l - d e f i n e d
o p t i c a l s t a t e s o f a-Si s i l i c o n are produced by i o n i m p l a n t a t i o n ; presumably t h e thermal p r o p e r t i e s may a1 so be somewhat d i f f e r e n t f o r t h e two states. b.
Density, Thermal Conductivity, and S p e c i f i c Heat The d e n s i t i e s o f Si , Ge, and most 111-V semiconductors decrease
s l o w l y w i t h temperature i n t h e c r y s t a l 1 ine phase , increase from
5-13% on m e l t i n g ( S i = lo%, Ge = 5%, GaAs = l l X ) , and then decrease
4.
MELTING MODEL OF PULSED LASER PROCESSING
w i t h i n c r e a s i n g T above Tc (Glasov e t al.,
1969).
191
As i n d i c a t e d
above, t h e d e n s i t y o f an amorphous semiconductor depends s t r o n g l y on t h e method used f o r p r e p a r i n g t h e m a t e r i a l . compaction,
i.e.,
I n a-Si of ideal
w i t h o u t v o i d s and i n c l u s i o n s o f i m p u r i t i e s o r
c - S i , t h e d e n s i t y has been r e p o r t e d t o be from 2-101 l e s s t h a n i t i s f o r c-Si.
Although v a r i a t i o n s i n d e n s i t y from one phase t o
another a r e g e n e r a l l y i m p o r t a n t because o f t h e i n s i g h t t h e y p r o v i d e i n t o changes i n chemical bonding, t h e s e v a r i a t i o n s a r e n o t l a r g e enough t o have a s i g n i f i c a n t impact on t h e agreement between c a l c u l a t i o n s and experiments a t t h i s time.
A s t h e experimental mea-
surements a r e r e f i n e d and o t h e r p h y s i c a l parameters a r e determined more p r e c i s e l y , i t may be necessary t o i n c l u d e d e n s i t y changes i n t h e heat f 1ow c a l c u l a t i o n s . Thermal c o n d u c t i v i t i e s o f semiconductors have n o t been accuratel y measured over t h e extended temperature range ( i n c l u d i n g t h e l i q u i d phase) r e q u i r e d i n m e l t i n g model c a l c u l a t i o n s .
The measurements o f
Glassbrenner and Slack (1964) a r e g e n e r a l l y considered t o have p r o v i d e d t h e most a c c u r a t e data a v a i l a b l e on t h e thermal c o n d u c t i v i t y o f c r y s t a l l i n e S i and Ge up t o t h e i r m e l t i n g p o i n t s .
I n f a c t , small
d i f f e r e n c e s between t h e thermal c o n d u c t i v i t y data of Glassbrenner and Slack and o t h e r s e t s o f data (such as t h o s e o f F u l k e r s o n e t al., 1968) do n o t make l a r g e d i f f e r e n c e s i n most o f t h e c a l c u l a t e d r e s u l t s . However, i n e f f o r t s t o o b t a i n t h e b e s t p o s s i b l e agreement between c a l c u l a t e d and measured d u r a t i o n s o f s u r f a c e m e l t i n g o f s i l i c o n Lowndes e t a l . (1983) found t h a t t h e Glassbrenner and Slack conduct i v i t y data p r o v i d e d somewhat b e t t e r agreement t h a n d i d e a r l i e r d a t a g i v e n i n Goldsmith e t a l . (1961).
Amos and Wolfe (1978) have
measured t h e temperature dependence o f t h e thermal c o n d u c t i v i t y o f GaAs up t o a few hundred degrees below t h e m e l t i n g p o i n t ; here t h e i r d a t a was s t r a i g h t f o r w a r d l y e x t r a p o l a t e d t o Tc. More s e r i o u s d i f f i c u l t i e s a r e encountered i n t h e molten s t a t e where, t o o u r knowledge, n e i t h e r t h e magnitude nor t h e temperature dependence o f K f o r any semiconductor has been measured w i t h s u f f i c i e n t accuracy t o be u s e f u l (see,
however, t h e comnents i n Ho
192
R. F. WOOD ET AL.
e t al.,
1974).
In t h e absence o f t h i s i n f o r m a t i o n , m e l t i n g model
c a l c u l a t i o n s have o f t e n used t h e Wiedemann-Franz (W-F) law t o r e l a t e t h e thermal c o n d u c t i v i t y t o t h e e l e c t r i c a l c o n d u c t i v i t y
U,
which
has been measured i n m o l t e n S i , Ge, GaAs, and o t h e r semiconductors j u s t above t h e i r m e l t i n g temperatures by Glasov e t a l . (1969).
This
usage i s based on t h e f a c t t h a t t h e s e semiconductors behave as good m e t a l s i n t h e molten s t a t e and t h e r e f o r e t h e charge and heat c a r r i e r s a r e expected t o be t h e same.
The temperature dependence
of K i n t h e m o l t e n s t a t e can a l s o be e s t i m a t e d i n t h i s way s i n c e t h e W-F law p r o v i d e s t h e expression K = L*uT,
i n which t h e L o r e n t z
number L* i n good metals i s 2.45 x 10-8 Wn/deg* (see K i t t e l , 1971). The work o f Glasov e t a l .
i n d i c a t e s t h a t u decreases very s l o w l y
w i t h T above t h e m e l t i n g p o i n t i n S i and GaAs, and somewhat more r a p i d l y i n Ge.
The thermal c o n d u c t i v i t i e s o f t h e s e t h r e e semi-
conductors a r e shown on Fig. 2 as a f u n c t i o n o f temperature. The thermal c o n d u c t i v i t y o f amorphous semiconductors p l a y s an i m p o r t a n t r o l e i n d e t e r m i n i n g t h e response o f an amorphous l a y e r t o a l a s e r p u l s e (Chapter 6).
U n f o r t u n a t e l y i t has been d i f f i c u l t
t o o b t a i n r e l i a b l e d a t a f o r Ka. d e p o s i t e d on mica s u b s t r a t e s , Ka
P
For 7000-A t h i c k a-Ge f i l m s
Nath and Chopra (1974) found t h a t
0.13 W/cm deg a t room temperature and i n c r e a s e d o n l y s l i g h t l y
a t h i g h e r temperatures.
From Fig. 2 i t can be seen t h a t t h i s v a l u e
i s r a t h e r c l o s e t o t h a t o f Kc near t h e c r y s t a l l i n e m e l t i n g p o i n t . The s i t u a t i o n i s q u i t e d i f f e r e n t f o r s i l i c o n .
Goldsmid e t a l .
(1983) measured Ka a t room temperature f o r a 1.15-pm
thick a-Si
l a y e r d e p o s i t e d on a s a p p h i r e s u b s t r a t e and found a v a l u e o f 0.026 W/cm deg w i t h a r e p o r t e d p r o b a b l e e r r o r o f e t al.
(1984),
2
15%.
Lowndes
i n work discussed i n Chapter 6 and l a t e r i n t h i s
chapter, found Ka = 0.02 W/cm deg which i s i n s a t i s f a c t o r y agreement w i t h t h e r e s u l t s o f Goldsmid e t a l .
E x t e n s i v e comparisons o f
t h e r e s u l t s o f experiments and c a l c u l a t i o n s on t h e p u l s e d l a s e r m e l t i n g of a-Si l a y e r s by Webber e t a l . (1983) gave an e s t i m a t e o f Ka o f
- 0.01
W/cm deg.
4.
193
MELTING MODEL OF PULSED LASER PROCESSING
The temperature dependence o f t h e s p e c i f i c heat o f S i , Ge, and GaAs i s q u i t e modest a f t e r an i n i t i a l sharp i n c r e a s e a t low temperatures, and w i l l n o t be shown here.
The d a t a on c-Si used i n
t h e c a l c u l a t i o n s were t a k e n f r o m Goldsmith e t a l . (1961) and i s i n reasonably good agreement w i t h o t h e r measurements o f t h e s p e c i f i c h e a t (see, f o r example, Shanks e t al.,
1963.; H u l t g r e n e t al.,
S p e c i f i c heat data f o r Ge can be found i n H u l t g r e n e t al., GaAs i n Amos and Wolfe (1978).
1973). and f o r
We do n o t know of r e l i a b l e data f o r
t h e s p e c i f i c heat o f m o l t e n semiconductors and i n a l l o f t h e c a l c u l a t i o n s discussed h e r e i t was assumed t o remain c o n s t a n t a t i t s v a l u e i n t h e s o l i d a t t h e m e l t i n g temperature o r t o decrease by
- 10% from t h a t
value (Hultgren e t al.,
1973).
Chen and T u r n b u l l
(1969) have measured t h e s p e c i f i c heat o f b o t h c- and a-Ge f i l m s ; t h e d i f f e r e n c e was o n l y about 5% between t h e two forms. c.
Laser-Re1 a t e d Parameters The r e f l e c t i v i t y R and t h e a b s o r p t i o n c o e f f i c i e n t a o f c r y s -
t a l l i n e s i l i c o n a r e discussed a t l e n g t h i n Chapter 3.
The r e s u l t s
g i v e n t h e r e were used as a guide i n t h e c h o i c e o f values f o r t h e c a l c u l a t i o n s d e s c r i b e d below, b u t s p e c i f i c values o f R and a ( o r k )
w i l l be discussed as we proceed.
( R e c a l l t h a t a d i f f e r s from k,
t h e "heat a b s o r p t i o n c o e f f i c i e n t " o n l y i f t h e c a r r i e r s c r e a t e d by t h e i n c i d e n t l i g h t d i f f u s e s i g n i f i c a n t l y b e f o r e g i v i n g t h e i r energy t o t h e l a t t i c e ; h e r e we assume t h a t d i f f u s i o n i s n e g l i g i b l e and t h a t
k = a.)
The r e s u l t s o f Brodsky e t a l . (1970) were used as a guide
t o t h e o p t i c a l p r o p e r t i e s o f amorphous s i l i c o n .
We t a k e n o t e a g a i n
o f t h e f a c t t h a t values o f a measured a t low l i g h t i n t e n s i t i e s may n o t be t h e same as t h e values a p p r o p r i a t e under t h e i n t e n s i t i e s used i n p u l s e d l a s e r annealing.
Nonlinear e f f e c t s i n t h e i n t e n s i t y ( I )
t h a t do occur w i l l presumably always a c t t o i n c r e a s e k over t h e values a p p r o p r i a t e a t low l i g h t l e v e l s .
The remaining l a s e r - r e l a t e d
parameter i s t h e temporal p u l s e shape, f o r t h e i n d i v i d u a l cases considered.
and i t t o o w i l l be given
The l a s e r - r e l a t e d parameters
and t h e i r n o t a t i o n a r e sumnarized i n T a b l e 11.
194
R. F. WOOD ETAL. TABLE I1
O p t i c a l and Laser-Related Parameters f o r S i l i c o n C a l c u l a t i o n s ~
~~~
~
~
Quantity
Symbol
Units
a,k
cm-1
Absorption c o e f f i c i e n t c r y sta1 amorphous 1i q u i d
aC
IV. 9.
x-
T-,
A- T-,
aa
all
R
Ref 1e c t iv i t y c r y st a 1 amo rp hous 1i q u i d
%
RC
Ra Rll
Laser p u l s e d u r a t i o n (FWHM) shape energy d e n s i t y
Comment s
nsec
T,t,
E,
J/cm2
and I-dependent and I-dependent 106
and T-dependent Ra = R c 0.70 f o r most c a l c u l a t i o n s A-
varied discussed i n t e x t varied
Results o f Melting Model Calculations in Silicon
CRYSTALLINE
AND AMORPHOUS MODELS
A p p l i c a t i o n s o f t h e m e l t i n g model d e s c r i b e d i n t h e preceding s e c t i o n a r e complicated by t h e wide v a r i a t i o n s i n t h e p h y s i c a l prope r t i e s o f semiconductors which can be produced by doping, implantation, heating, etc.
ion
For convenience o f d i s c u s s i o n , a major
d i s t i n c t i o n w i l l be made between c r y s t a l l i n e - and amorphous-model calculations.
However, such a c l e a r c u t d i s t i n c t i o n ,
based on t h e
presence o r absence o f long-range c r y s t a l l i n e o r d e r i n a l l o r p a r t o f t h e sample,
does n o t adequately cover a l l t h e cases t h a t can
a r i s e i n p r a c t i c e , as t h e f o l l o w i n g d i s c u s s i o n w i l l make c l e a r . a.
C r y s t a l l i n e Models I n many model c a l c u l a t i o n s r e p o r t e d i n t h e l i t e r a t u r e ,
the
a b s o r p t i o n c o e f f i c i e n t was assumed t o have some average, constant (temperature-independent) v a l u e t h r o u g h o u t t h e sample.
The use of
195
4. MELTING MODEL OF PULSED LASER PROCESSING
such an approximation o r i g i n a l l y r e f l e c t e d a l a c k o f knowledge o f
1) t h e temperature dependence o f t h e o p t i c a l p r o p e r t i e s o f s i l i c o n and o t h e r semiconductors a t e l e v a t e d temperatures, 2) t h e change i n t h e o p t i c a l p r o p e r t i e s due t o moderate l a t t i c e damage (produced f o r example by i o n i m p l a n t a t i o n o f l i g h t i o n s such as boron), 3) t h e effects dopants,
of
various
c o n c e n t r a t i o n s and t y p e s o f s u b s t i t u t i o n a l
and 4) t h e dependence o f t h e a b s o r p t i o n c o e f f i c i e n t on
the l i g h t intensity.
The work o f J e l l i s o n and Modine (1982,1983)
d e s c r i b e d i n Chapter 3 has added g r e a t l y t o o u r knowledge o f t h e temperature-dependent
optical properties o f c r y s t a l l i n e s i l i c o n ;
comparable data f o r o t h e r semiconductors i s n o t y e t a v a i l a b l e .
A
l i m i t e d amount of i n f o r m a t i o n on t h e e f f e c t s o f l a t t i c e damage on t h e o p t i c a l p r o p e r t i e s i s now a v a i l a b l e ,
b u t t h e wide range o f
dopant species and c o n c e n t r a t i o n s t h a t may be p r e s e n t i n samples used f o r s t u d i e s o f l a s e r p r o c e s s i n g make i t i m p r a c t i c a l t o measure t h e temperature dependence o f t h e o p t i c a l p r o p e r t i e s f o r each new s i t u a t i o n as i t a r i s e s (see Chapter 3, S e c t i o n 11.3).
I t i s known
t h a t t h e i n t e n s i t y dependence o f t h e a b s o r p t i o n c o e f f i c i e n t f o r wavelengths below o r very near t h e band gap i s an i m p o r t a n t e f f e c t (discussed i n Chapter 9), b u t l i t t l e i s known about i t s importance f o r wavelengths i n t h e r e g i o n above t h e band gap.
O f course, t h e r e
a r e cases when t h e use of a s i n g l e constant v a l u e o f a i s an e n t i r e l y acceptable approximation.
As shown i n Chapter 3, t h e o p t i c a l
p r o p e r t i e s f o r l a s e r s o p e r a t i n g a t wavelengths l e s s than show p r a c t i c a l l y no temperature dependence whatsoever. t h e value o f a i s so h i g h
(2
106 cm-1)
- 360 nm
Moreover,
a t t h e s e wavelengths t h a t
heavy doping, l a t t i c e damage, and even complete amorphization a r e u n l i k e l y t o s i g n i f i c a n t l y change it. Once
re1 i a b l e e s t i m a t e s o f t h e temperature-dependent b e h a v i o r
of r e f l e c t i v i t i e s and a b s o r p t i o n c o e f f i c i e n t s a t many d i f f e r e n t wavelengths became a v a i l a b l e ,
t h e y were i n c l u d e d i n t h e m e l t i n g
model c a l c u l a t i o n s i n t h e manner d e s c r i b e d i n S e c t i o n 111.7.
In
f a c t , t h e disagreement between m e l t i n g model c a l c u l a t i o n s based on a constant a b s o r p t i o n c o e f f i c i e n t and t h e d a t a which was b e g i n n i n g
196
R. F. WOOD ET AL.
t o accumulate on t h e d u r a t i o n o f s u r f a c e m e l t i n g d u r i n g p u l s e d ruby l a s e r i r r a d i a t i o n (see Chapter 6 ) was one o f t h e main m o t i v a t i o n s f o r c a r r y i n g o u t t h e measurements o f temperature-dependent o p t i c a l properties.
The reasonably good agreement between t h e r e s u l t s o f
t i m e - r e s o l v e d r e f l e c t i v i t y and t r a n s m i s s i v i t y experiments and t h e me1t i ng model ca 1cu 1a t ions wh ich i nc 1uded t emperat u re-dependent o p t i c a l p r o p e r t i e s , was a c l e a r i n d i c a t i o n o f t h e need t o t r e a t t h e o p t i c a l p r o p e r t i e s i n a more s a t i s f a c t o r y way t h a n t h a t d e s c r i b e d i n t h e p r e c e d i n g paragraph. t i o n s w i t h c o n s t a n t a and temperature-dependent
For s i m p l i c i t y ,
both t h e c a l c u l a -
R i n t h e s o l i d and t h e c a l c u l a t i o n s w i t h
o p t i c a l p r o p e r t i e s w i l l be r e f e r r e d t o h e r e
as c-model c a l c u l a t i o n s .
A l l such c a l c u l a t i o n s w i l l have i n common
t h e use o f t h e thermal p r o p e r t i e s ( l a t e n t heat, m e l t i n g temperature, thermal c o n d u c t i v i t y ) o f c r y s t a l l i n e s i l i c o n . b.
Amorphous Models C a l c u l a t i o n s on samples w i t h s u f f i c i e n t l y t h i c k amorphous l a y e r s
a r e somewhat s i m p l i f i e d ,
i n so f a r as t h e o p t i c a l p r o p e r t i e s a r e
concerned, because t h e a b s o r p t i o n c o e f f i c i e n t o f t h e amorphous l a y e r i s an o r d e r o f magnitude o r more g r e a t e r t h a n t h e c r y s t a l l i n e value and i s p r o b a b l y l e s s temperature- and intensity-dependent. example, t h e a b s o r p t i o n c o e f f i c i e n t a t X = 0.694 wn i s i n c r y s t a l l i n e and
- 5x104 cm-1
For
- 2 . 5 ~ 1 0 cm-1 ~
i n amorphous s i l i c o n a t 2OOC.
For
a b s o r p t i o n c o e f f i c i e n t s t h i s l a r g e , t h e temperature dependence o f t h e o p t i c a l p r o p e r t i e s should n o t p l a y as i m p o r t a n t r o l e as i t does i n c r y s t a l l i n e material.
However, t h e e f f e c t s o f l i g h t i n t e n s i t y
on t h e a b s o r p t i o n o f amorphous m a t e r i a l s may s t i l l be s i g n i f i c a n t . Hence, f o r t h i c k , u n i f o r m l y amorphous l a y e r s , i n which a l l of t h e l i g h t i s absorbed i n t h e amorphous l a y e r , a s i n g l e constant value o f a i s p r o b a b l y again a reasonable approximation. layer i s thin,
I f t h e amorphous
t h e d i f f e r e n t a b s o r p t i o n i n t h e c r y s t a l l i n e and
amorphous m a t e r i a l s should i n p r i n c i p l e be taken i n t o account, b u t
197
4. MELTING MODEL OF PULSED LASER PROCESSING
modeling o f such a s i t u a t i o n becomes q u i t e complex, e s p e c i a l l y s i n c e changes i n t h e thermal p r o p e r t i e s a t t h e a-c i n t e r f a c e must a l s o be included. The c-model i s used h e r e i n c a l c u l a t i o n s on undoped, c r y s t a l l i n e m a t e r i a l and temperature-dependent c o e f f i c i e n t s a r e o f t e n used.
r e f l e c t i v i t i e s and a b s o r p t i o n
The a-model i s used o n l y when a w e l l -
d e f i n e d amorphous l a y e r i s present, and t h e presence o f such a l a y e r i s i n d i c a t e d much more s t r o n g l y by t h e a l t e r e d thermal p r o p e r t i e s t h a n by t h e o p t i c a l p r o p e r t i e s .
The r e s u l t s discussed i n t h i s
s e c t i o n were chosen p r i m a r i l y t o i l l u s t r a t e s a l i e n t f e a t u r e s o f t h e m e l t i n g model c a l c u l a t i o n s and t h e y do n o t n e c e s s a r i l y r e p r e s e n t t h e most r e c e n t o r most a c c u r a t e f i t s t o t h e r e s u l t s o f p a r t i c u l a r experiments. 10.
CALCULATIONS FOR RUBY AND FREQUENCY-DOUBLED Nd LASERS I n t h i s subsection, we i l l u s t r a t e some of t h e more i m p o r t a n t
r e s u l t s o b t a i n e d from t h e m e l t i n g model by c o n s i d e r i n g c a l c u l a t i o n s f o r c r y s t a l 1ine s i 1 icon ir r a d i a t e d w i t h p u l ses f r o m frequencydoubled YAG ( A = 532 nm) and ruby ( A = 694 nm) l a s e r s .
I n these
wavelength ranges t h e o p t i c a l p r o p e r t i e s a r e known t o be s t r o n g l y temperature-dependent
(Chapter 3) and t o o b t a i n s a t i s f a c t o r y agree-
ment w i t h t i m e - r e s o l v e d r e f l e c t i v i t y and t r a n s m i s s i o n experiments t h i s temperature dependence should be included.
However, t h e pos-
s i b i l i t y t h a t t h e a b s o r p t i o n i s a l s o i n t e n s i t y dependent should be k e p t i n mind. F i g u r e 3 shows t h e temperature as a f u n c t i o n o f d i s t a n c e from t h e f r o n t s u r f a c e o f a sample i r r a d i a t e d w i t h a p u l s e o f energy density
E,
= 1.2
J/cm2 and d u r a t i o n T~ = 18 nsec (FWHM).
The
a b s o r p t i o n c o e f f i c i e n t and r e f l e c t i v i t y i n t h e s o l i d were taken t o be uC = 5x105 cm-1 and R c = Ro + 5x10-5 calculation.
T, r e s p e c t i v e l y f o r t h i s
When t h e s u r f a c e melted, t h e r e f l e c t i v i t y was i n -
creased t o 0.70 and u t o 106 cm-1.
The break i n some o f t h e curves
a t T = 141OOC g i v e s t h e p o s i t i o n o f t h e l i q u i d - s o l i d i n t e r f a c e , o r
198
R. F. WOOD ETAL.
-
I
I
I
- ..
Fig.
3.
I
I
I
I
Xa = 532 nrn 2 El = 4.2 J/crn
4800 2...,
I
................
I
TIME (nsec)
12
-
-
Temperature as a function of depth in silicon during and a f t e r
irradiation with a 18-nsec a frequency-doubled YAG
~
1 .2-J / c m 2 pulse representative o f the pulses from nm) laser.
( A = 532
m e l t f r o n t , a t t h e t i m e f o r which t h e curve i s drawn.
From t h e
r e s u l t s o f a s e r i e s o f c a l c u l a t i o n s l i k e t h o s e represented by Fig. 3, t h e p o s i t i o n o f t h e m e l t f r o n t as a f u n c t i o n o f t i m e f o r a g i v e n p u l s e energy d e n s i t y can be determined and p l o t t e d t o y i e l d t h e curves o f Fig. 4.
Also, t h e d e r i v a t i v e w i t h respect t o t i m e a t any
p o i n t on one o f t h e curves o f F i g . 4 g i v e s t h e v e l o c i t y v o f t h e l i q u i d - s o l i d i n t e r f a c e a t t h a t time. values
2
It can be seen t h a t f o r EQ
0.4 J/cm2 t h e m e l t f r o n t p e n e t r a t e s very r a p i d l y i n t o t h e
s o l i d b e f o r e r e a c h i n g i t s maximum p e n e t r a t i o n .
Near t h e maximum
p e n e t r a t i o n , t h e m e l t - f r o n t v e l o c i t y f i r s t drops s h a r p l y , and t h e n changes s i g n , and t h e m e l t f r o n t recedes back t o t h e s u r f a c e a t s e v e r a l meters p e r second.
These are t h e p r i n c i p a l f e a t u r e s o f t h e
m e l t - f r o n t c a l c u l a t i o n s and t h e i r e s s e n t i a l v a l i d i t y i s confirmed by a v a r i e t y o f experiments discussed i n o t h e r chapters o f t h i s volume.
199
4. MELTING MODEL OF PULSED LASER PROCESSING
-
E
0.70 0.60
7 -
3.
$
+ Q [r
0.50
-
..............
Al = 532 nm
2
El ( J / c m 2 )
0.4
= 18 nsec
0.6 -----0.8 .-.-.-
-
--_----
4.0 1.2 4.4 f.6
---- 4.8 -
0.40
z
W
a
+ z
0.30
-
0.20
-
0.40
-
5!
lL
'
3 W 5
-
I
0
Fig. 4. Melt-front position as a function of time a f t e r initiation o f an 18-nsec pulse o f various energy densities from a frequency-doubled YAG laser.
Additional information o f i n t e r e s t
in the interpretation o f
p u l s e d l a s e r m e l t i n g i s i l l u s t r a t e d i n Fig. 5, which g i v e s t h e s u r f a c e temperature Ts of t h e sample as a f u n c t i o n o f t h e t i m e a f t e r t h e b e g i n n i n g of pulses of s e v e r a l d i f f e r e n t energy d e n s i t i e s . see t h a t T,
We can
increases very r a p i d l y u n t i l i t reaches t h e m e l t i n g
temperature a t 1 4 1 0 ° C ,
where i t pauses momentarily u n t i l t h e l a t e n t
heat o f m e l t i n g i s absorbed and t h e n begins t o r i s e again ( i f Eg i s g r e a t enough) t o some maximum value. On c o o l i n g , t h e process i s reversed except t h a t T, f o r long periods o f time.
drops q u i c k l y t o 1 4 1 O o C , where i t remains The reason f o r t h i s behavior of Ts i s
t h a t t h e f o r m u l a t i o n o f t h e heat conduction and phase-change problem used here a l l o w s t h e r e l e a s e o f l a t e n t heat o n l y a t t h e m e l t i n g temperature, and r e q u i r e s t h a t a l l of t h e l a t e n t heat i n any part i c u l a r f i n i t e - d i f f e r e n c e c e l l be g i v e n up b e f o r e t h e temperature o f t h a t c e l l can b e g i n t o decrease.
This i m p l i e s t h a t no under-
c o o l i n g o f t h e l i q u i d below t h e c r y s t a l l i z a t i o n temperature i s
200
R. F. WOOD ET AL.
2000
I
I
I
1
I
I
I
I
I
1900 -
-Yw
-
............... 0.6 -
4700 -
-
4000
--.-.-.- 0.0 --- 1.0 4.2
-
-
1500 -
-
1600
(r
3
2 [r
4400 W
4300 4200 -
(400 0
10
20
30
40
50
TIME
Fig. 5. 18-nsec,
60
70
00
90
400
(nsec)
Surface temperature as a function of time a f t e r the initiation o f 532-nm
laser pulses of d i f f e r e n t energy densities.
allowed, b u t t h i s may n o t be a s a t i s f a c t o r y approximation f o r some s i t u a t i o n s which can a r i s e i n l a s e r a n n e a l i n g (see Chapter 5). We now t u r n t o a b r i e f discusson o f a d d i t i o n a l i l l u s t r a t i v e r e s u l t s t a k e n from e a r l i e r c a l c u l a t i o n s r e l a t e d t o t i m e - r e s o l v e d o p t i c a l experiments w i t h a ruby l a s e r ; t h e d e t a i l s o f t h e e x p e r i ments and c a l c u l a t i o n s are g i v e n i n a paper by Lowndes e t a l . (1982a). F i g u r e 6 shows a s e r i e s o f m e l t - f r o n t p r o f i l e s (i.e., t i m e ) which,
when compared t o t h e curves on F i g . 4,
t h a t i r r a d i a t i o n s w i t h 15-nsec p u l s e s a t X p u l s e s a t X = 532 nm g i v e s i m i l a r r e s u l t s . however,
=
p o s i t i o n vs demonstrate
693 nm and 18-nsec It should be noted,
t h a t t h e t h r e s h h o l d f o r m e l t i n g i s s i g n i f i c a n t l y lower
w i t h t h e s h o r t e r wavelength l i g h t , as might be expected from t h e differences i n the absorption coefficients.
F i g u r e s 7a and 7b show
t h e temperature a t v a r i o u s t i m e s a f t e r t h e b e g i n n i n g o f a 1.2-J/cm2 ruby l a s e r p u l s e as a f u n c t i o n o f depth on two d i f f e r e n t depth scales.
We n o t e t h a t f o r depths as s h a l l o w as 5 um t h e temperature
4. MELTING MODEL OF PULSED LASER PROCESSING
201
0.4
f
I
Y
E
.-P 0.3
E
0
Fig. 6.
40
100
0
20
Melt-front
position as a function of time a f t e r the beginning o f a
15-nsec ruby laser pulse.
60 TIME (nsec)
80
The temperature dependences o f the r e f l e c t i v i t y and
absorption coefficient are indicated by Rc and k c , respectively.
does n o t exceed
- 2OOOC a t any time.
Such low temperatures a t such
s h a l l o w depths i s t h e b a s i s f o r r e f e r r i n g t o l a s e r p r o c e s s i n g as " c o l d processing" s i n c e t h e b u l k o f a 200-pm t h i c k s i l i c o n w a f e r
i s r a i s e d o n l y very s l i g h t l y above t h e ambient temperature. 11.
CALCULATIONS FOR ULTRAVIOLET EXCIMER LASERS As discussed i n Chapter 1, rare-gas h a l i d e (RGH) excimer l a s e r s
a r e b e g i n n i n g t o emerge as e x c e l l e n t r a d i a t i o n sources f o r p u l s e d l a s e r p r o c e s s i n g o f semiconductors.
F o r t h e study o f fundamental
problems a s s o c i a t e d w i t h u l t r a r a p i d me1t i n g and s o l i d i f i c a t i o n , excimer l a s e r s p r o v i d e s e v e r a l d e s i r a b l e f e a t u r e s n o t found i n s o l i d s t a t e lasers. XeCl l a s e r i s
F o r example, a o f c-Si a t t h e 308-nm wavelength o f a
2 lo6
cm-'
and v i r t u a l l y independent o f temperature
(Chapter 3, Table I ) ; a s i m i l a r v a l u e h o l d s f o r b o t h a- and a-Si.
202
R. F. WOOD E T A L .
? ]'
1400
0
1200
1000
O -
W
a
2 Q
a
800 600
LU
a
-----K$-
L- -------
2 0 0 ~
0
\s_
--*-------
400
0
, 0.2
1
I
I
0.4
0.6
1
, , , 1.0
0.8
1.2
1.4
1400
-
0 0,
w
a
.................
---------.ns --zoo
1000
100 ns 150
800
tn'
?
ns
600 400 200
-
-
-
0 0
F i g . 7.
-
(b)
1200
1
2 3 4 DEPTH (micrometers)
5
6
a . Temperature as a function o f depth a t various times a f t e r the
beginning o f a 15-nsec pulse from a ruby laser. extended depth scale.
b. Same as Fig. 7a but on an
The results show the rapid temperature fall-off
with
distance f r o m the front surface o f the sample.
Moreover, t h e r e f l e c t i v i t y i s s i m i l a r i n t h e c r y s t a l l i n e , amorphous, and m o l t e n s t a t e s ,
a p p a r e n t l y changing by o n l y
- 10% on m e l t i n g .
Therefore, a t t h e wavelength o f t h e XeCl l a s e r , l a r g e e f f e c t s due t o d i f f e r e n c e s i n o p t i c a l p r o p e r t i e s and t h e i r temperature dependences i n t h e v a r i o u s phases a r e n o t i n t r o d u c e d , so t h a t t h e d i f f e r e n c e s
203
4. MELTING MODEL OF PULSED LASER PROCESSlNG
t h a t occur e x p e r i m e n t a l l y can be a t t r i b u t e d almost e n t i r e l y t o t h e d i f f e r i n g thermal p r o p e r t i e s o f t h e phases. I n t h i s subsection, several aspects o f excimer l a s e r p r o c e s s i n g t h a t have become apparent from a combination o f experimental anneali n g s t u d i e s and s u p p o r t i n g model c a l c u l a t i o n s w i l l be discussed. More s p e c i f i c a l l y , i t w i l l f i r s t be shown how such work can p r o v i d e i n f o r m a t i o n about t h e thermal p r o p e r t i e s o f molten s i l i c o n ; t h e n evidence w i l l be g i v e n t h a t m e l t depths o f
>
1 urn may be a t t a i n a b l e
w i t h excimer l a s e r s w i t h o u t p r o d u c i n g s u r f a c e damage; and f i n a l l y i t w i l l be demonstrated t h a t t h e c a p a b i l i t y o f changing t h e p u l s e shape and d u r a t i o n o f an excimer l a s e r (by changing t h e gas m i x t u r e ) p r e s e n t s some i n t e r e s t i n g p o s s i b i l i t i e s f o r c o n t r o l o f s o l i d i f i c a t i o n r a t e s from t h e molten phase.
a.
I n f o r m a t i o n About t h e Thermal P r o p e r t i e s o f L i q u i d S i l i c o n It was mentioned i n S e c t i o n 111.8 t h a t wide v a r i a t i o n s i n some
o f t h e thermal p r o p e r t i e s o f s i l i c o n a r e found i n t h e l i t e r a t u r e .
In
t h e work d e s c r i b e d i n t h e paper by Wood and G i l e s (1981), t h e vapori z a t i o n temperature and l a t e n t heat o f v a p o r i z a t i o n o f s i l i c o n were t a k e n t o be 2315°C and 2535 c a l / g ,
respectively;
b o t h o f these
values are a t t h e l o w e r end o f t h e ranges o f r e p o r t e d values o f T v and Lv.
Also,
i n Wood and G i l e s t h e thermal c o n d u c t i v i t y o f
l i q u i d s i l i c o n was r e l a t e d t o t h e e l e c t r i c a l c o n d u c t i v i t y by t h e Wiedemann-Franz
law
(Sec.
111.8b),
but f o r
simplicity
KQ was
assumed t o be constant r a t h e r t h a n t o vary l i n e a r l y w i t h T as t h e W-F r e l a t i o n s h i p would r e q u i r e .
When t h e s e assumptions about Tv,
L v , and Kp, were used i n m e l t i n g model c a l c u l a t i o n s , s u r f a c e vapori z a t i o n was p r e d i c t e d t o occur a t
- 2 J/crn2 f o r 15-25-nsec
f r o m ruby and frequency-doubled Nd:YAG l a s e r s .
pulses
Since several exper-
iments s t r o n g l y i n d i c a t e d t h a t damage d i d indeed occur a t about t h e s e energy d e n s i t i e s ( a l t h o u g h o t h e r experiments suggested somewhat h i g h e r t h r e s h o l d s ) , t h e assumptions seemed t o be s a t i s f a c t o r y . However, when m e l t i n g model c a l c u l a t i o n s were c a r r i e d o u t i n support o f t h e f i r s t d e t a i l e d experiments on t h e l a s e r a n n e a l i n g o f s i l i c o n
204
R. E WOOD ET AL.
w i t h a XeCl l a s e r (Lowndes, e t al.,
1982b; Young e t al.,
1982), i t
became apparent t h a t t h e c a l c u l a t i o n s were p r e d i c t i n g damage a t much lower energy d e n s i t i e s t h a n those a t which damage was a c t u a l l y By r a i s i n g Tv and L v t o t h e upper values o f t h e i r r e p o r t e d
observed.
KQ,
ranges and by i n c l u d i n g t h e l i n e a r T-dependence o f
i t was pos-
s i b l e t o suppress t h e onset o f v a p o r i z a t i o n t o t h e e x t e n t t h a t reasonably good agreement w i t h experiment c o u l d be obtained. F i g u r e 8 shows m e l t - f r o n t p r o f i l e s f o r a XeCl l a s e r o p e r a t i n g w i t h a p u l s e shape which c o u l d be approximated c l o s e l y by t h e t r a p e z o i d shown on t h e f i g u r e .
Even a t an energy d e n s i t y of 4 J/cm2,
v a p o r i z a t i o n o f t h e s u r f a c e d i d n o t occur, w h i l e a m e l t - f r o n t penetration of
>
1 vm and a m e l t d u r a t i o n o f
>
400 nsec were obtained.
The c a l c u l a t i o n s showed t h a t v a p o r i z a t i o n d i d occur a t an energy d e n s i t y o f 5 J/cm2 so t h a t t h e onset o f s u r f a c e damage should p r e sumably o c c u r between 4 and 5 J/cm2. 4.6 i.4
Both Lowndes e t a1
1
n
I-
. (1982b)
-
0 0
a 0.8 t-
z
0
w 0.6 LL I
0.2 0
Fig. 8.
0
50
400
150
300
350
400
450
Melt-front profiles produced by radiation from a XeCl laser with the
indicated pulse shape and duration. J/cm2,
200 250 TIME (nsecl
Surface damage occurs between 4 and 4 . 5
in good agreement with experiment,
as discussed in the text.
205
4. MELTING MODEL OF PULSED LASER PROCESSING and Young e t a l .
(1982) have r e p o r t e d t h a t t h e onset o f s u r f a c e
damage d i d n o t occur u n t i l used d i f f e r e n t
> 4 J/cmZ.
EQ
Even though t h e two s t u d i e s
l a s e r s w i t h d i f f e r e n t p u l s e shapes and d u r a t i o n s ,
c a l c u l a t e d m e l t - f r o n t curves were n o t g r e a t l y d i f f e r e n t . Although a d e f i n i t i v e s t u d y of t h e onset o f damage produced by u l t r a v i o l e t pulses has n o t y e t been c a r r i e d o u t , t h r e e c o n c l u s i o n s f r o m t h e work t h a t has been r e p o r t e d seem c l e a r : I n f o r m a t i o n about t h e thermal p r o p e r t i e s o f l i q u i d semiconduct o r s can be e x t r a c t e d from a c a r e f u l comparison o f t h e r e s u l t s o f experiments and c a l c u l a t i o n s on excimer l a s e r m e l t i n g once the optical properties (especially
RE) a r e w e l l e s t a b l i s h e d .
The s p a t i a l homogeneity o f excimer l a s e r pulses can be made good enough
t h a t energy d e n s i t i e s
>, 4 J / c d
(depending
on
p u l s e shape and d u r a t i o n ) can be used w i t h o u t p r o d u c i n g s u r f a c e damage;
c a l c u l a t i o n s i n d i c a t e t h a t m e l t i n g t o depths
g r e a t e r t h a n 1 pm can t h e n be achieved. As a c o r o l l a r y t o
Z),
i t now seems t h a t t h e beam homogeneity
problems a s s o c i a t e d w i t h s o l i d s t a t e l a s e r s a r e even more severe than previously realized. Comparison o f Nd:YAG, Ruby, and XeCl M e l t - f r o n t P r o f i l e s Although t h e r e a r e s u b s t a n t i a l d i f f e r e n c e s between t h e o p t i c a l p r o p e r t i e s o f s i l i c o n a t t h e wavelengths o f t h e Nd and ruby l a s e r s and t h o s e o f t h e u l t r a v i o l e t l a s e r s , t h e a n n e a l i n g c h a r a c t e r i s t i c s i n terms o f t h e energy d e n s i t y r e q u i r e d t o m e l t t o a c e r t a i n depth a r e r o u g h l y comparable f o r comparable p u l s e d u r a t i o n s .
This i s
i l l u s t r a t e d by comparing t h e m e l t - f r o n t p r o f i l e s o f Fig. 4 (Nd:YAG, 18 nsec) and Fig. 6 (ruby,
c.
15 nsec) w i t h Fig. 8 (XeC1, 41 nsec).
E f f e c t s o f Pulse Shape and D u r a t i o n The temporal p u l s e shape o f excimer l a s e r s , though q u i t e com-
p l i c a t e d a t times, trapezoids.
can g e n e r a l l y be approximated r a t h e r w e l l by
F i g u r e 9 shows m e l t f r o n t s f o r two d i f f e r e n t excimer
l a s e r p u l s e shapes; t h e approximate p u l s e shapes are a l s o shown on
206
R. E WOOD ET AL.
t h e figure.
I t can be seen from t h e f i g u r e t h a t f o r t h e same energy
density, t h e longer pulse duration r e s u l t s i n a substantially shallower m e l t - f r o n t penetration.
However,
t h e d u r a t i o n o f surface
m e l t i n g f o r t h e two d i f f e r e n t p u l s e shapes a t t h e same energy denT h i s w i l l n o t always be t h e case.
s i t y may be q u i t e comparable.
It should be p o s s i b l e , as o t h e r c a l c u l a t i o n s n o t i l l u s t r a t e d here suggest,
t o arrange combinations o f p u l s e energy,
d u r a t i o n , and
shape such t h a t t h e m e l t d u r a t i o n i s g r e a t l y prolonged by a l o n g Under such c o n d i t i o n s , t h e r e t u r n o f t h e melt
" t a i l " on t h e pulse.
f r o n t t o t h e s u r f a c e can be s e n s i t i v e l y c o n t r o l l e d by t h e i n t e n s i t y i n t h e t a i l o f t h e pulse.
As a r e s u l t , w i t h pulses from an appro-
p r i a t e l y designed excimer l a s e r , i t should be p o s s i b l e t o o b t a i n a wide range o f regrowth v e l o c i t i e s w i t h which t o conduct both fundamental and a p p l i e d s t u d i e s o f l a s e r processing. 0.9
I
0.8
2
-
I
1
I
I
XeCl LASER _ _ _ - -25.5 _ nsec ---70.5 nsec
5 a z
I
-
0.7
I
I T- (nsec) 25.5 25.5 25.5 70.5 70.5 70.5
---i.5
3.
$ fa
I
El (J/crn2) i.0 -------1.5 ---2.0
---2.5
0.6
--ZO
-
0.5 0.4
0
a
5 W z:
0.3 0.2 0.1
0
0
20
40
60
100 I 2 0 TIME (nsec)
80
I40
160
(80
200
Fig. 9. Variation o f melt-front penetration with pulse duration and shape (light lines) for two different pulses from a XeCl laser.
4. 12.
207
MELTING MODEL OF PULSED LASER PROCESSING
EFFECTS OF AMORPHOUS LAYERS Recent t i m e - r e s o l ved r e f l e c t i v i t y measurements , t r a n s m i s s i o n
e l e c t r o n microscopy s t u d i e s ,
and model c a l c u l a t i o n s f o r s i l i c o n
samples c o n t a i n i n g amorphous s u r f a c e l a y e r s (Lowndes e t a l . Wood e t al.,
, 1984;
1984) have p r o v i d e d c o n s i d e r a b l e i n s i g h t i n t o t h e r o l e
p l a y e d by amorphous l a y e r s i n t h e l a s e r - a n n e a l i n g process.
The
work, which i s d e s c r i b e d i n d e t a i l i n Chapter 6, e s t a b l i s h e d t h a t t h e thermal c o n d u c t i v i t y o f a - S i magnitude l e s s t h a n t h a t o f c-Si
i s a p p r o x i m a t e l y an o r d e r of (see a l s o Webber e t al.,
1983).
Furthermore, t h e c a l c u l a t i o n s showed t h a t t h e response o f t h e amorphous l a y e r t o t h e a n n e a l i n g l a s e r p u l s e i s determined p r i m a r i l y by t h i s g r e a t l y reduced thermal c o n d u c t i v i t y and t h a t t h e r e d u c t i o n o f Ta and La from Tc and Lc a r e c o m p a r a t i v e l y unimportant.
However,
s i n c e t h e a - S i m e l t s a t Ta, which may be s e v e r a l hundred degrees l a s e r m e l t i n g o f a-Si
lower t h a n Tc,
l a y e r s on c-Si
substrates
p r o v i d e s a unique method f o r f o r m i n g h i g h l y undercooled molten silicon.
The TEM c a r r i e d o u t i n c o n n e c t i o n w i t h t h e t i m e - r e s o l v e d
o p t i c a l measurements show t h a t ift h e m e l t f r o n t does n o t p e n e t r a t e t h e a-Si l a y e r , a f i n e - g r a i n e d (FG) p o l y c r y s t a l l i n e ( p ) S i l a y e r i s u s u a l l y formed,
f o l l o w e d by a r e g i o n o f l a r g e - g r a i n e d
e x t e n d i n g t o t h e surface.
(LG) p-Si
The presence o f t h e f i n e - g r a i n e d m a t e r i a l
suggests t h a t homogeneous ( o r heterogeneous) b u l k n u c l e a t i o n has
some T
occurred a t
between Ta and Tc.
5 Tn,
t h e b u l k n u c l e a t i o n temperature,
lying
It i s apparent f r o m t h e s e r e s u l t s t h a t t h e
dynamics o f m e l t i n g and r e s o l i d i f i c a t i o n o f amorphous l a y e r s i s a very complex s u b j e c t , and one t h a t we can n o t hope t o cover i n much d e t a i l here.
Therefore,
we w i l l l i m i t o u r s e l v e s t o a d i s c u s s i o n
o f c e r t a i n aspects o f t h e s u b j e c t f o r which m e l t i n g model c a l c u l a t i o n s may p r o v i d e a t l e a s t some u s e f u l i n s i g h t s . F i g u r e 10 shows a schematic i l l u s t r a t i o n o f t h e cases which a r i s e i n t h e l a s e r m e l t i n g o f a w e l l - d e f i n e d a-Si l a y e r on a c-Si substrate;
t h e amorphous l a y e r may have been formed f o r example
by s e l f - i o n i m p l a n t a t i o n o f S i i n t o S i .
The m e l t i n g temperature
208
R. F. WOOD E T A L .
, a-Si To = 1 1 50°C
C-Si Tc = 1410°C
cose o
cose b
cose c
Fig. 10.
Schematic of the three cases of melt-front
(rnf) penetration that
can arise when amorphous overlayers are present on crystalline substrates.
and thermal c o n d u c t i v i t y i n t h e amorphous and c r y s t a l 1i n e r e g i o n s a r e designated by T and K w i t h a p p r o p r i a t e s u b s c r i p t s . f r o m Table
I,
Ta = 1 1 5 O o C ,
Case a.
F o r example,
F i g . 2, and S e c t i o n I I I . 8 b Tc = 1 4 1 O o C , Kc = K c ( T ) , and Ka = 0.02 W/deg cm.
The p u l s e energy d e n s i t y i s s u f f i c i e n t l y low t h a t t h e
m e l t f r o n t does n o t p e n e t r a t e t h r o u g h t h e a-Si l a y e r .
T h i s means
t h a t a pool o f h i g h l y undercooled molten S i i s formed, separated from t h e c-Si s u b s t r a t e by an a-Si l a y e r o f low thermal c o n d u c t i v i t y .
Case b.
The energy d e n s i t y i s s u f f i c i e n t f o r t h e m e l t f r o n t
t o p e n e t r a t e t o t h e a-c i n t e r f a c e .
The m e l t f r o n t w i l l pause a t
t h e i n t e r f a c e u n t i l t h e heat f l o w adjusts t o t h e d i f f e r e n c e s i n l a t e n t heats,
AL
= Lc-La,
i n t h e a and c regions.
and m e l t i n g temperatures,
AT = Tc-Ta,
Further complicating t h e calculations i s
t h e b e h a v i o r o f t h e thermal c o n d u c t i v i t y which may change by an o r d e r o f magnitude a t t h e a-c i n t e r f a c e .
I n t h i s case,
as i n
case a, a pool of undercooled % - S i w i l l be p r e s e n t i n t h e sample.
209
4. MELTING MODEL OF PULSED LASER PROCESSING Case c.
The m e l t f r o n t p e n e t r a t e s beyond t h e a-c i n t e r f a c e .
I n t h i s case,
s i g n i f i c a n t undercooling o f t h e molten S i i s not
expected f o r any prolonged p e r i o d s , and t h e p h y s i c a l c o n d i t i o n s a r e c l o s e l y s i m i l a r t o t h o s e which e x i s t when c-Si m e l t s and r e solidifies.
For a given
ER, t h e d i f f e r e n c e i n l a t e n t heat between
a- and c-Si a c t s as an a d d i t i o n a l heat source t o i n c r e a s e t h e m e l t f r o n t p e n e t r a t i o n when an amorphous l a y e r i s present. I t appears t h a t m e l t i n g model c a l c u l a t i o n s o f t h e t y p e d e s c r i b e d
i n Sec. I11 can deal reasonably s a t i s f a c t o r i l y w i t h case c, b u t i t i s n o t a t a l l c l e a r how t o best deal w i t h s o l i d i f i c a t i o n f r o m a h i g h l y undercooled m e l t i n which b u l k n u c l e a t i o n may occur.
The
major conceptual d i f f i c u l t y encountered i n addressing t h e problem i s t o f i n d a s a t i s f a c t o r y way t o i n c l u d e t h e e f f e c t s o f b u l k nucleat i o n o f t h e c r y s t a l l i n e phase.
The major computational d i f f i c u l t i e s
a r e t h a t o f i n c l u d i n g s o l i d i f i c a t i o n a t a temperature o t h e r t h a n t h e m e l t i n g temperature and o f i n t r o d u c i n g b u l k n u c l e a t i o n e f f e c t s i n t o an e s s e n t i a l l y one-dimensional c a l c u l a t i o n .
I n the following,
we w i l l d i s c u s s how t h e s e e f f e c t s can be r o u g h l y s i m u l a t e d i n a one-dimensional t r e a t m e n t , b u t i t must be emphasized t h a t t h e c a l c u l a t i o n s do represent o n l y a s i m u l a t i o n and a r e n o t based on a s a t i s f a c t o r i l y rigorous t h e o r e t i c a l formulation.
Nevertheless, t h e
c a l c u l a t i o n s and t h e experiments taken t o g e t h e r g i v e very u s e f u l i n s i g h t s i n t o t h e u n d e r l y i n g p h y s i c a l mechanisms. W i t h t h e f o r e g o i n g i n mind, t h e more standard heat f l o w c a l c u l a t i o n s d e s c r i b e d i n S e c t i o n 111.5 were m o d i f i e d t o i n c l u d e simul a t i o n o f t h e e f f e c t s o f b u l k n u c l e a t i o n as f o l l o w s . While t h e m e l t f r o n t i s s t i l l advancing i n t o t h e a-Si l a y e r , La, Ta, and Kay Kc,
or
K,
a r e used depending on t h e phase o f t h e m a t e r i a l i n a
given f i n i t e - d i f f e r e n c e c e l l .
The l a t e n t heat i s always switched
f r o m La t o Lc as soon as an a-Si c e l l has melted; t h i s means t h a t when c r y s t a l l i z a t i o n occurs due t o b u l k n u c l e a t i o n o r o t h e r w i s e an amount o f energy equal t o Lc-La becomes a v a i l a b l e .
I f t h e melt
f r o n t does n o t p e n e t r a t e t o t h e a-c i n t e r f a c e , a l a y e r o f a-Si w i t h i t s low thermal c o n d u c t i v i t y remains. The R - S i i n a g i v e n c e l l i s
210
R. F. WOOD ET AL.
a l l o w e d t o s o l i d i f y p r o v i d e d i t s temperature i s l e s s t h a n Tn; t h i s c o n d i t i o n s i m u l a t e s b u l k n u c l e a t i o n and a l l o w s t h e i n c r e m e n t a l l a t e n t heat AL t o i n c r e a s e t h e m e l t - f r o n t p e n e t r a t i o n . m o l t e n c e l l s i n which T i s i n i t i a l l y
> Tn
but
<
For those
Tc, c r y s t a l l i z a t i o n
cannot occur u n t i l T drops below Tn, o r u n t i l t h e r a p i d l y growing r e g i o n o f FG p-Si p r o v i d e s n u c l e a t i o n s i t e s f o r t h e l i q u i d w i t h T
>
Tn.
When t h e m e l t f r o n t i n i t i a l l y c o n t a c t s t h e a-c i n t e r f a c e ,
t h e temperature o f t h e i n t e r f a c e , Tac, w i l l be Ta.
The m e l t f r o n t
cannot p e n e t r a t e f u r t h e r u n t i l Tac r i s e s t o Tc and enough l a t e n t heat i s s u p p l i e d t o b e g i n t o m e l t t h e c-Si region.
I f E,
i s not
g r e a t enough f o r Tac t o reach Tc, c r y s t a l l i z a t i o n w i l l occur a t temperatures between Ta and Tc. formed i f Tac
<
Tny b u t o t h e r w i s e o n l y l a r g e - g r a i n e d m a t e r i a l w i l l
F i n a l l y , when Tac
be formed.
A t h i n l a y e r o f FG p-Si may be
2
Tc, e p i t a x i a l regrowth o f s i n g l e -
c r y s t a l S i f r o m t h e c-Si s u b s t r a t e can proceed j u s t as i t would f o r an e n t i r e l y c-Si sample. The r e s u l t s o f t h e c a l c u l a t i o n s a r e shown i n Figs. 11 and 12; t h e y can be i n t e r p r e t e d as f o l l o w s .
F i g u r e 11 i n d i c a t e s t h a t t h e
temperature o f t h e a-Si f o r 0.2 J/cm2 does n o t r i s e above Tny so t h a t o n l y FG p-Si i s expected a t t h e s u r f a c e ( i n agreement w i t h TEM), f o l l o w e d by t h e remainder o f t h e a-Si. n e a r e s t t h e surface,
T > Tn
F o r 0.3 J/cm2, i n t h e r e g i o n
and a t h i n r e g i o n o f LG m a t e r i a l i s
expected t o grow f r o m t h e u n d e r l y i n g , b u l k - n u c l e a t e d FG m a t e r i a l . As E,
increases, t h e LG r e g i o n grows i n s i z e w h i l e t h e FG r e g i o n J/cm2 o n l y -200 A o f FG p-Si remains; f o r
s h r i n k s u n t i l a t -0.6
E Q > 0.8 J/cm2 t h e m e l t f r o n t p e n e t r a t e s c o m p l e t e l y t h r o u g h t h e
a-Si.
The k i n k a t -0.10
because Ta
<
Tc and La
pm
<
i n each of t h e curves on Fig. 12 occurs
Lc, and t h e m e l t f r o n t pauses u n t i l enough
energy i s absorbed f o r i t t o p e n e t r a t e t h e a-c i n t e r f a c e . 0.6
;5 E,
0.8 JIcm2,
n o t p e n e t r a t e it. a l l of t h e &-Si i s
the f r o n t contacts t h e i n t e r f a c e
For
b u t does
;5 0.6 J/cm2, t h e temperature o f p a r t o r Tn and n u c l e a t i o n occurs i n an extended r e g i o n
F o r E,
<
f o r which no w e l l - d e f i n e d m e l t f r o n t e x i s t s , as i n d i c a t e d by t h e cross h a t c h i n g on Fig. 12.
211
4. MELTING MODEL OF PULSED LASER PROCESSING
h Tn= 1210OC
El= 0.8 J/cm2
- 1500
-
i
To= 1150'C
1600
Y
1400
(I:
3 I-
a 1300
(I:
W
a
5 1200 I-
1100
1000
900
600 DEPTH
400
(8)
800
1000
Temperature as a function o f depth in a c-Si sample with a O.l-lrm
Fig. 11. thick a-Si
200
0
layer at the surface.
T
undercooled and
0.30
For low energy densities the 8-Si
may be below Tn,
1
'
-E 0.25
i s highly
the temperature for bulk nucleation.
E 1 2 2 J/cm2
I
/ \
I
I 0 - 5
I
I
ON c-Si
-
1
I 0.20 n W n I-
5
0.15
0
a
I&.
5 w
0.10
r 0.05 0
0
10
20
30
40
50
60
TIME (nsec)
70
80
90 100
Fig. 12. M e l t front p r o f i l e s f o r a silicon sample with an -O.l-!~m layer a t the front surface.
thick a-Si
The melt-front position i n the cross-hatched area
i s not defined because o f bulk nucleation,
as discussed i n the text.
212
R. F. WOOD E T A L
13. a.
MISCELLANEOUS ILLUSTRATIVE CALCULATIONS Substrate Heating F i g u r e s 13 and 14 i l l u s t r a t e t h e t y p e of r e s u l t s which can be
o b t a i n e d when t h e sample i s e i t h e r c o o l e d o r heated w h i l e t h e l a s e r i r r a d i a t i o n i s c a r r i e d out.
The c a l c u l a t i o n s were made f o r t h e
same excimer l a s e r p u l s e used i n c a l c u l a t i n g t h e curves o f Fig. 8. I t can be seen f r o m t h e f i g u r e s t h a t t h e c a l c u l a t i o n s p r e d i c t a
n e a r l y l i n e a r i n c r e a s e i n m e l t - f r o n t p e n e t r a t i o n w i t h s u b s t r a t e temp e r a t u r e Tsub, and from Fig. 13 t h a t t h e regrowth v e l o c i t y decreases w i t h i n c r e a s i n g TsUb.
From t h e s e r e s u l t s , i t i s apparent t h a t sub-
s t r a t e h e a t i n g may be o f i n t e r e s t f o r several reasons:
1) I f , f o r
example, an a v a i l a b l e l a s e r cannot supply t h e energy r e q u i r e d t o m e l t t h e near-surface r e g i o n t o a r e q u i r e d depth, sample h e a t i n g may e x t e n d t h e m e l t i n g range t o t h a t depth.
The c a l c u l a t i o n s of F i g . 14
i n d i c a t e t h a t f o r a given EL t h e maximum m e l t - f r o n t p e n e t r a t i o n may be extended by a f a c t o r o f two or more by i n c r e a s i n g t h e sample temperature from 20°C t o 500°C.
2 ) Even i f a l a s e r i s s u f f i c i e n t l y
p o w e r f u l i t may s t i l l be advantageous t o use s u b s t r a t e h e a t i n g so h
0.5
E
3-
0.4
--- PULSE
0 c
S 0.3
SHAPE
-
a
t-
z 0
-
0.2
LT
W
= o
0
40
80
120 TIME (ns)
160
200
Fig. 1 3 . E f f e c t s of substrate heating on melt-front profiles for the excimer laser pulse used in calculating the profiles o f Fig. 8 .
4.
-200
Fig. 14.
213
MELTING MODEL OF PULSED LASER PROCESSING
0 200 400 600 Tsub, SUBSTRATE TEMPERATURE CC)
Maximum melt-front
000
penetration as a function o f substrate tem-
perature and energy density for the laser pulse o f Fig.
13.
t h a t t h e energy d e n s i t y f o r a n n e a l i n g can be reduced and t h e magnit u d e o f t h e s p a t i a l inhomogeneities i n t h e l a s e r energy b e t t e r controlled.
3 ) Probably t h e most i n t e r e s t i n g use o f s u b s t r a t e h e a t i n g
and c o o l i n g i s t o c o n t r o l t h e m e l t - f r o n t v e l o c i t y d u r i n g r e s o l i d i f i c a t i o n of t h e molten near-surface r e g i o n ( C u l l i s e t al., Wood and G i l e s , 1981). b.
1980;
T h i s w i l l be discussed i n more d e t a i l below.
E f f e c t s o f Varying t h e A b s o r p t i o n C o e f f i c i e n t Curves of m e l t - f r o n t p e n e t r a t i o n as a f u n c t i o n o f an average
a b s o r p t i o n c o e f f i c i e n t i n t h e c model a r e g i v e n i n Fig. 15. f e a t u r e s o f t h e s e curves a r e o f p a r t i c u l a r i n t e r e s t .
Two
F i r s t , we
n o t e t h a t t h e p e n e t r a t i o n depths appear t o r a p i d l y approach l i m i t i n g values as a i s increased.
This implies t h a t u l t r a v i o l e t r a d i a t i o n
214
R. F. WOOD ET AL.
, from
(e.g.
excimer l a s e r s ) p r o v i d e s no p a r t i c u l a r advantage j u s t
because i t i s absorbed i n a t h i n s u r f a c e l a y e r .
T h i s r e s u l t , which
t o some may seem s u r p r i s i n g , comes about p r i m a r i l y because o f t h e i n t e r p l a y between a b s o r p t i o n and t h e r e f l e c t i v i t y jump on m e l t i n g . The h i g h e r t h e a b s o r p t i o n c o e f f i c i e n t , t h e q u i c k e r t h e near-surface r e g i o n m e l t s and decreases t h e percentage o f t h e i n c i d e n t l i g h t t h a t The second f e a t u r e of t h e r e s u l t s which i s o f i n t e r e s t
i s absorbed.
i s t h e b e h a v i o r o f t h e curves a t low values o f a. As a i s decreased, t h e l a s e r energy i s d e p o s i t e d more u n i f o r m l y throughout t h e sample and e v e n t u a l l y , f o r a g i v e n
E Q and
TQ,
a v a l u e o f a i s reached f o r
which enough heat t o cause m e l t i n g cannot be s u p p l i e d t o any volume element o f t h e sample.
It i s i n t h e c a l c u l a t i o n s w i t h small values
o f a and/or l o n g p u l s e times, t h a t t h e f o r m a t i o n o f " s l u s h " zones become important. The dashed c u r v e on Fig. 15 f o r
EL
= 1.2 J/cm2 shows t h e e f f e c t
o f n o t i n c r e a s i n g a t o 106 cm-1, t h e m e t a l l i c - l i k e value, on m e l t i n g .
Only a t t h e l o w e s t values o f a does t h i s have a n o t i c e a b l e e f f e c t 0.7 c
-z 0.6 E 3-
a a
z
---
n
01 01
- lo6 cm-' ON MELTING - UNCHANGED ON MELTING
-
0.5 0.4
LI
a 0.3 I-
2
0
a 0.2 U I
5
LI
I
0.4
I
0 0
Fig.
15.
2
I
I
I
I
I
I
Al
I
4 6 8 10 12 t4 25 50 a , ABSORPTION COEFFICIENT (104 cm-')
I 75
900
Illustration o f the effects of the absorption coefficient on the
maximum melt-front position. The dashed curve shows the results for a pulse o f
1.2 J / c m 2 if a is not switched to lo6 cm'l
when the surface melts.
215
4. MELTING MODEL OF PULSED LASER PROCESSING on t h e m e l t - f r o n t p e n e t r a t i o n .
A more i m p o r t a n t e f f e c t i s t h e be-
h a v i o r o f t h e r e f l e c t i v i t y which f o r very s h o r t wavelengths may drop r a p i d l y as t h e plasma frequency o f L-Si i s approached. E f f e c t s o f Pulse Shape and D u r a t i o n
c.
F i g u r e 16 shows m e l t - f r o n t h i s t o r i e s f o r p u l s e s o f r a d i c a l l y d i f f e r e n t shape f o r which t h e enerqy d e n s i t y i n c i d e n t on t h e sample i s t h e same.
I t i s c l e a r from these r e s u l t s t h a t l a r g e v a r i a t i o n s
i n t h e p u l s e shape can a l t e r t h e m e l t - f r o n t p e n e t r a t i o n .
However,
p u l s e shapes which a r e reasonably c l o s e t o one another w i l l g i v e quite similar melt-front histories.
These r e s u l t s a r e s i g n i f i c a n t
because i t i s seldom p o s s i b l e t o o b t a i n a t r u e gaussian p u l s e shape, e s p e c i a l l y a f t e r frequency doubling, and t h e p u l s e shape can f l u c t u a t e somewhat from p u l s e t o pulse.
-
0.5
E
ri
v
-
0.4
L
5
0 Q
I-
0.3
z
8
,
LL
0.2
0.4
0
0
400
2 00 TIME (nsec)
300
400
Fig. 1 6 . Illustration o f the e f f e c t s o f widely differing pulse shapes on the melt-front profiles for a 1 .25-J/cm2 pulse. The dashed curve represents a gaussian pulse.
216
R. F. WOOD ET AL.
The r e s u l t s o f c a l c u l a t i o n s t o t e s t t h e s e n s i t i v i t y o f m e l t f r o n t penetration t o t h e pulse duration o f a gaussian-like pulse a r e given i n Fig. 17.
The f i g u r e shows t h a t f o r a g i v e n v a l u e o f
E, t h e m e l t - f r o n t h i s t o r y i s f a i r l y s e n s i t i v e t o l a r g e v a r i a t i o n s Nonetheless, i t would seem t h a t pulses o f
i n t h e pulse duration.
a w e l l - t u n e d l a s e r can be c o n t r o l l e d t o t h e e x t e n t needed i n most l a s e r - a n n e a l i n g a p p l i c a t i o n s ; f o r example, t h e d i f f e r e n c e s between m e l t - f r o n t h i s t o r i e s f o r a 15-nsec and a 20-nsec p u l s e a r e n o t l i k e l y t o be i m p o r t a n t .
The c a l c u l a t i o n s do show t h a t l a s e r p u l s e s o f t h e
same energy d e n s i t y Eg b u t w i d e l y d i f f e r e n t d u r a t i o n w i l l produce quite different melt-front profiles. time-resolved Chapter 6,
optical
experiments,
Moreover,
h i g h l y accurate
such as t h o s e d e s c r i b e d i n
can r e s o l v e t h e d i f f e r e n c e s i n t h e t i m e o f onset o f
m e l t i n g between 15- and 20-nsec pulses.
0.6
-
-
0.5
E
k
0.4
v)
2 t-
0.3
z
0
lx
y
0.2
5 W 2
0.4
0
0
400
200
300
400
TIME (nsec)
Effects of pulse duration on melt-front profiles for a pulse of 1 . 2 5 J/crn2; the "slush zoneff i s discussed in the text. Fig.
17.
217
4. MELTING MODEL OF PULSED LASER PROCESSING
The dashed l i n e on t h e 200 nsec c u r v e shows t h e ''slush'' o r t r a n s i t i o n zone discussed i n S e c t i o n 111.6.
A l l o f t h e curves on
t h e f i g u r e have s l u s h zones a s s o c i a t e d w i t h them, b u t t o a v o i d conf u s i o n t h e y have n o t been shown.
For s h o r t p u l s e s t h e y do n o t
p e n e t r a t e deeper t h a n t h e f u l l y m e l t e d zone and t h e y e x i s t f o r o n l y b r i e f periods.
The hook on t h e s h o r t - t i m e edge o f each curve i s an
i n d i c a t i o n o f t h e s l u s h zone.
By i n c r e a s i n g t h e p u l s e t i m e beyond
200 nsec, t h e d u r a t i o n o f t h e s l u s h zone can be i n c r e a s e d f o r t h i s
1.25-J/cm2
pulse, b u t e v e n t u a l l y a p u l s e l e n g t h w i l l be reached f o r
which t h e m e l t i n g temperature i s never a t t a i n e d . 14. a.
ADDITIONAL DISCUSSION OF RESULTS Thermal Gradients a t t h e L i q u i d - S o l i d I n t e r f a c e Most phenomenological t h e o r i e s o f c r y s t a l growth ( T u r n b u l l ,
1956; Jackson and Chalrners, 1956; Thurmond, 1959; T u r n b u l l andCohen, 1960) r e l a t e t h e c r y s t a l l i z a t i o n r a t e t o t h e "temperature o f t h e l i q u i d - s o l i d interface,"
as discussed i n Chapter 5.
However, i n
terms o f t h e macroscopic heat d i f f u s i o n equations used h e r e (see Eq. 4), i t i s t h e temperature g r a d i e n t a t t h e m e l t f r o n t which i s important i n determining t h e m e l t - f r o n t velocity. I n Table 111, we show b o t h s p a t i a l and temporal g r a d i e n t s i n t h e s o l i d a t t h e i n t e r f a c e f o r a number o f d i f f e r e n t s o l i d i f i c a t i o n v e l o c i t i e s . I t can be seen t h a t t h e s e g r a d i e n t s a r e very l a r g e .
The m e l t i n g
v e l o c i t i e s a r e even g r e a t e r , as a l l o f t h e m e l t - f r o n t p r o f i l e s g i v e n i n t h e f i g u r e s i n t h i s c h a p t e r show. The g r a d i e n t s i n t h e l i q u i d r a p i d l y decrease and become p r a c t i c a l l y n e g l i g i b l e f o r l o n g p e r i o d s o f t i m e w h i l e t h e m e l t f r o n t i s r e t u r n i n g t o t h e s u r f a c e ; t h i s cond i t i o n had been r e a l i z e d f o r t h e v e l o c i t i e s i n Table 111. b.
Control o f Melt-Front Velocity I t i s i m p o r t a n t i n t e s t i n g v a r i o u s models of n o n e q u i l i b r i u m
s e g r e g a t i o n d u r i n g u l t r a r a p i d r e c r y s t a l l i z a t i o n t o be a b l e t o cont r o l the melt-front velocity.
The r e s u l t s o f m e l t i n g model c a l -
c u l a t i o n s suggest s e v e r a l methods o f d o i n g t h i s .
These i n c l u d e
218
R. F. WOOD E T A L .
Table I 1 1 S p a t i a l and Temporal Gradients i n t h e S o l i d a t the Interface
V
(m/sec) 3 6 9 12 15
ia
Spatial Gradient (107 deg/cm)
Temporal Gradient (1010 deg/sec)
0.57 1.14 1.71 2.29 2. a6 3.43
0.17 0.68 1.54 2.75 4.29 6.17
1) s u b s t r a t e h e a t i n g , 2 ) a l t e r a t i o n o f t h e thermal c o n d u c t i v i t y o f t h e base r e g i o n , and 3) v a r i a t i o n o f
Et and
T ~ .
An average v e l o c i t y Va f o r each o f t h e m e l t - f r o n t p r o f i l e s on Fig. 13 i s g i v e n i n Table I V ; Va was d e f i n e d by t h e s l o p e o f t h e s t r a i g h t l i n e drawn from t h e t i m e t h e m e l t f r o n t r e t u r n e d t o t h e s u r f a c e and t a n g e n t t o t h e p r o f i l e j u s t a f t e r t h e maximum m e l t f r o n t penetration.
It can be seen t h a t v a r i a t i o n s o f f a c t o r s o f 2-3 i n
t h e average values d u r i n g r e c r y s t a l l i z a t i o n can be produced by subs t r a t e h e a t i n g i n e a s i l y a c c e s s i b l e temperature ranges.
Variations
o f t h i s magnitude have been s u f f i c i e n t t o a l l o w l i m i t e d s t u d i e s (White e t al.,
1980, 1981; Poate, 1981) o f t h e e f f e c t s o f i n t e r f a c e
v e l o c i t y on dopant s e g r e g a t i o n d u r i n g l a s e r annealing. Another e f f e c t i v e method f o r c o n t r o l l i n g v would be t o modify t h e thermal c o n d u c t i v i t y o f t h e s u b s t r a t e region. done by d e p o s i t i n g t h i n ( < 1
pm)
w i t h very low thermal c o n d u c t i v i t y .
T h i s c o u l d be
f i l m s o f s i l i c o n on s u b s t r a t e s M e l t - f r o n t c a l c u l a t i o n s on a
sample c o n s i s t i n g o f a 0.5-pm f i l m o f S i on an SiO, s u b s t r a t e showed t h a t v c o u l d be reduced t o a t l e a s t 1.5 m/sec by t h i s technique. I n p r a c t i c e , a d i f f i c u l t y w i t h t h e method i s t h a t s i n g l e c r y s t a l s i l i c o n f i l m s cannot be grown on most i n s u l a t i n g s u b s t r a t e s , and t h e o p t i c a l and thermal p r o p e r t i e s o f p o l y c r y s t a l l i n e f i l m s grown
4.
MELTING MODEL OF PULSED LASER PROCESSING
219
Table I V Average Sol i d i f i c a t i o n V e l o c i t i e s f o r t h e Melt-Front P r o f i l e s o f F i g . 13 Tsub
("c)
va (m/sec)
20 200 400 600
4.9 3.7 2.7 1.9
under a v a r i e t y o f c o n d i t i o n s are n o t w e l l e s t a b l i s h e d .
The case o f
s i l i c o n - o n - s a p p h i r e (SOS) i s an e x c e p t i o n because c-Si f i l m s can be grown on sapphire, b u t t h e thermal c o n d u c t i v i t y o f A1203 i s high. A t h i r d method f o r c o n t r o l l i n g v i s suggested by t h e curves on Fig. 17 which i n d i c a t e t h a t t h e r e i s a t l e a s t a weak dependence o f v on
T ~ .
Because i t i s p o s s i b l e t o v a r y t h e p u l s e d u r a t i o n f r o m t h e
nanosecond t o t h e femtosecond regime, i t i s p o s s i b l e t o g r e a t l y a l t e r
v i n t h i s manner.
By u s i n g combinations o f s h o r t pulses and low
energy d e n s i t i e s , values o f v as h i g h as 15-20 m/sec can be obtained. A t t h e s e regrowth v e l o c i t i e s , form ( L i u , e t al.,
s i l i c o n s o l i d i f i e s i n an amorphous
1979; Tsu e t al.,
1979; C u l l i s e t al.,
1982), as
discussed i n Chapter 5. I t should be noted t h a t attempts t o modify t h e m e l t - f r o n t veloc-
i t y w i l l a l s o produce v a r i a t i o n s i n t h e m e l t - f r o n t p e n e t r a t i o n and t h i s i n t u r n w i l l e f f e c t t h e dopant r e d i s t r i b u t i o n i n i o n - i m p l a n t e d layers.
Because o f t h i s ,
i t appears t h a t meaningful r e s u l t s o f
experiments designed t o t e s t t h e e f f e c t s of m e l t - f r o n t
v e l o c i t y on
t h e s e g r e g a t i o n o f dopants d u r i n g l a s e r a n n e a l i n g can o n l y be o b t a i n e d a f t e r f a i r l y e x t e n s i v e c a l c u l a t i o n s and t h e o r e t i c a l f i t s t o t h e experimental data. c.
Importance o f C a r r i e r D i f f u s i o n As a l r e a d y mentioned, t h e c a l c u l a t i o n s o f Yoffa (1980a) seem
t o i m p l y t h a t t h e d i f f u s i o n o f t h e h o t , dense gas o f e l e c t r o n s and holes generated d u r i n g l a s e r a n n e a l i n g p l a y s a dominant r o l e i n
220
R. F. WOOD E T A L .
d e t e r m i n i n g t h e temperature r i s e o f t h e l a t t i c e .
The f o l l o w i n g
e x p r e s s i o n f o r t h e e l e c t r o n d e n s i t y i n t h e near-surface r e g i o n o f a l a s e r - i r r a d i a t e d sample was d e r i v e d by Yoffa,
I n t h i s expression,
g i s t h e c a r r i e r generation rate, 6 i s t h e
a b s o r p t i o n l e n g t h ( = a-l) f o r l a s e r r a d i a t i o n o f frequency uR, Te i s t h e phonon emission r a t e due t o c o l l i s i o n s o f t h e p h o t o e x c i t e d electrons with the l a t t i c e , and
fiiw
i s t h e energy o f t h e e m i t t e d phonons,
B i s the characteristic d i f f u s i o n length o f the carriers. E q u a t i o n (11) has a form analogous t o I( x ) = (l-R) Ioa exp(-ax)
,
(12)
which can be o b t a i n e d from Eq. (8) when a ( o r k ) i s constant and t h e r e f l e c t i v i t y f a c t o r l-R i s included.
I n Eq. (ll), gftwa i s t h e r a t e
a t which t h e i n c i d e n t r a d i a t i o n i s absorbed i n t h e sample by e l e c t r o n i c excitations, Eq.
and i s t h e r e f o r e e q u i v a l e n t t o (l-R)Ioa i n
(12). The q u a n t i t y Ne,ss(X)(fU/Te)
from Eq. (11) can be i n t e r -
p r e t e d as t h e r a t e a t which t h e absorbed energy i s given up t o t h e l a t t i c e through e l e c t r o n - l a t t i c e i n t e r a c t i o n s and i s equiva e n t t o I ( x ) i n Eq. (12). 1 (x
=
Equation (11) can t h u s be r e w r i t t e n as
(l-R)I o ( ~-6)'l
(8+6)-'[eXp-
(x/8)
-
6exp ( - x / 6 ) ]
.
I f c a r r i e r d i f f u s i o n i s n e g l i g i b l e , 8 = 0, and Eq. (13) reduces t o Eq.
(12) as i t should.
6 i s n o t n e g l i g i b l e , Eq.
Wood and G i l e s (1981) have shown t h a t when
(13) can s t i l l be approximated q u i t e w e l l
by t h e expression i n Eq. ( 1 2 ) , b u t w i t h an e f f e c t i v e a b s o r p t i o n coefficient.
I t f o l l o w s t h e n from r e s u l t s o f c a l c u l a t i o n s o f m e l t -
f r o n t p e n e t r a t i o n as a f u n c t i o n o f a b s o r p t i o n c o e f f i c i e n t on Fig. 15, t h a t t h e maximum p e n e t r a t i o n depth o f t h e m e l t f r o n t in t h e c-model i s e s s e n t i a l l y constant f o r wide ranges o f t h i s e f f e c t i v e absorpt i o n coefficient.
Hence, w i t h i n l i m i t s , i t does n o t r e a l l y m a t t e r
221
4. MELTING MODEL OF PULSED LASER PROCESSING whether o r n o t c a r r i e r d i f f u s i o n , e s t i m a t e d by Yoffa, i s present.
even o f t h e l a r g e magnitude
On t h e o t h e r hand, t h e r e may very
w e l l be circumstances i n which c a r r i e r d i f f u s i o n becomes i m p o r t a n t f o r a fundamental understanding o f t h e i n t e r a c t i o n o f l a s e r r a d i a t i o n w i t h semiconductors.
These a r e most l i k e l y t o occur i n s i l i c o n
f o r very s h o r t , i n t e n s e pulses o f high-frequency r a d i a t i o n , as d i s cussed i n S e c t i o n 11.
V. 15.
Dopant Redistribution i n th e Melting Model
INTRODUCTION One o f t h e most r e a d i l y apparent c h a r a c t e r i s t i c s o f p u l s e d l a s e r
p r o c e s s i n g o f semiconductors i s t h e e x t e n s i v e dopant r e d i s t r i b u t i o n t h a t i s observed t o occur as a r e s u l t o f t h e l a s e r a n n e a l i n g o f ion-implanted layers.
The e x t e n t of t h e r e d i s t r i b u t i o n cannot be
e x p l a i n e d by known mechanisms o f s o l i d - s t a t e d i f f u s i o n , b u t i t i s e a s i l y e x p l a i n e d q u a n t i t a t i v e l y by d i f f u s i o n i n t h e l i q u i d s t a t e . Examples o f t h e agreement which can be o b t a i n e d between experimental and c a l c u l a t e d dopant p r o f i l e s a r e given i n several chapters o f t h i s book and o t h e r examples w i l l be discussed here.
An i m p o r t a n t
purpose o f t h e s e c t i o n i s t o review macroscopic concepts o f s o l i d i f i c a t i o n o f a two-component l i q u i d when s e g r e g a t i o n o f t h e d i l u t e component (dopant) occurs.
It w i l l be seen t h a t nonequil ib r i u m
s e g r e g a t i o n o f t e n p l a y s a major r o l e in the-dopant r e d i s t r i b u t i o n process.
I n t h i s s e c t i o n we w i l l a l s o show how t h e r e s u l t s o f t h e
m e l t i n g model c a l c u l a t i o n s d e s c r i b e d above,
enter i n t o t h e cal-
c u l a t i o n s o f dopant r e d i s t r i b u t i o n d u r i n g l a s e r processing. r a p i d l y moving 1 i q u i d - s o l i d
The
i n t e r f a c e and t h e dopant d i f f u s i o n
which occurs w h i l e t h e near-surface r e g i o n o f t h e sample i s molten g i v e r i s e t o y e t another moving-boundary problem t h a t ,
l i k e the
heat conduction problem, cannot be solved e x a c t l y except i n s p e c i a l cases.
I t i s necessary, t h e r e f o r e , t o have approximate a n a l y t i c a l
222
R. F. WOOD ETAL
and f i n i t e - d i f f e r e n c e methods a v a i l a b l e t o c a l c u l a t e dopant r e d i s t r i b u t i o n once m e l t - f r o n t data o f t h e t y p e g i v e n i n S e c t i o n I V i s available.
Another o b j e c t i v e o f t h i s s e c t i o n t h e n i s t o d e s c r i b e
s e v e r a l such methods and t o g i v e i n d i c a t i o n s of t h e i r accuracy. The t o p i c s and c a l c u l a t i o n s considered h e r e w i l l be l i m i t e d m a i n l y t o t h o s e which i l l u s t r a t e c e r t a i n general p o i n t s t h a t should be k e p t i n mind when a t t e m p t i n g t o e x t r a c t s e g r e g a t i o n c o e f f i c i e n t s f r o m e x p e r i m e n t a l data. 16.
SEGREGATION COEFFICIENTS An i m p o r t a n t concept i n t h e macroscopic t h e o r y o f s o l i d i f i c a -
t i o n o f a d i l u t e two-component a l l o y i s t h a t o f t h e " s e g r e g a t i o n " o r "distribution" coefficient.
A number o f d i f f e r e n t usages o f
t h e t e r m i n o l o g y appear i n t h e l i t e r a t u r e and i t may be u s e f u l t o t h e r e a d e r t o review some o f t h e s e b r i e f l y . The i n t e r f a c e r e g i o n i s c u s t o m a r i l y assumed t o be o f n e g l i g i b l e t h i c k n e s s i n t h e macroscopic t h e o r y and t h e i n f o r m a t i o n about micros c o p i c k i n e t i c processes a t t h e i n t e r f a c e i s expressed i n terms o f t h e i n t e r f a c e segregation ( o r d i s t r i b u t i o n ) c o e f f i c i e n t .
This
q u a n t i t y i s d e f i n e d as t h e r a t i o o f t h e c o n c e n t r a t i o n o f t h e d i l u t e component o f t h e a l l o y i n t h e s o l i d Cs t o i t s c o n c e n t r a t i o n i n t h e l i q u i d Ca. immediately i n f r o n t o f t h e moving i n t e r f a c e .
It i s
i m p o r t a n t t o recognize t h a t t h e i n t e r f a c e s e g r e g a t i o n c o e f f i c i e n t k i and t h e q u a n t i t y f r e q u e n t l y r e f e r r e d t o as t h e e f f e c t i v e segreg a t i o n c o e f f i c i e n t keff a r e q u i t e d i f f e r e n t .
This l a t t e r q u a n t i t y
i s g e n e r a l l y d e f i n e d as t h e r a t i o o f t h e i m p u r i t y c o n c e n t r a t i o n i n t h e s o l i d t o an i n i t i a l , u n i f o r m i m p u r i t y c o n c e n t r a t i o n Co i n t h e l i q u i d solvent.
T h i s c o n c e n t r a t i o n w i l l g e n e r a l l y be o b t a i n e d o n l y
a t some d i s t a n c e from t h e i n t e r f a c e .
Thus, f o r k i and k e f f we have
4. MELTING MODEL OF PULSED LASER PROCESSING Equation (14) i n d i c a t e s t h a t C,
and C,
223
a r e t o be e v a l u a t e d a t t h e
" i n t e r f a c e " i n t h e macroscopic t r e a t m e n t o f segregation.
Since
d i f f u s i o n i n t h e s o l i d i s i m p e r c e p t i b l e on t h e t i m e s c a l e i n v o l v e d i n p u l s e d l a s e r processing,
C,
a t the interface i s essentially
t h e same as t h a t observed i n t h e s o l i d l o n g a f t e r t h e i n t e r f a c e has passed a g i v e n p o i n t .
We w i l l r e s e r v e t h e symbol k g f o r t h e
i n t e r f a c e s e g r e g a t i o n c o e f f i c i e n t when c r y s t a l growth occurs so s l o w l y t h a t a c o n d i t i o n o f l o c a l o r quasi-thermodynamic e q u i l i b r i u m i s m a i n t a i n e d a t a l l times. I n a d d i t i o n t o t h e s e q u a n t i t i e s , a number o f o t h e r " s e g r e g a t i o n c o e f f i c i e n t s " appear i n t h e l i t e r a t u r e .
F o r example, Burton e t a l .
(1953) o b t a i n e d t h e e x p r e s s i o n
,
k = kl/[k'+(l-k')exp(-vab/D,)1
f o r an " e f f e c t i v e d i s t r i b u t i o n c c e f f i c i e n t . "
I n t h i s equation, k '
can be e i t h e r k i o r k y of t h e p r e c e d i n g paragraph, v i s t h e m e l t f r o n t velocity, D
a.
i s t h e d i f f u s i o n c o e f f i c i e n t i n t h e l i q u i d , and
&b i s r e f e r r e d t o , somewhat a r b i t r a r i l y , d i f f u s i o n boundary l a y e r .
as t h e t h i c k n e s s o f t h e
Equation ( 1 6 ) has caused some c o n f u s i o n
because i t appears t o g i v e a r e l a t i o n s h i p between k and k' i n terms o f v, and t h i s has l e d some t o b e l i e v e t h a t i t d e s c r i b e s t h e l a s e r a n n e a l i n g processes.
I n f a c t , Eq. (16) appl i e s t o C z c h r o l s k i - t y p e
growth of c r y s t a l s i n which a r o t a t i o n a l v e l o c i t y i s superimposed on t h e l i n e a r c r y s t a l growth,
and i t has no d i r e c t r e l a t i o n s h i p
w i t h t h e s i t u a t i o n encountered i n l a s e r annealing.
I n order t o
c l a r i f y t h e physics underlying t h e various segregation c o e f f i c i e n t s appearing i n t h e l i t e r a t u r e , we w i l l f i r s t c o n s i d e r one-dimensional s o l i d i f i c a t i o n i n which t h e r e i s an i n i t i a l l y u n i f o r m c o n c e n t r a t i o n of s o l u t e atoms i n t h e l i q u i d . 17.
ONE-DIMENSIONAL SOLIDIFICATION There a r e many macroscopic (i.e.
t i o n s ) t r e a t m e n t s o f one-dimensional ature,
, employing
d i f f e r e n t i a l equa-
solidification i n the l i t e r -
but we have found t h e development g i v e n by Smith e t a l .
224
R. F. WOOD ETAL.
(1955) p a r t i c u l a r l y u s e f u l and w i l l f o l l o w i t here.
It i s assumed
t h a t t h e l i q u i d - s o l i d i n t e r f a c e moves w i t h c o n s t a n t v e l o c i t y i n t h e x - d i r e c t i o n and t h a t ,
as i n t h e p r e c e d i n g subsection,
Co i s t h e
i n i t i a l l y uniform concentration o f solute i n t h e l i q u i d .
It i s
f u r t h e r assumed t h a t
1) i m p u r i t y d i f f u s i o n i n t h e s o l i d i s n e g l i g i b l e ; 2 ) no c o n v e c t i v e m i x i n g o f s o l u t e and s o l v e n t occurs; 3)
t h e i n t e r f a c e s e g r e g a t i o n c o e f f i c i e n t k i i s c o n s t a n t and < 1;
4)
t h e l i q u i d - s o l i d i n t e r f a c e i s always p e r p e n d i c u l a r t o t h e x - a x i s ; and
5) t h e r e i s no c o u p l i n g between mass and heat d i f f u s i o n . These c o n d i t i o n s p r o b a b l y correspond c l o s e l y t o t h o s e encountered i n p u l s e d l a s e r a n n e a l i n g i n most instances.
However, t h e f o r m a t i o n
o f c e l l u l a r s t r u c t u r e i n t h e d i s t r i b u t i o n o f some i m p u r i t i e s has been observed i n laser-annealed samples (van Gurp e t al., Narayan,
1979;
1980); t h i s i s a c l e a r i n d i c a t i o n t h a t c o n d i t i o n s 2 and 4
may break down (see, f o r example, M u l l i n s and Sekerka, 1964).
It
s h o u l d be n o t e d t h a t c o n d i t i o n 3 does n o t s p e c i f y whether k i i s an e q u i 1ib r i um (ky ) o r a nonequi 1ib r i um value.
Coup1 ing between t h e
mass and heat d i f f u s i o n equations g i v e s r i s e t o t h e S o r e t and Dufour e f f e c t s (Woods, 1975).
Although t h e temperature and c o n c e n t r a t i o n
g r a d i e n t s may be very l a r g e d u r i n g l a s e r annealing, t h e t i m e s a r e s o s h o r t t h a t t h e s e e f f e c t s a r e a p p a r e n t l y n o t important.
Smith e t a l . (1955) s o l v e d t h e dopant d i f f u s i o n e q u a t i o n i n t h e l i q u i d , i.e.,
i n t h e presence o f t h e advancing s o l i d i f i c a t i o n f r o n t .
The motion
o f t h i s f r o n t makes i t i m p o s s i b l e t o s o l v e t h e e q u a t i o n a n a l y t i c a l l y f o r t h e general case, b u t an a n a l y t i c a l s o l u t i o n i s p o s s i b l e when t h e i n i t i a l c o n c e n t r a t i o n o f t h e s o l u t e i s uniform.
Then a t r a n s -
f o r m a t i o n t o a c o o r d i n a t e system moving w i t h t h e m e l t f r o n t can be made and t h e e q u a t i o n
4.
MELTING MODEL OF PULSED LASER PROCESSING
obtained; here front.
225
i s measured r e l a t i v e t o t h e p o s i t i o n o f t h e melt
XI
We are i n t e r e s t e d not only i n t h e s o l u t i o n o f t h i s equa-
t i o n which gives t h e steady-state behavior o f CQ but a l s o i n those which g i v e t h e i n i t i a l and terminal t r a n s i e n t s . from Eq.
It can be seen
(14) t h a t once CQ i s known a t t h e i n t e r f a c e , C s f o r a
given k i f o l l o w s inmediately.
A t e r m i n a l " t r a n s i e n t " occurs when
t h e m e l t f r o n t approaches t h e f r o n t surface o f t h e s e m i - i n f i n i t e sample ( s o l i d i f i c a t i o n completed); t h i s t r a n s i e n t i s o f p a r t i c u l a r i n t e r e s t i n l a s e r annealing, as we s h a l l see. The steady-state s o l u t i o n o f Eq. (18) occurs when aC,/at
= 0
a t x ' = 0. The boundary c o n d i t i o n s can be w r i t t e n as C,(Xl)
=
co
at
XI
=
,
(19a)
C,(x')
= Co/ki
at
XI
= 0
,
(19b)
with q = l-ki.
I t can be seen from t h i s equation t h a t under steady-
s t a t e c o n d i t i o n s t h e d i s t r i b u t i o n o f t h e dopant i n t h e l i q u i d immed i a t e l y i n f r o n t o f the interface f a l l s o f f exponentially w i t h x ' , u n t i l t h e value C o i s obtained.
The second boundary c o n d i t i o n
r e q u i r e s t h a t , a t t h e i n t e r f a c e ( X I = 0), C,(O) must be such t h a t t h e concentration i n t h e s o l i d i s e x a c t l y C o y t h a t i s , from t h e definition of k i
c,
= Co = kiC,(0)
, or
ki = Co/Cll(0)
.
(21)
It should be noted t h a t t h i s equation s t a t e s t h a t t h e concentration
o f dopant incorporated i n t o t h e s o l i d under steady-state growth i s e x a c t l y Co and does not depend on v e l o c i t y , regardless o f t h e value o f ki.
226
R. F. WOOD ET AL
A f t e r a t r a n s f o r m a t i o n back t o x, t h e e q u a t i o n f o r t h e i n i t i a l t r a n s i e n t as a f u n c t i o n o f x ( n o t x ' ) g i v e n by Smith e t a l . i s Cs(x) = (Co/2){1 + e r f ( f v / 2 ) erfc
( (2ki-1)
+ (2ki-l)exp
( -kiq(v/Da)x)
.
./m/}2)
C s ( x ) f o r s e v e r a l values of ki w i l l be g i v e n s h o r t l y . be n o t e d f r o m t h e d e f i n i t i o n o f k e f f [Eq.
I t should
( 1 5 ) ] t h a t Eq. ( 2 2 ) can
be t h o u g h t o f as g i v i n g an expression f o r a t r a n s i e n t component o f k e f f which i s v e l o c i t y dependent. The t e r m i n a l t r a n s i e n t i s expressed as a s e r i e s t h a t i s n o t p a r t i c u l a r l y transparent.
The s e r i e s d i v e r g e s a t x2 = 0, where x2
i s t h e d i s t a n c e f r o m t h e f r o n t s u r f a c e o f t h e sample,
if ki is
l e s s t h a n u n i t y ; however, as x2 increases f r o m z e r o i t converges rapidly.
Obviously, t h e r e a r e no t e r m i n a l o r i n i t i a l t r a n s i e n t s
when t h e r e i s no s e g r e g a t i o n (i.e.,
when k i = I ) .
The equations f o r t h e i n i t i a l and t e r m i n a l t r a n s i e n t s g i v e n by Smith e t a l . (1955) were used t o c a l c u l a t e t h e form o f C s ( x ) / C o f o r f o r s e v e r a l d i f f e r e n t values o f k i .
The r e s u l t s are shown i n Fig. 18
f o r a m e l t - f r o n t v e l o c i t y o f 4 m/sec and a t y p i c a l d i f f u s i o n coe f f i c i e n t o f 4x10'4
cm2/sec.
Except f o r k i = 0.15,
these values
correspond t o t h e e q u i l i b r i u m s e g r e g a t i o n c o e f f i c i e n t s f o r B y As, Sb, and I n i n S i g i v e n i n t h e c o m p i l a t i o n o f Trumbore (1960) and shown h e r e w i t h o t h e r data i n Table V o f Sec. V.19a.
I t i s obvious
f r o m t h i s f i g u r e t h a t , under t y p i c a l c o n d i t i o n s o f p u l s e d l a s e r annealing,
t h e s e g r e g a t i o n o f some dopants may never reach t h e
steady-state condition.
Thus, f o r k i = 0.15,
0.023,
and 0.0004,
b e f o r e t h e i n i t i a l t r a n s i e n t ( s o l i d l i n e ) i s over, t h e m e l t f r o n t has reached t h e v i c i n i t y o f t h e f r o n t s u r f a c e where a l l o f t h e dopant accumulated i n t h e l i q u i d ahead o f t h e m e l t f r o n t i s d e p o s i t e d i n t h e terminal transient, and 0.023.
as i n d i c a t e d by t h e dashed curves f o r k j = 0.15
T h i s p o s s i b l e f a i l u r e t o reach s t e a d y - s t a t e c o n d i t i o n s
4. MELTING MODEL OF PULSED LASER PROCESSING 1.6
227
I I I I l 1 1 1 1 1 1 1 l I I I I I I I I ki
1.4
A
t.2
*
V= 400 cm/sec D=4 X cm2/5ec
0.80 0.30 0.15
0.023
1.0 0
u
\ 0.0 W
0.6
-
\
\ \
\
0.4
0.2
0
Fig.
0.05
0
18.
0.10 0.15 DISTANCE FROM SURFACE ( p m )
0.20
C s / C o as a function o f distance from the surface.
Cs is the dopant
concentration in the solid and Co i s the i n i t i a l , uniform concentration in the liquid and unmelted material. occurred.
(Wood e t al.,
Melting to a depth o f 0.23 pm was assumed to have
1981a)
b e f o r e t h e m e l t f r o n t reaches t h e s u r f a c e must be k e p t i n mind when e x t r a c t i n g values o f k i f r o m e x p e r i m e n t a l l a s e r a n n e a l i n g data. F a i l u r e t o do so can g i v e r i s e t o m i s l e a d i n g r e s u l t s .
18. a.
MODELS AND APPROXIMATIONS FOR DOPANT-TRANSPORT CALCULATIONS The I n s t a n t a n e o u s Approximation I n t h o s e cases where t h e m e l t - f r o n t p e n e t r a t i o n i s deep and
t h e r e g i o n o f t h e i m p l a n t e d dopant p r o f i l e remains m o l t e n f o r an extended p e r i o d ,
s a t i s f a c t o r y f i t s t o t h e e x p e r i m e n t a l d a t a can
f r e q u e n t l y be o b t a i n e d by assuming t h a t t h e i m p l a n t e d r e g i o n i s i n s t a n t a n e o u s l y melted,
s t a y s molten f o r some t i m e , and t h e n r e -
s o l id i f ies ins tantaneous ly.
ns t a n t aneous a p p r o x i mat ion ‘’ T h i s “i
i n v o l v e s a s t r a i g h t f o r w a r d s o l u t i o n o f Eq.
(17) during the time
t h e s u r f a c e r e g i o n i s molten; i t a l s o corresponds, i n e f f e c t , t o an
228
R. F. WOOD ET AL.
i n f i n i t e me1t - f r o n t v e l o c i t y d u r i n g me1t i ng and r e c r y s t a l 1iz a t i o n . The dopant p r o f i l e i n t h e s o l i d a f t e r r e s o l i d i f i c a t i o n i s g i v e n by
where C i and Cs a r e t h e i n i t i a l (x=xo, r e s p e c t i v e l y and G(x,t
I
xo,to)
blem (Carslaw and Jaeger, 1959).
t = t o ) and f i n a l p r o f i l e s
i s t h e Green's f u n c t i o n o f t h e p r o -
I f t h e melt f r o n t penetrates w e l l
beyond t h e r e g i o n o f t h e i m p l a n t e d p r o f i l e , i t i s a good approximat i o n t o t a k e G(x,t
I xo,to)
t o be t h e Green's f u n c t i o n f o r d i f f u s i o n
i n a s e m i - i n f i n i t e sample, i.e., G(x,t
I xo,to)
=
[4nD ( t - t )]-1'2 t o 2
Because t h e m e l t i n g and r e s o l i d i f i c a t i o n a r e assumed t o occur i n t i m e s s h o r t compared t o t h e t o t a l m e l t d u r a t i o n , t h e i n i t i a l p r o f i l e i n t h e l i q u i d can be approximated by t h e i m p l a n t e d p r o f i l e and t h e f i n a l p r o f i l e i n t h e s o l i d i s very n e a r l y t h e same as t h a t i n t h e l i q u i d a t t h e end o f t h e m e l t d u r a t i o n . The instantaneous approxi m a t i o n d e s c r i b e d h e r e does n o t a l l o w f o r segregation, b u t i t i s q u i t e u s e f u l f o r s t u d y i n g r e d i s t r i b u t i o n o f dopants such as
P, B,
and As f o r which k i i s very n e a r l y u n i t y under most l a s e r - a n n e a l i n g c o n d i t i o n s (Wang e t al., b.
1978, 1979).
A M o d i f i e d Instantaneous Approximation The approach j u s t d e s c r i b e d above i s t o o r e s t r i c t i v e i n t h e
way t h e e f f e c t s o f m e l t - f r o n t motion a r e t r e a t e d .
I n particular,
when t h e m e l t f r o n t does n o t p e n e t r a t e beyond t h e i m p l a n t e d p r o f i l e , a s i n g l e t i m e d u r i n g which t h e dopant atoms i n t h e p r o f i l e a r e a l l o w e d t o d i f f u s e cannot be assigned i n even an approximately
4.
MELTING MODEL OF PULSED LASER PROCESSING
c o r r e c t manner.
229
C l e a r l y , i t i s necessary t o l i m i t t h e t i m e d u r i n g
which t h e dopant atoms i n any volume element of t h e i n i t i a l p r o f i l e a r e a l l o w e d t o d i f f u s e t o t h e t i m e t h e m a t e r i a l o f t h a t volume element i s molten.
An approximation which g i v e s s u r p r i s i n g l y good
( 2 3 ) , i s obtained
r e s u l t s , y e t remains w i t h i n t h e s p i r i t o f Eq.
f r o m t h e f o l l o w i n g procedure used by Wood e t a l . (1981a). t j - t o = A t j d u r i n g which a t h i n l a y e r a t a depth
X j
remains molten, as c a l c u l a t e d from t h e m e l t - f r o n t Section I V ) ,
i s s u b s t i t u t e d i n t o Eqs.
The t i m e
i n t h e sample p r o f i l e s (see
(23) and (24) and t h e i n t e This i n t e g r a t i o n
g r a t i o n over xo i s c a r r i e d o u t t o f i n d Cs(Xj).
i s performed over o n l y t h a t p a r t o f t h e sample which i s molten d u r i n g b t j s i n c e o n l y t h e m a t e r i a l i n t h i s r e g i o n can a c t as a source f o r r a p i d d i f f u s i o n .
Thus, when t h e l i q u i d - s o l i d i n t e r f a c e
does n o t p e n e t r a t e e n t i r e l y through t h e i m p u r i t y p r o f i l e , o n l y t h a t p a r t o f C i ( x ) which i s i n t h e molten zone w i l l undergo d i f f u s i o n . T h i s r e q u i r e s t h a t t h e Green's f u n c t i o n i n Eq. (23) be m o d i f i e d t o t h a t f o r a s l a b o f a p p r o p r i a t e t h i c k n e s s o r , more approximately, t h a t G(x,t
1 xo,to)
o f Eq. ( 2 4 ) be r e s t r i c t e d i n t h e range o f x and
xo and s u i t a b l y renormalized. c.
Q u a s i - S t a t i o n a r y F i n i t e - D i f f e r e n c e Approach I n t h e quasi - s t a t i o n a r y approximation t h e sample i s assumed t o
be d i v i d e d i n t o
N l a y e r s o r c e l l s of v a r i o u s thicknesses.
A l l of
t h e m a t e r i a l i n any one l a y e r i s assumed t o m e l t o r s o l i d i f y a t t h e same time.
The m e l t f r o n t t h u s advances i n t o t h e s o l i d and
recedes back t o t h e surface i n f i n i t e jumps, and between jumps i t i s h e l d s t a t i o n a r y w h i l e d i f f u s i o n i n t h e m o l t e n m a t e r i a l occurs. F o r example, b e g i n n i n g a t t h e surface l a y e r , t h e dopant d i f f u s i o n e q u a t i o n i s solved i n t h a t l a y e r f o r some t i m e i n t e r v a l and w i t h t h e i m p l a n t e d p r o f i l e as t h e s t a r t i n g p r o f i l e .
The m e l t f r o n t t h e n
jumps t o t h e next l a y e r where i t pauses f o r a chosen t i m e i n t e r v a l . During t h i s i n t e r v a l
,
d i f f u s i o n occurs w i t h t h e i n i t i a l p r o f i l e
g i v e n by t h e d i s t r i b u t i o n of i m p u r i t y i n t h e f i r s t l a y e r a f t e r t h e f i r s t time interval
,plus
t h e implanted p r o f i l e i n t h e second l a y e r .
230
R. F. WOOD E T A L .
I n t h i s way, t h e d i f f u s i o n e q u a t i o n i s s o l v e d i n steps as t h e m e l t f r o n t f i r s t p e n e t r a t e s i n t o t h e sample and t h e n r e t u r n s t o t h e s u r f ace. T h i s problem can be s o l v e d a n a l y t i c a l l y by u s i n g Green's f u n c t i o n s a p p r o p r i a t e t o s l a b s o f w i d t h s determined by t h e f i n i t e steps o f the melt front.
However, these Green's f u n c t i o n s a r e g i v e n as
i n f i n i t e s e r i e s and a r e r a t h e r cumbersome t o work w i t h . a quasi-stationary finite-difference by s e v e r a l groups ( K i r k p a t r i c k ,
Therefore
(QFD) approach has been used
e t al.,
1980; Wood e t al.,
Bakker and Hoonhout, 1981; B a e r i and Campisano, 1982).
1981a;
The formula-
t i o n o f t h e dopant d i f f u s i o n problem i n a f i n i t e - d i f f e r e n c e framework p a r a l l e l s c l o s e l y t h e heat d i f f u s i o n problem a l r e a d y discussed i n Sec. 111, and w i l l n o t be discussed here.
However, i n c o r p o r a t i o n o f
s e g r e g a t i o n e f f e c t s a t t h e moving i n t e r f a c e causes an a d d i t i o n a l cornp l i c a t i o n which, if n o t handled c o r r e c t l y , can l e a d t o convergence and accuracy problems.
F o r small values o f k i , a p r o p e r l y c o n s t r u c t -
ed computer program can a l s o r e q u i r e l a r g e amounts o f computer time. d.
An Approximate Treatment o f Segregation The f i n i t e - d i f f e r e n c e
approach mentioned above i s capable o f
a c h i e v i n g g r e a t accuracy, flexibility,
e.g.,
and i t can p r o v i d e a h i g h degree o f
t h e d i f f u s i o n c o e f f i c i e n t can be a l l o w e d t o
depend on p o s i t i o n and temperature.
However,
t h e computer t i m e
r e q u i r e d t o achieve a c c e p t a b l e accuracy i n c r e a s e s very r a p i d l y as To p a r t i a l l y
t h e i n t e r f a c e s e g r e g a t i o n c o e f f i c i e n t decreases. circumvent t h e s e drawbacks,
an approximate approach based on t h e
f o l l o w i n g model was used by Wood e t a l . (1981a).
A f t e r melting o f
t h e i m p l a n t e d r e g i o n has occurred, t h e dopant p r o f i l e i s a l l o w e d t o develop a c c o r d i n g t o some a n a l y t i c a l approximation, e.g., a l r e a d y d e s c r i b e d above.
those
The p r o f i l e i s t h e n f i x e d and s e g r e g a t i o n
i s a l l o w e d t o occur as t h e m e l t f r o n t sweeps back t h r o u g h t h i s profile.
The c o n c e n t r a t i o n C,(j)
i n the l i q u i d i n the j - t h cell
i s made up o f a c o n t r i b u t i o n C t Y p ( j ) f r o m t h e d i f f u s e d p r o f i l e and a component Cfi,seg(j)
a r i s i n g from segregation, i.e.,
4. MELTING MODEL OF PULSED LASER PROCESSING
231
C, ,P ( j ) i s g i v e n f r o m t h e f i r s t stage o f t h e c a l c u l a t i o n and C t Y s e g ( j ) i s c a l c u l a t e d by assuming t h a t t h e dopant d e p o s i t e d j u s t
i n f r o n t o f t h e advancing m e l t f r o n t d i f f u s e s a c c o r d i n g t o an expon e n t i a l behavior, as one would expect f o r t h e s t e a d y - s t a t e behavior i n a m e l t w i t h a u n i f o r m dopant c o n c e n t r a t i o n .
We have
The n - t h t e r m on t h e r i g h t s i d e o f t h i s e q u a t i o n g i v e s t h e c o n t r i b u t i o n t o t h e dopant c o n c e n t r a t i o n i n t h e j - t h c e l l due t o segregat i o n o f t h e dopant AX,.
in t h e n - t h c e l l as t h e m e l t f r o n t sweeps through
Equations (25) and (26) can be w r i t t e n i n i n t e g r a l form as
The presence of C,(x')
i n t h e i n t e g r a l i n t h i s equation i s cumber-
some, a l t h o u g h a simple i t e r a t i v e scheme might be s u f f i c i e n t t o deal with'it.
A somewhat s i m p l e r expression can be o b t a i n e d by s t r a i g h t -
forward manipulations; t h e r e s u l t i s C,(x)
= (v/D,)
I
X
exp(-(v/D,)ki
(x-x'))CpyP(x')dx'
0
+
X
1
exp (-(v/DL)ki (x-x ' )) (dC, ,p (x ' )/dx ')dx '
0
I f CQ,p(x'),
t h e t h e dopant c o n c e n t r a t i o n i n t h e l i q u i d a f t e r d i f -
f u s i o n , i s constant, t h e second t e r m on t h e r i g h t - h a n d s i d e of t h i s equation i s z e r o and t h e d i s t r i b u t i o n i n t h e s o l i d i s Cs(x) = kiC,(x)
= Co(l-q
exp(-(v/D,)kix))
.
(29)
232
R. F. WOOD ETAL.
T i l l e r e t al.
(1953) o b t a i n e d t h i s e q u a t i o n on an i n t u i t i v e b a s i s
and Smith e t a l . (1955) subsequently showed t h a t i t i s q u i t e a good a p p r o x i m a t i o n t o t h e c o r r e c t form g i v e n by Eq. (22).
When Ce,p(x')
i s n o t constant, t h e d i f f e r e n t i a t i o n and i n t e g r a t i o n s i n Eq. (28) can be c a r r i e d o u t n u m e r i c a l l y . Equations (22) and ( 2 9 ) do n o t g i v e t h e s e g r e g a t i o n s p i k e a t t h e s u r f a c e due t o t h e t e r m i n a l t r a n s i e n t .
T h i s s p i k e can be b u i l t
i n t o t h e c a l c u l a t i o n s i n an approximate b u t somewhat a r t i f i c i a l manner.
From F i g .
18 i t i s seen t h a t t h e t e r m i n a l t r a n s i e n t i s
c o n f i n e d almost e n t i r e l y t o t h e r e g i o n w i t h i n approximately 200 A o f t h e surface; s i n c e t h e experiments cannot g i v e t h e dopant p r o f i l e t h e r e r e l i a b l y , i t seems best j u s t t o d e p o s i t t h e f i n a l amount o f segregated dopant i n t h i s r e g i o n w i t h o u t undue concern f o r i t s exact d i s t r i b u t i o n .
Assuming no loss o f dopant a t t h e s u r f a c e , t h e
amount o f dopant t o be found i n t h i s s p i k e can be o b t a i n e d by i n t e g r a t i n g C s ( x ) up t o 200 A from t h e s u r f a c e and s u b t r a c t i n g t h e r e s u l t f r o m t h e i m p l a n t e d dose. 19.
CALCULATIONS AND RESULTS I n t h i s s u b s e c t i o n r e s u l t s o f c a l c u l a t i o n s by t h e methods
d e s c r i b e d above a r e given.
First,
however,
t h e i n p u t data f o r
t h e c a l c u l a t i o n s a r e discussed. a.
I n p u t Data and S c a l i n g o f M e l t - F r o n t P r o f i l e s The o n l y i n p u t data f o r these c a l c u l a t i o n s a r e t h e i n i t i a l
dopant d i s t r i b u t i o n s , dopant d i f f u s i o n c o e f f i c i e n t s i n molten S i , t h e i n t e r f a c e s e g r e g a t i o n c o e f f i c i e n t s , and t h e m e l t - f r o n t p r o f i l e s . A s discussed i n Chapter 2, d i s t r i b u t i o n s o f i o n - i m p l a n t e d dopants
have g e n e r a l l y been o b t a i n e d by secondary i o n mass spectroscopy (SIMS) o r by R u t h e r f o r d b a c k s c a t t e r i n g (RBS); t h e y can be s u p p l i e d t o t h e computer programs f o r t h e d i f f u s i o n c a l c u l a t i o n s i n numeric a l form or, more approximately, by a f i t t e d gaussian.
Two s e t s o f
values o f D, have appeared i n t h e l i t e r a t u r e . Wood e t a l . (1981a) found t h a t t h e s e t g i v e n by Kodera (1965) gave b e t t e r o v e r a l l
4. MELTING MODEL OF PULSED LASER PROCESSING
233
agreement between c a l c u l a t e d and measured p r o f i l e s t h a n d i d t h e one given by Shashkov and Gurevich (1968); b o t h s e t s a r e shown i n Table V.
I t should be n o t e d t h a t t h e r e a r e l a r g e d i f f e r e n c e s i n
t h e two s e t s f o r a number o f t h e dopants and t h a t t h e e r r o r l i m i t s e s t i m a t e d by Kodera a r e q u i t e l a r g e i n some cases. Table V Values o f kp, DQ, and k i f r o m t h e l i t e r a t u r e Dopant
kya
DRC
DRb
kid
(10-4 cm2/sec)
B
P As Sb Ga In Bi
0.8 0.35 0.3 0.023 0.008 0.0004 0.0007
2.4kO. 7 5.1k1.7 3.320.9 1.5k0.5 4.821.5 6.9k1.2
---
3.3k0.4 2.720.3
---
1.4k0.5 0.7k0.5 0.220.3
kie
v = 4 m/sec
---- -
1.00 0.7 0.2 0.15 0.4
~
0.9-1.0 0.9-1.0 0.9-1.0 0.8-1.0 0.15-0.3 0.10-0.20 0.25-0.35
a. Trumbore (1960); b. Kodera (1965); c. Shashkov and Gurevich (1968); d. White e t a l . (1980); e. Wood e t a l . (1981a) The thermal t r a n s p o r t c a l c u l a t i o n s d e s c r i b e d i n S e c t i o n s 111 and I V can consume l a r g e amounts of computer t i m e ,
and i t i s
i m p r a c t i c a l t o c a l c u l a t e m e l t - f r o n t p r o f i l e s f o r every p o s s i b l e variation
of
l a s e r energy d e n s i t y ,
p u l s e d u r a t i o n time,
etc.
Also, s i n c e t h e r e a r e u n c e r t a i n t i e s i n t h e thermal c o n d u c t i v i t y , r e f l e c t i v i t y b e f o r e and a f t e r annealing, a b s o r p t i o n c o e f f i c i e n t s , and o t h e r experimental q u a n t i t i e s ,
some v a r i a t i o n of m e l t - f r o n t
p r o f i l e s t o improve t h e f i t between c a l c u l a t e d and experimental dopant p r o f i l e s i s j u s t i f i e d .
A simple s c a l i n g procedure which i s
u s e f u l i n t h i s connection i s d e s c r i b e d by Wood e t a l . (1981a). b.
C a l c u l a t i o n s w i t h t h e A n a l y t i c a l Approximations F i g u r e 19 shows SIMS p r o f i l e s of boron b e f o r e and a f t e r l a s e r
a n n e a l i n g o f a S i sample implanted w i t h 35 keV B t o a dose o f 1.03 x 1016/cm2. The annealing was c a r r i e d o u t w i t h a ruby l a s e r u s i n g a
234
R. F. WOOD ET AL.
102'
IMPLANTED PROFILE
ANNEALED PROFILE -A-
--*--
ANALYTICAL FINITE DIFFERENCE
\
'
0
0.f
Y
0.2
0.3
0.4
0.5
0.6
DEPTH (,urn)
F i g . 1 9 . Concentration o f boron a s a function o f depth before and a f t e r laser annealing. The solid curve marked with triangles was calculated using the instantaneous approximation o f Eqs. ( 2 3 ) and ( 2 4 ) ; the dashed curve marked with diamonds was calculated by the QFD method. Both calculations assumed k i = 1 . (Wood e t a l . ,
1981a)
p u l s e energy d e n s i t y o f a p p r o x i m a t e l y 1.8 J/cm2.
The c i r c l e s and
squares g i v e t h e e x p e r i m e n t a l data b e f o r e and a f t e r l a s e r annealing, respectively.
The s o l i d
curve
marked w i t h t r i a n g l e s was c a l c u l a t e d
by u s i n g t h e as-implanted p r o f i l e f o r C i (xo,to) i n t h e i n s t a n t a n e o u s a p p r o x i m a t i o n o f Eqs. (23) and ( 2 4 ) and v a r y i n g t h e q u a n t i t y D g ( t - t o ) t o o b t a i n a s a t i s f a c t o r y f i t t o t h e laser-annealed p r o f i l e . With K o d e r a ' s v a l u e o f DR f o r boron f r o m Table
V,
t-to = 180 nsec, and
w i t h t h e Shashkov and Gurevich v a l u e t-to= 130 nsec.
These times
a r e c o n s i s t e n t w i t h t h e t i m e t h a t a t h i n l a y e r a t a depth o f vm remains m o l t e n a f t e r a 1.75-2.0-J/cm2 ruby l a s e r p u l s e .
- 0.40
The
4.
235
MELTING MODEL OF PULSED LASER PROCESSING
dashed c u r v e on t h e f i g u r e g i v e s t h e f i t t o t h e experimental d a t a when t h e a c t u a l motion o f t h e m e l t f r o n t i s t a k e n i n t o account i n a finite-difference calculation. results f o r
The r e s u l t s on Fig. 19 and s i m i l a r
P- and As-implanted samples e s t a b l i s h t h a t , even on
t h e s i m p l e s t model, d i f f u s i o n i n t h e m o l t e n s t a t e can e x p l a i n t h e observed dopant p r o f i l e spreading. F i g u r e s 20a and b show r e s u l t s o f e a r l y attempts t o f i t t h e dopant p r o f i l e s i n As-implanted (100 keV, 1.4 t h e experimental d a t a i s f r o m White e t a l .
x
1016 cm-2) samples;
(1978).
These r e s u l t s
a r e o f p a r t i c u l a r i n t e r e s t because t h e l a t e s t work on t h e m e l t i n g o f a-Si (see Sec. 111) has almost c e r t a i n l y s u p p l i e d t h e s o l u t i o n t o a p r e v i o u s l y unresolved d i f f i c u l t y i n f i t t i n g t h e p r o f i l e of F i g . 20a f o r
E,
= 0.82
From Fig. 20b i t can be seen t h a t
J/cm2.
t h e measured and c a l c u l a t e d p r o f i l e s a r e i n good agreement w i t h i n t h e probable e r r o r o f t h e
RBS measurements.
For values o f E,
on Fig. 20b, t h e m e l t f r o n t p e n e t r a t e d e n t i r e l y t h r o u g h t h e pm
shown
,., 0.16-
amorphous l a y e r produced by t h e i m p l a n t a t i o n , and e p i t a x i a l r e -
growth ensued.
F o r t h e 0.82-J/cm2
penetrated only t o
-
0.1
pm;
p u l s e of Fig. 20a, t h e m e l t f r o n t
n o t e t h a t t h e i m p l a n t e d and annealed
p r o f i l e s c o i n c i d e a f t e r t h i s depth.
C a l c u l a t i o n s o f dopant d i f -
f u s i o n w i t h k i = 1 and 0.7 gave r e s u l t s which c l e a r l y do n o t f i t t h e experimental d a t a w e l l .
Moreover,
f o r k i = 0.7
there i s a
s u f f i c i e n t l y l a r g e segregation spike a t t h e surface t h a t i t i s u n l i k e l y i t would have gone undetected i n t h e experiments. i s t h e o r i g i n o f these discrepancies?
What
When t h e c a l c u l a t i o n s were
c a r r i e d o u t t h e very low values o f t h e thermal c o n d u c t i v i t y o f a-Si d i s c u s s e d i n Sec.
I V had n o t been determined.
As discussed i n
Sec. I V , we now know t h a t t h e presence o f a-Si l a y e r s can have l a r g e and p r e v i o u s l y unforeseen impact on t h e s o l i d i f i c a t i o n b e h a v i o r when t h e m e l t f r o n t does n o t p e n e t r a t e t h r o u g h t h e amorphous region. M e l t d u r a t i o n s a r e prolonged by t h e heat f l o w b a r r i e r formed by t h e unmelted a-Si l a y e r , b u l k n u c l e a t i o n may occur and t h e s u r f a c e may s o l i d i f y b e f o r e t h e i n t e r i o r does.
Although d e t a i l e d c a l c u l a t i o n s
236
R. E WOOD ET AL. 20
I
I
I
0
0.04
0.08
INITIAL
I
I
0.46
0.20
i8
-
16
‘?E
o” !4 N
-0z
42
P + a 40 cc b-
z
8z 8
8 0 z
6
W
M
a
4
2
n
I
O.i2 DEPTH ( p m )
Fig. 20a.
Dopant profiles for As-implanted Si.
For a 0 . 8 2 J/cm2 pulse the
melt front does not penetrate through the implanted layer.
- 16
m I
g
14
%
0 12
EXPERIMENT
v c
o_
C A L C ULAT I0N
I0
I-
2 8
0
I-
z W 0
z
0
0
6
EXPERIMENT C A L C UL AT ION
4
0 6
v,
2
LT
a 0
0
Fig. 20b.
0.04
0.08
0.12 0.16 DEPTH ( p m )
0.20
0.24
Comparison o f experimental and calculated profiles obtained when
EQ i s great enough for the melt front to penetrate beyond the implanted layer.
237
4. MELTING MODEL OF PULSED LASER PROCESSING
t o e s t a b l i s h t h a t t h i s complex behavior i s t h e source o f t h e d i s crepancies between t h e c a l c u l a t e d and measured p r o f i l e s on Fig. 20a have not y e t been made, t h e r e seems l i t t l e doubt t h a t i t i s . C a l c u l a t i o n s w i t h t h e m o d i f i e d instantaneous approximation were c a r r i e d out t o f i t dopant p r o f i l e s obtained when As-doped l a y e r s o f a-Si, deposited by e l e c t r o n beam evaporation on c-Si substrates, were r e c r y s t a l l i z e d by laser-induced m e l t i n g (Young e t al.
, 1979).
The e x c e l l e n t f i t s between measured and c a l c u l a t e d p r o f i l e s f o r l a y e r s 0.1 and 0.2 p m t h i c k are shown i n Fig. 21.
The experimental
p r o f i l e s a f t e r annealing were determined by anodic o x i d a t i o n , 2
102’
5
2
1019
5 0.1
0.2
0.3
0.4
0.5
DEPTH ( p m )
Fig. 21. 0.2-pm
Dopant profiles in Si samples on which As-doped a-Si layers 0.1 and
thick were deposited and subsequently laser irradiated.
curves are the assumed idealized starting profiles.
The dashed
(Young et a l . ,
1979)
/ .a.
0
., I .I
0
N
W 0
n
I-
I
E
-3.
0 -
-
0
0
0
: 0
N
0
w
a
I-
I
-3.
E
0 -
0
0
0
iiic - 0
.E
00
co- x
0
239
4. MELTING MODEL OF PULSED LASER PROCESSING
s t r i p p i n g , and measurements o f t h e e l e c t r i c a l c a r r i e r c o n c e n t r a t i o n . The s t a r t i n g p r o f i l e s cannot be o b t a i n e d i n t h i s manner because t h e As i s e l e c t r i c a l l y i n a c t i v e i n t h e d e p o s i t e d amorphous l a y e r . However,
t h e d e p o s i t i o n c o n d i t i o n s were such t h a t t h e i n i t i a l As
c o n c e n t r a t i o n should have been f a i r l y uniform.
The u n i f o r m con-
c e n t r a t i o n s r e q u i r e d t o g i v e t h e c a l c u l a t e d p r o f i l e s a r e shown by t h e dashed l i n e s on t h e f i g u r e .
I n t h e s e experiments, t h e p u l s e
energy d e n s i t i e s (shown on Fig. 21) were w e l l above t h o s e r e q u i r e d t o m e l t c o m p l e t e l y t h r o u g h t h e amorphous l a y e r s and t h e e f f e c t s o f t h e small
v a l u e o f Ka a r e n o t expected t o be p a r t i c u l a r l y
significant.
c.
C a l c u l a t i o n s f o r Dopants Showing Large Segregation E f f e c t s U n l i k e t h e r e s u l t s f o r B-,
P-,
and As-implanted laser-annealed
S i , t h e p r o f i l e s o f Gay I n , B i , and t o a l e s s e r e x t e n t Sb, i n S i
a f t e r l a s e r a n n e a l i n g show c l e a r evidence o f segregation.
Surface
s e g r e g a t i o n s p i k e s a r e observed, b u t t h e y a r e n o t n e a r l y as l a r g e as t h e y would be i f t h e values o f k:
shown i n Table V were a p p l i -
cable.
ki
The n e c e s s i t y
of choosing
>
kp
t o o b t a i n agreement
between experimental and c a l c u l a t e d p r o f i l e s and t h e f a c t t h a t t h e e q u i l i b r i u m s o l u b i l i t y l i m i t can be g r e a t l y exceeded as a r e s u l t
o f l a s e r a n n e a l i n g show t h a t t h e process i s a h i g h l y n o n e q u i l i b r i u m one. C a l c u l a t i o n s by Wood e t a l . (1981a) o f p r o f i l e s i n B i - and I n i m p l a n t e d S i a r e used here as t h e p r i m a r y i l l u s t r a t i o n s o f t h e r e s u l t s o f t r e a t i n g ki periments (White e t a1 t o a dose of respectively.
- 1.5
as an a d j u s t a b l e parameter.
., 1980),
I n t h e ex-
bismuth and i n d i u m were i m p l a n t e d
1 . 2 ~ 1 0 cmm2 ~ ~ a t energies o f 250 keV and 125 keV, Ruby l a s e r pulses o f
J/cm2 were used f o r annealing.
- 15-nsec
d u r a t i o n (FWHM) and
The experimental r e s u l t s a r e
shown i n Figs. 22a and 22b f o r B i and In, r e s p e c t i v e l y .
When f i t -
t i n g t h e data, t h e experimental p o i n t s i n t h e f i r s t 200 A o f t h e sample where t h e very l a r g e s e g r e g a t i o n s p i k e s occur were excluded. A v a l u e o f t h e d i f f u s i o n c o e f f i c i e n t f o r B i i n S i i s n o t g i v e n by
240
R. F. WOOD ETAL.
Kodera; s e v e r a l values were t e s t e d b u t i n t h e c a l c u l a t i o n s d e s c r i b e d h e r e Dg had t h e v a l u e 2.4~10-4 cm2/sec. a maximum p e n e t r a t i o n depth o f phous l a y e r .
- 0.36
The m e l t - f r o n t p r o f i l e gave pm f o r a 0.18-pm
t h i c k amor-
A value o f approximately 0.35 f o r k i gave t h e best f i t
t o t h e B i p r o f i l e , and t h e r e s u l t s a r e shown on Fig. 22a by t h e s o l i d line.
A l s o shown on t h e f i g u r e i s t h e p r o f i l e f o r k i = 1.0,
i.e.,
w i t h no s e g r e g a t i o n and t h e p r o f i l e u s i n g t h e e q u i l i b r i u m v a l u e o f k y = 0.0007;
the l a t t e r i s clearly
c o m p l e t e l y unable
s a t i s f a c t o r y f i t t o t h e experimental data.
t o give a
An examination o f t h e
amount o f dopant segregated t o t h e s u r f a c e r e g i o n (depth
3
200 A )
g i v e s a u s e f u l check on t h e value o f k i o b t a i n e d f r o m f i t t i n g t h e smooth p a r t o f t h e p r o f i l e p r o v i d e d t h e depth o f t h e dopant d i s t r i b u t i o n i s g r e a t enough f o r t h e i n i t i a l t r a n s i e n t t o have d i e d out.
The experimental data show t h a t a p p r o x i m a t e l y 18% o f t h e
i m p l a n t e d dose i s l o c a t e d w i t h i n t h e f i r s t 200 A o f t h e surface. The c o r r e s p o n d i n g percentages from c a l c u l a t i o n s w i t h t h e f i n i t e d i f f e r e n c e method were 6, 11, 15, and 31 f o r k i values o f 1.0, 0.3,
and 0.1,
respectively.
Thus,
0.5,
b o t h t h e percentage o f dopant
i n t h e s u r f a c e peak and t h e l e a s t - s q u a r e s f i t t i n g o f t h e dopant p r o f i l e (which excludes t h e s u r f a c e peak) i n d i c a t e a v a l u e o f k i i n t h e range 0.25-0.35.
Calculations w i t h t h e modified instan-
taneous a p p r o x i m a t i o n method were a l s o c a r r i e d o u t w i t h r e s u l t s i n When kp = 0.0007 was good agreement w i t h t h o s e j u s t described. used i n t h e c a l c u l a t i o n s , g r e a t e r t h a n 99% o f t h e dopant was segregated t o t h e surface. The r e s u l t s f o r c a l c u l a t i o n s on I n i n S i w i t h D,
= 6.9~10-4
a r e shown i n Fig. 22b. The b e s t f i t t o t h e experimental d a t a was o b t a i n e d w i t h k i = 0.12 which i s q u i t e c l o s e t o t h e v a l u e o f 0.15 found by White e t a l .
(1980).
A c a l c u l a t i o n w i t h k j = 0.12 u s i n g
t h e f i n i t e - d i f f e r e n c e method gave t h e d o t t e d curve on Fig. 22b; t h e calculation
with
k y = 0.0004
a l s o shown on t h e f i g u r e .
u s i n g t h e method o f Sec. V.18d i s
The c a l c u l a t e d amount o f dopant segre-
g a t e d t o t h e s u r f a c e was-61$, i n e x c e l l e n t agreement w i t h t h e v a l u e o f 60% o b t a i n e d e x p e r i m e n t a l l y . When ky = 0.0004 was used i n t h e
4. MELTING MODEL OF PULSED LASER PROCESSING
241
c a l c u l a t i o n s , n e a r l y 100% o f t h e dopant was segregated t o t h e surface;
c l e a r l y e q u i l i b r i u m values o f t h e segregation c o e f f i c i e n t
g e n e r a l l y cannot be used f o r c a l c u l a t i o n s o f pulsed l a s e r annealing. An a d d i t i o n a l problem i s encountered when dopant p r o f i l e s o f Sb and Ga i n S i are considered, namely, t h a t o f dopant l o s s d u r i n g annealing (White e t al., Wood e t a l .
1980).
I n o r d e r t o study t h i s problem,
(1981a) compared t h e r e s u l t s obtained f o r t h r e e d i f -
f e r e n t assumptions about dopant loss.
I n one s e t o f c a l c u l a t i o n s ,
a l l o f t h e dopant l o s s occurred d u r i n g t h e i n i t i a l p a r t o f t h e surface m e l t i n g when t h e temperatures i n t h e l i q u i d S i were t h e highest.
Equally good f i t s t o t h e experimental data were found f o r
k i values o f 1.0 and 0.8 f o r Sb i n S i .
However, when t h e same
assumption was a p p l i e d t o t h e case o f Ga i n S i , s a t i s f a c t o r y f i t s
A second set o f c a l c u l a t i o n s assumed t h a t
could not be obtained.
dopant loss occurred o n l y when t h e surface concentration exceeded some t h r e s h o l d value.
Good f i t s t o both t h e Sb and Ga experimental
data were r e a l i z e d , b u t t h e f i t f o r Sb r e q u i r e d a value o f k i = 0.4 and a maximum m e l t - f r o n t p e n e t r a t i o n s u b s t a n t i a l l y l e s s than t h a t i n d i c a t e d by t h e heat t r a n s f e r c a l c u l a t i o n s .
The t h i r d s e t o f
c a l c u l a t i o n s assumed t h a t t h e dopant loss i s p r o p o r t i o n a l t o t h e surface concentration, as one would expect from k i n e t i c r a t e theory. I t was found t h a t s a t i s f a c t o r y f i t s t o b o t h Sb and Ga i n S i could
be obtained w i t h t h i s method; t h e r e s u l t s f o r Sb i n S i a r e shown i n Fig. 23. 20.
DISCUSSION AND
CONCLUSIONS
A number o f p o i n t s should be kept i n mind when c o n s i d e r i n g t h e r e s u l t s o f t h i s section.
F i r s t , on t h e experimental side, i t should
be recognized t h a t Rutherford backscattering, though a very powerful technique, y i e l d s f a i r l y s u b s t a n t i a l e r r o r bars on t h e data, unless very l o n g counting times can be used t o reduce t h e s t a t i s t i c a l f l u c tuations.
The c a l c u l a t i o n s discussed above u t i l i z e d RMS f i t t i n g s
t o t h e data as though no e r r o r s were involved.
Had t h e s t a t i s t i c a l
242
R. F. WOOD ET AL
IMPLANTED LASER ANNEALEO ki = t.0
0
-I
0.04 0.08
0
0.12
0.16 0.20 0.24
DEPTH
0.28
(pm)
Fig. 23. Antimony distribution in Sb-implanted Si before and a f t e r laser annealing. There i s some loss of Sb from the sample during laser annealing. The solubility limit of Sb in Si is - 7 ~ 1 0 1 ~ / c m ~ .
e r r o r s been t a k e n i n t o c o n s i d e r a t i o n , i t would have been d i f f i c u l t t o differentiate,
f o r example,
between a ki
o f 1.0 and 0.9 f o r
f i t t i n g t h e As data, o r between k i values o f 0.25-0.45 t h e smooth p a r t o f t h e B i p r o f i l e .
for fitting
Somewhat c o u n t e r b a l a n c i n g t h i s
d i f f i c u l t y a r e t h e p r e c i s e r e s u l t s t h a t b o t h t h e experiments and t h e c a l c u l a t i o n s g i v e f o r t h e amount o f dopant i n t h e s u r f a c e spike. Good agreement between experiment and t h e o r y f o r t h e percentage o f dopant i n t h i s s p i k e i s g e n e r a l l y o b t a i n e d , and t h i s percentage i s q u i t e c o n s i s t e n t w i t h t h e r e s u l t s o f t h e RMS f i t t i n g o f t h e smooth part of the profile.
Nevertheless,
i t s h o u l d be recognized t h a t
t h e e x t r a c t i o n o f k i from t h e e x p e r i m e n t a l data may be s u b j e c t t o some f a i r l y s u b s t a n t i a l e r r o r s . Turning t o t h e calculations,
we see f r o m Eqs.
(20) and ( 2 2 )
t h a t t h e r a t i o o f v/DR i s a fundamental q u a n t i t y i n t h e a n a l y t i c a l s o l u t i o n s f o r t h e case t h a t t h e i n i t i a l dopant p r o f i l e i s c o n s t a n t ;
243
4. MELTING MODEL OF PULSED LASER PROCESSING
t h i s r a t i o a l s o i s of b a s i c importance i n t h e more complicated case of nonuniform p r o f i l e s .
The heat t r a n s p o r t and m e l t i n g c a l c u l a t i o n s
can be assumed t o g i v e r a t h e r a c c u r a t e values o f v as l o n g as t h e dopant c o n c e n t r a t i o n remains low.
The values o f
D, however a r e
s u b j e c t t o f a i r l y l a r g e u n c e r t a i n t i e s as i n d i c a t e d by t h e e r r o r e s t i m a t e s of Kodera (1965) and t h e d i f f e r e n c e s between Kodera's values and those o f Shashkov and Gurevich (1968).
I n t h i s connec-
t i o n , Wood e t a l . (1981a) c a r r i e d o u t c a l c u l a t i o n s on b o t h B i and I n u s i n g t h e values o f
D,
from Shashkov and Gurevich and found t h e
r e s u l t s t o t a l l y unacceptable u n l e s s t h e m e l t - f r o n t p r o f i l e s were g r e a t l y a l t e r e d f r o m t h o s e d i c t a t e d by t h e heat t r a n s f e r c a l c u l a tions.
T h i s would seem t o be a c l e a r i n d i c a t i o n t h a t Kodera's
values a r e t h e more a c c u r a t e ones.
Apparently a u s e f u l way t o
D,
and k i i s t o employ whatever
proceed i n d e t e r m i n i n g values o f
means a v a i l a b l e t o a s c e r t a i n t h a t t h e c a l c u l a t i o n s o f m e l t - f r o n t p r o f i l e s give r e l i a b l e results. e t al.
Time-resol ved r e f l e c t i v i t y (Lowndes
(1983) and e l e c t r i c a l c o n d u c t i v i t y ( G a l v i n e t a l . ,
1982;
Thompson and G a l v i n , 1983) measurements a r e extremely u s e f u l i n t h i s connection.
Then t h e p r o f i l e s c a l c u l a t e d can be used i n t h e dopant
d i f f u s i o n c a l c u l a t i o n s t o determine r e f i n e d values o f values o f k i .
Thus,
D, and new
systematic e x p e r i m e n t a l s t u d i e s o f l a s e r
annealing, coupled w i t h r e f i n e d c a l c u l a t i o n s o f m e l t - f r o n t p r o f i l e s and f u r t h e r experiments and c a l c u l a t i o n s on dopant r e d i s t r i b u t i o n , can be expected t o l e a d t o improved values o f b o t h l a s t two columns i n Table V show t h e values o f t i o n s o f White e t a l . and from Wood e t a l .
kj
DQ and k i .
The
from t h e c a l c u l a -
When t h e v a r i o u s sources
o f p o s s i b l e e r r o r i n t h e experiments and c a l c u l a t i o n s a r e considered, t h e r e i s e s s e n t i a l l y complete agreement between t h e two s e t s o f c a l c u l a t i o n s which were done c o m p l e t e l y independently. The sketchy experimental r e s u l t s presented t h u s f a r on those cases i n which t h e m e l t f r o n t does n o t p e n e t r a t e e n t i r e l y through t h e implanted p r o f i l e may p o i n t t h e way toward a deeper unders t a n d i n g o f t h e dynamics o f u l t r a r a p i d c r y s t a l l i z a t i o n and b u l k nucleation. It w i l l r e q u i r e h i g h i n s t r u m e n t a l r e s o l u t i o n and a
244
R. F. WOOD E T A L .
combination o f c a r e f u l e x p e r i m e n t a t i o n and r e a l i s t i c modeling t o d i s e n t a n g l e b u l k n u c l e a t i o n e f f e c t s , t r a n s i e n t e f f e c t s a r i s i n g from s h a l l o w me1t - f r o n t p e n e t r a t i o n , and s t e a d y - s t a t e segregation.
The
c a p a b i l i t y o f t r e a t i n g transients i s already b u i l t i n t o the
QFD
approach, b u t n u c l e a t i o n p r e s e n t s a problem. An e s p e c i a l l y i m p o r t a n t e f f e c t o f t h e t r a n s i e n t s may come i n t o p l a y i n t h e i n t e r p r e t a t i o n o f experiments i n which t h e dependence o f k i on m e l t - f r o n t v e l o c i t y (discussed i n Cfiapter 5) i s studied. shows t h a t f o r k i
<
0.15,
As a l r e a d y noted, Fig. 18
a steady-state s i t u a t i o n i s not attained
b e f o r e t h e m e l t f r o n t reaches t h e surface.
I n such a case, t h e
amount o f dopant c o n t a i n e d i n t h e s u r f a c e peak i s n o t determined e n t i r e l y by k j , as i t i s when a s t e a d y - s t a t e s i t u a t i o n e x i s t s . t h e m e l t - f r o n t v e l o c i t y i s reduced (e.g.,
When
by s u b s t r a t e h e a t i n g ) t o
t h e p o i n t t h a t k j f a l l s below about 0.15-0.20,
o r when t h e m e l t -
f r o n t p e n e t r a t i o n i s very shallow, i t w i l l be necessary t o c o r r e c t f o r t h e e f f e c t o f t h e i n i t i a l t r a n s i e n t on t h e amount o f dopant segregated t o t h e s u r f a c e b e f o r e t h e t r u e dependence o f k i on v can be determined.
T h i s p o i n t seems t o have been g e n e r a l l y overlooked
i n the literature.
VI.
Summary and D i r e c t i o n s f o r F u t u r e Work
I n t h i s chapter, we have examined t h e t h e o r e t i c a l b a s i s f o r t h e m e l t i n g model o f p u l s e d l a s e r a n n e a l i n g o f semiconductors.
The
p r i n c i p a l a b s o r p t i o n mechanisms o f l i g h t o f t h e wavelengths o f i n t e r e s t (near i n f r a r e d t o near u l t r a v i o l e t ) i n S i , Ge, and GaAs a r e e l e c t r o n - h o l e p a i r p r o d u c t i o n and induced f r e e - c a r r i e r absorption.
Recombination processes and t h e c a r r i e r - l a t t i c e i n t e r a c t i o n
r e s u l t i n t h e l i g h t energy b e i n g c o n v e r t e d t o heat i n t i m e s o f t h e o r d e r o f 10-12 sec, making t h e m e l t i n g model presented here v a l i d f o r l a s e r s w i t h p u l s e d u r a t i o n s o f t h e o r d e r o f tens o f picoseconds and g r e a t e r . D e t a i l s o f t h e m e l t i n g model f o r m u l a t i o n o f p u l s e d l a s e r anneali n g have been given, i n c l u d i n g a d i s c u s s i o n o f t h e thermal and o p t i c a l p r o p e r t i e s t h a t go i n t o t h e numerical c a l c u l a t i o n s . The
4. MELTING MODEL OF PULSED LASER PROCESSING
245
m e l t i n g model was used t o c a l c u l a t e a number o f q u a n t i t i e s t h a t can be d i r e c t l y compared w i t h experimental r e s u l t s i n Chapters 2, 3, 6, and 8 i n c l u d i n g maximum me1t - f r o n t p e n e t r a t i o n , me1t d u r a t i o n times , m e l t - f r o n t v e l o c i t i e s , and dopant c o n c e n t r a t i o n p r o f i l e s .
The m e l t -
i n g model has been e x h a u s t i v e l y t e s t e d under a v a r i e t y o f e x p e r i mental c o n d i t i o n s , p a r t i c u l a r l y i n S i and GaAs (Chapter 8), and i t s e s s e n t i a l v a l i d i t y i n t h e nanosecond regime has been c o n v i n c i n g l y demonstrated.
The q u e s t i o n t h e n a r i s e s as t o t h e f u t u r e d i r e c t i o n
o f t h i s t y p e o f work. We b e l i e v e t h a t more work i s needed i n s i l i c o n t o completely " c a l i b r a t e " t h e model c a l c u l a t i o n s and t o , a t t h e same time, d e t e r mine more a c c u r a t e l y , some o f t h e p h y s i c a l parameters i n v o l v e d i n t h e modeling.
Both t i m e - r e s o l v e d and post-anneal i n g experiments
w i l l be i n v a l u a b l e i n t h i s connection.
Additional information
about t h e r e f l e c t i v i t y o f m o l t e n semiconductors a t t h e s h o r t wavel e n g t h s o f excimer l a s e r s i s b a d l y needed t o make t h e c a l c u l a t i o n s more r e a l i s t i c .
P a r t i c u l a r a t t e n t i o n should be p a i d t o t h e e f f e c t s
o f even small amounts o f i m p u r i t i e s on. t h e m e l t - f r o n t v e l o c i t i e s . T r a n s i e n t e f f e c t s must be c a r e f u l l y accounted f o r i n e x t r a c t i n g values o f k i f r o m t h e experimental data.
The q u e s t i o n o f whether
o r n o t k i as a f u n c t i o n o f v e l o c i t y s a t u r a t e s a t h i g h v e l o c i t i e s can o n l y be answered i n t h i s way (see Chapter 5).
The r o l e o f
i n t e r f a c i a l u n d e r c o o l i n g and i t s i n c o r p o r a t i o n i n t o t h e heat f l o w equations r e q u i r e s more thorough study, as does t h e whole q u e s t i o n of how t o t r e a t t h e h i g h l y undercooled l i q u i d formed when a - S i i s m e l t e d by l a s e r pulses. T u r n i n g t o o t h e r m a t e r i a l s , we n o t e t h a t very l i t t l e work has been done on m a t e r i a l s o t h e r t h a n s i l i c o n , w i t h t h e p o s s i b l e except i o n o f GaAs.
The work on GaAs d e s c r i b e d i n Chapter 8 shows con-
v i n c i n g l y t h a t p u l s e d l a s e r a n n e a l i n g can be used t o determine dopant d i f f u s i o n c o e f f i c i e n t s i n t h e l i q u i d s t a t e ; more s t u d i e s o f t h i s t y p e are needed i n o t h e r compound semiconductors.
Silicon
and GaAs have undoubtedly r e c e i v e d t h e most a t t e n t i o n because o f t h e i r p o t e n t i a l a p p l i c a t i o n s i n t h e semiconductor i n d u s t r y . From
246
R. F. WOOD E T A L
a more fundamental s t a n d p o i n t , i t would appear t h a t work on germanium now o f f e r s some very i n t e r e s t i n g p o s s i b i l i t i e s t o f u r t h e r t e s t o u r u n d e r s t a n d i n g o f t h e r o l e s p l a y e d by v a r i o u s p h y s i c a l parameters, e.g.,
o p t i c a l p r o p e r t i e s , m e l t i n g temperatures,
l a t e n t heats, and
thermal c o n d u c t i v i t i e s o f t h e v a r i o u s phases, t h e r o l e p l a y e d by i n t e r f a c i a l u n d e r c o o l i n g and t h e occurrence and e f f e c t s o f b u l k nucleation.
C l e a r l y , a l t h o u g h much has been accomplished, much more
remains t o be done.
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K i r k p a t r i c k , J. R., G i l e s , G. E., and Wood, R. F. (1980). I n "Heat T r a n s f e r and Thermal C o n t r o l " Vol. 78, p. 152. A I A A Progress i n A s t r o n a u t i c s and Aeronautics Series, New York. K i t t e l , C. (1971). " I n t r o d u c t i o n t o S o l i d S t a t e Physics" F o u r t h E d i t i o n , p. 263. Wiley and Sons, New York. Knapp, J. A., and Picraux, S. T. (1981). Appl. Phys. L e t t . 38, 873. Kodera, H. (1963), Jpn. J. Appl. Phys. 2, 212. Kokorowski, S. A . , Olsen, G. L., and Hess, L. D. (1982). J. Appl. Phys. 53, 921. L i e t o i l a , A., and Gibbons, J. F. (1979). Appl. Phys. L e t t . 34, 332. L i u , P. L., Yen, R., and Bloembergen, N. (1979). Appl. Phys. L e t t . 34, 864. L i u , J. M., Yen, R., Kurz, H., and Bloembergen, N. (1982). Appl. Phys. L e t t . 41, 643. Lo, H. W., and Compaan, A. (1980). Phys. Rev. L e t t . 44, 1604. Lo, H. W., and Compaan, A. (1981). Appl. Phys. L e t t . 38, 179. Lowndes, D. H., J e l l i s o n , G. E., Jr., and Wood, R. F. (1982a). Phys. Rev. 0 26, 6747. Lowndes, D. H., Cleland, J. W., C h r i s t i e , W. H., Eby, R. E., J e l l i s o n , G. E., Narayan, J., Westbrook, R. D., Wood, R. F., N i l s o n , J. A., and Dass, S. C. (1982b). Appl. Phys. L e t t . 41, 938. Lowndes, D. H., Wood, R. F., and Westbrook, R. D. (1983). Appl. Phys. L e t t . 43, 258. Lowndes, D. H. , Wood, R. F., and Narayan, J. (1984). Phys. Rev. Lett. 52, 561. M u l l i n s , W. W. , and Sekerka, R. F. (1964). J. Appl. Phys. 35, 444. Narayan, J. (1980). J. Met. 32, 15. Nath, P., and Chopra, K. L. (1974). Phys. Rev. B 10, 3415. Olson, G. L., Kokorowski , S. A., Roth, J. A., and Hess, L. D. (1983). Mat. Res. SOC. Symp. Proc. 13, 141. Poate, J. M. (1981). Mat. Res. SOC. Symp. Proc. 1, 46. Ready, J. F. (1971). " E f f e c t s o f High Power Laser R a d i a t i o n . " Academic Press, New York. Shank, C. V. , Yen, R., and H i r l i m a n , C. (1983). Phys. Rev. Lett. 5 0 , 454. Shanks, H. R., Maycock, P. D., S i d l e s , P. H., and Danielson, G. C. (1963). Phys. Rev. 130, 1743. Shashkov, Y. M., and Gurevich, V. M. (1968). Russ. J. Phys. Chem. 42, 1082. Smith, V. G., T i l l e r , W. A., and R u t t e r , J. W. (1955). Can. J. Phys. 33, 723. Sooy, W. R., G e l l e r , M., B o r t f e l d , D. P. (1964). Appl. Phys. L e t t . 5, 54. Spaepen F., and T u r n b u l l , D. (1978). I n "Laser-Sol i d I n t e r a c t i o n s and Laser Processing" (S. D. F e r r i s , H. J. Leamy, and J. M. Poate, eds.), p. 97. Am. I n s t . Phys., New York. Surko, C. M., Simons, A. L., Auston, D. H., Golovchenko, J. A., Slusher, R. E. , and Venkatesan, T. N. C. (1979). Appl. Phys. L e t t . 34, 635.
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Svantesson, K. G., Nilsson, N. G., and Huldt, L. (1971). S o l i d S t a t e Commun. 9, 213. Svantesson,-K. G., and Nilsson, N. G. (1978). Physica S c r i p t a 18, 405. Thompson, M. O., and Galvin, G. J. (1983). Mat. Res. SOC. Symp. Proc.
13, 57.
Thurmond, C. D. (1959). I n "Semiconductors" (N. B. Hannay, ed.), p. 145. Reinhold. T i l l e r , W. A., Jackson, K. A,, Rutter, J. W., and Chalmers, B. (1953). Acta Met. 1. 428. Trumbore, F. A. (1960). B e l l System Tech. J. 39, 205. Tsu, R., Hodgson, R. T., Tan, T. Y., and Baglin, J. E. (1979). Phys. Rev. L e t t . 42, 1356. T u r n b u l l , D. (1956). S o l i d S t a t e Physics 3, 225. T u r n b u l l and Cohen (1960). I n "Modern Aspects o f t h e Vitreous State" (J. D. Mackensie, ed.), p. 38. Butterworth, London. van D r i e l , H. M., Preston, J. S . , and Gallant, M. I, (1982). Appl. Phys. L e t t . 40, 385. van Gurp, G. J., Eggermont, G. E., Tamninga, Y., Stacy, W. T., and Gijsbers, J. R. M. (1979). Appl. Phys. L e t t . 35, 273. van Vechten, J. A. (1980). I n "Laser and E l e c t r o n Beam Processing of M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 53. Academic Press, New York. van Vechten, J. A. (1983). I n "Cohesive P r o p e r t i e s o f Semiconductors Under Laser I r r a d i a t i o n " (L. D. Laude, ed.), p. 429. Martinus Nighoff, The Hague. von Allmen, M. (1980). I n "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 6. Academic Press, New York. von Allmen, M., A f f o l t e r , K., and Wittmer, M. (1981). Mat. Res. SOC. Symp. Proc. 1, 559. von A1 lmen, M. (1982). I n "Laser Annealing o f Semiconductors" (J. M. Poate and J. W. Mayer, Eds.), p. 43. Academic Press, New York. von der Linde, D., and F a b r i c i u s , N. (1982). Appl. Phys. L e t t . 41,
991.
Wang, J. C., Wood, R. F. , and Pronko, P. P. (1978). Appl. Phys. L e t t . 33, 455. Wang, J. C., Wood, R. F., White, C. W., Appleton, B. R., Pronko, P. P., Wilson, S . R., and C h r i s t i e , W. H. (1979). I n "Laser-Solid I n t e r a c t i o n s and Laser Processing-1978" (S. D. F e r r i s , H. J. Leamy, and J. M. Poate, eds.), p. 127. A I P Conference Proceedings No. 50, New York. Webber, H. C. , C u l l i s , A. G., and Chew, N. G. (1983). Appl. Phys. L e t t . 43, 669. White, C. W., C h r i s t i e , W. H., Pronko, P. P., Appleton, B. R., Wilson, S . R., Young, R. T., Wang, J. C., Wood, R. F., Narayan, J., and Magee, C. W. (1978). Conf. on I o n Beam M o d i f i c a t i o n o f M a t e r i a l s . Proceedings published i n Radiat. E f f . 47, 37 (1980). White, C. W., Wilson, S. R., Appleton, B. R., and Young, F. W., Jr. (1980). J. Appl. Phys. 51, 738.
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White, C. W., Appleton, 8. R., S t r i t z k e r , G., Zehner, D. M., and Wilson, S. R. (1981). Mat. Res. SOC. Symp. Proc. 1, 59. Wilson, D. G., Solomon, A. D., and Biggs, P. T. , eds. (1978). "Moving Boundary Problems." Academic Press; New York. Wood, R. F., Wang, J. C., Giles, G. E., K i r k p a t r i c k , J. R. ( 980). I n "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 37. Academic Press, New York. Wood, R. F., and Giles, G. E. (1981). Phys. Rev. B 23, 2923 . Phys. Wood. R. F., K i r k p a t r i c k . J. R., and Giles, G. E. (1981a). Rev. B 23; 5555: Wood, R. F., Lowndes, D. H., and C h r i s t i e , W. H. (1981b). Mat. Res. SOC. Symp. Proc. 1, 231. Wood, R. F. (1982). Phys. Rev. B 25, 2786. Wood, R. F., Lowndes, D. H., and Narayan, J. (1984). Appl. Phys. L e t t . 44, 770. Woods, L. C. (1975). "Thermodynamics o f F l u i d Systems." Oxford U n i v e r s i t y Press, Oxford. Yoffa, E. J. (1980a). Appl. Phys. L e t t . 36, 37. Yoffa, E. J. (1980b). Phys. Rev. B 21, 2415. Young, R. T., Narayan, J., and Wood, R. F. (1979). Appl. Phys. L e t t . 35, 447. Young, R, T., van der Leeden, G. A., Narayan, J., C h r i s t i e , W. H., Wood, R. F., Rothe, D. W., and L e v a t t e r , J. I. (1982). IEEE E l e c t r o n Device L e t t e r s EDL-3, 280.
CHAPTER 5 N O N E Q U I L I B R I U M SOLIDIFICATION FOLLOW IN^ PULSED LASER MELTIN6 R. F. Wood F. W. Young, Jr.
I. I I.
111.
IV.
V. VI.
INTRODUCTION CLASSICAL PHENOMENOLOGICAL THEORY OF CRYSTAL GROWTH 1. Background. 2. Rate Equations f o r t h e I n t e r f a c e Velocity 3. I n t e r f a c i a l Undercooling 4. M u l t i l a y e r Treatments o f t h e I n t e r f a c i a l Region 5. Discussion. NONEQUILIBRIUM SEGREGATION 6. Segregation i n a Two-Component System 7. Velocity-Dependent Rate Constants and A c t i v a t i o n Energies 8. C a l c u l a t i o n s o f k i f o r Various Dopants i n S i l i c o n 9. S o l u b i 1it y L i m i t s 10. S o l u t e Trapping 11. Discussion. INTERFACE INSTABILITY AND FORMATION OF CELLULAR STRUCTURE. 12. Background. 13. M u l l i n s and Sekerka Theory of Interfacial Instability 14. C a l c u l a t i o n s o f S t a b i l i t y Diagrams and C e l l u l a r S t r u c t u r e . AMORPHOUS PHASE FORMATION DURING ULTRARAPID SOLIDIFICATION CONCLUSIONS AND DIRECTIONS FOR FUTURE WORK
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REFERE~CES
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Copyright 0 1984 by Acadernlc Press, Inc All rights of repruductlon In any form reserved ISBN 0-12-752 123-2
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R. F. WOOD E T A L .
Introduction
1.
I t was shown i n Chapter 2 t h a t t h e i n t e r f a c e s e g r e g a t i o n coe f f i c i e n t f o r some dopants i n s i l i c o n d u r i n g p u l s e d l a s e r a n n e a l i n g can be i n c r e a s e d by orders o f magnitude over t h e e q u i l i b r i u m values. It was a l s o demonstrated t h a t e q u i l i b r i u m s o l u b i l i t y l i m i t s can be exceeded by s i m i l a r f a c t o r s .
Both o f t h e s e e f f e c t s a r e c l e a r i n d i -
c a t i o n s t h a t t h e p h y s i c a l processes i n v o l v e d i n t h e r a p i d s o l i d i f i c a t i o n c h a r a c t e r i s t i c o f l a s e r a n n e a l i n g t a k e p l a c e w e l l away f r o m thermodynamic e q u i l i b r i u m .
The i n c r e a s e i n s t a b i l i t y a g a i n s t
c e l l u l a r formation a t t h e l i q u i d - s o l i d i n t e r f a c e w i t h i n t e r f a c e v e l o c i t y , mentioned i n Chapter 2, i s a c l o s e l y r e l a t e d m a n i f e s t a t i o n o f the highly nonequilibrium nature o f t h e u l t r a r a p i d s o l i d i f i c a t i o n induced by p u l s e d l a s e r i r r a d i a t i o n .
The o b s e r v a t i o n by s e v e r a l
groups o f t h e f o r m a t i o n o f t h i n amorphous s i l i c o n l a y e r s on s i n g l e c r y s t a l s u b s t r a t e s as a r e s u l t o f p u l s e d l a s e r i r r a d i a t i o n i s a f o u r t h dramatic i l l u s t r a t i o n o f t h e nonequilibrium nature o f t h e processes i n v o l v e d . I n t h i s c h a p t e r t h e s e m a n i f e s t l y n o n e q u i l i b r i u m * e f f e c t s a r e considered w i t h i n t h e c o n t e x t o f c l a s s i c a l phenomenol o g i c a l t h e o r i e s o f c r y s t a l growth. N o n e q u i l i b r i u m e f f e c t s t h a t occur d u r i n g r a p i d s o l i d i f i c a t i o n had been considered f r e q u e n t l y i n t h e l i t e r a t u r e b e f o r e t h e development o f l a s e r annealing.
Most o f t e n , t h e s t u d i e s o f such e f f e c t s
were concerned w i t h m e t a l l i c systems and t h e r a p i d s o l i d i f i c a t i o n was achieved e i t h e r by s p l a t c o o l i n g o r r e l a t e d techniques (Flemings, 1982; Mehrabian, 1982; Kroeger e t a l . 1982) o r by d e n d r i t i c growth i n supercooled me1t s (Langer, 1980).
*
The development o f p u l s e d 1aser
Throughout t h i s chapter, as i s f r e q u e n t l y done i n t h e l i t e r a t u r e , we use t h e t e r m " n o n e q u i l i b r i u m " i n t h e sense t h a t t h e r e a r e s i g n i f i c a n t departures from c o n d i t i o n s p r e v a i l i n g a t t r u e thermodynamic equi 1ibrium, where a f t e r a1 1 no n e t macroscopic changes can occur. More s p e c i f i c a l l y , " n o n e q u i l i b r i u m " can be i n t e r p r e t e d as i m p l y i n g t h a t t h e v e l o c i t y o f t h e phase i n t e r f a c e cannot be n e g l e c t e d i n discussing t h e e f f e c t s involved.
5.
253
NONEQUILIBRIUM SOLIDIFICATION
a n n e a l i n g has g r e a t l y extended t h e range o f s o l i d i f i c a t i o n v e l o c i t i e s a t t a i n a b l e under w e l l - c o n t r o l l e d e x p e r i m e n t a l c o n d i t i o n s and has made p o s s i b l e more meaningful t e s t s o f v a r i o u s proposed models o f r a p i d s o l i d i f i c a t i o n phenomena.
However, l a s e r a n n e a l i n g has been
most successful and most s t u d i e d i n t h e elemental semiconductors s i l i c o n and germanium.
These elements f o r m f o u r f o l d coordinated,
c o v a l e n t l y bonded semiconductors i n t h e s o l i d s t a t e , b u t on m e l t i n g t h e y t r a n s f o r m t o a close-packed s t r u c t u r e w i t h a c o o r d i n a t i o n It i s
number o f e i g h t o r g r e a t e r and e x h i b i t m e t a l l i c p r o p e r t i e s .
n o t c l e a r t h e e x t e n t t o which t h e simple c l a s s i c a l phenomenological models o f c r y s t a l growth a p p l y t o t h e r a p i d s o l i d i f i c a t i o n o f m a t e r i a l s such as these.
For example, Glasov e t a l .
(1969) have
discussed a t l e n g t h a p r e - c r y s t a l l i z a t i o n stage i n S i and Ge which t h e y observed f o r temperatures i n a range o f approximately a hundred degrees above t h e s o l i d i f i c a t i o n temperature.
In t h i s stage t h e
s h o r t range order, as probed by v i s c o s i t y measurements and x-ray d i f f r a c t i o n , was observed t o change i n a manner n o t observed f o r most metals.
T h i s may i m p l y t h a t t h e dynamical aspects o f t h e phase
t r a n s i t i o n i n f a c t t a k e p l a c e over an extended temperature range. The q u e s t i o n may a r i s e as t o how i t can be determined t h a t a system has undergone a nonequi 1ib r i u m thermodynamic process.
In
some cases t h i s q u e s t i o n i s r a t h e r easy t o answer on a p r a c t i c a l l e v e l , w h i l e i n o t h e r s t h e answer i s by no means obvious.
Thus,
i n a two-component system, e s p e c i a l l y one t h a t e x h i b i t s r e t r o g r a d e s o l u b i l i t y (Chapter Z ) , i t can be assumed t h a t t h e o b s e r v a t i o n o f s o l u b i l i t i e s w e l l above t h e thermodynamic e q u i l i b r i u m values i s an i n d i c a t i o n t h a t nonequi l i b r i u m processes have occurred. case o f a one-component system t h e answer i s , l e s s apparent.
I n the Here
i t w i l l be assumed t h a t i f any p h y s i c a l s i t u a t i o n s o t h e r t h a n t h o s e g e n e r a l l y a s c r i b e d t o s o l i d i f i c a t i o n a t thermodynamic e q u i l i b r i u m are realized,
a n o n e q u i l i b r i u m process must have occurred.
For
example, any f o r m a t i o n o f amorphous s i l i c o n must come about t h r o u g h a n o n e q u i l i b r i u m process because s i l i c o n forms a c r y s t a l l i n e l a t t i c e
254
R. F. WOOD ETAL.
i f s o l i d i f i c a t i o n occurs so s l o w l y t h a t l o c a l thermodynamic e q u i l i b r i u m holds. vacancies,
However,
interstitials,
i t i s also t r u e t h a t t h e formation o f and t h e i r c l u s t e r s a t c o n c e n t r a t i o n s
above t h e e q u i l i b r i u m c o n c e n t r a t i o n s a t a g i v e n temperature d u r i n g r a p i d s o l i d i f i c a t i o n would a l s o be i n d i c a t i v e o f a n o n e q u i l i b r i u m There i s evidence (Mooney e t a l .
process.
1980, Young e t a l .
, 1983)
1978, Benton e t al.,
t h a t such n o n e q u i l i b r i u m c o n c e n t r a t i o n s
of i n t r i n s i c d e f e c t s a r e produced d u r i n g p u l s e d l a s e r annealing. C o n s i d e r i n g a g a i n a two-component semiconductor system, i t may be n o t e d t h a t c r y s t a l l i z a t i o n of v i r t u a l l y p e r f e c t l a t t i c e s cont a i n i n g concentrations o f s u b s t i t u t i o n a l
dopants determined by
e q u i l i b r i u m s o l u b i l i t y l i m i t s i s a normal process i n device f a b r i cation.
Nevertheless, s i n c e e q u i l i b r i u m s o l u b i l i t i e s a r e observed
t o be g r e a t l y exceeded d u r i n g p u l s e d l a s e r annealing, even a t comp a r a t i v e l y low c r y s t a l l i z a t i o n v e l o c i t i e s when a v i r t u a l l y p e r f e c t l a t t i c e i s formed, i t must be concluded t h a t n o n e q u i l i b r i u m p r o cesses occur even when t h e c r y s t a l l i z a t i o n o f t h e two-component system o t h e r w i s e appears normal.
The presence o f s t r e s s e s and
s t r a i n s i n t h e l a t t i c e i s a p p a r e n t l y n o t enough t o d i s r u p t t h e c r y s t a l l i n i t y even i n h i g h l y s u b s t i t u t e d l a t t i c e s . hand,
as t h e s o l i d i f i c a t i o n
v e l o c i t y i s increased,
On t h e o t h e r a point i s
reached a t which molten s i l i c o n s o l i d i f i e s i n t h e amorphous phase. T h i s o b s e r v a t i o n o f two a p p a r e n t l y d i s t i n c t types o f n o n e q u i l i b r i u m processes i s c o n s i s t e n t w i t h t h e r e c o g n i t i o n by many a u t h o r s (see, f o r example, t h e review by Haubenreisser and P f e i f f e r , 1983) t h a t two t y p e s o f i n t e r f a c i a l processes must t a k e p l a c e d u r i n g s o l i d i f i cation.
A d i f f u s i v e - t y p e process i s r e q u i r e d f o r long-range atomic
transport,
and must g e n e r a l l y be p r e s e n t even i n a one-component
system s i n c e s e l f - d i f f u s i o n i s always present.
On t h e o t h e r hand,
most atoms a r e l i k e l y t o go from t h e bonding c h a r a c t e r i s t i c o f t h e l i q u i d t o t h a t c h a r a c t e r i s t i c o f t h e s o l i d by r e l a x a t i o n processes which i n v o l v e atomic displacements o f l e s s t h a n a l a t t i c e spacing. The simultaneous occurrence o f n o n e q u i l i b r i u m processes on two d i f f e r e n t l e v e l s o r t i m e s c a l e s i s demonstrated by an e x t e n s i v e body
5. of
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NONEQUILIBRIUM SOLIDIFICATION
r e s u l t s from laser-annealing studies o f silicon.
Thus,
as
a l r e a d y noted, n o n e q u i l i b r i u m s e g r e g a t i o n i s observed a t v e l o c i t i e s w e l l below t h o s e r e q u i r e d f o r t h e onset o f t h e liquid-amorphous t r a n s i t i o n , y e t when t h e m a t e r i a l s o l i d i f i e s i n an amorphous phase, n o n e q u i l i b r i u m s o l u b i l i t i e s and even c e l l u a r s t r u c t u r e s c o n t i n u e t o be observed ( C u l l i s e t a l .
1982a).
I n t h e n e x t s e c t i o n , t h e elementary phenomenological t h e o r y o f c r y s t a l growth o f a single-component m a t e r i a l i s b r i e f l y reviewed. I n S e c t i o n 111, t h e main f e a t u r e s o f a k i n e t i c r a t e model f o r none q u i l i b r i u m s e g r e g a t i o n i s described, as an i l l u s t r a t i o n of how t h e c l a s s i c a l t h e o r i e s o f c r y s t a l growth can be extended t o i n c l u d e none q u i l i b r i u m e f f e c t s a s s o c i a t e d w i t h dopant i n c o r p o r a t i o n .
Section
I V c o n t a i n s a d i s c u s s i o n o f another aspect o f n o n e q u i l i b r i u m c r y s t a l growth.
The t r e a t m e n t o f i n t e r f a c e s t a b i l i t y i n a two-component
system developed by M u l l i n s and Sekerka (1964) i s considered and i t i s shown how n o n e q u i l i b r i u m e f f e c t s can be i n t r o d u c e d d i r e c t l y
i n t o t h e treatment through t h e expression f o r t h e nonequilibrium segregation c o e f f i c i e n t given i n Section I V .
Section V contains
a d i s c u s s i o n o f t h e f o r m a t i o n o f amorphous o r d i s o r d e r e d l a y e r s d u r i n g s o l i d i f i c a t i o n a t very h i g h i n t e r f a c i a l v e l o c i t i e s (-15 rn/sec i n the
directions).
I t i s considered how t h i s n o n e q u i l i b r i u m
phenomena can be discussed b o t h on t h e b a s i s o f quasi-thermodynamic arguments and w i t h i n t h e framework o f k i n e t i c r a t e models.
The
c h a p t e r concludes w i t h a b r i e f summary and a few remarks about l i k e l y d i r e c t i o n s f o r f u t u r e work i n t h i s area.
11.
1.
Classical Phenomenological Theory o f Crystal Growth
BACKGROUND Over t h e y e a r s s i n c e t h e e a r l y 1950s, two c l o s e l y r e l a t e d
f o r m u l a t i o n s o f a phenomenological model o f m e l t i n g and s o l i d i f i c a t i o n o f single-component m a t e r i a l s have been used i n d i s c u s s i o n s
256
R. F. WOOD ET AL
o f a v a r i e t y o f e f f e c t s a s s o c i a t e d w i t h c r y s t a l growth.
One formu-
l a t i o n tends t o emphasize thermodynamic concepts (see, f o r example, Turnbull
,
1956; T u r n b u l l and Cohen,
1960; Spaepen and T u r n b u l l
,
1982) w h i l e t h e o t h e r i s based on k i n e t i c r a t e t h e o r y and emphasizes t h e k i n e t i c s o f atomic processes o c c u r r i n g a t t h e l i q u i d - s o l i d i n t e r f a c e (see, f o r example, Chalmers, 1954; Jackson and Chalmers, 1956; Thurmond, 1959).
Because some i n f o r m a t i o n about t h e k i n e t i c s
o f i n t e r f a c i a l processes must be i n c o r p o r a t e d i n t o t h e thermodynamic approach i n o r d e r t o have motion o f t h e l i q u i d - s o l i d i n t e r f a c e and because t h e k i n e t i c r a t e approach, i n t u r n , u t i l i z e s some thermodynamic i n f o r m a t i o n , t h e two f o r m u l a t i o n s a r e v i r t u a l l y i d e n t i c a l i n most respects.
Nevertheless, i t appears t h a t t h e k i n e t i c r a t e
approach i s t h e more f l e x i b l e o f t h e two and o f f e r s g r e a t e r i n s i g h t into solidification
processes,
especially i n the d i l u t e binary
a l l o y s t o be considered i n t h e n e x t s e c t i o n .
Also, as we w i l l show
below, t h e r a t e equations a r e e a s i l y extended t o m u l t i l a y e r t r e a t ments o f t h e i n t e r f a c i a l region.
For t h e s e reasons, and because a
k i n e t i c approach can b e t t e r t r e a t n o n e q u i l i b r i u m processes, t h e language o f t h e k i n e t i c r a t e f o r m u l a t i o n w i l l be employed t h r o u g h o u t t h e b u l k o f t h i s chapter. Our c o n s i d e r a t i o n s w i l l be r e s t r i c t e d t o t h o s e s i t u a t i o n s most commonly encountered i n s t u d i e s of p u l s e d l a s e r m e l t i n g and s o l i d i -
To be more s p e c i f i c , o n l y s t a b l e p l a n a r growth from a c r y s t a l l i n e s u b s t r a t e w i l l be considered i n t h i s s e c t i o n . Such
fication.
growth i m p l i e s t h a t homogeneous n u c l e a t i o n i n t h e b u l k o f t h e l i q u i d i s n o t r e q u i r e d f o r c r y s t a l growth, s i n c e t h e s u b s t r a t e i t s e l f p r o vides a n e a r l y p l a n a r , continuous d i s t r i b u t i o n o f n u c l e a t i o n s i t e s a l o n g t h e e n t i r e phase f r o n t .
These r e s t r i c t i o n s mean t h a t t h e
s i t u a t i o n s t r e a t e d h e r e d i f f e r s i g n i f i c a n t l y f r o m t h o s e f o r which many c r y s t a l growth models were o r i g i n a l l y developed. Those developments o f t e n assumed, e i t h e r e x p l i c t l y o r i m p l i c i t l y , t h a t a l i q u i d , i n i t i a l l y f r e e o f n u c l e a t i o n s i t e s and s u r f a c e s , c o u l d be c o o l e d so s l o w l y and u n i f o r m l y and t h e heat o f s o l i d i f i c a t i o n d i s s i p a t e d so q u i c k l y t h a t when s o l i d i f i c a t i o n d i d b e g i n t h e temperature o f
5 . NONEQUILIBRIUM SOLIDIFICATION t h e system remained u n i f o r m and n e a r l y constant.
257 These assumptions
l e d t o reasonably w e l l d e f i n e d n o t i o n s o f t h e temperature o f t h e i n t e r f a c e and o f t h e e x t e n t o f undercooling.
I n f a c t , under some
c o n d i t i o n s t h e u n d e r c o o l i n g c o u l d o f t e n be e s t i m a t e d and c o r r e l a t e d w i t h t h e observed r a t e s o f growth o f seeded c r y s t a l l i t e s and dendrites.
However, as T u r n b u l l and Cohen (1960) p o i n t e d o u t , once
a s t a b l e c o l l e c t i o n o f n u c l e a t i o n s i t e s i s i n t r o d u c e d and l a t e n t heat begins t o be l i b e r a t e d , t h e problem o f i n t e r p r e t i n g c r y s t a l growth i n a supercooled one-component difficult.
l i q u i d becomes extremely
I n s p i t e o f these d i f f i c u l t i e s , t h e c l a s s i c a l t h e o r y most
a p p r o p r i a t e t o l a s e r - p r o c e s s i n g problems w i l l be presented here as i t i s u s u a l l y encountered i n t h e l i t e r a t u r e on c r y s t a l growth.
2.
RATE EQUATIONS FOR THE INTERFACE VELOCITY The t r e a t m e n t o f Jackson and Chalmers (1956) and Thurmond ( 1 9 5 9 )
w i l l be f o l l o w e d here.
We d e f i n e t h e f o r w a r d r a t e c o n s t a n t K f as
t h e r a t e a t which atoms l e a v e t h e l i q u i d ( a ) and j o i n t h e s o l i d (s), and t h e r e v e r s e r a t e constant Kr as t h e r a t e a t which t h e atoms l e a v e t h e s o l i d and j o i n t h e l i q u i d ; f o r c l a r i t y we w i l l sometimes r e p l a c e t h e s u p e r s c r i p t s f by as and r by SE. A t t r u e thermodynamic equilibrium, interface.
Kf = Kr and t h e r e i s no m o t i o n o f t h e l i q u i d - s o l i d
During s o l i d i f i c a t i o n Kf
>
Kr,
and t h e i n t e r f a c e o r
m e l t - f r o n t v e l o c i t y v i s g i v e n by v = Kf Conversely, Kr
-
Kf.
- Kr.
(1)
d u r i n g m e l t i n g Kr
>
Kf,
and t h e m e l t i n g v e l o c i t y i s
The r a t e c o n s t a n t s a r e seen t o have u n i t s o f v e l o c i t y .
They a r e u s u a l l y w r i t t e n i n t h e f o r m K f = Af exp(-AHRS/kT)
,
Kr = Ar exp(-AHSa/kT)
.
(3)
258
R. F. WOOD E T A L .
The q u a n t i t i e s hHEs and hHsI a r e a c t i v a t i o n e n e r g i e s f o r t h e t r a n s formations L+S and s+a r e s p e c t i v e l y .
F i g u r e 1 shows AHRS and AHSa
and t h e i r r e l a t i o n s h i p t o Lc, t h e l a t e n t heat o f t h e phase change, i.e.,
Lc =
cation.
-
w i t h t h e l a t e n t heat n e g a t i v e f o r s o l i d i f i -
AHS~
The q u a n t i t y a s s o c i a t e d w i t h t h e abscissa r e p r e s e n t s a
g e n e r a l i z e d c o o r d i n a t e i n some sense d e s c r i b i n g a c o n f i g u r a t i o n a l average over t h e changing c o n f i g u r a t i o n s o f t h e system as t h e i n t e r f a c e r e g i o n i s traversed.
The i n t e r f a c e r e g i o n i t s e l f may be ex-
tended and m o l e c u l a r dynamics c a l c u l a t i o n s (Toxvaerd and Praestgaard, 1977; Landman e t al.,
1980; Cleveland e t al.,
1982) i n d i c a t e t h a t
i t s w i d t h may be several i n t e r l a y e r spacings. The i n t e r f a c e r e g i o n may be even more extended i f t h e p r e - c r y s t a l l i z a t i o n stage discussed by Glasov e t a l . (1969) i s indeed present. The p r e - e x p o n e n t i a l f a c t o r s i n Eqs. ( 2 ) and ( 3 ) were expressed by Jackson and Chalmers as a p r o d u c t o f f a c t o r s , f o r example, A
f
f f f f f = B G u N V
.
(4)
Bf i s an "accommodation c o e f f i c i e n t " and Gf a geometrical f a c t o r
which t o g e t h e r express t h e p r o b a b i l i t y t h a t an atom a t t h e i n t e r f a c e has a component o f v i b r a t i o n i n a d i r e c t i o n t o t r a v e r s e t h e
I
SOLID
Kr=KS'-
LIQUID t Kf
= K Is
: CONFIGURATION COORDINATE
Fig.
1.
interface.
Schematic illustration o f kinetic processes a t the liquid-solid W i s the Ilwidthll o f the interface.
5 . NONEQUILIBRIUM SOLIDIFICATION
259
i n t e r f a c e and t h a t t h e r e i s a p o s i t i o n f o r i t t o move t o on t h e
Nf i s t h e number o f atoms p e r cm2
other side o f t h e interface.
i n the l i q u i d a t the interface,
V f i s t h e atomic volume i n t h e
l i q u i d , and vf i s t h e v i b r a t i o n a l frequency i n t h e l i q u i d .
The A ' s
a r e c l e a r l y r e l a t e d t o t h e e n t r o p y o f t h e phases and t h i s r e l a t i o n The f a c t o r s Bf and B r
s h i p was discussed by Jackson and Chalmers.
a r e g e n e r a l l y assumed t o be v i r t u a l l y independent o f temperature.
I f t h e d e n s i t i e s and c o o r d i n a t i o n numbers i n t h e l i q u i d and s o l i d phases a r e approximately equal , t h e geometrical f a c t o r s f o r p l a n a r s o l i d i f i c a t i o n should a l s o be n e a r l y equal.
Then t h e accommodation
c o e f f i c i e n t s and t h e frequencies would l a r g e l y determine t h e d i f ferences between Af and A r . With t h e expressions f o r Kf Eq.
and Kr
( 2 ) and ( 3 ) ,
f r o m Eqs.
(1) becomes f r f f v = K (1-K /K ) = K 11
The r a t i o Ar/Af i b r i u m , i.e.,
-
r f (A /A ) exp(-Lc/kT)}
.
i s c u s t o m a r i l y f i x e d by examining Eq. ( 5 ) a t e q u i l To be more s p e c i f i c , we w i l l assume t h a t
when v = 0.
complete c r y s t a l l i z a t i o n (as opposed, e.g. t h e m e l t i s b e i n g discussed,
,t o
amorphization) f r o m
append t h e s u b s c r i p t c t o L,
denote t h e c r y s t a l l i n e m e l t i n g temperature by Tc. that
A r/A f = exp(Lc/kTc)
and
We t h e n have
.
(6)
a n d t h e expression f o r v becomes
v = Kf { 1
-
exp[-(Lc/kTcT)(Tc
- T) 3) .
(7)
T h i s e q u a t i o n can be w r i t t e n i n a s l i g h t l y d i f f e r e n t form s i n c e t h e change i n t h e Gibbs f r e e energy (AG = a t T,
i s zero.
Therefore, L,
(= AH,)
AH
- TAS) on c r y s t a l l i z a t i o n
= bSCTc, so t h a t
260
R. F. WOOD E T A L .
Note a l s o from Eq. ( 6 ) t h a t Ar/Af
= exp(ASc/k),
which e s t a b l i s h e s
t h e r e l a t i o n s h i p between t h e A ' s and t h e e n t r o p y change. The q u e s t i o n o f course a r i s e s about t h e values o f T and K f t o be used t o determine t h e v e l o c i t y .
The answer f o r T must be i n
some sense t h e "temperature o f t h e i n t e r f a c e , "
b u t i t i s extremely
d i f f i c u l t t o d i r e c t l y measure such a temperature and i t i s u s u a l l y i n f e r r e d from t h e observed growth r a t e by making c e r t a i n assumptions about t h e i n t e r f a c i a l k i n e t i c s .
I n any case,
i t i s customary t o
append a s u b s c r i p t i t o T and r e f e r t o T i as t h e i n t e r f a c e tempera t u r e and t o b T i s Tc-Ti as t h e degree o f undercooling.
Equation ( 7 )
can t h e n be w r i t t e n as
The q u a n t i t y Lc/kTc i s 3.61
i n silicon,
and f o r moderate under-
c o o l i n g s t h e e x p o n e n t i a l can be expanded and o n l y t h e l i n e a r t e r m in
AT^
kept t o give
v = K f (Lc/kTc)(ATi/Ti)
.
Note t h a t s i n c e Lc i s a n e g a t i v e q u a n t i t y f o r s o l i d i f i c a t i o n , v i s o p p o s i t e i n d i r e c t i o n t o Kf, o f t e n been suggested t h a t Kf,
i n agreement w i t h F i g .
1.
I t has
t h e r a t e constant f o r t r a n s i t i o n s
from t h e l i q u i d t o t h e s o l i d should be r e l a t e d t o d i f f u s i o n and/or viscosity coefficients i n the liquid.
However, as p o i n t e d o u t i n
t h e I n t r o d u c t i o n , many authors have recognized t h a t two t y p e s o f processes ( d i f f u s i v e and r e l a x a t i o n a l ) should g e n e r a l l y e n t e r i n t o t h e d e t e r m i n a t i o n o f t h e value o f K f and i t s temperature dependence. I n o r d e r t o make c o n t a c t w i t h o t h e r r e c e n t discussions, we n o t e t h a t T u r n b u l l and Cohen (19601, Spaepen and T u r n b u l l (1980,1982) , and o t h e r s have w r i t t e n f o r t h e i n t e r f a c e v e l o c i t y an e q u a t i o n o f t h e form v = f k; A 11
-
exp -AGc/kT
5.
261
NONEQUILIBRIUM SOLIDIFICATION
i n which f i s d e s c r i b e d as t h e f r a c t i o n o f i n t e r f a c i a l s i t e s a t which rearrangement can occur,
and k; as t h e
t h e displacement p e r rearrangement.
frequency and A as
T i i s t h e i n t e r f a c e temperature
and A G i~ s t h e f r e e energy o f c r y s t a l l i z a t i o n p e r atom.
The n o t a -
t i o n hGc i n Eq. ( 1 1 ) i s somewhat m i s l e a d i n g s i n c e i t i s n o t c l e a r what temperature dependence i s t o be assumed.
i t can be deduced t h a t A G ~i s given by t h e e q u a t i o n
Eqs. ( 7 ) - ( 9 ) , llGc(Ti)
-
= AH C
w i t h dHC = Lc and T i = Tc,
AH^
By comparison w i t h
=
T,AS,
Ti~Sc
AS^
,
e v a l u a t e d a t Tc.
and Eq.
Then s i n c e A G ~= 0 a t
(11) becomes
W i t h i n t h e framework o f t r a n s i t i o n s t a t e r a t e t h e o r y (see,
for
example, C h r i s t i a n , 1975) t h e f a c t o r f k ; A can be i n t e r p r e t e d as t h e f o r w a r d r a t e c o n s t a n t K f o f Eq. (13); t h e s i g n o f A G i~ n Eq. ( 1 1 ) was chosen t o agree w i t h t h i s i n t e r p r e t a t i o n and t h e c o n v e n t i o n o f Fig. 1. k!
The temperature-dependence o f t h e f a c t o r comes i n t h r o u g h
w r i t t e n i n an a c t i v a t e d f o r m (Spaepen and T u r n b u l l ,
F r a t e l l o e t al., o f Fig.
1.
1980) w i t h t h e a c t i v a t i o n energy t h e same as
1979;
AH^^
I t i s e v i d e n t t h e n t h a t t h e d i f f e r e n c e between t h e
k i n e t i c r a t e t h e o r y and t h e more thermodynamic approach i s more a matter o f viewpoint than o f t h e f i n a l r e s u l t s ,
a t l e a s t on t h e
s i m p l e phenomenological l e v e l d i s c u s s e d here. 3.
INTERFACIAL UNDERCOOLING The d i f f i c u l t y i n a p p l y i n g t h e above e q u a t i o n s f o r v t o s i t u -
a t i o n s a r i s i n g i n p r a c t i c e stems f r o m t h e l a c k o f knowledge o f K f and t h e problem o f s p e c i f y i n g t h e t e m p e r a t u r e Ti. As d i s c u s s e d i n t h e I n t r o d u c t i o n , i n t h o s e s i t u a t i o n s f o r which t h e phenomenological models were o r i g i n a l l y c o n s t r u c t e d , t h e s p e c i f i c a t i o n o f t h e temp e r a t u r e seemed s t r a i g h t f o r w a r d because a slow, u n i f o r m , measurable,
262
R. F. WOOD E T A L .
and u s u a l l y r a t h e r small u n d e r c o o l i n g o f t h e e n t i r e l i q u i d c o u l d be e s t a b l i s h e d :
T h i s i s f a r from t h e s i t u a t i o n t h a t p r e v a i l s i n
l a s e r - i n d u c e d m e l t i n g and s o l i d i f i c a t i o n .
I n any case, i n o r d e r t o
make some progress i n t h e a p p l i c a t i o n o f Eq. (9), and p a r t i c u l a r l y i n e s t i m a t i n g T i , t h e v e l o c i t y o f t h e l i q u i d - s o l i d i n t e r f a c e obt a i n e d from heat f l o w e q u a t i o n s can be i n t r o d u c e d . The c a l c u l a t i o n s i n Chapter 5 o f t h e temperature o f t h e nears u r f a c e r e g i o n d u r i n g p u l s e d l a s e r a n n e a l i n g show t h a t ( f o r t h e boundary c o n d i t i o n chosen a t t h e l i q u i d - s o l i d i n t e r f a c e ) t h e temp e r a t u r e o f t h e l i q u i d drops q u i c k l y a f t e r t h e l a s e r p u l s e t o t h e m e l t i n g p o i n t and remains t h e r e w h i l e t h e f l o w o f l a t e n t heat r e leased by s o l i d i f i c a t i o n i s determined by t h e temperature g r a d i e n t i n t h e s o l i d and t h e thermal p r o p e r t i e s o f t h e m a t e r i a l .
For t h e
p r e s e n t , i t w i l l be assumed t h a t t h i s c o n d i t i o n has been reached. The v e l o c i t y o f t h e p l a n a r i n t e r f a c e f o r c r y s t a l l i z a t i o n can t h e n be o b t a i n e d from t h e one-dimensional
heat ( Q ) f l u x boundary con
d i t i o n a t t h e i n t e r f a c e , i.e.,
,
d Q / d t = ApLcdx/dt = KcGis with Gis
z [grad T]i,s
(14
.
Adx i s t h e volume o f m a t e r i a l from which l a t e n t heat i s l i b e r a t e d during the time i n t e r v a l dt,
p
i s t h e d e n s i t y , and Kc i s t h e thermal
conductivity o f the crystalline solid.
The s u b s c r i p t s on t h e
square b r a c k e t s mean t h a t t h e g r a d i e n t o f T i s t o be e v a l u a t e d i n t h e t h e s o l i d a t the l i q u i d - s o l i d interface. Eq.
For u n i t area (A = l ) ,
(14) becomes s i m p l y
Equation ( 9 ) r e l a t e s v t o t h e temperature o f t h e i n t e r f a c e whereas Eq. (16) r e l a t e s v t o t h e g r a d i e n t o f T a t t h e i n t e r f a c e .
5.
263
NONEQUILIBRIUM SOLIDIFICATION
The S t e f a n boundary c o n d i t i o n expressed by Eq.
(14) assumes t h a t
l a t e n t heat i s r e l e a s e d so q u i c k l y t h a t t h e i n t e r f a c e v e l o c i t y
is
l i m i t e d o n l y by t h e r a t e a t which heat can be conducted away. Equation ( 9 ) , on t h e o t h e r hand,
reflects the reality of inter-
f a c i a l k i n e t i c s and t h e f a c t t h a t some i n t e r f a c i a l undercool i n g i s necessary t o have s o l i d i f i c a t i o n .
If the interfacial kinetics
a r e r a p i d enough t o keep up w i t h t h e h e a t f l o w r a t e , t h e v e l o c i t y p r e d i c t e d by Eqs. ( 9 ) and (16) should be v e r y n e a r l y t h e same and e s t i m a t e s o f T i can be o b t a i n e d by t a k i n g v from t h e h e a t - f l o w calculations.
I n t h i s case t h e i n t e r f a c e m o t i o n may be viewed as
"heat f l o w l i m i t e d . "
I f t h e i n t e r f a c i a l k i n e t i c s become s l u g g i s h
and t h e r a t e o f l a t e n t heat r e l e a s e i s s i g n i f i c a n t l y l e s s t h a n t h e heat c o n d u c t i v i t y r a t e ,
t h e v e l o c i t y c a l c u l a t e d from h e a t f l o w
equations may be i n c o r r e c t because t h e l i q u i d has t i m e t o undercool s i g n i f i c a n t l y before s o l i d i f i c a t i o n .
I n t h i s case t h e i n t e r -
f a c e motion i s s a i d t o be " i n t e r f a c e l i m i t e d . "
From t h e s t a n d p o i n t
o f t h e heat f l o w equations t h e presence o f u n d e r c o o l i n g may be regarded as b e i n g approximately e q u i v a l e n t t o a r e d u c t i o n f r o m Tc t o T i during crystallization,
as we w i l l d i s c u s s s h o r t l y .
The r e s u l t s o f c a l c u l a t i n g T i , and t h e r e f o r e A T i , from Eq. (10) w i t h values o f v e x t r a c t e d from t h e c a l c u l a t i o n s o f Chapter 4 are shown i n Table I. We chose s e v e r a l values o f v f o r which t h e heat f l o w r e s u l t s showed t h a t t h e g r a d i e n t o f T i n t h e l i q u i d was n e g l i g i b l e , i n agreement w i t h t h e assumption l e a d i n g t o Eq. (14).
From
t h e s e values and t h e values Gis given by t h e heat f l o w c a l c u l a t i o n s i t was v e r i f i e d t h a t Eq. (16) h e l d very w e l l over an extended range
o f v, as expected.
The process was t h e n i n v e r t e d t o o b t a i n t h e
values o f Gis shown i n Table I. Because Eq.
( l o ) , which i s o n l y
an approximation t o Eq. ( 9 ) , has been used t o r e l a t e AT1 t o v, t h e correspondence between Eq.
AT^
and G i s w i l l n o t be t h a t p r e d i c t e d by
( 9 ) a t l a r g e AT^; nevertheless,
i l l u s t r a t i v e purposes. values o f Kf,
AT^
i t i s e n t i r e l y adequate f o r
and T i were c a l c u l a t e d f o r two constant
t h a t i s t o say, any temperature dependence o f K f was
not included i n t h e c a l c u l a t i o n s a t t h i s point.
The v a l u e o f
264
R. F. WOOD ETAL
Table I Values o f
V
(m/sec)
2 4 6 8 10 12 14 16 18 20 22
AT^
and T i f o r d i f f e r e n t i n t e r f a c e v e l o c i t i e s and two d i f f e r e n t values o f Kf.
Gis (106 deg/cm)
3.8 7.6 11.4 15.2 19.1 22.9 26.7 30.5 34.3 38.1 41.9
Kf = 50 m/sec
K f = 100 m/sec ATi
Ti
ATi
Ti
9.3 18.5 27.5 36.5 45.4 54.2 62.9 71.5 80.0 88.4 96.7
1673.7 1664.5 1655.5 1646.5 1637.6 1628.8 1620.1 1611.5 1603.0 1594.6 1586.3
18.5 36.5 54.2 71.5 88.4 105.0 121.2 137.1 152.7 168.0 183.0
1664.5 1646.5 1628.8 1611.5 1594.6 1578.0 1561.8 1545.9 1530.3 1515.0 1500.1
K f = 100 m/sec was t a k e n from work by Wood (1982) on nonequil ib r i u m
dopant segregation,
which w i l l be t r e a t e d i n t h e n e x t s e c t i o n .
T h i s v a l u e o f K f leads t o e s t i m a t e s o f A T i o f l e s s t h a n 100 deg a t s o l i d i f i c a t i o n v e l o c i t i e s o f -20 m/sec. that
AT^
It i s b e l i e v e d by many
must be s u b s t a n t i a l l y l a r g e r t h a n t h i s t o e x p l a i n t h e
observed 1iquid-to-amorphous phase t r a n s i t i o n which occurs f o r v =
15-20 m/sec i n t h e
d i r e c t i o n s
i n silicon.
More s p e c i f i c a l l y ,
from a p u r e l y thermodynamic s t a n d p o i n t i t would be necessary t o undercool t h e l i q u i d t o t h e m e l t i n g p o i n t o f amorphous s i l i c o n , which has been v a r i o u s l y e s t i m a t e d t o be f r o m 500 deg below Tc t o very n e a r l y equal t o Tc (see Chapter 4). were a l s o c a r r i e d o u t f o r K f
=
50 m/sec,
Therefore c a l c u l a t i o n s w i t h t h e r e s u l t shown i n
Table I t h a t A T i was a p p r o x i m a t e l y doubled. C l e a r l y from t h e f o r e g o i n g , t h e value o f K f i s q u i t e i m p o r t a n t i n relating the interface velocity t o
AT^, and i t i s apparent
t h e r e f o r e t h a t any s t r o n g T-dependence o f Kf w i l l a l s o be an import a n t and perhaps dominant f a c t o r .
I n Fig. 2 t h e v e l o c i t y has been
5.
265
NONEQUILIBRIUM SOLIDIFICATION
20 18 16
-
14
In
-‘i 12 2. I-
3 10 0
-I
w
.’
8 6 4
2
0
F.-.
I 0
40
I 80
I
I
I
I
120 160 200 240 ATi, UNDERCOOLING (deg)
I
J
280
320
tterface velocity as a function o f undercooling for severa. values
o f the activation energy AHEs in Eq.
(2).
c a l c u l a t e d as a f u n c t i o n o f ATi from Eq.
( 9 ) f o r a number o f d i f -
f e r e n t values o f t h e a c t i v a t i o n energy AHES.
For each v a l u e o f
AHgs shown, Af was determined by r e q u i r i n g t h a t Kf = 100 m/sec when v = 4 m/sec. T h i s procedure was chosen t o be c o n s i s t e n t w i t h work on dopant s e g r e g a t i o n discussed i n t h e n e x t s e c t i o n . o f Eq.
Whenever AHLs
( 2 ) i s small and t h e r a t e c o n s t a n t i t s e l f i s l a r g e , we can
expect v g i v e n by
Eqs. (9) and (16) t o be v e r y n e a r l y equal over
an extended range o f b T i because t h e s e c o n d i t i o n s w i l l g e n e r a l l y ensure t h a t t h e i n t e r f a c e motion i s p r i m a r i l y heat f l o w c o n t r o l l e d . The curve on Fig. 2 f o r which AHES = 0.5 eV g i v e s a reasonably c l o s e approximation t o t h e s t r i c t l y heat f l o w l i m i t e d case i n t h e range
266 of
R. F. WOOD E T A L
AT^
shown, and hence t h e o t h e r curves demonstrate t h e i n f l u e n c e
t h a t K f or A H E ~(i.e.,
t h e i n t e r f a c e k i n e t i c s ) can have on v.
The
r e s u l t s show t h a t v i s r e l a t i v e l y i n s e n s i t i v e t o AHas as l o n g as AT
,<
40 deg
and v
7-8 m/sec;
i t would be very d i f f i c u l t t o
detect t h e predicted differences i n v experimentally.
For l a r g e r
values o f A T i and v, t h e curves begin t o d i v e r g e r a p i d l y as a r e s u l t o f t h e temperature dependence o f K f coming s t r o n g l y i n t o play. The q u e s t i o n a r i s e s as t o how t h e f o r m u l a t i o n o f t h e heat flow problem can be m o d i f i e d t o t a k e i n t o account l a r g e values o f ATi.
In p r i n c i p l e , t h e S t e f a n boundary c o n d i t i o n o f Eqs. ( 1 4 ) - ( 1 6 ) can be r e p l a c e d by a more general p r e s c r i p t i o n f o r t h e v e l o c i t y o f t h e phase f r o n t such as t h a t g i v e n by Eq. (9).
However, t h e i n c o r p o r a -
t i o n o f t h i s more general boundary c o n d i t i o n i s n e i t h e r s t r a i g h t f o r w a r d i n p r a c t i c e nor p a r t i c u l a r l y en1 i g h t e n i n g when embedded i n
a complex computer program, and an approximate t r e a t m e n t o f t h e effects
o f l a r g e u n d e r c o o l i n g s would be u s e f u l f o r purposes o f
q u a l i t a t i v e discussions.
It was a l r e a d y suggested above t h a t t h e
sluggishness o f t h e i n t e r f a c i a l k i n e t i c s g i v e s r i s e t o a s i t u a t i o n r o u g h l y e q u i v a l e n t t o r e l e a s i n g t h e l a t e n t heat Lc a t T i r a t h e r t h a n Tc; i n o t h e r words a f t e r t h e u n d e r c o o l i n g i s e s t a b l i s h e d , t h e s o l i d i f i c a t i o n temperature i s e f f e c t i v e l y T i r a t h e r t h a n T.,
We
can t e s t t h i s equivalence by f i r s t a p p r o x i m a t i n g G i s i n Eq.
(16)
by Gis
= (Tc
-
Tam)’Xam
(17)
and then i n t r o d u c i n g T i i n p l a c e o f Tc,
as i n d i c a t e d on Fig. 3.
The q u a n t i t i e s Tam (ambient temperature) and Xam ( d i s t a n c e from i n t e r f a c e over which T drops t o Tarn) a r e n o t independent s i n c e t h e y should be chosen t o g i v e a good a p p r o x i m a t i o n t o Gis o b t a i n e d from t h e heat f l o w problem o r from Eq.
(16).
In e i t h e r case, an
i n t u i t i v e l y a p p e a l i n g c h o i c e f o r Tam i n t h e case o f t h e l a s e r a n n e a l i n g problem i s t h e temperature o f t h e s u b s t r a t e Tsub. t h i s c h o i c e of Tam, , ,X
With
can be determined f r o m t h e known values
5.
267
NONEQUILIBFWM SOLIDIFICATION
LIQUID
DISTANCE FROM INTERFACE Fig. 3.
Diagram for estimating the e f f e c t o f interfacial undercooling Tc-Ti
on the temperature gradient at the interface.
of Gis o r from assumed values o f v and Eq.
With Xam d e t e r -
(17) can t h e n be changed t o T i and v c a l c u l a t e d
mined, Tc i n E q . a g a i n f r o m Eq.
(16).
(16).
Table I 1 shows t h e r e s u l t s f o r
Vi 5
v(Ti) o f
such a c a l c u l a t i o n f o r s e v e r a l d i f f e r e n t assumed values o f v and a ATi o f 200°C.
I t i s apparent t h a t t h i s approximate method o f c a l -
c u l a t i n g t h e i n f l u e n c e o f ATj on the i n t e r f a c e v e l o c i t y r e s u l t s i n a l o w e r i n g o f v by -3m/sec when t h e heat f l o w c a l c u l a t i o n s g i v e a v e l o c i t y o f -20 m/sec.
This r e s u l t i s consistent w i t h t h e reduction Table I 1
E f f e c t o f a 200 deg u n d e r c o o l i n g on t h e i n t e r f a c e v e l o c i t y as c a l c u l a t e d f r o m Eqs. (16) a n d ( l 7 ) . V i is t h e v e l o c i t y when T i = 1210OC. v (m/sec) V i
(m/sec)
16
18
20
22
13.7
15.4
17.1
18.8
268
R. F. WOOD ETAL.
o f v due t o u n d e r c o o l i n g e s t i m a t e d by Thompson e t a l .
(1983) from
a comparison o f v o b t a i n e d from t r a n s i e n t e l e c t r i c a l c o n d u c t i v i t y measurements and from m e l t i n g model c a l c u l a t i o n s . The c o n s i s t e n c y o f t h e above r e s u l t s w i t h m e l t i n g model c a l c u l a t i o n s i n which t h e s o l i d i f i c a t i o n temperature i s decreased from Tc by ATi can be t e s t e d . pulses o f
x
C a l c u l a t i o n s were c a r r i e d o u t f o r l a s e r
= 347 nm (frequency doubled r u b y ) , 2.5-nsec
and energy d e n s i t i e s o f 0.3 and 0.4 J/cm*.
duration
The c a l c u l a t i o n s were
f i r s t made f o r a s o l i d i f i c a t i o n temperature o f 1410°C and t h e n f o r one o f 1210°C t o s i m u l a t e an u n d e r c o o l i n g o f
AT^
= 200 deg.
The
r e s u l t s a t a t i m e when t h e temperature g r a d i e n t i n t h e l i q u i d was e s s e n t i a l l y z e r o were examined, and values o f v and G i s e x t r a c t e d . Data from t h e s e c a l c u l a t i o n s a r e shown i n Table 111.
I t i s seen
t h a t t h i s procedure a l s o r e s u l t s i n t h e l o w e r i n g o f t h e i n t e r f a c e v e l o c i t y by 2-3 m/sec i n t h e v e l o c i t y regime o f i n t e r e s t .
The agree-
ment between t h e data i n Tables I 1 and I 1 1 i s encouraging i n so f a r as i t i n d i c a t e s t h a t u n d e r c o o l i n g s o f -200 deg. f o r v e l o c i t i e s o f
-20 m/sec can a p p a r e n t l y l e a d t o improved agreement between experiment and c a l c u l a t i o n s and t h a t t h e s e u n d e r c o o l i n g s can be i n c o r p o r a t e d i n t o t h e modeling by changing t h e boundary c o n d i t i o n g i v e n by Eq.
(16).
I t i s i m p o r t a n t t o r e c o g n i z e however t h a t t h i s does
n o t r e s o l v e t h e problem o f d e t e r m i n i n g K f and
AT^.
It should be
q u i t e c l e a r from F i g . 2 t h a t a given v a l u e o f h T i can l e a d t o a range o f values o f v depending on t h e values o f t h e a c t i v a t i o n energy appearing i n
~f.
Table I 1 1 E f f e c t o f a 200 deg r e d u c t i o n i n s o l i d i f i c a t i o n temperature on t h e i n t e r f a c e v e l o c i t y o b t a i n e d from m e l t i n g model c a l c u l a t i o n s . Eg(J/cm2) 0.3 0.4
I n t e r f a c e V e l o c i t y (m/sec) T~ = 1 4 1 0 0 ~ Tc = 1210°C 18.9 15.5
15.7 13.5
269
5 . NONEQUILIBRIUM SOLIDIFICATION 4.
MULTILAYER TREATMENTS OF THE INTERFACIAL REGION The elementary t h e o r y d e s c r i b e d above i g n o r e s t h e s t r u c t u r e o f
t h e i n t e r f a c i a l l a y e r and t h e r e f o r e a l l q u a n t i t i e s appearing i n t h e t h e o r y must be considered as h a v i n g been averaged i n some sense. There have been s e v e r a l developments e x t e n d i n g t h e k i n e t i c r a t e approach t o m u l t i l a y e r i n t e r f a c i a l r e g i o n s (Cahn, 1968; J i n d a h l and T i l l e r , 1968; Temkin, 1981).
1960; Jackson,
1966, 1981; Cherepanova,
Haubenreisser and P f e i f f e r (1983) have r e c e n t l y reviewed
t h e m i c r o s c o p i c t h e o r y of c r y s t a l growth. approach o f F l e t c h e r (1975,
Here we o u t l i n e t h e
1976) as an i l l u s t r a t i o n o f a m u l t i -
l a y e r treatment. F l e t c h e r employed a model i n which t h e l i q u i d i s viewed essent i a l l y as a s o l i d w i t h a l a r g e c o n c e n t r a t i o n o f d e f e c t s .
This
c o u l d be viewed as a weakness o f t h e model (as F l e t c h e r recognized), b u t i n f a c t t h e r e i s no e n t i r e l y s a t i s f a c t o r y t h e o r y o f t h e l i q u i d state.
Moreover, t h e e x a c t d e s c r i p t i o n o f t h e l i q u i d i s n o t c r u -
c i a l t o t h e conceptual f o u n d a t i o n o f t h e model; f o r example, t h e c o n c e n t r a t i o n o f i m p e r f e c t bonds, a d i s o r d e r parameter, e t c . c o u l d be i n t r o d u c e d i n s t e a d of t h e " c o n c e n t r a t i o n of defects."
I n any
case, F l e t c h e r w r o t e an e x p r e s s i o n f o r t h e f r e e energy o f t h e b u l k m a t e r i a l i n terms o f t h e c o n c e n t r a t i o n o f d e f e c t s C, t h e d e f e c t f o r m a t i o n energy
E,
and t h e temperature.
Because of t h e l a r g e
concentration o f defects present i n t h e l i q u i d , f u n c t i o n o f C, i.e.,
E
= E(C).
E
i s itself a
F o r p a r t i c u l a r values o f t h e quan-
t i t i e s appearing i n t h e e x p r e s s i o n f o r t h e f r e e energy, F l e t c h e r showed t h a t t h e s o l i d and l i q u i d c o u l d c o e x i s t a t some " m e l t i n g " temperature Tm.
The defect c o n c e n t r a t i o n i n t h e s o l i d was -10-11
and i n t h e l i q u i d i t was -0.5. Next, an expression f o r t h e f r e e energy o f m a t e r i a l c o n t a i n i n g a p l a n a r , layered, i n t e r f a c i a l r e g i o n was minimized t o o b t a i n t h e v a r i a t i o n o f t h e d e f e c t c o n c e n t r a t i o n across t h e e q u i l i b r i u m (stationary) interface.
The e x t e n t t o which t h e d e f e c t f o r m a t i o n
energy i n a g i v e n l a y e r depended on C determined whether t h e i n t e r f a c e was d i f f u s e o r sharp.
270
R. F. WOOD ET AL.
With t h e d e f e c t s t r u c t u r e o f t h e e q u i l i b r i u m i n t e r f a c e f i x e d , F l e t c h e r a l l o w e d t h i s i n t e r f a c e t o sweep t h r o u g h t h e s o l i d a t a g i v e n v e l o c i t y and s o l v e d t h e s e t o f simultaneous d i f f e r e n t i a l equations f o r t h e d e f e c t c o n c e n t r a t i o n i n t h e v a r i o u s l a y e r s .
For
example, f o r t h e N-th l a y e r t h e t i m e dependence o f t h e c o n c e n t r a t i o n i s g i v e n by
The t e r m q u a d r a t i c i n t h e C ' s i s an i n t r a l a y e r d e f e c t a n n i h i l a t i o n t e r m and K i s t h e k i n e t i c r a t e c o e f f i c i e n t f o r t h i s process.
The
o t h e r terms account f o r d i f f u s i v e jumps between t h e N-th l a y e r and i t s a d j a c e n t neighbors, N-1 and N+1, w i t h r a t e s determined by t h e
u's. When t h e s e t o f d i f f e r e n t i a l equations i s s o l v e d f o r times l o n g enough f o r steady s t a t e t o be e s t a b l i s h e d ,
i t i s found t h a t t h e
c o n c e n t r a t i o n o f d e f e c t s i n t h e s o l i d depends on t h e v e l o c i t y o f the interface.
The i m p l i c a t i o n s o f t h i s i n c o n n e c t i o n w i t h t h e
l a s e r - i n d u c e d e+a phase t r a n s f o r m a t i o n i n S i w i l l be discussed l a t e r i n t h i s chapter.
I n connection w i t h t h e d i s c u s s i o n i n t h i s
s e c t i o n , we n o t e t h a t s i n c e t h e l a t e n t heat o f a s o l i d is r e l a t e d t o t h e c o n c e n t r a t i o n o f d e f e c t s present (a-Si i s an extreme case), t h i s more e l a b o r a t e m u l t i l a y e r t r e a t m e n t p o i n t s up one o f t h e d i f f i c u l t i e s o f t h e s i m p l e t h e o r y i n which t h e l a t e n t heat i s t r e a t e d as a constant q u a n t i t y . 5.
DISCUSSION
In t h i s s e c t i o n c e r t a i n aspects o f t h e c l a s s i c a l phenomenological t h e o r y o f s o l i d i f i c a t i o n and c r y s t a l growth have been considered. I n p a r t i c u l a r , t h e k i n e t i c r a t e t h e o r y o f t h e i n t e r f a c e motion, t h e i n t e r f a c e temperature,
and t h e degree o f undercool i n g have been
5. discussed.
NONEQUILIBRIUM SOLIDIFICATION
271
T h i s d i s c u s s i o n w i l l form t h e b a s i s f o r t h e m a t e r i a l
i n t h e r e s t o f t h e chapter.
In t h e presence o f very l a r g e tem-
p e r a t u r e g r a d i e n t s , such as t h o s e o c c u r r i n g d u r i n g p u l s e d l a s e r i r r a d i a t i o n , t h e q u a n t i t i e s T i and
AT^
a r e u n l i k e l y t o be d i r e c t l y
measurable, and t h e r e f o r e t h e y a r e u s u a l l y i n t r o d u c e d i n such a way as t o g i v e agreement between t h e m e l t - f r o n t v e l o c i t y o b t a i n e d f r o m k i n e t i c r a t e equations and t h e v e l o c i t y o b t a i n e d from heat f l o w calculations.
Such a procedure, however,
i s n o t unique because
o f u n c e r t a i n t y about t h e i n t e r f a c i a l k i n e t i c s e n t e r i n g i n t o t h e d e t e r m i n a t i o n o f Kf.
I n silicon,
i t appears t h a t t h e e f f e c t s o f
i n t e r f a c i a l undercool i n g become apparent o n l y a t i n t e r f a c e v e l o c i t i e s o f a p p r o x i m a t e l y 8 m/sec and g r e a t e r .
It would appear t h a t
c a r e f u l measurements u s i n g t i m e - r e s o l ved techniques (Chapter 6 ) and r e f i n e d modeling (Chapter 4) w i l l p r o v i d e c o n s i d e r a b l e i n s i g h t i n t o and q u a n t i t a t i v e i n f o r m a t i o n about t h e r o l e o f i n t e r f a c i a l k i n e t i c s i n t h e s o l i d i f i c a t i o n o f elemental semiconductors such as s i l i c o n and germanium.
T h i s i n f o r m a t i o n should be p a r t i c u l a r l y u s e f u l i n
c o n s t r u c t i n g more e l a b o r a t e and s o p h i s t i c a t e d t h e o r i e s o f u l t r a r a p i d s o l i d i f i c a t i o n phenomena, such as t h o s e a r i s i n g from m u l t i l a y e r t r e a t m e n t o f t h e i n t e r f a c i a l r e g i o n and m o l e c u l a r dynamics calculations.
We emphasize once again t h a t t h e t r a n s f o r m a t i o n from
m e t a l l i c - l i k e bonding i n t h e l i q u i d t o c o v a l e n t bonding i n t h e s o l i d over d i s t a n c e s o f p r o b a b l y
RO
more t h a n s e v e r a l i n t e r a t o m i c
l a y e r s g r e a t l y complicates t h e t h e o r e t i c a l t r e a t m e n t o f semicond u c t o r systems.
On t h e o t h e r hand, t h i s same t r a n s f o r m a t i o n p r o -
v i d e s a c e r t a i n r i c h n e s s n o t found i n m e t a l l i c systems,
which
g e n e r a l l y m a i n t a i n t h e i r bonding c h a r a c t e r i s t i c s i n t h e l i q u i d and s o l i d states. The s i n g l e most i m p o r t a n t r e s u l t o f t h i s s e c t i o n i s c o n t a i n e d i n t h e curves d i s p l a y e d on Fig. 2.
We have n o t y e t discussed t h i s
f i g u r e i n any d e t a i l and t h e q u e s t i o n o f course a r i s e s as t o how i t i s t o be i n t e r p r e t e d .
A p r e c i s e q u a n t i t a t i v e answer t o t h i s
q u e s t i o n i s d i f f i c u l t , but t h e f o l l o w i n g q u a l i t a t i v e i n t e r p r e t a t i o n i s g e n e r a l l y advanced. As t h e temperature o f t h e l i q u i d f a l l s below
272
R. F. WOOD ETAL.
Tc, i t i s i n c r e a s i n g l y u n l i k e l y t h a t atoms w i l l be a b l e t o overcome t h e a c t i v a t i o n energy AH^^ and make t r a n s i t i o n s i n t o t h e c r y s t a l l i n e s t a t e (see Fig. 1).
The system w i l l t e n d t o become " f r o z e n " i n t h e
l i q u i d - l i k e s t a t e , o r , i n o t h e r words, an amorphous o r g l a s s y s o l i d o r an e x t r e m e l y viscous l i q u i d w i l l be formed.
The s i m p l e t h e o r y
i s t o o l i m i t e d t o p r e d i c t t h e i n t e r f a c i a l v e l o c i t i e s a t which t h e s e t r a n s f o r m a t i o n s occur.
A p u r e l y thermodynamic c r i t e r i o n would
assume t h a t t h e l i q u i d must be undercooled t o t h e temperature o f t h e liquid-to-amorphous phase t r a n s i t i o n i n s i l i c o n ( i f indeed t h e r e i s such a unique temperature).
T h i s i n t e r p r e t a t i o n w i l l be con-
s i d e r e d i n more d e t a i l i n S e c t i o n 5 o f t h i s chapter.
Here, we w i l l
c l o s e t h i s d i s c u s s i o n by r e i t e r a t i n g t h e o b s e r v a t i o n t h a t i n t e r f a c i a l u n d e r c o o l i n g i n s i l i c o n a p p a r e n t l y does n o t g r e a t l y a l t e r t h e r e s u l t s o f m e l t i n g model c a l c u l a t i o n s u n t i l i n t e r f a c i a l veloci t i e s o f -8 m/sec a r e exceeded.
T h i s i m p l i e s t h a t much o f t h e work
on dopant s e g r e g a t i o n , s o l u b i l i t y l i m i t s , and i n t e r f a c i a l i n s t a b i l i t i e s discussed i n t h e n e x t two s e c t i o n s need n o t e x p l i c i t l y account f o r i n t e r f a c i a l u n d e r c o o l i n g s i n a f i r s t approximation.
111. 6.
SEGREGATION
Nonequilibrium Segregation
I N A TWO-COMPONENT SYSTEM
The elementary k i n e t i c r a t e t r e a t m e n t o f t h e s o l i d i f i c a t i o n of a s i n g l e component m a t e r i a l g i v e n i n t h e l a s t s e c t i o n has been extended by Jackson (1958) t o two-component
systems.
Several
a u t h o r s and groups o f authors have a p p l i e d t h i s t y p e o f development t o t h e problem o f nonequil i b r i u m s e g r e g a t i o n i n a l a s e r - i r r a d i a t e d two-component
system; h e r e we f o l l o w t h e approach t a k e n by Wood
(1980, 1982).
T h i s approach a r r i v e s a t an expression f o r t h e segre-
g a t i o n c o e f f i c i e n t which i s an e x p l i c i t f u n c t i o n o f t h e v e l o c i t y o f t h e l i q u i d - s o l i d i n t e r f a c e v o f the preceding section. The i n c o r p o r a t i o n o f h o s t ( h ) and dopant ( d ) atoms i n t o t h e s o l i d can be d e s c r i b e d by t h e r a t e equations
5. R
j
-
NONEQUILIBRIUM SOLIDIFICATION
,
(19)
C i and C s a r e t h e c o n c e n t r a t i o n s
( i n atomic f r a c t i o n s ) o f
= KfC'
~j
KrCS
j j
,
273
j = h,d
with
Here,
t h e j - t h t y p e atom i n t h e l i q u i d and s o l i d a t t h e i n t e r f a c e , and f K j and KI a r e t h e forward and r e v e r s e r a t e c o n s t a n t s i n t r o d u c e d i n Sec. 11.2,
b u t now s u b s c r i p t e d t o i n d i c a t e h o s t o r dopant.
Pro-
v i d e d t h e r e i s no s i g n i f i c a n t d i f f u s i o n i n t h e s o l i d , t h e r a t e a t which t h e c o n c e n t r a t i o n o f h and d appear
in t h e s o l i d i s j u s t t h e
i n t e r f a c e v e l o c i t y t i m e s t h e c o n c e n t r a t i o n observed i n t h e s o l i d , S
Adding Rh and Rd from Eq. (19)
o r R j = vCj.
and u s i n g Eq. (20)
gives
w h i l e , f o r t h e dopant o n l y
vCd = KdCd
-
KiCi
.
The i n t e r f a c e s e g r e g a t i o n c o e f f i c i e n t i s g i v e n by S I I k i z Cd/Cd
.
It f o l l o w s d i r e c t l y from Eq.
f ki = Kd/(V+KL) f IfKd and K:
(22) t h a t
= (Kd/Kd)(l f r +
V/Kd)r - 1
.
a r e assumed n o t t o depend on v, t h e n k i w i l l decrease
as v increases, which i s completely c o n t r a r y t o t h e experimental r e s u l t s discussed i n Chapters 2 and 4. therefore,
It can be concluded,
t h a t one o r more o f t h e K's must depend on v and t h e
274
R. F. WOOD ETAL.
o r i g i n o f t h i s v-dependence w i l l now be i n v e s t i g a t e d . however,
We note f i r s t ,
t h a t t h i s dependence must be q u i t e s t r o n g i n o r d e r t o
e x p l a i n t h e observed s e n s i t i v i t y o f k i t o
V.
7. VELOCITY-DEPENDENT RATE CONSTANTS AND ACTIVATION ENERGIES It w i l l be assumed t h a t a l l o f t h e r a t e constants can g e n e r a l l y be o f t h e a c t i v a t e d form o f Eqs.
( 2 ) and (3).
However, here t h e
AH^^
w i l l be s i m p l i f i e d somewhat by r e p l a c i n g by Uh, f by uh, e t c . Then t h e expression f o r t h e r e v e r s e r a t e constant
notation SL
AHh
o f a dopant atom becomes
KL
= A:
i n which
exp(-Ud/kT) r
,
(25)
U i i s the e f f e c t i v e
b a r r i e r height
dopant atom from t h e s o l i d i n t o t h e l i q u i d . that the
K's
a g a i n s t jumps o f a
I t was j u s t e s t a b l i s h e d
must be v-dependent and t h e source o f t h i s dependence
w i l l now be t r a c e d , under t h e s i m p l i f y i n g assumption t h a t o n l y t h e v dependence o f K Z need be examined.
F i r s t , however, i t i s use-
f u l t o i n t r o d u c e a d d i t i o n a l n o t a t i o n t o d i s t i n g u i s h between e q u i l i b r i u m (v=O) and n o n e q u i l i b r i u m (v#O) q u a n t i t i e s .
Hence, c o r r e -
sponding t o t h e d e f i n i t i o n ( 2 3 ) , we have
k 01. z Cdso /C 1d10 and t o Eq. (25),
Kio
= ' A:
exp(-Uio/kT)
.
Then i n t h e l i m i t t h a t v+O, kq+kS, Kd+Kso, and SO f o r t h . t h e v e l o c i t y dependence o f q u a n t i t i e s A,:
:K
Clearly
must come f r o m one o r more o f t h e
Ug, o r T i n Eq. (25) and each o f these w i l l be con-
sidered i n turn.
5. a.
275
NONEQUILIBRIUM SOLIDIFICATION
Role o f I n t e r f a c i a l Temperature I t might be thought t h a t a v dependence o f Kd c o u l d come about
t h r o u g h t h e temperature, s i n c e as we saw i n Eqs.
( 7 ) and ( 9 ) t h e
k i n e t i c r a t e equations f o r s o l i d i f i c a t i o n r e s u l t i n a r e l a t i o n s h i p between v and t h e e x t e n t o f i n t e r f a c i a l u n d e r c o o l i n g ATi.
In
p r i n c i p l e , t h i s r e l a t i o n s h i p can be i n v e r t e d t o w r i t e T i = T i ( v ) y which can t h e n be used i n Eq. (25) t o o b t a i n Kd(v).
T h i s procedure
i s i n d i r e c t l y r e l a t e d t o t h a t from which t h e temperature dependence o f ky was o b t a i n e d by Thurmond and S t r u t h e r s (1953) and a development somewhat s i m i l a r t o t h e i r s w i l l be f o l l o w e d here. Writing
Eq.
under t h e assumption
(24)
that
v
<<
Kdr0 ( t h i s
should remain a good approximation f o r even f a i r l y l a r g e values o f v ) , and i n s e r t i n g t h e a c t i v a t e d expressions f o r K i o and Kdr0 g i v e s
The e q u i l i b r i u m values of t h e A ' s and U ' s appear i n Eq. (28) because h e r e t h e hypothesis t h a t t h e v-dependence o f k i comes o n l y from T i i s being examined.
I n r e g u l a r s o l u t i o n t h e o r y (see, f o r example,
C h r i s t i a n , 1965) a r e l a t i o n s h i p l i k e t h a t g i v e n i n Eq. ( 6 ) f o r t h e h o s t h o l d s a l s o f o r t h e dopant,
i.e.,
a t e q u i l i b r i u m (v=O)
i n which Ld i s t h e l a t e n t heat o f c r y s t a l l i z a t i o n and Td t h e m e l t i n g temperature o f t h e pure c r y s t a l l i n e dopant element. 5
Uzo
With AUd
- U i o , ki becomes
I f i d e a l l i q u i d and s o l i d s o l u t i o n s a r e assumed, AUd = Ld; i f t h e
s o l i d s o l u t i o n i s d i l u t e but not ideal, d i f f e r e n t i a l heat o f s o l u t i o n . Struthers, the notation
AUd w i l l a l s o c o n t a i n a
C o n t i n u i n g t o f o l l o w Thurmond and
276
R. F. WOOD E T A L .
kp
*
I ki(Th) 0
i s i n t r o d u c e d f o r k i a t t h e m e l t i n g p o i n t o f t h e h o s t m a t e r i a l ; we a l s o now use t h e n o t a t i o n Th and Lh i n s t e a d o f Tc and Lc o f Sec. 11. A f t e r adding and s u b t r a c t i n g t h e q u a n t i t y Th(kTi)-l(AUdT{l-ASd) t h e r i g h t - h a n d s i d e o f Eq. I n ki = ThTT1 i In kp = I n ky
*
+
*
to
(30), I n k i becomes
+ AsdATi(kTi)-l
[ l n ky
*
.
+ AS~/~](AT~/T~)
(32)
ASd = Ld/Td i s t h e e n t r o p y o f m e l t i n g o f t h e pure c r y s t a l l i n e dopant. Assuming t h a t t h e dopant c o n c e n t r a t i o n i s t o o d i l u t e t o s i g n i f i c a n t l y e f f e c t t h e i n t e r f a c e v e l o c i t y and u s i n g Eq. A T ~ / T=~ vkTh/KhfoLh
.
( l o ) , we have (33)
An e x p r e s s i o n f o r k i i s t h e n f i n a l l y o b t a i n e d t h a t e x p l i c i t l y contains the interface velocity ki(v)
= kp
*
exp{Clnkp
*
+ Ld/kT,](vkTh/LhK~o)]
Table I V shows values of I n ky
*
, and
.
(34)
Ld/kTd f o r common dopants i n
silicon.
I t i s apparent from t h e t a b l e t h a t , except f o r B, t h e
expression
in
square b r a c k e t s i n Eq. (34) i s n e g a t i v e and hence t h e
v dependence o f ki i n t r o d u c e d i n t h i s way i s such t h a t k i decreases w i t h i n c r e a s i n g v which i s again completely c o n t r a r y t o t h e observed behavior.
C l e a r l y t h e n t h e s t r o n g v-dependence o f k i i s n o t brought
about d i r e c t l y by t h e i n t e r f a c i a l undercool ing.
5.
Table I V
*
Values o f I n k p
*
I n kp
Ld/kTd
b.
277
NONEQUILIBRIUM SOLIDIFICATION
and Ld/kTd f o r common dopants i n s i l i c o n
B
As
P
Sb
Ga
Bi
In
-1.204
-0.223
-1.050
-3.772
-4.828
-7.264
-7.824
0.000
1.076
0.998
2.645
2.223
2.494
0.918
V e l o c i t y Dependence o f t h e Entropy-Related F a c t o r s Next, t h e p o s s i b i l i t y t h a t t h e A ' s a r e v e l o c i t y dependent i s
considered.
As discussed i n t h e p r e c e d i n g s e c t i o n , t h e A's a r e
e n t r o p y - r e l a t e d f a c t o r s t h a t can be w r i t t e n as a product o f ( 1 ) a geometrical f a c t o r , t i o n a l frequency,
( 2 ) an accommodation c o e f f i c i e n t , ( 3 ) a v i b r a -
(4) t h e number o f p o s s i b l e s i t e s p e r u n i t area a t
t h e i n t e r f a c e , and (5) t h e volume o f an atom.
For Groups I11 and
V dopants i n s i l i c o n , which p r e s e r v e t h e same t e t r a h e d r a l bonding
w i t h t h e h o s t atoms t h a t e x i s t s between t h e h o s t atoms themselves, i t i s d i f f i c u l t t o see how t h e f a c t o r s ( l ) , (2), (3), and (4) c o u l d produce any more t h a n a very weak v-dependence.
I n t h e case o f (3),
a dependence o f t h e v i b r a t i o n a l frequency on t h e mass o f t h e dopant atom i s expected, b u t i t seems u n l i k e l y t h a t t h e frequency would be h i g h l y v-dependent.
I n l i n e w i t h t h i s reasoning, i s t h e con-
s i s t e n t use o f t h e i n t e r f a c e e q u i l i b r i u m c o n d i t i o n t o determine t h e r a t i o o f Ar t o Af, as expressed i n S e c t i o n 11.2 by Eq. (6). Also, i n m u l t i l a y e r treatments o f s o l i d i f i c a t i o n , t h e basic geometrical s t r u c t u r e o f t h e i n t e r f a c e i s i n v a r i a b l y taken t o be independent o f velocity.
It i s apparent t h e r e f o r e t h a t t h e r e i s good reason f o r
assuming t h a t any dependence o f t h e A ' s ( o r a t l e a s t t h e i r r a t i o s ) on i n t e r f a c e v e l o c i t y i s very weak. We are now f o r c e d t o conclude t h a t a s t r o n g v-dependence of t h e
K-'s must come i n t h r o u g h t h e U's themselves.
The f a c t t h a t t h e
a c t i v a t i o n energies appear as arguments o f t h e e x p o n e n t i a l f u n c t i o n
278
R. F. WOOD ET AL.
i n s u r e s t h a t small changes i n them can induce l a r g e changes i n t h e f o r c e constants.
I n t h e r e s t o f t h i s subsection we s k e t c h how t h e
v-dependence o f t h e K ' s can be i n t r o d u c e d t h r o u g h t h e U's. c.
Velocity-Dependent A c t i v a t i o n Energies For s i m p l i c i t y , we c o n t i n u e t o assume t h a t o n l y K:
i s velocity
dependent and c o n c e n t r a t e on how t h i s can come about t h r o u g h
Ui.
We w r i t e
where U ':
i s t h e e q u i l i b r i u m v a l u e o f t h e a c t i v a t i o n energy.
velocity-dependent
component AU;(V)
o f v, b u t i t must go t o 0 as v
may be a c o m p l i c a t e d f u n c t i o n
0 and t o some p o s i t i v e f i n i t e l i m i t -
+
i n g v a l u e as v becomes l a r g e .
The
Equation (25) can now be w r i t t e n as
r ro ro r Kd = F Ad exp(-Ud /kT)exp(-AUd(v)/kT) = F .K;oexp(-AU;(v)/kT).
(36)
i n which F z As/Aso. U s i n g Eq. (36) i n Eq.
(24) and assuming again t h a t v i s small
r f compared t o Kd and t h a t Kd = Kdfoy g i v e s
o fo ro w i t h ki z Kd /Kd
. The c o n d i t i o n s on t h e A f a c t o r s f o l l o w i n g from
t h e d i s c u s s i o n i n Sec. I I I . 7 b r e q u i r e t h a t F = 1.
We can a l s o p u t
where f ( v ) i s a f u n c t i o n t h a t goes t o 0 as v + 0 and t o 1 as v + m y ra r and AUd i s t h e asymptotic v a l u e o f A U ~ ( V )as v becomes very l a r g e . O f course v can n o t exceed
f
Kh,
b u t f o r convenience t h e n o t a t i o n
5. v +
OD
279
NONEQUILIBRIUM SOLIDIFICATION
I f i t i s r e q u i r e d t h a t k i + 1 as v +
w i l l be used.
my
then
i t i s e a s i l y shown t h a t
A
.
U =~ -kT~ I n k:
m I f i t i s known t h a t k i goes t o some maximum v a l u e k i as v becomes l a r g e , t h e more general e x p r e s s i o n m o
A U i a = kT ln(ki/ki)
can be introduced. m a r i l y by
kg,
The numerical v a l u e o f A U g a i S determined p r i -
and even an
i n t e r f a c i a l u n d e r c o o l i n g o f a 100 deg
o r more w i l l n o t g r e a t l y i n f l u e n c e i t s v a l u e i n s i l i c o n . we can s e t T = Th i n Eq. ki = ki0 exp[(-kThln
Therefore,
(39a) and o b t a i n t h e simple e x p r e s s i o n kp)f(v)/kTi]
,
(40 1
i n which f ( v ) i s a f u n c t i o n whose l i m i t i n g b e h a v i o r f o r l a r g e and small v i s known b u t i s o t h e r w i s e undetermined a t t h i s p o i n t . I n s i g h t i n t o t h e form o f
AU:(V)
can be o b t a i n e d on t h e b a s i s
o f t h e p h y s i c a l process i l l u s t r a t e d s c h e m a t i c a l l y i n Fig. 4. steady-state s o l i d i f i c a t i o n ,
During
the material i n the v i c i n i t y o f the
advancing m e l t f r o n t i s d i v i d e d i n t o t h r e e regions.
I n regions S
and L, t h e m a t e r i a l has t h e p r o p e r t i e s o f t h e s o l i d and l i q u i d , r e s p e c t i v e l y , w h i l e i n t h e i n t e r f a c e r e g i o n I,t h e p r o p e r t i e s change f r o m those o f a l i q u i d t o those o f a s o l i d .
atom i s deposited a t t h e b e g i n n i n g o f I.
A t t i m e t = 0, a dopant
As s o l i d i f i c a t i o n proceeds
and r e g i o n I moves across t h e o r i g i n a l p o s i t i o n o f t h e d e p o s i t e d atom, a time-dependent, e f f e c t i v e p o t e n t i a l b a r r i e r U l ( v , t ) a g a i n s t escape back i n t o t h e l i q u i d i s b u i l t up because o f t h e t r a n s i t i o n from t h e atomic o r d e r i n g and bonding o f t h e l i q u i d t o t h a t o f t h e r s o l i d . Ud(v,t) can be thought o f as a r i s i n g from a s e r i e s o f potent i a l w e l l s i n r e g i o n I between which t h e atom makes t r a n s i t i o n s .
While r e g i o n I i s moving across t h e o r i g i n a l p o s i t i o n o f t h e deposi t e d atom, t h e atom i t s e l f w i l l t e n d t o d i f f u s e toward L and t h i s
280
R. F. WOOD ETAL.
I
SOLID (S) INTERFACE (1)
I
LIQUID ( L I
t,=O
Fig. 4. Illustration o f the progress o f a dopant atom through an interface region advancing with velocity v.
f u r t h e r complicates t h e problem.
I n any case, a time-independent
a c t i v a t i o n energy U i ( v ) can be o b t a i n e d f r o m an average o f U i ( v , t ) over t h e t i m e t i t h e dopant atom spends i n 1. Because o f d i f f u s i o n , t i may be much l o n g e r t h a n t h e t i m e r e q u i r e d f o r r e g i o n I t o pass
a stationary point.
There i s a s i g n i f i c a n t d i f f e r e n c e between t h e
k i n e t i c s o f i n t e r f a c e processes f o r dopant and h o s t atoms.
Host
atoms a r e c o n s t a n t l y undergoing t r a n s i t i o n s between t h e l i q u i d and s o l i d also,
b u t whenever a h o s t atom l e a v e s t h e s o l i d t h e r e i s
almost always another h o s t atom i n t h e i n t e r f a c e t o t a k e i t s place, whereas t h i s i s n o t t r u e f o r a dopant atom.
A dopant atom has a
h i g h p r o b a b i l i t y o f b e i n g r e p l a c e d by a h o s t atom as a consequence of an i n t e r f a c e exchange process, p a r t i c u l a r l y a t low v e l o c i t i e s where many interchanges occur b e f o r e t h e i n t e r f a c e passes.
Although
5.
281
NONEQUILIBRIUM SOLIDIFICATION
b o t h host and dopant atoms p a r t i c i p a t e i n r e l a x a t i o n a l and d i f f u s i v e t y p e i n t e r f a c i a l processes,
i t i s o n l y f o r t h e dopant atoms t h a t
t h e d i f f u s i v e process i s obvious through i t s e f f e c t on k i ( o r on t h e c o n c e n t r a t i o n o f dopant i n t h e s o l i d ) . Two simple a n a l y t i c a l forms f o r f ( v ) were used i n t h e c a l c u l a t i o n s r e p o r t e d by Wood (1980,
1982); t h e f o r m t h a t p r o v i d e d t h e
b e s t agreement w i t h t h e s t i 1 sparse experimental data i s f(v)
1
=
-
(vo/v)(l
-
exp (- v/vo 1)
(41)
T h i s e x p r e s s i o n can be obta ned by t i m e a v e r a g i n g t h e f u n c t i o n g(v,t)
= (1
-
exp(-vt/voti))
(42)
o v e r ti, t h e t i m e a dopant atom spends i n t h e i n t e r f a c e region. Although t i i s n o t known, t h e q u a n t i t y V o t i can be i n t e r p r e t e d q u a l i t a t i v e l y as t h e d i s t a n c e xo t h e atom d i f f u s e s w h i l e i n r e g i o n
I and vo can be r e l a t e d t o an average d i f f u s i o n c o e f f i c i e n t D i o f t h e dopant i n I by D i = voxo.
Since experimental values o f D i are
n o t a v a i l a b l e , Wood assumed t h a t D i was p r o p o r t i o n a l t o DQ, t h e d i f f u s i o n c o e f f i c i e n t i n t h e l i q u i d m a t e r i a l , and p u t vo = DQ/xA; f o r a g i v e n vo, x; 8.
CALCULATIONS
i s r e l a t e d t o xo i n an obvious manner.
OF ki FOR VARIOUS DOPANTS I N SILICON
C a l c u l a t i o n s o f k i w i t h t h e model d e s c r i b e d above i n v o l v e i n p u t data which a r e e i t h e r n o t known o r a r e of l i m i t e d accuracy. Trumbore (1960) has e s t i m a t e d t h a t t h e r e a r e t 10-20% e r r o r s a s s o c i a t e d w i t h even t h e b e s t d e t e r m i n a t i o n s o f t h e values o f k y c o n t a i n e d i n h i s compilation.
i n Table V.
Values o f ky t a k e n from t h i s c o m p i l a t i o n a r e g i v e n Values o f t h e d i f f u s i o n c o e f f i c i e n t s o f Group 111 and
V dopants i n l i q u i d S i measured by Kodera (1963) and by Shashkov and
G u r i v i c h (1968) a r e a l s o shown i n t h e t a b l e .
The s u b s t a n t i a l d i f -
ferences between t h e two s e t s o f values and t h e l a r g e e r r o r l i m i t s Kodera assigned t o h i s values p o i n t up t h e l a c k o f s a t i s f a c t o r y
282
R. F. WOOD ET AL.
Table V Experimental data. Values o f k i a r e n o t d i r e c t l y measurable; t h e y can be determined o n l y by t h e o r e t i c a l l y f i t t i n g experimental dopant p r o f i l e s .
Dopant
ky
a
0.8 0.35 0.3 0.023 0.008 0.0004 0.0007
B P
As
Sb Ga In Bi
kid
2.4k.7 5.121.7 3.35.9 1.52.5 4.821.5 6.921.2
---
3.3t.4 2.7?. 3
---
1.45.5 0.66+_.5 0 . 1 7 ~3
---
---
---
1.00 0.7 0.2 0.15 0.4
kie
-1.0 -1.0 -1.0 0.8- 1.0 0.15-0.3 0.10-0.20 0.25-0.35
a. Trumbore (1960); b. Kodera (1963); c. Shashkov and G u r i v i c h (1968) d. White e t a l . (1980); e. Wood e t a l . (1981) i n f o r m a t i o n about d i f f u s i o n c o e f f i c i e n t s i n t h e l i q u i d s t a t e o f s i l i c o n ; i n o t h e r semiconductors t h e s i t u a t i o n i s even worse.
The
c a l c u l a t i o n s o f dopant d i f f u s i o n d u r i n g p u l s e d l a s e r a n n e a l i n g r e p o r t e d by Wood e t a l .
(1981) and d e s c r i b e d h e r e i n Chapter 4
showed t h a t t h e Kodera values gave r e s u l t s c o n s i s t e n t l y c l o s e r t o t h e experimental dopant p r o f i l e s t h a n d i d t h o s e of Shashkov and Gurivich. F i g u r e 5 shows t h e r e s u l t s o f c a l c u l a t i o n s o f ki f o r most of t h e Group I 1 1 and V dopants i n S i as a f u n c t i o n o f v u s i n g Eq. (41) i n t h e e x p r e s s i o n f o r k i o f Eq. (40). o f D,
I n t h e c a l c u l a t i o n s , values
f o r t h e dopants considered by Kodera were a l l o w e d t o vary
w i t h i n t h e ranges g i v e n by him. expressed as Da/x;
The parameter vo o f E q . (41) was
and, by t r i a l and e r r o r , values o f x;
and o f D,
which gave reasonably good f i t s t o t h e experimental values o f k i shown i n Table V were determined.
The f i t s t o t h e experimental
dopant p r o f i l e s from which t h e values o f k i i n Table V were d e t e r mined assumed v -4 m/sec f o r a l l dopants except Sb, f o r which v was
5.
283
NONEQUILIBRIUM SOLIDIFICATION
4.0
0.8 c
z
w V
$, 0.6 8 z 0
t u t:
0.4
v)
x
0.2 0
0
Fig. 5.
EXPERIMENTAL DATA FOR Bi
I 0
2
I
6 €3 v, MELT-FRONT VELOCITY [ rnlsec)
4
40
Variation o f k i with v f o r group I l l and V dopants in S i .
(Wood,
1982)
-3 m/sec.
The v a l u e o f x;
found i n t h i s way was 850 A , b u t i t must
be emphasized t h a t t h i s value i s a s s o c i a t e d w i t h s u b s t a n t i a l l y i f values o f D i were a v a i l a b l e . between
DE and would change
Such values must l i e
DE and D, ( d i f f u s i o n c o e f f i c i e n t i n t h e s o l i d ) , although
p r o b a b l y much c l o s e r t o t h e values o f DE.
Since t h e range between
DQ and D, covers about f i v e o r d e r s o f magnitude f o r t h e dopants considered here, good e s t i m a t e s o f Di and xo cannot be made a t t h i s time. 9.
SOLUBILITY LIMITS The q u e s t i o n of t h e maximum c o n c e n t r a t i o n o f dopant atoms t h a t
can be i n c o r p o r a t e d i n t o t h e h o s t l a t t i c e d u r i n g t h e u l t r a r a p i d , nonequil i b r i u m , c r y s t a l growth c h a r a c t e r i s t i c o f l a s e r a n n e a l i n g i s o f c o n s i d e r a b l e i n t e r e s t from both a fundamental and a p p l i e d s t a n d p o i n t (see t h e d i s c u s s i o n i n Chapter 2). q u e s t i o n b r i e f l y here.
We c o n s i d e r t h i s
284
R. F. WOOD ET AL.
For an i d e a l , d i l u t e , b i n a r y s o l i d s o l u t i o n which i s c r y s t a l l i z i n g f r o m an i d e a l , s a t u r a t e d , l i q u i d s o l u t i o n , t h e molar concentration o f t h e host i n t h e l i q u i d i s
t h i s e x p r e s s i o n i s contained, f o r example, Eq.
(4.6).
i n Thurmond (1959) as
Then s i n c e
CdS = k.C% = ki ( 1-Ch)II i d
,
(44)
we have t h a t
f The e x p r e s s i o n i n b r a c k e t s i s a good approximation t o V/Kh o f Eq. (9). It f o l l o w s t h e n t h a t w i t h i n t h e framework o f t h e elementary approx-
i m a t i o n s made here, t h e s i m p l e e x p r e s s i o n C i = ki(v/Kh) f
i s obtained.
Equation (46) g i v e s an e s t i m a t e o f t h e maximum con-
c e n t r a t i o n o f dopant which w i l l appear i n t h e s o l i d i n s o l u t i o n (i.e.,
s u b s t i t u t i o n a l l y ) a t a given c r y s t a l l i z a t i o n velocity.
The
e q u a t i o n does n o t p r e d i c t an a b s o l u t e s o l u b i l i t y l i m i t because t h e assumptions on which i t i s p r e d i c a t e d cannot h o l d a t a r b i t r a r i l y h i g h dopant c o n c e n t r a t i o n s i n t h e l i q u i d .
Moreover, t h e r e c r y s t a l -
l i z a t i o n v e l o c i t y w i l l depend on t h e dopant c o n c e n t r a t i o n u n l e s s t h e c o n c e n t r a t i o n i s very d i l u t e and t h i s must be t a k e n i n t o account. T a b l e V I shows some o f t h e q u a n t i t i e s r e l a t e d t o t h e c a l c u l a t i o n s o f C$/Cio
which have been made f o l l o w i n g t h e simple t h e o r y
o u t l i n e d above.
The second column g i v e s values o f t h e maximum
e q u i l i b r i u m c o n c e n t r a t i o n i n atoms/cm3,
N%O, t a k e n from Trumbore's
c o m p i l a t i o n and i n t h e t h i r d column t h e s e values have been conv e r t e d t o atomic percent.
Columns f o u r and f i v e g i v e experimental
285
5 . NONEQUILIBRIUM SOLIDIFICATION Table V I Experimental data and r e s u l t s o f c a l c u l a t i o n s o f t h e s o l u b i l i t y l i m i t s u s i n g Eqs. (40), (41), and (46). The r e c r y s t a l l i z a t i o n v e l o c i t y was t a k e n t o be 4 m/sec except f o r Sb where 3 m/sec was used. Dopant
Nioa (cm-3 )
B
6. Ox 1020 1 . 4 ~102 1 1 . 5 ~1021 7.0~1019 4.5~1019 8. Ox 1017 8.0~1017 2. ox 1019
P As Sb Ga In Bi A1
c;/c;o
:c
0.0122 0.029 0.031 0.0014 0.00092 0.000016 0.000016 0.00041
6.0~1021 1.3~1021 4.5~1020 1.5~1020 4.0~1020
a. Trumbore (1960); b. White e t a l . d a t a taken from White e t a l .
4 18 10 188 500
cal c
cal c
0.039 0.036 0.037 0.022 0.016 0.006 0.012 0.019
3 1 1 16 18 400 775 47
(1980); c. Wood e t a l .
(1981)
(1980). Values o f Cz c a l c u l a t e d f r o m
Eq. (46) u s i n g values o f k i from Eqs. (40) and (41) a r e l i s t e d i n t h e f s i x t h column. The r a t e c o n s t a n t Kh was assumed t o be 100 m/sec and v was 4 m/sec f o r a l l dopants except Sb f o r which i t was 3 m/sec. The l a s t column shows values o f and 6.
C:/Cio
f r o m t h e d a t a i n columns 3
The agreement between experiment and t h e o r y i s q u i t e s a t i s -
f a c t o r y i n view o f t h e v a r i o u s l a r g e u n c e r t a i n t i e s t o which b o t h t h e experiments and c a l c u l a t i o n s a r e s u b j e c t .
The g r e a t e s t apparent
discrepancy between t h e o r y and experiment i s f o r a r s e n i c .
This
d isag r eemen t p r oba b 1y occu r s because t h e as sumpt ion s u nde r ly in g Eq.
(43) a r e v i o l a t e d . The measured v a l u e o f N i f o r a r s e n i c g i v e s
C$ = 0.12, and t h e r e f o r e t h e c o n c e n t r a t i o n o f dopant atoms i s f a r from d i l u t e . Also, i t should be n o t e d t h a t Eq. (46) makes Ci + 0 as v + 0 and t h i s b e h a v i o r i s i n c o r r e c t .
F o r dopants w i t h s m a l l
values o f k i t h i s i s o f l i t t l e importance b u t f o r dopants w i t h l a r g e values o f kq needed.
an e x p r e s s i o n which behaves p r o p e r l y
a t small v i s
I n any case, t h e agreement between experiment and theory
can be expected t o improve as t h e t h e o r y , measurements, and c a l c u l a t ions a r e r e f ined.
286
R. F. WOOD E T A L .
10,
SOLUTE
TRAPPING
Baker and Cahn (1969,1971)
introduced t h e terminology " s o l u t e
t r a p p i n g " t o d e s c r i b e t h e s i t u a t i o n i n which a s o l u t e atom e x p e r i ences an i n c r e a s e i n chemical p o t e n t i a l as i t crosses t h e l i q u i d s o l i d interface.
Since, f o r a system i n thermodynamic e q u i l i b r i u m ,
t h e chemical p o t e n t i a l can n o t depend on p o s i t i o n , an i n c r e a s e i n t h e chemical p o t e n t i a l i m p l i e s t h a t a n o n e q u i l i b r i u m process has t a k e n place. Baker and Cahn i l l u s t r a t e d t h e c a l c u l a t i o n o f t h e change i n chemical p o t e n t i a l f o r t h e case i n which d i l u t e s o l u t i o n s a r e p r e s e n t i n both t h e s o l i d and l i q u i d .
I n t h i s case, t h e chemical
p o t e n t i a l s of t h e dopant i n t h e s o l i d and l i q u i d a r e given respect i v e l y by 'd
:p
- BS + kT l n ( y S C i ) = BE
+
kT I n (yRC:)
The constants Bs,
,'B
ys, y R
. depend on t h e temperature and r e f e r e n c e
s t a t e f o r t h e energy, but t h e y may be e l i m i n a t e d by u s i n g t h e equality of
~2 and 11%
a t e q u i l b r i u m when C i = C i 0 and C% = Ca0.
For an
i n d i v i d u a l dopant atom c r o s s i n g t h e i n t e r f a c e , t h e change i n t h e chemical p o t e n t i a l i s t h e n g i v e n by
U s i n g Eqs. (47a) and (47b) and t h e d e f i n i t i o n s i n Eqs. ( 2 3 ) and ( 2 6 ) , A U becomes ~ ~p~ =
kT I n (ki/kS)
.
(49)
Since t h e B ' s and y ' s are temperature dependent and were e l i m i n a t e d by u s i n g e q u i l i b r i u m c o n d i t i o n s , t h e T i n Eqs. (47a) and (47b) i s
5.
287
NONEQUILIBRIUM SOLIDIFICATION
t h e temperature a p p r o p r i a t e f o r s o l i d i f i c a t i o n under n e a r l y e q u i l i b r i u m conditions.
For d i l u t e s o l u t i o n s , T i s approximately T h y
t h e m e l t i n g p o i n t o f t h e h o s t m a t e r i a l , and t h e r e f o r e
Comparing t h i s t o Eq. (40), i t i s found t h a t
~u~ =
-
kTh I n k y f ( v )
.
Near e q u i l i b r i u m , v = 0, f ( v ) = 0, and t h e dopant experiences no change i n chemical p o t e n t i a l as i t crosses t h e i n t e r f a c e .
On t h e
o t h e r hand, a t h i g h i n t e r f a c e v e l o c i t i e s f ( v ) = 1, and t h e maximum v a l u e o f hpd i s A
U o ~f Eq. ~ (39a) o r (39b).
Thus, t h e model des-
c r i b e d here does s a t i s f y t h e requirement t h a t t h e r e be an i n c r e a s e i n t h e chemical p o t e n t i a l on s o l i d i f i c a t i o n i f k i i s observed t o 0
exceed k i .
11.
a.
DISCUSSION Other Models o f N o n e q u i l i b r i u m Segregation The developments d e s c r i b e d i n t h i s s e c t i o n have been based on
one p a r t i c u l a r approach t o t h e dopant s e g r e g a t i o n problem.
We
b e l i e v e t h i s approach t o be t h e s i m p l e s t and most d i r e c t manner o f a r r i v i n g a t t h e dependence o f t h e s e g r e g a t i o n c o e f f i c i e n t on t h e v e l o c i t y o f t h e l i q u i d - s o l i d i n t e r f a c e , b u t a number o f o t h e r t r e a t ments o f t h i s s u b j e c t have appeared.
These w i l l be d e s c r i b e d b r i e f l y
h e r e and references given f o r t h e reader i n t e r e s t e d i n p u r s u i n g t h e matter further. Jackson (1958) took an approach based on k i n e t i c r a t e t h e o r y b u t d i d n o t a l l o w h i s r a t e constants t o depend e x p l i c i t l y on t h e interface velocity.
Baker and Cahn (1969) subsequently p o i n t e d o u t
t h a t Jackson's t h e o r y d i d n o t s a t i s f y t h e thermodynamic c r i t e r i o n f o r " s o l u t e t r a p p i n g " and t h e r e f o r e c o u l d n o t be c o r r e c t .
Baker
288
R. F. WOOD ET AL.
and Cahn (1971) have a l s o g i v e n c r i t i q u e s o f f o u r t h e o r i e s o f s o l i d i f i c a t i o n based on i r r e v e r s i b l e thermodynamics (Borisov, 1962, 1966; B a r a l i s , 1968; Aptekar and Kamenetskaya, 1962; J i n d a l and T i l l e r , 1968).
Each t h e o r y i s shown t o have i t s shortcomings and t h e
i n t e r e s t e d reader should c o n s u l t t h e paper by Baker and Cahn f o r t h e d e t a i l s o f how t h e v a r i o u s arguments a r e made. Here, however, i t i s o f i n t e r e s t t o d i s c u s s a few f e a t u r e s o f t h e J i n d a l - T i l l e r t h e o r y (1968).
These a u t h o r s d i v i d e d t h e i n t e r -
f a c e r e g i o n i n t o a number o f l a y e r s each o f which i s regarded as an open thermodynamic system undergoing i r r e v e r s i b l e changes.
While
t h e l a y e r n e a r e s t t h e s o l i d i f i e d m a t e r i a l i s undergoing a phase t r a n s i t i o n , i t i s a l s o exchanging atoms w i t h t h e l a y e r next t o i t , and so f o r t h .
The major problem J i n d a l and T i l l e r encountered was
t h e d i f f i c u l t y o f s o l v i n g t h e simultaneous equations f o r any number o f l a y e r s g r e a t e r t h a n one.
T h e i r model resembles t h e m u l t i l a y e r
model o f c r y s t a l growth g i v e n by F l e t c h e r and d e s c r i b e d here i n S e c t i o n 11.
J i n d a l and T i l l e r o b t a i n e d an e x p r e s s i o n f o r k i which
has t h e r i g h t behavior a t l a r g e values o f v b u t does n o t go t o k? as v goes t o zero.
A t l e a s t two t h e o r i e s o f n o n e q u i l i b r i u m s e g r e g a t i o n based on m o d i f i c a t i o n s o f t h e d i f f e r e n t i a l e q u a t i o n approach t o s o l i d i f i c a t i o n t h e o r y have been proposed.
As m i g h t be expected, t h e modi-
f i c a t i o n s i n v o l v e t h e assumptions made about t h e i n t e r f a c e region. Whereas t h e more standard d e r i v a t i o n s based on t h e d i f f u s i o n equat i o n t r e a t t h e i n t e r f a c e as a p l a n a r d i s c o n t i n u i t y (Smith e t al., 1955) , t h e f o r m u l a t i o n s o f Chernov (1962) and Baker (1970) recognize t h a t t h e i n t e r f a c e r e g i o n must have a f i n i t e w i d t h and p r o p e r t i e s which a r e i n t e r m e d i a t e between t h o s e o f t h e s o l i d and l i q u i d . Cahn e t al.
(1980) have i l l u s t r a t e d t h e i n t e r f a c e v e l o c i t y dependence
o f k i which f o l l o w s from B a k e r ' s model when v a r i o u s assumptions about t h e i n t e r f a c e r e g i o n a r e made. A l l o f t h e models discussed above were f o r m u l a t e d b e f o r e t h e
i n c e p t i o n o f p u l s e d l a s e r a n n e a l i n g and a t a t i m e when l i t t l e o r no e x p e r i m e n t a l data were a v a i l a b l e as an a i d i n t h e development of t h e
5. models.
NONEQUILIBRIUM SOLIDIFICATION
289
Jackson e t a l . (1980) proposed a model i n which t h e y simply
added a t e r m p r o p o r t i o n a l t o t h e v e l o c i t y t o t h e numerator o f f fo r ro Eq. (24) ( w i t h Kd = Kd and Kd = Kd ), i.e.,
.
ki = (Kdf o +av)/(Ki0+v)
(53)
The parameter a was i n t e r p r e t e d as t h e f r a c t i o n of dopant atoms i n t h e l a y e r adjacent t o t h e i n t e r f a c e which i s trapped: i t was t a k e n equal t o u n i t y .
I n so f a r as t h i s model e x p l i c i t l y recognizes t h e
v e l o c i t y dependence o f t h e r a t e constants, i t i s i n accord w i t h t h e model a l r e a d y d e s c r i b e d here.
However, we n o t e t h a t merely adding
a t e r m l i n e a r i n v has very l i t t l e j u s t i f i c a t i o n and r e s u l t s i n a r a t h e r i n f l e x i b l e model t h a t has been c r i t i c i z e d on several grounds (Wood, 1982; Aziz, 1982). Morehead (1980) has r e p o r t e d t h e r e s u l t s o f a f i n i t e - d i f f e r e n c e calculation t h a t incorporates r a t e e f f e c t s a t t h e interface.
His
approach appears capable o f d e s c r i b i n g n o n e q u i l i b r i u m segregation, b u t t h e n a t u r e o f some o f t h e approximations made i s n o t c l e a r and h i s f o r m u l a t i o n o f t h e problem r e q u i r e s f u r t h e r development. A z i z (1982) has d e s c r i b e d two k i n e t i c r a t e models f o r dopant segregation.
One model y i e l d s a s i m i l a r f u n c t i o n a l form f o r k i ( v )
t o t h a t proposed by Jackson e t a l . (1980) b u t has a d i f f e r e n t physi c a l c o n t e n t ; Aziz g i v e s a d e t a i l e d comparison o f t h e two formulations.
The o t h e r model resembles i n some r e s p e c t s t h e one d e s c r i b e d
i n t h i s s e c t i o n , b u t d e r i v e d w i t h a p i c t u r e o f stepwise growth underl y i n g it. A z i z o b t a i n s an e q u a t i o n which can be p u t i n t h e f o r m 0
0
ki = ki + ( l - k g ) e x p ( - v / v ) ,
(54)
F i n a l l y , b e f o r e l e a v i n g t h i s d i s c u s s i o n o f macroscopic models and c a l c u l a t i o n s , i t i s o f i n t e r e s t t o n o t e t h a t over t h i r t y y e a r s ago H a l l (1952) proposed t h e expression ki
= k:
t (ki-ki)exp(-v m o
0/ v )
.
(55)
290
R. F. WOOD E T A L .
m k i i s a maximum v a l u e ( n o t n e c e s s a r i l y u n i t y ) a t t a i n e d by k i when
v+-
(see Eq. 39b).
A d e t a i l e d d e r i v a t i o n o f t h i s e x p r e s s i o n was
a p p a r e n t l y never given, b u t i t i s f o r m a l l y i d e n t i c a l t o Eq. ( 5 4 ) m except f o r t h e added g e n e r a l i t y o f i n t r o d u c i n g k i i n s t e a d o f 1. On a more m i c r o s c o p i c l e v e l , Gilmer (1983) has d e s c r i b e d c a l c u l a t i o n s based on Monte C a r l o s i m u l a t i o n w i t h a k i n e t i c I s i n g model. Although G i l m e r ' s c a l c u l a t i o n s seem more s u i t e d f o r modeling t h e solid-vapor interface, they are useful i n p r o v i d i n g i n s i g h t s i n t o t h e s e g r e g a t i o n e f f e c t s i n v o l v e d i n l a s e r annealing.
However, t h e
c a l c u l a t i o n s appear t o r e q u i r e i n o r d i n a t e l y h i g h i n t e r f a c i a l underc o o l i n g s t o o b t a i n t h e observed b e h a v i o r o f k j ( v ) w i t h v. S i m i l a r l y , t h e m o l e c u l a r dynamics c a l c u l a t i o n s c a r r i e d o u t by Cleveland e t a l . (1982) a r e l i m i t e d by t h e use o f lennard-Jones p o t e n t i a l s , which do n o t s i m u l a t e t h e d i r e c t i o n a l i t y o f t h e c o v a l e n t bonding i n semiconductors, y e t such c a l c u l a t i o n s
are also q u i t e useful i n
p r o v i d i n g i n s i g h t i n t o t h e nonequilibrium e f f e c t s described i n t h i s section. b.
S a t u r a t i o n o f t h e Segregation C o e f f i c i e n t a t High I n t e r f a c e Velocities There i s some experimental evidence ( B a e r i e t al.,
1981) t h a t
k i ( v ) saturates w i t h increasing v a t a value less than u n i t y f o r
B i i n S i . However, t h e r e i s a b a s i c c o n f l i c t between t h e s e r e s u l t s and s i m i l a r r e s u l t s from t h e measurements o f White e t a l . (1981) on t h e same system.
Poate (1982) has presented data which suggests
t h a t k i ( v ) f o r I n i n S i may a l s o s a t u r a t e w i t h i n c r e a s i n g v.
How-
ever, i n h i s d a t a f o r h i g h e r v (v = 7.5 m/sec) t h e r e i s c l e a r e v i dence t h a t t h e m e l t f r o n t d i d n o t p e n e t r a t e t h r o u g h t h e i m p l a n t e d r e g i o n , which would make t h e d e t e r m i n a t i o n o f an a c c u r a t e v a l u e o f k i ( v ) d i f f i c u l t because o f u n c e r t a i n t i e s about t h e c a l c u l a t i o n ( o r e s t i m a t i o n ) o f v when an a-Si l a y e r i s p r e s e n t (see Chap. 4, Sec. IVb).
On t h e o t h e r hand, f o r some o f t h e small e x t r a c t e d values o f
k i a t low v, steady s t a t e regrowth c o n d i t i o n s almost c e r t a i n l y had n o t been reached and t h i s c o u l d a l s o make an a c c u r a t e d e t e r m i n a t i o n
5.
291
NONEQUILIBRIUM SOLIDIFICATION
o f k i ( V ) d i f f i c u l t (see Chap. 4, Sec. V.20).
This s i t u a t i o n points
up t h e need f o r more comprehensive and c a r e f u l e x p e r i m e n t a t i o n and f u r t h e r r e f i n e m e n t o f t h e numerical methods f o r e x t r a c t i n g t h e values o f k i from t h e experimental data. There may be p h y s i c a l reasons why k i m i g h t s a t u r a t e w i t h v f o r h i g h i n t e r f a c e v e l o c i t e s , b u t these w i l l n o t be discussed here s i n c e t h e experimental s i t u a t i o n has n o t y e t been resolved.
I n s t e a d we
a r e more i n t e r e s t e d i n whether o r n o t t h e v a r i o u s t h e o r i e s and models can account f o r t h i s s a t u r a t i o n f r o m a t h e o r e t i c a l standp o i n t , i f i t does indeed occur.
A p p a r a n t l y some authors (see, e.g.,
Jackson, 1983; White and Appleton, 1982) a r e under t h e misconception t h a t o n l y t h e model o f Jackson e t a l .
(1980) accounts f o r such a
The model d e s c r i b e d here i n S e c t i o n 111.7 can i n c o r -
saturation.
porate a saturation demonstrated by Eq.
c o n d i t i o n i n a s t r a i g h t f o r w a r d manner, (39b),
and i t seems q u i t e l i k e l y t h a t such a
c o n d i t i o n can be b u i l t i n t o t h e o t h e r models as w e l l ; i n Eq. (55) k i
+
m
ki
as
as v becomes l a r g e .
I n fact,
obviously
the saturation
o c c u r r i n g i n t h e model o f Jackson e t a l . appears t o be an a r t i f a c t o f t h e i n f l e x i b i l i t y o f t h e i r model s i n c e s a t u r a t i o n o f k i a t values l e s s t h a n u n i t y i s p r e d i c t e d f o r a l l dopants, and t h i s i s n o t observed f o r c.
B,
P, As,
and p r o b a b l y S b (see Fig. 5 and Chap. 2).
Dependence o f k i ( v ) on O r i e n t a t i o n Several experimental s t u d i e s have e s t a b l i s h e d t h a t t h e i n t e r f a c e
s e g r e g a t i o n c o e f f i c i e n t depends n o t o n l y on v e l o c i t y , b u t a l s o on c r y s t a l l o g r a p h i c o r i e n t a t i o n ( B a e r i e t a1 1981; Poate, 1982). silicon i n the
,
., 1981;
White e t al.,
We emphasize t h a t t h e regrowth v e l o c i t i e s o f <110>, and <111> d i r e c t i o n s a r e e s s e n t i a l l y
equal, a t l e a s t up t o t h e v e l o c i t i e s a t which t h e amorphous phase o r a h i g h l y d e f e c t i v e c r y s t a l l i n e phase begins t o be formed.
From
t h e s t a n d p o i n t o f t h e phenomenological model presented i n t h i s section,
t h i s e f f e c t can be understood by r e c o g n i z i n g t h a t t h e
a c t i v a t i o n energy U$(v)
and perhaps t h e A f a c t o r i n Eq.
depend on o r i e n t a t i o n , as discussed by Wood (1982).
(25) can
On a somewhat
292
R. F. WOOD ETAL.
more m i c r o s c o p i c l e v e l
,
Gilmer (1983) has demonstrated t h a t t h e
Monte C a r l o approach can be made t o y i e l d t h e observed d i r e c t i o n a l dependence o f k i ( v ) by a s u i t a b l e choice o f t h e f i t t i n g parameters i n t h e k i n e t i c I s i n g model.
The p h y s i c a l c o n t e n t o f t h e approx-
i m a t i o n s r e q u i r e d t o o b t a i n agreement w i t h experiment needs much f u r t h e r study, b u t such study should e v e n t u a l l y l e a d t o a deeper understanding o f t h e r o l e o f i n t e r f a c e k i n e t i c s i n l i q u i d s t h a t change from m e t a l l i c t o c o v a l e n t bonding on s o l i d i f i c a t i o n .
Interface Instabilfty and Formation of Cellular Structure
IV. 12.
BACKGROUND The f o r m a t i o n o f c e l l u l a r s t r u c t u r e d u r i n g t h e s o l i d i f i c a t i o n
of m a t e r i a l s i s a well-known phenomenon (see, e.g., 1973 f o r copious i l l u s t r a t i o n s ) .
T a r s h i s e t al.,
As d e s c r i b e d i n Chapters 2 and 4,
c e l l u l a r s t r u c t u r e has been observed i n laser-annealingexperiments. The s t r u c t u r e c o n s i s t s o f r e g i o n s o f h o s t m a t e r i a l c o n t a i n i n g t h e s o l u t e o r dopant atoms a t r e l a t i v e l y low c o n c e n t r a t i o n s surrounded by c e l l w a l l s i n which t h e c o n c e n t r a t i o n o f dopants i s much higher. Q u e s t i o n s concerning t h e dependence o f s o l u b i l i t i e s on i n t e r f a c e v e l o c i t y and t h e s o l u b i l i t y l i m i t s which can u l t i m a t e l y be a t t a i n e d a r e r e l a t e d t o t h e s t a b i l i t y o f t h e p l a n a r i n t e r f a c e and how i t i s i n f l u e n c e d by t h e v a r i o u s p h y s i c a l parameters o f t h e t h e problem (Chapter 2).
The f a s c i n a t i n g aspect o f t h e c e l l u l a r s t r u c t u r e t h a t
i s observed d u r i n g laser-anneal i n g experiments i s t h a t t h e f o r m a t i o n
o f t h e c e l l s and t h e i r s i z e s can o n l y be e x p l a i n e d i f t h e none q u i l i b r i u m values o f k j are used i n t h e c a l c u l a t i o n s d e s c r i b e d next.
Thus,
t h e study o f c e l l u l a r f o r m a t i o n and t h e c o n d i t i o n s
under which i t occurs p r o v i d e another source o f i n f o r m a t i o n on none q u i l i b r i u m c r y s t a l growth and u l t r a r a p i d s o l i d i f i c a t i o n .
In this
s e c t i o n , we show how a f o r m u l a t i o n by M u l l i n s and Sekerka (1964) o f t h e problem o f t h e s t a b i l i t y o f a p l a n a r i n t e r f a c e moving i n a b i n a r y a l l o y can y i e l d good agreement between t h e o r y and experiment f o r c e l l u l a r f o r m a t i o n i n ion-implanted,
laser-annealed s i l i c o n .
5. 13.
293
NONEQUILIBRIUM SOLIDIFICATION
MULLINS AND SEKERKA THEORY
OF INTERFACIAL INSTABILITY
M u l l i n s and Sekerka (1964) considered t h e e f f e c t s o f an i n f i n i t e s i m a l s i n u s o i d a l p e r t u r b a t i o n o r r i p p l e superimposed on t h e moving p l a n a r i n t e r f a c e .
The d e t a i 1s o f t h e Mu1 1ins-Sekerka (M-S)
t r e a t m e n t a r e t o o i n v o l v e d t o p r e s e n t here,
b u t we w i l l discuss
b r i e f l y t h e t h r e e major assumptions u n d e r l y i n g it.
The f i r s t
assumption i s t h a t steady s t a t e s o l i d i f i c a t i o n has been reached and t h a t mass and heat t r a n s p o r t can be t r e a t e d by t h e a p p r o p r i a t e d i f f u s i o n equations.
As shown i n Chapter 4, t h e use o f d i f f u s i o n
equations t o d e s c r i b e t h e r e d i s t r i b u t i o n o f dopants i n t h e molten m a t e r i a l i s a p p a r e n t l y q u i t e a good a p p r o x i m a t i o n i n p u l s e d l a s e r annealing, b u t i t may happen, e s p e c i a l l y a t h i g h regrowth v e l o c i t i e s , t h a t steady s t a t e c o n d i t i o n s a r e n o t a t t a i n e d f o r some dopants i n t h e s h a l l o w s u r f a c e r e g i o n m e l t e d by t h e l a s e r r a d i a t i o n .
The
second assumption o f t h e M-S t h e o r y i s t h a t t h e superimposed s i n u s o i d a l p e r t u r b a t i o n i s so weak t h a t i t s e f f e c t s on t h e dopant conc e n t r a t i o n and temperature d i s t r i b u t i o n a few wavelengths ( o f t h e p e r t u r b a t i o n ) from t h e moving l i q u i d - s o l i d i n t e r f a c e a r e n e g l i g i b l e . The t h i r d assumption .of t h e M-S t r e a t m e n t c o n s i s t s o f two boundary c o n d i t i o n s imposed a t t h e l i q u i d - s o l i d
interface,
as discussed
next. The f i r s t c o n d i t i o n r e l a t e s t h e temperature a t t h e i n t e r f a c e t o t h e l i q u i d u s c u r v e on t h e phase diagram i n t h e f o l l o w i n g manner. L e t t h e p l a n a r m e l t f r o n t advance i n t h e x - d i r e c t i o n ,
l e t T4 be t h e
temperature o f t h e s i n u s o i d a l l y v a r y i n g i n t e r f a c e when t h e s o l u t e i s present, and l e t TN be t h e temperature i n t h e absence o f t h e solute. t o x.
Note t h a t T4 and TN vary i n t h e d i r e c t i o n s p e r p e n d i c u l a r Then t h e M-S t h e o r y r e q u i r e s t h a t
294
R. F. WOOD ET AL.
i n which m i s t h e s l o p e o f t h e l i q u i d u s
and
concentration i n t h e l i q u i d a t the interface.
i s t h e dopant dY4 TN i s r e l a t e d t o t h e C'
m e l t i n g temperature Th o f t h e h o s t a t a p l a n a r i n t e r f a c e by TN = Th
+
ThrK'
(57)
K' i s t h e average c u r v a t u r e a t a p o i n t on t h e i n t e r f a c e and r i s a s o - c a l l e d c a p i l l a r i t y c o n s t a n t which i s r e l a t e d t o t h e i n t e r f a c i a l f r e e energy.
(57), l o c a l e q u i l i b r i u m i s assumed
I n a p p l y i n g Eq.
t o h o l d i n t h e l i q u i d and so t h e s l o p e o f t h e l i q u i d u s can be t a k e n from t h e e q u i l i b r i u m phase diagrams.
It i s i m p o r t a n t t o n o t e t h a t
l o c a l e q u i l i b r i u m i s n o t assumed t o e x i s t i n t h e s o l i d . The second i n t e r f a c e c o n d i t i o n r e q u i r e s t h a t t h e i n t e r f a c e v e l o c i t y i n t h e dopant d i f f u s i o n c a l c u l a t i o n s must be c o n s i s t e n t w i t h t h e v e l o c i t y o b t a i n e d from t h e heat d i f f u s i o n equations.
This
c o n d i t i o n i s expressed by
i n which Ks and KQ a r e t h e thermal c o n d u c t i v i t i e s o f t h e p u r e h o s t i n t h e s o l i d and l i q u i d s t a t e s ,
r e s p e c t i v e l y , Ts and T,
are t h e
corresponding temperatures , and t h e o t h e r symbols have been p r e v i o u s l y defined.
The s u b s c r i p t
+
indicates t h a t t h e gradients are t o
be c a l c u l a t e d a t t h e s i n u s o i d a l l y v a r y i n g i n t e r f a c e . of v , Ts, Tg, and C:
The dependence
on z and y comes about because t h e i n t e r f a c e
i s no l o n g e r p l a n a r once t h e s i n u s o i d a l p e r t u r b a t i o n has been superimposed, and hence t h e q u a n t i t i e s vary from p o i n t t o p o i n t on t h e interface.
The r e l a t i o n s h i p between t h e v e l o c i t y o f t h e m e l t f r o n t
and t h e thermal g r a d i e n t s was g i v e n i n Chapter 4.
The r e l a t i o n s h i p
5. between v and aC;/ax
295
NONEQUILIBRIUM SOLIDIFICATION
comes from t h e s t e a d y - s t a t e s o l u t i o n o f t h e
dopant d i f f u s i o n e q u a t i o n f o r t h e case i n which t h e i n i t i a l dopant c o n c e n t r a t i o n C i O i s u n i f o r m and t h e m e l t - f r o n t moves w i t h c o n s t a n t v e l o c i t y ( T i l l e r e t al.,
1953); t h e s o l u t i o n i s (see Eq.
(20) o f
Chapter 4) =
C;[1'
+ (l-ki)kfl
,
exp(- vx/D,)]
(59)
i n which x i s measured r e l a t i v e t o t h e moving l i q u i d - s o l i d i n t e r f a c e = Cdeo/ki
The boundary c o n d i t i o n a t t h e i n t e r f a c e i s C;(O)
a x = -, Cd(-) = . 'C;
Differentiation of
Eq.
and a t
(59) and use o f t h e
i n t e r f a c e boundary c o n d i t i o n s g i v e t h e r e l a t i o n s h i p between v e l o c i t y and c o n c e n t r a t i o n a t t h e i n t e r f a c e expressed i n Eq. (58). and Sekerka e v i d e n t a l l y meant f o r k i i n Eq. i b r i u m i n t e r f a c e segregation c o e f f i c i e n t ,
Mullins
(59) t o be t h e e q u i l i.e.,
k? i n t h e nota-
t i o n used here, b u t t h e r e i s n o t h i n g i n t h e d e r i v a t i o n o f Eq. (59) t h a t r e s t r i c t s k i t o be kq.
T h i s i s an i m p o r t a n t p o i n t
and t o -
g e t h e r w i t h t h e f a c t t h a t Eq.
( 5 6 ) i n v o l v e s o n l y t h e slope o f t h e
l i q u i d u s i m p l i e s t h a t t h e M-S t r e a t m e n t i s n o t r e s t r i c t e d t o cond i t i o n s o f l o c a l thermodynamic e q u i l i b r i u m .
More s e r i o u s o b j e c t i o n s
t o applying t h e theory t o t h e l a s e r annealing o f ion-implanted samples might be t h a t (1) t h e i n i t i a l c o n c e n t r a t i o n o f dopant w i l l g e n e r a l l y n o t be u n i f o r m , and (2) f o r low values o f k i steady s t a t e c o n d i t i o n s w i l l n o t be a t t a i n e d b e f o r e t h e s o l i d i f i c a t i o n f r o n t reaches t h e s u r f a c e o f t h e sample.
The M u l l i n s and Sekerka t h e o r y
p r o v i d e s an expression f o r t h e t i m e d e r i v a t i v e o f t h e i n f i n i t e s i m a l o s c i l l a t o r y displacements o f t h e i n t e r f a c e (Eq. (20) o f M u l l i n s and Sekerka).
A n a l y s i s o f t h i s expression y i e l d s t h e s t a b i l i t y condi-
t i o n s f o r t h e planar interface. Cahn e t a l .
(1980) a p p l i e d t h e M-S t h e o r y t o an a n a l y s i s o f
c e l l u l a r f o r m a t i o n d u r i n g p u l s e d l a s e r anhealing.
Data now a v a i l -
a b l e show t h a t t h e s t a b i l i t y a g a i n s t c e l l u l a r f o r m a t i o n i s much g r e a t e r t h a n p r e d i c t e d and t h a t t h e c e l l s i z e s observed a r e cons i d e r a b l y d i f f e r e n t from t h o s e c a l c u l a t e d .
The thermal g r a d i e n t s
L
--
/
In I N S i CAPILLARY L I M I T
.
M-S THEORY
C: ( k y = 0.00041
0
2
4
3
4
v, MELT-FRONT VELOCITY ( m / s e c )
Fig. 6.
5
6
90-6
0
4
2
3
4
5
6
7
8
9
4
0
4
y MELT-FRONT VELOCITY ( m / s e c l
Interface stability diagrams for a ) Sb and b ) In in Si.
Comparison o f the solid
curves shows the dramatic increase in stability when k i ( v ) i s used in the calculations.
4
4
2
5.
297
NONEQUILIBRIUM SOLIDIFICATION
a t t h e i n t e r f a c e assumed by Cahn e t a l . were much s m a l l e r t h a n those c a l c u l a t e d by Wood and G i l e s , (1981), b u t t h e most s e r i o u s drawback o f t h e i r c a l c u l a t i o n s was t h e assumption t h a t l o c a l e q u i l i b r i u m has t o a p p l y i n t h e M-S t r e a t m e n t and t h a t as a consequence e q u i l i b r i u m o r n e a r - e q u i l i b r i u m values o f t h e s e g r e g a t i o n c o e f f i c i e n t should be used. 14.
CALCULATIONS OF STABILITY DIAGRAMS AND CELLULAR STRUCTURE The r e s u l t s o f s t a b i l i t y c a l c u l a t i o n s f o r Sb and I n i n S i u s i n g
Eq.
(20) o f M u l l i n s and Sekerka a r e i l l u s t r a t e d on Figs. 6a and b
(Wood, 1982).
The heavy s o l i d curves on t h e f i g u r e s separate t h e
s t a b l e and u n s t a b l e r e g i o n s o f t h e diagrams,
f o r which concentra-
t i o n s vs m e l t - f r o n t v e l o c i t i e s a r e p l o t t e d .
The l o w e r c u r v e was
c a l c u l a t e d w i t h ki = ky ki = ki(v)
and t h e upper curve w i t h one form o f t h e
r e l a t i o n s h i p g i v e n by Wood.
The s l o p e o f t h e l i q u i d u s
l i n e s were e s t i m a t e d from t h e phase diagrams o f Sb and I n i n S i and t h e c a p i l l a r i t y parameter was t a k e n f r o m Cahn e t a l .
(1980).
The
dashed curves on t h e f i g u r e s g i v e t h e " c a p i l l a r y l i m i t " f o r which C i = kiThrv/(m(ki-l)D,)
(see Eq.
,
( 2 7 ) o f M u l l i n s and Sekerka).
This absolute s t a b i l i t y
c r i t e r i o n h o l d s when t h e c a p i l l a r y e f f e c t completely dominates t h e e f f e c t s o f c o n s t i t u t i o n a l s u p e r c o o l i n g due t o t h e presence o f t h e solute.
I n t h e l i m i t t h a t thermal g r a d i e n t s o r c o n s t i t u t i o n a l
s u p e r c o o l i n g dominate t h e problem, C i i s g i v e n by C i = D k.G /(m(ki-1)v)
t 1 L
Curves f o r t h i s case do n o t appear on t h e f i g u r e because t h e y cann o t be r e s o l v e d from t h e o r d i n a t e on t h e s c a l e used. h o r i z o n t a l l i n e s l a b e l l e d C:
on t h e f i g u r e s
The l i g h t
give the equilibrium
s o l u b i l i t y l i m i t s ( v = 0) from Trumbore (see Table V I ) .
The o t h e r
298
R. F. WOOD ETAL.
h o r i z o n t a l l i n e s , l a b e l l e d Cs,
i n d i c a t e t h e approximate n o n e q u i l -
i b r i u m c o n c e n t r a t i o n s found by White e t a l .
(1980) and a l s o g i v e n
i n Table V I .
ky as v + 0, t h e s t a -
We p o i n t o u t t h a t because ki +
b i l i t y curves f o r k i = k i ( v ) and ki = ky must go t o t h e same values as v
+
0.
It i s p r e d i c t e d f r o m Fig. 6a t h a t when k i ( v ) i s used i n t h e c a l c u l a t i o n no c e l l u l a r s t r u c t u r e i s expected t o appear a t v = 3 m/sec f o r c o n c e n t r a t i o n s o f Sb which a r e t y p i c a l o f those f r e q u e n t l y used i n experiments.
I f , however, k i was n o t s t r o n g l y v-dependent
and had t h e e q u i l i b r i u m v a l u e o f ky,
t h e p l a n a r m e l t f r o n t would
become u n s t a b l e a t a c o n c e n t r a t i o n more t h a n an o r d e r o f magnitude lower than t h a t f o r k i = k i ( v ) .
S i m i l a r r e s u l t s f o r I n and o t h e r
dopants i n s i l i c o n e s t a b l i s h c o n c l u s i v e l y t h a t k i ( v ) i s e s s e n t i a l i n determining t h e s t a b i l i t y o f t h e planar i n t e r f a c e a t high s o l i d i f i c a t i o n velocities.
A l s o i t should be recognized t h a t when c e l l -
u l a r f o r m a t i o n does occur t h e amount o f dopant l e f t w i t h i n t h e c e l l s i s n o t n e c e s s a r i l y t h e maximum t h a t can be p l a c e d s u b s t i t u t i o n a l l y i n the l a t t i c e a t t h a t p a r t i c u l a r c r y s t a l l i z a t i o n velocity.
The
M-S t h e o r y does n o t g i v e q u a n t i t a t i v e i n f o r m a t i o n about t h e con-
centration i n e i t h e r t h e c e l l s o r the c e l l walls.
It would seem
t h a t t h e c o r r e c t way t o e s t a b l i s h t h e maximum c o n c e n t r a t i o n s i s t o approach t h e i n s t a b i l i t y curves on Figs. 6a and b from below, i.e.,
by keeping t h e v e l o c i t y f i x e d , and g r a d u a l l y i n c r e a s i n g the
concentration u n t i l t h e i n s t a b i l i t y j u s t sets in.
O f course, t h i s
would a l s o be t h e b e s t method f o r e s t a b l i s h i n g t h e c r i t i c a l c e l l s i z e s discussed next. The c a l c u l a t i o n s which l e a d t o t h e curves d i s p l a y e d on Figs. 6a and b a l s o p r o v i d e i n f o r m a t i o n about t h e dependence o f t h e c e l l s i z e on v a r i o u s parameters.
I n Fig. 7, t h e c e l l s i z e a t t h e onset
o f t h e i n t e r f a c e i n s t a b i l i t y i s shown as a f u n c t i o n o f m e l t - f r o n t velocity.
I t can be seen t h a t t h e c e l l s i z e i n i t i a l l y decreases
r a p i d l y w i t h v e l o c i t y and t h e n goes i n t o a r e g i o n o f much slower decrease.
The c o n c e n t r a t i o n s c o r r e s p o n d i n g t o t h e s e c e l l s i z e s
can be t a k e n from curves l i k e t h o s e o f Fig. 6.
It may be somewhat
5.
299
NONEQUILIBRIUM SOLIDIFICATION
40.0
5.0
1
t
L
2.0
CELL SIZE AT ONSET OF INSTABILITY i.o
-
E 3 0.5 W
N
v)
-I -1 LL'
0.2
u
0.4 Sb 0.05
0.02
0
Fig.
7.
2 4 6 6 v, MELT-FRONT VELOCITY (m/secl
C e l l size a t the onset o f the interface instability as a function
o f melt-front
velocity for Sb, G a , and I n in Si using velocity-dependent
The c r i t i c a l cell size decreases and the c r i t i c a l with v.
(0
(Wood,
concentration
ki.
increases
1982)
d i f f i c u l t e x p e r i m e n t a l l y t o determine c r i t i c a l c e l l s i z e s a c c u r a t e l y t o compare w i t h t h e c a l c u l a t e d values because o f t h e d i f f i c u l t y i n p r e p a r i n g samples w i t h t h e c o r r e c t c o n c e n t r a t i o n s and i n e s t a b l i s h i n g l a s e r - a n n e a l i n g c o n d i t i o n s which g i v e e x a c t l y t h e r i g h t m e l t f r o n t v e l o c i t i e s t o map o u t t h e boundary between t h e s t a b l e and u n s t a b l e regions.
Moreover, Fig. 7 shows t h a t as v increases, t h e
c r i t i c a l c e l l s i z e becomes less s e n s i t i v e t o v, which makes t h e p o s s i b i l i t y o f a c c u r a t e l y r e l a t i n g c e l l s i z e t o v more d i f f i c u l t .
300
R. F. WOOD E T A L
The curves d i s p l a y e d on Fig. 8 g i v e i n f o r m a t i o n o f a somewhat d i f f e r e n t type.
To c o n s t r u c t t h e s e curves, a m e l t - f r o n t v e l o c i t y
i s chosen and t h e dopant c o n c e n t r a t i o n a t which a p a r t i c u l a r c e l l s i z e appears i s determined.
F o r example, on Fig. 6b l e t us suppose
t h a t 4 m/sec has been chosen f o r v, and t h e n determine t h e c e l l s i z e f o r each c o n c e n t r a t i o n above t h e c r i t i c a l c o n c e n t r a t i o n f o r i n s t a bility.
T h i s leads t o curve 2 f o r t h e I n group o f curves on Fig. 8.
The p o i n t f u r t h e s t t o t h e r i g h t on each c u r v e on Fig. 8 corresponds t o t h e c e l l s i z e and dopant c o n c e n t r a t i o n s a t t h e onset of t h e interfacial instability.
It should be noted t h a t i f one h o l d s t h e
c o n c e n t r a t i o n f i x e d and i n c r e a s e s t h e m e l t - f r o n t v e l o c i t y , t h e c e l l s i z e i n c r e a s e s up t o a c e r t a i n maximum v a l u e c o r r e s p o n d i n g t o t h a t Ioo
c
5 MELT-FRONT VELOCITY Irn/sec) I - 4.5 2-4.0
2
2
I 0.08
I
0
Fig. 8. c e l l size.
0.04
I
I
0.12 0.16 CELL S I Z E ( p m )
Interrelationship o f dopant concentration
I
1
0.20
0.24
melt-front velocity
and
The c e l l size furthest t o the r i g h t f o r a given velocity occurs at
the onset o f the instability.
I f the concentration i s held fixed the c e l l size
increases as the velocity i s increased until, f o r a given velocity, face becomes stable.
(Wood,
1982)
the inter-
5 . NONEQUILIBRIUM SOLIDIFICATION
301
a t which t h e s t a b i l i t y c u r v e (Fig. 6 ) i s crossed.
It i s apparent
from Fig. 8 t h a t f o r a g i v e n c o n c e n t r a t i o n , t h e i n c r e a s e i n c e l l s i z e i s very r a p i d as t h e v e l o c i t y approaches t h a t o f t h e i n s t a b i l i t y curve.
As a l r e a d y mentioned above, t h i s means t h a t r e l i a b l e values
o f t h e c e l l s i z e a t t h e onset o f t h e i n s t a b i l i t y w i l l be d i f f i c u l t t o determine e x p e r i m e n t a l l y . The main p o i n t i n p r e s e n t i n g Fig. 6 i s t o demonstrate how t h e model o f Sec. I 1 1 leads t o r e s u l t s which a r e i n s a t i s f a c t o r y agreement w i t h t h e experimental data and t o show t h e reader t h a t c a l c u lations
0
u s i n g k i i n t h e M-S f o r m a l i s m
g i v e r e s u l t s which d i f f e r
f r o m t h e observed data by orders o f magnitude.
As s t a t e d e a r l i e r
M u l l i n s and Sekerka c a u t i o n e d a g a i n s t t h e r e l i a b i l i t y o f p r e d i c t i o n s about c e l l s i z e s u n l e s s t h e i n s t a b i l i t y i s known t o be weak. However, f r o m t h e sparse data t h a t i s now a v a i l a b l e i t appears t h a t t h e p r e d i c t i o n s o f c e l l s i z e a r e perhaps more a c c u r a t e t h a n might be expected.
For example, Narayan (1981) has r e p o r t e d c e l l diameters
o f 700 R , 520 A , and 350 A r e s p e c t i v e l y for Gay
In, and Fe i n S i
a f t e r l a s e r a n n e a l i n g w i t h p u l s e s which should have produced m e l t f r o n t v e l o c i t i e s o f approximately 4 m/sec; close t o those given
on F i g . 8.
these values a r e q u i t e
However, adequate t e s t s o f t h e
t h e o r y must a w a i t a d d i t i o n a l experimental data.
V.
Amorphous Phase Formation During Ultrarapid Solidification L i u e t a l . (1979) and Tsu e t a l . (1979) f i r s t demonstrated t h a t
i r r a d i a t i o n of c r y s t a l 7 ine S i w i t h u l t r a s h o r t and/or u l t r a v i o l e t l a s e r p u l s e s c o u l d r e s u l t i n t h e f o r m a t i o n o f t h e amorphous ( a ) phase.
The subsequent work by C u l l i s e t a l . (1982b) showed t h a t t h e
amorphous phase c o u l d be formed over r e l a t i v e l y l a r g e areas on b o t h (100) and (111) substrates.
With such l a r g e areas, one-dimensional
h e a t f l o w c a l c u l a t i o n s were j u s t i f i e d and t h e r e s u l t s o f such c a l culations indicated that, a f t e r melting, s o l i d i f i c a t i o n v e l o c i t i e s o f 15-20 m/sec a r e r e q u i r e d t o produce t h e e f f e c t on a (100) s i l i c o n s u r f a c e ; on a (111) surface, t h e r e q u i r e d v e l o c i t y i s s u b s t a n t i a l l y
302
R. F. WOOD E T A L .
l e s s b u t a-Si m a t e r i a l i s n o t formed i n i t i a l l y .
The o b s e r v a t i o n o f
t h e f o r m a t i o n o f t h e amorphous phase from t h e l i q u i d
(11)phase
at
v e r y h i g h regrowth v e l o c i t i e s i s perhaps t h e s i n g l e most remarkable example o f t h e n o n e q u i l i b r i u m s o l i d i f i c a t i o n phenomena encountered i n p u l s e d l a s e r processing.
Since t h e p r e c i s e n a t u r e o f t h e e f f e c t
i s s t i l l a m a t t e r o f s p e c u l a t i o n , we w i l l c o n f i n e ourselves here t o an a b b r e v i a t e d d i s c u s s i o n o f two mechanisms which have been proposed t o e x p l a i n it. An e x p l a n a t i o n based on a c o n v e n t i o n a l thermodynamic approach would s i m p l y r e q u i r e t h a t t h e l i q u i d s i l i c o n be c o o l e d below a temp e r a t u r e Ta (
A t r e a t m e n t o f t h e growth o f t h e amorphous l a y e r from t h e
l i q u i d would t h e n presumably f o l l o w a development s i m i l a r t o t h a t o f Sec. I1 f o r t h e II + c-Si t r a n s i t i o n .
One o f t h e most troublesome
q u e s t i o n s which a r i s e s w i t h such a t r e a t m e n t i s whether o r n o t a u n i q u e v a l u e o f Ta, t h e " m e l t i n g temperature" e x i s t s f o r a-Si.
This
q u e s t i o n a r i s e s independently o f t h e problem o f e s t a b l i s h i n g j u s t what c o n s t i t u t e s p e r f e c t a-Si,
b u t i t may be connected w i t h another
q u e s t i o n concerning whether o r n o t Ta might depend on t h e c o n d i t i o n s under which a - S i i s heated o r l i q u i d s i l i c o n cooled.
Certainly the
w i d e l y v a r y i n g e s t i m a t e s o f Ta (see Chapter 4) which have appeared i n t h e l i t e r a t u r e suggest t h a t t h e problem o f d e f i n i n g Ta needs much f u r t h e r attention. As noted above, numerous estimates o f Ta and La ( l a t e n t h e a t ) f o r a-Ge and a-Si have appeared (Chen and T u r n b u l l
, 1978;
Chen, 1978; Spaepen and T u r n b u l l , 1978; B a e r i e t al., Anderson,
1981; Olson e t al.,
1983).
Bagley and
1980; Fan and
The most r e c e n t and p r o b a b l y
t h e most r e l i a b l e v a l u e o f La has been o b t a i n e d by Donovan e t a l . (1983) u s i n g d i f f e r e n t i a l scanning c a l o r i m e t r y . AH ,,,
They found t h a t
t h e e n t h a l p y o f c r y s t a l l i z a t i o n from t h e amorphous phase, had
a v a l u e o f 424 J / g f o r t h e i r a - S i samples formed by i o n i m p l a n t a t i o n o f A r and Xe.
Using standard thermodynamic c a l c u l a t i o n s ,
other
thermodynamic data i n t h e l i t e r a t u r e , and a number o f approximations whose v a l i d i t y i s d i f f i c u l t t o assess a t t h i s time, t h e y concluded
5.
303
NONEQUILIBRIUM SOLIDIFICATION
t h a t a lower bound t o Ta was 1295 K w i t h La = 37.5 kJ/mole and t h a t more l i k e l y values were Ta = 1420 K (i.e., deg) and La = 37.04 kJ/mole.
an u n d e r c o o l i n g o f -260
The c a l c u l a t i o n s gave t h e temperature
dependence o f t h e Gibbs f r e e energy change, AG, o f t h e l i q u i d and amorphous phases r e l a t i v e t o t h e c r y s t a l l i n e phase.
The r e s u l t s
o f such c a l c u l a t i o n s can be d i s p l a y e d on a diagram s i m i l a r t o t h a t shown s c h e m a t i c a l l y i n Fig. 9 ( t h e temperature Tn on t h i s f i g u r e
w i l l be discussed s h o r t l y ) .
Although, s t r i c t l y speaking, such a
diagram i s v a l i d o n l y a t thermodynamic e q u i l i b r i u m , i t has f r e q u e n t l y been used i n d i s c u s s i o n s o f t h e h i g h l y n o n e q u i l i b r i u m a.
t r a n s i t i o n induced by p u l s e d l a s e r i r r a d i a t i o n .
+
a phase
I n so f a r as t h i s
usage i s even approximately v a l i d , a thermodynamic i n t e r p r e t a t i o n o f t h e a. + a t r a n s i t i o n would r e q u i r e an u n d e r c o o l i n g o f t h e l i q u i d o f -260 deg and g r e a t e r t o o b t a i n motion o f t h e a-a. phase f r o n t . B e a r i n g on t h e above thermodynamic i n t e r p r e t a t i o n , however, i s t h e r e c e n t work o f Lowndes e t a l . (1984) and Wood e t a l . (1984) on l a s e r - i n d u c e d m e l t i n g o f a-Si l a y e r s (see Chapter 6, Secs.
s <
0.1
Q
O
V.10
-
4-
-W2
I -0.1 800
1
1000
Ta
1
I
1200
1400 T
Fig. 9.
Tc
Tn 1600
I
1800
IK)
Schematic diagram of the Gibbs f r e e energy f o r liquid and amorphous
silicon as a function of temperature. AG f o r the liquid and amorphous phases are given with respect to G for the crystalline phase.
304
R. F. WOOD ET AL.
and 11). T h i s work s t r o n g l y suggests t h a t b u l k n u c l e a t i o n ( T u r n b u l l , 1950; Jackson and Chalmers, 1956) o f t h e c r y s t a l l i n e phase i n h i g h l y undercooled k-Si occurs a t some temperature Tn -50-100 t h a n Ta;
t h i s temperature i s i n d i c a t e d on Fig.
deg h i g h e r
9.
Since b u l k
n u c l e a t i o n o f t h e c r y s t a l l i n e phase can occur i n t h e system when i t i s undercooled so s l o w l y as t o remain always near thermodynamic e q u i l i b r i u m , u n d e r c o o l i n g must occur so r a p i d l y i n t h e l a s e r - m e l t e d samples t h a t b u l k n u c l e a t i o n i s suppressed.
I f t h i s i s t h e case,
i t would seem t h a t a k i n e t i c r a t h e r t h a n a thermodynamic d e s c r i p t i o n
o f t h e a m o r p h i z a t i o n phenomenon i s more a p p r o p r i a t e . An a l t e r n a t i v e view o f t h e a m o r p h i z a t i o n process,
based on a
k i n e t i c r a t e approach, has been proposed by Wood (1983).
A similar
view i s c o n t a i n e d imp1 i c i t l y i n F l e t c h e r ' s m u l t i l a y e r i n t e r f a c i a l model o f s o l i d i f i c a t i o n discussed b r i e f l y i n Sec. 11.4.
Basically,
t h e i d e a i s t h a t when t h e s o l i d i f i c a t i o n v e l o c i t y becomes g r e a t enotigh, t h e r e i s n o t enough t i m e f o r t h e atoms o f t h e m a t e r i a l t o f i n d t h e i r p e r f e c t c r y s t a l alignment and d e f e c t s b e g i n t o be p r o duced.
The n a t u r e o f t h e d e f e c t s need n o t be s p e c i f i e d , b u t p r e -
sumably t h e y a r e r e l a t e d t o incomplete bonding ( d a n g l i n g bonds) and bond angle d i s t o r t i o n s .
I n any case, as t h e c o n c e n t r a t i o n o f d e f e c t s
i n c r e a s e s r a p i d l y (and e v e n t u a l l y p r e c i p i t o u s l y ) w i t h i n c r e a s i n g s o l i d i f i c a t i o n v e l o c i t y , t h e amorphous phase i s so t o speak t r a p p e d i n a manner s i m i l a r t o t h a t o f s o l u t e t r a p p i n g .
It should be empha-
s i z e d t h a t t h i s i s an i n h e r e n t l y n o n e q u i l i b r i u m process because i t occurs when t h e v e l o c i t y o f t h e l i q u i d - s o l i d i n t e r f a c e i s no l o n g e r n e g l i g i b l e compared t o t h e " r a t e o f c r y s t a l l i z a t i o n . " The model used by Wood i n t r o d u c e s a r a t e c o n s t a n t t h a t desc r i bes r e l a x a t i o n processes ( r e o r i e n t a t i o n s ,
recombinations,
re-
arrangements) which can o c c u r on s u b s t a n t i a l l y s h o r t e r t i m e and d i s t a n c e s c a l e s t h a n those a s s o c i a t e d w i t h t h e d i f f u s i v e processes i n v o l v e d i n t h e dopant s e g r e g a t i o n problem o f Sec. 111. t i v e velocity-dependent
An e f f e c -
a c t i v a t i o n energy i s i n t r o d u c e d i n t h i s
model, as i t was i n t h e model f o r dopant segregation.
The v e l o c i t y
dependence comes about because an atom b e i n g swept over by t h e
5 . NONEQUILIBRIUM SOLIDIFICATION
305
10
0
I-
z
w
2 4 0 V I-
z
2
W
0
0
0
8
4
12
f6
MELT-FRONT VELOCITY ( r n l s e c )
20
Fig. 10. Concentration of lldefectstl and onset o f amorphization as a function o f velocity and regrowth direction.
As discussed in the text, amorphization at
a defect concentration o f -5% is largely speculative.
(Wood, 1983)
advancing l i q u i d - s o l i d i n t e r f a c e f i n d s i t i n c r e a s i n g l y d i f f i c u l t t o r e l a x i n t o i t s c r y s t a l l i n e c o n f i g u r a t i o n as t h e i n t e r f a c e veloci t y increases.
S t a t e d i n another way, t h e s o l i d i f i c a t i o n f r o n t
tends t o r u n away from t h e c r y s t a l l i z a t i o n f r o n t and c r y s t a l l i z a t i o n cannot occur i f t h e temperature o f t h e a-c i n t e r f a c e t h e n drops q u i c k l y enough. velocity-dependent
W i t h t h i s p i c t u r e i n mind, Wood r e l a t e d t h e
r e l a x a t i o n r a t e constant t o t h e r a t e constants
f o r solid-phase r e c r y s t a l l i z a t i o n o f a-Si l a y e r s on c-Si s u b s t r a t e s o b t a i n e d by Csepregi e t a l .
(1978).
The r e s u l t s t h a t emerge from
a model o f t h i s t y p e a r e i l l u s t r a t e d i n Fig. 10.
The o r i e n t a t i o n
dependence o f t h e d e f e c t c o n c e n t r a t i o n i s a d i r e c t consequence o f u s i n g t h e r e s u l t s o f Csepregi e t a l .
t o determine t h e b e h a v i o r
p r e d i c t e d by t h e model a t h i g h r e g r o w t h v e l o c i t i e s .
Questions
concerning t h e c o n c e n t r a t i o n s o f d e f e c t s which a S i l a t t i c e can s u s t a i n b e f o r e d i s o r d e r i n g have been c o n s i d e r e d i n t h e l i t e r a t u r e ( S t e i n e t al.,
1970; Crowder e t al.,
1970; and van Vechten, 1982),
b u t no d e f i n i t i v e answers f o r s i t u a t i o n s c l o s e l y r e l a t e d t o t h e one
306
R. F. WOOD ET AL,.
discussed h e r e a r e a v a i l a b l e .
The c o n c e n t r a t i o n o f approximately
5% i n d i c a t e d on F i g . 10 i s o n l y a guess and c o u l d e a s i l y be t o o h i g h by an o r d e r o f magnitude o r more.
Also, we emphasize again
t h a t " d e f e c t " i s used h e r e i n a very general sense. L e t us c o n s i d e r b r i e f l y Eq.
(18) which i s t h e main e q u a t i o n
( o r r a t h e r system o f e q u a t i o n s ) o f F l e t c h e r ' s m u l t i l a y e r model. F l e t c h e r found t h a t f o r a p a r t i c u l a r c h o i c e o f t h e s e t o f K's and v ' s i n t h a t equation, t h e n o n e q u i l i b r i u m c o n c e n t r a t i o n o f d e f e c t s
trapped i n t h e c r y s t a l
increased approximately l i n e a r l y w i t h v
i n i t i a l l y , but subsequently a range o f v i n which t h e c o n c e n t r a t i o n i n c r e a s e d much more r a p i d l y was reached, i n a manner s i m i l a r t o t h a t shown on F i g . 10. Table V I I .
Some o f t h e d a t a g i v e n by F l e t c h e r i s shown i n
Although we do n o t expect t h e numbers i n t h i s t a b l e ( o r
t h e i n p u t d a t a used by F l e t c h e r ) t o correspond, even approximately t o t h e case o f s i l i c o n , i t i s c l e a r t h a t F l e t c h e r ' s t r e a t m e n t p r o v i d e s a framework i n which t o c o n s t r u c t a m u l t i l a y e r , m i c r o s c o p i c model o f t h e observed R
+
a transition.
H i s model, l i k e t h a t of
Wood, suggests t h a t t h e r e i s an i n i t i a l f a i r l y slow i n c r e a s e o f t h e d e f e c t c o n c e n t r a t i o n w i t h v a f t e r which a v e r y r a p i d i n c r e a s e i n t h e d e f e c t c o n c e n t r a t i o n occurs, l e a d i n g t o amorphization. Regardless o f t h e n a t u r e o f t h e t h e o r y t h a t e v e n t u a l l y proves most s u i t a b l e f o r d e s c r i b i n g t h e a m o r p h i z a t i o n process,
it i s
apparent even now t h a t d e t a i l e d experimental s t u d i e s o f t h e micros t r u c t u r e o f t h e s o l i d formed i n a v e l o c i t y range around t h e onset o f amorphization would be very u s e f u l .
I t i s hoped t h a t such s t u d i e s
w i l l be c a r r i e d o u t i n t h e near future. Table V I I C o n c e n t r a t i o n C o f quenched-in d e f e c t s a t s e v e r a l i n t e r f a c i a l v e l o c i t i e s v as g i v e n by F l e t c h e r . v (cm/sec)
0
10-5
10-4
10-3
10-2
C (%)
8x10'12
8x10'11
8x10-10
9x10'9
3x10-'+
5.
VI.
307
NONEQUILIBRIUM SOLIDIFICATION
Conclusions and Directions f o r Future Work
The regrowth v e l o c i t i e s t h a t can be a t t a i n e d as a r e s u l t o f p u l s e d l a s e r - i n d u c e d m e l t i n g p u t t h e s i m p l e c l a s s i c a l phenomenol o g i c a l models o f c r y s t a l growth t o a severe t e s t , and i t i s n o t y e t c l e a r t h a t t h e t h e o r i e s can pass t h i s t e s t , except perhaps i n q u a l i t a t i v e terms.
I n p a r t i c u l a r , methods f o r q u a n t i t a t i v e e v a l -
u a t i o n o f t h e r e l a t i o n s h i p between v and ATi g i v e n by Eq. (9) o v e r an extended range o f v need t o be developed.
Unfortunately, A T i
i s a q u a n t i t y t h a t i s e x t r e m e l y d i f f i c u l t t o measure d i r e c t l y under t h e c o n d i t i o n s o f p u l s e d l a s e r annealing.
I n d i r e c t measurements
such as t h o s e r e l a t e d t o t i m e - r e s o l ved x-ray s c a t t e r i n g (Larson e t al.,
1982, 1983; see d i s c u s s i o n i n Chapter 6 ) i n v o l v e assump-
t i o n s and c a l c u l a t i o n s which themselves must be c a r e f u l l y tested. Nevertheless, we can be h o p e f u l t h a t t h e s e i n d i r e c t methods may be r e f i n e d t o t h e e x t e n t t h a t r e l i a b l e i n f o r m a t i o n about underc o o l i n g o f t h e l i q u i d can be obtained.
The t i m e - r e s o l v e d e l e c t r i c a l
c o n d u c t i v i t y measurements ( G a l v i n e t al.,
1982; Thompson and Galvin,
1983; Chapter 6) g i v e a f a i r l y d i r e c t measurement o f t h e s o l i d i f i c a t i o n v e l o c i t i e s i n many cases.
I f v and A T i i n Eq. (9) can be
determined over an extended range,
t h e r a t e constant K f and t h e
a c t i v a t i o n energy appearing i n i t c o u l d t h e n be determined.
It
would appear t h a t t h i s w i l l c o n t i n u e t o be a f r u i t f u l area o f research f o r some t i m e t o come. I n t h e area of
nonequilibrium segregation,
c l o s e l y r e l a t e d t o t h e above need t o be emphasized.
several t o p i c s F o r example,
e x t e n s i v e a d d i t i o n a l experimental work i s needed t o determine t h e b e h a v i o r o f k i w i t h v e l o c i t y and t o e s t a b l i s h whether o r n o t k i s a t u r a t e s a t values l e s s than u n i t y f o r some dopants a t concentrat i o n s f o r which t h e approximations o f Sec. I11 remain v a l i d . a saturation,
i f i t does occur,
s o l i d i f i c a t i o n o f the host i t s e l f .
Such
m i g h t p r o v i d e i n f o r m a t i o n about With t h e a v a i l a b l i l i t y o f addi-
t i o n a l experimental data, more s o p h i s t i c a t e d modeling, m u l t i l a y e r treatments o f t h e i n t e r f a c i a l
including
r e g i o n and m o l e c u l a r
308
R. F. WOOD ET AL.
dynamics w i t h
angular-dependent
becomes more meaningful.
i n t e r a c t i o n s among t h e atoms,
A l s o e x t e n s i v e work i s needed on t h e
i n f l u e n c e o f dopants on t h e regrowth v e l o c i t y and t h e e x t e n t t o which t h e dopant c o n c e n t r a t i o n i n f l u e n c e s t h e values o f k i e x t r a c t e d from t h e experimental d a t a (see t h e d i s c u s s i o n i n Sec. Chapter 6).
11.2 o f
Obviously t h e q u e s t i o n o f s o l u b i l i t y l i m i t s i s c l o s e l y
t i e d t o t h e foregoing. There a r e s e v e r a l q u e s t i o n s a s s o c i a t e d w i t h t h e f o r m a t i o n o f c e l l u l a r s t r u c t u r e whose r e s o l u t i o n c o u l d g i v e a d d i t i o n a l i n s i g h t i n t o s e v e r a l areas o f i n t e r e s t here.
Many o f t h e s e q u e s t i o n s are
i n t i m a t e l y connected w i t h problems a l r e a d y s e t f o r t h above and w i l l n o t be discussed f u r t h e r .
However, we p o i n t o u t t h a t s i n c e t h e
f o r m a t i o n o f c e l l u l a r s t r u c t u r e does mean t h a t t h e u n d e r l y i n g t h e o r y g e n e r a l l y assumed i n t h i s c h a p t e r and i n Chapter 4 no l o n g e r holds, i t i s obvious t h a t t h e r e i s a g r e a t deal t o be l e a r n e d f r o m s t u d i e s i n t h i s l i m i t i n g t r a n s i t i o n regime. Studies o f t h e R
+
a phase t r a n s i t i o n s h o u l d be d i r e c t e d toward
e s t a b l i s h i n g more c l e a r l y t h e n a t u r e o f t h i s phase t r a n s i t i o n . Q u e s t i o n s concerning t h e t r a n s i t i o n temperature, i t can be w e l l - d e f i n e d and/or unique, v a l i d i t y o f t h e AG(T) diagram,
such as whether
need t o be resolved.
The
t h e e x t e n t t o which i t can g i v e
u s e f u l i n f o r m a t i o n about n o n e q u i l i b r i u m processes,
and q u e s t i o n s
r e l a t i n g t o b u l k n u c l e a t i o n c l e a r l y need much f u r t h e r study.
A
r e c o n c i l i a t i o n between t h e thermodynamic and k i n e t i c approaches s h o u l d be e x p l o r e d t o e s t a b l i s h i f t h e r e r e a l l y i s any b a s i c conf l i c t between t h e t w o and,
i f so, t o r e s o l v e it.
I n t h i s area
a l s o , t h e q u e s t i o n o f t h e p r o p e r t i e s o f t h e i n t e r f a c i a l r e g i o n and development o f m u l t i l a y e r t r e a t m e n t s o f t h e i n t e r f a c e need t o be studied i n great d e t a i l .
The t r a n s f o r m a t i o n o f t h e e l e c t r o n i c
s t r u c t u r e o f t h e m a t e r i a l from m e t a l l i c bonding t o c o v a l e n t bonding w i t h i n a t h i n i n t e r f a c i a l r e g i o n and i n very s h o r t t i m e s
is a
p a r t i c u l a r l y i n t r i g u i n g aspect o f t h e s e phenomena. We do n o t expect any o f t h e above problems t o be p a r t i c u l a r l y easy t o solve, b u t when t h e y a r e s o l v e d we w i l l f o r t h e f i r s t t i m e
309
5 . NONEQUILIBRIUM SOLIDIFICATION
have a q u i t e complete p i c t u r e o f u l t r a r a p i d s o l i d i f i c a t i o n and nonequilibrium
e f f e c t s i n one o r more w e l l - d e f i n e d elemental systems
( S i , Ge) and t h e i r d i l u t e a l l o y s .
Acknowl edglnents Much o f t h e work d e s c r i b e d i n t h i s c h a p t e r has b e n e f i t t e d from d i s c u s s i o n s w i t h a number o f our c o l l e a g u e s a t Oak Ridge. l i k e t o thank p a r t i c u l a r l y Cooke, M.
D. H. Lowndes,
G.
E.
We would
Jellison,
E. M o s t o l l e r , T. Kaplan, B. C. Larson, and M.
J. F.
J. Aziz.
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C h r i s t i a n , J. W. (1975). "The Theory o f Transformations i n M e t a l s and A l l o y s , " Pergamon Press, Oxford. Cleveland, C. L., Landman, U., and B a r n e t t , R. N. (1982). Phys. Rev. L e t t . 49, 790. Crowder, B. L., T i t l e , R. S., Brodsky, M. H., and P e t i t , G. D. (1970). Appl. Phys. L e t t . 16, 205. and Sigmon, T. W. Csepregi, L., Kennedy, E. F., Mayer, J. W., (1978). J. Appl. Phys. 49, 3906. C u l l i s , A. G., Weber, H. C., and Chew, N. G. (1982a). Appl. Phys. L e t t . 40, 988. C u l l i s , A. G., Weber, H. C., Chew, N. G., Poate, J. M., and B a e r i P. (1982b). Phys. Rev. L e t t . 49, 219. Donovan, E. P., Spaepen, F., T u r n b u l l , D., Poate, J. M., and Jacobson, D. C. (1983). Appl. Phys. L e t t . 42, 698. Fan, J. C. C., and Anderson, H. (1981). J. Appl. Phys. 52, 4003. Flemings, M. C. (1981). I n " M e t a l l u r g i c a l T r e a t i s e s " (J. K. T i e n and J. F. E l l i o t , eds.) p. 291. The M e t a l l u r g i c a l S o c i e t y o f AIME, Warrendale, Pa. F l e t c h e r , N. H. (1975). J. C r y s t a l Growth 28, 375. F l e t c h e r , N. H. (1976). J.Crysta1 Growth 35, 39. F r a t e l l o , V. J., Hays, J. F., Spaepen, F., and T u r n b u l l , D. (1980). J. Appl. Phys. 51, 6160. G a l v i n , G. J., Thompson, M. O., Mayer, J. W., Hammond, R. G., P a u l t e r , N., and Peercy, P. S. (1982). Phys. Rev. L e t t . 48, 33. Gilmer, G. H. (1983). Mat. Res. SOC. Symp. Proc. 13, 249. Glasov, V. M., Chizhevskaya, S. N., and Glagoleva, N. N. (1969). " L i q u i d Semiconductors. I' Plenum Press, New York. H a l l , R. N. (1952). Phys. Rev. 88, 139. and P f e i f f e r , H. (1983). I n "Modern Theory of Haubenreisser, W., C r y s t a l Growth 1" (A. A. Chernov and H. MUller-Krumbhhaar, eds.), p. 43. Heidelberg. Jackson, K. A. and Chalmers, B. -(1956). Can. J. Phys. 34, 473. Jackson, K. A. (1958). Can. J. Phys. 36, 683. Jackson, K. A. (1968). J. C r y s t a l Growth 3-4, 507. Jackson, K. A., Gilmer, G. H., and Leamy, H. J. (1980). I n "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 104. Academic Press, New York. Jackson, K. A. (1983). I n "Surface M o d i f i c a t i o n and A l l o y i n g " (J. M. Poate, G. F o t i , and D. C. Jacobson, eds.) Chap. 3. J i n d a l , B. K., and T i l l e r , W. A. (1968). J. Chem. Phys. 49, 4632. Kodera, H. (1963) Jpn. J. Appl. Phys. 2, 212. Kroeger, D. M., Coghlan, W. A. Easton, D. S. , Koch, C. C. , and Scarbrough, J. 0. (1982). J. Appl. Phys. 53, 1445. Landman, U., Cleveland, C. L., Brown, C. S . (1980). Phys. Rev. L e t t . 45, 2032. Langer, J. S. (1980). Rev. Mod. Phys. 1 , 52. Larson, B. C., White, C. W., Noggle, T. S . , and Mills, D. M. (1982). Phys. Rev. L e t t . 48, 337. Noggle, T. S., B a r h o r s t , J. F., and Larson, B. C., White, C. W., M i l l s , D. M. (1983). Appl. Phys. L e t t . 42, 282.
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NONEQUILIBRIUM SOLIDIFICATION
311
L i u , P. L., Yen, R., Bloembergen, N., and Hodgson, R. T. (1979). Appl. Phys. L e t t . 34, 864. Lowndes, D. H., Wood, R. F., and Narayan, J. (1984). Phys. Rev. L e t t . 52, 561. Mehrabian, R. (1982). I n t . Met. Rev. 27, 185. Mooney, P. M., Young, R. T., K a r i n s , J., Lee, Y. H., and Corbett, J. W. (1978). Phys. S t a t . Sol ( a ) 48, K31. Morehead, F. (1980). I n "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 143. Academic Press, New York. M u l l i n s , W. W., and Sekerka, R. F. (1964). J. Appl. Phys. 35, 444. Narayan, J. (1981). J. Appl. Phys. 52, 1289. Olson, G. L., Kokorowski, S. A., Roth, J. A., and Hess, L. D. (1983). Mat. Res. Sac. Symp. Proc. 13, 141. Poate, J. M. (1982). Mat. Res. SOC. Symp. Proc. 4, 121. Spaepen, F. , and T u r n b u l l , D. (1979). I n " L a s e r - S o l i d I n t e r a c t i o n s and Laser Processing-1978" (S. D. F e r r i s , H. J. Leamy, and J. M. Poate, eds.), p. 73. Am. I n s t . Phys., New York. Spaepen, F., and T u r n b u l l , D. (1982). I n "Laser Annealing o f Semiconductors" (J. M. Poate and J. W. Mayer, eds.), p. 15. Academic Press, New York. Shashkov, Y. M., and Gurevich, V. M. (1968). Zh. F i z . Khim. 42, 2058. (Russ. J. Phys. Chem. 42, 1082). S t e i n , H. J . , Vook, F. L., B r i c e , D. K., Borders, J. A., and Picraux, S . T. (1970). R a d i a t i o n E f f e c t s 6 , 19. T a r s h i s , L. A., Walker, J. L., and R u t t e r , J. W. (1973). I n "Metallography, S t r u c t u r e s , and Phase Diagrams," M e t a l s Handbook 8, p. 150. American S o c i e t y f o r Metals, Ohio. Temkin, D. E. (1966). I n " C r y s t a l l i z a t i o n ProcessesIn p. 15. C o n s u l t a n t s Bureau, New York. Temkin, D. E. (1981). J. Cryst. Growth 52, 299. Thompson, M. O., and Galvin, G. J. (1983). Mat. Res. SOC. Symp. Proc. 13, 57. Thompson, M. O., Mayer, J. W., C u l l i s , A. G., Webber, H. C., Chew, N. G., Poate, J. M., and Jacobson, D. C. (1983). Phys. Rev. L e t t . 50, 896. Thurmond, C. D., and S t r u t h e r s , J. D. (1953). J. Phys. Chem. 57, 831.
Thurmond, C. D. (1959). I n "Semiconductors" (H. €3. Hannay, ed.), p . 145. Reinhold, New York. T i l l e r , W. A., Jackson, K. A., R u t t e r , J. W. , and Chalmers, B. (1953). Acta M e t a l l . 1, 428. Toxvaerd, S., and Praestgaard, E. (1977). J. Chem. Phys. 67, 5291. Trumbore, F. A. (1960). Be77 Syst. Tech. J. 39, 205. Tsu, R., Hodgson, R. T., Tan, T. Y., and B a g l i n , J. E. (1979). Phys. Rev. L e t t . 42, 1356. T u r n b u l l , D. (1950). J. Appl. Phys. 21, 1022. T u r n b u l l , D. (1956). S o l i d S t a t e Phys. 3, 225.
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T u r n b u l l , D., and Cohen, M. (1960). I n "Modern Aspects o f t h e V i t r e o u s S t a t e " (J. D. McKensie, ed.), p. 38. B u t t e r w o r t h , London. van Vechten, J. A. (1982). B u l l . Amer. Phys. SOC. 27, 365. White, C. W., Wilson, S. R., Appleton, B. R., and Young, F. W., Jr. (1980). J. Appl. Phys. 51, 738. White, C. W., Appleton, B. R., S t r i t z k e r , B., Zehner, D. M., and Wilson, S . R. (1981). Mat. Res. SOC. Symp. Proc. 1, 31. White, C. W., and Appleton, B. R. (1982). I n "Laser Annealing o f Semiconductors" (J. M. Poate and J. W. Mayer, eds.), p. 111. Academic Press, New York. Wood, R. F. (1980). Appl. Phys. L e t t . 37, 302. Wood, R. F., and G i l e s , G. E. (1981). Phys. Rev. B 23, 2923. Wood, R. F., K i r k p a t r i c k , J. R., and G i l e s , G. E. (1981). Phys. Rev. B 23, 5555. Wood, R. F. (1982). Phys. Rev. B 25, 2786. Wood, R. F. (1983). Mat. Res. SOC. Symp. Proc. 13, 83. Wood, R. F., Lowndes, D. H., and Narayan, J. (1984). J. Appl. Phys. 44, 770. Young, R. T., Narayan, J., C h r i s t i e , W. H., van d e r Leeden, G. A., L e v a t t e r , J. I., and Cheng, L. J. (1983). S o l i d S t a t e T e c h n o l . 26, 183.
TIME-RESOLVED MEASUREMENTS DURING PULSED LASERI R R A D I A T I O N OF S I L I C O N D. H. Lowndes G. E. Jellison, Jr.
I. I I.
............. ....... . . . . . . .. .. .. .. .. .. .. . . . . . . . . . . .. ....... ........... ........... .... ............. ............ ............ .......... . .. .. .. . . . . . . . ..............
INTRODUCTION. EXPERIMENTAL TESTS AND APPLICATIONS OF THE THERMAL MELTING MODEL 1. Nanosecond Time-Resolved Optical Properties 2. Electrical Conductance 3. Synchrotron X-Ray Diffraction. 4. Optical Measurements of Lattice Temperature. I I I. PULSED RAMAN SCATTERING MEASUREMENTS. 5. Experimental Results 6. Complications in Pulsed Raman Measurements 7. Sumnary: Pulsed Raman Measurements IV. ENERGY TRANSFER FROM OPTICALLY EXCITED CARRIERS TO THE CRYSTAL LATTICE 8. Time Scale for Melting of the Lattice. 9. Observations of the Optically Excited El ectron-Hole P1 asma in Silicon V. SOLIDIFICATION OF HIGHLY UNDERCOOLED LIQUID SILICON. 10. Liquid-to-Amorphous Phase Transformation 11. Nucleation and Growth o f Polycrystal1 i ne Si 1 icon from Undercooled Liquid Silicon VI. CONCLUDING REMARKS. REFERENCES. 313
Copyright 01984 by Academic Press, Inc. All rights of reproduction in any form reserved ISBN 0-12-752123-2
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D. H. LOWNDES E T A L .
I.
INTRODUCTION
The f i e l d of l a s e r processing o f semiconductors now encompasses an impressi ve variety of new phenomena and potenti a1 appl i c a t i ons , as demonstrated in t h e overview presented in Chapter 1. The technological p o s s i b i l i t i e s have recently served t o stimulate anew t h e i n t e r e s t s of physicists and materials s c i e n t i s t s in t h e underlying fundamental physical processes t h a t a r e involved in the i n t e r a c t i o n between intense pulsed photon beams and semiconductor s u b s t r a t e s , p a r t i c u l a r l y a t photon f l uences t h a t are s u f f i c i e n t t o produce high c a r r i e r density s o l i d s t a t e plasmas and/or melting o f semiconductors. The experiments t h a t have been used t o evaluate t h e e f f e c t s of pulsed l a s e r i r r a d i a t i o n of semiconductors, and t o t e s t our t h e o r e t i c a l understanding of the processes involved, f a l l i n t o two broad categories : (1) Conventional post-irradiation measurements such as Rutherford backscattering (RBS), secondary ion mass spectroscopy (SIMS) , transmission and scanning electron microscopy (TEM and SEM) and sheet e l e c t r i c a l properties measurements; and ( 2 ) Time-resol ved measurements during and i m e d i a t e l y a f t e r pulsed l a s e r i r r a d i a t i o n , including measurements of optical r e f l e c t i v i t y and transmission, e l e c t r i c a l conductivity, and blackbody (thermal ) emission, as well as s t r u c t u r e - and/or temperatures e n s i t i v e measurements such as pulsed Raman s c a t t e r i n g , x-ray d i f f r a c t i o n measurements of l a t t i c e s t r a i n , optical second harmonic generation, and electron d i f f r a c t i o n . Post-irradiation measurements have been used t o reveal a wide range of new phenomena, p a r t i c u l a r l y i n connection with understanding the process of ul t r a r a p i d recrystal 1 i z a t i o n and the nonequi 1 i bri um sol i di f i cation phenomena t h a t r e s u l t from pul sed 1aser melting of ion-implanted semiconductors. The wealth of information t h a t has been obtained regarding dopant segregation and sol ubi 1 i t y e f f e c t s , and nonequil ibrium crystal growth generally, i s described i n Chapters 2-5 of t h i s book and in more d e t a i l in the recent s e r i e s
6.
315
TIME-RESOLVEDMEASUREMENTS
.
of Materi a1 s Research Society Symposia Proceedings ( F e r r i s et a1 , 1979; White and Peercy, 1980; Gibbons et a l . 1981; Appleton and 1983; Fan and Johnson, 1984). Celler, 1982; Narayan e t a1 However, post-irradiation measurements s u f f e r from the limitat i o n t h a t they f a i l t o provide d i r e c t , dynamical information about t h e process of absorption of optical energy by a semiconductor, or about the subsequent energy relaxation processes t h a t lead t o u l t r a r a p i d melting and s o l i d i f i c a t i o n . Such information i s provided by time-resol ved optical measurements. Time-resolved optical measurements provide a d i r e c t means f o r studying the process of formation of a dense electron-hole plasma in semiconductors, as photons are absorbed i n d i r e c t or i n d i r e c t t r a n s i t i o n s across the energy bandgap, and f o r determining the time scale f o r t h e t r a n s f e r of energy from photoexcited c a r r i e r s t o the crystal l a t t i c e , leading t o melting of the l a t t i c e a t l a t e r times and f o r s u f f i c i e n t l y high l a s e r fluences. However, a t the high c a r r i e r d e n s i t i e s ((1021/cm3) produced by pulsed l a s e r s , the carrier-phonon relaxation r a t e i s very h i g h (>1012/sec) and i s f u r t h e r enhanced by a nonlinear dependence on c a r r i e r concentration, via t h e Auger recombination process. Thus, in order t o observe the formation and decay of the high c a r r i e r density photoexcited electron-hole plasma i n s i l i c o n , measurements on t h e picosecond---or even shorter---time scale are required. Such measurements have now been carried out, using pulse-and-probe optical techniques, v i s i b l e and near-infrared probe pulses, and both pico- and femto-second l a s e r s (Liu et a1 , 1982a; Shank e t a1 , 1983a; Lompr6 et a1 , 1984a, b; van Driel e t a1 , 1984). Direct measurements of t h e s t r u c t u r a l change accompanying pulsed l a s e r melting a l s o were c a r r i e d out recently on t h e picosecond time s c a l e , v i a electron d i f f r a c t i o n from aluminum t h i n f i l m s (Mourou and Wi 11 iamson, 1982; Wi 11 iamson e t a1 , 1984), and with 90 femtosecond resolution, via time-resolved second harmonic generation a t a (111) s i l i c o n surface (Shank e t a l . , 1984).
.,
.
.
.
.
.
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D. H. LOWNDES E T A L
Time-resolved measurements provide a d i r e c t means f o r evaluating our understanding of the physical mechanisms involved i n pulsed 1a s e r anneal i ng of ion-imp1 anted semiconductors , on the nanosecond and longer time s c a l e s ; these measurements provide an especially s t r i n g e n t test of t h e o r e t i c a l models f o r the laser-anneal i n g process. For exampl e , nanosecond resol u t i on time-resol ved measurements of optical r e f l e c t i v i t y have revealed how differences in the i n i t i a l s t a t e of c r y s t a l l i n i t y or amorphization of a semiconductor modify the onset of melting, the duration of melting and
the overall time s c a l e f o r anneal i ng and recrystal 1 i z a t i on (Auston e t a1 , 1979; Lowndes et a1 , 1984a, b; Wood et a1 , 1984). Similar e f f e c t s may r e s u l t from changes in chemical composition in t h e near-surface region (Lowndes and Wood, 1981). Time-resol ved measurements a1 so were used t o moni t o r t h e sol id-phase r e c r y s t a l l izat i o n process r e s u l t i n g from cw l a s e r annealing, though i n t h i s case the c h a r a c t e r i s t i c time scale f o r the measurements was a fact o r of 106-108 longer (Olson et a1 , 1980). Time-resolved measurements have been a t t h e focal point of a controversy regarding t h e physical mechanism f o r pulsed l a s e r annealing, i n connection with "thermal" versus "nonthermal" models f o r t h e annealing process. The thermal melting model assumes t h a t t h e l a s e r energy absorbed by t h e sample i s transferred t o the l a t t i c e i n a time comparable t o or l e s s than t h e typical annealing l a s e r pulse duration (of order 10-100 nsec) and t h a t t h e r e a f t e r normal heat t r a n s f e r and melting occur. Thermal melting calcul at i o n s carried out by several groups (see Chapter 4 ) found e a r l y support in an impressive a r r a y of primarily post-anneal ing experi mental r e s u l t s , including ( a ) the i n t e r p r e t a t i o n of SIMS and RBS measurements of implanted dopant r e d i s t r i b u t i o n p r o f i l e s in both s i l i c o n and GaAs, a f t e r pulsed l a s e r annealing, in terms of liquidphase diffusion (White et al., 1980; Lowndes et al., 1981; Wood e t al., 1981a, b ) and (b) the presence w i t h i n dopant d i s t r i b u t i o n s i n s i l i c o n , a f t e r pulsed l a s e r annealing, of c e l l u l a r s t r u c t u r e ,
.
.
.
.
6.
317
TIME-RESOLVED MEASUREMENTS
which i s a well-known c h a r a c t e r i s t i c o f t h e breakdown o f a p l a n a r liquid-solid
i n t e r f a c e d u r i n g r e c r y s t a l l i z a t i o n (Poate e t a l .
1978; van Gurp e t al.,
1979; Narayan, 1980).
,
E a r l y time-resolved
r e f l e c t i v i t y measurements i n s i l i c o n (Auston e t a1 and i n GaAs (Lowndes and Wood, 1981; Wood e t a1
., 1978,
1979)
., 1981b) were a l s o
i n t e r p r e t e d as g i v i n g a d i r e c t measure o f s u r f a c e me1t d u r a t i o n , v i a t h e d u r a t i o n o f t h e h i g h r e f l e c t i v i t y phase (HRP),
and were
found t o be g e n e r a l l y i n accord w i t h m e l t i n g model c a l c u l a t i o n s . However, t h e thermal me1t i n g model was n o t u n i v e r s a l l y accepted. Van Vechten and co-workers (Van Vechten e t al.,
1979; Van Vechten,
1980; Van Vechten and Compaan, 1981) suggested t h a t annealing occurs by l a t t i c e s o f t e n i n g , n o t m e l t i n g , t h r o u g h promotion o f e l e c t r o n s f r o m bonding t o a n t i - b o n d i n g s t a t e s and accompanied by f o r m a t i o n o f a long-1 i v e d
(-10-100
nsec) , h i g h - d e n s i t y
(-1021-1022/~m3)
e l e c t r o n - h o l e plasma. Several t i m e - r e s o l v e d measurements on s i l i c o n were i n t e r p r e t e d as c o n t r a d i c t i n g t h e m e l t i n g model and p r o v i d i n g support f o r t h e " p l asma anneal ing" model.
Compaan and co-workers
used t h e r a t i o o f Stokes t o a n t i - S t o k e s i n t e n s i t i e s i n attempts t o measure t h e l a t t i c e temperature o f c r y s t a l 1 i n e s i l i c o n ( c - S i ) s h o r t l y a f t e r t h e end o f t h e h i g h r e f l e c t i v t y phase.
These e x p e r i -
ments and s i m i l a r ones by von d e r L i n d e and co-workers,
which
e v e n t u a l l y brought t h e Raman r e s u l t s i n t o agreement w i t h o t h e r temperature measurements, w i l l be discussed i n S e c t i o n 111. Compaan and co-workers a l s o c a r r i e d out a s e r i e s o f t i m e - r e s o l v e d t r a n s m i s s i o n and r e f l e c t i v i t y measurements u s i n g b o t h c-Si and s i l i c o n -
.,
on-sapphire (SOS) samples (Lee e t a1 1981; A y d i n l i e t al., 1981). As discussed i n Sec. 11.1, t h e i r i n i t i a l o b s e r v a t i o n o f f i n i t e
(-25%) t r a n s m i s s i on through c-Si d u r i n g t h e HRP was subsequently shown t o be wrong by Lowndes (1982) o b s e r v a t i o n o f zero t r a n s mission. I n response t o t h e suggestion o f a plasma-annealing mechanism, a v a r i e t y o f a d d i t i o n a l experimental techniques were developed f o r i n d i r e c t l y measuring t h e l a t t i c e temperature o f t h e near-surface
318
D.H. LOWNDES ETAL.
region of a semiconductor during pulsed laser irradiation. Direct temperature measurements are experimentally quite difficult because of the very short times sec) and small laser-beam dimensions used in typical experiments, and the presence of very large ternporal and spatial temperature gradients. A further compl ication i s the need t o select a thermometric parameter, X(T), t h a t can be measured under these constraints b u t whose dependence on T can be calibrated independently (usually under steady-state conditions). Ideally, X should be independent of other variables (e.g., plasma density) t h a t may a1 so change rapidly d u r i n g and imediately after pulsed laser irradiation. During the past several years, timeresolved experimental techniques have been developed f o r re1 i ably estimating l a t t i c e temperature and for placing an upper bound on the el ectron-hole plasma temperature, using as the thermometric parameter (1) the velocity distribution (Stritzker et al., 1981; Pospieszczyk et al., 1983) or (2) total charge (Liu et al., 1982b; 1984) of electrons and positive ions emitted from Malvezzi et a1 1 aser-heated semiconductor surfaces, (3) x-ray diffraction measure1982a,b, 1983a, b ) , (4) ments of l a t t i c e strain (Larson et a1 changes in the complex index o f refraction of silicon films a t visible wavelengths (Murakami et a1 , 1981; Lompr6 et a1 , 1983), and (5) the emission of thermal (blackbody) radiation (Kemmler et a1 1984). Measurements made using a l l of these techniques have now established t h a t surface temperatures do reach, and exceed, the melting p o i n t of silicon d u r i n g the high reflectivity phase t h a t accompanies the pulsed laser-anneal i n g process. I n this chapter we survey the results o f a selection of the time-resolved measurements performed during pulsed laser i r r a d i a t i o n . Particular emphasis has been placed on those experimental results t h a t can be directly compared w i t h predictions o f model calculations, i n order t o provide as complete a picture as possible o f the current state of agreement between experiments and calculations. For this reason the discussion is restricted t o results
.
.,
.
.)
.
6. obtained
319
TIME-RESOLVED MEASUREMENTS
using s i l i c o n ,
thus avoiding complications t h a t
are
associated w i t h d e v i a ti o n s from s to i c h i o m e t r y i n compound semiconductors (Lowndes and Wood,
1981).
E f f e c t s o f pulsed l a s e r
r a d i a t i o n on GaAs are discussed separately i n Chapters 7 and 8.
11. Experimental Tests and A p p l i c a t i o n s o f t h e Thermal M e l t i n g Model 1.
NANOSECOND TIME-RESOLVED OPTICAL PROPERTIES
a.
Experimental Considerations Time-resol ved measurements o f t h e o p t i c a l transmission
(T) o r
r e f l e c t i v i t y (R) d u ri n g pulsed l a s e r i r r a d i a t i o n are complicated by t h e simul taneous i mp o s i ti o n o f several
requirements;
these
i n c l u d e ( I ) det e c to r response time, (2) s p a t i a l and temporal q u a l i t y o f t he annealing l a s e r beam, (3) s p a t i a l alignment o f low-power cw probe l a s e r beams w i t h the pulsed beam, and ( 4 ) t h e need f o r accur a t e energy den s i ty measurements i n t h e ( f r e q u e n t l y small ) r e g i o n whose o p t i c a l p r o p e r t i e s are monitored by t h e probe beam.
Figure 1
shows schematically two s i m p l i f i e d y e t t y p i c a l experimental setups used i n measurements on t h e nanosecond time scale w i t h v i s i b l e and n e a r-inf rared cw probe beams. The pulsed l a s e r beam must have a smooth temporal and s p a t i a l profile.
Pronounced temporal spikes o r m u l t i p l e pulses are unac-
ceptable because accurate q u a n t i t a t i v e comparisons w i t h model c a l c u l a t i o n s (which f o r convenience g e n e ra l l y assume an i d e a l i z e d temporal pulse) become d i f f i c u l t and because excessive power dens i t i e s may cause damage t o t h e sample. For t h i s reason most experiments have been c a r r i e d out u s i ng pul sed 1asers operated i n t h e TEMoo mode, w i t h attempts also made t o minimize temporal modul a t i o n o f t h e pulse shape (due, t u d i n a l modes).
f o r example, t o m u l t i p l e l o n g i -
Pulses t h a t are n e a r l y Gaussian i n s p a t i a l cross
320
D. H. LOWNDES ETAL.
TIME RESOLVED TRANSMISSION EXPERIMENT OSCILLOSCOPE h, 1 nnr)
-
COLLECTION LENS SYSTEM,
I
THIN SHEET WITH
2 OR 3 m m diam APERTURE PULSED RUBY LASER SILICON
FOCUSING
LENS^ ',/ / '_'
IR HdNe PROBE LASER (1 mW A = 1.152 rm)
m.
0.44-
f"::$p)
-A -
TIME OF ONSET-OF-MELTING MEASUREMENT
THREE h = 0.633 Nrn
PLANAR VACUU
LENS SYSTEM
FOCUSING LENS
(3.4 mW CW. A = 0.633 wrn
Fig. 1. Schematic representation o f experimental setups for time-resolved ( a ) transmission (Lowndes e t al., 198213) and (b) reflectivity (Lowndes and Wood, 1981) measurements.
6.
321
TIME-RESOLVED MEASUREMENTS
section and s u f f i c i e n t l y smooth temporally t o be e a s i l y approximated i n model calculations a r e obtained with ruby, Nd:YAG, Nd:glass, and excimer lasers. Pulsed excimer l a s e r s possess the very s i g n i f i c a n t advantage of having only low s p a t i a l coherence so t h a t d i f f r a c t i v e modulat i o n of the l a s e r beam by d u s t p a r t i c l e s or apertures is not present t o nearly the extent i t i s w i t h pulsed s o l i d - s t a t e l a s e r s . Local damage t o specimens a t the positions of d i f f r a c t i o n maxima and/or beam inhomogeneity on a f i n e s c a l e can therefore be avoided by using a pulsed excimer l a s e r . Multi-mode pulsed s o l i d - s t a t e l a s e r operation can a l s o be used, provided t h a t s p a t i a l beam inhomogeneities a r e removed by placing t h e sample behind a d i f f u s e r p l a t e or diffusing l i g h t pipe. However, d i f f u s e r plates can cause microscopic damage t o even a s i l i c o n sample surface i f the sample i s too close ( < l o cm) t o the p l a t e , or i f the grinding/etching procedure i s not optimized (Young e t a1 1982). Since t h e pulsed energy density f a l l s off rapidly with distance beyond a d i f f u s e r p l a t e , i t s application is probably r e s t r i c t e d t o multi-mode l a s e r s capable of delivering s u b s t a n t i a l l y more energy per pulse than i s needed. Diffusing l i g h t pipes have the disadvantage in tirneresolved measurements using cw probe beams t h a t t h e sample must be placed very close (<1mn) t o t h e end o f t h e l i g h t pipe t o obtain homogeneous i r r a d i a t i o n , t h u s r e s t r i c t i n g probe beam access and limiting one t o measurements with a l a r g e probe beam angle of incidence. I t i s also usually desirable t o focus down a cw probe beam t o a small (10-200 pm diam) spot on t h e sample and t o center the probe beam i n the most homogeneous part of the pulsed beam. The use of a large (-1 nun) probe beam spot s i z e r e s u l t s i n measurements of a r t i f i c i a l l y long r e f l e c t i v i t y rise and f a l l times, possibly due t o s p a t i a l averaging over regions t h a t contain d u s t p a r t i c l e s or 1979), as well microscopic surface i r r e g u l a r i t i e s (Auston et a1 as over the pulsed beam s p a t i a l energy density profile. However,
.,
.,
322
D. H. LOWNDES ET AL.
t h e use o f a t i g h t l y focused cw probe beam c o n f l i c t s t o some e x t e n t w i t h t h e requirement o f a c c u r a t e pulsed energy d e n s i t y m a s u r e ments-necessary
f o r comparison with model c a l c u l ations-in
the
r e g i o n monitored by t h e probe beam. Accurate c a l o r i m e t r i c measurements o f pul sed 1a s e r energy u s i n g c o m o n v o l ume-absorbi ng c a l o r i m e t e r s r e q u i r e measuring t h r o u g h an a p e r t u r e w i t h a diameter o f
-1 m, c e n t e r e d on t h e pulsed and probe beams. Since t h e probe beam i s g e n e r a l l y focused t o a s u b s t a n t i a l l y s m a l l e r spot, one i s forced t o use t h e most homogeneous p o s s i b l e pulsed beam and a l s o t h e smallest possible aperture consistent w i t h accurate c a l o r i m e t r i c measurements. The d e t e c t o r s r e q u i r e d f o r nanosecond o p t i c a l experiments must have s h o r t (nanosecond o r l e s s ) response times, h i g h s e n s i t i v i t y t o low and r a p i d l y changing l i g h t l e v e l s ,
and good l i n e a r i t y o f
t h e d e t e c t o r ' s e l e c t r i c a l response i n f o l l o w i n g b o t h r i s i n g and f a l l i n g l i g h t signals.
Recent experience on t h e p a r t o f several
r e s e a r c h groups demonstrates t h a t many o f t h e s o l i d - s t a t e d e t e c t o r s t h a t do p r o v i d e s u f f i c i e n t l y s h o r t response t i m e f a i l t o meet t h e s e n s i t i v i t y and/or l i n e a r i t y requirements.
Lowndes e t a l . (1982a,
b ) t e s t e d a S i avalanche photodiode (APD;
a c t i v e r e g i o n 1.5
mn
diam, -1 nsec t i m e c o n s t a n t ) , a Ge P I N photodiode, a Ge APD, and an InAs photodiode f o r use i n t i m e - r e s o l v e d T measurements; o n l y t h e S i APD met t h e combined requirements o f h i g h s e n s i t i v i t y , a p p r o x i m a t e l y nanosecond response time, good l i n e a r i t y on b o t h r i s i n g and f a l l i n g l i g h t s i g n a l s , and n e g l i g i b l e b a s e l i n e overshoot.
Moss and Marquardt (1982) have s y s t e m a t i c a l l y i n v e s t i g a t e d t h e response o f v a r i o u s f a s t p h o t o d e t e c t o r s t o r e f 1 e c t e d and t r a n s m i t t e d probe beams o f 0.633,
1.06,
1.34,
and 3.39
p wavelength
d u r i n g pul sed ruby l a s e r anneal i n g o f 100 keV As-imp1 anted s i l icon. They found t h a t t r a n s i e n t r e f l e c t i v i t y s i g n a l s were independent o f t h e d e t e c t o r used, except f o r small d i f f e r e n c e s a t t r i b u t a b l e t o known d i f f e r e n c e s i n d e t e c t o r response time. However, t r a n s i e n t T measurements were dependent upon t h e d e t e c t o r used.
Measurements
6.
TIME-RESOLVED MEASUREMENTS
323
performed with a fast Ge PIN photodiode or a Au-doped Ge photodetector exhibited an abrupt i n i t i a l drop i n T, followed by a continuous f a l l in T d u r i n g the HRP and then slow recovery over a longer period o f time, with the apparent T never reaching zero. The Ge APD also d i d not respond t o photoluminescence from the sample in the presence o f 1.06- or 1.34-pn~ probe beams, although i t did detect photo1 umi nescence i n their absence, during pulsed laser irradiation. I n contrast, measurements using a Si PIN diode or a Ge APD showed T decreasing abruptly t o a value indistinguishable from zero , remaining a t zero during the HRP and then recovering. Thus, although all the detectors tested were able t o follow transient reflectivity (increasing l i g h t ) signals, some detectors failed t o faithfully follow a light signal falling t o zero. I n addition, some response nonlinearities in the presence o f superimposed light signals have been observed. The need for considerable care in using solid-state detectors for transient optical properties measurements i s clear. b.
Transmission Measurements
A systematic comparison of the predictions o f thermal melting model calculations with measurements of the time-resolved optical transmission ( T ) and reflectivity ( R ) of c-Si has been carried out by Lowndes and co-workers. Their T measurements (Lowndes, 1982;
.
Lowndes et a1 , 1982a, b ) used a pulsed ruby laser [A = 694 nm, 14 f 1-nsec (FWHM) pulse d u r a t i o n ] and c-Si wafers t h a t were polished on b o t h sides t o eliminate scattering of a 2 mW cw He/Ne 1152 nm probe laser beam. The experiment i s shown schematically i n Fig. la. The laser beam was focused t o produce only a slow variation o f energy density, E l , over a 4-m diam central region. Calorimetric measurements of ,E were carried out through a 1-mm diam aperture t h a t was centered on the region sampled by the probe laser beam. A 1152-nm bandpass f i l t e r (10-nm bandwidth and -40% T ) was necessary directly in front of the Si APD detector in order t o prevent
324
D. H. LOWNDES E T A L .
intense near-bandgap photo1 umi nescence emi tted by the sampl e f r o m swamping the detector when i t was close (-9 mn) t o the sample (see be1 ow).
I
0.65t0.06J/crn2
I
I
0.87k0.07 J/cm2
I
I
t.16t0.08 J/cm2
I
Fig. 2. Transmission o f an unfocused 1.1 5-pm probe beam through c-Si, a t a s e r i e s o f pulsed laser El and viewed on two d i f f e r e n t timescales (20 and 50 nsecldiv.). The dashed horizontal lines indicate the i n i t i a l To. The top The arrows mark the figure i s for El l e s s than the threshold for the HRP. position o f the peak o f the ruby laser pulse (plus or minus a few nanoseconds) (Lowndes 1982).
,
6.
325
TIME-RESOLVED MEASUREMENTS
Three separate storage osci 11oscope recordings (on successive ruby laser shots) w i t h ( a ) both probe beam and ruby beam present, ( b ) probe beam blocked, (c) both beams blocked, allowed a clean separation of the T (or R ) signal from near-bandgap photoluminescence and radiated electromagnetic noise (associated w i t h f i r i n g the pulsed ruby laser). The measured quantities were ratios of T or R during the HRP t o the initial To or Ro; these were converted t o absolute T or R values u s i n g separate measurements o f To and RO
Figures 2 and 3 show typical results of a series o f T measurements, w i t h EL b o t h below and above the threshold (En = 0.8 J/cm2) for the HRP, using both unfocused ( l / e 2 diam = 970 pm) and focused ( l / e 2 diam = 230 pn) probe beams, respectively. T drops t o zero, 0.6
0.5
I-
EXPERIMENTAL
0.2
......,....Ep = 0.62
---
0.1
0
EQ = 0.86
CALCULATION v EQ = 0.8
Ep = 1.07 Ep = 1.24
0
50
100 150 TIME AFTER PULSE (nsec)
A
o o
EQ = 1.0 EP = 1.2 EQ = 1.4
200
250
Fig. 3. Time-dependent transmission o f a 1 . 1 5 pm focused probe laser beam through c-Si during and after pulsed ruby laser irradiation. Lines: experimental data. Discrete points: model calculations (see t e x t ) . En i s given in J/cm2. To = 0 . 5 1 8 i s also shown (Lowndes e t al., 1 9 8 2 b ) .
326
D. H. LOWNDES ETAL.
and remains a t zero,
f o r a p e r i o d o f t i m e t h a t increases w i t h
i n c r e a s i n g ER; r e c o v e r y t o t h e i n i t i a l value o f To occurs i n 2500 nsec (Fig. 3). F o r t h e measurements with t h e focused probe beam (Fig. 3), a background s i g n a l
,primarily
near-bandgap photo1umi nescence t r a n s -
m i t t e d t h r o u g h t h e 1%bandpass f i l t e r ,
was s u b t r a c t e d from t h e
measured T ( u s i n g successive l a s e r shots, as d e s c r i b e d above) t o o b t a i n t h e data o f Fig.
3.
T h i s background s i g n a l s u b t r a c t i o n
procedure was found t o be h i g h l y r e p r o d u c i b l e on successive l a s e r shots,
as i l l u s t r a t e d by Fig.
2.5
I \0
t
3
f
5 1.5 -
\
L (D
1.0
z
-
v)
I-
\
0
Z
0
a
0 BACKGROUND
0
L
I 0 TRANSMISSION
\
(D
[r
I 1.24 J/cm2
0 \ 0 2.0
v)
z
I
The minimum T observed was
4.
0.5
0
-10
0
10
20
30
40
50
TIME (nsec) Fig. 4. Time-dependent T signals on successive pulsed ruby laser shots ( 0 ) with and ( 0 ) without the 1 1 5 2 nm probe laser beam, illustrating the reproducibility of the procedure for subtracting residual near-bandgap photoluminescence and radiated electromagnetic noise from the T signal (Lowndes e t al.,
1982b).
6.
327
TIME-RESOLVED MEASUREMENTS
i n d i s t i n g u i s h a b l e f r o m zero,
w h i l e t h e maximum T t h a t c o u l d be
present and remain undetected d u r i n g t h e zero-T p e r i o d was e s t i mated t o be <1%. A v a r i e t y o f probe beam f o c u s i n g and c o l l e c t i o n c o n d i t i o n s were used w i t h no m o d i f i c a t i o n o f t h e zero-T r e s u l t . Thus, t h e zero o f T i s genuine and i s n o t t h e r e s u l t o f s c a t t e r i n g , deflection,
and/or
self-defocusing
o f t h e probe beam o f f t h e
d e t e c t o r due, f o r example, t o a change i n index o f r e f r a c t i o n d u r i n g t h e HRP (Lee e t a1
., 1981;
A y d i n l i e t a1
., 1981).
We note a l s o
t h a t o b s e r v a t i o n s of f i n i t e (-25%) T t h r o u g h b o t h c-Si (Lee e t al., 1981) and SOS ( A y d i n l i e t a1 mental a r t i f a c t ,
., 1981) are now known t o be an e x p e r i -
probably due t o a s p a t i a l l y dependent d e t e c t o r
response t i m e o r " d i f f u s i o n t a i l " (Compaan e t al.,
1982a; see a l s o
S e c t i o n 1I.l.a). F i g u r e 3 a l s o shows r e s u l t s o f c a l c u l a t i o n s o f t h e timedependent T t h r o u g h c-Si a t several
,E values.
M e l t i n g model c a l -
c u l a t i o n s (Wood and G i l e s , 1981), c a r r i e d o u t using t h e macroscopic d i f f u s i o n e q u a t i o n c a s t i n t o f i n i t e - d i f f e r e n c e form f o r numerical s o l u t i o n on a c l o s e l y spaced mesh o f p o i n t s i n space and time, are d e s c r i b e d i n Chapter 4.
The values used f o r t h e v a r i o u s thermal
and o p t i c a l parameters i n t h e present c a l c u l a t i o n s are g i v e n i n Table I.
[The temperature dependences o f t h e o p t i c a l a b s o r p t i o n
c o e f f i c i e n t and t h e r e f l e c t i v i t y ( a t h = 694 nm) f o r c-Si ( J e l l i s o n and Modine,
1982a,
1983) were i n c l u d e d i n t h e c a l c u l a t i o n s f o r
Fig. 3, which was not t r u e o f e a r l i e r c a l c u l a t i o n s . ] The p r i m a r y t h e o r e t i c a l r e s u l t s of these m e l t i n g model c a l c u l a t i o n s are p l o t s o f (a) m e l t depth vs t i m e and (b) time-dependent p r o f i l e s o f temp e r a t u r e vs depth i n s o l i d , c-Si (see Chapter 4). as i n p u t data f o r t h e T ( t ) c a l c u l a t i o n s .
o p t i c a l constants f o r m o l t e n S i (Shvarev e t a1 a1
These were used
Published values f o r t h e
., 1977;
Lampert e t
., 1981) were used t o c a l c u l a t e t h e a t t e n u a t i o n due t o t h e m o l t e n
layer,
w h i l e measurements
( J e l l i s o n and Lowndes,
o p t i c a l absorption c o e f f i c i e n t
1982) o f t h e
as a f u n c t i o n o f temperature a t
1152 nm were used f o r t h e h o t c-Si (See a l s o Chapter 3.)
f r e e parameters are i n v o l v e d i n t h e T ( t ) c a l c u l a t i o n s .
Thus, no
328
D. H. LOWNDES E T A L .
Table I Input data f o r c r y s t a l l i n e s i l i c o n calculations appropriate t o ruby (694 nm) and Nd:YAG (532 nm) l a s e r s . See Chapters 3 and 4 f o r d e t a i l s . Quantity
Value Used and Comments
Thermal Conductivity and Specific Heat
Temperature-dependent (see Chapter 4 )
Density
2.3 g/cm3 (-10% change on me1 t i n g ignored).
Latent Heat of Me1 t ing
1799 J / g
Me1 t i ng Temperature
1410°C
Substrate Temper a t ure
20°C
Ref 1e c t ivi t y (a) s o l i d (694 nm) (532 nm) 1 iquid
R = 0.34 + 5 x 10-5 T R = 0.37 t 5 x 10-5 T R = 0.70
Absorption Coefficient ( a ) (694 nm) (532 nm)
2540 exp (T/427'C) 9380 exp (T/430°C)
Pulse Duration (FWHM) (694 nm) (532 nm)
15 nsec 18 nsec
Pulse Energy Density
varied
(oc) (oc)
( a ) J e l l i s o n and Modine (1982a; 1983)
As Fig. 3 shows, both the d e t a i l e d shape of the calculated return t o f i n i t e T (following the zero-T period) and the El dependence of T a r e in good agreement w i t h the experimental r e s u l t s . However, careful comparison of the experimental and calculated T i n Fig. 3, f o r times well a f t e r the zero-T period, reveals t h a t t h e experimental T i s 10.05 lower than t h a t
calculated.
This suggests the
6.
329
TIME-RESOLVED MEASUREMENTS
presence of i 1 0 % a d d i t i o n a l f r e e - c a r r i e r absorption (not included i n the melting calculations). I n order t o estimate t h e magnitude o f f r e e c a r r i e r absorption that i s
l i k e l y t o be present, due t o
carriers
remaining i n t h e
near-surface region f o r times >_lo0 nsec, use was made of t h e f a c t t h a t any i n i t i a l h i g h ( l a s e r melting-induced) c a r r i e r c o n c e n t r a t i o n
w i l l be r a p i d l y reduced by Auger recombination and, secondarily, by c a r r i e r d i f f u s i o n away from t h e surface.
This r e s u l t s i n a t o t a l
absorption i n t h e c a r r i e r d i f f u s i o n depth L = Lafc = 0.042,
i.e.,
-5% (Lowndes e t a1
-
6 p of order
., 1982b).
I hIlII
This estimate
i s i n unexpectedly good agreement (considering the roughness o f t h e c a l c u l a t i o n ) w i t h t h e experimentally observed " e x t r a " absorpt i o n t10% f o r t > 100 nsec (Fig. 3). I n c o n t r a s t , t h e a d d i t i o n a l f r e e c a r r i e r absorption c a l c u l a t e d assuming only a t h e r m a l l y generated " i n t r i n s i c " c a r r i e r p o p u l a t i o n
7, Chapter 4), i s <0.5%. Thus, t h e " e x t r a " absorption appears t o be due t o r e l a t i v e l y l o n g - l i v e d c a r r i e r s t h a t are t h e remnant of those c a r r i e r s generated i n i t i a l l y by absorption o f t h e ruby l a s e r pulse. corresponding t o t h e t = 100 nsec temperature p r o f i l e (Fig.
The presence of these long-1 i v e d c a r r i e r s was d i r e c t l y confirmed i n these experiments , through observation o f t h e i r near-bandgap photo1 uminescence (Lowndes e t a1 c.
., 1982b).
R e f l e c t i v i t y Measurements: Surface M e l t Duration and Time o f Onset o f M e l t i n g o f c - S i l i c o n
The f i r s t studies o f l a r g e t r a n s i e n t increases i n t h e r e f l e c t i v i t y o f semiconductors d u r i n g pulsed l a s e r i r r a d i a t i o n were c a r r i e d out by Sooy and co-workers (1964) and by Birnbaum (1964). These authors were i n t e r e s t e d i n t h e r e f l e c t i v i t y increase phenomenon p r i m a r i l y f o r i t s p o s s i b l e use i n t h e modulation and c o n t r o l o f Q-switched lasers. Sooy e t a l . (1964) used a cw HeNe probe beam t o observe an approximate doubling and s a t u r a t i o n o f t h e magnitude o f the r e f l e c t i v i t y o f several semiconductors d u r i n g pulsed ruby laser irradiation.
They observed i n c r e a s i n g d u r a t i o n o f t h e HRP
330
D. H. LOWNDES E T A L .
with increasing l a s e r pulse energy and were apparently t h e f i r s t t o suggest t h a t me1 t i n g occurs. They a1 so noted the c h a r a c t e r i s t i c “orange peel I‘ t e x t u r e of laser-damaged semiconductor surfaces. Blinov et a l . (1967) also observed a r e f l e c t i v i t y change in GaAs t h a t they a t t r i b u t e d t o melting, b u t were unable t o see a similar e f f e c t in s i l i c o n , which they ( c o r r e c t l y ) a t t r i b u t e d t o i t s higher melting threshold energy density. Precise time-resolved r e f l e c t i v i t y measurements during pulsed l a s e r annealing were f i r s t c a r r i e d out by Auston and co-workers (Auston et a l . , 1978, 1979), using both the fundamental (1064 nm) and frequency-doubled (532 nm) Nd:YAG wavelengths as the pulsed beam. The principal conclusion of t h e i r study was t h a t t h e mechanism f o r pulsed l a s e r annealing i s melting followed by liquidphase epitaxial regrowth. Analytical estimates of the melting threshold energy density, and model calculations of surface melt duration and r e c r y s t a l l i z a t i o n velocity were also made. These were found t o be in agreement, t o within b e t t e r than a f a c t o r of two, w i t h the duration of the high r e f l e c t i v i t y phase and t h e recryst a l l i z a t i o n velocity inferred from time-resolved r e f l e c t i v i t y measurements. However, Auston et a l . noted t h a t melt duration calcul a t i o n s giving t h e best agreement with t h e i r experiments required using an anomalously low value (R = 0.57) f o r t h e r e f l e c t i v i t y of molten s i l i c o n (R = 0.72). Measurements of pulsed l a s e r energy density in the small area sampled by t h e cw probe l a s e r beam were probably not of high enough precision in these i n i t i a l experiments t o j u s t i f y more detailed comparisons of measured and calculated melt durations (0. H. Auston, private communication). More recently, Lowndes, Wood, and co-workers have used ruby (Lowndes e t a1 , 1982a, b ) and frequency-doubled Nd:YAG l a s e r s (Lowndes et a1 , 1983, 1984a, b) t o carry out systematic and detai 1ed compari sons of the predi c t ions of thermal me1 t i ng model c a l c u l a t i o n s ( a s described above) with r e s u l t s of nanosecondresolution time-resolved r e f l e c t i v i t y measurements during pulsedl a s e r i r r a d i a t i o n of both c r y s t a l l i n e and amorphous s i l i c o n . This
. .
6.
331
TIME-RESOLVED MEASUREMENTS
work i s noteworthy because o f t h e development and use together o f two d i f f e r e n t
r e f l e c t i v i t y measurement techniques:
(1) measure-
ments o f t h e p r e c i s e time, tm, o f t h e onset o f melting, w i t h i n t h e d u r a t i o n o f t h e i n c i d e n t l a s e r pulse, combined w i t h (2) conventional measurements o f t h e d u r a t i o n o f t h e HRP, corresponding t o t h e surface m e l t duration.
They have demonstrated t h a t t h e time of onset
o f m e l t i n g i s p a r t i c u l a r l y s e n s i t i v e t o d e t a i l s of t h e energy absorption process i n t h e s o l i d phase (i.e.,
sensitive t o the
values assumed i n model c a l c u l a t i o n s f o r t h e thermal and o p t i c a l p r o p e r t i e s o f t h e s o l i d ) p r i o r t o t h e occurrence o f melting. For example, Lowndes e t a l . (1983) used t m measurements t o i n f e r t h a t photoexci t e d
free
carriers
enhance
the
temperature-dependent
absorption c o e f f i c i e n t l e a d i n g t o an intensity-dependent a and t h u s decreasing t h e m e l t i n g t h r e s h o l d El.
They a l s o used t,
measurements, t o g e t h e r w i t h model c a l c u l a t i o n s f o r ion-imp1 anted a-Si layers, t o demonstrate t h e dominant r o l e played by t h e low value o f t h e thermal c o n d u c t i v i t y , dynamics o f m e l t i n g o f a-Si;
Ka, o f a-Si
i n governing t h e
t h e i r work r e s u l t e d i n a determina-
t i o n o f Ka for a-Si (Lowndes e t al.,
1984a, b; see Sec. 1I.l.d).
For these time-resolved r e f l e c t i v i t y measurements a frequencydoubled Nd l a s e r followed by d i c h r o i c m i r r o r s was used t o provide a harmonically pure 532-nm l a s e r beam w i t h (18 duration.
2
1) nsec pulse
The l a s e r was operated a t f i x e d a m p l i f i e r gain t o avoid
s l i g h t gain-dependent changes i n second-harmonic pulse width;
,E
was v a r i e d using lenses. Special care was taken t o a c c u r a t e l y monitor t h e pulse energy d e n s i t y and duration, both because it was d e s i r e d t o make absolute comparisons w i t h r e s u l t s o f thermal m e l t i n g model c a l c u l a t i o n s ,
and because p r e l i m i n a r y measurements
f o r c-Si had i n d i c a t e d s y s t e m a t i c a l l y s h o r t e r m e l t d u r a t i o n s f o r a given E,,
when compared w i t h t h e e a r l i e r measurements by Auston
and co-workers (1979).
A beam spl i t t e r and microjoulemeter were
used f o r i n s i t u m o n i t o r i n g of t h e energy o f each l a s e r shot; t h e microjoulemeter was c a l i b r a t e d using a 1-mm diam aperture and a
332
D. H. LOWNDES ET AL.
volume-absorbing c a l o ri m e te r a t t h e p o s i t i o n o f t h e sample.
A
separate vacuum planar photodiode was used t o monitor t h e temporal shape o f each pulse.
Only l a s e r pulse shapes t h a t were t e m p o r a l l y
n e a r l y Gaussian were used i n th e analyses. beam,
a t near-normal
A cw HeNe probe l a s e r
incidence and focused t o a l / e spot s i z e
v a r i a b l e from 35 pm t o about 100 pm upon t h e sample's surface, was used t oget h e r w i t h a s i l i c o n avalanche photodiode t o monitor t h e time-resolved r e f 1 e c t i v i t y . F igure 5 shows r e s u l t s of measurements o f surface m e l t d u r a t i o n
for (100) c-Si (Lowndes e t a1 was found t o be
ER
= 0.38
., 1983).
The t h r e s h o l d f o r t h e HRP
J/cm2 t o o b t a i n R (633 nm) = 70%, w i t h
R reaching t he constant value 73 (+1)%f o r ,E > 0.45 J/cm2; t h e
140
-
v)
0
120
THIS WORK
C
v
g=a
100
3
80
2z
60
3
40
D
w
LL
a
2
20 0 0
0.2
0.4
0.6
0.8 1.0 E l (J/cm2 1
1.2
1.4
1.6
1.8
Fig. 5. Measured surface melt duration vs frequency-doubled ( 5 3 2 n m ) pulsed Nd:YAG laser energy density, EA: (0)Lowndes e t a l . ( 1 9 8 3 ) ; (---) Austonetal. ( 1 9 7 9 ) (smoothed). Also shown are results of thermal melting model calculations o f surface melt duration: ( A ) 20-ns pulse duration; (-) 18-11s pulse duration; (-) 18-ns pulse duration, a = 5 x l o 5 crn-l.
6.
TIME-RESOLVEDMEASUREMENTS
333
l a t t e r value i s in good agreement w i t h the value R = 72% a t 633 nm for molten Si under near-equilibrium melting conditions (Shvarev e t al., 1977; Lampert e t al., 1981). For 0.33 < El < 0.38 J/cm2, a ,reflectivity r i s e indicative of melting was also observed, b u t w i t h the maximum R in the 50-69% range. [The 633-nm reflectivity of c-Si just below i t s 1410°C melting temperature i s R z 41-433 (Sato, 1967; Lampert e t al., 1981).] Thus, these experiments demonstrate t h a t the El "width" of the transition t o the HRP, near and above the melting threshold, i s actually > 0.1 J/cm2, probably corresponding t o formation o f either a shallow me1 t , comparable in depth t o the optical skin depth (-10 rim at 633 nm wavelength), or t o spatially nonuniform melting near the melting threshold 1984). Lowndes e t al. (1983) point out that (Combescot e t a1 the breadth of t h i s transition t o the HRP may be significant for the interpretation of pulsed Raman l a t t i c e temperature measurements, i f these measurements are carried out for El very near the melting threshold (see, for example, Compaan e t al., 1982a,b), since such measurements are presumably i n danger o f sampling b o t h mol ten and solid regions even during the HRP. The differences (see Fig. 5) between the melt durations measured by Auston et al. (1979) and those of Lowndes et al. (1983) are not believed t o be due t o the different pulse durations used in their experiments, but instead are believed t o result from the experimental difficulty of accurately measuring El through an aperture having a diameter approaching t h a t of the focused cw probe beam spot s i z e o r of ensuring uniform ER over a larger measurement aperture (Lowndes e t a1 , 1983). ThE results of thermal melting model calculations of surface melt duration for c-Si are also shown in Fig. 5; these were carried out (as described in Chapter 4) using the values for the temperature-dependent optical and thermal parameters o f Tab1 e I. The results in Fig. 5 i l l u s t r a t e the effect on model calculat i o n s of changes i n the assumed laser pulse duration or near-surface optical properties; significant differences between the calculations
.,
.
334
D. H. LOWNDES ETAL.
appear i n t h e p h y s i c a l l y i n t e r e s t i n g r e g i o n near and above t h e m e l t i n g t h r e s h o l d E J , but disappear almost e n t i r e l y f o r EJ J/cm2.
>
1.2
For example, small b u t s i g n i f i c a n t d i f f e r e n c e s occur depend-
i n g upon whether t h e absorption c o e f f i c i e n t , a(T), and r e f l e c t i v i t y , R(T),
are switched instantaneously from t h e values f o r c-Si
(at
1410OC) t o those f o r molten S i when surface m e l t i n g occurs, o r i f t h e y are i n s t e a d switched more g r a d u a l l y as t h e melt f r o n t i n i t i a l l y penetrates t o a depth o f about 2 o p t i c a l s k i n depths (-20
As shown i n Fig.
nm) i n t o t h e sample.
5,
stepwise s w i t c h i n g
t o g e t h e r w i t h t h e experimentally c o r r e c t 18-nsec pulse d u r a t i o n r e s u l t s i n good agreement between experimental and c a l c u l a t e d me1t d u r a t i o n s , but i n a c a l c u l a t e d m e l t i n g t h r e s h o l d EJ t h a t i s s l i g h t l y high. The p o s s i b i l i t y t h a t a i s i n t e n s i t y dependent a t t h e pulsed l i g h t l e v e l s used i n these experiments was a l s o recognized i n these model c a l c u l a t i o n s .
Nonlinear absorption, due t o photoexcited f r e e
c a r r i e r s , would be expected t o increase a, l e a d i n g t o more r a p i d m e l t i n g o f t h e near-surface region.
Lowndes e t a l .
(1983) used
known Auger recombination r a t e s and estimated t h e f r e e - c a r r i e r a b s o r p t i o n cross-section a t 532 nm, t o conclude t h a t f r e e - c a r r i e r a b s o r p t i o n i s a t l e a s t comparable t o t h e absorption given by a(T). The e f f e c t o f t h i s intensity-dependent absorption was modeled by setting a = 5
x
lo5 cm-1 (a value i n t e r m e d i a t e between t h e a values
f o r molten S i and s o l i d S i a t i t s m e l t i n g p o i n t ) . Fig.
5,
As shown i n
t h i s c a l c u l a t i o n gives an e x c e l l e n t r e p r o d u c t i o n o f t h e
measured EJ dependence o f t h e surface melt duration,
including
b o t h a decrease i n t h e m e l t i n g t h r e s h o l d and a s l i g h t decrease o f m e l t d u r a t i o n a t higher Ex,
r e l a t i v e t o c a l c u l a t i o n s t h a t use t h e
a(T) v a l i d a t low l i g h t i n t e n s i t i e s . F i g u r e 6 shows r e s u l t s o f measurements o f t h e p r e c i s e time o f t h e onset o f m e l t i n g , l a s e r pulse.
t m y
o f c-Si d u r i n g an i n c i d e n t 18-nseca 532-nm
The zero o f time f o r these measurements was taken t o
be t h e center o f t h e i n c i d e n t l a s e r pulse; p o s i t i v e times correspond t o m e l t i n g before t h e peak o f t h e pulse i s reached.
Also
6.
335
TIME-RESOLVED MEASUREMENTS
15
10
B 0 -5
-10
Fig. 6.
I 0.2
I
I
I 1 1.0
I
0.4 0.6 0.8 ENERGY DENSITY iJ / c m P1
Measured (open c i r c l e s ) and calculated (-,
--
-)
times o f onset
o f melting f o r c-Si vs pulsed laser energy density (Lowndes e t a l . , text and the caption for Fig. 5 ) .
1983; see
shown i n Fig. 6 a r e t h e r e s u l t s o f two s e t s o f model c a l c u l a t i o n s o f t,,,, cm-l.
u s i n g e i t h e r a ( T ) f o r c-Si o r t h e h i g h e r value a = 5
x
105
Although b o t h c a l c u l a t i o n s are i n qua1 i t a t i v e agreement w i t h
t h e experimental data,
i t is t h e enhanced-absorption c a l c u l a t i o n
t h a t g i v e s e x c e l l e n t q u a n t i t a t i v e agreement w i t h experiment.
Thus,
t h e new o n s e t - o f - m e l t i n g t y p e o f r e f l e c t i v i t y measurement p r o v i d e s a s e n s i t i v e means o f d i s t i n g u i s h i n g between model c a l c u l a t i o n s t h a t d i f f e r o n l y i n t h e i r assumptions r e g a r d i n g t h e values o f thermal o r o p t i c a l parameters p r i o r t o t h e occurrence o f m e l t i n g ; f o r c-Si t h i s technique p r o v i d e s evidence f o r t h e i n f l u e n c e o f i n t e n s i t y dependent a b s o r p t i o n on t h e pul sed l a s e r me1t i ng process.
336 d.
D. H. LOWNDES E T A L .
R e f l e c t i v i t y Measurements:
Thermal C o n d u c t i v i t y o f a - S i l i c o n
Recent t h e o r e t i c a l and experimental e f f o r t s ( t h e l a t t e r using b o t h pulsed and continuous heating) t o determine thermodynamic p r o p e r t i e s o f a-Si, such as i t s m e l t i n g temperature, Ta, o r l a t e n t heat of fusion,
Lay have r e s u l t e d i n values f o r Ta ranging from
Ta = Tc ( t h e 1685 K m e l t i n g temperature o f c - S i ) t o Ta as much as 500 K below Tc (Bagley and Chen, 1979; Spaepen and Turnbull , 1979; Baeri e t a1
., 1980;
Knapp and Picraux, 1981; Olson e t al.,
.,
1983a,
1983). Lowndes and Wood r e c e n t l y i n v e s t i b; and Donovan e t a1 gated whether an a l t e r n a t i v e approach, using a combination o f timeresolved r e f l e c t i v i t y measurements and model c a l c u l a t i o n s , could provide additional
i n f o r m a t i o n about
e i t h e r t h e thermodynamic
parameters o r thermophysical p r o p e r t i e s o f a-Si (such as i t s thermal c o n d u c t i v i t y , Ka), o r h e l p t o i l l u m i n a t e t h e reasons f o r t h e wide range o f values o f Ta obtained i n previous studies. p r i n c i p a l r e s u l t s o f t h e i r work was t o measure Ka.
One o f t h e By comparing
time-resolved model c a l c u l a t i o n s w i t h t i m e o f onset-of-me1 t i n g ( t m ) measurements (see t h e preceding s e c t i o n ) , they were able t o demo n s t r a t e t h a t both pulsed l a s e r m e l t i n g o f ion-implanted a-Si
, and
t h e subsequent r e s o l i d i f i c a t i o n process i n p a r t i a l l y molten a-Si l a y e r s a t higher En,
are r e l a t i v e l y i n s e n s i t i v e t o t h e values o f
Ta and La; instead, both processes are dominated by t h e low value o f Ka.
F i g u r e 7 shows measurements o f t,,, f o r a- and c-Si as a funct i o n o f .El
The data were obtained by simultaneously m o n i t o r i n g
t h e time e v o l u t i o n o f t h e r e f l e c t i v i t y signal and o f an i n c i d e n t 532-nmY 18-nsec (FWHM) l a s e r pulse, and d e f i n i n g t m as the time when t h e HRP i s f i r s t reached d u r i n g t h e pulse. I n contrast t o t h e behavior o f c-Si, a 100-nm t h i c k a-Si l a y e r was found never t o m e l t as l a t e as t h e center o f t h e l a s e r pulse, even a t low E;l f o r high
El t h e a-Si l a y e r melts w i t h i n t h e f i r s t few nanoseconds
o f t h e pulse.
6.
TIME-RESOLVED MEASUREMENTS
337
> t (I) z W
n
0 L W
z W
t&--J
0.0 O.l-6
-4 -2 0
2
4
8
0
10 12 14
ONSET OF MELTINQ (nrec)
Fig. 7. Measured times of onset o f melting ( t m ) for c-Si (solid circles) and for a-Si (open circles). Also shown a r e the results o f model calculations o f t, (see t e x t ; Lowndes et a l . , 1 9 8 4 a ) .
As was shown above f o r c-Si, m e l t i n g model c a l c u l a t i o n s o f t, a r e q u i t e s e n s i t i v e ( e s p e c i a l l y near t h e m e l t i n g t h r e s h o l d ) t o t h e values assumed f o r o p t i c a l o r thermal p r o p e r t i e s i n t h e s o l i d phase, p r i o r t o melting.
Fig. 7 a l s o shows t h e r e s u l t s o f model calcu-
l a t i o n s o f t m vs En f o r a 100-nm t h i c k i o n - i m p l a n t e d a-Si l a y e r These c a l c u l a t i o n s were c a r r i e d o u t on a (100) c-Si s u b s t r a t e . u s i n g as o p t i c a l p r o p e r t i e s f o r a-Si a r e f l e c t i v i t y R = 0.42 and absorption c o e f f i c i e n t a = 5 x
lo5
cm-1,
independent o f
T i n the
s o l i d phase; when m e l t i n g o f t h e f i r s t c e l l (-40 nm) i n t h e f i n i t e d i f f e r e n c e c a l c u l a t i o n s occurred, R was increased t o 0.7 and a t o l o 6 cm-’. E x p l o r a t o r y c a l c u l a t i o n s were made with many d i f f e r e n t values of La and Ta o f a-Si
, but
t h e base1 i n e c a l c u l a t i o n s assumed
La = 1320 J / g and Ta = 115OoC, i n c l o s e agreement w i t h t h e r e c e n t r e s u l t s o f Donovan e t a l . (1983).
338
D. H. LOWNDES E T A L .
The i n i t i a l c a l c u l a t i o n s assumed t h a t Ka o f a-Si was c o n s t a n t w i t h T and equal t o t h e high-temperature c r y s t a l l i n e value, K, = 0.25 W/cm-K, b u t disagreement between t h e measured and c a l c u l a t e d values o f t m showed t h a t t h i s assumption c o u l d n o t be c o r r e c t . Ka was t h e n s u c c e s s i v e l y h a l v e d u n t i l i t was i n t h e range 0.01-0.03 W/cm-K, whereupon s a t i s f a c t o r y agreement w i t h experiment was f i n a l l y obtained.
The v a l u e 0.02 W/cm-K was t h e n used as a b e s t e s t i m a t e
o f Ka i n c a l c u l a t i o n s (see below) i n which values o f o t h e r parameters were v a r i e d ; t h i s Ka v a l u e r e p r e s e n t s an average over t h e weakly T-dependent Ka (T) expected f o r an amorphous semiconductor between 300 K and Ta.
The agreement t h a t can be o b t a i n e d between e x p e r i -
ment and c a l c u l a t i o n s o f t m vs E l i s shown i n Fig. 7 by t h e two s o l i d curves f a l l i n g w i t h i n o r c l o s e t o t h e measured d a t a f o r a-Si and c-Si.
The dashed c u r v e i n t h e c-Si r e g i o n shows t h e r e s u l t s
o f c a l c u l a t i o n s u s i n g Ta = 1150°C and La = 1320 J/g, b u t Ka = Kc. I t i s apparent t h a t t h e r e d u c t i o n o f La and Ta from t h e c o r r e -
sponding c r y s t a l l i n e values (Lc = 1799 J / g and Tc = 1410°C) has a r e l a t i v e l y minor e f f e c t on tm(E1).
As another example, t h e dashed
c u r v e i n t h e a-Si r e g i o n o f Fig.
7 was o b t a i n e d by p u t t i n g Ta =
Tc = 1410°C and La = 1320 J/g, and keeping Ka = 0.02 W/cm-K.
These
r e s u l t s p r o v i d e c o n v i n c i n g evidence f o r t h e dominant r o l e o f Ka i n d e t e r m i n i n g tm.
The v a l u e Ka = 0.02 W/cm-K f o r i m p l a n t a t i o n -
arnorphized S i i s i n good agreement w i t h t h e v a l u e Ka (293 K) = 0.026 W/cm-K f o r a 1.15-pin-thick d e p o s i t e d f i l m o f a-Si on sapphire (Goldsmid e t al.,
1983) and w i t h t h e e s t i m a t e o f Ka
- 10-2 W/cm-K,
i n f e r r e d from measurements o f t h e m e l t i n g t h r e s h o l d E l o f a-Si l a y e r s (Webber e t a1
., 1983).
Lowndes e t a1
mated a l a t t i c e thermal c o n d u c t i v i t y , t r u m f o r a-Si, 2.
. (1984a)
also e s t i -
u s i n g a Debye phonon spec-
of Ka (300 K) = 0.011 W/cm-K.
ELECTRICAL CONDUCTANCE The d u r a t i o n o f m e l t i n g and t h e v e l o c i t y o f t h e r e c r y s t a l l i z i n g
i n t e r f a c e produced by p u l s e d - l a s e r i r r a d i a t i o n o f s i l i c o n have been i n f e r r e d from t i m e - r e s o l v e d r e f l e c t i v i t y measurements and frommodel
6.
339
TIME-RESOLVED MEASUREMENTS
calculations (see F i g . 6, Chapter 4 ) . However, more d i r e c t measurements of t h e velocity of the liquid-solid i n t e r f a c e and of the t o t a l depth of molten s i l i c o n have been carried out using a timeresolved e l e c t r i c a l conductivity technique (Galvin et a1 1982, 1983, 1984; Thompson et a1 1982, 1983a, b, 1984; Thompson and Gal vi n , 1983). The original method used by Galvin e t a l . (1982, 1983) was t o measure the conductance of a s i l i c o n bar during and a f t e r uniform i r r a d i a t i o n of t h e bar by a 30-nsec ruby l a s e r pulse, taking advantage of the f a c t t h a t Si undergoes a l a r g e decrease in e l e c t r i c a l r e s i s t i v i t y upon melting. Low l i f e t i m e Au-doped s i l i c o n was used t o prevent photoconductance due t o photogenerated f r e e c a r r i e r s from dominating the sample conductance f o r long times a f t e r i r r a diation. By using a sample with a l a r g e length/width r a t i o , t h e resistance of t h e sample can be made much l a r g e r than the contact resistance (-2-3 Q ) , even a t the time of maximum melt depth. After t h e photoconductance has decayed (-135 nsec a f t e r i r r a d i a t i o n , see Fig. 8) the sample conductance G a (w/l) ( d l p ) , where w and 1 a r e t h e width and length of the sample, d i s the depth of the molten layer, and p i s the r e s i s t i v i t y of molten s i l i c o n (80p~-crn). Because of the steep temperature gradient near t h e recrystal 1 i zi ng i n t e r f a c e , thermally generated f r e e c a r r i e r s in the s o l i d s i l i c o n contribute l e s s than a 10% correction t o the conductance of the molten layer (Galvin et a1 , 1982). T h u s , a plot of sample conductance vs time can be scaled t o give melt depth v s time, with t h e velocity of the liquid-solid i n t e r f a c e during r e c r y s t a l l izat i o n given by t h e slope of the plotted curves. For i r r a d i a t i o n with El l e s s than the melting threshold (-0.9 J/cm2 i n these experiments) t h e current t r a n s i e n t s observed by Galvin e t a1 were nearly i d e n t i c a l , showing only the photoconduct i v e response. However, f o r El > 0.9 J/cm2 the current t r a n s i e n t s broadened in time, r e s u l t i n g in time-resolved sample conductances ( o r melt depths) t h a t were deconvoluted from these current trans i e n t s as shown in Fig. 8. The v e r t i c a l broken l i n e a t 135 nsec
.,
.
.
.,
340
D. H. LOWNDES E T A L .
0.8 0.10
-
0.6
0.08
ut
-2 ut
E
v
c
e .-
2 W
- 0.4
0.06
a
0.04 z"
0
E
I-
I t-
V 3
W
0
8
0.2 0.02
0
0
0.8 0.10
-
0.6
0.08 ;
In
0
r E
c
e --
- 0.4 0
0.06
E
1
0 Z
a
0.04 0
z
0.2
0
8 0.02 0
100
200 TIME (ns)
300
0 400
Fig. 8. ( a ) Conductance vs time from experiments o f Galvin e t al. ( 1 9 8 2 ) . The depth scale i s derived by assuming the conductance i s determined only by ( b ) Calculated curves, based on a thermal heat the conductivity o f molten Si. f l o w model, o f the conductance o f liquid and near-interface solid S i (upper The dashed line i s the experimental curve) and liquid Si only (lower curve). curve for En = 2.6 j / c m 2 .
6.
341
TIME-RESOLVED MEASUREMENTS
shows t h e approximate d u r a t i o n o f photoconductance; times t h e measured conductance decreases time. G a l v i n e t a1
. (1982,
f o r longer
n e a r l y 1in e a r l y w i t h
1983) a l s o c a r r i e d out numerical c a l c u l a -
t i o n s [ s i m i l a r t o those o f Wang e t a l . (1978), Wood and G i l e s (1981) and Baeri e t a l . (1978, 1979)] o f t h e time-dependent conductance o f molten s i l i c o n (excluding photoconductance) f o r comparison w i t h t h e i r experimental curves.
Fig. 8 ( b ) shows t h e c a l c u l a t e d conduc-
tance ( o r m e l t depth) o f molten s i l i c o n w i t h t h e i n c i d e n t power
(2.5 J/cm2) chosen so t h a t t h e melt depth matched t h a t o f t h e experimental data a t 2.6 J/cm2. The e f f e c t o f i n c l u d i n g t h e cond u c t i v i t y due t o t h e r m a l l y generated c a r r i e r s i n t h e s o l i d s i l i c o n , below t h e l i q u i d - s o l i d i n t e r f a c e , was also estimated (Fig. 8(b)) w i t h t h e r e s u l t t h a t t h e i r conductance c o n t r i b u t i o n was equivalent t o a molten s i l i c o n thickness o f o n l y about 50 nm.
Thus, t h e con-
ductance c a l c u l a t i o n s appear t o g i v e a good estimate of actual melt depths.
Since t h e slopes o f t h e two c a l c u l a t e d curves i n
Fig. 8(b) are n e a r l y p a r a l l e l , t h e c a l c u l a t i o n s should a l s o g i v e a good estimate o f regrowth v e l o c i t y .
The regrowth v e l o c i t y c a l -
c u l a t e d from t h e lower curve i n Fig. 8(b) i s 2.7 m/sec,
i n good
agreement w i t h t h e slope o f t h e experimental curve a t 2.6 and w i t h t h e average experimental value o f 2.8 m/sec f o r 1.9
<
2.6 J/cm2.
J/cm2
<
El
The regrowth v e l o c i t y i s determined by t h e tempera-
t u r e gradient a t t h e growing m e l t - s o l i d i n t e r f a c e , a c t u a l l y a f u n c t i o n o f time and o f l a s e r E,
and so i s
and pulse duration.
G a l v i n e t a l . (1983) have shown t h a t f o r times l o n g compared w i t h t h e pulse d u r a t i o n (so t h a t t h e temperature gradient i s determined by thermal d i f f u s i o n ) t h e v e l o c i t i e s measured a t various times from conductance data are i n e x c e l l e n t agreement w i t h model c a l c u l a t i o n s , decreasing w i t h i n c r e a s i n g time a f t e r i r r a d i a t i o n . Both t h e pulsed l a s e r c o n d i t i o n s and t h e samples used f o r t r a n s i e n t conductance measurements have undergone s u b s t a n t i a l changes since t h e i n i t i a l experiments, i n order t o be able t o measure v a r i a t i o n s i n t h e m e l t - s o l i d i n t e r f a c e v e l o c i t y d u r i n g m e l t - i n
342
D. H. LOWNDES ET AL
and s o l i d i f i c a t i o n , and i n order t o d i r e c t l y measure t h e t h r e s h o l d
v e l o c i t y a t which t h e laser-induced 1iquid-amorphous phase t r a n s i t i o n ( L i u e t al.,
1979) o f s i l i c o n occurs.
discussed i n Sec. V.10).
(The l a t t e r t o p i c i s
Silicon-on-sapphire
(SOS) samples were
used f o r these studies because t h e very s h o r t photoconductive l i f e t i m e (-200 psec) o f c a r r i e r s i n SOS permits t h e e n t i r e m e l t - i n and s o l i d i f i c a t i o n process t o be d i r e c t l y observed (Thompson e t a1 1982, 1983a, b).
.,
Photolithographic patterning o f s i l i c o n resis-
t i v e channels i n t h e t y p i c a l l y 0 . 5 - p
thick S i films results i n
l a r g e l e n g t h / w i d t h r a t i o s (50-loo), w h i l e c o n f i n i n g t h e sample t o an area o f 1 mn2.
Thus, contact r e s i s t a n c e c o r r e c t i o n s are made
n e g l i g i b l e and homogeneity requirements on t h e l a s e r beam are minimized.
The use o f o n l y -0.5 p t h i c k S i f i l m s causes i n t e r f e r -
ence e f f e c t s t o s t r o n g l y modify t h e absorption o f 694 nm ruby l a s e r r a d i a t i o n (Thompson e t a1
., 1982);
however, by using UV r a d i a t i o n
from a frequency-doubled (347 nm) and pul se-chopped (2.5-10
nsec
pulse d u r a t i o n ) ruby l a s e r t h i s problem can be avoided. Thompson e t a l .
(1983a) p o i n t out t h a t under short-pulse UV
i r r a d i a t i o n t h e m e l t - i n processes should be very s i m i l a r i n SOS and i n b u l k S i , since both t h e UV absorption l e n g t h (a few tens o f nm) and t h e thermal d i f f u s i o n l e n g t h ( 4 . 4 pm i n 10 nsec a t 1685 l e s s than t h e f i l m thickness.
K) are
On t h e o t h e r hand, when using 694-nm
ruby l a s e r r a d i a t i o n , regrowth v e l o c i t i e s are found t o be s i m i l a r f o r SOS and bulk S i , as a r e s u l t o f a c o i n c i d e n t a l competition between t h e o p t i c a l and thermal p r o p e r t i e s o f t h e sapphire substrate.
Transient conductance measurements on SOS using a 25-nsec,
694-nm ruby l a s e r pulse allowed t h e e n t i r e m e l t - i n and e p i t a x i a l regrowth process t o be observed (Thompson e t al.,
1983b); m e l t - i n
v e l o c i t i e s were found t o be 5-13 m/s ( i n c r e a s i n g l i n e a r l y w i t h l a s e r Ex u n t i l melt-through occurred) w h i l e s o l i d i f i c a t i o n veloci t i e s were 2.8-3.3
m/s (decreasing w i t h i n c r e a s i n g Ex).
An approximately l i n e a r increase o f m e l t - i n v e l o c i t y w i t h i n c i dent l a s e r
El was observed i n
(1983a) using 2.5-nsec
SOS by Thompson and co-workers
pulses o f 347-nm r a d i a t i o n .
This l i n e a r
6.
343
TIME-RESOLVED MEASUREMENTS
proportionality resulted in a nearly constant time a t which the maximum melt depth was reached (see Fig. 9 ) , corresponding t o melt-in velocities as h i g h as 40 m/s. To obtain s t i l l higher melt-in velocities, Thompson e t a l . (1983a) used 2.5-ns pulses of the fundamental (694 nm) ruby wavelength. The longer absorption length a t 694 nm produces a shallower temperature gradient near the surface, both "pre-heating" deeper regions of the bulk SOS and reducing heat conduction into the bulk. Combined with the higher pulse energies available at 694 nm, t h i s allowed melt-in velocities as h i g h as 200 m/s t o be obtained. From their 200 m/s maximum melt-in velocity measurement, Thompson e t al. (1983a) inferred a minimum thermal conductivity for molten Si of 1.1 W/cm-K. (This i s presumably a temperature-averaged value.) Since transient conductance measurements provide continuous monitoring of melt depth vs time, they make i t possible, in p r i n ciple, t o measure variations in the velocity of the regrowing 0.16
0.10 I
n m
" !i c
HJ 0
0.05
0.00
0
10
20
30 TIME
40
50
(ns)
Fig. 9. Current transients and depth o f melting for 2 . 5 n s e c , 347-nm irradiation of SOS. The photolithographically patterned sample is shown in the inset (Thompson e t a t . , 1983a).
344
D. H. LOWNDES ET AL
liquid-solid interface as a function of depth. This possibility was recently exploited by Galvin e t al. (1984) i n order t o carry out direct measurements of the effects of various solute atoms on the regrowth velocity. The incorporation of solute atoms i n t o a growing crystal depends strongly on the velocity, Vi, o f the liquid-solid interface. The segregation coefficient, k i (the ratio of the solute concentration in the solid t o t h a t i n the liquid a t the interface), provides a measure of the departure from equilibrium growth conditions (see CHapter 5 for a detailed discussion). ki values inferred from solute redistribution profiles following pulsed laser melting of ion-implanted silicon, a t regrowth velocities of several m/s, can exceed equilibrium k i values by several orders of magnitude and increase w i t h increasing V i (White e t a1 , 1980; Wood e t a1 1981a). A number of studies have related k i t o Vi b u t have gene r a l l y used thermal melting model calculations of V j for pure s i 1 icon, without considering the effects of the solute atoms on Vi i t s e l f . I n contrast, very recent measurements by Galvin e t al. (1984) appear t o show t h a t V i depends on both the type and the concentrat i o n of the solute atoms present, V i being substantially reduced ( r e l a t i v e t o pure S i ) for solute concentrations >, a few atomic percent. Their measurements were carried out on photo1 i thographically patterned SOS samples t h a t were doped by ion implantation; the samples *re recrystallized by pulsed laser annealing, prior t o the Vi measurements, i n order t o clearly separate solute atom effects on Vi from other effects associated w i t h solidification of laser-irradiated amorphous Si (see Section V). Transient conductance measurements on subsequent laser s h o t s were used t o o b t a i n Vi vs depth; separate backscattering measurements of solute atom depth profiles, between successive laser shots, made i t possible t o obtain V i as a function of solute concentration. A reduction of Vi by 25 (18)%was observed for an In concentration of 3.0 (1.5)
.
.,
6.
345
TIME-RESOLVED MEASUREMENTS
a t . %; f o r 0, v i r e d u c t i o n s o f 18 and 28% r e s u l t e d from 0 concen-
%, r e s p e c t i v e l y . Similar effects have been observed f o r As, Gay and N ( G a l v i n e t a1 , 1984). An
t r a t i o n s o f 7.2
and 14.6
at.
.
e x p l a n a t i o n f o r these s u b s t a n t i a l r e d u c t i o n s i n
i s not available
Vi
a t t h i s time, b u t t h e reduced m e l t i n g p o i n t o f t h e a l l o y ( i m p l y i n g s m a l l e r undercool i n g , r e l a t i v e t o pure S i ) , t h e i n c r e a s e i n e n t h a l p y o f m i x i n g ( i n c r e a s i n g t h e l a t e n t heat o f f u s i o n ) and t h e p o s s i b l e presence o f " s o l u t e drag" (Aziz, 1983) a l l may c o n t r i b u t e ( G a l v i n e t a1
., 1984).
Ifthese measurements o f a s u b s t a n t i a l s o l u t e con-
c e n t r a t i o n dependence o f
V i
are confirmed by f u t u r e experiments,
and a l s o shown t o be a general phenomenon f o r lower s o l u t e concent r a t i o n s , then t h e
Vi
dependence o f k i w i l l need t o be re-examined,
t a k i n g i n t o account t h e e f f e c t o f t h e s o l u t e on
V i e
Transient
conductance measurements p r o v i d e a means f o r c a r r y i n g o u t such a study. I n summary, t h e s i m p l i c i t y o f i n t e r p r e t a t i o n o f t h e t r a n s i e n t conductance technique p r o v i d e s s t r o n g c o n f i r m a t i o n o f t h e b a s i c thermal m e l t i n g aspect o f pulsed l a s e r annealing on t h e nanosecond timescale.
Moreover,
t h e r e s u l t s o b t a i n e d t o d a t e demonstrate
t h a t t h e technique p r o v i d e s a powerful means f o r observing l i q u i d s o l i d i n t e r f a c e motion under a wide v a r i e t y o f r a p i d quenching c o n d i t i o n s and f o r a wide range o f specimens.
I t s use r e q u i r e s
o n l y a m a t e r i a l w i t h a s h o r t f r e e - c a r r i e r l i f e t i m e and a substant i a l i n c r e a s e o r decrease i n c o n d u c t i v i t y upon m e l t i n g (e.g.,
some
m e t a l s ) . The technique w i l l c l e a r l y be used t o study t h e i n t e r r e 1 a t ions h i ps o f sol Ute s e g r e g a t i o n and t r a p p i ng , me1t undercool ing and thermodynamic e f f e c t s , d u r i n g r a p i d s o l i d i f i c a t i o n o f semicond u c t o r s f o l l o w i n g pulsed l a s e r m e l t i n g .
However,
r e a l l y i m p l i e s no r e s t r i c t i o n t o pulsed l a s e r s :
t h e technique
Indeed, t r a n s i e n t
conductance measurements have a1 ready been used t o measure me1t depths, m e l t d u r a t i o n s , and regrowth v e l o c i t i e s d u r i n g pulsed p r o t o n beam annealing o f i o n - i m p l a n t e d S i (Fastow e t al., 1983). S i m i l a r l y , e x t e n s i o n s o f t h e t e c h n i q u e t o more concentrated serniconductor a l l o y s , and t o l a y e r e d semiconductor s t r u c t u r e s , can be
346
D.H. LOWNDES E T A L
anticipated.
Finally, it i s only a matter o f time u n t i l s i m i l a r
t r a n s i e n t conductance measurements are c a r r i e d out f o r metal s and metal a l l o y s f o r which t h e r e i s a s i g n i f i c a n t d i f f e r e n c e between t h e l i q u i d and s o l i d phase e l e c t r i c a l c o n d u c t i v i t i e s . 3.
SYNCHROTRON X-RAY DIFFRACTION Time-resol ved x-ray d i f f r a c t i o n p r o v i d e s a d i r e c t means f o r
p r o b i n g t h e near-surface
structure o f a crystal l a t t i c e during
pulsed l a s e r i r r a d i a t i o n .
Using t h i s method, t h e temperature can
be measured t h r o u g h thermal expansion, and t h e onset o f m e l t i n g and r e c r y s t a l 1 i z a t i o n can be monitored t h r o u g h t h e disappearance and reappearance o f Bragg d i f f r a c t e d i n t e n s i t y f r o m t h i s region. The requirement o f
nanosecond r e s o l u t i o n i n such measurements
prevented t h e use o f c o n v e n t i o n a l x-ray sources f o r such measurements;
however,
Larson and co-workers
(1982a,
b;
1983a) demon-
s t r a t e d t h a t s y n c h r o t r o n x-ray pulses c o u l d be used t o c a r r y o u t nanosecond r e s o l u t i o n x-ray d i f f r a c t i o n on s i l i c o n d u r i n g pulsed l a s e r annealing.
These s y n c h r o t r o n x-ray measurements were t h e
f i r s t nanosecond r e s o l u t i o n s t u d i e s o f t r a n s i e n t s t r u c t u r a l phenomena i n c r y s t a l s ,
and t h e y p r o v i d e d t h e f i r s t depth-resolved
informat ion on s i 1 i c o n d u r i ng pul sed 1aser anneal ing
.
These mea-
surements y i e l d e d depth-dependent thermal s t r a i n p r o f iles, from which l a t t i c e temperature p r o f i l e s were i n f e r r e d f o r t i m e s b o t h d u r i n g and a f t e r r u b y l a s e r i r r a d i a t i o n , and measurements on boroni m p l a n t e d s i l i c o n showed s i l i c o n l o s e s c r y s t a l l i n i t y d u r i n g t h e anneal ing process. The experiments o f Larson e t a l .
(1982a, b; 1983a) were c a r -
r i e d o u t u s i n g t h e C o r n e l l High Energy Synchrotron Source (CHESS), which p r o v i d e d monochromatic p r o b i n g pulses o f 1.5-8 x-rays. Time r e s o l u t i o n was o b t a i n e d by s y n c h r o n i z i n g t h e l a s e r such t h a t t h e p r o b i n g x-ray pulses reached t h e c r y s t a l s u r f a c e a t d e s i r e d times a f t e r t h e a r r i v a l o f ruby l a s e r pulses (694-nm wavelength, 15-nsec
6.
347
TIME-RESOLVED MEASUREMENTS
LASER-SYNCHROTRONTIMING SCHEME
SVNCH
MONITOR
Fig. 10. Schematic view of the iaser-synchrotron timing scheme used by Larson e t at. (1982a, b).
d u r a t i o n ) , as i s shown s c h e m a t i c a l l y i n Fig. 10.
The t i m e d e l a y
between t h e a r r i v a l o f t h e l a s e r and x-ray pulses was c o n t r o l l e d t o a p r e c i s i o n o f 25 nsec.
The s h o r t d u r a t i o n o f t h e s y n c h r o t r o n
x - r a y pulses ( 4 . 1 5 nsec) allowed t h e use o f p u l s e - h e i g h t a n a l y s i s t o determine t h e number o f s c a t t e r e d x-rays per pulse, and repeated measurements were made a t each t i m e d e l a y p o i n t t o improve t h e s t a t i s t i c a l precision. The p r i n c i p l e o f t h e measurement from t h e d i f f r a c t i o n standp o i n t i s i l l u s t r a t e d i n Fig. 11.
The >20-p p e n e t r a t i o n depth o f
1.5 A x-rays i n s i l i c o n a l l o w s x-rays t o probe cornpletely t h r o u g h t h e near-surface
r e g i o n i n v o l v e d i n t h e l a s e r annealing.
The
e f f e c t o f n e a r - s u r f a c e thermal s t r a i n on x-ray s c a t t e r i n g from p u r e c-Si i s represented i n t h e m i d d l e panel o f Fig. 11 where l o c a l Bragg s c a t t e r i n g from t h e s t r a i n e d r e g i o n o f t h e c r y s t a l r e s u l t s i n an angular e x t e n s i o n o f t h e normal Bragg s c a t t e r i n g .
Positive
s t r a i n (heating) gives r i s e t o a d i s t r i b u t i o n o f s c a t t e r i n g a t n e g a t i v e AO, g i v e n by AO = -Etan(oB)
where
E
=
(1)
E ( D ) i s t h e s t r a i n , D i s t h e depth i n t h e sample, and oB
i s t h e Bragg angle.
348
D. H. LOWNDES ETAL.
STRAIN SCATTERING
SCATTERING
PURE SILICON (SCHEMATIC)
h
1.5 c .v)
5
1.0
L z1
e
c
?? 0.5 0
v
> k $
W
0
I-
z
.
DEPTH (pm)
BORON IMPLANTED SILICON ( SCH EM AT I C 1
0 f.5 w I-
0
a a
-
f.0
0
0.5 -
0
I
0 -2000
./
/
/
I
I
-4000
0
A9
1000 (sec)
2000
3000
Fig. 11. Illustration of the x-ray scattering expected from pure silicon and from boron-implanted silicon with thermally induced strain distributions superimposed (Larson et al., 1982b).
6.
349
TIME-RESOLVED MEASUREMENTS
The bottom panel i n Fig. 11 i l l u s t r a t e s a method f o r " t a g g i n g " t h e s u r f a c e l a y e r o f s i l i c o n t h r o u g h boron doping.
Since boron
c o n t r a c t s t h e s i l i c o n l a t t i c e (Larson and Barhorst,
1980),
the
s c a t t e r i n g from t h e boron-doped s u r f a c e l a y e r appears a t p o s i t i v e
AO, and as such can be monitored s e p a r a t e l y f r o m t h e s u b s t r a t e s c a t tering.
Thermal s t r a i n superimposed on t h e boron c o n t r a c t i o n l e a d s
t o a s h i f t i n t h e p o s i t i o n o f t h e s c a t t e r i n g from t h e boron-doped l a y e r and, i n t h e case o f s u r f a c e m e l t i n g , t h e Bragg s c a t t e r i n g f r o m t h e boron-doped l a y e r vanishes a l t o g e t h e r . The a n a l y s i s o f t h e x-ray measurements on pure c-Si was c a r r i e d out by i t e r a t i v e l y f i t t i n g x - r a y s c a t t e r i n g c a l c u l a t i o n s based on t h e t h e o r y o f s c a t t e r i n g f r o m c r y s t a l s with one-dimensional s t r a i n (Larson and Barhorst, 1980) t o t h e measured x-ray d a t a w i t h t h e s t r a i n p r o f i l e as t h e f i t t i n g parameter. were
Temperature p r o f i l e s
subsequently deduced from t h e s t r a i n p r o f i l e s u s i n g t h e
e x p r e s s i o n (Larson e t al. 1983a,b) E
=
T
J
va(T) dT
TO
where T i s t h e temperature r e q u i r e d t o a t t a i n a s t r a i n
E
relative
t o t h e l a t t i c e a t t h e ambient temperature To and a(T) i s t h e temperature-dependent thermal expansion c o e f f i c i e n t .
For t h e case
o f t h i n heated l a y e r s on t h i c k s u b s t r a t e s , t h e heated l a y e r i s n o t a b l e t o expand l a t e r a l l y because o f t h e c o n s t r a i n t of t h e unheated s u b s t r a t e ; t h e r e f o r e , t h e f a c t o r T) i s i n c l u d e d t o account f o r t h e enhanced expansion normal t o t h e surface. The enhancement f a c t o r i s a f u n c t i o n o f t h e e l a s t i c c o n s t a n t s (Fukahara and Takano, 1977) and i s g i v e n by orientations,
T)
=
1.69
and 11 = 1.34 f o r t h e
and <111>
respectively.
Larson e t a l . (1982a, b ) f i r s t r e p o r t e d temperature p r o f i l e s i n l a s e r - i r r a d i a t e d s i l i c o n f o r times a f t e r t h e HRP; t h e y l a t e r (Larson e t al.,
1983a, b) r e p o r t e d d e t a i l e d measurements i n which
temperature p r o f i l e s were obtained d u r i n g , as w e l l as a f t e r , t h e
350
D. H. LOWNDES E T A L .
HRP.
Fig. 12 shows x-ray measurements o f Larson e t al. (1983a, b)
made a t t h e (111) r e f l e c t i o n o f c-Si d u r i n g t h e HRP induced by 1.5-J/crn2
ruby l a s e r pulses o f 15-nsec
(FWHM) pulse duration.
( S i m i l a r measurements were made using t h e (400) r e f l e c t i o n but are n o t shown here.)
The times were reckoned from t h e center o f t h e
l a s e r pulse so t h a t t h e 20-nsec measurements correspond t o a time j u s t a f t e r t h e end o f t h e l a s e r pulse and t h e 55-nsec measurements correspond t o a time approximately midway through t h e HRP phase o f t h e laser-annealing cycle. -100 nsec f o r 1.5-J/cm2
The HRP l i f e t i m e was measured t o be
l a s e r pulses.
I n a l l cases, s i z a b l e t h e r -
mal expansions were i n f e r r e d from t h e existence o f s c a t t e r i n g a t negative 80, and according t o Larson e t a1
., t h e
higher i n t e n s i -
t i e s measured a t 55 nsec, r e l a t i v e t o t h e i n t e n s i t i e s a t 20 nsec, x
1500
1200
g> 900 -
5 W -I
600
[L
A6 fs) Fig. 12. o f a 1 5-nsec,
al.,
Measured x-ray scattering a t 2 0 , 5 5 , and 1 5 5 nsec a f t e r the center 1.5-1 /cm2 ruby laser pulse for the Si ( 1 1 1 ) reflection (Larson e t
1 9 8 3 a , b).
6.
351
TIME-RESOLVED MEASUREMENTS
imply lower temperature g r a d i e n t s d u r i n g t h e regrowth phase than a t t h e end o f t h e l a s e r pulse. F i g u r e 13 shows time-resolved
temperature p r o f i l e s obtained
through a n a l y s i s o f t h e x-ray measurements i n Fig. 12.
The analyses
in c l uded t h e Debye-Wal 1 e r factors and t h e measurement r e s o l u t i o n (Larson e t a1
., 1983a,
b),
and they were c a r r i e d out under t h e
c o n s t r a i n t o f f i t t i n g both t h e (400) and (111) measurements w i t h t h e same s t r a i n p r o f i l e s .
We see d i r e c t l y t h a t t h e l a t t i c e tem-
peratures reached t h e m e l t i n g p o i n t (141OOC) o f s i l i c o n d u r i n g t h e l a s e r pulse and t h a t t h e temperature o f t h e l i q u i d - s o l i d i n t e r f a c e remained a t t h e me1t i n g p o i n t d u r i n g t h e anneal i n g process. a b i l i t y t o f i t t h e data f o r both t h e
The
and <111>o r i e n t a t i o n s
w i t h t h e same temperature p r o f i l e s suggests t h a t t h e physical processes and t h e thermal gradients are s i m i l a r f o r t h e two o r i e n t a t i o n s under these conditions.
1400
1.5 J/cmz o t, = 20 ns t, = 55 ns A t, = 155 n s Silicon
1200 1000 n
,u
W
800
600 400
200 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
DEPTH (pm)
1.4
1.6
1.8
2.0
Fig. 13. Time-resolved temperature profiles corresponding t o the x-ray measurements in Fig. 1 2 (Larson et at., 1983a, b).
352
D. H. LOWNDES E T A L
F i g u r e 12 a l s o shows x-ray measurements made on <111> s i l i c o n about 55 nsec a f t e r t h e end o f 155 nsec a f t e r l a s e r pulses (i.e., t h e HRP).
The f i t t o these data y i e l d e d the 155-nsec temperature
p r o f i l e shown i n Fig. 13 where t h e -1050°C surface temperature and t h e low temperature g r a d i e n t s i n d i c a t e surface c o o l i n g and heat f l o w i n t o t h e bulk, as would be expected f o l l o w i n g r e s o l i d i f i c a t i o n o f a melted surface l a y e r .
The 155-nsec measurements f o r 1.5 J/cm2
pulses are analogous t o t h e 100-nsec measurements f o r 1.3 J/cm2 pul ses r e p o r t e d i n t h e e a r l i e r work o f Larson e t a1
. (1982a) , i n
t h a t i n both cases the measurements were made s u b s t a n t i a l l y a f t e r t h e end o f t h e HRP.
Surface temperatures -300°C below t h e m e l t i n g
p o i n t were r e p o r t e d i n both cases; however, steeper temperature g r a d i e n t s were reported i n t h e e a r l i e r work (Larson e t al., Larson e t a l .
1982a).
p o i n t e d out t h a t t h e steep temperature gradients
were mainly a r e s u l t o f not i n c l u d i n g Debye-Waller f a c t o r s and measurement r e s o l u t i o n i n t h e o r i g i n a l a n a l y s i s (Larson e t a1 1982a, b; M i l l s e t al.,
.,
1983); they f u r t h e r pointed out t h a t both
experimental measurements and t h e o r e t i c a l c a l c u l a t i o n s y i e l d -75nsec HRP l i f e t i m e s f o r 1.3-J/cm2 1982a , b)
.
ruby l a s e r pulses (Lowndes e t a1
.,
The consistency of t h e thermal s t r a i n i n t e r p r e t a t i o n was t e s t e d i n t h e second experiment o f Larson e t al. (1983a) by comparing t h e amount o f l a s e r energy r e t a i n e d i n t h e s i l i c o n w i t h t h e thermal energy i n t h e temperature p r o f i l e s i n Fig. 13. By measuring t h e amount o f energy r e f l e c t e d from t h e s i l i c o n surface as well as t h e amount o f energy i n c i d e n t on the sample, Larson e t a l . t h a t 0.58 J/cm2 o f t h e 1.5-J/cm2
inferred
pulses was absorbed; on t h e other
hand, thermal energy d e n s i t i e s i n t h e temperature p r o f i l e s o f Fig. 13 were found t o be 0.26,
0.45,
and 0.57
(20.10) J/cm2 f o r t h e 20-,
55-, and 155-nsec measurement times, r e s p e c t i v e l y .
Recognizing t h a t
molten l a y e r s would not c o n t r i b u t e t o Bragg s c a t t e r i n g , t h e energy s h o r t f a l l i n t h e 20- and 55-nsec measurements was a t t r i b u t e d t o energy contained i n molten surface l a y e r s o f 4 . 4
and -0.2
pm,
6.
TIME-RESOLVED MEASUREMENTS
353
respectively. These me1 t depths , a1 though quite approximate, a r e e n t i r e l y consistent w i t h the melt-front penetration values calcul a t e d (Lowndes et a1 , 1982a, b ) using the thermal melting model and shown in Fig. 6 of Chapter 4. The 155-nsec temperature p r o f i l e , which corresponds t o a time when no melted layer should exist, accounts f o r a l l of t h e absorbed energy. The -lo7 K/cm temperature gradients a t 55 nsec (Fig. 13) are a l s o consistent with meltingmodel c a l c u l a t i o n s a s they imply a <,7-m/s regrowth velocity (Larson e t a l . , 1983a), which i s near the <5-m/s range calculated f o r experimental conditions similar t o those of the x-ray d i f f r a c t i o n experiments. In order t o d i r e c t l y monitor the s c a t t e r i n g from the surface l a y e r , Larson et a l . (1982a) performed measurements on boronimplanted s i l i c o n u s i n g t h e scheme i n the lower panel of Fig. 11. Both a thermal s h i f t in angle and a vanishing of the s c a t t e r i n g from the boron-doped layer were observed, as expected f o r thermal melting. The angular s h i f t of the s c a t t e r i n g from the boroncontracted l a y e r , 100 nsec a f t e r 1.3-J/cm2 l a s e r pulses, was found t o correspond t o an average temperature of -1100°C over the -0.4pm depth of the boron-doped layer. Although the thermal s h i f t o f the peak gives only a rough estimate of the average temperature o f the boron-doped l a y e r , t h e s h i f t i s consistent with temperatures t h a t reached the melting point and with l a r g e thermal gradients. By monitoring the s c a t t e r i n g from the boron-doped sample a s a function of time i n two angular ranges, A@ = 600-1100 sec (the heated boron peak position) and AO = 2300 sec (the ambient temperature boron peak position) , me1 t i ng of the boron-doped 1ayer The r e s u l t s a r e shown in Fig. 14 in the form of was observed. t h e r a t i o , R, of the s c a t t e r i n g with l a s e r pulses t o the s c a t t e r i n g without l a s e r pulses as a function of time. Larson e t a l . found t h a t the s c a t t e r i n g in the AO = 600-1100 sec range vanished comp l e t e l y a t about 40 nsec a f t e r the l a s e r pulses and recovered rapidly t o a value about twice t h a t without the l a s e r pulses. The
.
354
D. H. LOWNDES E T A L .
3.0
0
CK
2.0
A'
CK
4.0
0 2.0
-
-
CK0
\
-J [r
4.0
!t
0 -
I
1
I
I
A 0 = 2300 sec 2.5X4046 B+/cm2
l? \\b--A-h---j----
I
I
1
I I
It -r----
Fig. 14. Scattering from the boron-implanted t i m e a f t e r laser pulses (Larson e t a l . , 1 9 8 2 b ) .
I -
+. I
layer as a function o f the
vanishing of the i n t e n s i t y was interpreted as a r e s u l t of l a s e r rnelti ng o f the entire boron-doped layer; t h e subsequent increase i n i n t e n s i t y was interpreted in terms of r e s o l i d i f i c a t i o n o f the boron-doped layer. The return t o ambient conditions was indicated by t h e decay of t h e r a t i o t o unity a f t e r about a microsecond. As indicated in t h e bottom o f Fig. 14, the ambient temperature peak (ao = 2300 sec) vanished almost immediately a f t e r the l a s e r pulse. This behavior is consistent with a rapid temperature rise, large temperature gradients, and melting beginning a t t h e surface. I t should be pointed out t h a t thermal melting model calculations f o r pure c-Si predict melting t o a depth -0.27 p f o r 1.3-J/cm2 l a s e r pulses, b u t they predict a melt duration of only -20 nsec f o r
6.
355
TIME-RESOLVED MEASUREMENTS
p e n e t r a t i o n depths g r e a t e r t h a n 0.2 p. (See Fig. 6 o f Chapter 4.) However, h e a v i l y doped c-Si would be expected t o m e l t t o a g r e a t e r depth (and remain melted f o r a l o n g e r t i m e ) due t o i t s h i g h e r a b s o r p t i o n c o e f f i c i e n t a t 694 nm ( J e l l i s o n e t al.,
1981).
I n summary, s y n c h r o t r o n x-ray measurements p r o v i d e d i r e c t e v i dence s u p p o r t i n g t h e thermal m e l t i n g model o f l a s e r annealing. Temperatures r e a c h i n g t h e m e l t i n g p o i n t o f s i l i c o n d u r i n g t h e HRP, t h e temperature g r a d i e n t s , t h e thermal energy w i t h i n t h e temperat u r e p r o f i1es , t h e thermal s h i f t i n g and t h e temporary vani s h i ng o f t h e s u r f a c e s c a t t e r i n g d u r i n g t h e annealing process a l l argue s t r o n g l y f o r t h e thermal m e l t i n g model , and are c o n s i s t e n t w i t h p r e s e n t thermal model heat f l ow c a l c u l a t i ons.
OF LATTICE TEMPERATURE
4.
OPTICAL MEASUREMENTS
a.
Temperature-Induced O s c i l l a t i o n s i n t h e R e f l e c t i v i t y o f T h i n C r y s t a l 1ine F i 1ms Murakami e t a l .
(1981) r e p o r t e d a simple t e c h n i q u e f o r mea-
s u r i n g t h e average s i l i c o n l a t t i c e temperature d u r i n g and a f t e r nanosecond pulsed l a s e r i r r a d i a t i o n o f SOS samples f o r which t h e temperature i s averaged over t h e f i l m t h i c k n e s s . c o n s i s t s o f a time-dependent
T h e i r method
r e f l e c t i v i t y measurement u t i l i z i n g
t h e i n t e r f e r e n c e o f l i g h t r e f l e c t e d from t h e f r o n t ( S i - a i r ) and back ( S i - s a p p h i r e ) surfaces. The c o n d i t i o n f o r t h e appearance o f i n t e r f e r e n c e maxima o r minima i n t h e r e f l e c t e d l i g h t i n t e n s i t y i s
w i t h m = odd (even) f o r r e f l e c t i o n maxima (minima),
where d i s
t h e s i l i c o n f i l m t h i c k n e s s , n(h,T)
i s t h e index o f r e f r a c t i o n o f
silicon,
reflection.
and 0 i s t h e angle o f
experiments
using
static
furnace
heating,
I n preliminary Murakami
et
al.
356
D. H. LOWNDES.ETAL.
measured t h e temperature dependence o f n(h,T) l e n g t h o f a cw HeNe probe l a s e r , length i n silicon, i.e.,
a t t h e 633-nm wave-
f i n d i n g t h a t (h/n),
t h e wave-
becomes s h o r t e r w i t h i n c r e a s i n g temperature,
n increases w i t h temperature. Measurements o f r e f l e c t i v i t y i n t e r f e r e n c e o s c i l l a t i o n s d u r i n g
pulsed l a s e r i r r a d i a t i o n were c a r r i e d out by Murakami e t a l . u s i n g a ruby l a s e r (pulse d u r a t i o n 40-50 nsec) and a 633 nm HeNe probe l a s e r t o monitor t h e changing r e f l e c t i v i t y .
Two d i f f e r e n t
SOS samples were used, w i t h s i l i c o n l a y e r thicknesses o f 0.5
2.0 pm.
and
Using a value o f 0.3 cm2/s f o r t h e thermal d i f f u s i v i t y , D,
o f c - s i l i c o n , t h e energy i n t h e heating l a s e r pulse can d i f f u s e a d i s t a n c e o f order L = ( D t ) ’ / 2
- 1 p d u r i n g t h e 40-nsec
duration
o f t h e heating pulse; thus, a d e s c r i p t i o n i n terms o f an average f i l m temperature, r a t h e r than a temperature p r o f i l e , seems t o be
j u s t i f i e d on t h i s time scale and f o r these f i l m thicknesses. A t low ruby l a s e r EA, i n s u f f i c i e n t t o produce t h e HRP, r e f l e c t i v i t y o s c i l l a t i o n s corresponding t o l a t t i c e heating ( i n c r e a s i n g n) were observed.
For higher Ex, an increase o f r e f l e c t i v i t y t o
t h e 70% value c h a r a c t e r i s t i c o f molten s i l i c o n was observed f o r periods o f 130-400 nsec f o l l o w e d by a drop i n r e f l e c t i v i t y ( t o a value o f 40%) a f t e r which t h r e e r e f l e c t i v i t y maxima and two minima were observed,
over a t i m e p e r i o d extending w e l l beyond a psec
a f t e r l a s e r i r r a d i a t i o n . An was again found t o be p o s i t i v e , corresponding t o l a t t i c e heating, not t h e negative An expected f o r a simple plasma w i t h a Drude-like d i e l e c t r i c f u n c t i o n (Murakami e t a1
., 1981;
see Sec. IV.9).
Combining t h e known temperature depen-
dence o f n, t h e measured maxima and minima, and equation ( 3 ) , a mean temperature o f 900°C was found f o r the 2.0-p
t h i c k sample
n e a r l y 700 nsec a f t e r t h e onset o f a ruby l a s e r pulse t h a t prosurface melt duced a HRP o f -400-nsec duration. (For a given E,l d u r a t i o n s are longer f o r SOS samples than f o r c-Si, because o f Although t h e much lower e f f e c t i v e thermal c o n d u c t i v i t y o f SOS.) t h i s method o f temperature measurement i s l i m i t e d by averaging over
6.
357
TIME-RESOLVED MEASUREMENTS
t h e f i l m thickness and by t h e r e s t r i c t i o n t o measurement times near t h e occurrence o f i n t e r f e r e n c e maxima o r minima, it i s p o s s i b l e t o conclude from t h e data o f Murakami e t al. t h a t t h e s i l i c o n l a t t i c e temperature i n SOS samples i s a t l e a s t 1000°C, a t times
>
100 nsec
a f t e r t h e end o f t h e HRP. b.
Subnanosecond Excite-and-Probe L a t t i c e Temperature Measurements Pulsed l a s e r i r r a d i a t i o n o f a semiconductor such as s i l i c o n
produces changes i n t h e complex d e l e c t r i c constant, T = q + q ) t h a t r e s u l t both from t h e presence o f a l a s e r - e x c i t e d e l e c t r o n -
It i s shown i n Sec.
h o l e plasma and from heating o f t h e l a t t i c e .
IV.9 t h a t : (1) I f excite-and-probe measurements o f t h e o p t i c a l p r o p e r t i e s o f l a s e r - i r r a d i a t e d s i l i c o n are c a r r i e d o u t a t a probe wavelength i n t h e m i d - v i s i b l e region (e.g.,
532 nm), then v a r i a t i o n s i n
E~
are t h e
r e s u l t o f t h e opposing e f f e c t s o f i n c r e a s i n g l a t t i c e temperature and i n c r e a s i n g plasma density, w h i l e c2 i s governed mainly by t h e l a t t i c e tempera(2)
t u r e a t these frequencies. A low-level 532-nm e x c i t a t i o n pulse w i l l produce a plasma t h a t decays i n about 100 ps i n s i l i c o n (see Fig. 21).
Thus, a t l a t e r times t h e o p t i c a l p r o p e r t i e s o f l a s e r - i r r a d i a t e d s i l i c o n are mainly those o f t h e laser-heated l a t t i c e w i t h a minimal p l asma c o n t r i b u t ion. Recognition o f these f a c t s l e d Lompr6 and c o l l a b o r a t o r s (1983) t o c a r r y out l a t t i c e temperature measurements f o l l owing a 532-nm e x c i t a t i o n pulse (using e x c i t a t i o n l e v e l s below t h e m e l t i n g thresho l d ) by probing t h e R and T o f 0.1 and 0.5 pm t h i c k SOS samples, a t a f i x e d time delay o f 200 ps a f t e r t h e e x c i t a t i o n pulse.
The
probe pulse was a l s o a t 532 nm, w i t h special care taken t o avoid spurious effects
caused by t h e i n t e r f e r e n c e o f pulse and probe
358
D. H. LOWNDES ET AL.
beams a t t h e same wavelength.
Because s i l i c o n i s s t r o n g l y absorbing
a t 532 nm (see Table I ) t h e probe pulse fluence was l i m i t e d t o about 0.1% o f t h e e x c i t a t i o n pulse fluence, t o avoid s i g n i f i c a n t h e a t i n g by t h e probe pulse. The v a r i a t i o n s i n n and k t h a t were i n f e r r e d from R and T measurements by Lompr6 and c o l l a b o r a t o r s were i n t e r p r e t e d i n terms o f temperature changes using t h e known i n t r i n s i c v a r i a t i o n s i n n and k ( a r i s i n g from changes i n t h e i n d i r e c t bandgap and i n m a t r i x elements f o r t h e i n d i r e c t t r a n s i t i o n ) ,
using J e l l i s o n and Modine's
(1982a, 1983) measurements o f n and k f o r temperatures up t o 1000 K and an extrapol a t ion o f these r e s u l t s f o r higher temperatures. S l i g h t v a r i a t i o n s i n t h e thickness o f t h e SOS samples were found t o produce l a r g e changes i n R and T across i n d i v i d u a l samples, even w i t h o u t o p t i c a l pumping; t h e independence o f t h e d e r i v e d n and k values from t h e f i l m thickness a c t u a l l y used provided an additional
check
on
experimental
procedures
(Lompr6 e t
a1
.,
1983). I t was found t h a t t h e o p t i c a l absorption c o e f f i c i e n t o f the
SOS samples increased by about an order o f magnitude from room
temperature t o temperatures j u s t be1ow t h e me1t i ng p o i n t (me1t i ng was produced f o r fluences above 0.16 J/cm2). fraction, A
(A
=
1- R
- T),
The absorbed energy
was found t o depend s t r o n g l y on f i l m
t h i c k n e s s a t low fluence, but f o r higher fluences ( s t i l l below t h e m e l t i n g f l u e n c e ) t h e d i f f e r e n c e i n A f o r 0.1-pm and 0 . 5 - p t h i c k specimens was reduced.
If t h e actual temperature p r o f i l e ,
~ ( x ) , does vary s i g n i f i c a n t l y across t h e thickness o f a sample, then the optical density
depends s e n s i t i v e l y on T ( x ) ;
c o r r e l a t i n g R and T then r e q u i r e s
knowledge o f t h e actual p r o f i l e o f a(x) o r T ( X ) (Jacobsson, 1966; Lompre e t al.,
1983).
[The
r i g h t hand side o f Eq.
J e l l i s o n and Modine's form f o r a(x).]
( 4 ) uses
Lompre' and coworkers were
6.
TIME-RESOLVED MEASUREMENTS
359
unable t o f i t their R and T data by assuming a uniform temperature, even for the 0.1-w thick SOS sample. However, by numerically calculating T(X) (200 ps after excitation pulses of various fluences), taking into account the temperature dependences of optical absorpt i o n , thermal conductivity and heat capacity, and converting these T ( X ) profiles into absorption profiles a ( x ) and then optical density, using Eq. (4), they were able t o f i t their R and T data for the entire range of fluences below the melting transition. Figure 15 shows the resulting surface temperatures, T ~ ,and average temperatures, F, where
SURFACE TEMP Ts 0
AVERAGE TEMPT
0.05
0.10
0.15
El (J/cm2) Fig. 1 5 . Average temperatures and surface temperatures at t = 200 ps vs incident fluence of ~ O - P S , 532-nm heating pulses, for a 0.5-pn thick SOS film (Lompr; et a l . , 1 9 8 3 ) .
360
D. H. LOWNDES E T A L .
versus 532 nm e x c i t a t i o n f l u e n c e f o r a 0.5-IJ~ t h i c k SOS specimen. L a t t i c e temperatures i n excess o f 1500 K are reached a t a f l u e n c e o f 0.15 J/cm2,
i n good agreement w i t h t h e i r observation o f an R
jump i n d i c a t i v e o f m e l t i n g (expected t o occur a t Tc = 1685 K ) f o r fluences i n excess o f 0.16 J/cm2. c.
Time-Resol ved Pyrometry Thermally e m i t t e d r a d i t i o n provides another p o s s i b i l i t y f o r
t h e measurement o f t h e temperature, T, o f t h e near-surface region
of s i l i c o n d u r i n g pulsed l a s e r i r r a d i a t i o n .
According t o Planck's
1aw,
where c1 = 3.7413 x
erg-cm2/sec,
c2 = 1.4388 cm-deg, A i s t h e
wavelength i n cm, and W i s t h e t h e r m a l l y r a d i a t e d energy f l u x per u n i t wavelength.
C l e a r l y , a d e t a i l e d examination o f t h e wavelength
dependence o f emitted r a d i a t i o n can be used t o determine t h e temperature o f t h e near-surface region. I n a recent set o f experiments,
K e m l e r e t al.
(1984) have
measured t h e time-resolved thermal r a d i a t i o n erni t t e d d u r i n g pul sed 1aser heating o f s i 1 icon. They used t h e frequency-doubl ed output ( A = 532 nm) from a p a s s i v e l y Q-switched Nd:YAG l a s e r operated a t 20 Hz, w i t h a pulse w i d t h o f 10 nsec and l e s s than 5%
fluctuation.
rms energy
The t h e r m a l l y e m i t t e d r a d i a t i o n from t h e s i l i c o n
surface was observed i n a backward s c a t t e r i n g geometry w i t h a double monochromator and a p h o t o m u l t i p l i e r tube using a s i n g l e photon counting technique w i t h 400-ps time r e s o l u t i o n .
Measure-
ments were p o s s i b l e from 900 nm ( t h e p h o t o m u l t i p l i e r c u t o f f ) down t o about 500 nm (where t h e signal was l e s s than one photon per 500 l a s e r pulses).
6.
361
TIME-RESOLVED MEASUREMENTS
Setting the monochromator for specific wavelengths, Kemmler et al. measured the intensity of thermally emitted l i g h t over a 50-ns These observation time t h a t was divided i n t o 2-ns intervals. measurements were reanalyzed (correcting for the measured spectral response of the detection system) and then plotted a t specified times as the logarithm of intensity vs photon energy; the experimental points were found to l i e approximately on a straight line, from which the surface temperature was obtained by f i t t i n g w i t h Planck's Law, Eq. (6). Intensity ratios, a t different measurement times, were also found t o be in accord w i t h Planck's Law. The temperature prof i 1e resulting from such measurments , for a laser Ex = 0.54 J/cm2, i s shown in Fig. 16. The beginning and end of the HRP (marked by the arrows) are seen t o delineate very 1.7 1.6 17
(.r? 1.5
g
U
w 1.4
a
t
3
1.3
a
3I-
1.2 1.1
1.0
0.9
-10
-5
0
5 10 TIME [nsJ
15
20
25
Fig. 16. Solid circles: measured pyrometric temperature vs time. (The solid line i s a guide for the eye.) Dashed curve: laser heating pulse. The arrows mark the beginning and end o f the high r e f l e c t i v i t y phase (Kemmler e t al., 1984).
362
D. H. LOWNDES E T A L .
accurately t h e period of time f o r which surface temperatures of 1500-1700 K were measured. The i n i t i a l temperature peak r e s u l t s from overheating of the l i q u i d surface during absorption of the 1a s e r pul se , whi 1e the l a t e r plateau represents the equi 1b r i um temperature of t h e melt. The difference between t h e measured 1500 K and Tc = 1685 K f o r molten s i l i c o n was presumed t o be due t o a systematic e r r o r in the absolute temperature s c a l e , a r i s i n g primarily from uncertainties in the filament temperature of the tungsten lamp t h a t was used t o c a l i b r a t e t h e detection system; t h i s e r r o r was estimated t o be t300 K. Redefining t h e plateau temperature as being Tc = 1685 K r e s u l t s in a maximum l i q u i d s i l i con temperature, near the beginning of the HRP, of nearly 2000 K (Kern1er e t a1 , 1984). Kemmler e t a l . also point out t h a t t h i s type of thermal emission measurement has two important advantages f o r the determi nation of surface temperatures: (1) Spatial averaging of temperatures, in the presence of large temperature gradients normal t o t h e sample surf a c e , i s minimal because of t h e small escape depth of v i s i b l e thermal r a d i a t i o n ; and, ( 2 ) s p a t i a l averaging of temperatures i s f u r t h e r reduced by t h e strong increase i n thermally emitted i n t e n s i t y with temperature, which biases the measurement toward t h e highest temperature region. This l a t t e r behavior i s precisely the opposite of the s i t u a t i o n t h a t occurs i n pulsed Raman experiments, f o r which t h e Raman s c a t t e r i n g efficiency decreases rapidly with increasing l a t t i c e temperature, which may lead t o an underestimate o f surface temperature. Thus , thermal emission measurements seem especially we1 1 -suited f o r the determination of surface temperatures under t h e conditions of l a r g e temporal and s p a t i a l temperature gradients t h a t prevail in typical pulsed l a s e r experiments.
.
111.
Pulsed Raman Scattering Measurements
Much of the controversy regarding the physical mechanism f o r pulsed l a s e r annealing originated in a s e r i e s of time-resolved Raman s c a t t e r i n g measurements t h a t were intended t o d i r e c t l y probe
6.
TIME-RESOLVED MEASUREMENTS
363
the temperature of silicon during and immediately a f t e r the laserThe i n i t i a l experiments were induced high reflectivity phase. carried out by Lo and Compaan (1980a, 1981) but were subsequently repeated w i t h substantial changes in experimental apparatus (Compaan e t al., 1982a) and at different probe wavelengths (von der Linde and Wartmann, 1982, von der Linde et. a1 1983b). I t i s possible for Raman experiments t o measure l a t t i c e temperature because the Raman interaction involves either the absorption (Stokes, S, process) or the emission (anti-Stokes, AS, process) of a phonon. The population o f phonons in the l a t t i c e i s
.,
= l/Cexp(h~~/kT)-ll
no(wo,T)
(7 1
where wo i s the phonon frequency, T i s the l a t t i c e temperature, and k i s Boltzmann's constant. Since the intensity of the Stokes process will be proportional t o (no + l ) , while the intensity of the anti-Stokes process will be proportional t o no, the Stokes t o antiStokes intensity ratio, R(S/AS) , provides a temperature probe. If R(S/AS) were t o depend only on phonon population, then R(S/AS)
=
F ( n o + l ) / n o = F exp(h%/kT)
(8 1
w i t h the prefactor F = 1. In general, however, F i s a function of temperature and of the optical properties of the material being studied. I t is given for an experiment a t constant temperature by F(T) =
-
7 oana
(91
where w i s the photon frequency, u i s the Raman matrix element, a i s the optical absorption coefficient, n is the index of refraction and R is the reflectivity, for the subscripted processes. The subscript L refers t o the probe laser frequency and the barred If the quantities are the ratios indicated by the notation.
364
D. H. LOWNDES ET AL.
sample i s not a t a constant temperature, as i s the case f o r a general laser-annealing experiment, t h e n Eq. (8) cannot be used and a much more complicated formulation must be employed (see J e l l i s o n et a l . , 1983b). [Of course, i f temperature variations a r e not too severe, the use of Eq. (8) may be j u s t i f i e d . ] In this section, we shall b r i e f l y discuss the experiments of e Compaan and co-workers and of von der Linde and co-workers. W s h a l l a l s o discuss several complications involved i n these measurements. I t i s not possible t o give a d e t a i l e d accounting here o f t h e controversy t h a t has resulted from these measurements; instead, we include a comprehensive l i s t of references t o which t h e interested reader i s referred: Lo and Compaan, 1980a, b y 1981; Compaan e t a1 , 1982a, b y 1983a, b; Compaan and Trodahl , 1984; Lee et a1 , 1982; Bhattacharyya et a1 1982; von der Linde and Wartmann, 1982; von der Linde et a l . 1983a, b, 1984; Kemmler e t a1 1984; Wood e t al., 1982a, b y c; J e l l i s o n , et a l . , 1983a, b; J e l l i s o n and Wood, 1984.
.
.,
5.
EXPERIMENTAL RESULTS
a.
Compaan and Co-workers
.
.,
Measurements of the Stokes t o anti-Stokes i n t e n s i t y r a t i o were f i r s t made by Lo and Compaan (1980a), from which they inferred a l a t t i c e temperature of only 3OOOC in c-Si some 10 nsec a f t e r a 1.0 J/cm2 heating pulse, which was s u f f i c i e n t t o produce a t r a n s i t i o n t o t h e HRP, normally interpreted as indicating melting. A twobeam configuration was used f o r these experiments, consisting of an intense heating beam ( h = 485 nm, 90 gm diameter) and a weak probe beam ( h = 405 nm, 50 pm diameter), with the probe beam delayed in time by 10 nsec. The pulse widths of both the heating and probe beams were -7 nsec. The probe beam energy was 0.06 J/cm2, which was s u f f i c i e n t by i t s e l f t o increase the l a t t i c e temperature by -15OOC. Pulse-to-pulse energy v a r i a t i o n s were measured in the
6.
365
TIME-RESOLVED MEASUREMENTS
center o f each l a s e r beam and were reported t o be tlOX and 27% f o r t h e heating and probe l a s e rs , re s p e c ti v e l y .
A l a t e r refinement o f t h e experiment (Compaan e t al., r e s u l t e d i n improved s p a t i a l and temporal
1982a)
resolution.
These
experiments used a frequency-doubled Nd:YAG l a s e r ( A = 532 nm, 20-nsec pulse, -1 0 0 0 -p diameter) as t h e heating beam, and a l a r g e r diameter probe beam ( A = 405 nm, -200-p diameter, -7-nsec
pulse),
which reduced considerably t h e heating e f f e c t s o f the probe beam. A v a r i a b l e e l e c t r o n i c delay was used t o t r i g g e r t h e probe pulse
a t various times a f t e r th e heating pulse, and t h e sample r e f l e c t i v i t y was monitored w i t h a cw Ar+-ion l a s e r (A = 514 nm).
In
order t o o b t a i n a reasonable signal -to-noise r a t i o , roughly 10,000 l a s e r shots were averaged. I n t h e l a t t e r set o f experiments, no Raman signal was observed f o r t h e f i r s t 80 nsec, corresponding t o t h e d u r a t i o n o f t h e h i g h r e f 1e c t i v i t y phase.
The f i r s t observable Raman signal was observed
w i t h a 110 nsec delay, where t h e r e f l e c t i v i t y had returned t o i t s o r i g i n a l value; using Eq. (8), a temperature o f from R(S/AS).
-4OOOC
was obtained
Compaan e t a l . (1982a) concluded t h a t " t h e f a c t t h a t
t h e l a t t i c e temperature i s so low immediately a f t e r t h e h i g h r e f l e c t i v i t y period shows t h a t t h i s enhanced r e f l e c t i v i t y phase cannot be t he usual 1400°C molten phase o f s i l i c o n unless u n r e a l i s t i c a l l y l a r g e c o o l i n g r a t e s are assumed."
I n a s i m i l a r experiment, Lo and
Compaan (1981) measured R(S/AS) f o r samples implanted w i t h 200 keV As ions a t a dose of lO15/cm2 (a-Si); they r e p o r t e d t h a t t h e highest temperature obtained was 600 k 200°C a t 20 nsec a f t e r t h e end o f t he HRP. Compaan e t a l . r e versal
(1982b) have used a technique i n v o l v i n g time-
invaria n c e t o determine e x p e ri m e n t a l l y t h e c o r r e c t i o n
f a c t o r needed t o re1 a t e the Stokes-to-anti-Stokes Raman i n t e n s i t y r a t i o t o the l a t t i c e temperature. Three measurements were performed: (1) t he Stokes and (2) th e anti-Stokes i n t e n s i t y measurements a t t h e o r i g i n a l probe wavelength, and (3) t h e Stokes i n t e n s i t y
366
D. H. LOWNDES E T A L .
measurement a t a probe wavelength equal t o t h e previous anti-Stokes wavelength.
The c o r r e c t i o n f a c t o r i s t h e r a t i o o f t h e i n t e n s i t i e s
i n experiments (1) and (3) i f i t can be assumed t h a t e i t h e r t h e Raman m a t r i x element o f experiment (2) equals t h a t o f experiment (3),
o r i f t h e i n p u t and output channels o f experiment (3) are
reversed from those o f experiments (1) and (2). b.
von der Linde and Co-Workers The measurements o f von der Linde and co-workers (von der Linde
and Wartmann, 1982; von der Linde e t a1 one- and two-beam c o n f i g u r a t i o n s .
., 1983b,
1984) used both
I n t h e two-beam c o n f i g u r a t i o n ,
t h e heating beam was a frequency-doubled Nd:YAG l a s e r ( h = 532 nm, 0.5 mn diameter, -10-nsec
pulse), w h i l e t h e probe beam was s p l i t
o f f from t h e 1.06-w Nd:YAG fundamental and then f r e q u e n c y - t r i p l e d ( h = 355 nm).
beam was used.
I n t h e one-beam experiment, o n l y t h e 532-nm h e a t i n g The Raman-scattered photons were detected w i t h a
f a s t (-1 nsec) p h o t o m u l t i p l i e r , a l l o w i n g measurements o f t h e Stokes and anti-Stokes i n t e n s i t i e s t o be made as a f u n c t i o n o f time.
This
experiment d i f f e r s from those o f Compaan and co-workers i n t h a t Raman s c a t t e r i n g was measured continuously as a f u n c t i o n o f time, b u t o n l y f o r times when t h e l a s e r pulse was on.
The measurements
o f von der Linde and co-workers also r e q u i r e d signal averaging over several thousand laser shots. I n t h e f i r s t s e t o f experiments, von der Linde and Wartmann (1982) adjusted t h e spectrometer bandw i d t h t o 0.5 M t o c o l l e c t most o f t h e Stokes o r anti-Stokes s h i f t e d photons.
I n t h e i r second s e t o f experiments (von der Linde e t al.,
1984), they reduced t h e bandwidth o f t h e i r spectrometer and recorded t h e e n t i r e Raman spectrum. von der Linde and Wartmann i n f e r r e d a maximum temperature o f -800 K using j u s t t h e heating pulse ( s i n g l e beam experiment), but a maximum temperature o f -1600-3000 K from t h e two-beam experiment. These r e s u l t s were obtained using Eq.
( 8 ) , w i t h F = 0.95 f o r t h e
single-beam experiment, and F = 2.5 f o r t h e two-beam experiment,
6.
367
TIME-RESOLVED MEASUREMENTS
determined from room-temperature measurements of
G
(Renucci et a1
.,
1975) and of a (Jellison and Modine, 1982b). I n a l a t e r revision of these inferred temperatures, von der Linde e t al. (1983a, b ) determined the factor F as a function o f temperature, using constant-temperature measurements, and concluded t h a t the factor F decreased considerably as the temperature was increased for h = 355 nm photons; t h i s resulted in the two-beam experiments yielding a temperature of -600 K. In the experiments of von der Linde et al. (1984), both the time and frequency dependences o f the Raman-scattered photons were recorded. These experiments revealed that i n the experiments of von der Linde and Wartmann not all of the Raman-scattered photons were collected, due t o the spectrometer bandwidth t h a t was used. When the full line was recorded, von der Linde et a1 (1984) found t h a t the R(S/AS) measurements, as well as the Raman line shifts, resulted in temperatures that were consistent w i t h me1 t i ng model calculations, a1 though the associated error bars were very large.
.
6.
COMPLICATIONS I N PULSED RAMAN MEASUREMENTS
Several complications arise in attempting t o infer the l a t t i c e temperature from R(S/AS). a.
Pul se-to-Pul se Fluctuations
All of the pulsed Raman measurements performed t o date involved signal averaging, since the number of photons per laser pulse was general ly small (-0.5-10 photons/pul se) Signal averaging would be acceptable if each laser pulse were exactly the same and if the Stokes and anti-Stokes intensities were reproducible. Unfortunately, t h i s i s not generally the case; the degree of nonreproducibility of the laser pulses depends on the laser being used. However, a1 1 lasers exhibit pul se-to-pul se differences i n energy density, pulse shape, and t i m i n g . In cases of large temperature-time
.
368
D. H. LOWNDES ETAL.
o r temperature-di stance gradients,
signal averaging over these
f l u c t u a t i o n s may not be a good approximation. I n t h e experiments o f Compaan e t a l . (1982b), t h e heating pulse had an
Ex u n c e r t a i n t y o f -108, t h e probe pulse o f -5%.
Therefore,
I n addition,
nsec wide w i t h a time j i t t e r o f -4 nsec.
t h e probe pulse was -7
f o r probe delays very c l o s e t o t h e end o f t h e h i g h
r e f l e c t i v i t y phase, where t h e temperature-time g r a d i e n t i s extremely h i g h (-4 x 1O1O K/sec according t o m e l t i n g model c a l c u l a t i o n s , see Chapter 4), t h e temperature can vary several hundred degrees from p u l s e t o pulse and several hundred degrees from beginning t o end o f t h e probe pulse.
Signal averaging under these circumstances
i s suspect (see J e l l i s o n e t a1 b
.
., 1983b f o r d e t a i l e d c a l c u l a t i o n s ) .
Beam I nhomogenei t ies Heat w i l l d i f f u s e only -1-3
nsec d u r a t i o n o f t h e HRP.
i n s i l i c o n d u r i n g t h e -20-200-
Therefore, any transverse energy f l u c t u a -
t i o n s over distances g r e a t e r than several microns become important. Since it i s v i r t u a l l y impossible t o a s c e r t a i n t h e p r e c i s e energy
of t h e l a s e r pulse as a f u n c t i o n of time, transverse dimensions and pulse number, adequate r e p r e s e n t a t i o n o f these f l u c t u a t i o n s i n t h e general case i s impossible. c.
Temperature-Dependent Parameters I t i s now w e l l e s t a b l i s h e d t h a t t h e o p t i c a l f u n c t i o n s o f silicon
a r e very strong f u n c t i o n s o f temperature, p a r t i c u l a r l y f o r photon energies near t h e d i r e c t band gap (-3.4
eV a t room temperature; see
Chapter 3 and J e l l i s o n and Modine, 1983).
Furthermore, as t h e tem-
p e r a t u r e o f t h e s i l i c o n l a t t i c e i s increased, t h e energy p o s i t i o n o f t h e d i r e c t bandgap moves t o lower photon energies, and many o f t h e s p e c t r a l f e a t u r e s o f t h e o p t i c a l f u n c t i o n s become broadened. The r e f l e c t i v i t y and index o f r e f r a c t i o n can both change s i g n i f i c a n t l y as t h e temperature i s increased, changing t h e value o f t h e p r e f a c t o r F [see Eq.
(9) and Fig.
171 away from 1; however, the
6.
TIME-RESOLVED MEASUREMENTS
369
1.6 1.4
1.2 I.o
0.8 0.6 3.0
2.5 2.0 1.5
1.o 1
200
I 400
I
I
I
I
600 800 TEMPERATURE (K1
1
I 1000
Fig. 17. Correction factor ratios a, 6, R , and n for the Stokes to anti-Stokes intensity ratio [Eq. (9)] vs temperature for the two probe wavelengths 405 and 355 nm (Jellison et al., 1 9 8 3 a ) .
largest effects are observed i n the optical absorption coefficient (Jellison and Modine, 1982a; see Chapter 3 , Fig. 2) and the Raman matrix element (Jell ison e t a1 , 1983a; Compaan and Trodahl , 1984). I t i s expected theoretically that the Raman matrix element CJ can be represented (Renucci et a l . , 1975) in the resonant region by the expression
.
a(E,T)
=
CD
a2
370
D. H. LOWNDES E T A L
where C i s a numerical f a c t o r , D i s a l i n e a r combination of deformation p o t e n t i a l s , E i s the phonon amplitude [ = 1 / 2 + no], a i s t h e l a t t i c e constant, and E i s the complex d i e l e c t r i c function. versus photon energy, Figure 18 shows a plot of de(T)/dE determined from t h e optical data of J e l l i s o n and Modine (1982a, 1983) ( s e e Chapter 3, Fig. 1). Also shown in Fig. 18 are the data o f Renucci e t a l . (1975), taken a t 300 K. As can be seen, t h e f i t
I
I
I
I
Fig. 18. Raman matrix element squared compared with d e / d E for several temperatures. The arrows a t the bottom o f the figure indicate the positions o f the two probe wavelengths 4 0 5 and 3 5 5 nm, respectively. The data taken from Renucci e t a l . ( 1 9 7 5 ) a r e shown by the closed circles ( 0 ) (jellison et al., 1983a).
371
6. TIME-RESOLVED MEASUREMENTS i s s u r p r i s i n g l y good, except i n t h e r e g i o n near 3.1 eV. because Renucci e t al.
This i s
used t h e o l d e r a data o f Dash and Newman
(1955) i n reducing t h e raw Raman data; i f the more recent a data o f J e l l i s o n and Modine (1982b) i s used i n performing t h e data r e d u c t i o n i n t h i s r e g i o n (shown by t h e open t r i a n g l e s i n Fig. 18), a better fit o f
I de(T
= 300)/dE
I*
w i t h experimental data i s
obtained. I n a d d i t i o n t o t h e temperature dependences o f t h e o p t i c a l prope r t i e s , which g e n e r a l l y a f f e c t o n l y t h e coup1 i n g constants o f t h e Raman i n t e r a c t i o n , t h e r e also i s a temperature dependence t o t h e
r2,-, o p t i c a l phonon t h a t mediates t h e Raman i n t e r a c t i o n , thereby s t r o n g l y changing t h e l i n e p o s i t i o n and l i n e shape (Balkanski e t a1
., 1983).
I n experiments t h a t i n c l u d e a range o f temperatures
extending over several hundred degrees, these e f f e c t s should be in c l uded. d.
Stress E f f e c t s As described i n Section 11.3 o f t h i s chapter, time-resolved x-
r a y d i f f r a c t i o n measurements show t h a t t h e near-surface r e g i o n of t h e s i l i c o n l a t t i c e i s severely s t r a i n e d immediately a f t e r t h e HRP. This s t r a i n i s thought t o be due t o t h e simultaneous normal thermal expansion and l a t e r a l clamping o f t h e l a t t i c e ( t h e l a t t e r due t o t h e cool p a r t o f t h e sample o u t s i d e t h e l a s e r beam), r e s u l t i n g i n a u n i a x i a l s t r e s s w i t h symmetry a x i s perpendicular t o t h e sample surface. This s t r e s s s h i f t s t h e frequency o f t h e rZ5, o p t i c a l phonon and s p l i t s i t from a 3 - f o l d degenerate l i n e i n t o a s i n g l e t and a doublet.
Furthermore, t h e a p p l i e d s t r e s s reduces t h e symnetry
o f t h e near-surface r e g i o n from Oh ( f o r unstressed m a t e r i a l ) t o D2d ( f o r m a t e r i a l stressed along t h e d i r e c t i o n ) o r t o C3,, ( f o r m a t e r i a l stressed along t h e <111> d i r e c t i o n ) .
One r e s u l t o f
s t r e s s i s t h a t t h e band s t r u c t u r e no l o n g e r possesses t h e unstressed symmetry, making t h e conduction band v a l l e y s i n e q u i v a l e n t , s p l i t t i n g t h e normally degenerate bands a t t h e
r
p o i n t o f the B r i l l o u i n zone,
372
D.H. LOWNDES E T A L .
and a s y m e t r i c a l l y s h i f t i n g t h e bands a t a general p o i n t i n t h e zone.
Other m a n i f e s t a t i o n s o f an a p p l i e d s t r e s s are t h a t t h e Raman
s c a t t e r i n g tensor, which i s normally s y m e t r i c f o r cubic, unstressed
m a t e r i a l , becomes r i g o r o u s l y nonsymmetric , and t h e o p t i c a l propert i e s are changed (Gobeli and Kane, 1965; Chandrasekhar e t al., 1978).
A1 though stress-induced b i r e f r i n g e n c e i s measurable a t a1 1
v i s i b l e wavelengths,
t h e e f f e c t becomes even l a r g e r f o r photon
energies near t h e d i r e c t bandgap. For pulsed l a s e r annealing experiments,
t h e stress-re1 ated
change i n t h e phonon frequency i s p o s i t i v e and can be as l a r g e as 15 an-1 f o r s i l i c o n a t t h e m e l t i n g p o i n t ( J e l l i s o n and Wood, 1984). I f s t r e s s i s not included (as was t h e case w i t h Compaan e t al.,
1983b) , then t h e temperature i n f e r r e d from l i n e - p o s i t i o n measurements w i l l be too low. e.
E f f e c t s o f Excess Electron-Hole P a i r s Large numbers o f e l e c t r o n - h o l e p a i r s are created d u r i n g pulsed
1 aser i r r a d i a t i o n of semiconductors using above-bandgap r a d i a t i o n . However, i n s i l i c o n Auger recombination very q u i c k l y reduces t h e e l ectron-hol e pai r concentrat i o n t o -1019 e-h pai rs/cm3 (see Sec. IV.9).
It i s known t h a t very h e a v i l y p-doped s i l i c o n e x h i b i t s a
Fano s h i f t (Fano, 1961) due t o t h e i n t e r a c t i o n between excess holes i n t h e continuous band s t a t e s and t h e d i s c r e t e o p t i c mode. Although i t i s known t h a t t h e combined e f f e c t s of u n i a x i a l s t r e s s and heavy
doping can a f f e c t t h e Raman lineshape and i n t e n s i t y i n s i l i c o n (Cerdeira e t al.,
1973), e f f e c t s s p e c i f i c t o t h e case o f pulsed
l a s e r annealing are not known a t t h i s time. f.
Resonant E f f e c t s I f Raman s c a t t e r i n g experiments are performed a t photon ener-
g i e s near c r i t i c a l p o i n t s i n t h e B r i l l o u i n zone, t h e n t h e Raman s c a t t e r i n g i n t e n s i t i e s are enhanced by resonant e f f e c t s .
The Raman
s c a t t e r i n g m a t r i x element becomes very l a r g e i n t h i s region, but
i t a1 so becomes c r i t i c a l l y dependent upon wave1 ength , temperature ,
6.
TIME-RESOLVED MEASUREMENTS
373
.,
and stress (Jellison e t a1 1983a; Compaan and Trodahl , 1984). In silicon, t h i s resonant enhancement occurs near the direct band-
gap (-3.4 eV or 370 nm). When pulsed Raman experiments are performed w i t h probe photon energies in the resonant Raman regime, extreme care must be taken t o include all these effects. Unfortunately, much o f the required data i s not available so t h a t only guesses can be made concerning these effects. g.
Temperature-Depth Profile Effects
If the photon energy of the probe beam used in pulsed Raman measurements i s much smaller t h a n the direct bandgap, then reson a n t Raman effects are not very important. However, the probe light t h e n penetrates much deeper i n t o the material and can conceivably sample materi a1 whose temperature varies by hundreds of degrees. This i s the case for the single-beam experiments of von der Linde e t al. (1983b, 1984), for which both the heating and probe beam are at 532 nm. In t h i s case, the probe beam penetrates -2 pm a t room temperature, b u t only -0.4 pn at 700°C (Jellison and Modine, 1982a). Since thermal diffusion will n o t distribute heat from the front surface region t o depths greater than -0.5 p during the duration of the -10-nsec laser pulses used in Raman scattering experiments, large spatial temperature gradients are set up (see Chapter 4, Fig. 7 ) . These gradients are extremely important for probe-beam photon energies t h a t sample deep into the material (e.g., 532 nm), b u t are not nearly as important for highly absorbed probe wavelengths (such as 405 n m ) , for times a f t e r the HRP. 7.
SUMMARY:
PULSED RAMAN MEASUREMENTS
We have described recent pul sed Raman temperature measurements performed during pulsed laser irradiation of silicon. These measurements can be generally classified by the probe wavelength used: (1) Resonant experiments, where the probe laser wavelength i s near or above the direct band edge and ( 2 ) nonresonant experiments,
374
D. H. LOWNDES E T A L
where the probe laser wavelength i s well below the direct band edge. Fluctuations of the pulse-to-pulse energy density, beam inhomogeneities, and stress (Secs. III.6aY b y and d ) complicate both types of measurements; in addition, the resonant experiments are complicated mainly by the temperature dependence of quantities entering i n t o R(S/AS) (Secs. 111.6~ and f ) , while the nonresonant experiments, because of the probe wavelength, must take the extremely large spatial temperature gradients (Sec. 111.6s) i n t o account. Therefore, there does not appear t o be an ideal probe wave1 ength for time-resol ved Raman temperature measurements i n pulsed laser-irradiated silicon. I t is our belief t h a t these complications are so numerous, and so difficult t o treat, t h a t the l a t t i c e temperatures inferred from all R(S/AS) measurements t o date are subject t o large errors. I n particular, apparent low l a t t i c e temperatures should n o t be believed in the face of the overwhelming evidence of other experiments (see Secs. I1 and IV) t h a t indicate t h a t silicon does i n fact melt if laser energy sufficient t o produce the HRP i s incident upon the silicon surface. The HRP can therefore be identified w i t h the normal no1 ten phase of si 1 icon. IV.
Energy Transfer f r o m Optically Excited Carriers t o the Crystal Lattice
The central assumption of the thermal melting model calculations described i n Chapter 4 is t h a t the transfer of energy from laser-excited carriers t o the crystal l a t t i c e takes place i n a time t h a t is comparable t o or less t h a n the nanosecond or longer d u r a t i o n light pulses used for laser annealing of semiconductors. However, the time scale for establ ishment of this local thermodynami c equi 1 ib r i urn, between 1aser-exci ted carriers and the phonon system, is itself of fundamental interest; this time scale also determines the ultimate limit of applicability of a thermal melting model.
6.
375
TIME-RESOLVED MEASUREMENTS
A d e t a i l e d understanding o f t h i s thermal i z a t i o n process a c t u a l l y r e q u i r e s t r e a t i n g two processes t h a t occur simul taneously:
(1) The
dynamics o f a hot e l e c t r o n - h o l e plasma whose d e n s i t y may be varying r a p i d l y i n time and (2) t h e proce5s o f energy t r a n s f e r from these e x c i t e d c a r r i e r s t o t h e l a t t i c e v i a t h e electron-phonon i n t e r a c t i o n . Bloembergen e t al. (1982) and Yoffa (1980) have discussed t h e carr i e r generation and r e l a x a t i o n processes t h a t are i n v o l v e d and have estimated t h e i r c h a r a c t e r i s t i c t i m e scales (see Chapter 4). B r i e f l y , these are as f o l l o w s :
E l e c t r o n - h o l e p a i r s are produced by t h e
absorption o f l a s e r l i g h t i n d i r e c t o r i n d i r e c t t r a n s i t i o n s across t h e energy bandgap; f r e e - c a r r i e r i n c r e a s i n g c a r r i e r density.
absorption a1 so increases w i t h
The hot c a r r i e r s r e s u l t i n g from both
processes then thermalize, w i t h o t h e r c a r r i e r s and w i t h t h e l a t t i c e .
A common c a r r i e r temperature f o r e l e c t r o n s and holes i s expected t o be e s t a b l i s h e d i n
sec) as a r e s u l t ps = o f e l e c t r o n - e l e c t r o n and ( s e c o n d a r i l y ) electron-plasmon c o l l i s i o n s ;
concurrent w i t h these i n t r a - c a r r i e r e q u i l ib r a t ion processes, hot c a r r i e r s can
t r a n s f e r energy t o t h e l a t t i c e v i a
s c a t t e r i n g , i n a t i m e expected t o
1982).
electron-phonon
be S 1 ps (Bloembergen e t al.,
A s e r i e s o f electron-phonon i n t e r a c t i o n s r e s u l t s i n elec-
t r o n s t r i c k l i n g down t o t h e bottom o f t h e conduction band (holes t o t h e t o p o f t h e valence band), a t which p o i n t t h e remaining bandgap e x c i t a t i o n can be l o s t by e l e c t r o n - h o l e recombination.
For
picosecond times and h i g h c a r r i e r d e n s i t i e s , e l e c t r o n - h o l e recomb i n a t i o n i s expected t o occur predominantly by t h e Auger process, i n which t h e bandgap (recombination) energy o f an e l e c t r o n - h o l e p a i r i s given t o a t h i r d c a r r i e r (an e l e c t r o n o r a hole), a t a r a t e given by dN/dt = CN3, where C = 4 x 300
K (Dziewior and Schmidt, 1977).
cm6/s f o r S i a t
Although t h e Auger process
does not reduce t h e net energy o f t h e c a r r i e r system,
it does
reduce t h e c a r r i e r d e n s i t y and creates hot c a r r i e r s whose excess k i n e t i c energy can again be given up t o t h e l a t t i c e v i a f u r t h e r electron-phonon s c a t t e r i n g .
376
D. H. LOWNDES E T A L .
Several experiments have been carried out using laser pulse durations i n the 15-20 ps range, t o determine whether there are any apparent differences i n electron and l a t t i c e temperatures t h a t would indicate a lack of thermal equilibrium between carriers and the l a t t i c e on this time scale. For example, Yen et al. (1982) studied the melting and amorphization o f (111) crystalline silicon by 20 ps, 532 nm pulses. They found t h a t their observed melting threshold fluence (0.2 J/cm2) and amorphization fluences (0.2-0.26 J/cm2) were in agreement with melting model calculations, provided t h a t the electron-lattice energy relaxation time was taken t o be g o ps. Measurements of the photoelectric emission o f electrons and of the evaporation of positive ions have also been used t o investigate possible differences between electron and 1a t t i c e temperatures, since thermionic emission depends sensitively on the electron temperature, Te, while the onset of evaporation of positive ions from the surface indicates a high l a t t i c e temperature. In addition, the temperatures probed in particle emission measurements are very L i u , Yen , Ma1 vezzi , Kurz and nearly surface temperatures. Bloembergen ( L i u et a1 , 1982a, b; Malvezzi et a1 , 1984) measured the emission of electrons and of positive ions from both (100) and (111) silicon surfaces, following irradiation by visible (20 ps, 532 nm) and ultraviolet (15 p s , 266 nm) l i g h t pulses. Using the UV excitation, they observed three different photoelectric regimes over a wide range of incident laser fluence (10-5-3 x 10-1 J/cm2). One of the principal results of their study was the complete absence of any detectable thermionic effects on electron emission over the entire range of laser fluences, resulting i n establishment of an upper limit of Te = 3000 K f o r the average electronic temperature during the laser pulse. They also found t h a t sharp increases occurred i n both ion and electron emission near the threshold fluence for amorphization of the surface; these sharp increases, suggesting a sudden modification i n the sample surface, are consistent w i t h the melting transition t h a t is independently revealed
.
.
6.
TIME-RESOLVED MEASUREMENTS
by amorphization a t t h e same fluence.
377
Malvezzi e t a l . (1984) con-
cluded t h a t t h e i r data were c o n s i s t e n t w i t h t h e establishment o f thermal equi 1ib r i um between c a r r i e r s and t h e 1a t t i ce w i t h i n t h e d u r a t i o n o f t h e i r 15-ps l a s e r pulse. Fauchet and Siegman (1983) c a r r i e d out experiments and m e l t i n g model c a l c u l a t i o n s i n which two separate heating pulses o f 30- and 100-ps d u r a t i o n were used, a t wavelengths o f 532 nm and 1.06 p, w i t h t h e i n f r a r e d pulse delayed by times o f up t o several nanoseconds.
Absorption o f 1 . 0 6 - p
i s due mainly t o f r e e c a r r i e r s .
radiation i n crystal1 ine s i l i c o n Thus, t h e 1.06-pm fluence t h a t
i s r e q u i r e d t o produce m e l t i n g depends s e n s i t i v e l y on both t h e i n i t i a l 532-nm f l u e n c e and on t h e decay i n t h e 532-nm induced c a r r i e r d e n s i t y d u r i n g t h e delay period.
The authors s t a t e t h a t
t h i s two-pulse technique remains sensitive- t o plasma e f f e c t s a t plasma d e n s i t i e s w e l l below those t h a t are e a s i l y studied using t h e pul se-and-probe technique.
Conventional me1t i n g model calcu-
l a t i o n s were found t o accurately describe t h e dependence o f t h e m e l t i n g t h r e s h o l d on t h e combined v i s i b l e and i n f r a r e d fluences and time delay. It i s c l e a r from t h e above discussion t h a t experiments on a
picosecond, o r s h o r t e r , t i m e s c a l e are r e q u i r e d i n order t o c l e a r l y r e s o l v e t h e process o f energy t r a n s f e r from a l a s e r - e x c i t e d e l e c t r o n h o l e plasma t o t h e l a t t i c e , as w e l l as t o study t h e o p t i c a l prope r t i e s and time e v o l u t i o n o f such a high-density plasma p r i o r t o t h e occurrence o f melting. a r e sumnari zed be1ow. 8.
TIME SCALE
The r e s u l t s of a few such experiments
FOR MELTING OF THE LATTICE
Shank and c o l l a b o r a t o r s (Shank e t a1
., 1983a,
b; 1984) have
used 90 f s o p t i c a l pulses t o study t h e process o f energy t r a n s f e r from a l a s e r - e x c i t e d plasma t o t h e s i l i c o n c r y s t a l l a t t i c e and t o observe t h e m e l t i n g phase t r a n s i t i o n i n s i l i c o n w i t h subpicosecond resolution.
Although others (von der Linde and Fabricius, 1982)
378
D. H. LOWNDES ET AL.
had e a r l i e r obtained direct optical evidence of plasma formation followed by melting in silicon, Shank e t al. were the f i r s t t o observe melting occurring well a f t e r , rather t h a n during, the excitation pulse. Their experiments were carried o u t using amplified 620-nm pul ses from a coll idi ng-pul se , mode-1 ocked dye 1aser t o excite the silicon surface. Time-resolved reflectivity measurements were carried o u t by the excite-and-probe technique, using probe wavelengths o f 440, 678, and 1000 nm. Figure 19 shows transient reflectivity measurements vs time as a function of the excitation pulse energy density, where E t h = 0.1 J/cm2 i s the apparent melting threshold energy density. Shank e t al. (1983a) point out that the vibrational period for LO phonons in Si i s comparable t o the duration of their excitation pulse; thus, the electron-hole plasma i s expected t o dominate optical properties i n i t i a l l y , w i t h the eventual occurrence o f melting being signaled by the optical properties approaching those for molten silicon. The Drude expression for the refractive index of an undamped plasma is
where no i s the high frequency refractive index of silicon and wP = ( 4 ~ N e ~ / ~ m *i )sl /the ~ plasma frequency. A t a probe wavelength of 1 pm, the reflectivity ( F i g . 19a) i n i t i a l l y decreases, for the lower excitation energy densities, b u t a t the highest excitations i t increases immediately a f t e r the excitation pulse. This behavi o r can be understood as resulting from differences in the i n i t i a l plasma carrier density, i f wP < w (reduced np and reflectivity,
Eq. (11)) a t low excitation, b u t wP > w (imaginary np and high r e f l e c t i v i t y ) a t high excitation. Shank e t al. (1983a) used the expression for wP and the magnitude of the reflectivity change t o estimate N = 5 x 1021/cm3 for an excitation of 0.63Eth, assuming m* equal t o the free electron mass. [Other estimates of m* indicate t h a t a value 2-4 times smaller, and a corresponding reduction
6. TIME-RESOLVED MEASUREMENTS
379
I
I
0
-03 -
Fig. 19. Transient r e f l e c t i v i t y data at three probe wavelengths following femtosecond excitation. The solid lines a t 0.63 Eth are calculated assuming c a r r i e r diffusion into the bulk; f o r E > Eth, the calculations use the thin-film melting model. The decrease in r e f l e c t i v i t y a t the highest excitation i s due t o surface damage; the dashed curve i s a guide t o the eye (Shank e t al. , 1983a).
380 in al.
D. H. LOWNDES ET AL.
N, i s more n e a r l y c o r r e c t ; see van O r i e l (1984) and Lomprg e t (1984b)l.
Shank e t al.
(1983a) a l s o c a l c u l a t e d t h e decay o f
t h e r e f l e c t i v i t y a t 0.63 Eth f o r models based on c a r r i e r d i f f u s i o n and on Auger recombination; t h e y a t t r i b u t e d t h e observed slow decay (Fig. 19a) t o c a r r i e r d i f f u s i o n and concluded t h a t t h e i r measurements provide evidence f o r t h e s a t u r a t i o n o f Auger recombination a t h i g h c a r r i e r d e n s i t i e s , as described by Yoffa (1980).
This
conclusion has been challenged by Combescot and Bok (1983) who used a lower value o f
N and Auger recombination t o o b t a i n good
agreement w i t h t h e r e f l e c t i v i t y decay observed by Shank e t a1
.
The increase o f t h e r e f l e c t i v i t y t o a plateau value a f t e r a
.
few picoseconds (Fig. 19a) i s explained by Shank e t a1 as r e s u l t i n g from a t h i n l a y e r o f molten s i l i c o n forming on t h e sample surface and expanding i n t o i t s b u l k w i t h a v e l o c i t y
V i a
By t a k i n g t h e
o p t i c a l constants o f t h i s expanding f i l m t o be those o f molten silicon,
and using
Vi
and t h e plateau value o f r e f l e c t i v i t y as
f r e e parameters, Shank e t a l . (1983a) were able t o simultaneously f i t t h e t h r e e sets o f r e f l e c t i v i t y data i n Fig. 19, f o r each e x c i -
t a t i o n i n t e n s i t y , using a s i n g l e o f 6.2
x
lo3,
9 x
lo3,
and 2.5
Vi
value.
They obtained
Vi
values
x lo4 m/s f o r e x c i t a t i o n s o f 1.0
Eth, 1.26 Eth, and 2.5 Eth, r e s p e c t i v e l y .
The phenomenon of a m e l t -
i n v e l o c i t y p r o p o r t i o n a l t o e x c i t a t i o n i n t e n s i t y i s w e l l known from laser-anneal i n g experiments and model c a l c u l a t i o n s on t h e nanosecond time scale, though a t much lower v e l o c i t i e s (see Chapter 4,
Fig.
9 and Thompson e t al.,
value obtained by Shank e t al.
1983a).
However, t h e lowest
V i
i s already near t h e v e l o c i t y o f
sound i n c r y s t a l l i n e s i l i c o n ; they speculate t h a t a g r a d i e n t i n m e l t i n g r a t e vs depth may e x i s t .
A d i r e c t measurement o f t h e time scale f o r t h e disappearance o f c r y s t a l l i n e s t r u c t u r e was also made by Shank and c o l l a b o r a t o r s (1983b), who used a 90-fs, 620-nm o p t i c a l pulse t o e x c i t e a (111) c r y s t a l l i n e s i l i c o n surface and then measured t h e second harmonic r a d i a t i o n generated by a second much weaker and time-delayed probe
6. pul se.
381
TIME-RESOLVED MEASUREMENTS
S i nce s i 1i c o n has i n v e r s i o n symmetry , second harmonic gen-
e r a t i o n r e q u i r e s higher-order terms i n i t s nonlocal o p t i c a l susc e p t i b i l i t y , i n c l u d i n g both i s o t r o p i c and a n i s o t r o p i c second harmonic c o n t r i b u t i o n s ; it i s t h e l a t t e r , a n i s o t r o p i c ,
terms which
c a r r y i n f o r m a t i on regarding t h e exi stence o f c r y s t a l 1 ine symnetry (Bloembergen e t a1
., 1968;
Shank e t a1
., 1983b).
Although t h e
exact o r i g i n o f t h e second harmonic r a d i a t i o n i s s t i l l a subject f o r study, it i s known t o be generated i n t h e experiments o f Shank and co-workers w i t h i n a near-surface l a y e r t h a t i s no m r e than
70 A deep,
corresponding t o t h e escape depth f o r t h e i r second
harmonic l i g h t . Figure 20 shows t h e observed second harmonic r a d i a t i o n as a f u n c t i o n o f time and o f t h e angle o f r o t a t i o n 4 about t h e <111> axis;
t h r e e - f o l d symmetry,
i n t e r v a l s o f 120 degrees,
w i t h maxima and minima repeating a t i s apparent a t a low e x c i t a t i o n o f 0.5
Eth (Fig. 20a) where Eth = 0.1 J/cm2 i s t h e t h r e s h o l d f o r formation
o f an amorphous surface l a y e r , i n d i c a t i v e o f melting.
However,
a t a higher e x c i t a t i o n of 2.0 Eth, t h e second harmonic r a d i a t i o n
measured a t t h e maximum ( 6 = 120O) d r a s t i c a l l y decreases w h i l e t h a t a t t h e minimum p o s i t i o n ( 4 = 60')
increases s l i g h t l y b u t decreases
again w i t h i n about 500 fs (Fig. 20c).
As shown i n Fig. 20b, t h e
surface has l o s t considerable order a f t e r 240 fs; t h e second harmonic r a d i a t i o n becomes completely i s o t r o p i c w i t h i n 1 ps.
Shank
e t a l . (1983b) speculate t h a t since t h e second harmonic energy (4.0 eV) i s very c l o s e t o a sharp absorption peak o f s i l icon, t h e l a r g e decrease i n i n t e n s i t y i s probably due t o t h e l o s s o f resonant enhancement o f second harmonic generation t h a t would be expected t o accompany m e l t i n g . I n conclusion, t h e measurements o f Shank and c o l l a b o r a t o r s demonstrate a loss of c r y s t a l l i n e order on t h e (111) s i l i c o n surf a c e t h a t i s c o n s i s t e n t w i t h m e l t i n g o c c u r r i n g on a time scale of s u b s t a n t i a l l y l e s s than one picosecond. T h e i r r e f l e c t i v i t y measurements show t h a t t h e m e l t i n g process i s apparently i n i t i a t e d a t
382
D. H. LOWNDES E T A L
a1
120.
1.2
r
I
-1
0
I
I
I
1
I
2
3
4
J
5
t (PSI
Fig. 20. Polar plot of second harmonic intensity vs angle 41, at several times, for excitation energies o f ( a ) 0.5 Eth and ( b ) 2.0 Eth. ( c ) Normalized second harmonic intensity vs time for an excitation energy o f 2.0 Eth. Upper curve: 4 = 120° (maximum). Lower curve: 4 = 60° (minimum) (Shank et a l . ,
1983b).
6.
TIME-RESOLVED MEASUREMENTS
383
the surface, b u t is preceded by formation of a dense (>1021/cm3) electron-hole plasma in the near-surface region. 9.
OBSERVATIONS OF THE OPTICALLY EXCITED ELECTRON-HOLE PLASMA IN SILICON
From Eq. (11) and the expression for the plasma frequency, 9, i t follows t h a t a plasma-induced high reflectivity signal is expected only for optical probing frequencies w < 9.For E = 11.8 (the high-frequency dielectric constant of silicon), m* = 0.3 m, (mo = free electron mass) and w i t h N (=Ne 'Nh) = 5 x 1020/cm3being the electron-hole pair density, this cutoff frequency for the observation of plasma effects corresponds t o photon energies less t h a n 1.55 ev or t o probing wavelengths longer t h a n about 800 nm. Furthermore, since N varies both during creation and decay of the plasma, optical probing a t near-i nfrared wave1 engths should reveal two reflectivity minima, on either side ( i n time) of a plasmainduced reflectivity maximum (Lietoila, 1981). This effect provides a characteristic "signature" for plasma formation and decay and a sort of "holy grail" for experimentalists, w i t h which t o unambiguously distinguish between plasma formation and melting i n timeresolved reflectivity measurements. van Driel e t al. (1984) have pointed out that although a pl asma-i nduced ref1 ectivi t y mi nimum has been observed i n several semiconductors, the expected plasmon-resonance reflectivity maximum has not been observed in all cases. (However, see Gallant and van Driel (1982) for such observations in germanium.) For silicon, the plasma-induced reflectivity minimum was not observed until 1982, apparently because of the limited time resolution of e a r l i e r measurements. Figure 21 shows results of time-resolved reflectivity ( R ) and transmission ( T ) measurements carried o u t by von der Linde and Fabricius (1982) using 25-ps, 532-nm pulses for excitation of (100) silicon, and using weaker, time-delayed probe pulses at 1.06 pm, with the probe pulses focused t o a small central part of the
384 1.0 L
I
I I
-
0.6‘k
a4
-
0.2
-
0
.-
I
-
0.8
I I
It
.
&
:
c-Si
o.xi~m-2
. -. - - -‘I’. *
1
-150 -100
-50
50
0
100
150
200
DELAY TIME [ps]
I
1.0 0.8
I I I I I
-
ol
r-Si
I
I
-150
-100
-50
I
0
I
50
100
0.11 Jcm-2
150
200
DELAY TIME [ p s ]
Fig. 21. Reflectivity and transmission vs time delay for excitations of ( t o p ) 0.35 J / c m 2 and (bottom) 0.11 J / c m 2 (von der Linde and Fabricius, 1982).
excited area of the sample surface. von der Linde and Fabricius found t h a t there was a d i s t i n c t energy density threshold a t E t h = 0.21 (+ 0.01) J/cm2, separating two entirely different types of R and T behavior. For E l > E t h (Fig. 21, t o p ) the i n i t i a l 32%reflect i v i t y drops t o 28% about 20 ps before the peak of the excitation pulse, then rises t o a flat-topped plateau value of 76%, equal t o the r e f l e c t i v i t y of molten silicon a t 1.06 pm; simultaneous with the l a t t e r transition, the T signal drops t o about 5%, uncorrected f o r photoluminescence (see Fig. 4) or for l i g h t scattered from
6. evaporating material
TIME-RESOLVED MEASUREMENTS
(Liu
385
et al., 1982b), both of which may be
recorded along with transmitted probe light. I n contrast, for ER E t h (Fig. 21, bottom) a distinct R minimum was observed about 10 ps after the excitation pulse, with no l a t e r transition t o a
<
high-reflectivity phase, while the T signal also exhibited a minimum (very slightly l a t e r t h a n that in R ) b u t then recovered over several hundred picoseconds. The previously unobserved R minimum in both parts of Fig. 21 clearly reveals the formation of a dense electron-hole plasma; the rapid recovery of R i n the low excitation experiments indicates that the plasma recombination time i s less than the 25-ps excitation pulse duration. Finally, the jump t o R = 76% for E l > E t h can only be due t o melting: Attributing i t t o the plasma would require a three-orders-of-magnitude increase i n the plasma lifetime over only a 10%range in ER (von der Linde and Fabricius, 1982). Similar R results t o those shown in Fig. 21 were obtained for bulk sil icon by L i u e t a1 (1982b), who also carried out R and T measurements using SOS specimens. They point
.
o u t that the t h i n ( 4 . 5 pn) Si film provided by SOS samples is useful in t h a t i t minimizes carrier diffusion effects while promoting uniform temperatures and plasma profiles within the penetration depth of the excitation pulse; multiple interferences of the probing pulse at the air-sil icon and silicon-sapphire interfaces also result i n h i g h sensitivity t o time-dependent plasmainduced changes i n the complex index of refraction of silicon. However, the interpretation of T measurements i s a1 so made slightly less direct by the presence of these multiple reflections from the f r o n t and back surfaces of specimens (see also Lowndes e t al., 1982b). More quantitative information about the behavior of optically induced electron-hole plasmas in silicon can also be obtained from exci te-and-probe experiments by varying the frequency of the probing pulse, as demonstrated by Lompr6, Liu, Kurz, Bloembergen 1984a, b; van Driel e t al., 1984). and van Driel (Lompr6 e t a1
.,
386
D. H. LOWNDES E T A L .
I n p a r t i c u l a r , changes i n o p t i c a l p r o p e r t i e s due t o v a r i a t i o n s i n l a t t i c e temperature (Lomprb e t al.,
1983) can be separated from
changes due t o v a r i a t i o n s i n c a r r i e r d e n s i t y (Lomprb e t al., 1984b; van O r i e l e t a1
., 1984).
Assumi ng a Drude treatment of t h e e l e c t r o n - h o l e p l asma, t h e r e a l and imaginary p a r t s o f t h e complex d i e l e c t r i c constant ( T = €1 + i
~ may ~ be w r) i t t e n as
- k[
E~
= nL
E~
= 2nL k L
L
- aN/w2 ,
(12)
+ bN/w3 ,
(13)
where
and where R = n + i k , t h e absorption c o e f f i c i e n t a = 4nk?/, t h e d e n s i t y of e l e c t r o n - h o l e p a i r s , me (mh) and t h e e l e c t r o n ( h o l e ) mass with
m* =
(m,'
+ mi1)-'
N is
()
are
and mean s c a t t e r i n g time, r e s p e c t i v e l y , and
the subscript
frequency l a t t i c e p r o p e r t i e s of
L refers
t o the high
(unexcited) c r y s t a l l i n e s i l i c o n
i n t h e absence o f a laser-induced plasma. From Eqs. (12)-(15) i t f o l l o w s t h a t (1) EL increases w i t h l a t t i c e temperature (dn/dT > 0) b u t decreases w i t h N, w h i l e (2) and N.
E~
increases w i t h both temperature
These dependences can be separated e x p e r i m e n t a l l y by
c a r r y i n g out measurements b o t h a t s h o r t probe wavelengths (e.g., 532 nm),
f o r which plasma c o n t r i b u t i o n s become small,
and f o r
l o n g e r wavelengths (2-3 p ) , f o r which plasma e f f e c t s dominate. Thus, time-resolved n e a r - i n f r a r e d r e f l e c t i v i t y and transmission measurements a c t u a l l y provide a means f o r studying t h e time evolut i o n o f t h e c a r r i e r d e n s i t y i n l a s e r - e x c i t e d e l e c t r o n - h o l e plasmas, and are not r e s t r i c t e d t o simply d i s t i n g u i s h i n g between plasma f o r m a t i o n and melting.
However, o p t i c a l measurements are not able
6.
387
TIME-RESOLVED MEASUREMENTS
t o determine N and m* s e p a r a t e l y , b u t o n l y t h e r a t i o N/m*, t h e s c a t t e r i n g t i m e s ( < T ~ > , ) a l s o e n t e r Eqs.
since
(12)-(15)
as
unknown parameters. van D r i e l e t a l .
(1984) r e c e n t l y used t i m e - r e s o l v e d R and
T
measurements t o observe t h e plasmon resonance i n s i l i c o n and i n germanium, pm.
v i a measurements a t probe wavelengths o f 1.9
The plasmas were
pul ses,
excited
and 2.8
u s i n g <30 ps, 532 nm Nd:YAG l a s e r
w h i l e t h e probe wave1 engths were generated by s t i m u l a t e d
Raman s c a t t e r i n g i n 0.5-m l o n g c e l l s c o n t a i n i n g 50 atm o f 1.9 pin) o r CHI, ( f o r 2.8
pm).
H2 ( f o r
F i g u r e 22 shows t h e i r r e s u l t s o f R
and T measurements f o r S i a t e x c i t a t i o n l e v e l s w e l l below t h e m e l t i n g threshold.
A t 1.9
pm and f o r e x c i t a t i o n <0.04 J/cm2,
s i n g l e broad R minimum was observed (Fig. o f t h i s R minimum,
22a).
From t h e depth
and u s i n g t h e Drude model [Eqs.
van D r i e l e t a l . e s t i m a t e d t h a t N/m*
=
3.6
a
(12)-(15)1,
x 1048/g-~m3, though
n o t i n g t h a t t h i s cannot be p r e c i s e l y c o r r e c t s i n c e t h e depth o f t h e R minimum was c o n v o l u t e d by t h e f i n i t e w i d t h o f t h e probe pulse.
Assuming an m* range from 0.123 m, ( t h e low d e n s i t y v a l u e )
up t o 0.25
nb, t h i s r e s u l t s i n an estimated peak plasma d e n s i t y
o f 4-8 x 1020/cm3.
A t 2.8
pm t h e plasmon resonance i n s i l i c o n
was c l e a r l y r e s o l v e d (Fig. 22b), t h e R minimum a t low (0.015 J/cm2) e x c i t a t i o n evolving a t higher e x c i t a t i o n i n t o t h e c h a r a c t e r i s t i c resonance s i g n a t u r e o f two minima on e i t h e r s i d e o f an R peak ( t h e l a t t e r near t h e zero of t i m e d e l a y ) , as t h e near-surface c a r r i e r d e n s i t y passes t w i c e through t h e c r i t i c a l c a r r i e r d e n s i t y . van D r i e l e t a l . (1984) n o t e t h a t t h e y would expect a d e c o n v o l u t i o n o f t h e d a t a o f Fig. 22b with r e s p e c t t o t h e e x c i t a t i o n p u l s e width t o r e s u l t i n a peak plasmon r e f l e c t i v i t y o f >0.9. Lompr6 and co-workers (1984a, b) have also r e p o r t e d r e s u l t s o f t i m e - r e s o l v e d experiments i n which t h e y used a t h r e e - p u l s e techn i q u e t o show t h a t plasma h e a t i n g by f r e e c a r r i e r a b s o r p t i o n a p p a r e n t l y does n o t r e s u l t i n any s i g n i f i c a n t i n c r e a s e i n t h e plasma c a r r i e r d e n s i t y by impact i o n i z a t i o n , on t h e -20 picosecond
388
D. H. LOWNDES ET AL.
,
1-
-
5 Q8-
1 -
.
ul Z
Q6-
s t
ti
0.04 J/cm2
?I
-t
I I
REFLECTIVITY a, 19pm TRANSMISSION
-
I
-- -- -
2'
2 W
-
= 0.2-
I
1.o
-*'. -
-
-+
0.4-
-
--.--
\
I
>
t
,
\
1
I
I
I
I
I
BULK SILICON WAFER
Ib)
0.04J/cm* REFLECTIVITY A TRANSMISSION
at 2.8pm
-
_---*------J -a
Fig. 22. Reflectivity and transmission o f bulk silicon as functions o f probe t i m e delay, for probe wavelengths of ( a ) 1.9 km and ( b ) 2.8 pm and excitation pulse ( 5 3 2 n m ) energy densities of 0.015 and 0.04 J / c m 2 .
6.
389
TIME-RESOLVED MEASUREMENTS
t i m e s c a l e o f t h e i r experiments.
The f i r s t p u l s e (20 ps, 530 nm)
was used t o c r e a t e a plasma; a second p u l s e (30 ps, 1.06 pm) a t a f i x e d t i m e d e l a y o f 100 ps was used t o add energy t o t h e plasma by f r e e c a r r i e r a b s o r p t i o n ; a t h i r d p u l s e t h e n probed t h e r e s u l t i n g changes i n R and T as a f u n c t i o n o f i t s own t i m e delay.
The d a t a
r e s u l t e d i n changes i n R and T t h a t c o u l d be e x p l a i n e d e n t i r e l y by l a t t i c e h e a t i n g (see Sec. i n c r e a s e o f N.
11.4.b),
w i t h no evidence o f an
T h i s r e s u l t enabled Lompr6 e t a1
. (1984)
t o cal-
c u l a t e t h e h i g h e s t p o s s i b l e plasma d e n s i t y t h a t can be reached, assuming instantaneous thermal i z a t i o n o f a 20-ps, 0.1-J/cm2, e x c i t a t i o n p u l s e and no impact i o n i z a t i o n .
530-nm
The peak c a r r i e r den-
s i t y c a l c u l a t e d a t t h e s i l i c o n s u r f a c e was s l i g h t l y l e s s t h a n 8 x lO2O/crn3 ( i n good agreement with t h e o t h e r e s t i m a t e s above f o r lower e x c i t a t i o n levels).
V. 10.
S o l i d i f i c a t i o n o f H i g h l y Undercooled L i q u i d S i l i c o n
LIQUID-TO-AMORPHOUS PHASE TRANSFORMATION
I t i s now w e l l known t h a t t h i n l a y e r s o f amorphous s i l i c o n can be formed by u l t r a r a p i d s o l i d i f i c a t i o n from a s h a l l o w pool o f m o l t e n s i l i c o n t h a t i s produced by pulsed l a s e r i r r a d i a t i o n o f a c-Si surface; e i t h e r nanosecond pulses o f u l t r a v i o l e t l i g h t o r picosecond pulses o f v i s i b l e and n e a r - i n f r a r e d l i g h t may be used ( L i u e t al., 1979, 1981; Tsu e t a1 1979; C u l l i s e t a1 1982a, b; Boyd e t al., 1984). The amorphous phase forms as a r e s u l t o f u l t r a r a p i d
.,
.,
c o o l i n g o f a v e r y t h i n m o l t e n l a y e r by conduction t o t h e u n d e r l y i n g c r y s t a l l i n e substrate.
The c r i t i c a l i n f l u e n c e o f c o o l i n g r a t e on
amorphous phase f o r m a t i o n i s i l l u s t r a t e d by t h e f a c t t h a t t h e amorphous l a y e r can be formed o n l y i f t h e pulsed l a s e r f l u e n c e l i e s w i t h i n a narrow Ea window, j u s t above t h e m e l t i n g t h r e s h o l d fluence. Higher fluence r e s u l t s
i n m e l t i n g t o a s u f f i c i e n t depth t h a t
thermal energy s t o r e d i n t h e l i q u i d l a y e r prolongs t h e d u r a t i o n
390
D. H. LOWNDES E T A L
o f m e l t ng, w i t h c o o l i n g and s o l i d i f i c a t i o n t h e n o c c u r r i n g s u f f i c i e n t l y s l o w l y t h a t e p i t a x i a1 regrowth from t h e c r y s t a l 1 ine substrate
e s u l t s ( L i u e t al.,
1979, 1981; Yen e t al.,
1982).
Thus,
amorphous phase f o r m a t i o n i s f a v o r e d by t h e use o f a pulsed l a s e r source having a s h o r t a b s o r p t i o n l e n g t h and s h o r t p u l s e d u r a t i o n , i n o r d e r t o minimize thermal d i f f u s i o n and produce t h e h i g h e s t p o s s i b l e temperature g r a d i e n t s .
Amorphous l a y e r s a r e a1 so formed
more e a s i l y on a (111) s i l i c o n s u r f a c e t h a n on (100).
However, a
q u a n t i t a t i v e understanding o f t h e d i f f e r e n c e s observed on (100) and (111) s u r f a c e s has n o t been achieved so f a r ,
because o f ( 1 )
u n c e r t a i n t i e s r e g a r d i n g t h e e x t e n t o f undercool i n g i n t h e me1t near t h e growing i n t e r f a c e (Jackson; see Poate, 1982) and ( 2 ) because o f fundamental d i f f i c u l t i e s i n m i c r o s c o p i c a l l y modeling i m p u r i t y s e g r e g a t i o n and c r y s t a l growth phenomena w i t h i n t h e growi ng i n t e r f a c e i t s e l f (Wood, 1982, 1983; Poate, 1982). Large me1t undercool ings (below Tc = 1685 K, t h e normal f r e e z i n g temperature o f c-Si ) a r e expected t o accompany t h e l a r g e temperature g r a d i e n t s and c o o l i n g r a t e s t h a t r e s u l t i n amorphous phase f o r m a t i o n , b u t t h e p r e c i s e r o l e o f m e l t u n d e r c o o l i n g i s not understood (see Chapter 5,
Sec.
Thompson e t a1
11).
., 1983a)
One model
(Spaepen and T u r n b u l l ,
1982;
d e s c r i b e s t h e process o f amorphous phase
f o r m a t i o n i n e s s e n t i a l l y thermodynamic terms,
and assumes t h a t
amorphous phase f o r m a t i o n can occur o n l y when t h e undercooled m e l t temperature fa1 1 s we1 1 below t h e me1t i ng temperature o f a-Si , Ta, which i s e s t i m a t e d t o l i e some 200-250 K below Tc (see t h e f o l l o w i n g s e c t i o n o f t h i s c h a p t e r and Chapter 4.12).
An a l t e r n a t i v e model
(Wood, 1983) emphasizes k i n e t i c , r a t h e r t h a n p u r e l y thermodynamic, effects.
Using k i n e t i c r a t e t h e o r y and t h e concept o f a r a t e con-
s t a n t w i t h an a c t i v a t i o n energy dependent on regrowth v e l o c i t y , t h i s model a t t r i b u t e s amorphization t o t h e " t r a p p i n g " o f macroscopic c o n c e n t r a t i o n s o f d e f e c t s a t a r a p i d l y moving i n t e r f a c e , when t h e r e g r o w t h v e l o c i t y exceeds some c r i t i c a l value; t h i s c r i t i c a l velocit y does n o t necessari l y correspond t o undercool ing t o be1 ow Ta.
6.
391
TIME-RESOLVED MEASUREMENTS
Thermal m e l t i n g model c a l c u l a t i o n s have been used t o estimate t h a t t h e liquid-to-amorphous phase t r a n s i t i o n occurs f o r melt-sol i d i n t e r f a c e v e l o c i t i e s g r e a t e r than about 18 m/s ( C u l l i s e t al., 1982b);
however,
undercooling.
these c a l c u l a t i o n s neglect t h e e f f e c t o f me1t K i n e t i c models f o r c r y s t a l growth,
on t h e o t h e r
hand, d i r e c t l y r e l a t e t h e regrowth v e l o c i t y t o t h e free energy d i f f e r e n c e between t h e l i q u i d phase and regrowing s o l i d phase and, w i t h some approximations, t o m e l t undercooling (see Chapter 5 and a l s o Spaepen and Turnbull,
1982, f o r discussions o f t h i s p o i n t ) .
Thus, d i r e c t measurements o f t h e m e l t - s o l i d i n t e r f a c e v e l o c i t y , under c o n d i t i o n s l e a d i n g t o amorphous regrowth , can i n p r i n c i p l e be used t o i n f e r t h e amount o f undercooling, i f t h e form o f t h e equation connect ing v e l o c i t y and undercool ing i s known. Thompson e t al.
(1983a) have r e c e n t l y a p p l i e d t h e t r a n s i e n t
conductance technique
(see Sec.
11.2)
t o d i r e c t l y measure t h e
v e l o c i t y o f t h e s o l i d i f y i n g m e l t - s o l i d i n t e r f a c e under c o n d i t i o n s such t h a t t h e liquid-to-amorphous Using 2.5-ns,
t r a n s i t i o n o f s i l i c o n occurs.
347-nm l a s e r pulses, amorphous regrowth was observed
o n l y f o r El between 0.2 and 0.3 J/cm2 (compare w i t h Fig. 9), w i t h a maximum depth o f a-Si o f 14 nm (measured by RBS and TEM) being obtained f o r El
=
0.27 J/cm2.
F i g u r e 23 (Thompson e t a1
., 1983a)
shows t h e regrowth v e l o c i t y p l o t t e d as a f u n c t i o n o f maximum melt depth.
By v i s u a l l y observing t h e r e f l e c t i v i t y change character-
i s t i c o f formation o f a-Si,
a f t e r t h e t r a n s i e n t conductance mea-
surements, Thompson e t al. were able t o associate t h e occurrence as
o f amorphization w i t h a p a r t i c u l a r regrowth v e l o c i t y range;
shown i n Fig. 23, amorphization o f a (100) SOS surface occurs f o r regrowth v e l o c i t i e s exceeding 15 m/s.
I f t h e measurement o f a c r i t i c a l amorphization v e l o c i t y o f 15 m/s i s i n t e r p r e t e d s t r i c t l y w i t h i n t h e l i m i t s o f t h e thermodynamic model f o r a-Si formation, 200-250
K (i.e.,
Thompson e t a1
then a melt undercooling o f a t l e a s t
t o Ta) i s i m p l i e d a t t h i s v e l o c i t y .
. (1983a)
However,
p o i n t out t h a t t h e i r measurements may a1 so
392
D. H. LOWNDES E T A L .
01
I
0
50
1
I
I50 Melt depth (nm) 100
I
200
250
Fig. 23. Regrowth velocity vs melt depth for pulsed uv-irradiated SOS. The hatched area and solid data points represent the regime in which a-Si forms from the melt (Thompson et al., 1 9 8 3 a ) .
p r o v i d e i n d i r e c t support f o r t h e idea t h a t t h e r e a r e s t r o n g k i n e t i c
1i m i t a t i o n s on n u c l e a t i o n o f t h e amorphous phase: Considerably more Si was found t o melt (40-50 nrn) and be subjected t o h i g h regrowth v e l o c i t i e s than t h e maximum amorphous l a y e r thickness o f 14 nm t h a t t h e y observed.
One e x p l a n a t i o n f o r t h i s i s given by TEM micro-
graphs t h a t revealed undulations i n amorphous l a y e r thickness and i s l a n d s of a-Si,
suggesting t h a t n u c l e a t i o n o f a-Si may be i n i -
t i a t e d by i n t e r f a c e breakdown a t h i g h regrowth v e l o c i t i e s , a k i n e t i c limitation.
A l t e r n a t i v e l y , t h e discrepancy between maximum
m o l t e n and amorphous l a y e r thicknesses could be explained p u r e l y thermodynamically
i f t h e r e were a s u b s t a n t i a l
delay t i m e [-30
nm/(15 m/s) = 2 ns] r e q u i r e d t o undercool t h e l i q u i d t o Ta (Thompson e t a1
., 1983a).
6.
393
TIME-RESOLVED MEASUREMENTS
Closely r e l a t e d measurements o f m e l t - s o l i d i n t e r f a c e v e l o c i t y were also c a r r i e d out by Bucksbaum and Bokor (1984), who used a 248-nm, 15-psec m e l t i n g l a s e r pulse and a delayed 1.64-pm i n f r a r e d probe pulse t o determine t h e thickness o f t h e molten s i l i c o n f i l m on t h e picosecond time scale, using both r e f l e c t i v i t y and t r a n s mission measurements.
P r o f i l e s of m e l t depth vs time were obtained
f o r both (100) and (111) c-Si surfaces f o r fluences o f 0.03-0.11 J/cm2, corresponding t o maximum m e l t depths o f 2-55 nm, melt durat i o n s from -200 ps t o >2 ns, and r e s u l t i n g i n a n e a r l y constant s o l i d i f i c a t i o n v e l o c i t y o f about 25 m/s over about 80% o f t h e d u r a t i o n o f amorphous regrowth.
I n these experiments,
liquid
f i l m s up t o 40-nm t h i c k were found t o be f u l l y amorphized upon resolidification.
Bucksbaum and Bokor a l s o found t h a t they were
unable t o f i t t h e i r set o f measured melt depth vs time p r o f i l e s ( f o r v a r i ous 1aser f l uences) u s i ng a conventional thermal me1t ing model c a l c u l a t i o n t h a t assumed t h a t s o l i d i f i c a t i o n occurred a t
Tc = 1685 K.
However, by modifying t h e i r c a l c u l a t i o n t o i n c l u d e
a c o n s t r a i n t re1 a t i ng me1t - s o l i d i n t e r f a c e v e l o c i t y t o me1t undercooling, w i t h separate c o n s t r a i n t s f o r c r y s t a l l i n e s o l i d i f i c a t i o n ( f o r v e l o c i t i e s below 15 m/s) and amorphous s o l i d i f i c a t i o n (above t h e c r i t i c a l v e l o c i t y o f 15 m/s) they were able t o f i t t h e i r data. The undercool i n g a t t h e crossover p o i n t between c r y s t a l 1 ine and amorphous s o l i d i f i c a t i o n , i n t h i s f i t t e d c a l c u l a t i o n , was found t o be AT = 365-415
K.
However, t h i s c a l c u l a t i o n appears not t o
have taken i n t o account t h e very low thermal c o n d u c t i v i t y o f t h e growing a-Si l a y e r . Other c a l c u l a t i o n s , using a thermal conduct i v i t y value of 0.02 W/cm-K f o r a-Si (see Sec. II.l.d),
were a b l e
t o reproduce Bucksbaum and Bokor's experimental r e s u l t s w i t h much s m a l l e r m e l t undercooling
(R. F. Wood, p r i v a t e communication).
Bucksbaum and Bokor (1984) have discussed a number o f discrepancies between t h e i r experiments and model c a l c u l a t i o n s , but t h e p r i n c i p a l experimental conclusions appear t o be t h a t o p t i c a l measurements can be used t o i n f e r t h e p o s i t i o n o f t h e m e l t - s o l i d i n t e r f a c e w i t h
394
D. H. LOWNDES ET AL
-20 psec resolution, and t h a t t h e regrowth velocity s a t u r a t e s a t
about 25 m/s under conditions leading t o regrowth of r e l a t i v e l y thick amorphous films. 11.
NUCLEATION AND GROWTH OF POLYCRYSTALLINE SILICON FROM UNDERCOOLED LIQUID SILICON
.
Recent experiments by Thompson et a1 (1984) appear t o confirm t h e e a r l i e r estimate by Donovan and co-workers (1983) t h a t the melting temperature, Ta, of a-Si l i e s some 200-300 K below Tc = 1685 K f o r c-Si. The average thermal conductivity of a-Si has a l s o been found t o be Ka 0.02 W/cm-K, between one and two orders of magnitude l e s s than Kc f o r c-Si (see Sec. 1I.l.d). Lowndes and Wood have recently shown t h a t these differences in the properties o f the a- and c-phases of Si provide t h e basis f o r a pulsed l a s e r method f o r transforming an ion-implanted a-Si layer t o a highly undercooled l i q u i d (1)phase, and f o r studying the subsequent rapid s o l i d i f i c a t i o n process in a time-resolved way (Lowndes e t a1 1984a, b y c; Wood e t a1 1984). According t o these authors, i f a low fluence nanosecond l a s e r pulse i s used t o melt only p a r t i a l l y through an implanted a-Si layer, t h e molten Si t h a t i s produced remains a t a r e l a t i v e l y low temperature (T >, Ta)y corresponding t o undercooling of the e n t i r e pool of ,t-Si; the undercooled melt a1 so remains thermally and physically isolated from t h e c-Si beneath by t h e remaining, unmelted, low-Ka a-Si b a r r i e r layer. The absence of a c r y s t a l l i n e (or polycrystalline) s u b s t r a t e in contact with the melt prevents e p i t a x i a l regrowth, making i t possible t o observe what they i n t e r p r e t as bulk (volume) nucleation and crystal growth, d i r e c t l y from t h e highly undercooled melt. (This volume nucleation process i s not necessarily identical with homogeneous nucleation, since e i t h e r chemical impurities or small c r y s t a l l i t e s may be present in t h e i n i t i a l ion-implanted a-Si layer.) Time-resol ved r e f l e c t i v i t y ( R ) measurements showed t h a t the flat-topped h i g h - r e f l e c t i v i t y phase t h a t characterizes pulsed l a s e r melting o f c-Si was not observed following melting of r e l a t i v e l y
-
.,
.,
395
6 . TIME-RESOLVED MEASUREMENTS
t h i c k a-Si l a y e r s a t low Ea (near and above t h e a-Si m e l t i n g threshold).
Instead, w i t h i n a few nanoseconds o f reaching t h e maximum
R, t h e R s i g n a l began t o decay continuously t o lower values, behavi o r t h a t Lowndes e t a l . ( 1 9 8 4 ~ )a t t r i b u t e d t o t h e bulk n u c l e a t i o n and growth o f f i n e c r y s t a l l i t e s o c c u r r i n g w i t h i n t h e depth o f t h e
a-Si t h a t was o p t i c a l l y accessible t o t h e i r cw probe l a s e r beam. The importance of t h e thermal i s o l a t i o n provided by t h e unmelted a-Si was i l l u s t r a t e d by R measurements o f surface melt duration, T ~ ,vs
Ell f o r a-Si l a y e r s o f several thicknesses.
range o f
El,
T~
Over a l i m i t e d
was observed t o decrease w i t h i n c r e a s i n g E l ( f o r
an a-Si thickness o f -200 nm), o r t o e x h i b i t a l o n g plateau vs EJ ( f o r thinner a-layers)
,
behavior t h a t model c a l c u l a t i o n s showed
r e s u l t e d from s u b s t a n t i a l changes i n t h e r a t e o f heat l o s s t o t h e s u b s t r a t e when t h e 1 - S i had n e a r l y melted through t h e a-Si l a y e r t o t h e c-Si s u b s t r a t e beneath (Lowndes e t al., e t a1
., 1984).
Cross-sectional
1984a, b y c; Wood
TEM micrographs revealed t h a t a t low l a s e r Ex
t h e r e s o l i d i f i e d near-surface r e g i o n was composed o f very small (-100 A diam), equiaxed and randomly o r i e n t e d g r a i n s o f polycryst a l l i n e S i (Narayan and White, 1984; Lowndes e t a1
., 1984b).
At
h i g h e r Ex, t h i s f i n e - g r a i n e d m a t e r i a l was found o n l y deeper w i t h i n t h e r e s o l i d i f i e d region, w i t h large, columnar g r a i n s o f polycryst a l l i n e s i l i c o n (p-Si) extending back t o t h e surface.
The f i n e -
grained p-Si i s b e l i e v e d t o r e s u l t from t h e bulk n u c l e a t i o n process,
-
o c c u r r i n g a t a temperature Tn Ta + 50 K. The t h e o r e t i c a l model used by Wood e t a1 (1984) t o simulate bulk n u c l e a t i o n and growth
.
i n undercooled S i , and t o account f o r t h e two d i f f e r e n t types o f p-Si
regrowth,
i s summarized i n Sec.
IV.12,
Chapter 4 o f t h i s
book. An a l t e r n a t i v e t o b u l k n u c l e a t i o n as t h e explanation f o r t h e f o r m a t i o n o f f i n e - g r a i n e d p-Si , d u r i n g s o l i d i f i c a t i o n of l a s e r melted a-Si layers, has been given by Thompson e t a l . (1984). They p o s t u l a t e t h a t (1) t h e i n c i d e n t pulsed l a s e r energy produces an
396
D. H. LOWNDES ETAL.
i n i t i a l l i q u i d layer t h a t s o l i d i f i e s and forms large-grained p-Si b u t t h a t (2) the deeper-lying fine-grained p-Si i s formed by an explosive c r y s t a l l i z a t i o n process when a t h i n , self-propagating l a y e r t r a v e l s through the remaining unmelted a-Si a t a velocity estimated t o l i e i n the 10-20-m/s range. As evidence o f the existence of t h i s t h i n , propagating l a y e r , they c i t e t r a n s i e n t conductance measurements which, a t l e a s t f o r very low l a s e r E l , e x h i b i t two peaks in the plot of melt depth vs time. The second peak occurs a f t e r the surface has r e s o l i d i f i e d ( a s determined by separate monitoring of the surface r e f l e c t i v i t y ) and i s believed t o represent this t h i n , propagating molten layer. The energy t o d r i v e a nearly s e l f - s u s t a i n i n g molten layer i s expected t o be provided by the release of l a t e n t heat of fusion when c r y s t a l l i z a t i o n occurs (see Leamy e t a1 , 1981). However, Wood et a1 (1984) had e a r l i e r pointed out t h a t mu1 t i p l e simultaneously propagating phase f r o n t s , including buried molten layers driven by the release of 1a t e n t heat, a r e present in t h e i r cal cul a t i ons simul a t i ng bul k nucleation (Chapter 4, Sec. IV.12). T h u s , t h e suggestion of Thompson e t a1 (1984) may be a special case of t h e more general modeling of Wood e t al. Lowndes e t a l . (1984b) have a l s o stressed t h e f a c t t h a t TEM micrographs of the fine-grained p-Si region show no evidence of the l i n e a r f e a t u r e s t h a t might indicate explosive regrowth or t h e existence o f a pl anar regrowi ng i n t e r f a c e during i t s formation. Instead, only randomly oriented, and apparently equiaxed, grains are observed, suggesting t h a t a planar i n t e r f a c e did not e x i s t during formation of t h i s region. If a planar recryst a l l i z a t i o n front i s not present, then the a t t r a c t i v e simplicity of i n t e r p r e t i n g t r a n s i e n t conductance changes in terms of an equivalent melt d e p t h may be l o s t , especially so i f a mixture of nucleating undercooled 1iquid and growing grains i s present.
.
.
.
397
6. TIME-RESOLVED MEASUREMENTS VI.
Concluding Observations
There now e x i s t s a l a r g e body o f remarkably p r e c i s e experimental data,
t o g e t h e r w i t h model c a l c u l a t i o n s ,
which e s t a b l i s h
t h a t t h e mechanism f o r pulsed l a s e r annealing o f s i l i c o n (and o t h e r semiconductors) i s thermal m e l t i n g f o l l o w e d by r a p i d s o l i d i f i c a tion.
D e t a i l e d comparisons between experiments and model calcu-
l a t i o n s demonstrate t h a t c a l c u l a t i o n s using t h e known o p t i c a l and thermal parameters o f s i 1 icon, augmented by f r e e - c a r r i e r absorption effects,
account very w e l l
f o r t h e observed time o f onset o f
melting, f o r m e l t i n g t h r e s h o l d l a s e r fluences, and f o r t h e d u r a t i o n
o f me1t ing , i n experiments using both nanosecond and picosecond l a s e r pulses.
This agreement i s now s u f f i c i e n t l y good t h a t com-
parisons between experiments and c a l c u l a t i o n s on t h e nanosecond t i m e scale a c t u a l l y - have been used t o i n f e r p r o p e r t i e s o f t h e s o l i d phase, p r i o r t o m e l t i n g (e.g.,
t h e thermal c o n d u c t i v i t y o f
As a r e s u l t o f t h i s success, f u t u r e d e t a i 1ed comparisons between me1t ing model c a l c u l a t i ons and timean amorphous s i l i c o n l a y e r ) .
resolved measurements seem l i k e l y t o focus on o b t a i n i n g a d e t a i l e d understanding o f t h e r a p i d m e l t i n g and s o l i d i f i c a t i o n phase t r a n s i t i o n s themselves.
Overheating i n t h e s o l i d phase, p r i o r t o melting,
has not been observed t o date; t h e r e l a t i o n s h i p between melt underc o o l i n g and t h e v e l o c i t y o f t h e regrowing m e l t - s o l i d i n t e r f a c e has n o t been established; t h e r e l a t i v e r o l e s and importance o f k i n e t i c vs thermodynamic e f f e c t s , a t t h e h i g h i n t e r f a c e v e l o c i t i e s r e s u l t i n g i n amorphous regrowth, are not understood; an experimental technique f o r measuring m e l t undercooling
( o r s o l i d overheating)
a t the
growing ( o r m e l t i n g ) i n t e r f a c e i s needed. The widespread
fundamental
interest
i n the p o s s i b i l i t y o f
p l asma-i nduced , r a t h e r than thermal l y induced, phase t r a n s i t i o n s a l s o s t i m u l a t e d t h e development o f a wide v a r i e t y o f time-resolved experimental techniques f o r d i f f e r e n t i a t i n g between t h e two, and has helped t o push t h e time scale f o r o p t i c a l measurements down i n t o t h e 10-100 femtosecond range.
I n i t i a l a p p l i c a t i o n s o f these
398
D. H. LOWNDES E T A L
t e c h n i q u e s were l i m i t e d almost e x c l u s i v e l y t o s i l i c o n .
Similarly,
much o f t h e e f f o r t t o d a t e focused on t h e s i n g l e problem o f making t i m e - r e s o l v e d l a t t i c e and/or c a r r i e r temperature measurements, i n o r d e r t o d i r e c t l y v e r i f y t h a t thermal m e l t i n g o f t h e l a t t i c e , r a t h e r t h a n k o l d " p l asma anneal ing , occurred, and t o e s t a b l ish whether c a r r i e r s and t h e l a t t i c e were a t s i m i l a r temperatures. T h i s narrow focus was u s e f u l , s i n c e i t r e s u l t e d i n t h e c o m p e t i t i v e e v a l u a t i o n o f a v a r i e t y o f d i f f e r e n t experimental techniques, t h e b e s t o f which are now capable o f making i n d i r e c t l a t t i c e temperat u r e measurements on a t l e a s t t h e nanosecond t i m e scale. Recent t i m e - r e s o l v e d o b s e r v a t i o n s o f t h e l o s s o f c r y s t a l 1 i n e s t r u c t u r e accompanying me1t i n g demonstrated t h a t t h e t r a n s f e r o f energy from pulsed l a s e r - e x c i t e d c a r r i e r s t o t h e l a t t i c e ( r e s u l t i n g i n m e l t i n g ) a c t u a l l y occurs i n l e s s t h a n con.
The f i r s t o b s e r v a t i o n s o f v e r y h i g h
one picosecond i n s i l i density
(N >, 1021/cm3)
s o l i d s t a t e plasmas, p r i o r t o t h e occurrence o f m e l t i n g , have a l s o been c a r r i e d out.
I t i s c l e a r t h a t a f e r t i l e area f o r f u t u r e
r e s e a r c h i s t h e study o f t h e p r o p e r t i e s and t i m e e v o l u t i o n o f such h i gh-densi t y plasmas,
and o f el ectron-phonon
coup1 i n g and t h e
e l e c t r o n - l a t t i c e energy r e l a x a t i o n processes t h a t r e s u l t i n l a t t i c e melting.
The study o f energy and momentum r e l a x a t i o n processes
o c c u r r i n g e n t i r e l y w i t h i n an i s o l a t e d , h i g h - d e n s i t y , l a s e r - i n d u c e d plasma w i l l a l s o soon become p o s s i b l e , as t i m e - r e s o l v e d techniques a r e pushed t o t h e low end o f t h e femtosecond range. Finally, some developments beyond t h e Drude model w i l l be needed i n t h e i n t e r p r e t a t i o n o f these experiments, i n o r d e r t o a c c u r a t e l y r e l a t e plasma p r o p e r t i e s t o t h e semiconductors i n which t h e y a r e produced.
Acknowledgments We would especi a1 l y 1ike t o acknowledge t h e c o n t r i b u t i o n s , t o o numerous t o number and e x t e n d i n g over several years, r e s u l t i n g from many e n j o y a b l e c o n v e r s a t i o n s and c o l l a b o r a t i o n s w i t h R. F. Wood.
6.
TIME-RESOLVED MEASUREMENTS
399
We a l s o f e e l a s p e c i a l debt o f g r a t i t u d e t o J u l i a Luck f o r h e r seemingly boundless patience w i t h r e v i s i o n s , attention t o detail,
and h e r continued
throughout t h e t y p i n g o f t h i s manuscript.
F i n a l l y , we would l i k e t o acknowledge h e l p f u l conversations w i t h
D. H. Auston, A. A y d i n l i , M. J. Aziz, P. B a e r i , A. Compaan, A. G. C u l l i s , G. J. Galvin, H. Kurz, B. C. Larson, D. von der Linde, C. L. Marquardt, S. C. Moss, G. L. Olson, M. 0. B. R. Appleton,
Thompson, and J. A. Van Vechten, and t h e h e l p o f a number of c o l leagues
who provided f i g u r e s f o r use i n t h i s chapter.
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6.
401
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J e l l i s o n , G. E., Jr., Modine, F. A., White, C. W., Wood, R. F., and Young, R. T. (1981). Phys. Rev. L e t t . 46, 1414. J e l l i s o n , G. E., Jr., and Modine, F. A. (1982a). Appl. Phys. L e t t . 41, 180. J e l l i s o n , G. E., Jr., and Modine, F. A. (1982b). J. Appl. Phys. 53, 3745. J e l l i s o n , G. E. , Jr., and Lowndes, D. H. (1982). Appl Phys. L e t t 41, 594. J e l l i s o n , G. E., Jr., and Modine, F. A. (1983). Phys. Rev. B 27, 7466. J e l l i s o n , G. E., Jr., Lowndes, D. H., and Wood, R. F. (1983a). Phys. Rev. B 28, 3272. J e l l i s o n , G. E. , Jr., Lowndes, D. H. , and Wood, R. F. (1983b). Mat. Res. SOC. Proc. 13, 35. J e l l i s o n , G. E. , Jr., and Wood, R. F. (1984). Mat. Res. SOC. Proc. 23, 153. Kemmler, M. , Wartmann, G., and von der Linde, D. (1984). Appl Phys. L e t t . 45, 159. Knapp, J. A., and P i c r a u x , S. T. (1981). Appl. Phys. L e t t . 38, 873. Koebel , J. M., and S i f f e r t , P. (1981). J. Appl. Lampert, M. O., Phys. 52, 4975. Larson, B. C., White, C. W., Noggle, T. S. , and M i l I s , D. (1982a). Phys. Rev. L e t t . 48, 337. Noggle, T. S., B a r h o r s t , J. F., and Larson, B. C. , White, C. W., M i l l s , D. (1982b). Mat. Res. SOC. Symp. Proc. 4 , 13. Larson, B. C., and Barhorst, J. F. (1980). J. Appl. Phys. 51, 3181. Noggle, T. S., Barhorst, J. F., and Larson, B. C., White, C. W., M i l l s , D. M. (1983a). Appl. Phys. L e t t . 42, 282. Larson, 6. C., White, C. W. , Noggle, T. S. , Barhorst, J. F. , and M i l l s , D. M. (1983b). Mat. Res. SOC. Symp. Proc. 13, 43. Leamy, H. J. , Brown, W. L., Cel l e r , G. K., F o t i , G., Gilmer, G. H., and Fan, J. C. C. (1981). Appl. Phys. L e t t . 38, 137. Lee, M. C., Lo, H. W., A y d i n l i , A., and Compaan, A. (1981). Appl. Phys. L e t t . 38, 499. Lee, M. C. , Aydinl i, A., Lo, H. W. , and Compaan , A. (1982). J. Appl Phys. 53, 1262. L i e t o i l a , A. (1981). Ph.D. Thesis, S t a n f o r d U n i v e r s i t y . L i u , J. M., Yen, R., Kurz, H., and Bloembergen, N. (1981). Appl. Phys. L e t t . 39, 755. L i u , J. M., Yen, R., Kurz, H., and Bloembergen, N. (1982a). Mat. Res. SOC. Symp. Proc. 4, 29. L i u , J. M., Kurz, H., and Bloembergen, N. (1982b). Appl. Phys. L e t t . 41, 643. L i u , P. L. , Yen, R. , Bloembergen, N., and Hodgson, R. T. (1979). Appl. Phys. L e t t . 34, 864. Lo, H. W., and Compaan, A. (1980a). Phys. Rev. L e t t . 44, 1604. Lo, H. W. , and Compaan, A. (1980b). J. Appl Phys. 51, 1565. Lo, H. W. , and Compaan, A. (1981). Appl Phys. L e t t . 38, 179. Lompre', L. A., L i u , J. M., and Bloembergen, N. (1983). Appl. Phys. L e t t . 43, 168.
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Shank, C. V., Yen, R., and H i r l i m a n n , C. (1983a). Phys. Rev. L e t t . 50, 454. Shank, C. V., Yen, R., and H i r l i m a n n , C. (1983b). Phys. Rev. L e t t . 51, 900. Yen, R., and H i r l i m a n n , C. (1984). Mat. Res. SOC. Shank, C. V., Symp. Proc. 23, 53. Shvarev, K. M. , Baum, B. A., and Gel 'd, P. V. (1977). High Temp. 15, 548. G e l l e r , M. , and B o r t f e l d , D. P. (1964). Appl. Phys. Sooy, W. R., L e t t . 5, 54. Spaepen, F., and T u r n b u l l , D. (1979). A I P Conf. Proc. 50, 73 ( A I P , New York. Spaepen, F., and T u r n b u l l , D. (1982). I n "Laser Annealing o f Semiconductors" (J. M. Poate and J. W. Mayer, eds.), Chapter 2. Academic Press, New York. and Tagle, J. A. (1981). Phys. S t r i t z k e r , B., Pospieszczyk, A., Rev. L e t t . 47, 356. Thompson, M. 0. , G a l v i n , G. J. , Mayer, J. W. , Hammond, R. B. , P a u l t e r , N., and Peercy, P. S. (1982). Mat. Res. SOC. Symp. Proc. 4, 209. Thompson , M. 0. , Mayer , J. W. , C u l l i s , A. G. , Webber , H. C. , Chew, N. G., Poate, J. M., and Jacobson, D. C. (1983a). Phys. Rev. L e t t . 50, 896. Peercy, P. S., and Thompson, M. O., G a l v i n , G. J., Mayer, J. W., Hammond, R. 6. (1983b). Appl. Phys. L e t t . 42, 445. Thompson, M. O., Galvin, G. J., Mayer, J. W., Peercy, P. S., Poate, J. M., Jacobson, D. C., C u l l i s , A. G., and Chew, N. G. (1984). Phys. Rev. L e t t . 52, 2360. and G a l v i n , G. J. (1983). Mat. Res. SOC. Symp. Thompson, M. O., Proc. 13, 57. Thurmond, C. D. (1975). J. Electrochem. SOC. 122, 1133. Tsu, R. , Hodgson, R. T. , Tan , T. Y., and B a g l i n , J. E. E. (1979). Phys. Rev. L e t t . 42, 1356. van D r i e l , H., Lompr6, L.-A,, and Bloembergen, N. (1984). Appl. Phys. L e t t . 44, 285. van D r i e l , H. M. (1984). Appl Phys. L e t t . 44, 617. Van Gurp, G. J., Eggermont, G. E., Tamminga, Y., Stacy, W. J., and G i j s b e r s , J. R. M. (1979). Appl. Phys. L e t t . 35, 273. Van Vechten, J. A. Tsu, R., and S a r i s , F. (1979). Phys. L e t t . A 74, 422. Van Vechten, J . A. (1980). J. Phys. 41, C4-15. Van Vechten, J. A., and Compaan, A. (1981). S o l i d S t a t e Commun. 39, 867. von der Linde, D. , and Wartmann , G. (1982). Appl Phys. L e t t . 41, 700. von der Linde , D. , and Fabricus , N. (1982). Appl Phys. L e t t . 4 1 , 991. von der Linde, D. , Wartmann, G., and Compaan, A. (1983a). J. Appl Phys. 43, 613. von der Linde, D., Wartmann, G., and 0207s~A. (1983b). Mat. Res. SOC. Symp. Proc. 13, 17.
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von der Linde, D., Wartmann, G., Kemmler, M., and Zhu, Z . 4 . (1984). Mat. Res. SOC. Symp. Proc. 23, 123. Wang, J. C. , Wood, R. F. , and Pronko, P. P. (1978). Appl Phys. L e t t . 33, 455. White, C. W., and Peercy, P. S., eds. (1980). "Laser and E l e c t r o n Beam Processing o f M a t e r i a l s." Academic Press, New York. White, C. W., Wilson, S. R., Appleton, B. R., and Young, F. W., Jr. (1980). J. Appl. Phys. 51, 738. Williamson, S., Mourou, G., and L i , J. C. M. (1984). Phys. Rev. L e t t . 52, 2364. Wood, R. F., and G i l e s , G. E. (1981). Phys. Rev. B 23, 2923. Wood, K. F., K i r k p a t r i c k , J. R., and G i l e s , G. E. (1981a). Phys. Rev. B 23, 5555. Wood, R. F. , Lowndes, D. H., and C h r i s t i e , W. H. (1981b). Mat. Res. SOC. Symp. Proc. 1, 231. Wood, R. F. (1982). Phys. Rev. B 25, 2786. Wood, R. F., Lowndes, D. H., J e l l i s o n , G. E., Jr., and Modine, F. A. (1982a). Appl. Phys. L e t t . 41, 287. Wood, K. F. , Lowndes , D. H. , and G i l e s , G. E. (1982b). Mat. Res. SOC. Symp. Proc. 4, 67. Wood, R. F., Rasolt, M., and J e l l i s o n , G. E., Jr. ( 1 9 8 2 ~ ) . Mat. Res. SOC. Symp. Proc. 4, 61. Wood, R. F. (1983). Mat. Res. SOC. Symp. Proc. 13, 83. Wood, R. F., Lowndes, D. H., and Narayan, J. (1984). Appl. Phys. L e t t . 44, 770. Yen, R., L i u , J. M., Kurz, H., and Bloembergen, N. (1982). Appl. Phys. A 27, 153. Yoffa, E. J. (1980). Phys. Rev. B 21, 2415. Young, R. T. , Wood, R. F. , and C h r i s t i e , W. H. (1982). J. Appl. Phys. 53, 1178.
.
CHAPTER 7 SURFACE STUDIES SEMICONDUCTORS
OF
PULSED LASER IRRADIATED
D. M. Zehner
. . . . .. .. .. .. .. .. .. .. .. .. .. . . . . . .. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. ... ... ........
I. INTRODUCTION. 11. EXPERIMENTAL APPROACH 1. Sampl e P r e p a r a t i o n 2. C h a r a c t e r i z a t i o n Techniques. 111. PRODUCTION OF ATOMICALLY CLEAN SURFACES 3. S i l i c o n . 4. Germanium. 5. Group 111-V Compounds. GEOMETRIC SURFACE STRUCTURE IV. 6. Ordered Surfaces 7. Metastable Surfaces. 8. V i c i n a l Surfaces 9. Defects. SURFACE AND SUB-SURFACE STUDIES OF V. ION-IMPLANTED SILICON 10. S u b s t i t u t i o n a l Implants. 11. I n t e r s t it i a1 Imp1a n t s APPLICATIONS. VI. CONCLUSIONS VII. REFERENCES.
. . . .. .. .. .. .. .. .. .. . . .. .. .. .. .. .. .. .. ..
............... . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . ................ ................ I.
Introduction
The process of pulsed l a s e r annealing p r o v i d e s a way o f very r a p i d l y t r e a t i n g t h e near-surface
r e g i o n o f semiconductors.
t h i s a d i a b a t i c mode o f thermal processing, 1 iquid-phase
epitaxial
In
m e l t i n g f o l l o w e d by
regrowth from t h e s u b s t r a t e occurs w i t h
growth v e l o c i t i e s o f t h e o r d e r o f meters/s.
Thus t h e h e a t i n g and
c o o l i n g r a t e s achieved by t h i s form o f processing are o r d e r s o f
405
Capyright 01984 b i Academic Press, Inc. All rights of reproduction in any form reserved.
ISBN 0-12-752123.2
406
D. M. ZEHNER
magnitude f a s t e r t h a n t h o s e achieved by more c o n v e n t i o n a l t r e a t ments.
i t has been shown t h a t
With proper annealing conditions,
r e g i o n s f r e e o f extended d e f e c t s can be formed and s u b s t i t u t i o n a l i m p u r i t i e s can be i n c o r p o r a t e d i n t o t h e l a t t i c e f a r i n excess o f t h e e q u i l i b r i u m s o l u b i l i t y l i m i t s (see Chapters 1-4).
The f i n a l
a c t o f s o l i d i f i c a t i o n i s t h e f r e e z i n g o f t h e surface.
I n view o f
the
region,
i t may be
(impurities,
geometric
results
expected
that
structure, ductors
obtained f o r the
the
surface
near-surface
properties
e l e c t r o n i c energy l e v e l s ) o f l a s e r - a n n e a l e d semicon-
can
be
significantly
altered
with
respect
to
those
o b t a i n e d by c o n v e n t i o n a l h e a t i n g treatments. R e s u l t s o f experiments d i s c u s s e d i n t h i s c h a p t e r show t h a t p u l s e d l a s e r a n n e a l i n g can be used t o produce a t o m i c a l l y c l e a n s u r f a c e s , remove damage i n t h e outermost s u r f a c e l a y e r s , and a l t e r t h e e l e c t r o n i c p r o p e r t i e s i n t h e s u r f a c e region.
D e t a i l s con-
cerned w i t h p r o c e s s i n g o f t h e s u r f a c e i n o r d e r t o achieve these conditions
and t h e measurement
discussed i n Section
11.
of
the surface properties are
R e s u l t s which show t h a t
unwanted i m p u r i t i e s i n semiconductors (0, C, etc.) to
near
the
practical
detection
limits
of
levels of
can be reduced
surface
sensitive
I n Section I V the
s p e c t r o s c o p i e s a r e presented i n S e c t i o n 111.
s u b j e c t o f o r d e r i n t h e outermost l a y e r s f o r b o t h f l a t and v i c i n a l s u r f a c e s i s discussed, and
changes
in
along w i t h the production o f defects
stoichiometry
for
compound
semiconductors.
S e c t i o n V d e a l s w i t h t h e changes i n b o t h geometric and e l e c t r o n i c p r o p e r t i e s o f t h e s u r f a c e r e g i o n which occur when i o n i m p l a n t a t i o n
is
combined w i t h
laser
annealing.
Finally
in
Section
YI,
examples o f how t h e unique s u r f a c e p r o p e r t i e s achieved w i t h l a s e r a n n e a l i n g can discussed.
be a p p l i e d t o o t h e r s u r f a c e
investigations
are
7.
407
PULSED LASER IRRADIATED SEMICONDUCTORS
11.
Experimental Approach
For most i n v e s t i g a t i o n s concerned w i t h l a s e r p r o c e s s i n g o f semiconductors,
t h e i r r a d i a t i o n o f t h e sample has been performed
i n a standard atmospheric environment.
It i s g e n e r a l l y assumed
t h a t t h e m o d i f i c a t i o n o f t h e subsurface p r o p e r t i e s i s u n a f f e c t e d by i n t e r a c t i o n s a t t h e g a s - s o l i d i n t e r f a c e .
However, when one i s
concerned w i t h o b t a i n i n g i n f o r m a t i o n about t h e p r o p e r t i e s o f t h e s u r f a c e r e g i o n (1-20 A ) and t h e changes which occur due t o l a s e r annealing,
t h e i r r a d i a t i o n o f t h e sample and subsequent a n a l y s i s
must t a k e p l a c e i n an u l t r a h i g h vacuum (UHV) environment Torr).
I n t h i s s e c t i o n , t h e experimental d e t a i l s concerned w i t h
b o t h l a s e r annealing o f semiconductors i n UHV and t h e subsequent s u r f a c e c h a r a c t e r i z a t i o n a r e presented.
1.
SAMPLE PREPARATION
A v a r i e t y o f l a s e r s has been used i n i n v e s t i g a t i o n s concerned w i t h surface s t u d i e s o f laser-annealed semiconductors.
The most
f r e q u e n t l y used l a s e r s are e i t h e r p u l s e d ruby o r p u l s e d Nd:YAG, a l t h o u g h UV excimer and t u n a b l e dye l a s e r s have a l s o been employed. The procedures f o l l o w e d i n p e r f o r m i n g t h e l a s e r a n n e a l i n g i n a
UHV environment are very s i m i l a r i n a l l i n v e s t i g a t i o n s and w i l l be i l l u s t r a t e d by d i s c u s s i n g t h e approach used w i t h p u l s e d ruby l a s e r s (Zehner e t al.,
1980a,b).
A f t e r bakeout,
t h e background
p r e s s u r e i n t h e chamber which c o n t a i n e d t h e sample was t y p i c a l l y l e s s than 2 x 10-10 Torr.
The l i g h t from a Q-switched ruby l a s e r
( A = 694 nm,
FWHM), t r a n s m i t t e d i n t o t h e UHV system
T
= 15 nsec,
t h r o u g h a glass window, vacuum environment.
was used t o i r r a d i a t e t h e sample i n t h e
The samples were p o s i t i o n e d so t h a t any evap-
o r a t e d m a t e r i a l o r s c a t t e r e d l i g h t was c o n t a i n e d i n an enclosure which s h i e l d e d a1 1 s u r f a c e a n a l y s i s
instruments.
i r r a d i a t e d u s i n g t h e single-mode (TEMoo)
Samples were
o u t p u t o f t h e ruby l a s e r
a t energy d e n s i t i e s t h a t c o u l d be v a r i e d between -0.2 J/cm2.
The beam diameter was t y p i c a l l y between 3.0
and -4.0
and 6.0 mn.
Energy d e n s i t i e s , which have been c o r r e c t e d f o r t h e r e f l e c t i v i t y
408
D. M. ZEHNER
( - 10%) o f t h e g l a s s window,
were determined by measuring t h e
photon energy d e l i v e r e d through an a p e r t u r e o f known diameter positioned i n f r o n t o f a calorimeter. was
measured
with
an
in-1 i n e
against the calorimeter.
The energy o f each p u l s e
photodi ode
assembly
c a l ib r a t e d
Implanted samples were prepared i n a
separate i o n i m p l a n t a t i o n f a c i 1it y which was a1 so equipped f o r making R u t h e r f o r d b a c k s c a t t e r i n g (RBS) measurements.
T h i s tech-
n i q u e was used t o determine t h e i m p l a n t p r o f i l e and t o charact e r i z e t h e changes i n t h e subsurface r e g i o n t h a t occurred w i t h An e x t e n s i v e d i s c u s s i o n o f these r e s u l t s i s
l a s e r annealing.
c o n t a i n e d i n Chapter 2.
2.
CHARACTERIZATION TECHNIQUES Many o f t h e s u r f a c e s e n s i t i v e s p e c t r o s c o p i c techniques used t o
i n v e s t i g a t e t h e s u r f a c e r e g i o n o f laser-annealed semiconductors employ e i t h e r e l e c t r o n s o r photons as t h e i n c i d e n t probe. t h e s e cases t h e d e t e c t e d p a r t i c l e i s an e l e c t r o n .
In
As a con-
sequence o f t h e s h o r t mean f r e e path o f e l e c t r o n s w i t h energies between 20 and 1000 eV, o n l y t h e outermost s u r f a c e region, -20 A , i s probed. Auger
Several d i f f e r e n t techniques have been used. electron
spectroscopy
(AES)
was
used t o m o n i t o r t h e
l e v e l s of b o t h i m p u r i t i e s and implanted species i n t h e s u r f a c e r e g i o n of t h e sample. elements w i t h Z > 3. in
terms
of
the
T h i s technique i s capable o f d e t e c t i n g a l l L e v e l s o f i m p u r i t y c o n t a m i n a t i o n a r e quoted
ratios
of
the
peak-to-peak
signals
of
the
i m p u r i t y Auger t r a n s i t i o n s t o a p r i n c i p a l Auger t r a n s i t i o n o f t h e substrate. this
Although one must be c a r e f u l
technique
to
make
quantitative
i n a t t e m p t i n g t o use
measurements,
in
many
s i t u a t i o n s reasonable estimates o f t h e upper l i m i t o f t h e amount o f a p a r t i c u l a r species present
i n t h e s u r f a c e r e g i o n can be
made. Low-energy e l e c t r o n d i f f r a c t i o n (LEED) was employed t o d e t e r mine geometric o r d e r i n t h e s u r f a c e r e g i o n o f t h e sample.
By
examining t h e p o s i t i o n s o f t h e r e f 1e c t e d beams ( s p o t p a t t e r n s ) ,
7.
409
PULSED LASER IRRADIATED SEMICONDUCTORS
t h e symmetry and s i z e ( i n t r a - a t o m i c spacing) o f t h e s u r f a c e u n i t c e l l can be determined. with
Thus, any changes i n these spot p a t t e r n s are a
surface modification
r e f l e c t i o n o f changes
i n the
geometric arrangement o f atoms i n t h e outermost s u r f a c e l a y e r s . P h o t o e l e c t r o n spectroscopy
(PES)
was
used t o o b t a i n i n f o r -
m a t i o n about t h e e l e c t r o n i c p r o p e r t i e s o f t h e s u r f a c e r e g i o n o f t h e sample.
I n f o r m a t i o n about t h e valence and conduction bands o f
t h e s o l i d can be obtained
by employing photons w i t h energies
t y p i c a l l y l e s s than 50 eV.
With t h e use o f angle-resolved tech-
niques, i t i s p o s s i b l e t o map o u t bands and c h a r a c t e r i z e t h e symmetry o f s u r f a c e s t a t e s .
By employing photons o f h i g h e r energies
i t i s p o s s i b l e t o measure c o r e - l e v e l
b i n d i n g energies f o r b o t h
s u r f a c e and subsurface atoms.
111.
P r o d u c t i o n o f A t o m i c a l l y Clean Surfaces
The c r e a t i o n o f an a t o m i c a l l y c l e a n s u r f a c e i s one o f t h e obvious b u t f r e q u e n t l y d i f f i c u l t t a s k s t h a t must be performed p r i o r t o conducting experiments i n t h e f i e l d o f s u r f a c e science. I n v e s t i g a t i o n s concerned w i t h examining t h e p h y s i c a l and chemical properties
o f s u r f a c e s i n o r d e r t o understand s u r f a c e - r e l a t e d
phenomena r e q u i r e t h a t t h e l e v e l o f unwanted contaminants i n t h e f i r s t few monolayers be <1 at.%.
S i m i l a r requirements e x i s t i n
many areas o f device t e c h n o l o g i e s where t h e presence o f s u r f a c e contaminants e i t h e r d u r i n g f a b r i c a t i o n o r d u r i n g a p p l i c a t i o n can c o n t r i b u t e g r e a t l y t o t h e degradation o f device performance.
In
t h e s e areas, t h e near-surface r e g i o n w i l l become even more import a n t as d e v i c e dimensions become i n c r e a s i n g l y smaller. The t r a d i t i o n a l methods o f g e n e r a t i n g a t o m i c a l l y c l e a n s o l i d s u r f a c e s i n UHV i n c l u d e p h y s i c a l s p u t t e r i n g ,
reactive sputtering,
chemical r e a c t i o n s , e l e c t r o n scrubbing, thermal desorption, s i t i o n o r growth o f a f i l m i n s i t u , t u r e (Roberts,
1963).
depo-
and vacuum c l e a v i n g o r f r a c -
For a s i n g l e c r y s t a l
it i s
frequently
410
D. M. ZEHNER
necessary t o anneal t h e c r y s t a l a t h i g h temperatures i n o r d e r t o remove damage produced i n t h e s u r f a c e r e g i o n by c l e a n i n g t e c h niques such as s p u t t e r i n g .
However, i m p u r i t i e s may d i f f u s e from
t h e s u r f a c e i n t o t h e b u l k , o r indeed from t h e b u l k t o t h e surface; i n many cases, this
such as i n t h e study o f semiconductor
surfaces,
r e d i s t r i b u t i o n can be p a r t i c u l a r l y troublesome.
Another
disadvantage i s t h a t t h e t i m e r e q u i r e d t o c l e a n a s u r f a c e can be measured i n hours and sometimes days when c y c l i n g between sputt e r i n g and annealing i s required.
Furthermore,
f o r experiments
i n which t h e c r y s t a l i s h e l d below room temperature,
adsorption
o f background gases d u r i n g t h e t i m e t h e sample c o o l s anneal
at
an
tamination.
elevated
temperature
Therefore,
can produce
from an
unwanted
con-
improved ways o f p r e p a r i n g atomical l y
c l e a n and ordered surfaces are needed. I t i s w e l l known t h a t a focused l a s e r beam can remove macro-
scopic
quantities
(Ready,
1965),
o f material
from a s u r f a c e by v a p o r i z a t i o n
and i t has been shown t h a t l a s e r s are capable o f
r a i s i n g t h e temperature i n t h e near-surface conductor t o t h e m e l t i n g p o i n t (see, f o r e.g., Wang e t al.,
White e t al.,
1978;
1978, Chapter 4) i n a w e l l c o n t r o l l e d manner.
From
t h e s e observations,
i t was c l e a r t h a t l a s e r s might be u s e f u l i n
g e n e r a t i n g c l e a n surfaces. pose
in
r e g i o n o f a semi-
1969 (Bedair,
Although f i r s t u t i l i z e d f o r t h i s pur-
1969),
only
recently
has a
number
of
d e t a i l e d i n v e s t i g a t i o n s o f l a s e r c l e a n i n g been c a r r i e d out (see f o r e.g. al.,
Zehner e t al.,
1980;
Cowan e t al.,
1980a; McKinley e t al., 1980).
1980; Rodway e t
While most s t u d i e s have been
concerned w i t h S i , o t h e r elemental and compound semiconductors, as w e l l as metals, have a l s o been examined.
3.
SILICON
To i l l u s t r a t e t h e a p p l i c a t i o n o f l a s e r i r r a d i a t i o n i n t h e p r o d u c t i o n o f a t o m i c a l l y c l e a n surfaces, r e s u l t s obtained from S i
7.
411
PULSED LASER IRRADIATED SEMICONDUCTORS
surfaces are shown i n Fig.
1 (Zehner e t al.,
1980a).
The Auger
e l e c t r o n spectrum o b t a i n e d from t h e n a t i v e o x i d e o f an air-exposed S i sample, a f t e r i n s e r t i o n i n t o a UHV system and f o l l o w i n g bakeout,
i s shown a t t h e t o p o f Fig. 1. carbon (272 eV),
and S i
s i l i c o n dioxide,
Auger s i g n a l s from oxygen (510 eV),
(70-100
eV),
characteristic of S i
in
a r e r e a d i l y d e t e c t e d on t h e s u r f a c e o f t h e asMeasurements made by RBS i m p l y a
i n s e r t e d sample.
o x i d e t h i c k n e s s , t y p i c a l o f air-exposed S i .
M A native
A substantial reduction
i n t h e l e v e l s o f 0 and C present i n t h e s u r f a c e r e g i o n i s observed a f t e r i r r a d i a t i o n w i t h one l a s e r p u l s e (-2.0
1.
Fig.
J/cmz),
as shown i n
A f t e r exposing t h e same area t o f i v e l a s e r pulses, t h e
Auger e l e c t r o n spectrum shown i n Fig. same d e t e c t i o n c o n d i t i o n s , noise level.
1 indicates that f o r the
t h e 0 and C s i g n a l s a r e w i t h i n t h e
The Auger e l e c t r o n spectrum obtained from t h e same
area a f t e r i r r a d i a t i o n w i t h t e n l a s e r pulses i s shown a t t h e b o t 1.
Although t h e 0 and C s i g n a l s a r e n o t observable
i n t h i s trace,
by i n c r e a s i n g t h e e f f e c t i v e s e n s i t i v i t y o f t h e
tom o f Fig.
e l e c t r o n d e t e c t i o n system, these i m p u r i t i e s were determined t o be p r e s e n t i n s u r f a c e c o n c e n t r a t i o n s o f
o f a monolayer.
It
should a l s o be noted t h a t t h e l i n e shape o f t h e S i (70-100 eV) Auger
transition
is
that
expected
from
a clean S i
surface.
Although a hydrogen Auger t r a n s i t i o n cannot be d e t e c t e d w i t h AES, PES r e s u l t s (Zehner e t al.,
f r e e o f hydrogen.
1981c) show t h e s u r f a c e r e g i o n t o be
Consequently,
by i r r a d i a t i n g t h e c r y s t a l w i t h
s e v e r a l (t5) l a s e r pulses, t h e r e l a t i v e c o n c e n t r a t i o n s o f 0 and C i n t h e n e a r - s u r f a c e r e g i o n have been reduced by f a c t o r s of a t l e a s t 500 and 50,
respectively.
Thus,
contaminant l e v e l s com-
p a r a b l e t o those o b t a i n e d by repeated s p u t t e r i n g and c o n v e n t i o n a l thermal annealing over a p e r i o d o f several days can be o b t a i n e d i n a l a s e r processing t i m e o f <1 s. Because p h y s i c a l s p u t t e r i n g i s commonly employed as p a r t o f cleaning
procedures
i n s u r f a c e science,
the e f f e c t o f
pulsed
l a s e r i r r a d i a t i o n on samples t h a t had been s p u t t e r e d w i t h A r + i o n s (1000 eV,
5 PA,
30 min) was a l s o i n v e s t i g a t e d .
The Auger
412
D.M. ZEHNER I
I
I
I
c
AS INSERTED
-
f PULSE
9
5 PULSES
-
V
10 PULSES L A S E R ANNEALED
AES Si (100) PRIMARY BEAM: 2 keV, 5 p A MODULATION: 2 V p - p
I
0
Fig. 1 .
I00
I
I
200 300 400 ELECTRON ENERGY ( e V 1
I
500
600
Auger electron spectra obtained from an uncleaned Si ( 1 0 0 ) surface
and a f t e r pulsed laser annealing a t -2.0
J / c m 2 for 1 , 5 , and 10 pulses.
7.
413
PULSED LASER IRRADIATED SEMICONDUCTORS
e l e c t r o n spectrum a c q u i r e d f o l l o w i n g s p u t t e r i n g s t i l l showed t h e presence o f 0 and C, a l t h o u g h s u b s t a n t i a l l y reduced i n i n t e n s i t y , as w e l l
as t h e presence o f implanted Ar.
sample w i t h l a s e r pulses o f -2.0 tion
i n t h e contaminant-level
I r r a d i a t i o n o f the
J/cm2 produced a s i m i l a r reducAuger
signals
observed f o r t h e u n s p u t t e r e d surface.
for
0 and C as
Complete e l i m i n a t i o n o f
t h e A r Auger s i g n a l s occurred a f t e r two o r t h r e e pulses. Contamination m a i n l y CO, H,
of
the
Si
surface
background
These contaminants can be e a s i l y removed by I n addi-
i r r a d i a t i o n o f t h e sample w i t h one o r two l a s e r pulses. tion,
i f an a t o m i c a l l y
(1-1000 L),
gases,
i s i n e v i t a b l e even with a
H20, and hydrocarbons,
10-10 T o r r vacuum.
from
c l e a n s u r f a c e i s exposed t o 0,
o r CO
t h e s u r f a c e can be r e t u r n e d t o an a t o m i c a l l y c l e a n
s t a t e by i r r a d i a t i o n w i t h about f i v e l a s e r pulses. Increases
i n t h e UHV chamber,
i n t h e background pressure
measured by b o t h a nude i o n i z a t i o n gauge and a quadrupole mass spectrometer,
indicate
laser irradiation.
removal
of
s u r f a c e contaminants d u r i n g
Using l a s e r pulses o f -2.0
J/cmz and s t a r t i n g
w i t h a background pressure o f 2 x 10-1° T o r r , t h e f i r s t l a s e r p u l s e on an a s - i n s e r t e d o r f r e s h l y s p u t t e r e d sample caused a t r a n s i e n t p r e s s u r e r i s e i n t o t h e 5 x 10-8 t o 3 x
T o r r range.
Subsequent
p u l s e s on t h e same area were accompanied w i t h pressure r i s e s i n t o 10-9
the
Torr
range,
and these
c o n t i n u e d t o drop u n t i l
the
p r e s s u r e stayed i n t h e 10-10 T o r r range by about t h e f i f t h pulse. Because o f t h e p o s i t i o n o f t h e i o n i z a t i o n gauge r e l a t i v e t o t h e sample l o c a t i o n and t h e sampling r a t e o f t h e mass spectrometer, these
increases
did
not
provide
an
a c c u r a t e measure
of
the
p r e s s u r e i n c r e a s e i n f r o n t o f t h e sample, which was probably much h i g h e r than measured. The e f f e c t o f energy d e n s i t y on t h e e f f i c i e n c y o f removal o f i m p u r i t i e s was a1 so qua1 i t a t i v e l y i n v e s t i g a t e d . sity
range
i n v e s t i g a t e d was
one p u l s e a t -0.3
J/cm2,
from -0.3
to
3.2
The energy denJ/cm2.
After
t h e 0 and C s i g n a l s were reduced by a
f a c t o r o f o n l y 2 r e l a t i v e t o those observed on t h e a s - i n s e r t e d
414
D. M. ZEHNER
sample, and on a f r e s h l y s p u t t e r e d surface, embedded A r was s t i l l detected.
Two
general
energy d e n s i t y .
t r e n d s were
First,
observed
as
a function
of
t h e h i g h e r t h e energy d e n s i t y t h e more
e x t e n s i v e t h e removal o f i m p u r i t i e s by t h e f i r s t pulse.
Second,
a t any p u l s e energy d e n s i t y t h e l a r g e r t h e number o f pulses t h e more
complete
the
removal
of
impurities.
Energy d e n s i t i e s
g r e a t e r than 1.0 J/cm2 were r e q u i r e d t o produce a t o m i c a l l y c l e a n s u r f a c e s u s i n g ruby l a s e r i r r a d i a t i o n .
V i s i b l e s u r f a c e damage,
as observed o p t i c a l l y , occurred a t an energy d e n s i t y o f 3.2 J / c d b u t was n o t observed f o r energy d e n s i t i e s ~3.0 J/cm2. The process o f l a s e r annealing, which i n v o l v e s r a p i d m e l t i n g and r e s o l i d i f i c a t i o n (on a - 1 - u ~ t i m e s c a l e ) , suggests t h a t removal of
impurities
mechanisms,
from t h e s u r f a c e
r e g i o n can t a k e p l a c e by two
d e s o r p t i o n o f t h e v o l a t i l e contaminants and absorp-
t i o n and d i f f u s i o n o f o t h e r i m p u r i t i e s through t h e l i q u i d l a y e r . Both
mechanisms-desorption
and
diffusion-probably
influence
t h e r e d i s t r i b u t i o n o f a given i m p u r i t y , w i t h t h e r e l a t i v e import a n c e o f t h e two
processes depending on experimental c o n d i t i o n s .
F o r t h e c o n d i t i o n s discussed above,
t h e f a c t t h a t a pronounced
p r e s s u r e r i s e i s observed d u r i n g t h e f i r s t l a s e r p u l s e suggests that
t h e 0 and C contaminants a r e desorbed
during irradiation.
from t h e s u r f a c e
However, i t i s d i f f i c u l t t o q u a n t i f y these
o b s e r v a t i o n s and determine t h e amount o f m a t e r i a l desorbed from o n l y t h e c r y s t a l surface.
A b s o r p t i o n and d i f f u s i o n deeper i n t o
t h e sample occur predominantly f o r i m p u r i t i e s t h a t have a r e l a t i v e l y h i g h segregation c o e f f i c i e n t from t h e m e l t (White e t a l . , 1980a; Wood e t al., t i o n coefficient
1981a).
I m p u r i t i e s which have a low segrega-
from t h e m e l t have been shown t o segregate t o
t h e s u r f a c e d u r i n g annealing (White e t al., and 0 t h e e q u i l i b r i u m c o e f f i c i e n t s tively,
suggesting
that
these
1979,
a r e 0.07
1980a).
and 0.5,
For C respec-
i m p u r i t i e s may be r e d i s t r i b u t e d
over a depth i n t e r v a l e q u i v a l e n t t o t h e m e l t depth (-5000 A f o r l a s e r energy d e n s i t i e s o f -2.0
J/cm2).
Complete r e d i s t r i b u t i o n
o f t h e i m p u r i t i e s found on t h e surface, as i n s e r t e d i n t o t h e UHV
7. chamber,
415
PULSED LASER IRRADIATED SEMICONDUCTORS
over t h i s depth would g i v e r i s e t o a remaining s u r f a c e
c o n c e n t r a t i o n o f -0.3%
o f a monolayer f o r 0 and -0.1% of a mono-
b o t h of which a r e near t h e d e t e c t i o n l i m i t s f o r
l a y e r f o r C,
Auger e l e c t r o n spectroscopy.
These values are c o n s i s t e n t w i t h
t h e measured values discussed above.
Whether these i m p u r i t i e s
a r e desorbed from t h e s u r f a c e o r r e d i s t r i b u t e d i n depth can be a s c e r t a i n e d by experiments designed t o determine C and 0 depth p r o f i l e s o r t h e i r t o t a l c o n c e n t r a t i o n s i n t h e near-surface r e g i o n b e f o r e and a f t e r i r r a d i a t i o n . An i n v e s t i g a t i o n o f oxygen i n d i f f u s i o n d u r i n g l a s e r anneali n g o f s i l i c o n has been performed u s i n g RBS and t h e 160(a,ao)160 resonance s c a t t e r i n g a t 3.042 MeV (Westendorp e t a l . , e t al.,
1983).
T h i s technique,
1982; Wang
i n c o n j u n c t i o n w i t h AES, was used
t o determine t h e oxygen c o n c e n t r a t i o n on t h e s u r f a c e and i n t h e near-surface (3000 A ) region, both b e f o r e and a f t e r pulsed l a s e r annealing.
From t h e oxygen peak
area i n t h e b a c k s c a t t e r i n g
spectrum shown i n Fig. 2a, t h e i n i t i a l n a t i v e oxide l a y e r a t t h e s u r f a c e was determined t o have an a r e a l c o n c e n t r a t i o n o f 5.2 x 1015 atoms/cm2.
A f t e r i r r a d i a t i o n of t h e
Si(100) sample i n UHV w i t h
e i g h t pulses a t an energy d e n s i t y o f -1.5
J/cm2 (x=694 nm), Auger
measurements showed an oxygen c o n c e n t r a t i o n a t t h e s u r f a c e o f ~0.3% The oxygen c o n c e n t r a t i o n a t a depth o f 1200 A
o f a monolayer. (depth i n t e r v a l atoms/cm3,
500-1700 A ) was determined t o be 63.1
u s i n g t h e data shown i n Fig.
2b.
x lo1*
Assuming t h a t t h e
s i l i c o n s u r f a c e l a y e r i s m e l t e d completely d u r i n g t h e l a s e r p u l s e and t h a t t h e oxygen atoms i n t h e n a t i v e o x i d e l a y e r are homogeneously d i s t r i b u t e d over t h e depth o f t h e molten l a y e r (-3000 A ) , an oxygen Concentration o f 1.6 x 1020 atom/cm’ would be achieved i n t h a t molten l a y e r .
Thus, these r e s u l t s show no evidence f o r oxy-
gen i n d i f f u s i o n from t h e n a t i v e oxide l a y e r d u r i n g pulsed l a s e r annealing o f s i l i c o n .
I n fact,
t h e amount o f oxygen i s found t o
be equal t o o r l e s s than t h e s o l i d s o l u b i l i t y o f oxygen i n s i l i con,
which i s 5-8 x 1018 atomsjcm3 ( d e Kock,
1980).
This r e s u l t
can be made p l a u s i b l e by c o n s i d e r i n g t h e t i m e needed f o r l i q u i d
416
D. M. ZEHNER
CHANNEL N U M B E R
CHAW N iL NIJ M B E R
F i g . 2.
Resonance at ( a ) center o f native oxide l a y e r , ( b ) a t 1200
pulsed laser cleaned silicon.
BL = 164O.
in
7. SiO,
t o be d i s s o l v e d i n s i l i c o n .
i n t h e range o f 10-5-10-4
The d i s s o l u t i o n v e l o c i t y l i e s
cm/min f o r s i l i c o n s u b s t r a t e tempera-
t u r e s between 1700-1775 K (Chaney e t a1 a 30 A
Thus,
a thermal
., 1976; H i r a t a e t a1 ., 1980).
n a t i v e o x i d e l a y e r would d i s s o l v e a t about t h e
m e l t i n g p o i n t o f s i l i c o n i n 1.8-0.18 of
417
PULSED LASER IRRADIATED SEMICONDUCTORS
m e l t i n g model,
s.
Consequently,
a native
oxide
i n terms
l a y e r cannot be
d i s s o l v e d i n s i l i c o n s i n c e t h e m e l t d u r a t i o n a f t e r pulsed l a s e r annealing, f o r t h e c o n d i t i o n s used, i s measured t o be -100 ns. I f t h e n a t i v e o x i d e l a y e r on t o p o f s i l i c o n does n o t d i f f u s e
i n t o t h e bulk, it has t o evaporate d u r i n g t h e c l e a n i n g procedure. This
can
be
shown
to
be
reasonable
by
calculating
the
Si02 m o l e c u l a r e v a p o r a t i o n r a t e f o r d i f f e r e n t s i l i c o n s u b s t r a t e temperatures.
Using equi 1 ib r i u m thermodynamics, t h e r a t e a t 2000 K
i s c a l c u l a t e d t o be 3.4 x 1012 mol/cm2 ns (Westendorp e t al., Wang e t al.,
Since a t y p i c a l a r e a l d e n s i t y o f a n a t i v e
1983).
o x i d e on s i l i c o n i s 2-5 x
lo15
mol/cm2,
t h e complete removal by
e i g h t l a s e r pulses r e q u i r e s 5 x 1014 mol/cm2 d u r i n g each pulse.
1982;
t o be evaporated
T h i s e v a p o r a t i o n r a t e r e q u i r e s a m e l t dura-
t i o n o f t h e s u r f a c e o f -150 ns, which i s t h e same o r d e r o f magnit u d e as obtained from thermal m e l t i n g model c a l c u l a t i o n s . When
a
silicon
surface
is
covered w i t h
an o v e r l a y e r
of
i m p u r i t i e s much t h i c k e r t h a n t h e t y p i c a l n a t i v e o x i d e t h i c k n e s s (20 A ) ,
complete e l i m i n a t i o n o f t h e i m p u r i t i e s by l a s e r i r r a d i a -
tion i s difficult. (Bermudez,
Experiments on h e a v i l y o x i d i z e d S i surfaces
1982) have shown t h a t t h e minimum a t t a i n a b l e l e v e l o f
c o n t a m i n a t i o n i s dependent on t h e i n c i d e n t energy d e n s i t y and on t h e i n i t i a l i m p u r i t y coverage.
From these r e s u l t s i t i s con-
c l u d e d t h a t although d e s o r p t i o n may be t h e dominant mechanism f o r removal o f i m p u r i t i e s i n t h i n o v e r l a y e r s , d i f f u s i o n and r e d i s t r i b u t i o n o f i m p u r i t i e s i n t h e melted r e g i o n occur when t h i c k l a y e r s a r e i n i t i a l l y present. I n v e s t i g a t i o n s have a l s o been performed t o examine t h e i n c o r p o r a t i o n o f oxygen i n t o S i d u r i n g p u l s e d l a s e r annealing i n an atmospheric environment.
While some i n v e s t i g a t o r s c l a i m evidence
418
D. M. ZEHNER
f o r i n c o r p o r a t i o n ( G a r u l l i e t al.,
1980; Hoh e t al.,
a1
incorporation
.,
1981),
others
(Westendorp e t a1
argue
., 1982;
that
Wang e t a1
1980; L i u e t
does
., 1983).
not
occur
To minimize con-
c e r n about t h e p o s s i b l e r e d i s t r i b u t i o n o f i m p u r i t i e s i n a UHV t h e s a f e s t procedure t o f o l l o w i s t o s p u t t e r t h e
environment,
s u r f a c e m i l d l y t o reduce s u b s t a n t i a l l y t h e c o n c e n t r a t i o n o f these i m p u r i t i e s and then t o l a s e r anneal t h e c r y s t a l t o produce an a t o m i c a l l y clean surface region.
4.
GERMANIUM I n v e s t i g a t i o n s concerned w i t h t h e removal o f t h e n a t i v e o x i d e
l a y e r from Ge samples w i t h pulsed l a s e r i r r a d a t on have produced r e s u l t s s i m i l a r t o those o b t a i n e d w i t h S i samples (Zehner e t a1
.,
1 9 8 0 ~ ) . A t o m i c a l l y c l e a n surfaces can be o b t a i n e d by i r r a d i a t i o n w i t h about f i v e l a s e r pulses (-1.9
J/cm2).
As observed w i t h S i ,
s p u t t e r i n g can be used i n c o n j u n c t i o n w i t h l a s e r i r r a d i a t i o n t o produce a t o m i c a l l y c l e a n surfaces also.
The range o f energy den-
s i t y t h a t can be used t o remove i m p u r i t i e s from t h e s u r f a c e w i t h o u t p r o d u c i n g macroscopic damage was found t o be -0.5
t o -2.1
J/cm2.
Damage t o t h e s u r f a c e occurred above an energy d e n s i t y o f 2.2 J/cm*,
and complete removal o f s u r f a c e i m p u r i t i e s d i d n o t occur
below an energy
5.
d e n s i t y o f -0.5
J/cm2.
III-V COMPOUNDS
GROUP
The p r o d u c t i o n o f a t o m i c a l l y c l e a n surfaces on GaAs c r y s t a l s was i n v e s t i g a t e d
u s i n g procedures s i m i l a r t o those p r e v i o u s l y
d e s c r i b e d (Zehner e t a1
.,
1982).
The Auger e l e c t r o n spectrum
o b t a i n e d from an a s - i n s e r t e d sample and p l o t t e d a t t h e t o p o f F i g . 3 shows t h a t l a r g e amounts o f C (272 eV) and 0 (510 eV) are present,
as w e l l as a small t r a c e o f Ca (291 eV).
impurities
from t h e s u r f a c e r e y i o n
m u l t i p l e pulses,
irradiation with
and t h e e f f i c i e n c y o f c l e a n i n g i n c r e a s e d w i t h
i n c r e a s i n g eneryy d e n s i t y . following multiple-pulse F i g . 3.
required
Removal o f
An example o f a spectrum o b t a i n e d
i r r a d i a t i o n o f -0.6
J/cm2 i s shown i n
The carbon and oxygen s i g n a l s are w i t h i n t h e n o i s e l e v e l
7.
419
PULSED LASER IRRADIATED SEMICONDUCTORS
AES GaAs (100) PRIMARY BEAM: 2 keV, 5 p A
I
-
15 PULSES -0.6 J/cm2
4
AFTER Ar' SPUTTERING
-
15 PULSES -0.3 J/cm2 0
100
200 300 400 ELECTRON ENERGY (eV)
500
F i g . 3. Auger electron spectra obtained from an uncleaned GaAs ( 1 0 0 ) surf a c e , a f t e r sputtering and a f t e r pulsed laser annealing a t 4 . 6 and 4 . 3 J /cm2.
o f t h e Auger detected.
e l e c t r o n spectrum,
and no o t h e r i m p u r i t i e s were
However, t h e r e i s a d i f f e r e n c e i n t h e Ga (55 eV) t o As
( 3 1 eV) Auger t r a n s i t i o n i n t e n s i t y r a t i o and i n t h e Ga Auger t r a n s i t i o n l i n e shape when compared w i t h t h a t obtained a f t e r A r + i o n s p u t t e r i n g , a l s o shown i n t h i s f i g u r e ( A r Auger s i g n a l a t 215 eV). Complete removal o f t h e C and 0 i m p u r i t i e s from an a s - i n s e r t e d sample c o u l d n o t be o b t a i n e d when u s i n g energy d e n s i t i e s o f t0.5
420
D. M. ZEHNER
J/cm2.
However,
by f i r s t s p u t t e r i n g w i t h Ar+ i o n s , i t was found
t h a t a s u r f a c e w i t h no i m p u r i t i e s present c o u l d be produced by laser J/cm2,
i r r a d i a t i o n w i t h energy d e n s i t i e s between 0.15 as i l l u s t r a t e d a t t h e bottom of Fig.
f i f t e e n pulses a t -0.3 intensity
J/cm2.
and 0.4
3 f o r t h e case o f
Although t h e Ga/As Auger t r a n s i t i o n
r a t i o observed f o l l o w i n g
such
surface treatment
is
s i m i l a r t o t h a t observed a f t e r s p u t t e r i n g , t h e Ga Auger l i n e shape showed t h e same t y p e o f change as t h a t observed a t h i g h e r energy densities.
T h i s change, a r e d u c t i o n i n t h e degree o f s p l i t t i n g
w i t h i n t h e l i n e shape (due t o t h e s p i n - o r b i t s p l i t ,M,,
level),
observed a f t e r i r r a d i a t i o n a t a l l energy d e n s i t i e s i s due t o t h e presence o f Ga i n l o c a l regions which a r e n o n s t o i c h i o m e t r i c . I n o r d e r t o c o n f i r m t h i s e x p l a n a t i o n f u r t h e r , a f t e r pulsed l a s e r a n n e a l i n g i n UHV, t h e samples were t r a n s f e r r e d i n a i r t o a highvacuum (HV)
chamber, where t h e y were analyzed u s i n g 1.5-MeV
ion scattering.
Het
For purposes o f comparison, s i m i l a r measurements
were made on samples t h a t were n o t exposed t o l a s e r i r r a d i a t i o n (virgin).
A r e p r e s e n t a t i v e r e s u l t o f such a comparison i s shown
i n Fig. 4 f o r t h e (110) face o f GaAs.
The s p e c t r a obtained from
t h e v i r g i n sample can be i n t e r p r e t e d as s c a t t e r i n g from an ordered crystal
covered w i t h an a i r - f o r m e d
<111> c h a n n e l i n g spectra,
oxide.
shown i n Fig.
4,
Comparison o f t h e before ( s o l i d l i n e )
and a f t e r (dashed l i n e ) annealing w i t h f i v e l a s e r pulses a t an energy d e n s i t y o f -0.3 J/cm2 shows t h a t (1) t h e c o n c e n t r a t i o n o f As atoms a t t h e surface i s e s s e n t i a l l y t h e same i n t h e v i r g i n and laser-annealed
samples;
( 2 ) t h e y i e l d from Ga s u r f a c e atoms
i n c r e a s e s about a f a c t o r o f 2 a f t e r annealing; and ( 3 ) t h e scatt e r i n g y i e l d versus depth ( d e c r e a s i n g energy) increases s l i g h t l y a f t e r a n n e a l i n g w i t h no n o t i c e a b l e change i n t h e r a t e o f dechanneling.
The measured y i e l d s can be a t t r i b u t e d t o s c a t t e r i n g from
a s u r f a c e which c o n t a i n s r e g i o n s w i t h t h e c o r r e c t s t o i c h i o m e t r y i n c o n j u n c t i o n w i t h r e g i o n s which c o n t a i n excess Ga, c o n s i s t e n t w i t h t h e i n t e r p r e t a t i o n o f t h e measured Ga Auger l i n e shape. cleaning
results
are q u a l i t a t i v e l y
similar
t o those
The
obtained
7.
PULSED LASER IRRADIATED SEMICONDUCTORS
421
Backscattering-channeling spectra from a GaAs ( 1 1 0 ) surface. (---) a f t e r pulsed laser annealing at -0.3 J / c r n 2 .
(3
to4
103
v)
I-
$
8
402
10'
10° 1
Fig. 4 . As grown,
u s i n g glass-bonded [ 100) GaAs t r a n s m i s s i o n photocathodes (Rodway e t al.,
1980), and t h e d e t e r m i n a t i o n o f s t o i c h i o m e t r y i s c o n s i s t e n t
w i t h RBS t e s t s of GaAs decomposition due t o p u l s e d l a s e r i r r a d i a t i o n [de Jong e t al.,
1982a).
422
D. M. ZEHNER
S i m i 1a r
problems w i t h
respect
to
s t o i c h i ometry
have
been
encountered i n i n v e s t i g a t i o n s o f l a s e r - i r r a d i a t e d cleaved InP (110) s u r f a c e s exposed t o a i r (McKinley e t a1
., 1980).
Clean s u r f a c e s
were produced, and t h e AES s p e c t r a o b t a i n e d were q u i t e s i m i l a r t o t h o s e f o r c l e a n cleaved surfaces.
However, t h e s u r f a c e s were n o t
ordered, and d e t a i l e d s t u d i e s u s i n g scanning AES ( w i t h a 1-pin spatial
resolution)
indicated that,
although the overall
surface
showed a s t o i c h i o m e t r y c o n s i s t e n t w i t h t h a t o f t h e cleaved face, l o c a l r e g i o n s e x i s t e d where t h e d e v i a t i o n from s t o i c h i o r n e t r y was large.
The d e p a r t u r e from s t o i c h i o m e t r y subsequent
t o laser
i r r a d i a t i o n has been observed i n i n v e s t i g a t i o n s o f InP (100) s u r f a c e s a1 so (Moison e t a1 , 1982).
IV.
Geometric Surface Structures
The p r o c e s s i n g o f semiconductor m a t e r i a l s w i t h l a s e r i r r a d i a t i o n has been i n v e s t i g a t e d e x t e n s i v e l y .
O f m a j o r importance i s t h e f a c t
t h a t l a s e r a n n e a l i n g can be used t o anneal c o m p l e t e l y d i s p l a c e ment
damage
1978).
in
ion-imp1 anted
semiconductors
(Narayan
et
a1
.,
I n t h i s a p p l i c a t i o n t h e l a s e r r a d i a t i o n causes t h e s u r -
f a c e r e g i o n o f t h e c r y s t a l t o be m e l t e d t o a d e p t h o f several thousand
angstroms.
The m e l t e d
l a y e r then
regrows
from t h e
u n d e r l y i n g s u b s t r a t e by means o f l i q u i d - p h a s e e p i t a x i a l regrowth, and t h e regrown r e g i o n has t h e same c r y s t a l l i n e p e r f e c t i o n as t h e substrate.
I n s u r f a c e - r e l a t e d experiments, depending on t h e m e l t i n g
p o i n t , t y p e s and q u a n t i t i e s o f i m p u r i t i e s p r e s e n t i n t h e b u l k , reac-
t i v i t y t o background gases , and o t h e r f a c t o r s , c o n v e n t i o n a l thermal a n n e a l i n g o f c r y s t a l s i n UHV, e i t h e r t o c l e a n o r t o remove s p u t t e r damage,
i n v o l v e s h e a t i n g and subsequent c o o l i n g o f t h e sample f o r
p e r i o d s t h a t can range from minutes t o hours.
Calculations indicate
x=
694 nm, s e v e r a l t e n s o f
t h a t f o r p u l s e energies o f 1-2 J/cm2 a t
microseconds elapse between t h e i r r a d i a t i o n o f t h e sample a t room t e m p e r a t u r e w i t h t h e l a s e r beam and t h e r e t u r n o f t h e sample t o -600 K.
Thus, t h i s a n n e a l i n g t e c h n i q u e p r o v i d e s t h e c a p a b i l i t y f o r
7.
423
PULSED LASER IRRADIATED SEMICONDUCTORS
d o i n g experiments i n which t h e t i m e f o r thermal p r o c e s s i n g i s r e duced t o a minimum.
Since t h e p r e v i o u s s e c t i o n has shown t h a t
p u l s e d l a s e r i r r a d i a t i o n can be used t o produce a t o m i c a l l y c l e a n surfaces, i t i s then o f i n t e r e s t t o determine t h e annealing capab i l i t y o f t h i s technique w i t h respect t o o r d e r i n t h e s u r f a c e region o f a single crystal. 6.
ORDERED SURFACES I n o r d e r t o i n v e s t i g a t e t h i s q u e s t i o n , r e s u l t s o b t a i n e d from
samples o f s i l i c o n c r y s t a l s a r e presented (Zehner e t al.,
1980b).
These samples r e c e i v e d no c l e a n i n g t r e a t m e n t o t h e r t h a n a r i n s e i n a l c o h o l p r i o r t o i n s e r t i o n i n t o t h e UHV system.
Examination o f t h e
a s - i n s e r t e d samples w i t h LEED showed t h a t d i f f r a c t i o n p a t t e r n s c o u l d be observed o n l y a t r e l a t i v e l y h i g h energies (>250 eV) and t h a t t h e y c o n t a i n e d an i n t e n s e backround r e s u l t i n g from d i f f u s e s c a t t e r i n g . This
o b s e r v a t i o n i s c o n s i s t e n t w i t h t h e presence o f a n a t i v e
o x i d e l a y e r c o n t a i n i n g 0 and C as determined by AES and shown i n F i g . 1.
The LEED p a t t e r n s shown i n Fig. 5 i l l u s t r a t e t h e e f f e c t s
o f m u l t i p l e - p u l s e i r r a d i a t i o n on a S i ( 100) sample. d i a t i o n w i t h one l a s e r p u l s e o f -2.0
J/cm2,
A f t e r irra-
a (2x1) LEED p a t t e r n
w i t h moderate background i n t e n s i t y due t o d i f f u s e s c a t t e r i n g was obtained,
as shown a t t h e t o p o f Fig.
5.
Improvements i n t h e
q u a l i t y o f t h e d i f f r a c t i o n p a t t e r n occurred w i t h subsequent l a s e r pulses.
After
diffraction observed,
f i v e pulses,
reflections
a LEED p a t t e r n e x h i b i t i n g
and very
as shown i n F i g .
5.
sharp
low background i n t e n s i t y was
The f a c t t h a t w e l l - d e f i n e d LEED
p a t t e r n s can be obtained i n d i c a t e s t h a t c r y s t a l l i n e o r d e r extends to
t h e outermost monolayers
regrowth process.
after
the
1iquid-phase
epitaxial
No d e t e c t a b l e change i n t h e LEED p a t t e r n s was
observed w i t h a d d i t i o n a l pulses, as can be seen by comparing t h e patterns
for
five
and t e n pulses
shown i n Fig.
5.
Similar
r e s u l t s were o b t a i n e d from samples t h a t were i n i t i a l l y s p u t t e r cleaned by A r + bombardment.
Although t h e LEED p a t t e r n o b t a i n e d
subsequent t o one l a s e r p u l s e on a s p u t t e r e d s u r f a c e was
of
7.
PULSED LASER IRRADIATED SEMICONDUCTORS
h i g h e r q u a l i t y t h a n t h a t shown i n Fig.
5,
425
m u l t i p l e pulses were
a l s o r e q u i r e d t o o b t a i n t h e sharpest d i f f r a c t i o n p a t t e r n s . The LEED p a t t e r n s obtained from t h e t h r e e low-index o r i e n t a t i o n s o f S i subsequent t o l a s e r i r r a d i a t i o n w i t h f i v e pulses a r e shown i n F i g . 6.
A l l p a t t e r n s show sharp d i f f r a c t i o n r e f l e c t i o n s accompanied
by low background i n t e n s i t y , and i n a l l cases t h e q u a l i t y o f t h e p a t t e r n obtained improved w i t h m u l t i p l e - p u l s e f i v e shots.
i r r a d i a t i o n up t o
The (2x1) and ( 1 x 2 ) LEED p a t t e r n s o b t a i n e d from t h e
(100) and (110) surfaces,
respectively,
are s i m i l a r t o those ob-
t a i n e d u s i n g conventional thermal t r e a t m e n t s and show t h e presence o f r e c o n s t r u c t e d surfaces.
These o b s e r v a t i o n s i n d i c a t e t h a t t h e
atoms i n t h e outermost l a y e r s have enough t i m e a t a temperature, under t h e l a s e r annealing c o n d i t i o n s used, t o r e o r g a n i z e i n t o t h e r e c o n s t r u c t e d arrangements f r o m w h i c h t h e LEED p a t t e r n s a r e obtained. T h i s i s c o n s i s t e n t w i t h t h e proposed s u r f a c e s t r u c t u r e models f o r t h e (100) s u r f a c e t h a t i n v o l v e o n l y small l a t e r a l and v e r t i c a l d i s placements o f t h e atoms i n t h e f i l l e d outermost monolayers.
Laser
a n n e a l i n g o f e i t h e r (100) o r (110) S i samples cooled t o 100 K (Zehner e t a1
., 1980d)
produced surfaces from which LEED p a t t e r n s
i d e n t i c a l t o those shown i n Fig. 6 were obtained. The LEED p a t t e r n o b t a i n e d from t h e (111) s u r f a c e suggests t h a t as a r e s u l t o f l a s e r annealing t h e normal s u r f a c e s t r u c t u r e ( t r u n c a t i o n o f t h e b u l k ) i s obtained,
and t h e r e i s no evidence o f any
ordered l a t e r a l r e c o n s t r u c t i o n (Zehner e t a1
., 1 9 8 0 ~ ) . T h i s p o i n t
w i l l be discussed i n more d e t a i l l a t e r . Although a ( 2 x 1 ) LEED p a t t e r n can be obtained from a cleaved S i ( l l 1 ) surface, t h e (7x7) p a t t e r n shown i n Fig. 7 i s always observed on a clean, t h e r m a l l y annealed c r y s t a l surface.
A f t e r i r r a d i a t i o n w i t h t h e l a s e r and structure,
t h e S i sample was
t h e r m a l l y annealed a t e l e v a t e d temperatures.
The o b s e r v a t i o n o f
production o f the (1x1)
surface
1/7-order d i f f r a c t i o n spots, i n d i c a t i v e o f t h e r e c o n s t r u c t e d surface, occurred a f t e r annealing a t temperatures g r e a t e r t h a n -800 K. By h e a t i n g f o r a s u f f i c i e n t t i m e (>30 rnin) a t these temperatures,
a well-defined
( 7 x 7 ) p a t t e r n s i m i l a r t o t h a t shown i n Fig. 7 was
426
Fig. 6.
D. M. ZEHNER
LEED patterns from clean ( a )
(loo),
( b ) ( 1 1 0 ) , and ( c ) ( 1 1 1 ) Si
surfaces a t primary beam energies o f ( a ) 4 9 , ( b ) 9 2 , and ( c ) 47 eV. are shown subsequent to laser annealing at -2.0
Patterns
J / c m 2 for 5 pulses.
7.
Fig. 7.
PULSED LASER IRRADIATED SEMICONDUCTORS
427
LEED pattern from a clean thermally annealed ( 1 1 1 ) Si surface at a
primary beam energy o f 1 1 1 eV.
observed.
Subsequent i r r a d i a t i o n a t room temperature w i t h t h e
l a s e r resulted i n a (1x1) surface structure, possible t o cycle Moreover,
back and f o r t h between t h e two s t r u c t u r e s .
i t was determined t h a t t h e sample c o u l d be h e l d a t a
temperature between -100
and 700 K,
and a f t e r i r r a d i a t i o n w i t h a
l a s e r p u l s e t h e (1x1) s t r u c t u r e was observed.
LEED
defined
showing t h a t i t i s
patterns
single-crystal (Zehner e t a1
were
obtained
from
s u r f a c e s o f Ge subsequent
., 1 9 8 0 ~ ) .
As w i t h S i , w e l l low-index-oriented
t o laser irradiation
Mechanisms o f energy a b s o r p t i o n and r e d i s t r i b u t i o n i n t h e s u r f a c e r e g i o n , as w e l l as t h e development o f a comprehensive unders t a n d i n g o f t h e s t a t e o f t h e s u r f a c e under l a s e r annealing conditions,
have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n i n r e c e n t years.
The
f a c t t h a t ordered surfaces can be produced w i t h l a s e r annealing, as j u s t discussed,
suggests t h a t a n a t u r a l probe o f t h e s u r f a c e
r e g i o n which would y i e l d s t r u c t u r a l i n f o r m a t i o n on t h e f i r s t few atomic l a y e r s i s t i m e - r e s o l v e d LEED.
By measuring t h e i n t e n s i t y
o f a d i f f r a c t e d beam d u r i n g t h e l a s e r a n n e a l i n g process, i n f o r m a t i o n about t h e s t a t e o f t h e s u r f a c e can be obtained.
Such measurements
in conj u n c t i on w i t h time- r e s o l ved opt ic a l r e f 1 e c t iv i t y measurements have r e c e n t l y been made u s i n g a 'Ge (111) sample (Becker e t al., 1984a,b).
The LEED i n t e n s i t y was measured i n a temporal window
428
D.M. ZEHNER
e x t e n d i n g from a few nanoseconds b e f o r e t h e l a s e r p u l s e t o 1000 ns a f t e r t h e l a s e r pulse.
These i n t e n s i t i e s were then compared
t o those o b t a i n e d from a sample r a i s e d t o successive s t e a d y - s t a t e temperatures by r a d i a t i v e h e a t i n g from a f i l a m e n t - t y p e Results
show an e x t i n c t i o n
o f the diffracted
sistent
with
increase
the
observed
in
heater.
intensity,
optical
con-
reflectivity,
c l e a r l y i n d i c a t i n g t h a t t h e Ge s u r f a c e i s n o n c r y s t a l l i n e d u r i n g t h e l a s e r a n n e a l i n g process.
These changes i n i n t e n s i t y can be
c o m p l e t e l y accounted f o r i n t h e m e l t i n g model. To i l l u s t r a t e t h e a p p l i c a t i o n o f l a s e r a n n e a l i n g i n producing o r d e r e d s u r f a c e s t r u c t u r e s on c r y s t a l faces o f compound semicond u c t o r s , e s p e c i a l l y those i n which one o f t h e components i s v o l a tile,
r e s u l t s o b t a i n e d from t h e low-index faces o f GaAs c r y s t a l s
w i l l be presented (Zehner e t a1
., 1982).
A l l r e s u l t s t o be d i s -
cussed were o b t a i n e d from surfaces t h a t were i n i t i a l l y s p u t t e r e d i n o r d e r t o remove t h e C and 0 i m p u r i t i e s ,
s i n c e t h i s procedure
p e r m i t t e d t h e use o f r e l a t i v e l y low l a s e r p u l s e energy d e n s i t i e s i n o r d e r t o o b t a i n c l e a n surfaces. the
(loo),
The LEED p a t t e r n s obtained f o r
(110), and (111) o r i e n t a t i o n s o f GaAs c r y s t a l s f o l l o w i n g
i r r a d i a t i o n a t an energy d e n s i t y o f -0.3
J/cm2 a r e shown i n Fig. 8.
The q u a l i t y o f t h e s u r f a c e s t r u c t u r e r e s u l t i n g from l a s e r annealing, as r e f l e c t e d i n these p a t t e r n s , d i f f e r e d s i g n i f i c a n t l y from t h a t o b t a i n e d from elemental semiconductors.
The h i g h e s t qua1 i t y p a t -
t e r n s were o b t a i n e d from t h e (110) o r i e n t a t i o n ,
and reasonable
q u a l i t y p a t t e r n s were o b t a i n e d from both A- and B-type (111) o r i e n t a tions.
Very poor q u a l i t y p a t t e r n s were o b t a i n e d from t h e (100)
orientations.
A l l LEED p a t t e r n s were b a s i c a l l y (1x1)
,
suggesting
no long-range ordered r e c o n s t r u c t i o n as n o r m a l l y observed a f t e r c o n v e n t i o n a l thermal annealing.
I n a l l cases t h e o b s e r v a t i o n o f
d i f f u s e background i n t e n s i t y and/or s t r e a k i n g i n d i c a t e d t h e presence
o f d i s o r d e r i n t h e s u r f a c e region.
These o b s e r v a t i o n s a r e consis-
t e n t w i t h b o t h AES and RBS r e s u l t s , which i n d i c a t e t h e e x i s t e n c e o f excess Ga i n l o c a l r e g i o n s which a r e n o n s t o i c h i o m e t r i c i n t h e n e a r - s u r f a c e region.
Although a range o f energy d e n s i t i e s and a
7.
Fig. 8.
PULSED LASER IRRADIATED SEMICONDUCTORS
LEED patterns from clean laser-annealed
429
( a ) ( l o o ) , ( b ) 1 1 0 ) , and
( c ) ( 1 1 1 ) GaAs surfaces a t primary beam energies o f ( a ) 1 1 3 eV, ( b ) 1 2 3 eV, and ( c ) 95 eV.
D. M. ZEHNER
v a r i a t i o n o f t h e number o f pulses were t r i e d , i t was n o t p o s s i b l e t o produce surfaces from which b e t t e r q u a l i t y LEED p a t t e r n s c o u l d be observed.
Similar
r e s u l t s have been o b t a i n e d from t h e InP
( 100) s u r f a c e (Moison e t a1 7.
., 1982).
METASTABLE SURFACES The o b s e r v a t i o n t h a t a (1x1) LEED p a t t e r n i s o b t a i n e d from t h e
( 1 1 1 ) s u r f a c e o f S i a f t e r l a s e r i r r a d i a t i o n i n a UHV environment and t h a t t h e s u r f a c e i s a t o m i c a l l y c l e a n a f t e r such t r e a t m e n t suggests t h a t t h i s s u r f a c e p r o v i d e s t h e o p p o r t u n i t y f o r i n v e s t i g a t i n g a c l e a n semiconductor s u r f a c e t h a t e x h i b i t s no ordered l a t e r a l reconstruction.
The understanding o f t h i s s t r u c t u r e i s o f v i t a l
importance i n view o f t h e o r e t i c a l d e s c r i p t i o n s o f t h e S i (111) surface.
I f t h e s u r f a c e i s t r u l y b u l k - l i k e except f o r s u r f a c e r e -
l a x a t i o n , i t should d i f f e r from t h e d i s o r d e r e d high-temperature (1x1)
., 1981) and i m p u r i t y - s t a b i l i z e d (1x1) (Eastman e t a1 ., F l o r i o e t a1 ., 1971) surfaces. Furthermore, o t h e r i n v e s t i -
( B e n n e t t e t a1 1980a,b;
g a t i o n s o f t h e (111) s u r f a c e subsequent t o i r r a d i a t i o n w i t h l a s e r p u l s e s have i n d i c a t e d t h a t some degree o f d i s o r d e r i s present. T h i s s u b j e c t w i l l be discussed i n d e t a i l l a t e r i n t h i s s e c t i o n . W h i l e i n f o r m a t i o n about t h e symmetry and s i z e o f t h e twodimensional
unit
cell
diffraction
patterns,
on
the
surface
information
about
can
be
obtained
surface
from
relaxations
r e q u i r e s t h e measurement o f t h e i n t e n s i t i e s o f t h e d i f f r a c t e d e l e c t r o n beams as a f u n c t i o n o f i n c i d e n t e l e c t r o n energy ( I - V profile).
The e x p e r i m e n t a l l y measured p r o f i l e s must t h e n be com-
pared w i t h r e s u l t s o b t a i n e d from f u l l y converged dynamical LEEU c a l c u l a t i o n s assuming v a r i o u s s t r u c t u r a l models f o r t h e geometric arrangement i n t h e outermost l a y e r s . between t h e experimental
A measure o f t h e agreement
r e s u l t s and t h e p r e d i c t i o n o f model
c a l c u l a t i o n s i s p r o v i d e d by t h e R f a c t o r ( t h e lower t h e R f a c t o r value, t h e
b e t t e r t h e agreement).
A d e t a i l e d LEED a n a l y s i s f o r
l a s e r - a n n e a l e d (111)-( 1x1) s u r f a c e s o f S i has been performed, and t h e r e s u l t s are discussed below (Zehner e t al.,
1981a).
7.
431
PULSED LASER IRRADIATED SEMICONDUCTORS
A S i (111) s u r f a c e t h a t had been i r r a d i a t e d w i t h t h e o u t p u t o f t h e l a s e r a t an energy d e n s i t y o f -2.0 investigations.
J/cmz was used i n these
The i n t e n s i t i e s o f t h e d i f f r a c t e d beams were
measured as a f u n c t i o n o f e l e c t r o n energy u s i n g a Faraday cup operated as a r e t a r d i n g f i e l d analyzer.
Data were obtained f o r
a l l o f t h e { l o } , {01}, {20}, and {02} beams and f o r t h r e e each o f t h e { l l } and {21} beams.
Based on o b s e r v a t i o n s and c o n c l u s i o n s
drawn from p r e v i o u s s t u d i e s , s y m m e t r i c a l l y e q u i v a l e n t beams were averaged t o p r o v i d e a data base c o n t a i n i n g s i x average p r o f i l e s . The experimental data base has been compared w i t h t h e r e s u l t s o b t a i n e d from f u l l y converged dynamical LEED c a l c u l a t i o n s .
Details
o f these c a l c u l a t i o n s can be found elsewhere, and o n l y t h e r e s u l t s
w i l l be summarized here. t h e dynamical
Comparison o f p r o f i l e s o b t a i n e d from
LEED c a l c u l a t i o n s t o t h e measured I - V
suggests t h a t t h e f i r s t i n t e r l a y e r spacing, d,
profiles
i s c o n t r a c t e d by
25.5 a 2.5% w i t h respect t o t h e b u l k value and t h a t t h e second i n t e r l a y e r spacing, d,, b u l k value.
i s expanded 3.2 r 1%w i t h respect t o t h e
P r o f i l e s c a l c u l a t e d u s i n g these values a r e shown i n
Fig. 9, which a l s o c o n t a i n s t h e corresponding experimental p r o f i l e s and single-beam r e l i a b i l i t y f a c t o r s ( R ) determined f o r each comparison.
The six-beam R f a c t o r corresponding t o Fig. 9 i s 0.115.
T h i s value i n d i c a t e s a very good agreement between c a l c u l a t e d and experimental p r o f i l e s i n a conventional LEED a n a l y s i s and suggests t h a t t h e proposed s t r u c t u r a l model i s h i g h l y probable.
Furthermore,
t h i s R value i s s i g n i f i c a n t l y lower than any r e p o r t e d value o b t a i n e d i n a LEED a n a l y s i s o f any semiconductor surface.
The changes i n
i n t e r l a y e r spacings determined from t h i s a n a l y s i s correspond t o n e a r e s t - n e i g h b o r bond l e n g t h changes o f -0.058 and t0.075 A.
These
r e s u l t s are c o n s i s t e n t w i t h a t o t a l energy c a l c u l a t i o n f o r such a s u r f a c e which g i v e s an inward r e l a x a t i o n o f t h e outermost l a y e r ( N o r t h r u p e t al.,
1981).
I n a separate
investigation
of
a Si
(111) s u r f a c e l a s e r
annealed w i t h pulses from a doubled Nd:YAG l a s e r ( A = 530 nm), a V i d i c o n camera was used t o scan t h e LEED p a t t e r n recorded on
432
D. M. ZEHNER
(40)BEAM R = 0.466
( 0 2 ) BEAM R = 0.095
I
I
(24) BEAM R = 0.088
-
CALCULATED AVERAGE EXPERIMENTAL
4 20
00
00
(60
420
460
200
ENERGY (eV)
Fig. 9 .
A comparison o f the averaged experimental I-V p r o f i l e s with calcu= -25.5%
lated results for Adl2
and Ad23 = 3.2%.
P o l a r o i d f i l m i n o r d e r t o o b t a i n t o o b t a i n angular i n t e n s i t y prof i l e s (Chabal e t al., weak peak [-0.02 ha1 f - o r d e r
1981a).
I n these measureriients, a broad and
t i m e s t h e (11) i n t e n s i t y ]
position,
characteristic
was p r e s e n t a t t h e
o f a (2x1)
reconstruction.
From these data i t was concluded t h a t no long-range o r d e r e x i s t s b u t t h a t d i s o r d e r e d domains w i t h a buckled ( 2 x 1 ) - l i k e r e c o n s t r u c t i o n are present.
The absence o f such o b s e r v a t i o n s i n t h e pre-
v i o u s l y discussed LEED a n a l y s i s suggests t h a t surfaces prepared w i t h d i f f e r e n t l a s e r a n n e a l i n g parameters may d i s p l a y d i f f e r e n c e s i n t h e d e t a i l s o f s u r f a c e order. To examine t h e q u e s t i o n o f s u r f a c e order, scattering
medium energy i o n
combined w i t h channel i n g and b l o c k i n g ,
a technique
which is a l s o s e n s i t i v e t o geometrical s t r u c t u r e i n t h e s u r f a c e r e g i o n , has been used t o i n v e s t i g a t e t h e S i ( l l 1 ) s u r f a c e (Tromp
433
7 . PULSED LASER IRRADIATED SEMICONDUCTORS e t a1
., 1982).
I n t h i s study,
data were obtained b o t h from a
s u r f a c e e x h i b i t i n y a sharp (7x7) LEED p a t t e r n ,
prepared by con-
v e n t i o n a l procedures, and from a s u r f a c e e x h i b i t i n g a sharp (1x1) LEED p a t t e r n , prepared by i r r a d i a t i n g t h e sample w i t h a s i n g l e p u l s e from a ruby l a s e r .
From an a n a l y s i s o f t h e data i t was concluded
t h a t t h e atomic displacements on both s u r f a c e s a r e r e s t r i c t e d t o two monolayers,
probably t h e f i r s t double l a y e r o f t h e c r y s t a l .
T h i s c o n c l u s i o n i s c o n s i s t e n t w i t h t h e r e s u l t s o f t h e LEE0 a n a l y s i s . However, i n t h i s model t h e atoms i n t h e f i r s t two monolayers occupy w e l l - d e f i n e d p o s i t i o n s and should g i v e r i s e t o a s t r o n g b l o c k i n g effect.
This blocking e f f e c t
i s n o t reproduced i n t h e data,
suggesting t h a t t h e atoms may occupy d i f f e r e n t l a t e r a l p o s i t i o n s and g i v e r i s e t o less e f f i c i e n t and smeared-out b l o c k i n g .
Thus, t h e
r e s u l t s are i n c o n s i s t e n t w i t h a simple r e l a x a t i o n model and i n d i c a t e some degree o f d i s o r d e r i n t h e s u r f a c e region.
A s i m i l a r LEED a n a l y s i s has been performed on a laser-annealed Ge (111) s u r f a c e (Zehner e t a1
., 1981b).
As w i t h S i , t h e b e s t agree-
ment i s o b t a i n e d f o r a s t r u c t u r a l model i n which atoms i n t h e o u t e r most l a y e r are d i s p l a c e d inward and t h o s e i n t h e second l a y e r a r e d i s p l a c e d outward r e l a t i v e t o t h e i r b u l k p o s i t i o n s , r e s p e c t i v e l y . The corresponding nearest-nei ghbor bond l e n g t h changes are -0.037 and +0.066 A . An examination o f t h e e l e c t r o n i c s t r u c t u r e i n t h e s u r f a c e r e g i o n o f t h e laser-annealed S i (111) and Ge (111) s u r f a c e s i s o f i n t e r e s t i n view o f t h e r e s u l t s o f both t h e LEED analyses and i o n s c a t t e r i n g r e s u l t s j u s t discussed.
P h o t o e l e c t r o n spectroscopy
d i r e c t l y y i e l d s i n f o r m a t i o n about t h e l o c a l bonding b u t i s l e s s s e n s i t i v e t o t h e long-range o r d e r than LEED.
Therefore,
resolved
studies
and
anyle-i ntegrated
photoemi s s i o n
valence band s u r f a c e s t a t e s and s u r f a c e c o r e - l e v e l been performed f o r t h e f o l l o w i n g s u r f a c e s :
of
angleboth
s h i f t s have
( 1 ) laser-annealed S i
and Ge (111)-(1x1)
surfaces prepared as f o r t h e LEED s t u d i e s and
(2) S i (lll)-(7x7)
and Ge ( l l l ) - ( 2 x 8 )
s u r f a c e s prepared by t h e r -
m a l l y annealing t h e (1x1) surfaces (Himpsel e t al.,
1981).
The
434
D. M. ZEHNER
measurements were made u s i n g t h e d i s p l a y - t y p e spectrometer a t t h e s y n c h r o t r o n r a d i a t i o n source, Tantalus I. I n Fig.
10, a n g l e - i n t e g r a t e d photoemission s p e c t r a a r e pre-
sented f o r laser-annealed
(1x1) surfaces ( f u l l
curves) and f o r
t h e t h e r m a l l y annealed surfaces (dashed curves) o f Ge (111) and Si
The d o t t e d l i n e s show t h e s p e c t r a o b t a i n e d a f t e r a
(111).
hydrogen exposure, which r e s u l t s i n about a s a t u r a t i o n monolayer coverage o f hydrogen.
Below -4 eV, hydrogen induces e x t r a s t a t e s
t h a t are w e l l understood b u t n o t i m p o r t a n t i n t h i s c o n t e x t . difference
between
the
solid
(dashed)
curves
The
and t h e d o t t e d
A l l four
curves above -3 eV r e p r e s e n t s s u r f a c e - s t a t e emission.
s u r f a c e s have a d o u b l e t o f s t a t e s near t h e t o p o f t h e
clean
valence band which i s quenched by hydrogen exposure.
Relative t o
t h e t o p o f t h e valence band, these s t a t e s l i e a t -0.4
and -1.3
f o r t h e annealed S i (111) s u r f a c e s and a t -0.7 annealed Ge (111) surfaces. dependent
photoelectron
and -1.3
By u s i n g a n g l e - r e s o l v e d p o l a r i z a t i o n -
spectroscopy,
the
surface
states
determined t o have d i s t r i b u t i o n s i n momentum (Ell)-space
i n Fig.
are
and sym-
m e t r i e s which are s i m i l a r f o r a l l f o u r annealed surfaces. results
eV
f o r the
These
a r e summarized r e l a t i v e t o t h e hexagonal B r i l l o u i n zone
11.
I t is remarkable t h a t t h e predominant s u r f a c e s t a t e s f o r t h e
t h e r m a l l y annealed Ge (111) and S i (111) surfaces match t h e (1x1) s u r f a c e B r i l l o u i n zone and show no i n d i c a t i o n o f t h e small r e c i p r o cal
(2x8)
that
or
(7x7)
photoemission
unit
cells.
(This
can indeed sense
by t h e l a r g e r (1x1) u n i t c e l l i n b,, space.) t i o n f o r the S i ( l l l ) - ( 7 x 7 ) appears near t h e Fermi l e v e l
surface:
, which
observation
confirms
t h e short-range o r d e r given There i s one excep-
a weak t h i r d s u r f a c e s t a t e
makes t h i s s u r f a c e m e t a l l i c , i n
c o n t r a s t t o t h e o t h e r t h r e e surfaces.
This exception i s consistent
w i t h a band p i c t u r e , wherein t h e S i ( l l l ) - ( 7 x 7 )
s u r f a c e has t o be
m e t a l l i c because t h e r e i s an odd number o f e l e c t r o n s i n t h e ( 7 x 7 ) unit cell. tially
Each band holds two e l e c t r o n s ,
filled
band.
The
extra
surface
which leaves a parstate
for
the
Si
7.
F i g . 10.
PULSED LASER IRRADIATED SEMICONDUCTORS
435
Angle-integrated photoelectron spectra f o r the annealed G e ( 11 1 )
and S i ( l l 1 ) surfaces showing emission from two surface states near the top o f the valence band which i s quenched by hydrogen exposure (dotted l i n e s ) . denotes the valence-band
maximum.
Ev
436
D. M. ZEHNER
LOWER STATE
UPPER STATE
n
EXTRA STATE
EF
AT
F OR
Si (111)- (7x7)
Fig. 1 1 . Characteristic locations (dashed areas) of different surface states in the ( 1 x 1 ) surface Briliouin zone (hexagon) for the annealed G e ( l l 1 ) and S i ( l l 1 ) surfaces. At the zone center, the lower surface state has A 3 ( P ~ , ~character ) and the upper state has A, ( s , p z ) character.
(lll)-(7x7)
i s c o n c e n t r a t e d near t h e m i d d l e o f t h e edges o f a
( 7 x 7 ) surface B r i l l o u i n zone as shown i n Fig.
11, and i t s i n t e n -
s i t y i s s e n s i t i v e t o t h e long-range (7x7) order. Additional
information
about
the
surface
geometry
can be
o b t a i n e d by measuring t h e s h i f t s i n energy and i n t e n s i t y of c o r e l e v e l s f o r s p e c i f i c surface
atoms.
The s u r f a c e - s e n s i t i v e angle-
i n t e g r a t e d photoemission s p e c t r a f o r Ge(3d) and S i ( 2p) core l e v e l s ( w i t h experimental mean-free paths o f 5.9 S i , r e s p e c t i v e l y ) a r e shown i n Fig.
12.
and 5.4
A f o r Ge and
By comparing s p e c t r a f o r
c l e a n ( f u l l l i n e s ) and hydrogen-covered ( d o t t e d l i n e s ) surfaces, i t i s c l e a r t h a t t h e r e are c o r e l e v e l s a t lower b i n d i n g energies which are c h a r a c t e r i s t i c o f t h e c l e a n s u r f a c e (marked by arrows
7.
PULSED LASER IRRADIATED SEMICONDUCTORS
Si(111) hv=120 eV
Ge(ll1) h v = 7 0 eV
7x7
2x8
-1.0
Fig.
12.
437
- j.0 0 1 .O 0 1.0 I N I T I A L STATE ENERGY RELATIVE TO BULK (eV)
Surface-sensitive
core-level
spectra f o r the, annealed Ge( 1 1 1 )
and S i ( l l 1 ) surfaces showing s h i f t e d c o r e levels f o r special surface atoms. The Ge data consits o f spin-orbit-split
3 d 3 / 2 and 3 d g / 2 levels, whereas in
t h e S i data the 2 p 1 / 2 levels have been removed by spin-orbit D o t t e d lines are f o r hydrogen-covered Si(ll1)
-
( 2 x 1 ) + H, r e s p e c t i v e l y ] ,
l o w e r binding energies are removed.
deconvolution. ( 1 x 1 ) + H and wherein the surface core levels at
surfaces [ G e ( l l l )
-
438
D. M. ZEHNER
i n Fig.
The r e s u l t s o f a l e a s t - s q u a r e s f i t t o t h e data a r e
12).
g i v e n i n Table 1 and can be summarized as f o l l o w s :
t h e annealed
Ge (111) and S i (111) s u r f a c e s have r o u g h l y 1/4 o f a monolayer o f s u r f a c e atoms,
w i t h l a r g e core-level
l o w e r b i n d i n g energy.
s h i f t s (0.6-0.8
L i t t l e difference
eV) t o w a r d
i s observed between
t h e r m a l l y annealed and l a s e r - a n n e a l e d surfaces. TABLE I S p e c i a l S u r f a c e Atoms f o r t h e Annealed Ge( 111) and S i ( 111) Surfaces Core-level s h i f t (towards lower binding energy, M.1 e v ) (ev)
Number o f atoms in v o l ved (20.05 l a y e r ) ( 1dyer)
Ge( 111)-( 2x8)
0.75 0.35
0.28 >O. 25
Ge( 111)-( 1x1)
0.60
0.37
S i ( 111)-(7 x 7 )
0.70
0.16
S i ( 111)-( 1x1)
0.80
0.23
The s t r o n g s i m i l a r i t y o f t h e valence band s u r f a c e s t a t e s and surface core-level s p e c t r a f o r b o t h t h e l a s e r - a n n e a l e d and t h e r m a l l y annealed S i and Ge s u r f a c e s i n d i c a t e s t h a t these s u r f a c e s have very s i m i l a r l o c a l bonding geometries and d i f f e r m a i n l y i n long-range o r d e r i n v o l v i n g g e o m e t r i c a l arrangements t h a t a r e o n l y a p e r t u r b a t i o n o f t h e average l o c a l bonding geometry.
An i n t e r -
e s t i n g q u e s t i o n t h e n i n v o l v e s t h e LEED analyses (Zehner e t al., 1981a; Zehner e t al.,
1 9 8 l b ) , which g i v e such good agreement w i t h
d a t a u s i n g a model ( 1 x 1 ) geometry t h a t appears t o be d i f f e r e n t f r o m t h a t needed t o d e s c r i b e t h e s u r f a c e e l e c t r o n i c s t r u c t u r e . One p o s s i b l e e x p l a n a t i o n i s t h a t LEED i s n o t p a r t i c u l a r l y s e n i t i v e t o long-range d i s o r d e r i f i t i s p r e s e n t on t h e ( 1 x 1 ) surface. Thus,
t h e i n t e r l a y e r displacements determined may be considered
439
7 . PULSED LASER IRRADIATED SEMICONDUCTORS
t o be averages over t h e coherence l e n g t h o f t h e e l e c t r o n beam. Another relaxed,
explanation
is
that
photoemission
can
rule
out
the
ordered ( 1 x 1 ) geometry o n l y i f t h e s u r f a c e s t a t e s a r e
band-1 ike as assumed i n one-el e c t r o n band c a l c u l a t i ons [ Pandy e t 1974; S c h l i i t e r e t a1
al.,
calculations
predict
., 1975;
that
such
C i r a c i e t al.,
1975).
These
a s u r f a c e would be m e t a l l i c ,
w i t h a h a l f - f i l l e d band o f d a n g l i n g bond s t a t e s a t t h e Fermi energy, EF, and t h i s i s i n c o n s i s t e n t w i t h t h e data, which show no emission near EF f o r t h e (1x1) surfaces.
However,
correlation
e f f e c t s might be very i m p o r t a n t f o r these narrow s u r f a c e l e v e l s .
A number o f researchers (Duke e t al., al.,
1981; Lannoo e t al.,
f o r t h theoretical
1981,
1982; L o u i s e t al.,
proposals
1982; Del Sole e t 1982, 1983) have p u t
t h a t would make t h e photoemission
d a t a from t h e laser-annealed S i (111) s u r f a c e c o n s i s t e n t w i t h t h e u n r e c o n s t r u c t e d r e l a x e d s u r f a c e p r e d i c t e d by t h e LEED a n a l y s i s . I n t h e s e models i t i s assumed t h a t s t r o n g c o r r e l a t i o n s dominate the
surface
state
band
structure,
and
they
predict
a
low-
temperature a n t i f e r r o m a g n e t i c ground s t a t e and downward d i s p e r s i o n o f t h e d a n g l i n g bond s t a t e s along r-J. have n o t been t e s t e d e x p e r i m e n t a l l y .
These p r e d i c t i o n s
Nevertheless,
e f f e c t s cannot e x p l a i n t h e s i m i l a r i t y i n c o r e - l e v e l
correlation shifts for
b o t h laser-annealed and t h e r m a l l y annealed surfaces. B o t h a n g l e - i n t e g r a t e d (McKinley e t a l . (Chabal e t al.,
, 1981) and angle-resolved
1981a) photoemission data have been obtained from
laser-annealed S i (111) s u r f a c e s u s i n g d i f f e r e n t annealing conditions.
I n agreement w i t h t h e r e s u l t s j u s t discussed and w i t h
r e s u l t s o b t a i n e d i n an independent i n v e s t i g a t i o n u s i n g a ruby l a s e r (Dernuth e t al.,
1984), no occupied s t a t e s a t EF are observed.
However, t h e energies of t h e s u r f a c e s t a t e s and t h e i r d i s p e r s i o n , o b t a i n e d a f t e r i r r a d i a t i o n w i t h e i t h e r a XeCl o r frequency-doubled Nd:YAG d i f f e r somewhat from t h e r e s u l t s presented u s i n g a ruby laser.
I n fact,
i t i s argued t h a t t h e laser-annealed
surface
examined i n these s t u d i e s i s buckled w i t h no long-range o r d e r b u t w i t h a short-range ( 2 x 1 ) r e c o n s t r u c t i o n .
From these r e s u l t s and
440
D. M. ZEHNER
t h o s e o b t a i n e d from t h e r m a l l y
quenched S i
(111)
surfaces,
it
appears t h a t d i f f e r e n t l a s e r a n n e a l i n g c o n d i t i o n s (depth o f m e l t , r e g r o w t h v e l o c i t y ) can r e s u l t i n d i f f e r e n t l o c a l bonding arrangements.
8.
VICINAL SURFACES The
chemical
influence o f
steps
reactivity of
on t h e e l e c t r o n i c
properties
semiconductor s u r f a c e s a r e well
and
known.
Stepped ( v i c i n a l ) s u r f a c e s can be prepared by i n s i t u c l e a n i n g o r i o n etching,
but t h e control o f step density,
step height,
and
ease o f r e p r o d u c i b i l i t y has proved d i f f i c u l t u s i n g t h e s e convent i o n a l procedures.
The r a p i d m e l t i n g and regrowth achieved w i t h
l a s e r a n n e a l i n g suggest t h a t t h i s procedure can be used w i t h vicinal
surfaces t o produce s u r f a c e s c o n t a i n i n g monatomic steps
and u n i f o r m t e r r a c e widths. To demonstrate t h a t such s u r f a c e s can be produced, o b t a i n e d from a S i ( l l 1 ) f r o m a (111) plane (Zehner e t a1
c r y s t a l whose s u r f a c e was c u t a t 4.3'
toward t h e
., 1980b).
results
[ i i 2 ] d i r e c t i o n w i l l be discussed
F o r t h i s d i r e c t i o n , t h e edge atoms have
o n l y two n e a r e s t neighbors.
The w e l l - d e f i n e d (1x1)
LEED p a t t e r n
o b t a i n e d from t h e c l e a n s u r f a c e and shown i n Fig. 13 ( a ) was observed after
i r r a d i a t i n g t h e s u r f a c e with f i v e p u l s e s a t -2.0
J/cm2.
The p a t t e r n i n d i c a t e s t h e e x i s t e n c e o f a stepped s u r f a c e which can be indexed [ 1 4 ( 1 1 1 ) x ( i i 2 ) ] . energy, and [ O l ]
By v a r y i n g t h e p r i m a r y e l e c t r o n
t h e t h r e e f o l d spot s p l i t t i n g a l t e r n a t e s between t h e r e f l e c t i o n s a t s p e c i f i c e l e c t r o n energies.
[lo]
The energies
a t which a g i v e n r e f l e c t i o n i s s p l i t or n o n s p l i t g i v e s p e c i f i c i n f o r m a t i o n on t h e s t e p h e i g h t , and t h e angular s e p a r a t i o n between s p l i t spots provides i n f o r m a t i o n on t h e t e r r a c e width.
An a n a l y s i s
o f t h e spot s p l i t t i n g s i n t h i s p a t t e r n u s i n g o n l y a k i n e m a t i c t r e a t m e n t o f s i n g l e s c a t t e r i n g from t h e t o p l a y e r (Henzler, 1970) i n d i c a t e s t h a t t h e s u r f a c e c o n s i s t s o f monatomic s t e p s w i t h an average s t e p h e i g h t o f one double l a y e r (3.14 w i d t h s -45 A as i l l u s t r a t e d i n Fig. 14.
A)
with terrace
The absence o f f r a c t i o n a l
7.
Fig.
13.
441
PULSED LASER IRRADIATED SEMICONDUCTORS
LEED
patterns from clean vicinal
beam energies o f ( a ) 40 and ( b ) 68 eV.
Si(ll1 )
surfaces a t primary
( a ) Laser annealed, (b) thermally
annealed.
LASER ANNEALED
-
( 4 x 4 ) WITH SPLIT SPOTS
4;3"
THERMALLY ANNEALED
-
(7 x 7 )
4.30
Fig.
14.
Schematic
the (710) plane.
illustration o f the vicinal
Top view i s for the laser-annealed
surface projected into surface.
Bottom view
illustrates a possible configuration obtained with thermal annealing.
442
D. M. ZEHNER
order r e f l e c t i o n s ,
i n d i c a t i v e of
reconstruction,
suggests t h a t
t h e l o c a l atomic arrangement produced by t h i s a n n e a l i n g procedure may be s i m i l a r t o t h a t produced on f l a t (111) surfaces. I n o r d e r t o achieve such a h i g h step d e n s i t y c o n f i g u r a t i o n , a l a r g e amount o f atom motion has t o t a k e place.
T h i s movement can
be accomplished e i t h e r by e v a p o r a t i o n o f s u r f a c e atoms o r by d i f f u s i o n i n t h e molten phase.
R e s u l t s o f r e c e n t experiments w i t h
stepped s u r f a c e s (Osakabe e t al., evaporation 1475
K.
1980, 1981) show t h a t some
t a k e s p l a c e a t step edges a t temperatures as low as
Assuming m e l t i n g occurs d u r i n g t h e l a s e r a n n e a l i n g con-
d i t i o n s used, about
lo9
atoms/cm2 evaporate i n a 10-ns p u l s e f o r
an e v a p o r a t i o n r a t e o f 1017 atoms/cmzs [vapor p r e s s u r e 5 x 10-3 T o r r (Chabel e t al., monolayer,
T h i s corresponds t o o n l y 10-6 o f a
1982)].
which i s n o t enough t o account f o r t h e l a r g e atomic
rearrangements over hundreds o f angstroms. mechanism must dominate, be e s t i m a t e d D
2
Thus,
the diffusion
and a s u r f a c e d i f f u s i o n c o e f f i c i e n t can’
(100 A ) 2 / ( 10 ns) -loe4 cm2/s.
This high d i f -
f u s i o n c o e f f i c i e n t would be q u i t e i n c o m p a t i b l e w i t h a nonthermal model
of
l a s e r a n n e a l i n g b u t i s c o n s i s t e n t w i t h experimental
measurements ( N i shizawa e t a1 the melting
point,
the
., 1972).
surface
A t temperatures c l o s e t o
arrangement
i s dominated
by
e n t r o p y , which i s r e s p o n s i b l e f o r a s t e p - s t e p r e p u l s i o n (Gruber et al., disorder
1967).
is
As t h e c r y s t a l c o o l s down and t h e e n t r o p y - d r i v e n
reduced,
the
surface d i f f u s i o n
decreases
t o the
e x t e n t t h a t t h e steps cannot recombine; t h e y remain f r o z e n i n t h e high-temperature c o n f i g u r a t i o n . The s t a b i l i t y o f t h e r e g u l a r a r r a y o f steps was i n v e s t i g a t e d by s u b j e c t i n g t h e laser-annealed s u r f a c e t o a s e r i e s o f thermala n n e a l i n g t r e a t m e n t s a t h i g h e r and h i g h e r temperatures. f o r f l a t (111) S i surfaces,
As observed
thermal annealing o f t h e c r y s t a l t o
temperatures g r e a t e r than -800 K r e s u l t e d i n a s u r f a c e from which t h e ( 7 x 7 ) d i f f r a c t i o n p a t t e r n shown i n Fig. 13 ( b ) was o b t a i n e d i n accord w i t h p r e v i o u s o b s e r v a t i o n s (Olshanetsky e t a l .
, 1979).
The
absence o f s p l i t t i n g of i n d i v i d u a l spots i n d i c a t e s t h e e l i m i n a t i o n
7.
443
PULSED LASER IRRADIATED SEMICONDUCTORS
o f t h e r e g u l a r a r r a y o f monatomic steps, and t h e sharpness o f t h e integral-order
reflections
i s c o n s i s t e n t w i t h a s u r f a c e having
t e r r a c e s wider than -200
A.
macroscopic
inclination,
multilayer
illustrated
i n Fig.
14.
I n o r d e r t o m a i n t a i n t h e average steps must be present
as
A s u r f a c e c o n t a i n i n g monatomic steps
c o u l d be regenerated by i r r a d i a t i n g t h e t h e r m a l l y annealed surface with the laser.
These o b s e r v a t i o n s i n d i c a t e t h a t i t i s
p o s s i b l e t o produce r e p e a t e d l y a p a r t i c u l a r s t e p arrangement by i n i t i a l l y c u t t i n g the crystal t o the desired orientation. I n v e s t i g a t i o n s o f v i c i n a l S i (111) s u r f a c e s c u t a l o n g t h e [ i i 2 ] d i r e c t i o n have produced r e s u l t s very s i m i l a r t o those discussed above (Chabal e t al.,
1981b).
Steps along t h i s d i r e c t i o n c o n t a i n
edge atoms t h a t have t h r e e nearest neighbors.
Although d e t a i l e d
s t u d i e s on t h e angular p r o f i l e s show t h e step h e i g h t t o be 3.06 A i n t h i s d i r e c t i o n , somewhat l e s s than t h e d o u b l e - l a y e r separation, t h e o v e r a l l behavior f o r laser-annealed v i c i n a l surfaces i s t h e same f o r b o t h types o f steps.
9.
DEFECTS As a consequece o f m e l t i n g d u r i n g t h e l a s e r annealing process,
atoms a r e evaporated from t h e s u r f a c e region.
I n f a c t , measure-
ment o f S i p a r t i c l e emission d u r i n g e v a p o r a t i o n u s i n g a c l a s s i c a l t i m e - o f - f l i g h t technique has been used t o determine t h e l a t t i c e temperature and t o demonstrate t h a t me1 t i n g occurs ( S t r i t z k e r e t al.,
1981).
I n a d d i t i o n t o n e u t r a l p a r t i c l e emission, b o t h i o n
and e l e c t r o n e j e c t i o n s have been d e t e c t e d (Moison, e t a1
., 1982).
The t h r e s h o l d f l u e n c e r e q u i r e d f o r d e t e c t i o n o f such p a r t i c l e emission bas been determined f o r a number o f m a t e r i a l s (Moison e t al.,
1983).
R e s u l t s o b t a i n e d f o r InP and GaAs s i n g l e c r y s t a l s
a r e c o n s i s t e n t w i t h AES and RBS observations, erential Si,
l o s s o f t h e more v o l a t i l e component.
indicating a prefI n t h e case o f
t h e amount o f m a t t e r removed was observed t o be orders o f
magnitude l e s s .
444
D. M. ZEHNER
The o b s e r v a t i o n t h a t e v a p o r a t i o n occurs d u r i n g l a s e r a n n e a l i n g i n d i c a t e s t h a t t h e c r e a t i o n o f d e f e c t s i s p o s s i b l e and t h a t d u r i n g t h e quenching p e r i o d a c o m p e t i t i o n t a k e s p l a c e between t h e e l i m i n a t i o n o f d e f e c t s c r e a t e d a t t h e m e l t i n g temperature and t h e growth o f an ordered s u r f a c e
region.
T h i s p o s s i b i l i t y may be par-
t i c u l a r l y i m p o r t a n t i n t h e case o f r e c o n s t r u c t e d s u r f a c e l a y e r s . I f t h e d e f e c t s a r e n o t e l i m i n a t e d f a s t enough,
t h e y may impede
growth o f t h e s u p e r s t r u c t u r e by v a r i o u s mechanisms.
Thus,
the
r e g r o w t h v e l o c i t y o f t h e m e l t f r o n t may p l a y an i m p o r t a n t r o l e i n the d e t a i l s o f the
f i n a l geometric o r d e r i n g .
been suggested (Chabel e t al.,
I n fact,
i t has
1982) t h a t t h e d i f f e r i n g photo-
emission r e s u l t s o b t a i n e d i n independent laser-anneal i n g s t u d i e s can be i n t e r p r e t e d as a consequence o f d i f f e r e n t f i n a l
state
geometric o r d e r i n g due t o d i f f e r e n t regrowth v e l o c i t i e s . I n order
t o e x p l o r e t h e dependence on regrowth v e l o c i t y ,
measurements have been made subsequent t o annealing w i t h a pulsed XeCl excimer l a s e r ( A = 308 nm) (Zehner e t a1
., 1984a).
Measurements
were made a f t e r l a s e r a n n e a l i n g t h e c r y s t a l w i t h an energy d e n s i t y i n t h e range 1.0-4.0 (Wood and G i l e s ,
J/cm2.
Standard heat f l o w
calculations
1981) have been used t o e s t a b l i s h t h a t a v a r i a -
t i o n i n regrowth v e l o c i t y from 1 m/s a t 4.0 J / c d t o 4.5 1.0
J/cm2
m/s a t
can be o b t a i n e d w i t h t h e excimer l a s e r used i n t h i s
experiment (see Chapter 4).
R e s u l t s o b t a i n e d from p h o t o e l e c t r o n
spectroscopy
determine
were
used
to
s t r u c t u r e o f v a r i o u s laser-annealed annealed S i ( l l 1 )
-
(7x7),
the
surface
-
S i ( 111)
and cleaved S i ( l l 1 )
-
electronic
(1x1)
,
thermally
( 2 x 1 ) surfaces.
The s u r f a c e s t a t e s near t h e t o p o f t h e band are i m p o r t a n t s i n c e t h e y have c h a r a c t e r i s t i c energies and angular d i s t r i b u t i o n s t h a t have been s t u d i e d p r e v i o u s l y (Zehner e t al.,
1 9 8 1 ~ ) . I n Fig.
15
t h e energy d i s t r f b u t i o n s o f s u r f a c e s t a t e s near t h e t o p o f t h e valence band a r e shown f o r v a r i o u s S i ( l l 1 ) surfaces.
As evidenced
by t h e s e n s i t i v i t y t o hydrogen exposure ( n o t shown), s u r f a c e s e x h i b i t t h r e e dominant Fig.
the (7x7)
s u r f a c e s t a t e s l a b e l e d 1-3 i n
15 and t h e ( 2 x 1 ) s u r f a c e i s dominated by two s u r f a c e s t a t e s
7.
445
PULSED LASER IRRADIATED SEMICONDUCTORS
-6
-5
-3
4
-2
-1
0
1
INITIAL ENERGY ( RELATIVE TO VALENCE BAND MAXIMUM )
Fig.
15.
Angle-integrated
spectra from freshly cleaved S i ( 1 1 1 )-( 2x1 )
,
UV (308 nm XeCI) laser-annealed Si( 1 1 1 )-( 1x1 ) produced with 1, 2, 3 , and 4 J / c m 2 pulses, ruby (694 n m ) laser-annealed Si( 1 1 1 )(lxl) produced with a 2 J /cm2 pulse, and
Si (1 1 1 )-(7x7 )
obtained by thermal annealing.
446
D. M. ZEHNER
l a b e l e d 4 and 5.
For t h e v a r i o u s laser-annealed surfaces,
two
s u r f a c e s t a t e s which c l o s e l y resemble s t a t e s 1 and 2 on t h e ( 7 x 7 ) s u r f a c e and a r e t o t a l l y d i f f e r e n t from those observed on t h e c l e a v e d (2x1) s u r f a c e were i d e n t i f i e d . change
of
variation
this of
surface
state
regrowth v e l o c i t y
Moreover, no s i g n i f i c a n t
structure from
was
1-4.5
observed over
m/s,
apart
a
from a
weakening o f t h e s u r f a c e s t a t e s f o r t h e f a s t e s t regrowth velocity.
T h i s weakening c o u l d be due t o t h e onset o f d i s o r d e r when
t h e energy d e n s i t y employed approaches t h e me1t t h r e s h o l d . I t i s known t h a t t h e S i ( l l 1 ) s u r f a c e undergoes a s t r u c t u r a l
change from (7x7) ( B e n n e t t e t al.,
t o (1x1) a t a c r y s t a l temperature o f 1150 K 1981).
For very low c o o l i n g r a t e s t h e s t r u c -
t u r a l t r a n s i t i o n i s reversible,
b u t i f quenching r a t e s exceed
approximately
lo2
irreversible.
Consequently, t h e quenching r a t e a t t h e t r a n s i t i o n
K / s (Hagstrum e t al.,
1973), t h e t r a n s i t i o n i s
temperature, subsequent t o l a s e r i r r a d i a t i o n , may be i m p o r t a n t i n When S i ( 111) i s quenched
d e t e r m i n i n g t h e s u r f a c e s t a t e spectra.
t h r o u g h t h e t r a n s i t i o n temperature a t 102 K/s, s u r f a c e s t a t e s p e c t r a s i m i l a r t o those f o r t h e laser-annealed surfaces shown i n Fig. 15 a r e observed (Eastman e t al.,
1980b).
Heat f l o w c a l c u l a t i o n s f o r
l a s e r a n n e a l i n g a t 1.0 J/cm2 p r e d i c t a quenching r a t e o f 1010 K / s a t 1150 K.
Thus,
s i m i l a r s u r f a c e s t a t e s p e c t r a are observed f o r
quenching r a t e s between 102 and 1010 K/s.
I f t h e quenching r a t e
a t t h e t r a n s i t i o n temperature i s o f importance i n d e t e r m i n i n g t h e surface
state
spectra,
rates
i n excess
of
1010 K / s
will
be
necessary t o produce s u r f a c e s t a t e f e a t u r e s s i m i l a r t o those o f t h e (2x1) surface.
Furthermore, f o r regrowth v e l o c i t i e s g r e a t e r
t h a n 15 m/s, where an amorphous l a y e r i s formed ( C u l l i s e t a l . , 1982),
one would expect t o see s u b s t a n t i a l
differences i n the
s u r f a c e s t a t e spectra. Additional
i n f o r m a t i o n about b o t h t h e s i m i l a r i t i e s and d i f -
ferences i n geometric s u r f a c e s t r u c t u r e f o r s u r f a c e s prepared by d i f f e r e n t t r e a t m e n t s can be obtained by i n v e s t i g a t i n g a d s o r p t i o n phenomena.
The technique o f h i g h - r e s o l u t i o n i n f r a r e d spectroscopy
7.
447
PULSED LASER IRRADIATED SEMICONDUCTORS
(Chabel , 1983) has been used t o study t h e v i b r a t i o n a l spectrum o f hydrogen chemisorbed on S i ( l l l ) - ( 7 x 7 ) prepared by thermal a n n e a l i n g and S i ( l l 1 ) - ( 1 x 1 )
prepared by l a s e r annealing.
T h i s technique
g i v e s d i r e c t i n f o r m a t i o n on t h e number, p o s i t i o n , and p o i a r i z a t i o n o f d a n g l i n g bonds, which a r e present a t t h e s u r f a c e o f a semiconductor.
For coverages as low as 1.5% o f a monolayer o f hydrogen
on t h e S i ( l l l ) - ( 7 x 7 ) observed.
surface,
two d i s i i n c t a d s o r p t i o n peaks are
Each peak corresponds t o a S i - H s t r e t c h i n g v i b r a t i o n
f o r hydrogen chemisorbed a t d i f f e r e n t s i t e s .
By i n v e s t i y a t i n g
t h e change i n i n t e n s i t y and energy o f these v i b r a t i o n s i t i s concluded t h a t a unique chemisorption s i t e e x i s t s on t h i s s u r f a c e and i s recessed from t h e outermost plane.
R e s u l t s o b t a i n e d from
t h e laser-annealed S i ( 1 1 1 ) - ( 1x1) s u r f a c e show o n l y one a d s o r p t i o n peak.
The peak a s s o c i a t e d w i t h t h e unique a d s o r p t i o n s i t e i s
absent.
This
observation
strongly
suggests
that
the
unique
a d s o r p t i o n s i t e on t h e (7x7) s u r f a c e i s a r e s u l t o f long-range rearrangement
which
i s absent
on t h e
1aser-anneal ed surface.
Since b o t h a (1x1) u n r e c o n s t r u c t e d b u t r e l a x e d s u r f a c e as d e t e r mined i n t h e LEED a n a l y s i s and a m o s t l y d i s o r d e r e d s u r f a c e as determined by PES would n o t c o n t a i n such a w e l l d e f i n e d hole, t h e s e r e s u l t s cannot be used t o d i s c r i m i n a t e between t h e proposed structures. Rare gas t i t r a t i o n i s another technique used t o i n v e s t i g a t e geometric s t r u c t u r e .
The approach employed i s based on t h e concept
t h a t d i f f e r e n t geometric a d s o r p t i o n s i t e s f o r r a r e gas atoms can have d i f f e r e n t l o c a l work f u n c t i o n s .
Such l o c a l work f u n c t i o n
d i f f e r e n c e s produce d i f f e r e n t e l e c t r o n b i n d i n g energies re1a t i v e t o EF f o r these adsorbed atoms,
which a l l o w t h e d e l i n e a t i o n o f
v a r i o u s s i t e s as w e l l as t h e d e t e r m i n a t i o n o f t h e i r r e l a t i v e conc e n t r a t i o n s when examined w i t h PES. Si(lll)-(7x7) show
Recent i n v e s t i g a t i o n s o f t h e
s u r f a c e f o r xenon a d s o r p t i o n (Demuth e t a1
coverage-dependent
changes
i n t h e measured
., 1984)
PES b i n d i n g
e n e r y i e s a t b o t h h i g h and low coverages i n e i t h e r a d s o r p t i o n (as l o n g as near e q u i l i b r i u m a d s o r p t i o n c o n d i t i o n s a r e maintained) or
448
D. M. ZEHNER
d e s o r p t i o n experiments.
The sequence and number o f a d s o r p t i o n
s i t e s found f o r t h i s s u r f a c e are c o n s i s t e n t w i t h ( 1 ) a s p e c i a l h i g h b i n d i n g energy s i t e a t low coverages,
(2) a majority o f
nearly
equivalent
sites
over
surface
higher
coverages
where
rare-gas
most o f
the
adatom
(including
interactions
become
and ( 3 ) another t y p e o f m i n o r i t y s i t e p r i o r t o f o r -
important),
m a t i o n o f condensed o r m u l t i l a y e r s .
These r e s u l t s are c o n s i s t e n t
w i t h proposed s t r u c t u r a l models f o r t h e (7x7) s u r f a c e which have adatoms. (ruby
S i m i l a r measurements have been made on a laser-annealed S i ( 111)-(1x1)
laser)
surface.
The s i m i l a r i t i e s
i n the
r e s u l t s o b t a i n e d from t h i s s u r f a c e and those from t h e (7x7) surf a c e suggest t h e e x i s t e n c e o f adatoms.
This conclusion i s i n
c o n t r a s t t o t h e LEED r e s u l t s s u p p o r t i n g a f l a t ,
compressed s u r -
face.
A s t e p can be t r e a t e d as a d e f e c t and ordered a r r a y s o f such d e f e c t s produced by l a s e r a n n e a l i n g have been considered i n t h e discussion o f v i c i n a l
It i s w e l l
surfaces.
known t h a t l a s e r -
annealed s u r f a c e s have a r i p p l e d topography when examined on a macroscopic s c a l e (Leamy e t al.,
1978).
T h i s i m p l i e s t h a t steps,
randomly d i s t r i b u t e d , must e x i s t on such surfaces.
The p o s s i b i l -
i t y t h a t t h e S i ( 111)-(1x1) s u r f a c e s t r u c t u r e observed a f t e r l a s e r
annealing
can
be
associated
(Haneman,
1982; Moisum e t al.,
m i n i m i z e i t s f r e e energy.
with
steps
1983).
has
been
A surface reconstructs t o
The l o w e r i n g i n f r e e energy achieved
by r e c o n s t r u c t i o n can be e s t i m a t e d t h e o r e t i c a l l y , g r e a t accuracy, entropy.
considered
but not w i t h
due t o d i f f i c u l t i e s w i t h c o r r e l a t i o n e f f e c t s and
The presence o f s t r a i n w i l l t e n d t o oppose t h i s e f f e c t .
Based on
results
suggested (Haneman,
from a v a r i e t y
o f experiments
it
has been
1982) t h a t a s t r a i n e d r e g i o n a t t h e base o f
s t e p s on laser-annealed ( 111) surfaces causes s u r f a c e r e c o n s t r u c t i o n t o be i n h i b i t e d ,
r e s u l t i n g i n a (1x1)
surface structure.
Furthermore, i t i s suggested t h a t t h e behavior o f (100) surfaces, where t h e laser-annealed s t r u c t u r e i s t h e same as t h a t produced by thermal
annealing,
i s then
not
unexpected s i n c e t h e s t e p
7.
449
PULSED LASER IRRADIATED SEMICONDUCTORS
s t r u c t u r e s are o f d i f f e r e n t c r y s t a l l o g r a p h y and t h e r e i s no s i m i l a r evidence f o r step-associated s t r a i n .
V.
Surface and Subsurface S t u d i e s o f Ion-Implanted S i l i c o n
P r e v i o u s i n v e s t i y a t i o n s (White e t al.,
1980b) have shown t h a t
group I11 o r V i m p l a n t s occupy s u b s t i t u t i o n a l s i t e s subsequent t o l a s e r annealing and t h a t , as a consequence o f b o t h t h e h i g h l i q u i d phase d i f f u s i v i t i e s and t h e h i g h values o f d i s t r i b u t i o n c o e f f i c i e n t s , t h e y are a b l e t o d i f f u s e i n t o t h e c r y s t a l d u r i n g t h e regrowth I n c o n t r a s t , i t has been shown (White
process a f t e r i r r a d i a t i o n . e t al.,
1980c) t h a t those i m p l a n t s which do n o t form c o v a l e n t
bonds e x h i b i t , dependiny on t h e i m p l a n t dose, s e g r e g a t i o n t o t h e s u r f a c e as w e l l as t h e f o r m a t i o n o f a c e l l s t r u c t u r e subsequent t o l a s e r annealing.
The RBS and secondary i o n mass spectroscopy
( S I M S ) techniques employed i n these i n v e s t i g a t i o n s p r o v i d e d e t a i l e d
i n f o r m a t i o n about t h e d i s t r i b u t i o n w i t h respect t o depth b u t prov i d e no i n f o r m a t i o n about t h e c o n c e n t r a t i o n i n t h e s u r f a c e r e g i o n (<20 A )
and t h e changes which occur w i t h m u l t i p l e - p u l s e
diation.
Thus,
irra-
it i s o f i n t e r e s t t o u t i l i z e surface s e n s i t i v e
techniques i n o r d e r t o p r o v i d e complementary i n f o r m a t i o n . By u s i n g m u l t i p l e - p u l s e
irradiation,
the levels o f the prin-
c i p a l contaminants on s u r f a c e s o f i o n - i m p l a n t e d S i c r y s t a l s , 0 and C, can be reduced t o t h e p o i n t where t h e y a r e n o t d e t e c t a b l e i n Auger
s p e c t r a o b t a i n e d from t h e - s u r f a c e region.
However,
s i n c e t h e e x t e n t o f d i f f u s i o n o r s e g r e g a t i o n o f t h e implanted species i s known t o be a f u n c t i o n o f t h e number o f l a s e r pulses, a more a p p r o p r i a t e procedure i s t o s p u t t e r t h e samples i n i t i a l l y . T h i s r e s u l t s i n removal o f most o f t h e 0 and C s u r f a c e contaminants, and t h e l e v e l o f c l e a n l i n e s s a f t e r one p u l s e i s h i g h e r than
that
obtained without
sputteriny.
material affected during sputteriny,
-50 A,
Since t h e depth o f i s much s m a l l e r than
t h e depth o f t h e i m p l a n t e d r e g i o n , t y p i c a l l y -1000-2000
A,
the
s p u t t e r i n g process has l i t t l e o r no e f f e c t on t h e subsequent chanyes
i n t h e i m p l a n t r e d i s t r i b u t i o n t h a t occur d u r i n g l a s e r
450
D. M. ZEHNER
irradiation.
This
has
been
verified
by
comparing
results
o b t a i n e d f o l 1owing l a s e r anneal ing o f imp1 anted c r y s t a l s t h a t had been s p u t t e r e d w i t h t h o s e t h a t had not. sputtering
species
laser irradiation,
i s removed
Moreover,
from t h e s u r f a c e
since t h e
reyion during
as discussed i n S e c t i o n 111, i t cannot have
any e f f e c t on t h e r e d i s t r i b u t i o n o f t h e implanted species i n t h e s u r f a c e region.
For most i m p l a n t c o n d i t i o n s , as a consequence o f
t h e Gaussian-like
distribution,
the concentration o f
implanted
species i n t h e s u r f a c e r e y i o n i s n o t d e t e c t a b l e w i t h AES e i t h e r a f t e r insertion
10.
or a f t e r s p u t t e r i n g .
SUBSTITUTIONAL IMPLANTS R e s u l t s obtained on a Si(100) sample i m p l a n t e d w i t h 75As(100 keV,
8.3 x 1.0’6/cm2)
u s i n g t h e RBS technique are shown i n Fig. 16 and
i l l u s t r a t e t h e r e d i s t r i b u t i o n o f implanted species t h a t occurs w i t h multiple-pulse
irradiation.
Auger data were a l s o obtained
from t h i s sample f o l l o w i n g s i m i l a r i r r a d i a t i o n c o n d i t i o n s .
With
t h e s e d a t a t h e r a t i o o f t h e i n t e n s i t y o f t h e As(31 eY) Auger t r a n s i t i o n t o t h a t o f t h e S i ( 9 l eV) t r a n s i t i o n i s shown i n F i g . 17, where i t i s p l o t t e d as a f u n c t i o n o f t h e number o f l a s e r pulses. The data show t h a t t h e r e l a t i v e amount o f As i n t h e s u r f a c e r e g i o n decreases w i t h an i n c r e a s i n g number o f pulses, s i m i l a r t o t h e RBS r e s u l t s obtained f o r t h e subsurface r e y i o n .
Although n o t shown i n
t h i s f i g u r e , l i t t l e change i s observed i n t h e s u r f a c e c o n c e n t r a t i o n a f t e r a l a r g e number (>15) o f pulses where RBS r e s u l t s i n d i c a t e uniform concentration liquid-solid
interface.
from t h e subsurface
r e y i o n down t o t h e
I t i s d i f f i c u l t t o q u a n t i f y t h e AES
r e s u l t s t o t h e same degree as can be done w i t h t h e RBS data. Thus, a l t h o u g h i t can be concluded t h a t a r e d u c t i o n i n concentrat i o n occurs w i t h m u l t i p l e - p u l s e i r r a d i a t i o n , t h e r e c o u l d s t i l l be a p o s s i b l e chanye i n c o n c e n t r a t i o n i n going from t h e s u r f a c e t o subsurface r e g i o n t h a t
i s a consequense o f t h e surface-vacuum
451
7 . PULSED LASER IRRADIATED SEMICONDUCTORS
5
2
5
Fig.
16.
in S i ( 1 0 0 )
E f f e c t o f laser annealing on dopant profiles for As implanted as determined by RBS.
Profile
results are
implanted condition and subsequent to laser annealing at -2.0
shown
for
J/cm2.
as-
452
D. M. ZEHNER
gJ O
2
I
I
I
I
I
I
I
4 6 0 10 12 NUMBER OF PULSES (E0-2.1 J/cm2)
14
Fig. 17. Plot of the ratio of the As M W ( 3 1 e V ) to Si L W ( 9 1 eV) 4,5 283 Auger transition intensities as a function of the number of laser pulses.
interface. v a r i e t y of
Similar
Auger
results
have
been
obtained
for
a
i m p l a n t e d doses o f s u b s t i t u t i o n a l dopants i n S i ( l 0 U )
and (111) c r y s t a l s .
To examine t h e e f f e c t o f t h e i m p l a n t e d s p e c i e s on s u r f a c e o r d e r , LEED p a t t e r n s have been o b t a i n e d from t h e same 7%-implanted S i ( 1 0 0 ) sample.
Only a very weak,
was observed a f t e r
one l a s e r pulse.
p o o r l y d e f i n e d LEED p a t t e r n F o l l o w i n g two p u l s e s o f
i r r a d i a t i o n , t h e p a t t e r n shown a t t h e t o p o f F i g . 18 was obtained. I n t e g r a l o r d e r beams a r e observed, as w e l l as weak s t r e a k s between them.
With a d d i t i o n a l l a s e r pulses t h e s t r e a k s b e g i n t o coalesce
7 . PULSED LASER IRRADIATED SEMICONDUCTORS
Fig. 18.
LEED patterns from an As-implated Si( 100) surface a t a primary
beam energy of 49 eV.
-2.0
453
Patterns are shown subsequent to laser annealing at
J / m 2for ( a ) 2 , ( b ) 5 , and ( c ) 10 pulses.
454
D. M.ZEHNER
i n t o half-order reflections, surface structure.
i n d i c a t i n g t h e f o r m a t i o n o f a (2x1)
They c o n t i n u e t o become sharper and more
i n t e n s e w i t h a d d i t i o n a l pulses, as shown i n t h e f i g u r e .
However,
t h e p a t t e r n observed a f t e r t e n l a s e r pulses i s n o t as good as t h a t o b t a i n e d from a v i r g i n Si(100) c r y s t a l as shown i n Fig. 5 and t h u s i n d i c a t e s t h e presence o f d i s o r d e r i n t h e s u r f a c e region. Nevertheless,
i t i s i n t e r e s t i n g t o note t h a t t h e (2x1) LEE0 pat-
t e r n shows t h e e x i s t e n c e o f t h e r e c o n s t r u c t e d surface, s i m i l a r t o that
obtained
results
have
from a been
virgin
obtained
Si(100) for
a
crystal.
variety
Similar
of
LEED
substitutional
dopants i n Si(100) w i t h t h e q u a l i t y o f t h e LEED p a t t e r n o b t a i n e d f o r a s p e c i f i c l a s e r annealing c o n d i t i o n decreasing w i t h i n c r e a s i n g i m p l a n t dose.
I n c o n t r a s t t o these o b s e r v a t i o n s , (1x1) LEED p a t t e r n s were o b t a i n e d from S i ( l l 1 ) c r y s t a l s i m p l a n t e d w i t h a group I11 o r V dopant and then l a s e r annealed.
The p a t t e r n s are o f much h i g h e r
q u a l i t y a f t e r a g i v e n number o f l a s e r pulses when compared w i t h t h o s e obtained from t h e (100) surfaces, and t h e y show no evidence o f ordered l a t e r a l r e c o n s t r u c t i o n . The o b s e r v a t i o n t h a t l a s e r anneal i n y can be combined w i t h i o n i m p l a n t a t i o n t o p r o v i d e semiconductor s u r f a c e r e g i o n s c o n t a i n i n g n o v e l doping c o n c e n t r a t i o n s ( s u p e r s a t u r a t e d a l l o y s ) suggests t h a t t h e s e t e c h n i q e s may be used t o a l t e r o r t a i l o r t h e e l e c t r o n i c s t r u c t u r e i n t h i s region.
To examine t h i s p o s s i b i l i t y , photoemis-
s i o n techniques have been used t o i n v e s t i y a t e h i g h l y degenerate n-type S i ( l l 1 )
-
(1x1) surfaces as a f u n c t i o n o f As c o n c e n t r a t i o n
up t o - 5 x lO21/cm3 (-10 a t . %) and degenerate p-type S i ( l l 1 )
-
( 1 x 1 ) surfaces as a f u n c t i o n o f B c o n c e n t r a t i o n up t o -1 x 1021/cm3
( - 2 at.
% ) (Eastman e t a1
centrations electrically
are
about
., 1981).
10 and
These maximum doping con-
3 times
the concentrations o f
a c t i v e As and B a c h i e v a b l e by c o n v e n t i o n a l t e c h -
niques, r e s p e c t i v e l y . Angl e - i n t e g r a t e d photoemi s s i on s p e c t r a f o r t h e valence bands a r e presented i n F i g . 19 f o r i n t r i n s i c S i ( l l 1 )
-
( l x l ) , degenerate
7.
455
PULSED LASER IRRADIATED SEMICONDUCTORS
I
I
I
I
h u = 21 eV s / p POL. ANGLE-INTEG.
I
I
I
1 I
A.R.
A
7% AS 1.1 eV
INTRINSIC^
X- POCKEl
I
-I!
‘\ \“‘z”
(EF-EvIs =0.5
1 -8
-6
-2
-4
ENERGY (eV) Fig. valence
19.
Photoemission spectra ( p a r t i a l density o f states PDOS) for the
bands
of
highly doped Si. states.
laser-annealed
( 1 11 )-( 1 x 1 )
The levels near -0.4
ES, ES, and E denote the v c F
band minimum, and Fermi-level
surfaces o f
and -1 .3
valence-band
intrinsic
and
eV are due to surface maximum,
positions at the surface.
conduction-
456
D. M.ZEHNER
n-type As-doped ( 4 and 7 a t . %) S i ( l l 1 ) ( 1 a t . %) S i ( l l 1 )
p-type B-doped are
normalized
to
constant
-
-
( l x l ) , and degenerate
(1x1) surfaces.
total
emission
The s p e c t r a
within
5
eV
of
EF, and e n e r y i e s are g i v e n r e l a t i v e t o t h e valence-band maximum a t the
surface
(E:).
EF i s seen t o eV above E:
from 0.25
(i.e.,
shift
for the
markedly w i t h
doping
B-doped sample t o t h e con-
d u c t i o n band minimum Ec = 1.1 eV f o r t h e 7% As-doped sample). Relative t o i n t r i n s i c Si, doping,
f o r h i g h l y degenerate ( 1 a t .
%) B
t h e two s u r f a c e s t a t e s are u n a l t e r e d , and t h e p r i n c i p a l
changes a r e t h a t EF moves down by 0.25 eV and t h e s u r f a c e becomes metallic. at.
More dramatic e f f e c t s are seen w i t h As doping.
At 4
% As doping,
t h e s u r f a c e s t a t e s have become s i g n i f i c a n t l y
EF
has i n c r e a s e d by 0.1 eV r e l a t i v e t o t h e i n t r i n -
altered, while sic Si.
That i s ,
t h e upper "sp,-like"
d a n g l i n g bond s t a t e has
become much weaker and s h i f t e d upward i n energy by -0.3 l o w e r -1.4
eV; t h e
eV s t a t e has i n c r e a s e d s i g n i f i c a n t l y i n i n t e n s i t y , b u t
i t i s u n s h i f t e d i n energy;
w i t h new s t a t e s near
EF.
and t h e s u r f a c e has become m e t a l l i c
As t h e dopiny i s f u r t h e r i n c r e a s e d from
4 t o 7 a t . %, EF r a p i d l y s h i f t s and becomes pinned a t t h e conduct i o n band minimum Ec.
Also,
t h e upper sp,-like
surface s t a t e
c o n t i n u e s t o d i m i n i s h i n i n t e n s i t y so as t o be n e a r l y impercept i b l e by 7 a t .
% doping,
extremely intense. become occupied,
and t h e lower s u r f a c e s t a t e becomes
The conduction-band minima
( A min)
near X
and emission from these minima i s observed as
i n t e n s e e l l i p t i c a l lobes i n angle-resolved photoemission s p e c t r a ( d o t t e d l i n e l a b e l e d "AR"
i n Fig. 19).
By d e p o s i t i n g a t h i n Au
f i l m on t h i s s u r f a c e i t was p o s s i b l e t o show v i a S i 2p c o r e - l e v e l
measurements t h a t EF remained unchanged ( w i t h i n -50 meV). a "zero-barrier-height" e l e c t r i c a l purposes,
Thus,
Schottky b a r r i e r was formed , a l t h o u g h f o r
t h e Au-Si
i n t e r f a c e i s undoubtedly shorted
because o f t h e extreme degenerate n-type doping.
457
7. PULSED LASER IRRADIATED SEMICONDUCTORS 11.
INTERSTITIAL IMPLANTS I n o r d e r t o determine t h e e f f e c t s o f i n t e r s t i t i a l i m p l a n t s on
surface properties,
i n v e s t i g a t i o n s o f t h e segregation and zone
r e f i n i n g o f i m p u r i t i e s t o t h e s u r f a c e r e g i o n f o l l o w i n g pulsed 1 aser anneal ing have been performed. a c q u i r e d i n these s t u d i e s , implanted w i t h lOl5/cm2,
To i11u s t r a t e t h e r e s u l t s
data o b t a i n e d u s i n g S i ( l l 1 ) samples
Fe t o doses o f 1.13
x 1015 atoms/cm2,
x
6.0
and 1.8 x 10’6 atoms/cm2 and w i t h Cu t o a dose o f 6.9 x
101s atoms/cm2 and l a s e r annealed a t -2.0
J/cm2 (Zehner e t a1
.,
1984b) w i 11 be discussed.
As mentioned p r e v i o u s l y , examination w i t h AES showed t h a t a l l samples were covered w i t h insertion
i n t h e UHV
large quantities
system,
as
shown f o r
i m p l a n t e d w i t h Fe a t t h e t o p o f Fig. 20.
o f 0 and C a f t e r a Si(ll1)
sample
(Compare w i t h s i m i l a r
o b s e r v a t i o n s f o r v i r g i n S i c r y s t a l s as shown i n Fig.
1.)
The
s u r f a c e s were then s p u t t e r e d with 1000 eV Ar+ ions, which r e s u l t e d i n t h e removal o f most o f t h e 0 and C s u r f a c e contaminants as shown i n Fig. 20.
Auger s i g n a l s from t h e implanted species c o u l d
n o t be detected a f t e r t h i s t r e a t m e n t .
Following i r r a d i a t i o n w i t h
one l a s e r pulse, AES s p e c t r a showed t h e i m p l a n t e d species t o be p r e s e n t i n t h e s u r f a c e region.
T h i s i s i l l u s t r a t e d i n Fig. 20,
where Fe Auger s i g n a l s a t 46 and 703 eV are r e a d i l y detected. F o r t h e low dose case, l i t t l e i n c r e a s e i s observed i n t h e i n t e n s i t y o f t h e Fe Auger s i g n a l o b t a i n e d from a s u r f a c e i r r a d i a t e d w i t h a d d i t i o n a l pulses.
A1 though a t i n t e r m e d i a t e doses several
p u l s e s (two o r t h r e e ) are s u f f i c i e n t t o produce t h e s u r f a c e conc e n t r a t i o n t h a t r e s u l t s i n t h e maximum Fe Auger s i g n a l i n t e n s i t y , i n t h e h i g h dose case a t l e a s t f i v e pulses are r e q u i r e d t o produce t h e same r e s u l t . w i t h multiple-pulse 20.
An example o f t h e i n c r e a s e t h a t occurs
i r r a d i a t i o n i s shown a t t h e bottom o f Fig.
These observations are c o n s i s t e n t w i t h p r e v i o u s KBS r e s u l t s ,
showing a dependence o f t h e s e g r e g a t i o n t o t h e s u r f a c e t h a t i s a f u n c t i o n o f t h e i m p l a n t dose and number o f l a s e r pulses used f o r a n n e a l i n g (White e t a1
., 1 9 8 0 ~ ) .
458
D. M.ZEHNER
-
Jt-
-
AFTER Ar" SPUTTERING
AES 56Fe (150 keV. 6 X 1015/cm 2) IN (111) Si PRIMARY BEAM: 2 keV, 5 p A MODULATION: 2 Vp-p
w
e z U
r/l
-
-
z
+
1 PULSE
5 PULSES
+ +
Fe Si
Fig.
+
a +
Ar
1
I
I
0
100
200
20.
Auger
+
0
C
1
I
I
500 ELECTRON ENERGY (eV)
300
electron spectra
400
Fe I
600
I 700
from an uncleaned S i ( l l 1 ) surface
implanted with 56Fe ( 1 5 0 KeV, 6 ~ 1 0 ~ ~ / c m af~ t e )r , sputtering and a f t e r pulsed laser annealing at -2.0 J / c m 2 .
7.
459
PULSED LASER IRRADIATED SEMICONDUCTORS
The e f f e c t o f segregation on s u r f a c e o r d e r was determined by LEE0 observations. Fe-implanted
The LEED p a t t e r n s o b t a i n e d from each o f t h e
samples
subsequent
l a s e r pulses are shown i n Fig. 21.
t o the
irradiation with
five
For purposes o f comparison, a
LEED p a t t e r n o b t a i n e d from a v i r g i n S i ( l l 1 ) c r y s t a l f o r t h e same i n c i d e n t e l e c t r o n energy i s a l s o shown i n t h i s f i g u r e .
Although
( 1 x 1 ) LEE0 p a t t e r n s were obtained a f t e r one p u l s e o f i r r a d i a t i o n on each sample, a h i g h e r backyround i n t e n s i t y was always observed r e l a t i v e t o t h a t obtained from t h e v i r g i n c r y s t a l . increased
segregation,
at
intermediate
and
The e f f e c t o f
h i g h doses,
with
m u l t i p l e l a s e r pulses was t o degrade t h e q u a l i t y o f t h e LEED patterns.
I n yeneral
i n Fig.
21,
, the
background i n t e n s i t y increased , as shown
although t h e symmetry o f t h e p a t t e r n observed was
s t i l l (1x1). I n c o n t r a s t t o t h e r e s u l t s o b t a i n e d from Fe-implanted samples, t h e LEED p a t t e r n o b t a i n e d from t h e Cu-implanted sample a f t e r one l a s e r p u l s e was a (1x1) w i t h h e x a g o n a l - l i k e
r i n g s around each
i n t e g r a l o r d e r r e f l e c t i o n as shown i n Fig. 22.
T h i s i s t o be com-
pared w i t h t h e
(5x5)
pattern,
shown a t t h e t o p o f F i g .
22,
o b t a i n e d from a t h e r m a l l y annealed (111) s u r f a c e which contained Cu e i t h e r due t o s e g r e g a t i o n from t h e b u l k o r as a r e s u l t o f beam deposition.
The r i n g s around t h e i n t e g r a l
became more i n t e n s e and sharp w i t h a d d i t i o n a l shown a t t h e bottom o f Fig.
22,
order
reflections
l a s e r pulses,
although a well-defined
as
(5x5)
LEED p a t t e r n was never obtained. T h i s sugyests t h a t t h e domains c o n t a i n i n g Cu on t h e laser-annealed s u r f a c e a r e n e i t h e r as w e l l o r d e r e d nor as l a r g e as those on t h e t h e r m a l l y annealed surface. Subsequent
examination o f t h e ion-imp1 anted laser-anneal ed
c r y s t a l s w i t h RBS (2.5-meV f o l l o w i n g features:
He+ i o n b a c k s c a t t e r i n g )
( 1 ) For t h e Cu-implanted c r y s t a l
showed t h e
, one
pulse
o f l a s e r r a d i a t i o n caused t h e t r a n s p o r t o f a l l Cu t o t h e near-
s u r f a c e region, and ( 2 ) f o r t h e low dose Fe-implanted c r y s t a l , one p u l s e i s s u f f i c i e n t t o cause t h e complete t r a n s p o r t o f Fe t o t h e near s u r f a c e region. A t i n t e r m e d i a t e doses, s u b s t a n t i a l segregation
460
Fig. 21. (a) Si(ll1 ) S6Feat ( b ) ( Patterns are j lcrn2.
D. M. ZEHNER
LEED patterns, at primary beam energy of 110 eV, from a surface and from ( 1 1 1 ) surfaces of crystals implanted with 1 . 3 ~ 1 0 ~ ~ / c, m ( c ~) () 6 . 0 ~ 1 O ~ ~ / c and m ~ ()d,) ( 1 . 8 x 1 0 1 6 / c m 2 ) . shown subsequent to five pulses o f laser annealing at -2.0
7.
Fig. 22.
PULSED LASER IRRADIATED SEMICONDUCTORS
461
LEED patterns, a t a primary beam energy o f 71 eV, from ( a ) a
thermally annealed S i ( l l 1 ) surface a f t e r -1
-
monolayer deposition o f Cu and
from a ( 1 1 1 ) surface o f a crystal implanted with 6 . 9 ~ 1 0 ~ ~ / and c m laser ~ annealed with ( b ) 1 and ( c ) 5 pulses at
2.0 J / c m 2 .
462
D. M. ZEHNER
t o t h e surface occurs d u r i n g t h e f i r s t pulse,
b u t two pulses
a r e r e q u i r e d t o c o m p l e t e l y segreyate t h e Fe t o t h e near-surface region.
F i n a l l y a t h i g h doses,
even a f t e r f i v e l a s e r pulses,
s u b s t a n t i a l q u a n t i t i e s o f Fe remain i n t h e f i r s t 1000 A o f t h e c r y s t a l a t an averaye c o n c e n t r a t i o n o f -2 x 1021/cm3.
Furthermore,
c h a n n e l i n g s t u d i e s showed t h a t Fe i n t h e b u l k o f t h e c r y s t a l i s not i n s o l i d solution. From t r a n s m i s s i o n e l e c t r o n microscopy s t u d i e s that,
i t i s known
i n t h e case o f a high-dose Fe-implanted c r y s t a l ,
a well-
d e f i n e d c e l l s t r u c t u r e (see Chapters 1 and 4) i s observed i n t h e n e a r - s u r f a c e r e y i on subsequent t o l a s e r anneal ing (White e t a1 1980~).
The i n t e r i o r o f each c e l l i s an e p i t a x i a l
.,
column of
s i l i c o n e x t e n d i n g t o t h e s u r f a c e (average c e l l diameter -250 A ) . Surrounding each column o f s i l i c o n i s a c e l l w a l l and e x t e n d i n g t o a depth o f -1000 A,
,
650 A t h i c k
c o n t a i n i n g massive quan-
t i t i e s o f segreyated Fey p o s s i b l y i n t h e form o f Fe s i l i c i d e s . These r e s u l t s show t h a t subsequent t o l a s e r a n n e a l i n g t h e Fe (and Cu) i s n o t u n i f o r m l y d i s t r i b u t e d i n t h e plane o f t h e near-surface reyion but instead i s h i g h l y concentrated i n the w a l l s o f t h e c e l l structure. Fe-implanted
Thus,
t h e (1x1) LEE0 p a t t e r n s observed f o r t h e
samples a r i s e from t h e b u l k t e r m i n a t i o n o f (111)
planes i n t h e columns o f s i l i c o n a t t h e surface.
The absence o f
any o t h e r w e l l - d e f i n e d d i f f r a c t i o n f e a t u r e s from t h e Fe-implanted r e g i o n shows t h a t no long-range o r d e r e x i s t s i n t h e t e r m i n a t i o n o f t h e c e l l w a l l s a t t h e surface. rings
i n t h e Cu-implanted
The presence o f h e x a g o n a l - l i k e
crystals
order e x i s t s i n those c e l l walls, scale.
The
high
background
o b t a i n e d from t h e v i r g i n c r y s t a l
shows t h a t ,
i t i s on an w+xxmel_y small
intensities,
,
i f long-range
relative
to
that
observed f o r a l l i m p l a n t con-
d i t i o n s f o r Fe and Cu i n d i c a t e t h e presence o f d i s o r d e r ( p o s s i b l y s t r a i n i n t h e r e y i o n o f t h e c e l l w a l l boundaries) i n t h e o u t e r most l a y e r s , which increases w i t h i n c r e a s i n g i m p l a n t dose. F o r t h e s e samples , s p u t t e r i n g f o l 1owi ng 1aser
ir r a d i a t i o n
r e s u l t e d i n t h e removal o f some o f t h e i m p l a n t from t h e s u r f a c e
7. region.
463
PULSED LASER IRRADIATED SEMICONDUCTORS
However,
subsequent i r r a d i a t i o n w i t h t h e l a s e r again
r e s u l t e d i n t h e segregation o f l a r g e q u a n t i t i e s o f t h e i m p l a n t t o t h e s u r f a c e region.
Furthermore, f o r samples i n which i n t e r s t i -
t i a l species such as Cu a r e present i n t h e b u l k as a r e s u l t o f t h e growth process, l a s e r i r r a d i a t i o n can be used t o zone r e f i n e t h e s e species t o t h e s u r f a c e r e g i o n from a depth e q u i v a l e n t t o t h e maximum m e l t
penetration.
These i m p u r i t i e s can t h e n be
removed from t h e s u r f a c e w i t h l i g h t i o n S p u t t e r i n g ,
l e a v i n g an
i m p u r i t y - f r e e subsurface r e g i o n ( t o a depth determined by t h e melt
front
penetration),
l a s e r annealing.
which
remains
such a f t e r
subsequent
I n many d e v i c e a p p l i c a t i o n s i n v o l v i n g s i l i c o n ,
Cu and Fe i m p u r i t i e s a c t as very e f f i c i e n t recombination c e n t e r s and adversely a f f e c t m i n o r i t y - c a r r i e r l i f e t i m e .
The above obser-
v a t i o n s show t h a t l a s e r annealing combined w i t h s p u t t e r i n g can be used as a r a p i d p u r i f i c a t i o n t r e a t m e n t i n o r d e r t o produce an i m p u r i t y - f r e e s u r f a c e region.
VI.
Applications
I n v e s t i g a t i o n s discussed i n S e c t i o n I11 and I V concentrated on examining s p e c i f i c s u r f a c e p r o p e r t i e s a s s o c i a t e d w i t h l a s e r annealing changes
while in
S e c t i o n V was p r i n c i p a l l y
these
properties
that
i m p l a n t a t i o n w i t h l a s e r annealing.
occurred
concerned w i t h t h e by
combining
ion
I n t h i s section the u t i l i z a -
t i o n o f laser-annealed surfaces i s discussed.
The most p r o m i s i n g
a p p l i c a t i o n o f t h e l a s e r annealing t e c h n i q u e f o r producing atomic a l l y c l e a n surfaces i n d e v i c e processing appears t o be i n preparing
surfaces
application,
for
molecular
beam e p i t a x y
(MBE).
In this
t h e high-temperature t r a n s i e n t induced by t h e l a s e r
o f f e r s a very a t t r a c t i v e and e f f i c i e n t a l t e r n a t i v e t o t h e present prolonged preheat t r e a t m e n t a t t h e moderate temperature r e q u i r e d t o c l e a n t h e semiconductor s u r f a c e t o t h e h i g h standard e s s e n t i a l f o r good q u a l i t y e p i t a x y . a p r o d u c t i o n technique.
T h i s b r i n g s MBE a s t e p nearer t o being
464
D. K.ZEHNER
I n a r e c e n t i n v e s t i g a t i o n (de J o n j e t al.,
1983) LEED was
used t o study t h e i n i t i a l stages o f e p i t a x i a l growth o f s i l i c o n on s i l i c o n .
Both thermal a n n e a l i n g and l a s e r i r r a d i a t i o n were
used f o r s u r f a c e p r e p a r a t i o n , 1-10 nm.
and S i d e p o s i t i o n s were t y p i c a l l y
Using LEEO p a t t e r n s , t h e e p i t a x i a l growth temperature
was d e f i n e d as t h a t p a r t i c u l a r s u b s t r a t e temperature a t which an e p i t a x i a l overlayer, same q u a l i t y
grown on t h e c l e a n s u b s t r a t e ,
of diffraction
p a t t e r n as t h e s u b s t r a t e
R e s u l t s o b t a i n e d from 1aser-anneal ed vicinal
exhibits the
(loo),
itself.
(110) , ( 111) , and
(111) S i o r i e n t a t i o n s showed t h a t e p i t a x i a l growth can
t a k e p l a c e on surfaces prepared by t h i s procedure.
The growth
temperature f o r t h e (100) s u r f a c e was i d e n t i c a l t o t h a t o b t a i n e d u s i n g t h e r m a l l y prepared surfaces.
For t h e (111)
surface the
growth temperature determined f o r t h e thermal l y annealed s u r f a c e was h i g h e r t h a t t h a t determined f o r t h e l a s e r - a n n e a l e d s u r f a c e and a l s o f o r generally
the
laser-annealed
vicinal
surface.
accepted growth mechanism i n Si:MBE
growth by s t e p f l o w ,
the
r e s u l t s obtained f o r
Since t h e
above 870 K i s (111)
surfaces
sugyest t h e presence o f steps on t h e laser-annealed surface. Using an approach s i m i l a r t o t h a t j u s t described, t h e growth o f epitaxial
m u l t i l a y e r f i l m s o f v a r y i n g t h i c k n e s s on s i l i c o n
s u r f a c e s has been i n v e s t i y a t e d (de Jong e t a l .
Laser-
(loo), ( l l o ) ,
(111) and v i c i n a l (111)
A f t e r preparation,
s i l i c o n f i l m s were de-
annealed and t h u s c l e a n S i s u r f a c e s were used.
, 1982b,c).
p o s i t e d and subsequently l a s e r annealed a t i n c r e a s i n y energy dens i t i e s i n o r d e r t o determine t h e t h r e s h o l d f o r growth. determined by LEEU t o be -0.9 these
experiments.
After
T h i s was
J/cm2 f o r t h e ruby l a s e r used i n
determining t h e threshold,
silicon
l a y e r s were s e q u e n t i a l l y d e p o s i t e d and l a s e r annealed on a l l s u r faces.
I n t h i s way e p i t a x i a l
l a y e r s up t o 800 nm were yrown,
b u i l t up out o f 1 t o 20 sublayers.
The reappearance o f a LEEU
p a t t e r n a l l over t h e annealed area a f t e r each i r r a d i a t i o n i n d i cated e p i t a x i a l
regrowth o f a l a y e r .
o r i e n t e d samples,
I n particular,
on S i ( l l 1 )
annealed d e p o s i t e d l a y e r s e x h i b i t e d a (1x1)
7.
465
PULSED LASER IRRADIATED SEMICONDUCTORS
p a t t e r n which
i n t h e case o f t h e v i c i n a l
s u r f a c e had charac-
t e r i s t i c spot s p l i t t i n y i n t h e same c r y s t a l l o g r a p h i c d i r e c t i o n and t o t h e same amount as a nondeposited sample.
T h i s means t h a t
t h e steps i n t h e s u r f a c e are preserved by d e p o s i t i o n and pulsed Spectra obtained w i t h RBS show t h e e p i t a x i a l l y
l a s e r annealing.
grown r e g i o n s t o be o f good q u a l i t y . e x t r a r o u t e t o three-dimensional
T h i s method may p r o v i d e an
s i l i c o n structures.
By combininy t h e i o n i m p l a n t a t i o n , l a s e r annealing techniques discussed i n S e c t i o n V with m o l e c u l a r beam e p i t a x y , i t i s p o s s i b l e t o produce b u r i e d doped l a y e r s .
T h i s approach has been f o l l o w e d
i n a recent i n v e s t i g a t i o n ( S m i t e t a1 was f i r s t implanted w i t h As.
., 1982).
A Si(100) wafer
A f t e r subsequent i n s e r t i o n i n t o a
UHV system,
t h e sample was i r r a d i a t e d w i t h f i v e pulses from a
ruby l a s e r .
I n a d d i t i o n t o producing a clean, ordered surface, as
determined by LEED, As was r e d i s t r i b u t e d i n depth, as p r e v i o u s l y i l l u s t r a t e d i n Fig.
16.
The sample was t h e n heated ( t y p i c a l l y
K), and s i l i c o n was deposited a t a r a t e on t h e o r d e r o f 0.1
-800 nm/s.
A t y p i c a l l a y e r t h i c k n e s s was 100 nm.
The samples were
R e s u l t s showed (1) good e p i t a x y w i t h i n
t h e n examined w i t h RBS.
t h e d e p o s i t e d r e g i o n and ( 2 ) t h e e x i s t e n c e o f a b u r i e d As l a y e r w i t h an abrupt doped-undoped substrate-epitaxy with
specific
interface.
dopant
i n t e r f a c e (<15 nm),
located a t the
D i f f e r e n t l y doped s i l i c o n l a y e r s
concentration
profiles
and w e l l - d e f i n e d
i n t e r f a c e s are an i n t e g r a l p a r t o f most semiconductor devices. The above r e s u l t s , a b u r i e d l a y e r o f As-doped s i l i c o n produced by low-temperature
silicon
a n n e a l i n y and cleaning,
MBE
in
combination
with
pulsed-laser
show t h a t such devices can be produced.
With respect t o devices, t h e r e s u l t s j u s t discussed may prove t o be i m p o r t a n t f o r very h i g h frequency devices,
because MBE i s
one o f t h e few techniques t h a t o f f e r s t h e p o s s i b i l i t y o f making t h e sequences o f very s h a l l o w l a y e r s (-2000 A) w i t h w e l l - c o n t r o l l e d dopiny l e v e l s and abrupt changes i n dopant c o n c e n t r a t i o n s (-200 A) required
by
some
of
the oscillator
and a m p l i f i e r
structures
designed t o operate a t frequencies i n excess o f 200 GHz.
A major
466
D.M.ZEHNER
problem w i t h
Schottky
diodes
i s that
t h e metal-semiconductor
i n t e r f a c e forms t h e p o t e n t i a l b a r r i e r , and so device o p e r a t i o n i s v e r y s u s c e p t i b l e t o t r a c e contamination o f t h e semiconductor surface.
Laser c l e a n i n g o f such s u r f a c e s p r i o r t o metal d e p o s i t i o n
may w e l l minimize r e p r o d u c i b i l i t y problems i n Schottky devices.
VII.
Conclusions
It has been shown t h a t l a s e r annealing p r o v i d e s a new method f o r c l eani ny semi conductor surfaces.
There are f o u r advantayes
(1) no f o r e i g n atoms are i n t r o d u c e d i n t o t h e
o f t h i s technique:
s u r f a c e o r subsurface region;
( 2 ) t h e c l e a n i n y procedure does n o t
s p o i l t h e vacuum c o n d i t i o n s s i n c e t h e l a s e r i s l o c a t e d o u t s i d e t h e system and t h e beam i s i n t r o d u c e d t h r o u g h an o p t i c a l l y t r a n s p a r e n t window; irradiation,
(3) s i n c e o n l y t h e s u r f a c e r e g i o n i s heated d u r i n g b u l k i m p u r i t i e s cannot m i g r a t e t o t h e surface;
and
( 4 ) w i t h t h e t o t a l p r o c e s s i n g t i m e being on t h e o r d e r o f tl s, investigation
can
begin
immediately
after
cleaning,
thereby
a v o i d i n y t h e p o s s i b i l i t y o f r e c o n t a m i n a t i o n by background gases d u r i n g t h e c o o l i n g phase.
The a b i l i t y t o remove t h e n a t i v e o x i d e
l a y e r from a semiconductor s u r f a c e opens up t h e p o s s i b i l i t y f o r u s i n g t h i s technique t o w r i t e on a wafer. d u c i n g a t o m i c a l l y c l e a n surfaces,
I n a d d i t i o n t o pro-
i t has been shown t h a t l a s e r
a n n e a l i n g r e s u l t s in r e s t o r a t i o n o f o r d e r t o t h e s u r f a c e r e g i o n o f a damaged c r y s t a l . faces
containing
The m e t a s t a b l e s u r f a c e s t r u c t u r e s and sur-
ordered
arrays
of
steps
produced
by
laser
a n n e a l i n y can be used i n i n v e s t i g a t i o n s aimed a t understanding r e c o n s t r u c t i o n and growth w i t h i n t h e outermost monolayers.
In
t h e area o f b a s i c research concerned w i t h t h e physics and chemi s t r y of
surfaces,
these o b s e r v a t i o n s i n d i c a t e t h a t i t may be
p o s s i b l e t o modulate s u r f a c e coverage ( c l e a n , adsorb-desorb) i n a d s o r p t i o n experiments and a l t e r ordered s u r f a c e s t r u c t u r e s f o r k i n e t i c s studies.
467
7 . PULSED LASER IRRADIATED SEMICONDUCTORS When combined w i t h i o n i m p l a n t a t i o n , laser
annealing
(interatomic region.
can be used t o t a i l o r
i t has been shown t h a t
t h e geometric
lattice
spacings) and e l e c t r o n i c s t r u c t u r e i n t h e s u r f a c e
T h i s combination p r o v i d e s a way o f i n v e s t i g a t i n g a l l o y s
t h a t cannot be obtained u s i n g conventional c r y s t a l growth techniques.
For t e c h n o l o g i c a l a p p l i c a t i o n s , t h e p r o d u c t i o n o f a l l o y s
(submicrometer r e g i o n s ) w i t h t a i l o r e d s u r f a c e p r o p e r t i e s ( b u r i e d 1 ayers) should prove t o be very u s e f u l . From t h e r e s u l t s presented i t i s obvious t h a t l a s e r annealing o f semiconductor surfaces i n UHV has a tremendous p o t e n t i a l as a tool
for
both s u r f a c e science and p r a c t i c a l
v a r y i n g t h e wavelength,
energy d e n s i t y ,
application.
and p u l s e d u r a t i o n ,
By it
should be p o s s i b l e t o c h a r a c t e r i z e and understand t h i s processing t e c h n i q u e move completely.
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Moison, J . M., and Bensonssan, M. (1983). Surf. Sci. 126, 294. Narayan, J., Young, R. T., and White, C. W. (1978). J. Appl. Phys. 49, 3912. Nishizawa, J., Terasaki, T., and Shimbo, M. (1972). J . Cryst. Growth 13/14, 297. Ihm, J., and Cohen, M. L. (1981) Phys. Rev. Northrup, J . E., L e t t . 47, 1910. and Shklyaev, A. A. (1978). Surf. Sci 82, Olshanetsky, 6. Z., 445. Osakabe, N., Yagi, K., and Honjo, G. (1980). Japan. J. Appl. Phys. 19, L309. Yagui, K., and Honjo, G. (1981). Osakabe, N., T a u i s h i r o , Y., S u r f . Sci 102, 424. Pandey, K. C., and P h i l l i p s , J . C. (1974). Phys. Rev. L e t t . 32, 1433. Ready, 3. F. (1965). J. Appl. Phys. 36, 462. Roberts, R. W. (1963). B r i t . J. Appl. Phys. 14, 537. C u l l i s , A. G., and Webber, H. C. (1980). Appl. Rodway, D. C., Surf. Sci. 6 , 76. S c h l i i t e r , M., Chelikowsky, J . R., Lonie, S. G., and Cohen, M. L. (1975). Phys. Rev. L e t t . 34, 1385; Phys. Rev. B 12, 4200. S m i t , L., de Jony, T., Hoonhout, D., and S a r i s , F. W. (1982). Appl Phys. L e t t . 40, 64. S t r i t z k e r , B., Pospieszczyk, A., and Tagle, J. A. (1981) Phys. Rev. L e t t . 47, 356. van Loenen, E. J . , Iwami, M., and S a r i s , F. W. Tromp, R. M., (1982). Sol i d S t a t e Commun. 44, 971. Wang, J . C. , Wood, R. F., and Pronko, P. P. (1978). Appl Phys. L e t t . 33, 455. Wang, Z. L., Westendorp, J. F. M., and S a r i s , F. W. (1983). Nucl. Instrum. Methods 211, 193. Westendorp, J. F. M., Wang, Zhong-Lie, and S a r i s , F. W. (1982). Mat. Res. SOC. Symp. Proc. 4, 255. White, C. W., C h r i s t i e , W. H., Appleton, B. R., Wilson, S. K., Pronko, P. P., and Magee, C. W. (1978). Appl. Phys. L e t t . 33, 662. Narayan, J., Appleton, B. R., and Wilson, S. R. White, C. W., (1979). J . Appl. Phys. 50, 2967. White, C. W., Wilson, S. R., Appleton, B. R., and Young, F. W., Jr. (1980a). J. Appl Phys. 51, 738. White, C. W., Wilson, S. R., Appleton, 6. R., Young, F. W., Jr., and Narayan, J. (198Ub). I n Laser and E l e c t r o n Beam Processing o f M a t e r i a l s , (C. W. White and P. S. Peercy, eds.) p. 111, Academic Press, New York. Wilson, S. R., Appleton, 6. R., and Narayan, J . White, C. W., ( 1 9 8 0 ~ ) . I n Laser and E l e c t r o n Beam Processing of M a t e r i a l s , (C. W. White and P. S. Peercy, eds.), p. 124, Academic Press, New York. and G i l e s , G. E. (1981a). Phys. Wood, R. F., K i r k p a t r i c k , J . R., Rev. B 23, 5555. Wood, R. F., and G i l e s , G. E. (1981b). Phys. Rev. B 23, 2923.
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Zehner, D. M., White, C. W., and Ownby, G. W. (1980a). Appl. Phys. L e t t . 36, 56. Zehner, D. M., White, C. W., and Ownby, G. W. (198Ob). Surf. S c i L e t t . 92, L67. Zehner, D. M., White, C. W., and Ownby, ti. W. ( 1 9 8 0 ~ ) . Appl. Phys. L e t t . 37, 456. and Ownby, G. W. (1980d). I n Laser Zehner, D. M., White, C. W., and E l e c t r o n Beam Processiny o f M a t e r i a l s , (C. W. White and P. S. Peercy, eds.), p. 201, Academic Press, New York. Zehner, D. M., Noonan, J. R., Davis, H. L., and White, C. W. (1981a). J . Vac. Sci Techno1 18, 852. Zehner, D. M., Noonan, J. R., Davis, H. L., White, C. W., and Ownby, G. W. (1981b). Mat. Res. SOC. Symp. Proc. 1 , 111. Zehner, D. M., White, C. W., Heimann, P., R e i h l , B., Himpsel, F. J . , and Eastman, D. E. ( 1 9 8 1 ~ ) . Phys. Rev. B 24, 4875. Zehner, D. M., White, C. W., Appleton, B. R., and Ownby, G. W. (1982). Mat. Res. SOC. Symp. Proc. 4 , p. 683. Zehner, 0. M., White, C. W., Wood, R. F., P o l l a k , R. A., Himpsel, F. J., H o l l i n g e r , G., Marks, P. F., and R e i h l , B. (1984a) t o be pub1 ished. Zehner, D. M., White, C. W., and Ownby, G. W. (1984b). To be published.
.
.
.
CHAPTER 8 PULSED
0.
BEAM
P R O C E S S I N G OF
GALLIUM
ARSENIDE
H. Lowndes
- .
I. INTRODUCTION . I I. PULSED-LASER MELTING OF GALLIUM ARSENIDE 1. Time-Resolved Ref1 ecti vi ty Measurements. 2. Model Cal cul ati ons 3. Comparison of Measurements and Calculations. 4. Dopant Redistribution During Pulsed Laser Melting. I I I. ELECTRICAL ACTIVATION OF IMPLANTED IONS 5. Energy Density "Window" for Annealing High-Dose Implants. 6. Thermal Stability of Activated Carriers. IV. DEFECTS AND DAMAGE INDUCED BY HIGH-INTENSITY LASER PULSES. 7. Crystal1i ni ty Fol 1owing Pul sed-Laser Melting 8. Near-Surface Loss of Stoichiometry: Ga-Rich Residues. 9. Laser Damage and Beam Homogenizers. 10. Photo1 uminescence Studies 11. Electrically Active Defects 12. Attempts to Eliminate Compensating Defects V. ROLE OF THE PMBIENT ATMOSPHERE 13. Incorporation of Oxygen 14. Effect of Annealing in a High-pressure Ambient VI. CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH REFERENCES
................. . . .. .. .. .. .. .. .. .. .. .. ............ ........ ................... .... .............. .......
...............
. . . . . . . . . .. . .. .. .. .. .. . . . . . . . . . .. .. .. .. .. .. ....... ........ . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 471
Copyright 019x4 hy Academic Press. Inc. All rights o f reproducrion in any form reserved ISBN 0-12-752123-2
472
D. H. LOWNDES
I. I n t r o d u c t i o n The demonstration since 1976 o f a wealth o f new, n o n e q u i l i b r i u m r a p i d s o l i d i f i c a t i o n phenomena and new m a t e r i a l s p r o p e r t i e s r e s u l t i n g from pulsed beam processing o f s i l i c o n has n a t u r a l l y l e d t o t h e a p p l i c a t i o n o f pulsed processing techniques t o compound semiconductors,
as we1 1 as other non-el emental m a t e r i a l s.
The
review presented i n t h i s chapter focuses on pulsed processing o f GaAs w i t h l a s e r s and, t o a l e s s e r extent, e l e c t r o n beams.
Both
pulsed annealing o f i o n i m p l a n t a t i o n damage i n GaAs, as well as fundamental studies o f t h e e f f e c t o f r a p i d s o l i d i f i c a t i o n on the electrical
p r o p e r t i e s o f c r y s t a l l i n e GaAs,
are covered.
r e s t r i c t i o n t o GaAs, alone, i s made f o r several reasons.
The
As d i s -
cussed below, t h e case o f GaAs i s i l l u s t r a t i v e o f t h e new problems t h a t are encountered i n applying pulsed annealing t o compound semiconductors (and probably t o a l l compounds) r a t h e r than t o an elemental s o l i d such as s i l i c o n . most
Gallium arsenide i s a l s o the
important compound semiconductor f o r
sol i d s t a t e device
a p p l i c a t i o n s and has been t h e subject o f t h e overwhelming m a j o r i t y o f compound semiconductor pulsed annealing studies.
As a r e s u l t ,
f a r m r e d e t a i l e d i n f o r m a t i o n i s c u r r e n t l y a v a i l a b l e regarding GaAs than f o r a l l other compound semiconductors together,
and
o n l y f o r GaAs have t h e r e been d e t a i l e d studies o f t h e d e f e c t s r e s u l t i n g from t h e r a p i d s o l i d i f i c a t i o n process. Recent work suggests t h a t t h e l i m i t e d success o f pulsed annealing o f compound semiconductors
( i n comparison w i t h s i l i c o n )
r e s u l t s from the
i m p o s i t i o n o f a r a p i d s o l i d i f i c a t i o n process upon t h e more complex physical and chemical s t r u c t u r e s o f compound semiconductors.
The
nature o f d e f e c t s introduced i n t o GaAs by r a p i d s o l i d i f i c a t i o n has been i n d i r e c t l y i n f e r r e d i n several cases, and methods f o r m i n i m i z i n g o r e l i m i n a t i n g these defects are being explored.
These
r e c e n t developments make an in-depth review o f selected r e s u l t s f o r GaAs e s p e c i a l l y desirable.
8.
473
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
For GaAs, fundamental i n t e r e s t i n new s o l i d i f i c a t i o n phenomena has been supplemented by a v e r y s t r o n g p r a c t i c a l m o t i v a t i o n :
The
d e s i r e on t h e p a r t o f t h e semiconductor i n d u s t r y t o a v o i d some o f t h e problems and 1i m i t a t i o n s o f c o n v e n t i o n a l encapsul a t e d thermal (furnace) annealing o f ion-imp1 anted GaAs f o r d e v i c e a p p l i c a t i o n s . As i s t r u e f o r pulsed annealing, these l i m i t a t i o n s are a d i r e c t consequence o f t h e g r e a t e r p h y s i c a l and chemical c o m p l e x i t y o f a c r y s t a l 1 i n e compound compared with an elemental sol i d .
The fundamental
d i f f e r e n c e i n t h e annealing o f elemental and compound semiconduct o r s i s t h e requirement t h a t s t o i c h i o m e t r y must be m a i n t a i n e d i n t h e l a t t e r materials. s a t i s f i e d both l o c a l l y
The c o n s t r a i n t o f s t o i c h i o m e t r y must be (correct
coordination w i t h neighboring
atoms) and a t l o n g range (atoms o f d i f f e r e n t species on t h e c o r r e c t sublattice), i f defect-free,
device q u a l i t y material i s t o r e s u l t .
I n t h e case o f 111-V compound semiconductors t h i s c o n s t r a i n t i s e s p e c i a l l y d i f f i c u l t t o s a t i s f y , i f even moderately e l e v a t e d proc e s s i n g temperatures are i n v o l v e d , because o f t h e h i g h v o l a t i l i t y o f t h e column V c o n s t i t u e n t (e.g.,
As).
Anderson (1982) has noted t h a t ,
despite the stringency o f
a n n e a l i n g requirements, a t l e a s t a p r o t o t y p e commercial GaAs i o n i m p l a n t a t i o n technology, u s i n g encapsulated thermal annealing, was s u c c e s s f u l l y developed over a p e r i o d o f more t h a n a decade. anneal t i m e s are 5-30 min a t 600-1000°C.
Typical
However, t h i s technology
i s complex and has a number o f l i m i t a t i o n s :
The need f o r encap-
s u l a t i o n , t o avoid As l o s s and s u r f a c e decomposition; t h e need t o develop h i g h l y r e p r o d u c i b l e e n c a p s u l a t i o n methods and encapsul a n t c h a r a c t e r i s t i c s ( p a r t i c u l a r l y good adherence)
, t o avoid i n c o n s i s -
t e n t r e s u l t s ; incomplete removal o f imp1 a n t a t i o n damage and incomp l e t e e l e c t r i c a l a c t i v a t i o n o f h i g h dose implants,
resulting i n
o n l y moderate (-mid 1018/cm3 range) c a r r i e r c o n c e n t r a t i o n s , which a r e n o t s u f f i c i e n t f o r f a b r i c a t i o n o f non-a1 l o y e d ohmic c o n t a c t s ; and, i n t e r a c t i o n s between encapsul a n t s and deep-level i m p u r i t i e s i n GaAs.
474
D. H. LOWNDES
Pulsed beam annealing, u s i n g e i t h e r e l e c t r o n o r l a s e r beams, has been recognized f o r some t i m e as a p o s s i b l e a l t e r n a t i v e t o thermal a n n e a l i n g f o r compound semiconductors. Russian papers on t h e s u b j e c t (Kachurin e t a1
One of t h e e a r l i e s t
., 1976) demonstrated
p u l s e d l a s e r a n n e a l i n g o f i m p l a n t a t i o n damage i n GaAs.
One reason
f o r t h e a t t r a c t i v e n e s s o f p u l s e d annealing i s t h e t e n o r d e r s - o f magnitude r e d u c t i o n i n c h a r a c t e r i s t i c a n n e a l i n g t i m e s :
-lP7 sec
f o r t h e s u r f a c e m e l t d u r a t i o n r e s u l t i n g from p u l s e d i r r a d i a t i o n vs -lo3
sec f o r t h e solid-phase
processes i n v o l v e d i n f u r n a c e
annealing.
It was hoped t h a t t h e r a p i d regrowth process o f pulsed
annealing
(now i d e n t i f i e d as u l t r a r a p i d l i q u i d phase e p i t a x y )
would r e s u l t i n minimal As l o s s , t o produce n e a r l y s t o i c h i o m e t r i c GaAs.
However, i t i s now recognized t h a t As l o s s i s n o t n e g l i g i b l e
even under c o n d i t i o n s o f p u l s e d m e l t i n g and, much more s i g n i f i c a n t l y , t h a t d e v i a t i o n s f r o m s t o i c h i o m e t r y can r e s u l t as an i n h e r e n t consequence o f r a p i d s o l i d i f i c a t i o n .
Indeed, t h e two p r i n c i p a l
shortcomings o f p u l s e d annealing o f GaAs, t h e i n a b i l i t y t o a c t i v a t e low-dose i m p l a n t s and t h e low c a r r i e r m o b i l i t y i n c o n d u c t i n g l a y e r s formed by pulsed a n n e a l i n g o f high-dose i m p l a n t s , are now r e c o g n i z e d as symptomatic o f "quenched-in" d e f e c t s . There e x i s t o t h e r a n n e a l i n g methods w i t h c h a r a c t e r i s t i c annealing t i m e s t h a t f a 1 1 between pulsed anneal ing and encapsulated thermal annealing, though t h e r e have been f a r more a p p l i c a t i o n s o f t h e s e t o s i l i c o n t h a n t o GaAs.
Whereas cw beam h e a t i n g o f S i
r e s u l t s i n s o l id-phase e p i t a x i a1 regrowth on t h e m i 11isecond ti mes c a l e , cw annealing o f GaAs on t h e m i l l i s e c o n d t i m e s c a l e r e s u l t s i n f o r m a t i o n of s l i p planes caused by temperature g r a d i e n t s assoc i a t e d w i t h t i g h t l y focused annealing beams (Fan e t a1
., 1981).
The more complex anneal ing requirements o f GaAs a p p a r e n t l y r e q u i r e t i m e s o f o r d e r 1-5 sec f o r complete damage removal i n t h e s o l i d phase, as demonstrated by Shah and co-workers (1981) u s i n g a r a p i d l y scanned e l e c t r o n beam, by A r a i e t a l . and by Davies e t a l .
(1981) u s i n g a lamp a r r a y ,
(1983) u s i n g t h e c o n c e n t r a t e d o u t p u t o f f i l -
amentary q u a r t z halogen lamps.
Annealing on a somewhat l o n g e r
8.
475
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
t i m e s c a l e (-30
sec) u s i n g a g r a p h i t e s t r i p h e a t e r has a l s o been
r e p o r t e d by Sealy e t a l .
(1978) and by Chapman e t a l .
(1982).
A
survey and summary o f a n n e a l i n g r e s u l t s f o r i o n - i m p l a n t e d GaAs, u s i n g some o f these techniques, has been g i v e n by W i l l i a m s (1983b) and by W i l l i a m s and H a r r i s o n (1981).
R e s u l t s o b t a i n e d w i t h thermal
(furnace) annealing , p u l s e d anneal ing and cw beam annealing o f GaAs have a l s o been compared i n several recent reviews ( W i l l i a m s and
., 1981;
Harrison,
1981; Fan e t a1
1983a,b).
The r e v i e w by Fan and co-workers (1981) c o n t a i n s a d i s -
Anderson,
1982; and W i l l i a m s ,
c u s s i o n o f t h e c o r r e l a t i o n between e l e c t r i c a l p r o p e r t i e s and extended c r y s t a l l o g r a p h i c d e f e c t s ,
w h i l e t h e a r t i c l e by W i l l i a m s
(1983b) i n c l u d e s a b r i e f summary o f t h e few r e s u l t s o b t a i n e d when t r a n s i e n t a n n e a l i n g methods ( b o t h pulsed and cw) have been a p p l i e d t o compound semiconductors o t h e r t h a n GaAs. F i n a l l y , t h e c o n t i n u i n g s e r i e s o f M a t e r i a l s Research S o c i e t y Symposia on l a s e r and e l e c t r o n beam processing of m a t e r i a l s p r o v i d e s t h e most c o n c e n t r a t e d source o f i n f o r m a t i o n on t r a n s i e n t p r o c e s s i n g o f semiconductors ( F e r r i s e t a1
., 1979; White and Peercy,
Celler,
1980; Gibbons e t al.,
1982; Narayan e t a1
., 1983;
1981; Appleton and
and Fan and Johnson,
The o u t l i n e of t h i s c h a p t e r i s as f o l l o w s .
1984).
Section I1 i s
devoted t o a summary o f t i m e - r e s o l v e d measurements and model c a l c u l a t i o n s , which p r o v i d e t h e b a s i c evidence t h a t t h e mechanism f o r p u l s e d l a s e r a n n e a l i n g o f i o n - i m p l a n t e d GaAs i s m e l t i n g , l i q u i d phase d i f f u s i o n o f dopant ions, and u l t r a r a p i d l i q u i d - p h a s e e p i t a x i a l regrowth.
S e c t i o n I11 reviews r e s u l t s obtained i n e l e c t r i c a l l y a c t i v a t i n g i m p l a n t e d dopant ions i n GaAs v i a p u l s e d a n n e a l i n g and i l l u s t r a t e s these r e s u l t s vJith data o b t a i n e d under c o n d i t i o n s t h a t p e r m i t d i r e c t comparison w i t h t h e fundamental S e c t i o n 11..
studies i n
S t i m u l a t e d b o t h by t h e importance o f GaAs f o r e l e c -
t r o n i c devices, and by t h e growing s u s p i c i o n t h a t d e f e c t product i on i s an i n h e r e n t consequence o f r a p i d s o l id i f ic a t ion o f compound
semiconductors from t h e m e l t , r e c e n t work has i n c r e a s i n g l y centered on c a r e f u l s t u d i e s of t h e e l e c t r i c a l p r o p e r t i e s o f pulse-annealed
476
D. H. LOWNDES
GaAs and on o b t a i n i n g a more detailed fundamental understanding of the nonequilibrium physical and chemical processes involved i n r a p i d solidification. Section I V provides a detailed review of selected experiments t h a t provide information connecting defects in pulse annealed GaAs w i t h the rapid solidification process i t s e l f . Section V i s devoted t o the closely related topic of the role of the ambient i n introducing defects d u r i n g pulsed annealing: The behavior o f the native oxide layer and the effect of the type of ambi ent atmosphere and of atmospheric pressure. Sect i on V I concludes the chapter w i t h a brief summary of the limitations and successes of pulsed annealing of GaAs, as i t has normally been carried o u t , and w i t h suggestions f o r the mst promising new approaches. 11.
Pulsed Laser Melting of Gallium Arsenide
Experimental and theoretical studies of the pulsed laserannealing process, i n b o t h crystalline and ion-implanted GaAs, demonstrate that laser irradiation of GaAs by -15 nsec pulses results i n melting of the near-surface region, rapid penetration of a melt front t o a depth of typically several hundred nanometers be1 ow the surf ace , and sol i d i f icat i on at vel oci t i es of order 2-4 m/sec as the melt-solid interface returns t o the surface. Results of time-resolved reflectivity measurements (Lowndes and Wood, 1981; Auston et a1 1979) have been used t o directly monitor the d u r a t i o n of surface melting o f GaAs, and have been compared w i t h results of thermal melting model calculations (Wood et a1 1981a) of the depth and d u r a t i o n of melting as a function of laser energy P o s t - i r r a d i a t i o n SIMS measurements and model calcudensity, E 2' lations show t h a t d u r i n g the brief period of near-surface melting, implanted dopant ions are redistributed by l i q u i d phase diffusion modifed, however, by nonequilibrium segregation effects t h a t result from the high velocity of the recrystallization interface (see Chapters 2 and 4 ) . Aside from evaluating the agreement t h a t can be
.,
.,
8.
477
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
obtained between experiments and model c a l c u l a t i o n s d e s c r i b i n g l a s e r m e l t i n g of
GaAs,
these studies o f dopant r e d i s t r i b u t i o n
made i t p o s s i b l e t o determine, apparently f o r t h e f i r s t time, p r o p e r t i e s o f dopant ions i n molten GaAs, such as d i f f u s i o n coeff i c i e n t s and nonequilibrium i n t e r f a c e segregation c o e f f i c i e n t s d u r i n g r a p i d s o l i d i f i c a t i o n (Wood e t al.,
1.
1981a).
TIME-RESOLVED REFLECTIVITY MEASUREMENTS Time-resolved r e f l e c t i v i t y (R) measurements (Lowndes and Wood,
1981; Wood e t al.,
1981a) were c a r r i e d out using apparatus s i m i l a r
t o t h a t used f o r measurements on s i l i c o n (see Chapter 6).
Both
c r y s t a l 1ine and ion-imp1 anted GaAs samples were i r r a d i a t e d using s i n g l e pulses (FWHM d u r a t i o n -12 nsec) from a ruby l a s e r (694 nm wavelength) operated i n TEMoo mode.
R e f l e c t i v i t y was monitored
u s i n g a low-power cw HeNe probe l a s e r beam, t o g e t h e r w i t h a s i l i c o n photodiode and a storage o s c i 11oscope (risetime -2 nsec).
Figure 1
I I
I
R=O
RISING EDGE OF RUBY LASER PULSE *TIME
1. Model used for analysis o f time-resolved reflectivity o f GaAs for j / c m 2 . Rs = 34% (33%) for implanted (crystalline) samples (Lowndes and Wood, 1981 ) Fig.
El < 0.8
.
478
D. H. LOWNDES
i l l u s t r a t e s t h e m d e l t h a t was used f o r a n a l y s i s o f these measurements: R r i s e s from R, f o r a time
''f'
,T ,
0
and t h e n
i n t i m e ' c ~t o f a l l s toward
RFx,where i t i s m a i n t a i n e d RY with a l / e f a l l time of
Time-resolved r e f l e c t i v i t y measurements r e v e a l an i m p o r t a n t
q u a l i t a t i v e d i f f e r e n c e between t h e e f f e c t s o f pulsed l a s e r r a d i a t i o n on GaAs and S i : f o r Ra;x,
With EA a d j u s t e d t o g i v e comparable d u r a t i o n s
GaAs e x h i b i t s a much l o n g e r f a l l t i m e f o r R ( t h e r e f l e c -
t i v i t y " t a i l " i n Fig. 1 ) t h a n does S i .
This longer t a i l i s believed
t o be due i n p a r t t o As v a p o r i z a t i o n accompanied by f o r m a t i o n o f a Ga-rich l i q u i d t h a t has a much lower m e l t i n g p o i n t t h a n GaAs i t s e l f (Lowndes and Wood, 1981). s p e c t r a (Barnes e t a1
., 1978;
R u t h e r f o r d b a c k s c a t t e r i ng (RBS)
K u l a r e t a7
., 1979) and t r a n s m i s s i o n
e l e c t r o n microscopy (TEM) p l a n view micrographs ( F l e t c h e r e t a1
.,
1981a, b; see S e c t i o n I V ) p r o v i d e d i r e c t evidence o f such a Ga-rich r e s i d u e f o l l o w i n g pulsed l a s e r i r r a d i a t i o n .
Since t h e d u r a t i o n
o f t h e " t a i l " forms a m r e i m p o r t a n t p a r t o f t h e t o t a l m e l t durat i o n f o r GaAs t h a n i t does f o r S i , b u t i s probably n o t c h a r a c t e r i s t i c o f s t o i c h i o m e t r i c GaAs, Lowndes and Wood took as t h e i r measure o f the t o t a l m e l t duration t h e quantity i s only s l i g h t l y longer than F i g u r e 2 shows
'C
vs El
T
1/2
'C
= ( ' c ~+ z f ) ,
which
( F i g . 1).
f o r c r y s t a l l i n e GaAs and f o r GaAs
i m p l a n t e d w i t h 160 keV Se i o n s a t a dose o f 5x1015 cm2 (Lowndes and Wood, 1981); a l s o shown a r e smoothed r e s u l t s o f Auston e t a1
.
(1979) f o r GaAs i m p l a n t e d w i t h 50 keV Te i o n s a t a dose o f l o L 6 ions/cm*. 2.
MODEL CALCULATIONS The thermal m e l t i n g m d e l and numerical techniques used f o r
c a l c u l a t i o n s on GaAs (Wood e t al.,
1981a) were e s s e n t i a l l y t h e same
as t h o s e used i n e x t e n s i v e c a l c u l a t i o n s o f thermal and mass t r a n s p o r t d u r i n g pulsed l a s e r i r r a d i a t i o n of s i l i c o n (see Chapter 4). As discussed i n Chapter 4, t h e experimental c o n f i g u r a t i o n d u r i n g
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
479
300
250
ul
-
200
C
1
+
150
bE
-
100 50
0 0
Fig. 2. 1015/~,2; 0 ,
0.2
0.4
Surface melt duration vs E l .
--- , 50 keV T e ,
model calculations.
0.6
Ep ( J/cm2 1
A, c-GaAs;
0.8
1.o
0 , 1 6 0 k e V Se, 5 x
1 0 1 6 / c m 2 (smoothed; Auston e t a l . ,
1979);
The solid lines a r e smooth curves through the experi-
mental points (Lowndes and Wood,
1981).
p u l s e d l a s e r annealing i s such t h a t t h e heat conduction problem i s w e l l represented by t h e one-dimensional d i f f u s i o n equation, once t h e energy i s t r a n s f e r r e d from t h e c a r r i e r system t o t h e l a t t i c e . The v o l u m e t r i c h e a t - g e n e r a t i o n f u n c t i o n i n t h e c a l c u l a t i o n s i s determined by t h e i n t e r a c t i o n o f t h e l a s e r r a d i a t i o n w i t h t h e c a r r i e r system, and t h e subsequent t r a n s f e r o f t h e energy i n t h e l a s e r pulse t o the l a t t i c e . the carrier-lattice
Although t h e i n f o r m a t i o n now a v a i l a b l e on energy-transfer
mechanisms i n GaAs i s n o t
n e a r l y as complete as i t i s i n S i , we would expect e s s e n t i a l l y t h e same t y p e o f b e h a v i o r t o occur i n t h e two m a t e r i a l s , w i t h t h e f o l l o w i n g p o s s i b l e exceptions. Because o f t h e d i r e c t n a t u r e o f t h e e l e c t r o n i c e x c i t a t i o n s i n GaAs, we expect t h e temperature dependence o f t h e a b s o r p t i o n c o e f f i c i e n t t o be weaker i n GaAs t h a n i n S i .
480
D. H. LOWNDES
Because t h e band gap i n GaAs i s g r e a t e r t h a n i t i s i n S i , f r e e c a r r i e r a b s o r p t i o n due t o t h e r m a l l y e x c i t e d c a r r i e r s should be even l e s s i m p o r t a n t i n GaAs t h a n i n Si.
On t h e whole,
i t would seem
t h a t t h e response o f GaAs, l i k e t h a t o f S i , t o b o t h nanosecond and picosecond l a s e r pulses i s so r a p i d t h a t t h e t r a n s f e r o f energy from t h e c a r r i e r system t o t h e l a t t i c e i s e s s e n t i a l l y i n s t a n t a n eous on t h e nanosecond t i m e s c a l e o f t h e experiments discussed here. For thermal t r a n s p o r t c a l c u l a t i o n s i n GaAs, b o t h t h e c r y s t a l l i n e (c-) used.
and amorphous (a-) models (Wood and G i l e s , 1981) were
I n t h e c-model,
t h e a b s o r p t i o n c o e f f i c i e n t k f o r t h e ruby
l a s e r r a d i a t i o n was assigned an average c o n s t a n t value kc througho u t t h e sample.
I n t h e a-model,
a h e a v i l y damaged amorphous o r
n e a r l y amorphous l a y e r was assumed t o have been c r e a t e d by i o n implantation.
I n t h i s l a y e r , o f t h i c k n e s s Xa, k was g i v e n a v a l u e
k d d i f f e r e n t from i t s v a l u e kc i n t h e u n d e r l y i n g undamaged region. I n an i n d i r e c t band gap semiconductor such as S i , k d i s expected t o be much g r e a t e r t h a n kc, whereas i n a d i r e c t band gap m a t e r i a l l i k e GaAs t h e i r d i f f e r e n c e should be f a i r l y small s i n c e kc i s already q u i t e large. The i n p u t data f o r t h e thermal c a l c u l a t i o n s c o n s i s t s o f t h e thermal c o n d u c t i v i t y , K; s p e c i f i c heat, c; d e n s i t y , p; r e f l e c t i v i r e s p e c t i v e l y ; k, t i e s i n t h e s o l i d and m o l t e n s t a t e s , Rs and ,R, and/or kd; l a t e n t heats o f f u s i o n Lf and v a p o r i z a t i o n Lv; and t h e corresponding temperatures Tf and Tv a t which t h e phase changes occur.
G e n e r a l l y speaking, t h e d a t a f o r GaAs a r e n o t as w e l l known
as t h o s e f o r S i (see Chapters 3 and 4); t h e values used by Wood e t a l . (1981a) are shown i n Table I.
The c h o i c e o f kc = 6 x
l o 4 cm-I
r e f l e c t s an attempt t o e s t i m a t e and i n c l u d e t h e temperature dependdence o f kc (kc = 3 x ture).
lo4
cm-I f o r GaAs a t 694 nm a t room tempera-
The values o f Rs and Rm were chosen a t a t i m e when l i t t l e
was known about t h e o p t i c a l p r o p e r t i e s o f molten GaAs.
Changes
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
481
Table I Input data for GaAs melting model calculations (Wood et al., 1981a).
Quantity
Symbol
Value Used and Comments
Thermal conductivity
K
Temperature-dependent; from Amos and Wolfe (1978) ; extrapolated at high T.
Specific heat
C
As above
Density
P
5.3 g/cm3
Ref1 ecti v i ty sol id (694 nm) molten
RS
Absorption coefficient crystal 1 i ne
k kC
damaged
R Rm
kd
0.35 0.60 (see text)
6 x l o 4 cm-1 at 694 nm (see text) 1.2 x 105 cm-1 (see text)
Latent heats fusion vaporization
L Lf LV
Phase change temperatures me1 ti ng vaporization
Tm TV
1238°C 1800°C
Substrate temperature
Tsub
2O0C
Laser pulse duration energy density
;i
15 nsec (FWHM) varied
T
131 cal/g 533 cal/g
482
D. H. LOWNDES
in these quantities are not expected t o have large effects, except a t high pulse-energy densities, which must be avoided in GaAs in any case t o prevent surface damage, as discussed below. 3.
COMPARISON OF MEASUREMENTS AND CALCULATIONS
a.
Crystalline GaAs
Figure 3 shows results of melting model calculations for c-GaAs. The duration o f melting at any depth in the sample can be obtained from these curves; the surface melt duration i s plotted as a function of El in F i g . 2. The "slush zone" (indicated by the dotted lines in the 0.2 J/cm2 curve i n Fig. 3 ) occurs in the model calculations when the GaAs over an extended region has reached the melting temperature b u t has not yet absorbed enough latent heat t o complete the melting 0.8 0.7 0.6 -
I
1
GoAs TI = 15nsec
I
R,= 0.35 R, = 0.60
k,=kd=t)xfO
Z
I
I
I
4
crn
-4
-
0 0.5 L
-
v)
0
a 0.4
m0 c
0.3
I
!g 0.2 -1
0.1 0
Fig. 3 . Calculated rneIt-front profiles for GaAs. duration; tS = surface melt duration (Wood et a l . ,
Tp, = FWHM laser pulse 1981a).
8.
As Fig. 3 i n d i c a t e s ,
process.
483
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
t h r e s h o l d El
t h e model c a l c u l a t i o n s p r e d i c t a
= 0.2 J/cm2 f o r m e l t i n g t o a depth o f 20 nm (two o p t i -
c a l s k i n depths a t t h e 633 nm probe l a s e r wavelength) i n e x c e l l e n t agreement w i t h
t h e measured
El
= 0.22
r e s o l v e d R s i g n a l f i r s t reaches Ra!x
J/cm2 f o r which t h e time-
= 0.65,
t h e f l a t - t o p p e d max-
imum v a l u e o f r e f l e c t i v i t y t h a t i s a l s o o b t a i n e d f o r deeper l a y e r s o f molten GaAs produced a t h i g h e r El For
(Lowndes and Wood,
1981).
El > 0.8 J/cm2, Lowndes and Wood (1981) noted t h a t t h e R s i g n a t u r e f o r GaAs changes both qua1 i t a t i v e l y and
time-resolved
quantitatively.
The t r a i l i n g edge o f t h e R s i g n a l f i r s t becomes
rounded and then, a t Ex
>
0.9 J/cm2, t h e d u r a t i o n o f Rl
d r a m a t i c a l l y ; almost as soon t o a value
decreases
as t h e GaAs becomes molten, R drops
below R g and RF, which
was i n t e r p r e t e d as
signaling
t h e onset o f s u r f ace damage accompanied by s i g n i f ic a n t v a p o r i z a t ion.
-
[A s i m i l a r e f f e c t i s observed w i t h s i l i c o n f o r E l 3.2 J/cm2 (Auston e t a1 1979).] That t h i s sudden drop i n R does s i g n a l a
.,
damage t h r e s h o l d has been confirmed by post-anneal ing measurements : I n s p e c t i o n w i t h an o p t i c a l microscope r e v e a l s s u r f a c e d i s c o l o r a t i o n and damage; t h e H a l l m o b i l i t y o f c a r r i e r s i n i o n - i m p l a n t e d and laser-annealed l a y e r s i s found t o decrease f o r El
>
0.8 J/cm2 (see
s e c t i o n 111); and, a sudden onset o f oxygen uptake f r o m t h e ambient atmosphere occurs f o r El
-
1 J/crn2 (see s e c t i o n V).
These p o s t -
a n n e a l i n g o b s e r v a t i o n s are a l s o i n good agreement w i t h t h e f l a t t e n i n g o f t h e t o p s o f t h e c a l c u l a t e d m e l t - f r o n t p r o f i l e s f o r 0.8 and 1.0 J/cm2 Fig. 3), which s i g n a l s t h e onset o f v a p o r i z a t i o n i n t h e model c a l c u l a t i o n s . As shown i n Fig. 2, t h e r e i s good agreement between t h e c a l c u l a t e d d u r a t i o n o f s u r f a c e m e l t i n g o f c-GaAs and t h e measured durat i o n o f t h e h i g h - r e f l e c t i v i t y phase, f o r a l l E l between t h e m e l t i n g and damage t h r e s h o l d s . The p o s s i b i l i t y t h a t t h e very h i g h v e l o c i t y o f e p i t a x i a l r e g r o w t h , f o l 1owi ng pul sed 1a s e r me1t ing , p l ays a s i g n i f ic a n t r o l e i n c o n n e c t i o n with l a s e r - i n d u c e d
defects
(such as quenched-in
484
D. H. LOWNDES
vacancies o r a n t i - s i t e d e f e c t s ) i s discussed i n s e c t i o n s I11 and Here we simply note t h a t t h e v e l o c i t y w i t h which t h e r e c r y s -
IV.
t a l l i z i n g i n t e r f a c e approaches t h e s u r f a c e may be e s t i m a t e d from t h e s l o p e o f t h e m e l t - f r o n t p r o f i l e s i n Fig. 3, and i s -3.5 m/sec a t En = 0.4
(0.8)
(-1.8)
J/cm2.
b. Ion-Imp1 anted GaAs As Fig. 2 shows, near t h e
t h r e s h o l d (En
- 0.2
J/cm2) f o r a t -
t a i n i n g RYax t h e measured m e l t d u r a t i o n s f o r Se- and Te-implanted GaAs samples are i n good agreement w i t h each o t h e r and a l s o d i f f e r s u b s t a n t i a l l y from m e l t d u r a t i o n s f o r c-GaAs.
The m e l t i n g t h r e s h -
o l d f o r these i m p l a n t e d samples i s a l s o lowered by 4 . 0 4 below t h e t h r e s h o l d f o r c-GaAs, Wood,
1981).
A t t h e time,
i.e.,
J/cm*
by about 20% (Lowndes and
i t was suggested t h a t these e f f e c t s
m i g h t r e s u l t from t h e h i g h e r f r e e energy o f t h e amorphous phase (i.e.,
energy s t o r e d i n t h e i o n - i m p l a n t e d r e g i o n ) , o r f r o m e i t h e r
a lower l a t e n t heat o f f u s i o n , La, o r l o w e r m e l t i n g temperature, Ta, f o r a-GaAs t h a n f o r c-GaAs. t h e l a t t e r suggestions,
I n o r d e r t o q u a n t i t a t i v e l y check
m e l t i n g m d e l c a l c u l a t i o n s were c a r r i e d
o u t by Wood e t a l . (1981a) u s i n g t h e a-model w i t h an i m p l a n t a t i o n damaged l a y e r assumed t o be 220 nm deep.
W i t h i n t h e amorphous
l a y e r , t h e l a t e n t heat and m e l t i n g temperature were b o t h assumed t o be reduced from t h e i r c r y s t a l l i n e values. It was found t h a t t h i s d i d n o t g r e a t l y modify e i t h e r m e l t - f r o n t p r o f i l e s ( s i m i l a r t o Fig. 3) o r s u r f a c e m e l t d u r a t i o n s except near t h e l o w e s t ,El where a reduced l a t e n t heat o r m e l t i n g temperature f o r t h e amorphous l a y e r d i d play a r o l e i n prolonging surface m e l t duration.
A more
r e c e n t d e t a i l e d comparison o f t h e dynamical b e h a v i o r o f c - and as i l i c o n d u r i n g pulsed l a s e r m e l t i n g a l s o demonstrated t h a t La and Ta have o n l y a small e f f e c t on t h e n e a r - t h r e s h o l d m e l t i n g b e h a v i o r , which i s dominated i n s t e a d by t h e d r a s t i c a l l y reduced thermal cond u c t i v i t y o f t h e amorphous phase o f s i l i c o n (Lowndes e t a1
., 1984;
see Chap. 6); a s i m i l a r c o n c l u s i o n r e g a r d i n g t h e importance o f t h e l o w thermal c o n d u c t i v i t y o f t h e a-phase may a l s o h o l d f o r GaAs.
8. However,
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
485
i t appears t h a t S i may be an unusual case i n t h a t t h e
d i f f e r e n c e i n K f o r a- and c-Si i s e s p e c i a l l y l a r g e ; i n Ge, and probably i n GaAs, t h i s d i f f e r e n c e i s much smaller. F u r t h e r support f o r t h e v a l i d i t y o f u s i n g t h e c-GaAs m e l t - f r o n t p r o f i l e s o f Fig. 3 t o d e s c r i b e p u l s e d l a s e r m e l t i n g o f i o n - i m p l a n t e d GaAs, a t h i g h e r 1981a).
.
El, comes f r o m TEM measurements ( F l e t c h e r e t a1 ,
As shown i n F i g u r e 4, as-implanted samples d i s p l a y heavy
l a t t i c e damage t o a depth o f 180 r u b y l a s e r i r r a d i a t i o n (Fig.
Fig. 4.
rm; TEM measurements a f t e r pulsed
4) show t h a t a minimum El
TEM views of 1 6 0 keV, 5 x 1 0 1 5 / c m 2 Se-implanted
implanted; ( b ) E a z O . 2 5 J / c m 2 ; ( c ) El = 0 . 3 6 J / c m 2 ; p a t t e r n corresponding t o ( c )
(Lowndes e t a l . ,
of 0.4
G A S : ( a ) as-
( d ) electrondiffraction
1981b).
486
D. H. LOWNDES
J/cm2 i s needed t o anneal i m p l a n t a t i o n damage and r e s u l t s i n good regrowth,
epit a x i a
i n e x c e l l e n t agreement w i t h t h e m e l t depth
c a l c u l a t d f o r c-GaAs a t El
= 0.4 J / c d (Fig.
3).
These r e s u l t s
a r e f u r t h e r supported by t h e minimum E,
o f 0.4
needed t o e l e c t r i c a l l y
i m p l a n t s i n GaAs (see
activate similar
J/cm2 t h a t was
S e c t i o n 111). I n t h e i n t e r m e d i a t e El
range (-0.5
J/cm2),
T
f o r t h e Se- and
Te-implanted samples d i f f e r s by a f a c t o r o f 2 (Fig.
2).
A con-
t r i b u t i n g f a c t o r i n t h i s d i f f e r e n c e might be d i f f e r i n g degrees o f a m o r p h i z a t i o n produced by t h e i m p l a n t a t i o n c o n d i t i o n s , r e s u l t i n g i n d i f f e r e n t values f o r t h e thermal c o n d u c t i v i t y i n t h e amorphous r e g i o n (Lowndes e t al.,
However, i f o n l y t h i s e f f e c t were
1984).
p r e s e n t , t h e n t h e m e l t d u r a t i o n s should have become n e a r l y equal again a t higher
En.
Model c a l c u l a t i o n s u s i n g reasonable combina-
t i o n s o f t h e o t h e r thermal and o p t i c a l parameters o f a-GaAs were a l s o unable t o reproduce t h e wide range o f i m p l a n t e d samples i n Fig. 2.
T
shown f o r t h e i o n -
To c l a r i f y t h i s r e s u l t , Lowndes and
Wood (1981) conducted an a d d i t i o n a l s e t o f experiments: Using E,
0.4-0.5
J/cm2,
Se-implanted
=
samples were s u b j e c t e d t o repeated
(up t o 5) l a s e r i r r a d i a t i o n s , and
T
was measured each time. Although
t h e r e was some v a r i a t i o n i n T f r o m p u l s e t o pulse, t h e d u r a t i o n s f o r a g i v e n sample never decreased t o t h e s u b s t a n t i a l l y l o w e r v a l u e f o r c-GaAs, even though TEM showed t h a t these i n i t i a l l y amorphous samples e p i t a x i a l l y r e c r y s t a l l i z e d a f t e r a s i n g l e i r r a d i a t i o n a t
EL > 0.36 J/cm2.
The d i f f e r e n c e s i n T f o r a l l t h r e e s e t s o f sam-
ples, f o r t h e higher
El, were t h e r e f o r e i n t e r p r e t e d as due p r i m a r i l y
t o chemical e f f e c t s a r i s i n g from s u b s t a n t i a l d i f f e r e n c e s i n t h e doping o f t h e near-surface r e g i o n . ( S I M S ) measurements (Wood e t a1
Secondary i o n mass spectroscopy
., 1981a)
demonstrated t h a t Se does
segregate toward t h e sample s u r f a c e as a r e s u l t o f pulsed l a s e r annealing.
F o l l o w i n g a s i n g l e 0.5
J/cm2 i r r a d i a t i o n ,
a mean Se
c o n c e n t r a t i o n > 2 x 1020 cm3 was found i n t h e f i r s t 0.1 pn below t h e surface.
Compared w i t h t h e GaAs atomic d e n s i t y o f 4.4 x 1022
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
487
atoms/cm3, these f i g u r e s demonstrate h i g h l y degenerate doping o f at. %.The higher-dose,
-0.5
e t al.
shallower Te implant used by Auston
(1979) should have r e s u l t e d i n a s t i l l higher dopant con-
centration.
The corresponding d i f f e r e n c e s i n
T
t h a t were observed
support t h e idea t h a t melt duration, i n t h e intermediate Ex range, i s c o n t r o l l e d p r i m a r i l y by the chemical composition o f t h e nearsurface region. Dopant r e d i s t r i b u t i o n p r o f i l e s r e s u l t i n g from d i f f u s i o n o f Mg and Zn implants i n l i q u i d GaAs were a l s o c a l c u l a t e d as f u n c t i o n s of
El, using c a l c u l a t e d melt d u r a t i o n s t h a t d i f f e r e d o n l y s l i g h t l y
from those f o r c-GaAs,
and found t o be i n good agreement w i t h
experimental SIMS p r o f i l e s (Wood e t al.,
1981a; see s e c t i o n 11.4).
This r e s u l t a l s o supports t h e conclusion t h a t the longer melt d u r a t i o n s f o r Se- and Te-implanted GaAs are more c h a r a c t e r i s t i c o f departures from stoichiometry, and o f t h e formation o f lower m e l t i n g p o i n t l i q u i d s very near t h e surface, than o f s t o i c h i o m e t r i c GaAs.
I n s p e c t i o n o f t h e Ga-Se (Ga-Te) phase diagram does reveal
a number o f intermediate a l l o y s and compounds w i t h m e l t i n g p o i n t s around 1000°C ( S O O O C ) ,
i n a d d i t i o n t o pure Ga ( 3 O O C ) .
I n summary, time-resolved r e f l e c t i v i t y measurements f o r GaAs show t h a t surface m e l t d u r a t i o n s are dramatical l y d i f f e r e n t f o r c-GaAs and for a-GaAs produced by Se and Te implantation. c-GaAs,
For
t h e r e i s good agreement between measured and c a l c u l a t e d
values o f t h e m e l t i n g threshold, t h e t h r e s h o l d f o r damage due t o v a p o r i z a t i o n , and t h e d u r a t i o n s o f m e l t i n g a t i n t e r m e d i a t e energy densities. The experiments and c a l c u l a t i o n s by Lowndes and Wood suggest t h a t the longer me1 t d u r a t i o n s observed f o r ion-imp1 anted GaAs are t h e r e s u l t o f s u b s t a n t i a l d e v i a t i o n s from s t o i c h i o m e t r y and t h e formation of lower m e l t i n g - p o i n t m a t e r i a l a t t h e surface o f the sarnpl es.
488 4.
D. H. LOWNDES
DOPANT REDISTRIBUTION DURING PULSED LASER MELTING
The e f f e c t of pulsed melting and subsequent rapid r e s o l i d i f i c a t i o n upon dopant solid s o l u b i l i t y l i m i t s , and upon dopant redist r i b u t i o n and segregation, has not been as extensively investigated f o r GaAs as f o r Si. However, following some i n i t i a l uncertainty, i t has recently become c l e a r t h a t nonequil ibrium i n t e r f a c e phenomena s i m i l a r t o those occuring f o r Si a l s o occur in GaAs. Early RBS and channeling studies of 50 keV, 1016/cm2 Te-implanted GaAs revealed b e t t e r than 90% s u b s t i t u t i o n a l i t y (Te atoms residing on e i t h e r a Ga or an As s i t e ) following pulsed l a s e r annealing, corresponding t o a s u b s t i t u t i o n a l s o l i d s o l u b i l i t y of more than 1021/cm3, which exceeds the equilibrium s o l i d s o l u b i l i t y value by more than an order of magnitude (Barnes et a1 , 1979). However, t h e r e was d i s agreement i n e a r l y s t u d i e s regarding the extent of implanted dopant r e d i s t r i b u t i o n following pulsed annealing: I n s i g n i f i c a n t dopant r e d i s t r i b u t i o n was observed in some studies (Golovchenko and Venkatesan, 1978; Campisano e t al., 1978) while others found subs t a n t i a l r e d i s t r i b u t i o n (Barnes e t al., 1979; Sealy e t a l . , 1979). There was a l s o the observation t h a t some implanted impurities apparently diffused appreciably, whereas others did not, during 30 nsec pulsed ruby l a s e r annealing (Sealy et a l . , 1979). Williams (1983b) has pointed out t h a t pulsed-anneal ing studies in GaAs have often used l a s e r conditions just s u f f i c i e n t t o remove implantation damage in order t o avoid laser-induced damage and defects (see s e c t i o n s 111, I V , and V ) a t higher El. Less dopant r e d i s t r i b u t i o n i s t o be expected under such conditions. More importantly, redist r i b u t i o n i s probably then s e n s i t i v e l y dependent upon both the l a s e r parameters and upon the type of damage e x i s t i n g i n the implanted region (e.g., f u l l y amorphized vs defective c r y s t a l l i n e ) , which can influence the melting threshold and melt duration, as was shown above. Wood e t a l . (1981a) more recently reported the r e s u l t s of q u a n t i t a t i v e SIMS measurements and model calculations of dopant
.
8.
489
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
r e d i s t r i b u t i o n (see Chapter 4) f o l l o w i n g l a s e r annealing o f ionimplanted GaAs.
One r e s u l t o f t h e i r study was t o e s t a b l i s h t h a t
t h e measured d o p a n t - r e d i s t r i b u t i o n p r o f i l e s could be i n t e r p r e t e d as r e s u l t i n g from l i q u i d - p h a s e d i f f u s i o n o f implanted i o n s d u r i n g m e l t i n g , by using surface m e l t d u r a t i o n s t h a t were c o n s i s t e n t w i t h those o f Fig. 2.
An e q u a l l y important r e s u l t o f t h i s study was t o
determine, apparently f o r t h e f i r s t time, values f o r liquid-phase d i f f u s i o n c o e f f i c i e n t s , ,O,
and n o n e q u i l i b r i u m i n t e r f a c e segrega-
t i o n c o e f f i c i e n t s , k i , f o r Zn and Mg ions i n molten GaAs d u r i n g rapid solidification.
I n these studies, unencapsulated GaAs
Sam-
p l e s implanted w i t h 150 keV Zn o r w i t h 35 keV Mg ions t o a dose o f 5 x l O l 5 ions/cm2 were i r r a d i a t e d a t room temperature w i t h s i n g l e pulses (FWHM d u r a t i o n = 15-25 nsec) from a ruby l a s e r , w i t h t h e l a s e r beam s p a t i a l l y homogenized using a bent d i f f u s i n g l i g h t p i p e ( C u l l i s e t al.,
1979).
Dopant l o s s d u r i n g annealing was estimated
from t h e i n t e g r a t e d number o f secondary i o n counts.
Essentially
no losses were observed f o r t h e Mg-implanted samples, but t h e Znimplanted samples showed losses monotonically i n c r e a s i n g from -2% t o -20% f o r 0.5 Wood e t a1
G
Ex < 1.0 J/cm2.
. (1981b)
have described several methods f o r calcu-
l a t i n g dopant d i f f u s i o n d u r i n g pulsed l a s e r melting.
A method
(described i n Chapter 4) designed t o reduce t h e computer time r e q u i r e d by f i n i t e - d i f f e r e n c e c a l c u l a t i o n s , w h i l e s t i l l m a i n t a i n i n g acceptable accuracy, was used t o c a l c u l a t e 1iquid-phase dopantd i f f u s i o n p r o f i l e s ; values o f Da f o r dopant ions i n l i q u i d GaAs were not known, b u t were assumed t o be of t h e same magnitude as DA i n Si. The experimental and c a l c u l a t e d dopant r e d i s t r i b u t i o n p r o f i l e s are shown i n Figs. 5 (Mg) and 6 (Zn). An i n i t i a l attempt t o f i t t h e Mg SIMS data w i t h 01 = 5 x
lo-'+ cm2/sec
gave r e s u l t s
incompatible w i t h t h e c a l c u l a t e d and measured m e l t - d u r a t i o n times (Figs.
2 and 3).
E v e n t u a l l y i t was found t h a t DA = 2.5
x 10-4
cm2/sec gave good f i t s t o t h e experimental curves, as shown i n Fig.
5.
Since t h e Mg p r o f i l e s show c l e a r evidence o f surface
490
D. H.LOWNDES 1022 5
Mg-IMPLANTED GOAS AS-IMPLANTED EXP. E1(J/cm21 CAL. 0.51 ---
: -
loz'
5
2
2
2
1020
-
2
z
-----
0.62 0.81
R
-
1.03
z 0 II-
z
z
u 0 5
2 1019
0
0.05
0.10
0.15 0.20 DEPTH ( p m l
0.25
0.35
0.30
Fig. 5. Experimental (SIMS) and calculated Mg redistribution profiles in G a A s (Lowndes et al., 1981a). 1022
I
I
I
I
I
Zn-IMPLANTED GaAs AS- IMPLANTED E1(J/crn2) EXP.
0.84
I
=4
CAL.
-
I
g L
c K
w z
P 1020
0
40'9
0
0.05
0.40
0.45 0.20 DEPTH ( p n )
0.25
0.30
0.3!
Fig. 6. Experimental (SIMS)and calculated Z n redistribution profiles in G a A s (Lowndes et al., 1981a).
8. segregation,
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
491
i t was also necessary t o determine t h e i n t e r f a c e
segregation c o e f f i c i e n t , k i .
Wood e t a l .
(1981a) p o i n t out t h a t
t h e experimental data make a very d e f i n i t e statement about t h e amount o f dopant segregated t o t h e surface:
The necessity t o f i t
both t h e surface segregation "spike" and t h e p r o f i l e f a r below t h e surface severely c o n s t r a i n s k i ; a value k i = 0.6 the f i t t i n g .
r e s u l t e d from
TEM measurements on Mg-implanted GaAs samples revealed
a h i g h d e n s i t y o f very small d e f e c t s present a f t e r l a s e r annealing, b u t confined t o a narrow band a t t h e sample surface.
These defects
were presumed t o be associated w i t h t h e surface segregation of Mg, s i n c e they were not observed f o r o t h e r samples ( F l e t c h e r e t al., 1981a,b). The t h e o r e t i c a l f i t s t o t h e Zn data (Fig. 6 ) were somewhat l e s s s a t i s f a c t o r y than t o t h e Mg data but r e s u l t e d i n determination o f a Zn d i f f u s i o n c o e f f i c i e n t o f 3.0 x c o e f f i c i e n t k i = 1.0
(no segregation).
cm2/sec and a segregation These authors p o i n t out
t h a t although t h e i r computer programs allowed f o r dopant l o s s ( i n t h e case o f Zn), such losses complicated t h e f i t t i n g procedure because o f inherent d i f f i c u l t i e s w i t h SIMS measurements i n t h e c l o s e v i c i n i t y o f the surface:
The sharp drop i n t h e experimental
p r o f i l e s j u s t a t t h e "surface"
i s an u n r e a l i s t i c experimental
a r t i f a c t and i m p l i e s t h e need t o determine t h e p r e c i s e p o s i t i o n o f t h e surface i n t h e experimental data w i t h m r e accuracy.
Despite
t h i s d i f f i c u l t y w i t h t h e Zn data, t h e f i t between experiments and c a l c u l a t i o n s (Fig. 6) was s a t i s f a c t o r y . I t should be emphasized t h a t t h e k i values o f 0.6 (Mg) and 1.0 (Zn) found by Wood e t a1
. (1981a)
are not a p p r o p r i a t e f o r c r y s t a l -
l i z a t i o n under e q u i l i b r i u m conditions, but are c h a r a c t e r i s t i c o f nonequil i b r i u m segregation a t t h e growing i n t e r f a c e d u r i n g r a p i d solidification.
(See Chapters 2 and 4 f o r a discussion o f t h e o r e t -
i c a l models and t h e much more extensive experimental r e s u l t s now a v a i l a b l e f o r s i l i c o n . ) For comparison, W i l l a r d s o n and A l l r e d (1967) measured n e a r - e q u i l i b r i u m d i s t r i b u t i o n c o e f f i c i e n t s o f 0.1 f o r Mg
492
D. H.LOWNDES
and 0.40 f o r Zn, f o r c r y s t a l s pulled from t h e melt by t h e Czochralski technique. The s t r i k i n g deviation of t h e ki values derived from Figs. 5 and 6, from these equilibrium values, confirms t h e highly nonequilibrium nature o f t h e s o l i d i f i c a t i o n process in pulse annealed GaAs. Surface segregation of Se, b u t not of S i , following pulsed l a s e r melting was a l s o reported by Wood et a1 (1981a), using SIMS measurements. Harrison and Wil l i ams (1980) reported hi gh-resol uti on channeling spectra f o r a 60 keV, 2 x lO15/cm* In' implant in GaAs. Clear evidence was found f o r In segregation a t t h e GaAs surface following pulsed ruby l a s e r annealing a t 0.3 J/cm2 (which was suff i c i e n t t o remove implantation damage), and the In remaining in the bulk GaAs was found t o be s u b s t i t u t i o n a l . Finally, a substant i a l e f f e c t of oxygen in the ambient atmosphere, and of atmospheric pressure, on k i f o r Si dopant ions has been observed during pulsed l a s e r melting of c-GaAs (Sato e t a l . , 1982; see section V). However, more extensive and d e t a i l e d studies of high i n t e r f a c e veloci t y zone-refining e f f e c t s , such as have been documented f o r lows o l u b i l i t y impurities in s i l i c o n (see Chapter 2 ) , have not been c a r r i e d out f o r GaAs. As a r e s u l t , l i t t l e accurate information i s a v a i l a b l e f o r GaAs regarding s o l i d s o l u b i l i t y l i m i t s or the occurrence of c e l l ul a r s t r u c t u r e s (anal agous t o those seen i n s i 1 i con) , and thei r dependence upon recrystal 1i z a t i on f r o n t velocity (Chapters 2 and 5 ) .
.
111.
E l e c t r i c a l Activation o f Implanted Ions
E l e c t r i c a l a c t i v a t i o n o f ions implanted in GaAs requires annealing the implanted layer. Several reviews o f furnace and t r a n s i e n t annealing techniques f o r GaAs (Anderson, 1982; Williams, 1983a, b; Wi 11 i ams and Harri son , 1981 ) have appeared. Conventional thermal annealing of GaAs involves extended (>5 min) exposure t o elevated ( >6OO0C) temperatures , with r e s u l t a n t arsenic l o s s and degradation of the GaAs surface, unless specimens are encapsulated. Anderson
493
8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
(1982) has p o i n t e d out t h a t , d e s p i t e t h e inconvenience o f encapsul a t i o n , several o r g a n i z a t i o n s have now developed complex b u t successful
annealing
technologies
annealing o f i o n - i m p l a n t e d GaAs.
based on encapsulated
thermal
D o n n e l l y (1977) has s t r e s s e d t h e
importance o f d e v e l o p i n g r e p r o d u c i b l e e n c a p s u l a t i o n techniques t o avoid inconsistent results.
This d i f f i c u l t y , together with e f f e c t s
o f encapsulants on deep-level
i m p u r i t i e s and t h e i n a b i l i t y t o
achieve h i g h - c a r r i e r c o n c e n t r a t i o n s (Anderson, 1982), have p r o v i d e d t h e p r i n c i p a l a p p l i c a t i o n s - o r i e n t e d m o t i v a t i o n t o develop a l t e r n a t i v e annealing techniques. Thermal annealing and cw beam annealing ( u s i n g both l a s e r and electron
beams)
have
( d O l 3 / c m 2 ) implants.
been
successful
i n activating
low-dose
However, pulsed anneal i n g becomes competi -
t i v e i n e l e c t r i c a l a c t i v a t i o n f o r h i g h e r (>1014/cm2) doses, and i s c l e a r l y s u p e r i o r t o f u r n a c e a n n e a l i n g f o r t h e h i g h e s t (>1015/cm2) doses.
Experience t o d a t e i n o b t a i n i n g e l e c t r i c a l a c t i v a t i o n v i a
pulsed annealing o f i o n - i m p l a n t e d GaAs can be sumnarized i n a few sentences:
High doses o f b o t h n- and p-type i m p l a n t s can be p u l s e
annealed t o produce c a r r i e r c o n c e n t r a t i o n s i n t h e 10l9/crn3 ( n - ) t o low lO20/cm3 (p-) annealing.
However,
range, f a r h i g h e r t h a n r e s u l t from f u r n a c e t h e f r a c t i o n o f implanted i o n s t h a t a r e
e l e c t r i c a l l y a c t i v e i s s u b s t a n t i a l l y l e s s t h a n 1002, and i s espec i a l l y low f o r n-type i m p l a n t s , d e s p i t e e x c e l l e n t s u b s t i t u t i o n a l i t y o f dopant i o n s on l a t t i c e s i t e s .
These pulse-annealed l a y e r s a l s o
have lower m o b i l i t i e s , t y p i c a l l y by a f a c t o r o f 2-5, t h a n those expected f o r a given c a r r i e r c o n c e n t r a t i o n i n high-qua1 i t y GaAs (see, f o r exampl e, Sze and I r v i n , 1965). l a y e r s a r e a l s o t h e r m a l l y unstable,
Pul se-anneal ed , n-type
e x h i b i t i n g n e a r l y an o r d e r -
of-magnitude decrease i n c a r r i e r c o n c e n t r a t i o n f o l l o w i n g o n l y lowtemperature 1981).
(-300°C)
thermal t r e a t m e n t ( P i a n e t t a e t a1
.,
1980,
F i n a l l y , low-dose (<1014/cm2) i m p l a n t s e x h i b i t n e g l i g i b l e
a c t i v a t i o n f o l l o w i n g pulsed annealing.
494 5.
D. H. LOWNDES
ENERGY-DENSITY "WINDOW" FOR ANNEALING HIGH-DOSE IMPLANTS The e l e c t r i c a l - a c t i v a t i o n experiments t h a t are d e s c r i b e d i n t h e
remainder o f t h i s s e c t i o n a r e n o t i n t e n d e d as a comprehensive review; f o r a d d i t i o n a l examples t h e reader should r e f e r t o t h e a r t i c l e s c i t e d above.
Rather, t h e r e s u l t s c i t e d here a r e i n t e n d e d t o i l l u s -
t r a t e b o t h t h e l i m i t a t i o n s and successes o f p u l s e d a n n e a l i n g i n o b t a i n i n g e l e c t r i c a l a c t i v a t i o n o f n- and p-type i m p l a n t s under i m p l a n t a t i o n and a n n e a l i n g c o n d i t i o n s t h a t p e r m i t d i r e c t comparisons with r e s u l t s o f s t r u c t u r a l and d e f e c t s t u d i e s and o f t i m e r e s o l v e d experiments and m d e l c a l c u l a t i o n s d e s c r i b e d i n o t h e r p a r t s o f t h i s chapter. The e l e c t r i c a l - a c t i v a t i o n experiments d e s c r i b e d i n t h i s s e c t i o n (Lowndes e t a1 ture,
., 1981a,b)
were c a r r i e d o u t i n a i r , a t room tempera-
and w i t h o u t encapsulation.
(Some e f f e c t s o f pulsed l a s e r
anneal i n g t h r o u g h a s p u t t e r - d e p o s i t e d SiO, d i s c u s s e d i n S e c t i o n IV.)
encapsulant l a y e r a r e
These experiments d i f f e r from most o t h e r
work r e p o r t e d on pul sed anneal ing o f ion-imp1 anted GaAs t h r o u g h t h e use o f i m p l a n t a t i o n i n t o semiconducting ( r a t h e r t h a n s e m i - i n s u l a t i n g ) GaAs s u b s t r a t e s .
T h i s made i t p o s s i b l e t o use e l e c t r i c a l measure-
ments o f p-n j u n c t i o n p r o p e r t i e s
(I-V
and C-V
characteristics,
Chapter 3 ) t o h e l p i n assessing t h e consequences o f p u l s e d l a s e r m e l t i n g and r e c r y s t a l l i z a t i o n ;
t h e p-n j u n c t i o n a l s o p r o v i d e d t h e
necessary e l e c t r i c a l i s o l a t i o n f o r measurements of t h e sheet e l e c t r i c a l p r o p e r t i e s o f a h i g h l y doped, t h i n near-surface l a y e r l y i n g a t o p an o p p o s i t e l y doped s u b s t r a t e . The i o n s implanted ( a t room temperature) and t h e e l e c t r i c a l c h a r a c t e r i s t i c s o f the s i ngl e-crystal
GaAs s u b s t r a t e s used are
summarized i n Tab1 e I1 ; r a t h e r s h a l l ow imp1 a n t s , w i t h two p r o j e c t e d ranges (-600 A and -350 A ) were used.
Samples were i r r a d i a t e d w i t h
s i n g l e pulses f r o m a pulsed ruby l a s e r (FWHM d u r a t i o n = 20-25 ns) f o r energy d e n s i t i e s i n t h e range 0.3
< Ex <
1.0 J/cm2.
Both s i n g l e
mode and multimode l a s e r o p e r a t i o n were used, b u t t h e l a s e r l i g h t
TABLE I1
Ion implants i n s i n g l e c r y s t a l (100) semiconducting GaAs wafers (Lowndes et a l . , 1981a). Implanted Ions
Substrate ~~
~
Dopant
Type
C a r r i e r b) concentration (cm-3)
Zn
P
-1 x 1017
1&5x1Ol5
Si
n
4x1016
260
1&5x1015
Zn
P
-1x1017
336
172
1&5x1Ol5
Si
n
4x1016
35
351
212
1&5x1Ol5
Si
n
-1x1017
85
323
157
1&5x1Ol5
Zn
P
-1x 1017
a)
a) MP
Dopant Type
Energy (keV)
(A)
(a)
Si
n
80
677
370
Zn
P
150
595
284
Se
n
160
564
Zn
P
80
Mg
P
Se
n
Ion
a)Rp
=
~
Rp
Dose(s) ( i ons/cm2) 5x1015
projected range and MP = s t a n d a r d d e v i a t i o n i n the p r o j e c t e d range, along the d i r e c t i o n normal t o the GaAs s u r f a c e (Lindhard e t a l . , 1963).
-
b)p-type s u b s t r a t e s may contain a s much a s 6 x 1017 s i l i c o n / c m 3 , since they c o n t a i n Si from the c r u c i b l e walls, which i s over-compensated by Zn doping.
496
D. H. LOWNDES
was always passed t h r o u g h a bent, d i f f u s i n g fused s i l i c a l i g h t p i p e i n o r d e r t o remove s p a t i a l inhomogeneities (see s e c t i o n I V ) and t o o b t a i n as u n i f o r m annealing as p o s s i b l e . F i g u r e s 7 and 8 summarize t h e r e s u l t s o f H a l l e f f e c t and sheet r e s i s t i v i t y measurements c a r r i e d out ( u s i n g t h e van d e r Pauw t e c h n i q u e ) t o e v a l u a t e t h e p r o p e r t i e s o f t h e s h a l l o w p+ and n+ l a s e r annealed l a y e r s on o p p o s i t e l y doped s u b s t r a t e s (Lowndes e t a1 1981a, b)
.
As Fig.
.,
7 shows, t h e mean e l e c t r i c a l a c t i v a t i o n ( p e r c e n t o f
i m p l a n t e d i o n s t h a t c o n t r i b u t e a c a r r i e r ) f o r t h e p-type i m p l a n t s a t a dose of 5 x 1015/cm2 i s 80 a c t i v a t i o n l y i n g between 0.5
k
5%, w i t h t h e El window f o r h i g h
and 0.8
r e s i s t i v i t y ( o b t a i n e d f o r E l = 0.8
.
J/cm2.
The minimum sheet
J/cm2) f o r these samples was
Q/U For t h e 1 x lOI5/crn2 p-type i m p l a n t s , e l e c t r i c a l a c t i v a t i o n was much lower (<50%) and i r r e p r o d u c i b l e , t y p i c a l l y w i t h ps > 600 Q / O . The high-dose, n-type Se i m p l a n t s r e s u l t e d i n l o w e l e c t r i c a l a c t i v a t i o n (<20%, Fig. 8a); f u r t h e r m o r e , t h e sheet c a r r i e r c o n c e n t r a t i o n i n these samples was s u b j e c t t o d e g r a d a t i o n ps = 55
under low-temperature thermal t r e a t m e n t (7 min a t 400°C i n f o r m i n g gas),
as was r e p o r t e d i n more d e t a i l by P i a n e t t a e t a l .
1981; see s e c t i o n 111).
(1980,
However, 80 keV S i and 85 keV Se i m p l a n t s
a t 1 x lO15/cm2 produce a c t i v a t i o n s approaching 40%, with S i t h e b e t t e r o f t h e two (Fig. 8a).
F o r t h e 80 keV S i i m p l a n t , t h e b e s t
sheet r e s i s t i v i t y o b t a i n e d was ps = 50 Q / O a t E l = 0.5 J/cm2. F i g u r e s 7b and 8b i l l u s t r a t e a p a r t i c u l a r l y s t r i k i n g r e s u l t : The H a l l m o b i l i t y i n t h e ion-implanted, laser-annealed near-surface r e g i o n i s a l i n e a r l y i n c r e a s i n g f u n c t i o n o f El, f o r b o t h t h e pand n-type i m p l a n t s t h a t g i v e h i g h e s t e l e c t r i c a l a c t i v a t i o n , up t o El
=
0.8 J/cm2.
sharply,
However, f o r El
> 0.8
J / c d t h e m o b i l i t y decreases
i n c o n j u n c t i o n w i t h t h e onset o f e a s i l y v i s i b l e l a s e r
damage t o t h e samples' surfaces.
Time-resolved r e f l e c t i v i t y mea-
surements and t h e o r e t i c a l model c a l c u l a t i o n s ( s e c t i o n 11) b o t h a l s o p l a c e t h e onset o f damage due t o v a p o r i z a t i o n a t E l
>
0.8 J/cm2 f o r
497
8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
p-TYPE DOPANT IONS
-
I
P - T Y P E DOPANT IONS
10
-
-
35keV Mg, 5 x 10'5/cm2 0 l5OkeV Zn, 5 x tO'5/cm2 BOkeV Zn, 5 x 10'5/cm2 0
0.5
0
4.0
EI(J/cm2
0.5
0
1.0
El (J/cm2)
Fig. 7. ( a ) Percent electrical activation and ( b ) H a l l mobility, vs high-dose p-type implants (Lowndes e t a l . , 1 9 8 1 a ) .
80
I
-
z 0
I
1
1
n-TYPE DOPANT IONS
I
-
0960 keV Se, 5 ~ 9 0 ' ~ / c r n ~ A 8 5 keV Se,5 ~ l O ' ~ / c r n ~ 6o - -80 keVSi, 4 ~ ( O ' ~ / c r n ~ - ----085 keV Se, I ~ 4 O ' ~ / c r n ~
&I
c
a
Y e 140; 8 20
\..=.-4J
0 -
I
€ I s *
8
A I
1
500
T
Z(2)
-
I
l
for
l
DOPANT IONS 0
400 -
G 300 -
>
-E
$200-
too
kev s e . 5 . 1 0 ' % m ~ ~A 8 5 keVSe,5~(G'~/crn~
----0t60
keVS1.t atG'%m2 keVSe.4 xlO'%m'
-080
----a5
A I
I "-TYPE
Ex
0
I
I
I
I
I
Fig. 8. ( a ) Percent electrical activation and (b) H a l l mobility, vs high-dose n-type implants (Lowndes e t at., 1 9 8 1 a ) .
Ex
for
D. H. LOWNDES
c r y s t a l l i n e GaAs,
and a sudden onset o f oxygen uptake from t h e
atmosphere i s a l s o observed i n t h i s El range (see s e c t i o n V ) .
Thus,
t h e r e i s a w e l l - d e f i n e d E l window w i t h i n which e l e c t r i c a l a c t i v a t i o n o f s h a l l o w i o n i m p l a n t s i n GaAs can occur,
0.4
< El
6 0.8
J/cm2, w i t h t h e upper bound determined by t h e onset o f c a t a s t r o p h i c damage due t o v a p o r i z a t i o n and t h e lower bound governed by t h e n e c e s s i t y t o m e l t c o m p l e t e l y t h r o u g h t h e h e a v i l y damaged i o n i m p l a n t e d region, so t h a t e p i t a x i a l r e g r o w t h f r o m t h e s i n g l e - c r y s t a l s u b s t r a t e beneath can occur. between 0.4
The i n c r e a s e i n c a r r i e r m o b i l i t y
and 0.8 J / c d i s not a s s o c i a t e d w i t h f u r t h e r damage
removal, b u t occurs because t h e me1 t depth, and t h e r e f o r e t h e depth o f dopant d i f f u s i o n ,
i n c r e a s e s w i t h i n c r e a s i n g .El
Thus,
the
average dopant c o n c e n t r a t i o n i n t h e h e a v i l y doped near-surface l a y e r decreases w i t h i n c r e a s i n g increase o f c a r r i e r mobility.
El, r e s u l t i n g i n t h e observed
Lowndes e t a l . (1981a) have p o i n t e d
o u t t h a t t h e h o l e m o b i l i t i e s i l l u s t r a t e d i n Fig. t o be low i n r e l a t i o n t o
t h e h i g h (-1020/cm3)
r i e r c o n c e n t r a t i o n t h a t i s present,
7 do n o t appear
uncompensated car-
i f one e x t r a p o l a t e s Sze and
I r v i n ' s (1968) data t o s i m i l a r values o f c a r r i e r d e n s i t y . annealed n-type l a y e r s , by f a c t o r s o f 2-5,
however,
I n pulse-
t h e m o b i l i t y i s low ( t y p i c a l l y
i n t h i s and o t h e r work) i n r e l a t i o n t o t h e
e l e c t r o n d e n s i t i e s present. I t should a l s o be noted t h a t t h e h i g h e l e c t r i c a l a c t i v a t i o n o b t a i n e d f o r high-dose Zn imp1 ants, t o g e t h e r w i t h t h e Zn concent r a t i o n p r o f i l e s i n Fig. 6, i m p l y a Zn s u b s t i t u t i o n a l s o l i d solub i l i t y i n excess o f 1020/cm3 f o r pulsed annealing, about an o r d e r
o f magnitude g r e a t e r than i n c o n v e n t i o n a l near-equi 1 ib r i um c r y s t a l growth. I n summary, t h e s e r e s u l t s o f e l e c t r i c a l p r o p e r t i e s measurements
a r e i n e x c e l l e n t agreement with m e l t i n g model c a l c u l a t i o n s o f m e l t depth vs E l (Fig. 3 ) and w i t h TEM micrographs o f t h e l a s e r - a n n e a l e d n e a r - s u r f a c e r e g i o n (Fig. 4) f o r several values o f E.l c a t e a t h r e s h o l d El
Both i n d i -
= 0.4 J/crn2 t o m e l t e n t i r e l y t h r o u g h and e p i -
t a x i a l l y r e c r y s t a l l i z e t h e implantation-damaged region.
For h i g h e r
8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
499
El, c a r r i e r mobility i s a l i n e a r l y increasing function of El (Figs. 7b and 8 b ) , but drops abruptly with t h e onset o f vaporization, surface damage, and oxygen uptake (El > 0.8 J/cm2). 6.
THERMAL STABILITY OF ACTIVATED CARRIERS
Pianetta and co-workers (1980, 1981) have shown t h a t t h e high n-type c a r r i e r concentrations produced i n GaAs by Se or Te implantat i o n and pulsed annealing a r e only metastable and can be d r a s t i c a l l y reduced by thermal annealing. Figure 9 i l l u s t r a t e s t h e e f f e c t of subsequent isochronal thermal anneal ing (15 seconds a t a given temperature) on the c a r r i e r concentration of samples implanted Te+/cm2 and annealed w i t h a 0.8 J/cm2 ruby with 250 keV, 5 x l a s e r pulse. A two-stage loss of c a r r i e r s was found, the i n i t i a l I
5 x 1OY5
250 keV Te ++ GaAs
ISOCHRONAL ANNEAL AFTER LASER ANNEALING
4
1013;
I
200
I
400
1
600
I
800
TEMPERATURE ("C) Fig.
9.
Loss o f sheet c a r r i e r concentration caused b y isochronal heating
see t e x t ) following pulsed laser annealing (Pianetta et a l . ,
1980).
so0
D. H. LOWNDES
stage b e g i n n i n g a t about 200°C and t h e second stage a t about 600°C. The same behavior was observed f o r samples p u l s e annealed w i t h e i t h e r a ruby l a s e r o r an e l e c t r o n beam.
Differential electrical
measurements a1 so r e v e a l e d t h a t t h e major l o s s o f c a r r i e r s occurred w i t h i n 0.1 pm o f t h e surface, and t h a t c a r r i e r p r o f i l e s below 0.2 pm were i d e n t i c a l b e f o r e and a f t e r thermal annealing.
The s i g n i f -
icance o f t h i s i n s t a b i l i t y ,
reliability
i n c r e a t i n g long-term
problems f o r pul se-annealed devices and i n causing d i f f i c u l t i e s
i n c o n t a c t i n g t h e i r s u r f a c e i s apparent.
P i a n n e t t a e t a l . (1981)
have a l s o r e p o r t e d r e s u l t s o f d e t a i l e d channeling s t u d i e s c a r r i e d o u t on Te-imp1 anted, laser-annealed GaAs, i n which t h e y searched f o r s t r u c t u r a l changes accompanying t h e two-stage c a r r i e r concentration.
reduction i n
B a c k s c a t t e r i n g s p e c t r a and h i g h - r e s o l u t i o n
a n g u l a r scans o f Te-imp1 anted samples s u b j e c t e d t o thermal t r e a t m e n t a t 45OOC a f t e r l a s e r annealing r e v e a l e d no change i n t h e minimum y i e l d o f GaAs, i n t h e f r a c t i o n o f n o n s u b s t i t u t i o n a l Te, o r i n t h e c h a n n e l i n g h a l f a n g l e o f Te; i.e.,
t h e m a j o r i t y o f Te atoms remained
on s u b s t i t u t i o n a l l a t t i c e s i t e s f o l l o w i n g 450°C thermal annealing, even though t h e e l e c t r i c a l l y a c t i v e f r a c t i o n o f Te decreased from
20% t o 6%.
P i a n e t t a e t a l . (1981) have suggested t h a t t h i s b e h a v i o r
i s c o n s i s t e n t w i t h thermal m i g r a t i o n o f As o r Ga vacancies, which a r e known t o be m o b i l e a t l o w temperatures, and t h a t t h e i n i t i a l stage o f r e d u c t i o n i n t h e e l e c t r o n c o n c e n t r a t i o n may be due t o t h e f o r m a t i o n o f Te-vacancy complexes. These complexes c o u l d r e s u l t f r o m subsequent thermal annealing, i f a l a r g e number o f vacancies a r e f r o z e n i n i n i t i a l l y by r a p i d s o l i d i f i c a t i o n from t h e melt. Such a model a l s o seems t o f i t i n w i t h o t h e r m i c r o s t r u c t u r a l and
i n t h a t good s u b s t i t u t i o n a l i t y o f h o s t and dopant atoms on l a t t i c e s i t e s i s n o r m a l l y found i n r a p i d l y
d e f e c t data (see s e c t i o n I V ) ,
s o l i d i f i e d GaAs; thus, a h i g h d e n s i t y of vacancies c o u l d account f o r b o t h t h e low c a r r i e r c o n c e n t r a t i o n s t h a t a r e found i n t h e nears u r f a c e r e g i o n f o l l o w i n g pulsed annealing and f o r t h e general f a i 1 u r e o f pul sed anneal ing t o a c t i v a t e 1ow-dose imp1 ants. However,
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
501
i n t h e absence o f d i r e c t evidence f o r a l a r g e number o f mobile quenched-in vacancies, t h i s suggestion must s t i l l be regarded as conjecture.
IV.
Defects and Damage Induced by High-Intens t y Laser Pulses The nature and depth d i s t r i b u t i o n o f l a s e r - nduced damage and
d e f ect s have been studied r e c e n t l y i n both cr: j t a l l i n e and ionimplanted GaAs.
The techniques used i n c l u d e o p t i c a l and e l e c t r o n
microscopy, photo1 uminescence, b a c k s c a tt e r i n g spectra, DLTS, and I - V and C-V measurements on both Schottky b a r r i e r s and p-n junc-
tions. As an a i d t o t h i n k i n g about t h e problem o f defects and damage i n GaAs f o l l o w i n g pulsed annealing, i t i s useful t o separate the problem i n t o two p a rts :
F i r s t , t h e problem o f r e p a i r i n g l a t t i c e
damage t h a t i s caused by i o n i m p l a n t a t i o n and, second, t h e problem o f damage t h a t i s i n h e re n t t o the pulsed m e l t i n g and r a p i d s o l i d i f i c a t i o n process i t s e l f (found,
f o r example,
f o l l o w i n g pulsed
i r r a d i a t i o n o f even c-GaAs). The most obvious d i f f e r e n c e between t h e anneal i n g requi rements f o r elemental and compound semiconductors i s t h a t f o r an elemental semiconductor a l l t h a t i s re q u i re d t o r e s t o r e a c r y s t a l l i n e s t r u c t u r e i s t h a t bo th host and dopant atoms should occupy s u b s t i t u t i o n a l l a t t i c e sites.
For a compound semiconductor, t h e r e i s t h e
a d d i t i o n a l requirement o f stoichiometry, both l o c a l l y and a t l o n g range: Ga and As atoms should be coordinated w i t h neighbors o f opposite type, and should each be found o n l y on the c o r r e c t subl a t t i c e . Dopant atoms a l s o need t o occupy t h e c o r r e c t s u b l a t t i c e , i f compensation i s t o be avoided and f u l l e l e c t r i c a l a c t i v a t i o n
obtained.
Thus, even though channel i n g measurements do normally
demonstrate t he occurrence of h i gh-qua1 it y epi t a x i a1 regrowth f o l l o w i n g pulsed annealing o f GaAs, i n t h e sense t h a t both host and dopant atoms occupy o n l y s u b s t i t u t i o n a l l a t t i c e s i t e s , t h i s does not imply t h a t pulse-annealed GaAs i s d e f e c t free.
502
D. H.LOWNDES
For example, i t may be t h a t t h e u l t r a r a p i d s o l i d i f i c a t i o n from t h e m e l t d u r i n g pulsed annealing i s i n h e r e n t l y n o n s t o i c h i o m e t r i c , i n t h a t l a r g e concentrations o f a n t i s i t e ( A s G ~ or GaAs) defects, as w e l l as dopant atoms "trapped" on t h e wrong s u b l a t t i c e , may r e s u l t from t h e h i g h v e l o c i t y o f t h e r e c r y s t a l 1 iz i ng f r o n t d u r i n g e p i t a x i a1 regrowth. Deviations from s t o i c h i o m e t r y can a l s o occur i n a second way f o r implanted compound semiconductors:
I f o n l y one type o f i o n i s
implanted, an i n h e r e n t nonstoichiometry i s created. Co-implantation o f two species provides a s o l u t i o n i n p r i n c i p l e t o t h i s problem, b u t has not been e x t e n s i v e l y studied. However, t h e most obvious o r i g i n o f d e v i a t i o n s from s t o i c h i o m e t r y i n pulsed (and o t h e r ) annealing o f compound semiconductors i s t h e h i g h vapor pressure o f t h e column V c o n s t i t u e n t r e l a t i v e t o t h e column I 1 1 c o n s t i t u e n t .
I n i t i a l s t u d i e s o f pulsed annealing o f
implanted GaAs were m t i v a t e d i n p a r t by t h e hope t h a t t h e s h o r t m e l t d u r a t i o n (-100 nsec) would minimize loss o f v o l a t i l e As and would make i t p o s s i b l e t o anneal i m p l a n t a t i o n damage i n GaAs w i t h o u t encapsulation.
However , t h e equi 1 ibrium vapor pressure o f As a t
t h e m e l t i n g p o i n t o f GaAs i s -1 bar, which corresponds t o an As f l u x a t t h e l i q u i d surface o f about 1014/cm2, o r about one monolayer i n f i v e nanoseconds. D i r e c t evidence o f p r e f e r e n t i a l As loss, d u r i n g t h e time t h a t the GaAs surface i s molten, and o f l o s s o f s t o i c h i o m e t r y i n t h e near-surface region, i s provided by channeling and TEM observations o f a Ga-rich surface residue f o l l o w i n g pulsed annealing.
It has a l s o been suggested, by various authors, t h a t
a r s e n i c l o s s g r e a t l y enhances quenched-in vacancy formation, w i t h t h e vacancies then a c t i n g as compensating defects. There i s now extensive evidence from sheet and d i f f e r e n t i a l e l e c t r i c a l p r o p e r t i e s measurements t h a t pulsed l a s e r n l e l t i n g o f ionimp1anted o r c-GaAs , under c o n d i t i o n s t h a t have normally been used , r e s u l t s i n q u i t e h i g h d e n s i t i e s o f quenched-in, e l e c t r i c a l l y a c t i v e p o i n t defects o r d e f e c t complexes.
The f a i l u r e t o a c t i v a t e low-dose
8.
503
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
impl ants , t h e low mobi 1i t i es observed f o r hi gher-dose impl ants , and observations of both low conductivity and semi-insulating l a y e r s , just below the surface, following pulsed annealing, a l l t e s t i f y t o t h e presence of these defects. Since a l l of these problems become more severe with increasing El, i t i s apparent t h a t defect-free annealing of deep implants may be impossible, and t h a t even f o r shallow implants the Ex "window" f o r successful annealing may be narrow. 7.
CRYSTALLINITY FOLLOWING PULSED LASER MELTING
A number of authors have noted t h e excellent c r y s t a l l i n i t y t h a t i s obtained following pulsed l a s e r melting and rapid s o l i d i f i c a t i o n of ion-implantation amorphized GaAs (Barnes et a1 1978; Campisano
.,
.,
.,
e t a1 1978; Golovchenko and Venkatesan, 1978; Sealy e t a1 1979). 1980; Williams and Williams and co-workers (Williams et a1 Harrison, 1981) used channeling spectra t o compare the effectiveness of cw argon-i on 1a s e r anneal i ng , furnace anneal i ng , and pul sed 1a s e r annealing f o r removal of implantation l a t t i c e damage, and concluded t h a t pulsed l a s e r s are f a r superior t o regrowth in t h e s o l i d phase. In Figure 10, backscattering spectra are used t o i l l u s t r a t e the conversion of an i n i t i a l l y amorphous GaAs layer t o a nearly perfect c r y s t a l 1 ine s t r u c t u r e . Such high-qua1 i t y crystal 1 i ne regrowth i s t y p i c a l l y obtained when the E, i s high enough f o r t h e melt front t o penetrate e n t i r e l y through the implanted l a y e r , so t h a t highvelocity, liquid-phase e p i t a x i a l regrowth from the crystal1 ine s u b s t r a t e can occur. However, i t has been shown t h a t i f El i s less than the value needed t o melt through the implanted region, then t h e quality of c r y s t a l l i n e regrowth a l s o depends upon the type of damage in the implanted region. Penetration of t h e melt f r o n t only p a r t i a l l y through a f u l l y amorphized implanted layer r e s u l t s in polycrystall i ne regrowth (Campi sano et a1 , 1980). However , i f t h e impl anted 1ayer i s not e n t i r e l y amorphous , b u t a1 so contai ns a highly defective c r y s t a l l i n e region t o which the melt f r o n t
.,
.
504
D. H. LOWNDES
2
0 4.6
9.4
1.8
2.0
2.2
ENERGY ( M e V )
Fig. 10. Backscattering spectra of 2.5-MeV He' ions incident in a random direction o f GaAs samples implanted with 400-KeVTe 1015 direction in the cm-2, after pulsed ruby laser irradiation o f energy ( a ) 0.2 to 0.8 J / c m 2 , ( b ) 0.9 j/crn2, ( c ) 1 .O-1.4 )/crn2. Curve ( d ) i s obtained from the unimplanted GaAs sample (Compisano e t at. , 1980).
penetrates , then poor-qua1 i t y c r y s t a l 1 ine regrowth wi 11 occur , with d e f e c t s propagated back t o t h e surface. This defective crystal1 ine type of regrowth i s i l l u s t r a t e d in F i g . 4 (Fletcher et al., 1981a, b ) ; i n t h i s case the implanted samples (Table 11, section 111) were general l y polycrystall i ne r a t h e r than amorphous , as a r e s u l t of beam heating during the high-dose b u t shallow room-temperature implantation. A polycrystall ine region extends from the surface t o a 40 nm depth (Fig. 4 a ) ; the region from 40-100 nm (near the peak of the implanted dopant p r o f i l e ) was r e l a t i v e l y defect f r e e , containing only small loops and defect c l u s t e r s ; but, the region
8.
505
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
from 100-180 nm c o n t a i n e d a h i g h d e n s i t y o f loops, d i s l o c a t i o n t a n g l e s and small t w i n s (-30 nm across). (Fig.
I r r a d i a t i o n a t 0.25 J / c d
4b) r e s u l t e d i n a m e l t - f r o n t p e n e t r a t i o n o f s l i g h t l y more
t h a n 100 nm, j u s t i n t o t h e deeper damage l a y e r ; e p i t a x i a l regrowth r e s u l t e d i n a h i g h d e n s i t y o f d i s l o c a t i o n s and f a u l t e d r e g i o n s i n t h e regrown l a y e r ,
b u t p o l y c r y s t a l l ine r i n g s were absent from
s e l e c t e d area d i f f r a c t i o n p a t t e r n s and t w i n spots were more e a s i l y distinguished.
For E,
= 0.36
J/cm2,
(Figs.
4c and d ) , t h e m e l t
f r o n t p e n e t r a t e d t o a depth o f 175 nm, almost t o t h e narrow band o f small loops t h a t mark t h e end o f i m p l a n t a t i o n damage.
Epitaxial
r e g r o w t h r e s u l t e d o n l y i n a low d e n s i t y o f d i s l o c a t i o n p a i r s propa g a t i n g i n t o t h e laser-regrown
r e g i o n , o r i g i n a t i n g from r e g i o n s
where t h e m e l t f r o n t o n l y p a r t i a l l y p e n e t r a t e d t h i s band o f small loops.
But f o r E,
>
0.36 J/cm2, t h e m e l t f r o n t p e n e t r a t e d beyond
t h e implantation-damaged r e g i o n , and t h i s i n i t i a l l y h i g h l y defect i v e c r y s t a l l i n e r e g i o n was observed t o e p i t a x i a l l y regrow w i t h no evidence o f imp1 a n t a t i on damage remai n i ng w i t h i n t h e
- 10-8, reso-
l u t i o n o f these TEM s t u d i e s . 8.
NEAR-SURFACE LOSS OF STOICHIOMETRY:
Ga-RICH RESIDUES
Although c h a n n e l i n g s p e c t r a and TEM micrographs c o n f i r m a h i g h l y substitutional
s t r u c t u r e f o l l o w i n g pulsed annealing,
these same
techniques a t h i g h e r r e s o l u t i o n r e v e a l a s i g n i f i c a n t d e v i a t i o n f r o m s t o i c h i o m e t r y i n t h e near-surface r e g i o n , i n t h e f o r m o f As l o s s and a Ga-rich s u r f a c e residue.
F i g u r e 11 (Barnes e t al.,
1978)
shows random and a1 igned channel ing s p e c t r a f o r Te-imp1 anted GaAs b e f o r e and a f t e r p u l s e d l a s e r annealing.
Over 90% o f t h e i m p l a n t e d
Te r e s i d e s on s u b s t i t u t i o n a l s i t e s , b u t t h e w e l l - r e s o l v e d Ga and As s u r f a c e peaks i n d i c a t e an excess o f s u r f a c e Ga. E t c h i n g w i t h warm HC1 removes most o f t h e excess Ga, r e s u l t i n g i n t h e r e t u r n t o a 1:l Ga:As r a t i o i n subsequent channeling measurements.
506
D. H. LOWNDES
I
+ Nd: YAG + Nd: YAG
-IMPLANTED A-IMPLANTED
HCI ETCH
ANNEAL ANNEAL
+
\
n
AS - IMPLANTED
I 01
-I
300
VIRGIN
--
1
I
I
1
1
350 CHANNEL NUMBER
Fig.
11.
Random and < l o o > channeling spectra for 50 keV, 1OI6 Te/crn2
implanted GaAs before and a f t e r Nd:YAG laser annealing (1.06 p ~1,2 5 nsec FWHM 20 M W / c r n 2 ) , and before and a f t e r removal o f surface Ga residue (Barnes
,
et al.,
1978).
Optical and transmission-electron micrographs have also been used t o study the formation and growth of these Ga-rich surface deposits w i t h increasing pulsed l a s e r El (Lowndes e t a l . , 1981; Fletcher et a1 1981a). These studies c l e a r l y demonstrate t h a t even though El 2 0.4 J/cm* i s s u f f i c i e n t t o remove implantation l a t t i c e damage and obtain high e l e c t r i c a l a c t i v a t i o n of p-type implants, degradation of t h e near-surface region occurs f o r a1 1
.,
l a s e r energy d e n s i t i e s above the melting threshold (Ex -0.2 J/cm*).
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
507
Figure 1 2 shows plan view TEM micrographs of the surface of Znimplanted GaAs a f t e r pulsed ruby l a s e r i r r a d i a t i o n . The darker regions, present only a f t e r l a s e r annealing, were determined by x-ray analysis t o be gallium rich. These regions are small b u t numerous a t low El; t h e i r s i z e increases with increasing El while t h e i r number density decreases. For El = 0.81 J/cm2, these Ga-rich regions a r e 1inked together t o form a network or "cell -type" struct u r e with a c e l l s i z e o f 1-2 pm. Samples annealed a t 0.81 J/cm2 or higher energy d e n s i t i e s a l s o displayed a much f i n e r network s t r u c t u r e of Ga-rich regions (on a scale of t h e order of 100 nm c e l l s i z e ) , some o f which c e l l s were associated with very shallow d i s l o c a t i o n s , extending i n t o only t h e top 50 nm of the samples; these represent another form of laser-induced damage f o r El > 0.8
Fig. 12. Plan-view electron micrographs showing Ga-rich regions (dark) after pulsed ruby laser annealing of Zn-implanted GaAs. ( a ) 0.25 J / c m 2 ; ( b ) 0.36 J / c m 2 ; ( c ) 0.49 J / c m 2 ; ( d ) 0.81 J / c m 2 (Lowndes e t al., 1 9 8 1 b ) .
508
D. H. LOWNDES
J/cm2. Thus, there i s good overall agreement between TEM observat i o n s and e l e c t r i c a l activation measurements: Both show t h a t Ex 0.4 J / c d i s required t o completely remove the shallow ionimplantation damage considered here and t o a c t i v a t e implanted dopant ions. However, TEM measurements a l s o make i t c l e a r t h a t t h e "onset" of " e l e c t r i c a l " damage a t about 0.8 J/cm2 i s r e a l l y just the culmination of damage processes t h a t r e s u l t in a loss of stoichiometry and occur e s s e n t i a l l y continuously f o r a l l Ex above the melting threshold, when samples are annealed in a i r . SIMS depth p r o f i l e s of normalized arsenic counts, in both c r y s t a l l i n e and ion-implanted GaAs a f t e r l a s e r annealing, a l s o reveal t h a t the arsenic loss r e s u l t i n g from pulsed l a s e r i r r a d i a t i o n i s much m r e serious f o r implanted GaAs than f o r c-GaAs (Lowndes et a l . , 1981). This l a s t r e s u l t i s consistent with photo1 uminescence s t u d i e s , which showed a gradual decrease i n photoluminescent i n t e n s i t y with increasing E l f o r c-GaAs, b u t no photoluminescence from ion-implanted GaAs, regardless of the laser-annealing conditions (Lowndes and Fel dman , 1982; Fel dman and Lowndes , 1982). Davies e t a1 (1981, 1982a) have compared the surfaces of samples pulse annealed with and without a deposited f i l m of As2Se,. (The As2Se, f i l m was used as a dopant source f o r pulsed diffusion of Se dopant ions.) They reported t h a t microscopic measurements showed Ga globules only on surfaces annealed without As2Se3, while high-resolution channeling measurements revealed no excess of the Ga surface peak over t h e As surface peak. Apparently As i s cont r i b u t e d from t h e diffusion source t o produce a more nearly s t o i chi omet r i c surface. Rose and co-workers (1983) have recently reported a method f o r accurately measuring As and Ga losses from GaAs during annealing. Their method uti 1i zes quartz "catcher" sl ides t h a t are located just above, b u t not touching, t h e GaAs samples during annealing; t h e Ga and As deposits are then subjected t o neutron activation analysis, followed by gamma-ray counting f o r q u a n t i t a t i v e determination of
-
.
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
deposited As and Ga c o n c e n t r a t i o n s .
509
For comparison, RBS measure-
ments were a l s o c a r r i e d o u t on some o f t h e s l i d e s .
Rose e t a1
.
p o i n t out t h a t a l t h o u g h RBS has a s e n s i t i v i t y o f 1013/cm2 f o r comb i n e d As and Ga, o v e r l a p o f t h e As and Ga peaks makes i t d i f f i c u l t t o a c c u r a t e l y measure t h e amounts of
As o r Ga i n d i v i d u a l l y ; f o r
t o t a l As and Ga l o s s e s g r e a t e r t h a n 5 x 1015/cm2, t h e Ga and As peaks cannot be separated a t a l l .
I n contrast,
t h e y found t h a t
n e u t r o n - a c t i v a t i o n a n a l y s i s had a c o n s e r v a t i v e l y e s t i m a t e d sensi t i v i t y o f 1 x 1013/cm2 f o r As and Ga i n d i v i d u a l l y , and no degradat i o n o f s e n s i t i v i t y with i n c r e a s i n g q u a n t i t y .
Combined As and Ga
l o s s e s measured by t h e two methods were found t o be i n good agreement.
Rose e t a l . compared t h e As and Ga l o s s e s from v i r g i n (100)
GaAs c r y s t a l s f o r t h r e e d i f f e r e n t annealing methods:
p u l s e d ruby
1aser anneal i ng, anneal ing w i t h a q u a r t z ha1ogen 1amp, and anneal ing w i t h a v i t r e o u s carbon s t r i p heater.
(25 nsec pulse,
i n air,
For ruby l a s e r annealing
with a q u a r t z beam homogenizer) t h e i r
p r i n c i p a l r e s u l t was t h a t As l o s s e s s u b s t a n t i a l l y exceeded Ga losses, t y p i c a l l y by a f a c t o r o f 2 t o 3, f o r 0.3 > El
> 1.1 J/cm2,
w i t h t h e l o s s e s t e n d i n g toward e q u a l i t y a t t h e h i g h e r
-
El b u t
El 1.4 J/cm2. Arsenic l o s s e s ranged from about 1 x 1015/cm2 a t El = 0.4 J/cm2 t o 4 0 x lO15/cm2 a p p a r e n t l y o n l y becoming equal f o r a t 1.1 J/cm2.
(For comparison,
t h e r e a r e about 6 x lOl4/cm2 As
atoms i n a s i n g l e (100) l a t t i c e plane.) Only a few measurements were made by Rose e t a l . u s i n g i o n i m p l a n t e d GaAs samples, so i t i s n o t p o s s i b l e t o determine from t h e i r r e s u l t s whether As l o s s d u r i n g l a s e r annealing i s enhanced b y imp1 a n t a t i o n .
However, many e a r l i e r thermal-anneal i n g s t u d i e s
(Lou and Somorjai, 1971; Picraux, 1973) have shown t h a t t h e e f f e c t of i m p l a n t a t i o n i s t o enhance t h e t o t a l r e l e a s e of As a t temperat u r e s below 500'C
by as much as a f a c t o r o f e i g h t , and t o lower
t h e temperature a t which s i g n i f i c a n t As r e l e a s e begins from >600°C t o about 200°C.
Lou and Somorjai (1971) concluded t h a t t h e r a t e -
l i m i t i n g step i n t h e v a p o r i z a t i o n o f As from c r y s t a l l i n e GaAs i s
510
D. H. LOWNDES
Fig. 13. Optical Nomarski micrographs of pulsed ruby laser annealed GaAs ( a ) 0.49J/cm2; (b) 0.62J/cm2; ( c ) 0.81 j/cm2; (d) 0 . 9 8 j / c m 2 (Fletcheret a l . , 1981a).
t h e a v a i l a b i l i t y ( i .e.y formation and d i f f u s i o n ) of e i t h e r vacancies or divacancies a t the GaAs surface. Picraux (1973) has argued t h a t the e f f e c t of implantation i n enhancing As vaporization can t h u s be understood in terms of implantation providing s i g n i f i c a n t conc e n t r a t i o n s of vacancies (as well as other d e f e c t s ) close t o the surface. 9.
LASER DAMAGE AND BEAM 'HOMOGENIZERS
Optical Nomarski interference micrographs of the surface of c-GaAs specimens a1 so reveal large-scale surface r i p p l e s t h a t become more pronounced with increasing pulsed ruby l a s e r El (Fig. 13). For the higher El, a high density o f f i n e r background structure a1 so appears, and occasional l a r g e r vaporization " c r a t e r s "
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
511
Fig. 14. Q t i c a l micrographs showing inhomogeneous i r r a d i a t i o n o f a &As surface under d i f f e r e n t pulsed ruby laser beam conditions. The samples were f i r s t coated with -100 A Sn, i n order t o visibly enhance the e f f e c t o f local variations i n energy density, for polarized light photography. A l l pictures taken following a single pulse a t 0.5 J / c m 2 (scale: long axis = 1 mm). ( a ) Microcraters resulting from the use o f a diffuser plate. ( b ) D i f f r a c t i o n e f f e c t s from the bare single-mode beam. (c)
Homogeneous irradiation obtained with a bent, d i f f u s i n g light pipe.
512
D. H. LOWNDES
a r e also present. I t should be noted t h a t the r i p p l e patterns in Fig. 13 a l l resulted from i r r a d i a t i o n s using a beam-homogenizing l i g h t pipe. Even larger-scale surface degradation can r e s u l t from i r r a d i a t i n g a polished GaAs surface with a bare single-mode ruby l a s e r beam (Figure 1 4 ( b ) ) or by homogenizing the beam with a d i f f u s e r p l a t e such as i s used in pulsed ruby l a s e r annealing of s i l i c o n (Figure 1 4 ( a ) ) . In the l a t t e r case "microfocusing" by p i t s i n the d i f f u s e r p l a t e r e s u l t s i n c r a t e r s i n the GaAs surface. Figures 13 and 14 emphasize the limited usefulness of pulsed solids t a t e lasers and beam homogenizers f o r pulsed annealing. The recent demonstration of high homogeneity annealing of ion-implanted s i l i c o n using pulsed-excimer l a s e r s without the need f o r d i f f u s e r p l a t e s or other beam homogenizers (Lowndes e t a l . , 1982, 1983; Young e t a l . , 1983) suggests t h a t excimer l a s e r s should be espec i a l l y useful f o r GaAs, which i s f a r more susceptible t o surface damage than i s s i l i c o n . 10.
PHOTOLUMINESCENCE STUDIES
Lowndes and Fel dman (1982) have studied the photo1 umi nescence ( P L ) spectra o f both c- and implanted-GaAs following pulsed ruby l a s e r annealing. Samples were i r r a d i a t e d without encapsulation, i n a i r , mounted on s u b s t r a t e s a t room temperature, using single pulses (with E l u p t o 0.6 J/cm2) from a ruby l a s e r t h a t was spat i a l l y homogenized by a f u s e d - s i l i c a l i g h t pipe. Their study was motivated by the f a c t t h a t previous experiments using s i l i c o n had shown t h a t PL measurements following l a s e r annealing are a quite s e n s i t i v e indicator of the degree o f successful l a t t i c e regrowth (Mizuta et a l . , 1981; Skolnick e t al., 1981; Uebbing e t a l . , 1980) and of the introduction of new defects by e i t h e r cw ( S t r e e t e t a1 1979) or pulsed (Skolnick et al., 1981) l a s e r annealing. Figure 15 i l l u s t r a t e s t h e decrease i n PL i n t e n s i t y t h a t occurred A a f t e r i r r a d i a t i n g e i t h e r p- or n-type GaAs w i t h increasing El. substantial reduction of PL i n t e n s i t y was found even a t El = 0.4
.,
513
8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE 50
-efe
GoAs: Si 4.2 K
I
u) .c
40
._
-g 30 m
5I-
$
-
a Ep=O.O J/crn2 b E p = 0.2 J/cm2 c Ed=0.4 J/cm2 d El =0.6 J/cm2
20
L)
z
w 0 m
z W
5, J 1.42 4.44 1.46 1.48 1.50 1.52 ENERGY ( eV I
40
0
0.8
1.0 4.2 ENERGY ( e V )
4.4
Fig. 1 5 . Photoluminescence spectra for ( a ) p-type and ( b ) n-type c-GaAs a t 4 . 2 K (Lowndes and Feldman, 1 9 8 2 ) .
J/cm*, the lowest En value t h a t produced good e l e c t r i c a l activat i o n of high-dose, p-type implants (Fig. 7), corresponding t o a calculated maximum melt depth in c-GaAs of 180 nm (Fig. 3 ) . By considering the l / e absorption length a t t h e i r photoexcitation wavelength of 514.5 nm, and by varying t h i s wavelength, Lowndes and Feldman were able t o show t h a t the observed f a l l o f f in PL i n t e n s i t y was representative of bulk (not surface) recombination processes for photoexcited el ectron-hole pairs created w i t h i n an increasingly thick layer of material t h a t was melted and recryst a l l i z e d by the pulsed l a s e r . They concluded t h a t PL measurements show t h a t pulsed melting of c-GaAs always r e s u l t s in creation o f more nonradiative defect s i t e s than a r e eliminated. Some differences were found in the El dependence of PL intens i t y f o r n- and p-type GaAs, a t low En. The integrated PL intens i t y peaked near the melting threshold f o r n-GaAs ( a t both 77 K and 300 K ) but dropped off rapidly w i t h increasing El i n p-GaAs
D. H. LOWNDES
( a t a l l temperatures).
This d i f f e r e n c e i n PL behavior was a t t r i -
buted by Lowndes and Feldman t o d i f f e r e n c e s i n t h e nature o f t h e t r a n s i t i o n s being observed, i.e.
, r a d i a t i v e t r a n s i t i o n s predomi-
n a n t l y v i a near-band edge l e v e l s f o r t h e p-type m a t e r i a l vs r a d i a t i v e t r a n s i t i o n s through deep l e v e l s f o r t h e n-type m a t e r i a l .
One
e x p l a n a t i o n i s t h a t pulsed l a s e r i r r a d i a t i o n creates t r a p s a t i n t e r m e d i a t e l e v e l s , and t h a t these can be reached by t u n n e l i n g o r p e r c o l a t i o n from near t h e band edge.
These traps, i n t u r n , can
populate deeper l e v e l s , a1 so by tunnel ing and percol ation.
Thus,
PLA o f c-GaAs would be expected t o quench PL from t r a p l e v e l s very near t h e band edge (as observed i n p-type GaAs and i n n-type GaAs a t 300
K), w h i l e i n i t i a l l y not much a f f e c t i n g ( o r even s l i g h t l y
enhancing) recombination through much deeper l e v e l s (as observed i n n-type GaAs a t 77 K and 300 K).
DLTS measurements provide
independent evidence o f both t h e c r e a t i o n and removal o f e l e c t r o n i c deep l e v e l s i n GaAs v i a pulsed l a s e r i r r a d i a t i o n ; 1V.ll.c.
see s e c t i o n
Lowndes and Feldman also observed t h a t decreasing t h e i r
p h o t o e x c i t a t i o n wavelength always r e s u l t e d i n a decrease o f PL i n t e n s i t y i n PLA GaAs , apparently because e l ectron-hol e pai r s are t h e n created c l o s e r t o t h e sample surface, where t h e highest d e f e c t d e n s i t y i n r e c r y s t a l l i z e d m a t e r i a l i s also found. The p r i n c i p a l r e s u l t o f Lowndes and Feldman’s study o f PL from
I 1 GaAs was t h a t no PL was observed i n GaAs subjected t o high-dose ions/cm2) i o n i m p l a n t a t i o n , e i t h e r w i t h o r w i t h o u t subsequent pulsed l a s e r annealing. Thus, PL provides more evidence t h a t pulsed l a s e r annealing o f I 1 GaAs, though i t produces e p i t a x i a l regrowth, does not remove d e f e c t s t h a t act as n o n r a d i a t i v e recomb i n a t i o n centers.
Since PL i s observed i n c-GaAs subjected t o
s i m i l a r pulsed l a s e r energy d e n s i t i e s , though w i t h reduced i n t e n s i t y , these measurements suggest t h a t II/PLA GaAs always contains higher near-surface r e g i o n d e f e c t concentrations than GaAs subjected t o PLA alone, regardless o f t h e l a s e r El
used.
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
515
The lack of PL from high-dose II/PLA GaAs i s in agreement with r e s u l t s of a depth-resol ved cathodol umi nescence study by Norri s and m *ions a t 200 Peercy (1981), u s i n g GaAs implanted with 2 ~ 1 0 ~ ~ / c Cd keV (projected ranged -53 nm). No "range zone" PL (from the top 300 nm of the samples) was detected following pulsed ruby l a s e r annealing, except f o r En = 0.15 2 0.05 J/cm*--below the Ell thresholds f o r melting and f o r e p i t a x i a l regrowth--in which case PL intens i t y recovered t o about 10% of the i n t e n s i t y observed f o r c-GaAs. However, f u r t h e r exposure of such an "optimally" annealed sample t o a second, higher-energy pulse a t El = 0.25 J/cm2 ( t o promote e p i t a x i a l regrowth) removed t h e PL recovery. 11.
ELECTRICALLY ACTIVE DEFECTS
Pribat (1982) has noted t h a t t h e experimental methods t h a t were most useful f o r characterizing dopant i o n - l a t t i c e locations and residual near-surface l a t t i c e damage (e.g. , RBS in t h e channeling mode), following pulsed l a s e r annealing of high-dose ion implants i n GaAs and s i l i c o n , cannot provide useful information f o r low(
516
D. H. LOWNDES
centers. O f these methods , d i f f e r e n t i a l sheet e l e c t r i c a l propert i e s measurements and I-V, C-V, and DLTS measurements (using both Schottky b a r r i e r and p-n junction s t r u c t u r e s ) have a l l recently been used t o characterize defects introduced i n t o c-GaAs, or remaining i n 11-GaAs, following pulsed annealing. a.
Compensating Defects from Pulsed I r r a d i a t i o n of Crystalline GaAs.
Davies and co-workers (1980b) pointed out several years ago t h a t the f a i l u r e of pulsed annealing t o a c t i v a t e low implant doses i n GaAs and t h e low mobility values reported f o r higher doses a r e observed regardless of the type of dopant atom used o r of the pulsed annealing method ( l a s e r or electron beam). They a l s o noted t h a t t h e e l e c t r i c a l c h a r a c t e r i s t i c s of GaAs doped by pulsed l a s e r or e-beam diffusion from a deposited surface film closely resemble those of pulse-annealed implanted layers in GaAs, so t h a t d i f f i c u l t i e s with pulsed annealing of GaAs are not associated simply with t h e ion-implantation process. Since a c a r r i e r reduction had a l s o been observed near t h e surface of samples following pulsed annealing of deposited films (contrary t o expectations f o r doping by a diffusion process), they suggested t h a t both c a r r i e r loss and mobility reduction might be results of compensating defects t h a t a r e i n t r i n s i c t o the pulsed annealing process. In order t o i s o l a t e the e f f e c t s o f pulsed annealing on e l e c t r i cal properties, Davies e t a l . (1980b) subjected 0.8-pm thick epit a x i a l layers of GaAs, doped t o -2 x 1018/cm3, t o pulsed annealing with a 50-nsec duration electron beam a t 1.0 J/cm2. P r o f i l e s of c a r r i e r density and mobility vs depth were obtained using d i f f e r e n t i a l Hal 1 measurements , with successive layers removed by anodi zation (Lorenzo et a l . , 1979). Figure 16 i l l u s t r a t e s the r e s u l t i n g pronounced c a r r i e r loss and mobi 1 i t y reducti on t h a t were observed i n the near-surface region, d e s p i t e evidence of good c r y s t a l 1i ni t y from backscattering measurements (following removal of surface Ga).
8.
517
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
I
I
I
I
I
I
2000
al 0
In I
>
iooo " $ Y
3-
10'6
0
0.2
0.1
0.4
0.3
0.5
0
DEPTH ( p m ) Fig. 16. Profiles o f uncompensated c a r r i e r density and mobility a f t e r pulsed electron beam annealing ( 1 .O J / c m 2 , 50 nsec) o f epitaxial GaAs (Davies e t a l . , 1980b).
F i g u r e 16 shows t h a t h a l f o f t h e i n i t i a l c a r r i e r d e n s i t y i s compensated a t a depth o f -0.3
pm (corresponding t o -1018/cm3
compen-
s a t i n g d e f e c t s a t t h i s depth), w i t h g r e a t e r compensation as t h e s u r f a c e i s approached.
Davies e t a l .
(1980b) p o i n t o u t t h a t t h i s
d i s t r i b u t i o n o f compensating d e f e c t s would account f o r (1) t h e observed decreasing a c t i v a t i o n o f pul se-annealed
imp1 anted i o n s
with p r o g r e s s i v e r e d u c t i o n s i n i m p l a n t a t i o n dose, (2) o b s e r v a t i o n s o f b e t t e r a c t i v a t i o n f o r h i g h e r energy i m p l a n t s (Badawi e t al.,
518
D. H. LOWNDES
1980b), since the higher-energy implants l i e beyond the most heavily compensated near-surface region, and ( 3 ) t h e observation of maximum c a r r i e r concentrations beyond, r a t h e r than a t the surface, following doping by pulsed diffusion from a deposited film (Davies e t a l . , 1980a). Davies e t al. (1980b) also found t h a t the t o t a l sheet c a r r i e r concentration o f a pulse-annealed epitaxial GaAs specimen was f u r t h e r reduced by successive 10-min anneals in H2 or H2/AsH, a t temperatures of 100-600°C. This reduction was interpreted in terms of expansion of a nonconducting region deeper i n t o the sample: The compensating d e f e c t s produced by pulsed annealing apparently migrate r e a d i l y a t temperatures -400°C. No such change in sheet c a r r i e r concentration was observed upon s i m i l a r l y heating a 6 x 1016/cm3 doped e p i t a x i a l layer t h a t had not been pulse annealed. Although Davies e t a l . do not comment on the nature of the compensatingdefectsthatareapparentlyintroduced by pulsed annealing, i t should be noted t h a t any e f f e c t due t o ambient oxygen (see sect i o n V.13) can be ruled out by t h e i r use o f pulsed electron-beam annealing in vacuum, though oxygen from a native oxide layer was almost c e r t a i n l y present. b.
Results Using Schottky Barrier Structures
Pri bat and co-workers have a1 so sought t o c l a r i f y whether the lack of a c t i v a t i o n of low-dose n-type implants in GaAs by PLA should be a t t r i b u t e d t o laser-induced d e f e c t s , or t o implantation They performed I-V, C-V, and DLTS d e f e c t s remaining a f t e r PLA. measurements on Au/GaAs Schottky b a r r i e r s fabricated on uniformly doped b u t unimplanted (100) GaAs wafers. Laser i r r a d i a t i o n was done through a beam-homogeni zi ng 1ight pi pe using s i ngl e pul ses from both ruby and Nd:YAG pulsed l a s e r s . The most important r e s u l t s of Pribat (1982) and Pribat et a l . (1982) were t o demonstrate ( i ) a lowering of the Schottky b a r r i e r height following low El i r r a d i a t i o n of unimplanted GaAs wafers and
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
519
(ii) t h e f o r m a t i o n o f a s e m i - i n s u l a t i n g near-surface l a y e r , e l e c t r i c a l l y i n s e r i e s with t h e Schottky b a r r i e r , increases with i n c r e a s i n g El.
whose t h i c k n e s s
[The presence o f a semi -i nsul a t i ng
n e a r - s u r f a c e l a y e r was p r e v i o u s l y observed under o t h e r c o n d i t i o n s b y Mooney e t a l . (1981), Lowndes e t a l . (1981b), and Davies e t a l . (1980b),
among others.]
Using a mixed green (20%) and i n f r a r e d
(80%) pul sed-laser beam, t h e y compared t h e I - V c h a r a c t e r i s t i c s o f S c h o t t k y devices f a b r i c a t e d on u n i r r a d i a t e d GaAs samples and on samples i r r a d i a t e d a t 0.5 and 1.0 J/cm2.
The lower Ex i r r a d i a t i o n
r e s u l t e d i n an i n c r e a s e o f t h e f o r w a r d c u r r e n t by some f i v e o r d e r s o f magnitude,
b u t no change i n shape o f t h e I - V c h a r a c t e r i s t i c ,
corresponding t o a decrease o f t h e Schottky b a r r i e r h e i g h t from 0.95
t o 0.65
the I-V
V.
The h i g h e r
characteristic,
El i r r a d i a t i o n changed t h e shape o f
corresponding t o f o r m a t i o n o f a high-
r e s i s t i v i t y near-surface l a y e r i n s e r i e s with t h e S c h o t t k y device.
A1 though no p r e c i se Val ue f o r t h e reduced b a r r i e r h e i g h t c o u l d be o b t a i n e d i n t h i s case, a v a l u e o f o r d e r 0.6 V was estimated. S c h o t t k y b a r r i e r h e i g h t s were a l s o i n f e r r e d from t h e v o l t a g e a x i s i n t e r c e p t o f p l o t s o f C-2 vs V;
values o f 1.0 V and 0.65
were obtained f o r u n i r r a d i a t e d and 0.5 respectively.
V
J/cm2 ir r a d i a t e d samples ,
L i t t l e dependence o f t h e measured capacitance on
measurement frequency was observed i n e i t h e r case; t h i s was taken as evidence o f r e l a t i v e l y few deep l e v e l s i n t h e near-surface region. However,
f o r t h e 1.0 J / c d i r r a d i a t e d samples i t was found t h a t
t h e C-V c h a r a c t e r i s t i c s c o u l d o n l y be i n t e r p r e t e d by p o s t u l a t i n g a near-surface i n s u l a t i n g l a y e r , i n s e r i e s w i t h t h e capacitance o f a d e p l e t e d semiconductor, such -1
was CM
applied
= CI
-1
-1
+ Csc.
measured capacitance CM
By making use o f t h e
v o l t a g e equal t o t h e
-1 -1 and CM = CI
that the
f a c t s t h a t (i)f o r an
b u i l t - i n potential
, and t h a t ( i i ) low-frequency
Csc
i s infinite
(10 kHz) C-V data ( t o
which most t r a p s c o u l d be expected t o respond) c o u l d be used t o e s t i m a t e v b i = 0.65 V, t h e n b o t h independently.
From
Csc(Vsc)
Csc(Vsc)
and CI c o u l d be e v a l u a t e d
the c a r r i e r concentration p r o f i l e
520
D. H. LOWNDES
behind the insulating layer was inferred, while the magnitude of CI gave an estimate of the thickness of the insulating layer. Figure 17 summarizes the results of these C-V measurements for a Si-doped GaAs wafer (n = 2-3 x 1016/cm3 initially): The inferred
"'* n
E
Iy
W
c;nnn
U -=J
w 4000
z
3000 v)
z
LL
CARRIER PROFILE
I of7
5 6ooo
1 [ I
GREEN A GREEN+INFRARED (80%)
1
t I
0
I
I
I
1
200 400 600 800 1000 LASER ENERGY DENSITY (mJ/cmZ)
I I200
Fig. 17. ( a ) Carrier profile inferred from C ( V ) measurements a f t e r pulsed irradiation ( 5 3 2 nm, 20 nsec, 0.5 J / c m 2 ) . ( b ) Thickness of insulating layer (inferred from C ( V ) measurements) vs Ex (mixed 1064 nm and 5 3 2 nm pulses) (Pribat e t al. 1 9 8 2 ) .
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
c a r r i e r concentration (Fig. 1 7 ( a ) ) below the insulating layer agrees w i t h t h a t measured in the virgin wafer, while the inferred insulating layer thickness (Fig. 1 7 ( b ) ) i s e n t i r e l y consistent with a melting model (Fig. 3 ) , i f one a n t i c i p a t e s an abrupt t r a n s i t i o n in e l e c t r i c a l properties between r e c r y s t a l l i z e d GaAs and GaAs t h a t has not melted ( P r i b a t , 1982). A high density of point defects i s apparently quenched i n t o the resol i d i f i e d GaAs; the i n i t i a l l y uniform c a r r i e r concentration i s heavily compensated in the recryst a l l i z e d l a y e r , b u t r i s e s t o i t s unperturbed value in the virgin material below the point of maximum me1 t - f r o n t penetration. Pribat has attempted t o describe these r e s u l t s in terms of a threshold E l f o r a t r a n s i t i o n from typical surface e f f e c t s ( b a r r i e r lowering) t o typical bulk e f f e c t s (formation of an insulating l a y e r ) w i t h increasing l a s e r El. However, such a d i s t i n c t i o n seems a r t i f i c i a l : Both TEM observations (section IV.8) and melting model c a l culations (section 11) imply t h a t the introduction of defects assoc i a t e d with l a s e r melting and rapid s o l i d i f i c a t i o n is an e s s e n t i a l l y continuous process, increasing in depth with increasing E l once the melting threshold i s exceeded. Such an i n t e r p r e t a t i o n agrees with P r i b a t ' s finding t h a t the threshold E, f o r s i g n i f i c a n t b a r r i e r lowering and f o r increasing insulating layer thickness i s 0.35 J/cm2 f o r ruby l a s e r (694 nm) i r r a d i a t i o n and 0.25 J / c d f o r 532 nm Nd:YAG radiation, each of which i s ~ 0 . 1J/cm2 above the correspondi ng me1 t i ng threshol d f o r c-GaAs
.
c.
DLTS Measurements
Deep Level Transient Spectroscopy (DLTS) might be expected t o provide a p a r t i c u l a r l y s e n s i t i v e technique f o r monitoring the introduction and El dependence of deep l e v e l s t h a t compensate an i n i t i a l l y uniform c a r r i e r concentration in c-GaAs. However, Pribat (1982) has pointed out t h a t DLTS defect spectroscopy, performed t y p i c a l l y a t MHz frequencies, i s operative only behind any insul a t i n g layer t h a t e x i s t s a t or near the surface. For only s l i g h t l y
522
D. H. LOWNDES
damaged l a y e r s , t h e l i m i t i n g depth i s t h e zero b i a s w i d t h o f t h e d e p l e t e d l a y e r , which depends on doping l e v e l b u t g e n e r a l l y cannot be l e s s t h a n -100 nm i f c a p a c i t o r breakdown i s t o be avoided ( c o r responding t o c a r r i e r c o n c e n t r a t i o n s -2x1017/cm3).
Shunt leakage
c u r r e n t s i n l a s e r - i r r a d i a t e d m a t e r i a l a l s o make i t d i f f i c u l t t o use f o r w a r d v o l t a g e pulses (Emerson and Sealy, 1980; Davies e t al., 1982a).
As a r e s u l t ,
DLTS does n o t probe t h e immediate near-
s u r f a c e , l a s e r - r e c r y s t a l l i z e d r e g i o n i n which l a s e r - i n d u c e d p o i n t d e f e c t c o n c e n t r a t i o n s are expected t o be t h e l a r g e s t a t low E.l I n s t e a d , DLTS sees o n l y t h e " t a i l " o f any such d e f e c t concentrat i o n , t o t h e e x t e n t t h a t i t extends i n t o p a r t i a l l y o r uncompensated m a t e r i a l below t h e l a s e r - i n d u c e d i n s u l a t i n g l a y e r .
Thus,
only
small changes (corresponding t o l a s e r - i nduced d e f e c t c o n c e n t r a t i o n s o f <1016/cm3) are observed i n comparing t h e DLTS s p e c t r a o f r e l a t i v e l y s h a l l o w (0.1-0.4 c-GaAs, e t a1
even f o r El
., 1981;
eV) l e v e l s i n v i r g i n and h i g h E a - i r r a d i a t e d
= 1.0 J/cm2 (Emerson and Sealy,
P r i b a t e t al.,
1982).
1980; Mooney
These o b s e r v a t i o n s may n o t
c o n t r a d i c t t h e f a c t t h a t p u l s e d l a s e r annealing o f t h e much deeper
(0.83
eV) El,
t r a p l e v e l has been observed (Emerson and Sealy,
1979; Gamo e t al.,
1980; Mooney e t al.,
1981); i t has been suggested
t h a t t h i s annealing t a k e s p l a c e much deeper i n t h e GaAs samples, below t h e m e l t e d and r e c r y s t a l l i z e d
layer,
via
a solid-phase
r a d i a t i o n - e n h a n c e d process ( P r i b a t , 1982). d.
p-n J u n c t i o n C h a r a c t e r i s t i c s Lowndes and co-workers (1981a,b)
investigated e l e c t r i c a l char-
a c t e r i s t i c s o f p+-n j u n c t i o n s formed by pulsed ruby l a s e r a n n e a l i n g o f high-dose p-type (Zn, Mg) implants.
The use o f p-type implanted
i o n s r e s u l t e d i n h i g h e l e c t r i c a l a c t i v a t i o n i n t h e p+ e m i t t e r r e g i o n (Fig.
7), b u t s t i l l p e r m i t t e d d e t e c t i o n of t h e e f f e c t s o f c a r r i e r
compensation on t h e l i g h t l y doped n - t y p e s i d e of t h e p-n j u n c t i o n , v i a I - V and C-V measurements.
8. For
EA
PULSED BEAM PROCESSINGOF GALLIUM ARSENIDE
= 0.5
J/cm2,
523
t h e I - V c h a r a c t e r i s t i c s o f Zn-implanted
mesa diodes (0.35 m diam) were found t o f o l l o w t h e diode equation I = Ioexp(qV/nkBT) over 2-3 decades o f forward current.
However,
t h e diode p e r f e c t i o n f a c t o r "n" was always found t o be greater than 2,
w i t h a minimum value o f about 2.3
(at
EA
= 0.5
Jjcmz),
i n c r e a s i n g t o values < 3.8 w i t h i n c r e a s i n g laser-energy d e n s i t y Reverse conductance c h a r a c t e r i s t i c s were a1 so
(up t o 0.8 J/cm2).
measured and found t o consi s t e n t l y e x h i b i t a we1 1-defined reverse breakdown v o l t a g e o f 13-15 v o l t s , i n good agreement w i t h t h e theor e t i c a l l y expected breakdown v o l t a g e f o r t h e base-region GaAs doping l e v e l - 1 ~ 1 0 ~ ~ / c m ~ . Since t h e forward I - V diode s t r u c t u r e ,
C-V
c h a r a c t e r i s t i c s i n d i c a t e d a non-ideal
measurements were c a r r i e d out t o help t o
determine t h i s s t r u c t u r e .
For EA = 0.5 J/cm2, t h e measured capaci-
tance was much l e s s than t h a t expected from t h e known area and base doping c o n c e n t r a t i o n o f t h e diodes, and showed only small v a r i a t i o n w i t h reverse b i a s over the 0-10 v o l t range.
Because o f t h e small
measured capacitance, p l o t s o f C-2 vs V gave e x t r a p o l a t e d voltagea x i s i n t e r c e p t s o f 5-10 v o l t s , r a t h e r than the t h e o r e t i c a l l y expected value -1 v o l t .
Model c a l c u l a t i o n s using t h e known substrate
doping l e v e l showed t h a t a l l o f these anomalies could be explained by t h e presence o f an a d d i t i o n a l i n s u l a t i n g l a y e r , w i t h a thickness o f 0.2-0.4
pm, i n s e r i e s w i t h and j u s t below t h e j u n c t i o n .
Carrier
d e n s i t y p r o f i1 i n g measurements revealed c a r r i e r d e n s i t i e s >1020/cm3 a t and j u s t below t h e surface o f t h e p+ region, i n good agreement w i t h SIMS dopant-ion p r o f i l e s (Figs. 5 and 6 ) and demonstrated t h a t t h e i n s u l a t i n g l a y e r was not a t t h e surface.
Measured C 2 - V p l o t s ,
when c o r r e c t e d f o r t h i s s e r i e s capacitance, gave slopes and i n t e r cepts i n good agreement w i t h t h e known substrate doping and expected b u i l t - i n potential , respectively.
For j u n c t i o n s formed a t higher
Ell, C-V measurements i n d i c a t e d t h e presence o f a t h i c k e r s e r i e s i n s u l a t i n g l a y e r (Lowndes e t al., 1981b). Thus, a l l o f these
524
D. H. LOWNDES
measurements indicate t h a t l a s e r annealing r e s u l t s in a s u f f i c i e n t l y high density of defects t o produce compensation and an i n s u l a t i n g l a y e r below the p+-n junction and t h e heavily doped p+ region. 12.
ATTEMPTS TO ELIMINATE COMPENSATING DEFECTS
In t h i s section we review the r e s u l t s of several recent e f f o r t s t o reduce t h e concentration of near-surface compensating defects induced by pulsed annealing. Methods used include minimization of As l o s s , via As-bearing deposited films or encapsulant l a y e r s , and control of r e c r y s t a l l i z a t i o n velocity via s u b s t r a t e heating. The e f f e c t s of changing the type and/or pressure of t h e ambient atmosphere, and t h e role of native oxides, a r e reviewed in section V. a.
Minimization of Arsenic Loss
Loss of the group V constituent during ordinary thermal processing of 111-V compounds i s well known; surface residues and Ga and As backscattering surface peaks show t h a t preferential As loss a l s o occurs during pulsed beam annealing. However, in the course o f experiments in which an As2Se3 f i l m was used as a source f o r pulsed diffusion of Se dopant ions i n t o GaAs, Davies et al. (1981) found t h a t surface As loss could be very s u b s t a n t i a l l y reduced. Surfaces covered with a f i l m of As2Se3 were found t o be f r e e of Ga-ri ch globules fol 1owing pul sed anneal i ng , and backscatteri ng measurements showed Ga and As surface peaks of similar s i z e , suggesting t h a t As from the As2Se3 d i f f u s i o n source s u b s t a n t i a l l y improved near-surface stoichiometry. In order t o d i r e c t l y invest i g a t e the e f f e c t of an As,Se, layer upon compensating defects in the near-surface region, Davies e t a l . (1982b) used d i f f e r e n t i a l Hal 1 measurements t o measure c a r r i e r concentration and mobi 1i t y depth p r o f i l e s following pulsed electron-beam annealing (1.1 J/cm*) of e p i t a x i a l GaAs layers ( i n i t i a l c a r r i e r concentration 2 x 1018/cm3) t h a t were coated with a 2000 A f i l m of As2Se3 during annealing. However, they found t h a t t h e i n i t i a l e p i t a x i a l c a r r i e r concentrat i o n was not maintained in the near-surface region, d e s p i t e the
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
525
presence of both As from the diffusion source and of additional Se dopant atoms. T h u s , substantial compensation occurred d e s p i t e using anneal i ng conditions t h a t were be1 ieved t o mi nimi ze As l o s s , leading Davies e t a l . t o conclude t h a t As l o s s per se i s not the cause of the heavily compensated surface layer produced by pulsed anneal i ng of epi t a x i a1 GaAs. b.
Control of S o l i d i f i c a t i o n Rate Via Substrate Heating
TEM cross-sectional micrographs (Fig. 4) and backscattering measurements following removal of surface Ga residue ( F i g . 11), demonstrate t h a t the rapid r e c r y s t a l l i z a t i o n following pulsed l a s e r me1 t i ng of GaAs does r e s u l t i n excel 1ent crystal 1i ni t y Neverthe1981b; Anderson e t a l . , l e s s , several authors (Lowndes e t a1 1980) have pointed out t h a t pulsed l a s e r melting and subsequent rapid s o l i d i f i c a t i o n from the melt of a compound semiconductor should be expected t o produce q u i t e d i f f e r e n t point or local defect types than f o r an elemental semiconductor such as s i l i c o n . Pulsed l a s e r annealing has the potential f o r producing large numbers of a n t i - s i t e defects (GaAs or ASGa), r e s u l t i n g from "trapping" of one atom type on the opposite s u b l a t t i c e , as a r e s u l t of t h e high velocity of r e c r y s t a l l i z a t i o n during e p i t a x i a l regrowth. For t h e same reason, regions of varying composition (local nonstaichiometry) may be present. Finally, mobile vacancies, i n concentrations t h a t a r e probably strongly enhanced by arsenic l o s s , are expected t o be present and t o d i f f u s e and form complexes w i t h dopant ions. Wood and Giles (1981) have pointed out t h a t the velocity of t h e recrystal 1i z i ng melt-sol id i n t e r f a c e , following pulsed l a s e r melting, i s controlled by t h e temperature gradient a t the i n t e r f a c e ; t h i s gradient, and the regrowth velocity, may be s u b s t a n t i a l l y reduced by heating the s u b s t r a t e during pulsed l a s e r annealing. Davies et a l . (1982b) recently used d i f f e r e n t i a l Hall measurements of c a r r i e r concent r a t i on and mobi 1 i t y prof i 1es , fol 1owi ng pul sed electron-beam annealing o f e p i t a x i a l GaAs, t o determine whether
.,
.
526
D. H. LOWNDES
compensating defect concentrations were affected by s u b s t r a t e heating (presumably, by regrowth v e l o c i t y ) . Annealing a 1 x lO18/cm3 e p i t a x i a l layer a t 0.55 J/cm* and e i t h e r 340 or 350°C s u b s t r a t e temperature resulted only in a compensated layer some 3500 4 deep. However, increasing the pulse energy t o 1.0 J/cm2 while e-beam annealing 2 x lOl8/cm3 e p i t a x i a l GaAs a t a s u b s t r a t e temperature of 375°C resulted in retention of most o f the i n i t i a l c a r r i e r concentration over the near-surface region, which had been compensated when pulsed annealing was c a r r i e d out a t room temperature (Figure 18). Annealing a 1014/cm2 200 keV Si+ implant under the same conditions a l s o resulted in b e t t e r than 50% e l e c t r i c a l a c t i v a t i o n , in comparison with the compensation t o a depth of -3000 4 t h a t was observed f o l l owing pul sed anneal i ng a t room temperature. However, the 375" anneal resulted in a c a r r i e r p r o f i l e (Figure 19)
i
i
I
R.T.
i I
I
.I
I
I
1
J
.2
.3
.4
.5
DEPTH ( p m )
Fig. 18. Uncompensated c a r r i e r p r o f i l e s f o r epitaxial GaAs annealed a t 1 j / c m 2 with and without substrate heating (Davies e t at., 1982b).
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
f x ;
I
/ I
- I I I
/
I
\
I I
I
10'4 si ?cm2
\
1 J/cm2 375OC
\ \
\
\
\ \
I
ti
\ \ \
a 4000
\
O
V
I
0
0
0.2
0.4
I
w
9
2000
4000 0.6
DEPTH ( p m l Fig. 1 9 . Uncompensated c a r r i e r density and mobility profiles following pulsed annealing o f Si-implanted GaAs with 375OC substrate heating (Davies e t al., 1982b).
t h a t i s n o t t y p i c a l o f i o n i m p l a n t a t i o n , f o r which a peak i n c a r r i e r c o n c e n t r a t i o n i s n o r m a l l y obtained.
Davies e t a1
. suggest
that
t h i s p r o f i l e i n d i c a t e s prolonged m e l t i n g t o a depth o f a t l e a s t
6000 A as a r e s u l t o f s u b s t r a t e heating, so t h a t d i f f u s i v e dopant r e d i s t r i b u t i o n and n e a r l y u n i f o r m doping c o u l d occur.
Carrier
m o b i l i t i e s obtained by annealing a t 375OC were 1300 cm*/V-sec f o r t h e e p i t a x i a l l a y e r ( F i g u r e 18) and 1350 cm2/V-sec f o r t h e s i l i c o n
528
D. H. LOWNDES
i m p l a n t (Fig.
19), b o t h s t i l l s u b s t a n t i a l l y l e s s t h a n t h e 2000-
3000 cm2/V-sec
values expected f o r GaAs doped t o 1-2 x 101*/cm3
(Davies e t a1
.) 1982b).
I n conclusion,
supplemental
substrate
h e a t i n g c e r t a i n l y appears t o improve sheet e l e c t r i c a l p r o p e r t i e s and t o reduce compensating d e f e c t c o n c e n t r a t i o n s .
However, whether
t h i s improvement can be a t t r i b u t e d t o l e s s r a p i d s o l i d i f i c a t i o n i s uncertain:
Sat0 e t a l . (1982) have suggested t h a t t h e improvement
b r o u g h t about by s u b s t r a t e h e a t i n g i s due i n s t e a d t o increased As l o s s r e s u l t i n g i n regrowth from a G a - r i c h m e l t , j u s t i n f r o n t o f t h e r a p i d l y growing i n t e r f a c e ,
i n which case a lower As p a r t i a l
p r e s s u r e i s s u f f i c i e n t f o r f o r m i n g n e a r l y s t o i c h i o m e t r i c GaAs f r o m t h e melt.
Sat0 e t a l .
suggest t h a t regrowth v e l o c i t y becomes
c r i t i c a l l y i m p o r t a n t o n l y when t h e pressure o f t h e ambient annealing atmosphere i s low (see s e c t i o n V.14). c.
Use o f Encapsulant Layers Encapsulant l a y e r s (Si3N4, SiO,)
have been used d u r i n g pulsed
a n n e a l i n g o f GaAs w i t h t h e i d e a o f m i n i m i z i n g As l o s s , j u s t as i n thermal annealing.
Improved e l e c t r i c a l a c t i v a t i o n ,
f o r b o t h n-
and p-type dopant i o n s , has been r e p o r t e d w i t h t h e use o f encaps u l a n t l a y e r s d u r i n g p u l s e d a n n e a l i n g (Badawi e t a1 e t a1
., 1979a,
b; K u l a r e t a1
., 1979;
Badawi e t a1
.
1979; Inada
., 1980a).
How-
ever, pul sed anneal ing t h r o u g h an encapsul a n t l a y e r can a1 so c r e a t e a d d i t i o n a l problems. Doping o f t h e GaAs s u b s t r a t e by c o n s t i t u e n t s o f t h e capping l a y e r (e.g.
S i ) was noted by Badawi e t a l .
(1979).
I n c e r t a i n cases, t h i s e f f e c t might be used t o advantage, e.g., t h e near-surface
r e g i o n c o u l d be d e l i b e r a t e l y doped w i t h S i t o
a v o i d t h e h e a v i l y compensated l a y e r t h a t would o t h e r w i s e r e s u l t . However , encapsul a n t l a y e r s t e n d t o be damaged o r c o m p l e t e l y blown
away w i t h i n c r e a s i n g pul sed-energy d e n s i t y , and can a1 so produce For a nonpl anar GaAs s u r f a c e f o l l o w i ng pul sed 1a s e r anneal i ng example , u s i n g s p u t t e r - d e p o s i t e d SiO, A thick,
optical
.
encapsul a n t l a y e r s -300-600
and TEM micrographs
(Lowndes e t al.,
1981b;
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
529
1981a, b) r e v e a l t h a t f r o z e n - i n r i p p l e s w i t h wave-
F l e t c h e r e t al.,
l e n g t h <1 pm and a m p l i t u d e o f o r d e r 1000 A a r e formed f o r any energy d e n s i t y above t h e i n e l t i n g t h r e s h o l d ( F i g u r e 20).
R i p p l i n g appar-
e n t l y r e s u l t s f r o m d i f f e r e n t i a l s t r e s s e s between t h e encapsul a n t l a y e r and t h e molten GaAs s u r f a c e p r i o r t o t h e t i m e t h a t i t freezes. It was a l s o found (Lowndes e t al.,
1981b) t h a t t h e melt-depth pro-
f i l e remains p l a n a r , even though t h e s u r f a c e i s r i p p l e d , so t h a t a s p a t i a l l y nonuniform p-n j u n c t i o n depth below t h e r i p p l e d s u r f a c e can r e s u l t .
TEM s t u d i e s showed t h a t f o r m a t i o n o f Ga-rich s u r f a c e
g l o b u l e s was suppressed a t low El by encapsulation, h i g h e r El ( F l e t c h e r e t al.,
V.
1981b; Lowndes e t a1
but not a t
., 1981b).
R o l e o f t h e Ambient Atmosphere
The u n a t t r a c t i v e e l e c t r i c a l p r o p e r t i e s r e s u l t i n g from II/PLA o f GaAs, i n comparison w i t h s i l i c o n , t o g e t h e r w i t h evidence t h a t compensating d e f e c t s a r e i n t r o d u c e d and t h a t a r s e n i c l o s s occurs near t h e s u r f a c e d u r i n g pulsed l a s e r m e l t i n g o f GaAs, l e d two groups t o i n v e s t i g a t e r e c e n t l y t h e r o l e p l a y e d by t h e ambient atmosphere and by n a t i v e s u r f a c e o x i d e l a y e r s d u r i n g PLA o f GaAs. B e n t i n i , B e r t i , Cohen, Drigo, J a n n i t t i , P r i b a t , and S i e j k a (BBCDJPS; B e n t i n i e t a1
., 1982;
and Cohen e t a1
., 1983,
1984) have c a r r i e d
Fig. 20. Cross-section TEM micrograph showing surface ripple o n a pulsed ruby laser annealed Si02-encapsulated GaAs sample (0.31 J /cm2; 5 0 2 layer removed) (Lowndes e t a l . ,
1981b).
530
D. H. LOWNDES
o u t a d e t a i l e d , q u a n t i t a t i v e a n a l y s i s o f t h e El dependence o f n a t i v e o x i d e s t a b i l i t y d u r i n g PLA, and o f oxygen i n c o r p o r a t i o n f r o m b o t h t h e n a t i v e o x i d e and from an ambient atmosphere c o n t a i n i n g oxygen. Sato, Sunada, and Chikawa (SSC;
1982) have s t u d i e d t h e e f f e c t o f
b o t h h i g h - and low-pressure argon ambients on As/Ga s t o i c h i o m e t r y , on dopant-ion segregation, and on e l e c t r i c a l a c t i v a t i o n o f S i i o n i m p l a n t s , and have c o n t r a s t e d t h e s e r e s u l t s with a n n e a l i n g c a r r i e d o u t i n a i r , f o r f i x e d Ell o f 0.8 and 1.0 J/cm2.
13.
INCORPORATION OF OXYGEN The work o f BBCDJPS was m o t i v a t e d by t h e r e a l i z a t i o n t h a t oxygen
i n c o r p o r a t i o n from " n a t i v e "
s u r f a c e oxides i n t o m o l t e n GaAs is
l i k e l y t o play a s i g n i f i c a n t r o l e i n defect generation during p u l s e d l a s e r m e l t i n g and s o l i d i f i c a t i o n .
The ' l n a t i v e " o x i d e on
GaAs i s t y p i c a l l y >20 A t h i c k and i s composed p r i m a r i l y o f As203 and Ga203, though i n p r o p o r t i o n s t h a t depend upon t h e c o n d i t i o n s
., 1980; Breeze e t a1 ., 1980). A s i n g l e (Matsuure e t a1 ., 1981) had i n d i c a t e d t h a t oxygen
o f growth (Thurmond e t a1 p r i o r experiment
t r a p p i n g can occur d u r i n g nanosecond PLA o f GaAs i n a 3-4 atm. oxygen ambient;
p r i o r s t u d i e s o f oxygen i n c o r p o r a t i o n d u r i n g PLA
o f s i l i c o n suggested l i t t l e oxygen i n c o r p o r a t i o n f o r v i s i b l e nanosecond pulses, b u t t h a t t h e process i s complex and s t r o n g l y dependent on parameters t h a t i n c l u d e t h e pulsed l a s e r wavelength ( v i s i b l e vs u v ) ,
pulse duration
p r e s s u r e ( L i u e t a1 1980; C u l l i s e t a1
(nanosecond vs picosecond), and ambient
., 1979,
., 1982).
1981; Tsu e t a1
., 1979;
Hoh e t al.,
The p r i n c i p a l issues addressed by BBCDJPS are: ( a ) The r e l a t i o n s h i p between t h e t h r e s h o l d energy d e n s i t y f o r p u l s e d l a s e r m e l t i n g o f GaAs and t h e ( d i f f e r e n t ) t h r e s h o l d energy d e n s i t i e s f o r oxygen i n c o r p o r a t i o n from a n a t i v e o x i d e o r from t h e ambient ;
8 . PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
531
( b ) elucidation of t h e role of the native oxide layer, e i t h e r a s a source of oxygen or as a possible b a r r i e r t o incorporation of oxygen from the ambient;
( c ) the composition (stoichiometry) of pulsed laser-formed oxides on GaAs and the mechanism f o r t h e i r formation; and ( d ) changes i n the composition of the native oxide, and i n the depth p r o f i l e in GaAs of oxygen derived from t h e native oxide, a t low En. In order t o distinguish between 0 originating from a native oxide layer and 0 from the ambient atmosphere, BBCDJPS formed -10 nm thick l80-enriched oxide layers by anodic oxidation in an l80enriched solution. A 15-nsec duration ruby l a s e r and a beam The l60 and l 8 O homogenizer p l a t e were used f o r i r r a d i a t i o n s . contents in a near-surface region o f about 1-2 pm thickness (i.e., somewhat thicker than t h e maximum me1 t depth) were determined using nuclear reactions with deuteron and proton beams, respectively. Depth p r o f i l e s of the l 8 O content were a l s o obtained via a f i t t i n g procedure applied t o the resonant l80 ( p , a ) 15N reaction a t 629 keV, with a d e p t h resolution of about 20 nm (Cohen e t a l . , 1983, 1984). a.
Oxygen from the Ambient Atmosphere
Measurements by BBCDJPS under an 0 pressure of 4 atm. on native oxide-covered samples revealed no change i n the t o t a l near-surface 0 content (No 20 x lOl5/cm2) f o r En < 1 J/cm*. However, a large uptake of oxygen was observed f o r E, 1.1 J/cm2, w i t h No approachi n g 150 x 1015/cm2 a t E, = 1.7 J/cm2. A t t h i s point s a t u r a t i o n of t h e t o t a l 0 content occurs, apparently due t o competition between 0 uptake and nearly simultaneous evaporation a t the high temperat u r e s reached by the GaAs melt. Studies of t o t a l 0 content a t a fixed El = 1.5 J/cm? a l s o revealed t h a t the 0 uptake i s proport i o n a l t o 0 overpressure, over t h e 0-4 atm pressure range (Bentini 1982). BBCDJPS i n t e r p r e t these r e s u l t s as showing t h a t e t a1 neither diffusion of 0 atoms in the l i q u i d nor t h e i r r e a c t i v i t y
-
.,
532
D. H. LOWNDES
are limiting steps i n 0 uptake; they also calculate that about 10% of the 0 atoms striking the liquid surface are trapped i n a surface oxide layer (Bentini et al., 1982). Time-resolved reflectivity measurements and me1 t i n g model calcul a t i ons show that me1 t i ng of GaAs occurs under these conditions for El >, 0.2 J/cm2 (Fig. 2), yet significant 0 uptake from ambient a i r (po2 = 0.2 atm) was not observed by BBCDJPS until El > 1.1 J/cm2. T h i s result suggested t o BBCDJPS t h a t the presence o f native oxide on a GaAs substrate could hinder 0 uptake and that there i s an energy threshold t o remove native oxide by evaporation, before 0 uptake from the ambient can occur. Figure 21 shows results of measurements of l6O, l s O , and total 0 content vs Ex, following i rradi a t ion of GaAs sampl es covered by -10 nm thick 180-enri ched
q-
X
z
W
U
>-
X 0
0
0.4
0.8
1.0
1.4
ENERGY DENSITY ( J I c m ' l Fig. 21. El-dependence of l60incorporation from a Po = 4 atm. ambient and 2 of l80loss from a surface oxide for GaAs. The full line shows total 0 content (Bentini et al., 1982).
8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE n a t i v e oxides i n a p = 4 atm. 1 6 0 ambient.
The r e s u l t s demonstrate
<
t h a t t h e t o t a l 0 c o n t e n t i s n e a r l y c o n s t a n t f o r El t h a t massive 0 uptake from t h e ambient occurs f o r El t h e t h r e s h o l d v a l u e (Eth
533
1 J/cmz,
but
greater than
1 J/cmz) f o r removal o f t h e s u r f a c e
oxide layer. It should be noted t h a t t h e v a l u e o f Eth corresponds almost
e x a c t l y t o t h e En a t which Lowndes (1981) observed a c h a r a c t e r i s t i c ''damage"
s i g n a t u r e (a r e f l e c t i v i t y r i s e i n d i c a t i v e o f m e l t i n g ,
f o l l o w e d by a sharp drop i n r e f l e c t i v i t y ) i n t i m e - r e s o l v e d rneasurements; i t i s a l s o o n l y s l i g h t l y h i g h e r t h a n t h e p r e d i c t e d t h r e s h o l d (-0.8
J/cm2) f o r v a p o r i z a t i o n o f GaAs found i n rnelting-model c a l -
c u l a t i o n s (see Fig. 3). b.
P r o d u c t i o n and S t o i c h i o m e t r y o f Pulsed Laser-Induced Oxides
I n o r d e r t o s t u d y t h e composition o f t h e n e a r - s u r f a c e r e g i o n f o l l o w i n g pulsed l a s e r i r r a d i a t i o n , t h e number o f (Ga
t
As) c a t i o n s ,
Neat, l o c a t e d o u t o f c r y s t a l l o g r a p h i c s i t e s i n t h e surface-damaged l a y e r was determined by BBCDJPS u s i n g RBS o f 1.8 MeV 4Het i n t h e c h a n n e l i n g geometry (Cohen e t a1
., 1983,
1984).
By comparing t h i s
1a s e r - i nduced d i s o r d e r w i t h independent n u c l e a r r e a c t i o n measurements o f l60 uptake and 1 8 0 c o n t e n t , and o f 1 8 0 depth p r o f i l e s (as d e s c r i b e d above), t h e y were a b l e t o m o n i t o r b o t h t h e s u r f a c e o x i d e c o m p o s i t i o n (i.e., VS
El, f o r E l
<
s t o i c h i o m e t r y ) , f o r El
>
Eth, and l80 movement
Eth.
F o r samples i r r a d i a t e d above t h e Eth f o r ambient 0 uptake, i t was found t h a t t h e s u r f a c e Ga and As peaks are n o t s e p a r a t e l y r e s o l v e d , b u t t h a t t h e o v e r a l l amount o f (Ga
t
As) under t h e s u r -
face peak and t h e w i d t h (depth) o f t h e peak, were c o n s i s t e n t with amorphization t a k i n g p l a c e i n t h e s u r f a c e region. uptake t o (Ga
t
The r a t i o o f 0
As) atoms i n t h e s u r f a c e peak, d e r i v e d f r o m a l a r g e
number o f measurements a t v a r i o u s El
>
Eth, was a l s o very near t h e
.,
v a l u e 1.5 expected f o r s t o i c h i o m e t r i c Ga203-As203 (Cohen e t a1 1984). Thus, most o f t h e 0 uptake remains i n t h e s u r f a c e r e g i o n
534
D. H. LOWNDES
and forms an oxide.
Annealing experiments ( 3 h r s , 10-6 t o r r , 200°C)
f o l owing 0 uptake r e v e a l e d no 0 l o s s ; t h i s was i n t e r p r e t e d as ind c a t i n g t h a t oxide f o r m a t i o n i n t h e vapor phase, f o l l o w e d by condensation on t h e surface,
was u n l i k e l y .
I n t h a t case,
heat
t r e a t m e n t would have been expected t o evaporate l o o s e l y bound As203 mol e c u l e s (pAs203 -1 t o r r a t 2OOOC).
The most probable mechanism
f o r o x i d e f o r m a t i o n i s b e l i e v e d t o be d i s s o l u t i o n o f oxygen i n m o l t e n GaAs f o l l o w e d by o x i d a t i o n d u r i n g s o l i d i f i c a t i o n (Cohen e t al., c.
1984). Behavior o f t h e N a t i v e Oxide D u r i n g Pulsed Laser M e l t i n g Another r e s u l t o f BBCDJPS' study was t o show t h a t p u l s e d l a s e r
i r r a d i a t i o n can evaporate ( o r ,
more l i k e l y ,
dissolve) a native
o x i d e and cause p e n e t r a t i o n o f l 8 0 i n t o a GaAs specimen a t
El values
f a r below those needed f o r l60 uptake from t h e ambient.
These
measurements were m o t i v a t e d by t h e r e a l i z a t i o n t h a t t h e t h r e s h o l d energy d e n s i t y f o r d e g r a d a t i o n o f t h e b u l k e l e c t r i c a l p r o p e r t i e s (-0.35
J/cm2,
P r i b a t e t al.,
1983; -0.2-0.3
J/cm2,
Lowndes and
Feldman, 1982) o f l a s e r - i r r a d i a t e d GaAs i s c o n s i d e r a b l y l o w e r t h a n t h e -1 J/cm2 t h r e s h o l d f o r i n c o r p o r a t i o n o f s i g n i f i c a n t amounts o f ambient 0.
T h i s suggests d i s s o l u t i o n o f n a t i v e oxide, a t El values
j u s t above t h e m e l t i n g t h r e s h o l d ,
as a p o s s i b l e mechanism f o r
.
A marked broadeni ng o f t h e l80 depth d i s t r i b u t i o n , accompanied by a 10-30% loss o f l80, was observed f o l l o w i n g i r r a d i a t i o n a t 0.8 J / c d i n a 4 atm. 0 ambient, under which c o n d i t i o n s t h e r e was no 160 uptake. A marked decrease
degradation o f e l e c t r i c a l properties
o f t h e As s u r f a c e peak i n a l i g n e d RBS s p e c t r a was a l s o observed a t low
El, i n d i c a t i n g p r e f e r e n t i a l As203 evaporation, so t h a t t h e
broadened * O p r o f i1e was a t t r i b u t e d t o Ga20,-deri ved 0 p e n e t r a t i n g i n t o t h e GaAs c r y s t a l d u r i n g m e l t i n g (Cohen e t al.,
1983, 1984).
I n o r d e r t o study t h e b e h a v i o r o f t h e n a t i v e o x i d e s e p a r a t e l y f r o m ambient 0 uptake, BBCDJPS i r r a d i a t e d 180-enriched o x i d e l a y e r s a t various
El i n vacuum, w i t h r e s u l t s t h a t a r e summarized by F i g u r e s
22 and 23.
F i g u r e 22 shows l80 c o n c e n t r a t i o n p r o f i l e s f o l l o w i n g
8. PULSED BEAM PROCESSING OF GALLIUM ARSENIDE 2500
1
-
I
2000
5 I:
I
I
I
I
500
'%(p,a) 'ON RESONANCE --O--UNIRRADIATED SAMPLE -c-O.8 J/cmL IN VACUUM
420 eV/CHANNEL
620 (rev AT CHANNEL 0
-
r 4500
400
300
(000
200
500
(00
0
535
(0
0
20
20 40 CHANNEL NUMBER
50
60
0
r\ LASER ENERGY DENSITY
0.5
P = 16' Torr
- 0.4 0.6 -- 0.8 J/cm2 -. - 1.0 J/cm2 ----
9-0.3
0 a
J/cm2 J/cm2
a
z 0.2 0
0
25
50 DEPTH (nm)
75
100
Fig. 22. ( a ) Experimental e x c i t a t i o n curves and f i t s t o them b e f o r e ( 0 , ---) and a f t e r (0, -) laser i r r a d i a t i o n i n vacuum o f GaAs samples w i t h an l80enriched oxide layer. ( b ) l80 concentration p r o f i l e s (derived f r o m f i t t i n g nuclear e x c i t a t i o n curves) f o r GaAs samples i r r a d i a t e d a t various Ex (Cohen e t al., 1984).
536
D. H. LOWNDES
!02*
lo2‘ \
1020
\
z
0 I-
2I-
40’’
\
z
W
1 \
0
\
8
0
z
l0l6
loq
Fig. 23.
--- REFERENCE
, -0.4 J /em2
0
L A S E R IRRADIATED
l80SIMS p r o f i l e s f o r virgin GaAs (---)
0.4 J / c m 2 (-)
(Cohen e t a l . ,
i r r a d i a t i o n s a t v a r i o u s El, ( p e n e t r a t i o n o f l80) and e t a1
0.2 0.3 DEPTH ( p n )
0.1
., 1984).
0.4
and for GaAs i r r a d i a t e d a t
1983).
and demonstrates t h a t b o t h broadening
loss o f l80 occur even a t 0.4 J / c d (Cohen
A t h i g h e r E,l
l80 p r o f i l e s d e r i v e d from n u c l e a r
a n a l y s i s extend as much as 75 nm i n t o t h e sample. e t al.
However, Cohen
(1983) p o i n t out t h a t t h e l80 d e t e c t i o n l i m i t u s i n g n u c l e a r
SIMS measurements o f l80 p e n e t r a t i o n r e v e a l l80 c o n c e n t r a t i o n s o f o r d e r 0.01% w i t h respect t o t h e m a t r i x a t a depth > 200 nm ( F i g u r e 23) f o r an E l o f o n l y 0.4 J/cm2. Although
a n a l y s i s i s about 0.4%.
t h e r e a r e some q u a n t i t a t i v e problems i n r e l a t i n g t h e n u c l e a r and
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
537
SIMS measurements o f l80 depth p r o f i l e s , i t i s c l e a r t h a t penetra-
t i o n o f n a t i v e oxide-derived
oxygen deep i n t o t h e GaAs m a t r i x
occurs even f o r low EA. The Ga and As surface peaks derived from channeling a n a l y s i s a l s o reveal a marked As l o s s a t low E l (0.4-0.6 al.,
1984).
J/cm2) (Cohen e t
The n a t i v e oxide l a y e r i s e l i m i n a t e d a t El = 0.8 J/cm2
( f o r i r r a d i a t i o n i n vacuo), and f o r higher E l t h e channeling surface peaks i n d i c a t e t h a t t h e surface re g i o n i s c o n s t i t u t e d mostly o f As atoms (Figure 24). The i n t e r p r e t a t i o n given t o these r e s u l t s by BBCDJPS i s t h a t at
low E l t h e main process o c c u rri n g i s evaporation o f As203
accompani ed by format i o n o f Ga-ri c h s u r f ace 1ayers , i n agreement
I
I
I
I
CHANNELING SURFACE PEAKS
1500 -
UNIRRADIATED 0 0.4 J/cm2 A 0.6 J/cm2
A
1.0 J/cm2
1000
cn
I-
z 3 0 0
500
0
390
400 410 CHANNEL NUMBER
420
Fig. 24. As (higher channel number) and Ga (lower channel number) surface peaks before and a f t e r pulsed laser irradiation i n vacuum a t various E l (Cohen e t al. 1984).
538
D. H. LOWNDES
0.6 J/cm2, with the TEM r e s u l t s described in section IV.8. For E, evaporat i on of Ga203 becomes important and competition a1 so occurs between evaporation of Ga20, and oxygen or oxide diffusion i n t o l i q u i d GaAs. The strong As surface peak observed i n channeling experiments a t higher El (Fig. 24) i s concurrent with complete removal of t h e native oxide layer. I t i s suggested t h a t observat i o n of t h i s As surface peak i s a l s o consistent with the r e s u l t s of Sato, Sunada, and Chikawa (1982) described in t h e following s e c t i o n : The As surface peak r e s u l t s from t h e f a c t t h a t GaAs grown from a pulsed laser-induced melt is Ga-rich; t h e high As equilibrium p a r t i a l pressure, aided by 0 penetration i n t o the melt (see below) causes As t o segregate toward t h e surface. Rapid quenching, however, prevents complete evaporation of As from the surface. Thus, pulsed annealing i n vacuo changes t h e surface composition of an o r i g i n a l l y oxide-covered specimen from Ga-rich a t low E, t o As-rich a t high En, according t o Cohen e t a1 (1983). I n summary, t h e available evidence suggests t h a t GaAs degradat i o n in pulsed l a s e r annealing can indeed r e s u l t from deep penetrat i o n of oxygen derived from a native oxide layer, even a t low E, (>0.4 J/cm2). For higher Ex ( s u f f i c i e n t t o remove a native oxide) i r r a d i a t i o n s carried out in vacuo r e s u l t in degradation accompanied by As vacancies in t h e bulk and As precipitated on the surface. In t h e presence of an O-containing ambient, expulsion of As i s accelerated by uptake of 0 from t h e ambient (see below) and a surf a c e oxide i s formed with t h e (Ga + As)/O r a t i o near t h e value of 1.5 expected f o r stoichiometric Ga203-As203, though very l i k e l y on t h e Ga-rich side as a r e s u l t of preferential As203 l o s s ( i n agreement with t h e TEM measurements quoted in section IV.8).
.
14.
EFFECT OF ANNEALING IN A HIGH-PRESSURE AMBIENT
Sato, Sunada, and Chikawa (SSC, 1982) have studied the e f f e c t of a high-pressure ambient by carrying out pulsed ruby l a s e r annealing of t h e (100) surface of unencapsulated GaAs specimens
539
8 . PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
i n a 100-bar argon environment using 1.0 J/cm2 pulses (20 nsec
FWHM).
E f f e c t s due t o h i g h pressure were i s o l a t e d by comparing
w i t h s i m i l a r annealing experiments c a r r i e d out i n a i r and i n l - b a r n i t r o g e n and argon environments.
A d i f f u s i n g l i g h t pipe and a
converging l e n s were used t o homogenize t h e l a s e r beam and t o i n t r o duce it through a window i n t o t h e pressure chamber c o n t a i n i n g t h e specimens. a.
Stoichiometry and I m p u r i t y Segregation Depth p r o f i l e s o f atomic r a t i o s o f As/Ga were obtained by SSC
u s i n g SIMS, i n t h e form o f normalized r a t i o s (As/Ga)irr/(As/Ga)un f o r l a s e r - i r r a d i a t e d and u n i r r a d i a t e d regions a t t h e same depth. The specimens used were r e l a t i v e l y h i g h l y doped n-type GaAs (1.6 x 1Ol8 S i atoms/cm3), so t h a t p r o f i l e s o f S i concentration vs depth c o u l d also be examined f o l l o w i n g l a s e r annealing i n the d i f f e r e n t environments.
Thus, t h e e f f e c t s o f d i f f e r e n t ambient gases and
pressures on S i dopant i o n segregation could also be examined.
As shown i n Figure 25, annealing i n a 100-bar A r ambient r e s u l t e d i n an As/Ga r a t i o of u n i t y , except w i t h i n about 20 nm o f t h e surface.
I n c o n t r a s t , specimens annealed i n l - b a r A r o r a i r
ambients showed As d e f i c i e n c y extending over e s s e n t i a l l y t h e f u l l depth o f m e l t i n g (-0.55
prn a t 1.0 J/cm2,
note t h a t t h e two p a r t s o f Fig.
see Fig. 3).
[However,
25 seem s l i g h t l y i n c o n s i s t e n t ,
i m p l y i n g a double d i p i n t h e As/Ga r a t i o . This may be an a r t i f a c t o f t h e d i f f e r e n t normalizations o f the SIMS p r o f i l e s , on depth scales d i f f e r i n g by an order o f magnitude.] The s i m i l a r p r o f i l e s f o r both o f t h e low-pressure ambients and the sharp c o n t r a s t w i t h apparent s t o i c h i o m e t r y over t h e e n t i r e m e l t depth f o r t h e 100-bar
A r ambient i n d i c a t e t h a t As l o s s i s suppressed by a high-pressure environment.
I n a d d i t i o n , t h e specimen annealed i n a l - b a r a i r
ambient shows a sharper As surface "spike" and more severe As l o s s i n the near-surface r e g i o n than do specimens annealed i n l - b a r A r
540
D. H. LOWNDES
ENVIRONMENT I -bar Air Y
0
Q 1.0 K
0
(3 2
a v
1.1
-
ENVIRONMENT o 100- bar Ar 0
-
:0.9--
A
I
I
I
I
I - bar Ar I
- bar
I
Air
I
_
Fig. 25. Depth profiles o f atomic ( A s / G a ) ratios measured by SIMS following pulsed laser irradiation o f uniformly Si-doped c-GaAs a t 1 . 0 J / c m 2 in various environments (see t e x t ) : ( a ) Near-surface and (b) deeper i n t h e same specimens (Sato e t a l . ,
1982).
or N2. SSC concluded t h a t 0 atoms from a i r penetrate deeply and occupy As s i t e s during surface melting, increasing t h e number of As atoms t h a t segregate t o the surface. Figure 26 (SSC, 1982) demonstrates t h a t t h e r e d i s t r i b u t i o n o f S i dopant ions r e s u l t i n g from dopant segregation a t t h e rapidly moving melt-solid i n t e r f a c e may a l s o be a strong function of t h e type o f ambient gas present and o f i t s pressure. SIMS dopant atom
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
5.01
0'
(V/D)x 0.5 I
0.2
I
10
1
I
1
26.
1
I 1 1 1
,
50 DEPTH x
Fig.
5
I
I
541
1
I
1
I
I00 200
(8)
Silicon dopant profiles measured by SlMS following pulsed laser
irradiation o f uniformly Si-doped c-GaAs a t 1 . O J/crn2 i n various environments. Solid curves a r e the results o f f i t t i n g a model to the data (see t e x t ) (Sato e t al.,
1982).
profiles vs depth were f i t t e d by SSC t o curves (solid l i n e s , Fig. 26) obtained from the theory of normal freezing, as functions of ( v / D ) x and of k i , where v is the solidification front velocity (taken t o be 2 m/sec; see Fig. 3 ) , D i s the diffusion coefficient f o r Si in mlten GaAs, x i s the depth, and ki i s the (nonequilibrium) interface segregation coefficient. SSC point out that the ki values of 0.35 and 0.2 found for 1-bar and 100-bar Ar ambients, respectively, are substantially increased over the equilibrium value ko = 0.1-0.14 for S i in GaAs (Willardson and Allred, 1967),
542
D. H. LOWNDES
as i s expected f o r h i g h - v e l o c i t y s o l i d i f i c a t i o n (see s e c t i o n 11.4). Moreover,
SSC i n t e r p r e t t h e l a r g e r k i value f o r 1-bar ambient
pressure, t o g e t h e r with t h e r e s u l t s i n Fig. 25, as evidence t h a t S i atoms a r e m r e e a s i l y i n c o r p o r a t e d i n t o c r y s t a l s having a h i g h c o n c e n t r a t i o n o f As vacancies.
( S i i s an amphoteric i m p u r i t y i n
GaAs, capable o f occupying e i t h e r As o r Ga s u b l a t t i c e s i t e s , though p r e f e r e n t i a l l y occupying Ga s i t e s and a c t i n g as a donor a t moderate d o p i n g l e v e l s i n n e a r - e q u i l i b r i u m c r y s t a l growth.)
I n striking
c o n t r a s t , t h e S i dopant p r o f i l e f o r annealing i n a i r corresponds t o a v e r y small s e g r e g a t i o n c o e f f i c i e n t ( k i
<<
1).
SSC i n t e r p r e t
t h i s small v a l u e as evidence t h a t S i dopant atoms, as w e l l as As (see Fig. 25), a r e t h r u s t o u t toward t h e s u r f a c e by 0 atoms penet r a t i n g t h e m e l t and occupying As s u b l a t t i c e s i t e s .
These segre-
g a t i o n e f f e c t s are c o n s i s t e n t w i t h BBCDJPS' o b s e r v a t i o n t h a t subs t a n t i a l uptake o f ambient 0 occurs f o r E, b.
>,
1 J/cm2.
E l e c t r i c a l A c t i v a t i o n o f Implanted Ions The SIMS measurements o f s t o i c h i o m e t r y d e s c r i b e d above do n o t
p r o v i d e s u f f i c i e n t r e s o l u t i o n t o determine whether GaAs w i t h e l e c trical
properties s u i t a b l e f o r device f a b r i c a t i o n r e s u l t s from
a n n e a l i n g i n a high-pressure i n e r t gas ambient.
I n o r d e r t o address
t h i s question, SSC compared t h e e l e c t r i c a l p r o p e r t i e s o f specimens o f n-type GaAs implanted with 180 keV, 1 x l O L 5 Si*/cm2, f o l l o w i n g p u l s e d ruby l a s e r annealing a t 0.8 J/crn2 i n a i r , 1-bar A r and 100b a r A r ambients.
H a l l e f f e c t and sheet r e s i s t i v i t y measurements
f o l 1owi ng anneal ing showed t h a t sampl es anneal ed i n a i r were coated w i t h a t h i n m e t a l l i c phase.
( T h i s was presumably t h e Ga-rich
residue shown i n Figs. 11 and 12; i t can be removed w i t h d i l u t e warm HC1.)
For samples annealed i n 1-bar and 100-bar A r ambients,
t h e measured m o b i l i t i e s (percentage e l e c t r i c a l a c t i v a t i o n s ) were
780 cm*/V-s (12%) and 750 cm2/V-s
(loo%),
respectively.
SSC a t t r i b -
u t e t h e low e l e c t r i c a l a c t i v a t i o n o f samples annealed i n t h e 1-bar
8.
543
PULSED BEAM PROCESSlNG OF GALLIUM ARSENIDE
A r ambient t o some o f t h e S i dopant atoms being incorporated i n t o t h e As s u b l a t t i c e (occupying vacant As s i t e s i n accord w i t h F,ig. 25) and compensating S i donors on Ga s u b l a t t i c e s i t e s . Assuming average c a r r i e r concentrations o f 5 x 1018/cm3 and
4 x 1019/cm3 i n t h e II/PLA l a y e r s (estimated using t h e i r SIMS dopant p r o f i l e s ) SSC c a l c u l a t e d t h e o r e t i c a l maximum m o b i l i t i e s o f 2000 (800) cm2/V-s f o r t h e 1- (100) bar samples. Thus, both t h e e l e c t r i c a l a c t i v a t i o n and c a r r i e r m o b i l i t y appear t o reach t h e i r theor e t i c a l maximum values as a r e s u l t o f annealing i n a 100-bar A r ambient. These r e s u l t s r e q u i r e c o n fi rm a t i o n and m r e d e t a i l e d i n v e s t i g a t i o n , b u t c e r t a i n l y appear t o be a l a r g e improvement over t h e e l e c t r i c a l a c t i v a t i o n s and m o b i l i t i e s obtained p r e v i o u s l y (see s e c t i o n 111) by annealing various implants i n a i r , e i t h e r w i t h o r w i th out subst rat e heating. c.
Interpretation It now seems t h a t t h e defects introduced d u r i n g pulsed l a s e r
annealing o f GaAs are t h e r e s u l t o f a complex i n t e r a c t i o n i n v o l v i n g t h e type o f ambient atmosphere present, t h e ambient pressure, t h e presence or absence o f a n a t i v e oxide surface l a y e r , t h e pulsed laser
El and t h e h i g h v e l o c i t y o f s o l i d i f i c a t i o n .
Nevertheless,
by drawing upon t h e r e s u l t s o f time-resolved measurements and model c a l c u l a t i o n s (se c ti o n I I ) , TEM measurements ( s e c t i o n IV.8),
and
experiments using d i f f e r e n t ambient gases and pressures (above), i t i s possible t o present a t l e a s t a q u a l i t a t i v e model w i t h which t o describe these i n t e r a c t i o n s , along t h e l i n e s suggested by Sat0 e t a1 (1982), Cohen e t a1 (1983, 1984), and Benti n i e t a1 (1982).
.
.
.
The depth p r o f i l e s o f As/Ga r a t i o i n Figure 25 show t h a t t h e r a p i d sol id i f ica t ion process f o l 1owi ng pul sed 1aser me1t ing o f GaAs i n a l - b a r ambient always r e s u l t s i n growth o f an As-lean s o l i d , accompanied by a surface "spike" of As-rich m a t e r i a l . This i s c l e a r evidence t h a t s o l i d i f i c a t i o n takes place from an As-rich l i q u i d j u s t i n f r o n t o f th e s o l i d i f y i n g i n t e r f a c e . However, i n a
544
D. H. LOWNDES
100-bar A r ambient t h e As/Ga r a t i o f o l l o w i n g r a p i d s o l i d i f i c a t i o n i s e s s e n t i a l l y u n i t y and th e re i s o n l y a small As surface spike, i n d i c a t i n g t h a t s o l i d i f i c a t i o n occurs from a much more n e a r l y s t o i c h i o m e t r i c , though s t i l l s l i g h t l y As-rich,
melt.
Sat0 e t a l .
(1982) argue t h a t t h i s i s a l s o c o n s i s te n t w i t h t h e known P-T proj e c t i o n f o r t h e GaAs system,
a pressure much h i g h e r than 1 bar
being required t o s o l i d i f y s t o i c h i o m e t r i c GaAs from an As-rich melt.
They also p o i n t out t h a t a high-pressure environment has
t h e e f f e c t o f in c re a s i n g th e As p a r t i a l pressure a t t h e molten GaAs surface.
An i d e a l gas theory c a l c u l a t i o n (Sato e t a ] . , 1982)
suggests t h a t t h e d i f f u s i o n l e n g th f o r As atoms i n t h e vapor phase decreases by about an order o f magnitude when P i s increased from
1 t o 100 bar ( t o -1,800 bar Ar).
A d u ri n g a 200-nsec m e l t d u r a t i o n i n 100-
Thus, once some As atoms have evaporated, an As p a r t i a l
pressure s u f f i c i e n t t o suppress f u r t h e r As evaporation may develop.
TEM micrographs (Figure 12) o f t h e f i n e l y dispersed c e l l u l a r s t r u c t u r e o f Ga-rich residue are a l s o explained i f a Ga-rich s o l i d grows from an As-rich melt:
The e q u i l i b r i u m Ga-As phase diagram
does not permit a Ga excess g re a te r than about 0.01% i n t h e s o l i d compound GaAs.
Excess Ga would be expected t o be r e j e c t e d i n t o
t h e surrounding m a t r i x as l i q u i d Ga and frozen a t lower temperature. TEM provides c l e a r evidence (see Fig. 12) o f t h e i n c r e a s i n g l y l a r g e
amounts of Ga t h a t are disposed o f i n t h i s way, w i t h i n c r e a s i n g El, when annealing i s c a r r i e d out i n a i r a t 1 bar.
VI.
Conclusions and Suggestions for Future Research
It has been e s ta b l i s h e d t h a t t h e mechanism f o r pulsed annealing
o f ion-implanted GaAs i s t h e r a p i d p e n e tr a t i o n of a m e l t i n g f r o n t through t he imp1a n t a t ion-damaged r e g i on,
f o l l owed by r a p i d (2-4
m/sec) e p i t a x i a l regrowth from t h e c r y s t a l l i n e s u b s t r a t e beneath. E a r l y u n c e r t a i n t i e s regarding t h e mechanism and e x t e n t o f dopant i o n r e d i s t r i b u t i o n d u ri n g pulsed annealing have been resolved; dopant r e d i s t r i b u t i o n occurs by l i q u i d -p h a s e d i f f u s i o n i n t h e
8.
545
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
m e l t , modified by n o n e q u i l i b r i u m s e g r e g a t i o n e f f e c t s a t t h e r a p i d l y moving r e c r y s t a l l i z i n g i n t e r f a c e ; and, t h e e x t e n t o f r e d i s t r i b u t i o n i s l i m i t e d by t h e maximum depth o f m e l t i n g . The few q u a n t i t a t i v e measurements o f t h e i n t e r f a c e s e g r e g a t i o n c o e f f i c i e n t , k i , f o r i n c o r p o r a t i o n o f dopant i o n s i n GaAs d u r i n g r a p i d s o l i d i f i c a t i o n reveal l a r g e increases o f k i from t h e values t y p i c a l o f n e a r - e q u i l i b r i u m c r y s t a l growth, as has been found f o r silicon.
There i s a l s o some evidence t h a t , f o r GaAs, k i depends
upon t h e ambient p r e s s u r e and upon i n t e r a c t i o n s between a r s e n i c atoms and oxygen from t h e ambient; these dependences appear t o be connected with t h e s u s c e p t i b i l i t y o f GaAs t o As l o s s and accelerat i o n o f As l o s s by p e n e t r a t i o n o f oxygen. A1 though o t h e r s have suggested t h a t s i g n i f i c a n t s u r f a c e decompos i t i o n o f GaAs occurs o n l y f o r h i g h p u l s e powers o r l o n g p u l s e durations, case:
h i g h r e s o l u t i o n TEM s t u d i e s show t h a t t h i s i s n o t t h e
A Ga-rich
stoichiometry,
residue,
i n d i c a t i v e o f near-surface
loss o f
forms f o r a l l p u l s e d energy d e n s i t i e s above t h e
m e l t i n g threshold.
Use o f a deposited A s - r i c h f i l m (e.g.,
AszSe3)
d u r i n g pulsed a n n e a l i n g can suppress f o r m a t i o n o f G a - r i c h g l o b u l e s . (Recent work i m p l i e s t h a t h i g h ambient pressure may a l s o be e f f e c t i v e , though t h i s has n o t been d i r e c t l y v e r i f i e d v i a TEM).
The
v o l a t i l i t y o f As makes GaAs more s u s c e p t i b l e than s i l i c o n t o damage r e s u l t i n g from pulsed l a s e r beam inhomogeneities; s u r f a c e topography f o l l o w i n g p u l s e d annealing w i t h s o l i d s t a t e l a s e r s i s s t r o n g l y dependent upon t h e use o f beam homogenizers b u t o p t i c a l micrographs r e v e a l f r o z e n - i n quasi - p e r i o d i c r i p p l e p a t t e r n s even when homogen i z e r s a r e used. The r e l a t i v e l y i n c o h e r e n t and h i g h l y homogeneous l i g h t pulses now a v a i l a b l e from pulsed excimer l a s e r s should be useful i n e l i m i n a t i n y these r i p p l e s , along w i t h t h e inconvenience o f u s i n g beam homogenizers. Most measurements o f e l e c t r i c a l a c t i v a t i o n o f implanted i o n s have followed pulsed annealing i n a i r o r vacuum ( t h e l a t t e r u s i n g e l e c t r o n beams).
Under these c o n d i t i o n s , pulsed annealing o f high-
dose i m p l a n t s produces h i g h e r c a r r i e r c o n c e n t r a t i o n s t h a n f u r n a c e
546
D. H. LOWNDES
a n n e a l i n g (-1020/cm3 f o r p-type and mid-1019/cm3 range f o r n-type i m p l a n t s ) . However, t h e f r a c t i o n o f i m p l a n t e d i o n s t h a t a r e e l e c trically type).
a c t i v a t e d i s never 100% (580% f o r
p-type,
<
50% f o r n-
Furthermore, t h e a c t i v a t i o n o f n-type i m p l a n t s i s o n l y
metastable, and decreases upon low temperature annealing.
Pulsed
m e l t i n g and r a p i d s o l i d i f i c a t i o n i s s u p e r i o r t o e i t h e r f u r n a c e a n n e a l i n g o r scanned cw beam a n n e a l i n g f o r removal o f i m p l a n t a t i o n damage.
Channeling measurements show a h i g h degree o f s u b s t i t u -
t i o n a l i t y o f b o t h h o s t and dopant atoms on l a t t i c e s i t e s f o l l o w i n g p u l s e d annealing.
Most i n t e r e s t i n g l y , channeling s t u d i e s show no
change i n t h e s u b s t i t u t i o n a l f r a c t i o n o f n-type dopant i o n s d u r i n g t h e low-temperature activation.
thermal
anneal i n g t h a t reduces e l e c t r i c a l
F i n a l l y , low-dose i m p l a n t s cannot be a c t i v a t e d w i t h
p u l s e d a n n e a l i n g and t h e c a r r i e r m o b i l i t i e s i n t h e c o n d u c t i n g l a y e r s produced by p u l s e d annealing o f high-dose i m p l a n t s a r e low i n r e l a t i o n t o measured c a r r i e r c o n c e n t r a t i o n s .
These o b s e r v a t i o n s a r e
c o n s i s t e n t with d i f f e r e n t i a l sheet e l e c t r i c a l p r o p e r t i e s , S c h o t t k y b a r r i e r and p-n extensive
j u n c t i o n measurements, a l l o f
compensation (,101*/cm3)
pm) r e g i o n o f even c-GaAs,
in
which demonstrate
t h e near-surface (0.1-0.3
f o l l o w i n g pulsed annealing.
T h i s com-
p e n s a t i o n r e s u l t s i n a low c o n d u c t i v i t y o r s e m i - i n s u l a t i n g nears u r f ace 1ayer. Considerable progress has been made q u i t e r e c e n t l y i n b e g i n n i n g t o i d e n t i f y t h e d e f e c t s r e s p o n s i b l e f o r c a r r i e r compensation i n p u l se-anneal ed GaAs.
The b e s t avai 1a b l e experimental evidence sug-
g e s t s t h a t compensation i s due t o ( i ) vacancies t h a t a r e quenched-in d u r i n g r a p i d s o l i d i f i c a t i o n ( w i t h vacancy f o r m a t i o n b e i n g enhanced by As l o s s near t h e s u r f a c e ) and t o (ii)oxygen from t h e n a t i v e o x i d e l a y e r ( a t low E l ) o r from t h e atmosphere ( a t h i g h e r El). There i s a1 so evidence t h a t oxygen-arsenic i n t e r a c t i o n enhances arsenic loss.
The observed deepening o f t h e compensated l a y e r upon
subsequent thermal annealing i s c o n s i s t e n t w i t h vacancy m i g r a t i o n and vacancy complexing w i t h dopant ions.
Q u a n t i t a t i v e measurements
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
o f oxygen-depth
547
p r o f i l e s show c l e a r l y t h a t oxygen i s present i n
pulse-annealed GaAs; t h i s b e h a v i o r i s u n l i k e t h a t o f s i l i c o n under s i m i l a r p u l s e d - l a s e r c o n d i t i o n s . A n t i - s i t e d e f e c t s r e s u l t i n g from r a p i d s o l i d i f i c a t i o n a l s o seem very l i k e l y b u t t h e r e appears t o be no d i r e c t evidence o f t h e i r presence. An attempt t o d e f i n e optimum pulsed l a s e r annealing c o n d i t i o n s f o r GaAs, keeping i n mind t h e r e c e n t d e f e c t - r e l a t e d s t u d i e s reviewed i n s e c t i o n s I V and V, l e a d s t h i s a u t h o r t o conclude t h a t almost a l l measurements t o d a t e of
e l e c t r i c a l a c t i v a t i o n f o l l o w i n g pulsed
a n n e a l i n g are c h a r a c t e r i s t i c o n l y o f non-optimal c o n d i t i o n s .
The
e x i s t e n c e o f o n l y a narrow E, window f o r annealing ( s e c t i o n 111) suggests t h a t o p t i m i z e d annealing should b e g i n w i t h s h a l l o w i o n implantation
and an
E,
just
sufficient
t o melt through the
implantation-damaged l a y e r , i n o r d e r t o minimize As l o s s and surf a c e decomposition. and
However, t h e s t u d i e s reviewed i n s e c t i o n s I V
V show t h a t c a r e should be taken t o remove t h e n a t i v e o x i d e
l a y e r , and t h a t annealing should be done i n an i n e r t atmosphere, t o avoid b o t h oxygen i n c o r p o r a t i o n from t h e atmosphere and (perhaps) As-0 i n t e r a c t i o n s t h a t enhance As l o s s .
However, a l a r g e body o f
evidence suggests t h a t quenched-in compensating d e f e c t s a r e i n h e r e n t t o r a p i d s o l i d i f i c a t i o n i n compound semiconductors under " o r d i n a r y " c o n d i t i o n s ( i n a i r , a t 1 bar, a t room temperature). Pulsed anneali n g i n an A s - r i c h environment (e.g., an As-bearing s u r f a c e f i l m ) can minimize As l o s s b u t a p p a r e n t l y does n o t e l i m i n a t e compensation r e s u l t i n g from r a p i d s o l i d i f i c a t i o n .
The l i m i t e d data a v a i l a b l e
suggest t h a t t h e most p r o m i s i n g t e c h n i q u e f o r m i n i m i z i n g ( h o p e f u l l y , e l i m i n a t i n g ) t h e problem o f compensating d e f e c t s i s t h e use o f a h i g h ambient pressure, perhaps supplemented by s u b s t r a t e heating. Finally,
use of an ambient atmosphere r u l e s out pulsed e l e c t r o n
beams; excimer l a s e r s appear t o be t h e most u s e f u l i n o r d e r t o o b t a i n s p a t i a l l y uniform annealing, though no data e x i s t a t present r e g a r d i n g annealing o f GaAs u s i n g p u l s e d - u l t r a v i o l e t l i g h t .
D. H. LOWNDES
Future Research This review demonstrates t h a t pulsed anneal i n g o f GaAs has reached t h e p o i n t a t which we have an a p p r e c i a t i o n o f what the problems are and,
i n general terms,
of their origin.
We are
l i m i t e d now by i n s u f f i c i e n t data upon which t o base a more d e t a i l e d fundamental understanding and t o guide t h e development o f annealing processes.
Further
research i n several
areas i s especi a1 l y
t i me1y :
Optimized Annealing Conditions.
The apparently b e n e f i c i a l
e f f e c t o f annealing i n a high-pressure,
i n e r t gas ambient, upon
t h e e l e c t r i c a l p r o p e r t i e s o f both c r y s t a l 1 i n e and ion-implanted GaAs, must be confirmed and then studied i n much more breadth and detail.
Pressure has been an underused v a r i a b l e f o r t h e improve-
ment o f e l e c t r i c a l p r o p e r t i e s and should be e x p l o i t e d p a r t i c u l a r l y i n d e a l i n g w i t h compound m a t e r i a l s .
The e f f e c t o f simultaneous
s u b s t r a t e heating should be explored t o determine whether t h e r e i s a t r a d e o f f between s u b s t r a t e heating and h i g h pressure t h a t could r e s u l t i n optimal GaAs pulsed annealing c o n d i t i o n s a t lower pressure.
It a l s o seems important t o check, a t an e a r l y stage, whether
t h e presence o r absence o f a n a t i v e oxide l a y e r has a d e t e c t a b l e e f f e c t on t h e e l e c t r i c a l p r o p e r t i e s o f pressure annealed c- o r 11GaAs.
F i n a l l y , t h e need f o r a high-pressure ambient could be turned
t o advantage, if photochemical doping can be c a r r i e d out from t h e ambient atmosphere f o r GaAs, as has been demonstrated (using pulsed u l t r a v i o l e t l a s e r s ) f o r s i l i c o n . Avoidance o f an i o n - i m p l a n t a t i o n step, which enhances arsenic loss, would be advantageous. Studies o f Quenched-in Defects.
More d e t a i l e d studies are
needed t o e s t a b l i s h t h e nature o f quenched-in compensating p o i n t d e f e c t s and d e f e c t c l u s t e r s i n GaAs and t o c o r r e l a t e p a r t i c u l a r d e f e c t types w i t h pulsed annealing c o n d i t i o n s : types (vacancies;
Studies o f d e f e c t
a n t i - s i t e d i s o r d e r ; oxygen-related),
of their
a s s o c i a t i o n w i t h p a r t i c u l a r dopant atoms o r w i t h oxygen, and o f t h e i r c o n c e n t r a t i o n as a f u n c t i o n o f annealing c o n d i t i o n s ( P , T,
8.
PULSED BEAM PROCESSING OF GALLIUM ARSENIDE
ambient atmosphere) are a l l needed.
549
The suggestion t h a t t h e poorer
e l e c t r i c a l a c t i v a t i o n and low m o b i l i t y f o r n-type implants i n GaAs might be due t o d e p l e t i o n regions surrounding extended defects, r e s u l t i n g i n p i n n i n g o f t h e Fermi l e v e l below midgap a t these " i n t e r n a l surfaces" and tested.
(Fan e t a1
., 1981),
should a l s o be explored
Wherever possible, e l e c t r i c a l - p r o p e r t i e s measurements
should be c a r r i e d out i n p a r a l l e l w i t h d e f e c t s t u d i e s using m a t e r i a l anneal ed under t h e same condi t ions.
Fundamental Studies o f Rapid Solidification i n Compound Semiconductors. This i s an e s p e c i a l l y r i c h area f o r f u t u r e research: Measurements o f s o l i d s o l u b i l i t y l i m i t s and o f i m p u r i t y segregation, as f u n c t i o n s o f t h e v e l o c i t y , v, o f t h e r e c r y s t a l l i z i n g i n t e r f a c e and f o r a v a r i e t y o f dopant ions, are needed i n order t o provide b a s l i n e data f o r modeling r a p i d - s o l i d i f i c a t i o n processes i n compound m a t e r i a l s generally,
not j u s t f o r compound semiconductors.
The
a v a i l a b i l i t y o f two d i f f e r e n t s u b l a t t i c e s f o r occupancy imp1 i e s both " s i t e " and " a n t i - s i t e " t r a p p i n g o f
dopant (and host) atoms.
Combining t h e s i t e - a n t i s i t e p o s s i b i l i t i e s w i t h evidence t h a t i n t e r face segregation c o e f f i c i e n t s and s o l i d - s o l u b i l i t y l i m i t s are funct i o n s o f v,
P, and p o s s i b l y also o f oxygen p a r t i a l pressure, it
seems l i k e l y t h a t we w i l l f i n d both new e f f e c t s and a r i c h e r v a r i e t y o f e f f e c t s than have been found f o r s i l i c o n . though:
A word o f caution,
Such fundamental studies must be c a r r i e d out w i t h c a r e f u l
a t t e n t i o n t o t h e c o n t r o l o f GaAs d i s s o c i a t i o n o r surface decomposit i o n , and w i l l c e r t a i n l y b e n e f i t from p r i o r d e f i n i t i o n o f "optimized pulsed-anneal i n g conditions,"
as suggested above.
Acknowledgments I t i s a pleasure t o be a b l e t o thank my colleagues G.
E.
J e l l i s o n , Jr. and R. F. Wood f o r s t i m u l a t i n g discussions and helpf u l suggestions over t h e course o f several years. F. W.
I a l s o thank
Young, Jr. f o r reading a f i r s t d r a f t o f t h i s chapter and f o r
550
D. H. LOWNDES
suggesting a number o f improvements t h a t were subsequently i n c o r porated.
To J u l i a Luck goes s p e c i a l thanks f o r t h e s k i l l and care
w i t h which she prepared t h e t y p e s c r i p t , making my j o b so much easier.
I would a l s o l i k e t o acknowledge h e l p f u l conversations
o r correspondence, C h r i s t i e , D. J.
over a p e r i o d o f several years, w i t h W.
H.
E. Davies, R. E. Eby, B. J. Feldman, P. H. Fleming,
Fletcher, L. 0. Hess, R. B. James, P. Pianetta, D. P r i b a t , F.
Sato, J. Siejka, and R. 0,. Westbrook.
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B e n t i n i , G. G., B e r t i , M., Cohen, C., Drigo, A. V., J a n i t t i , E., P r i b a t , D., and Siejka, J. (1982). J. de Physique 43, C1-229. Breeze, P. A., Hartnagel, H. L. , and Sherwood, P. M. A. (1980). J. Electrochem. SOC. 127, 454. Campisano, S. V., F o t i , G., Rimini, E., Eisen, F. H., and N i c o l e t , M. A. (1978). S o l i d S t a t e Elec. 21, 485.
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Campisano, S. V., F o t i , G. , Rimini, E., Eisen, F. H., Tseng, W. F., N i c o l e t , M. A., and Tandon, J. L. (1980). J. Appl. Phys. 51, 295. Chapman, R. L., Fan, J. L. C. , Donnelly, J. P. , Tsaur, B. -Y. (1982). Appl. Phys. L e t t . 40, 805. Cohen, C., Siejka, J., P r i b a t , D. , B e r t i , M., Drigo, A. V., B e n t i n i , G. G., and J a n n i t t i , E. (1983). J. de Physique Colloque. C5, 179. Cohen, C., Siejka, J. B e r t i , M., Drigo, A. V., B e n t i n i , G. G., P r i b a t , D., and J a n n i t t i , E. (1984). J. Appl. Phys. 55, 4081. C u l l i s , A. G., Webber, H. C., and B a i l e y , P. (1979). J. Phys. E: Sci. I n s t r u . 12, 688. C u l l i s , A. G., Weber, H. C., and Chew, N. G. (1982). Mat. Res. SOC. Symp. Proc. 4, 131. Davies, D. E., Ryan, T., and Lorenzo, J. P. (1980a). Appl. Phys. L e t t : 37, 443.Davies, D. E., Lorenzo, J. P., and Ryan, T. G. (1980b). Appl. Phys. L e t t . 37, 612. Davies, D. E., Ryan, T. G., and Lorenzo, J. P. (1981). Mat. Res. ~oc.-symp. Proi. 1, 247. Davies, D. E., Lorenzo, J. P. , Kennedy, E. F. , Ryan, T. G. (1982a). I n " I n t . Symp. GaAs and Related Compounds" ( I n s t . Phys. Conf. Ser. No. 63, Oiso, Japan), p. 255. I n s t i t u t e o f Physics, London. Davies, D. E., Lorenzo, J. P., Kennedy, E. F., and Ryan, T. G. (1982b). I n " I n t . Symp. GaAs and Related Compounds" ( I n s t . Phys. Conf. Ser. No. 63, Oiso, Japan), p. 389. I n s t i t u t e o f Physics, London. Davies, D. E. , McNally, P. J. , Ryan, T. G., Soda, K. J. , and Comer, J. J. (1983). I n " I n t . Symp. GaAs and Related Compounds" ( I n s t . Phys. Conf. Ser. No. 65, Albuquerque), p. 619. Institute o f Physics, London. Donnelly, J. P. (1977). I n "Int. Symp. GaAs and Related Compounds" ( I n s t . Phys. Conf. Ser. No. 33b, Oiso, Japan), p. 168. I n s t i t u t e o f Physics , London. Emerson, N. G., and Sealy, B. J. (1979). Electron. L e t t . 15, 553. Emerson, N. G., and Sealy, B. J. (1980). Electron. L e t t . 16, 512. Fan, J. C. C., Chapman, R. L., Donnelly, J. P., Turner, G. W., and Bozler, C. (1981). Mat. Res. SOC. Symp. Proc. 1, 261. Fan, J. C. C. and Johnson, N. M., eds. (1984). Mat. Res. SOC. Symp. Proc. 23. Feldman, B. J., and Lowndes, D. H. (1982). Appl. Phys. L e t t . 40, 59. F e r r i s , S. D., Leamy, H. J., and Poate, J. M., eds. (1979). Am. I n s t . Phys. Conf. Proc. 50. Fletcher, J . , Narayan, J., and Lowndes, D. H. (1981a). Mat. Res. SOC. Symp. Proc. 2, 421. Fletcher, J., Narayan, J., and Lowndes, D. H. (1981b). I n "Proceedings o f t h e 2nd Oxford Conference on Microscopy o f Semicond u c t o r Materials," p. 121. Oxford U n i v e r s i t y Press, England.
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Gamo, K., Yuba, Y., Oraby, A. H., Murakami, K., Namba, S., and Kawasaki, Y. (1980). I n "Laser and Electron-Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 322. Academic Press, New York. Gibbons, J. F., Hess, L. D., and Sigmon, T. W., eds. (1981). Mat. Res. SOC. Symp. Proc. 1. Golovchenko, J. A., and Venkatesan, T. N. C. (1978). Appl. Phys. L e t t . 32, 464 Harrison, H. B., and Williams, J. S. (1980). I n "Laser and Electron-Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 481. Academic Press, New York. Hoh, K., Koyama, H., Uda, K., and Miura, Y. (1980). Jap. J. Appl. Phys. 19, 1374. Inada, T. , Kato, S. , Maeda, Y. , and Tokunaga, K. (1979a). J. Appl. Phys. 50, 6000. Inada, T., Tokunaga, K. , and Taka, S. (1979b). Appl. Phys. L e t t . 35, 546. Kachurin, G. A., Pridachin, N. B., and Smirnov, L. S. (1976). Sov. Phys.-Semiconductors (Eng. t r a n s ? . f 9 , 946. Kular, S. S., Sealy, B. J. , Badawi, M. H., Stephens, G. K., Sadana, D., and Booker, G. R. (1979). Electron. L e t t . 15, 413. Lindhard, J., Scharff, M., and S c h i o t t , H. E. (1963). Kgl. Danske Videnskab Selskab., Mat. Fys. Medd. 33, No. 14. L i u , P. L., Yen, R., Bloembergen, N., and Hodgson, R. T. (1979). Appl. Phys. L e t t . 34, 864. L i u , Yung S., Chiang, S. U., and Bacon, F. (1981). Mat. Res. SOC. Symp. Proc. 1, 117. Lorenzo, J. P. , Davies, D. E. , and Ryan, T. G. (1979). Electrochem. SOC. 126, 118. Lou, C. Y., and Somorjai, G. A. (1971). J. Chem. Phys. 55, 4554. Lowndes, D. H., Cleland, J. W., C h r i s t i e , W. H., and Eby, R. E. (1981a). Mat. Res. SOC. Symp. Proc. 1, 223. Lowndes, D. H. , Cleland, J. W,, Fletcher, J. , Narayan, J. , Westbrook, R. D., Wood, R. F., C h r i s t i e , W. H., and Eby, R. E. (1981b). I n "Proceedings o f t h e F i f t e e n t h P h o t o v o l t a i c S p e c i a l i s t s Conference," p. 45. I E E E , New York. Lowndes, D. H., and Wood, R. F. (1981). Appl. Phys. L e t t . 38, 971. Lowndes, 0. H., and Feldman, B. J. (1982). Mat. Res. SOC. Symp. Proc. 4, 689. Lowndes, D. H., Cleland, J. W . , C h r i s t i e , W. H., Eby, R. E., J e l l i s o n , G. E., Jr. , Narayan, J . , Westbrook, R. D., Wood, R. F., Nilson, J. A., and Dass, S. C. (1982). Appl. Phys. L e t t . 41, 938. Lowndes, D. H., Cleland, J. W., C h r i s t i e , W. H., Eby, R. E., J e l l i s o n , G. E., Jr., Narayan, J., Westbrook, R. D., Wood, R. F., Nilson, J. A. , and Dass, S. C. (1983). Mat. Res. SOC. Symp. Proc. 13, 407. Lowndes, D. H., Wood, R. F., and Narayan, J. (1984). Phys. Rev. L e t t . 52, 561.
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Matsuure, M., Ishida, M., Suzuki, A., and Haza, K. (1981). Jap. J. Appl. Phys. 20, L726. Mizuta, M., Sheng, N. H., and Merz, J. L. (1981). Appl. Phys. L e t t . 38, 453. Mooney, P. M., Bourgoin, J. C., and I c o l e , J. (1981). Mat. Res. SOC. Symp. Proc. 1, 255. Narayan, J. Brown, W. L., and Lemons, R. A. , eds. (1983). Mat. Res. SOC. Symp. Proc. 13. Nojima, S. (1982). J. Appl. Phys. 53, 5028. N o r r i s , C. B., and Peercy, P. S. (1981). Appl. Phys. L e t t . 39, 351. Pianetta, P. A., S t o l t e , C. A., and Hansen, J. L. (1980). In "Laser and Electron-Beam Processing o f M a t e r i a l s " (C. W. White and P. S. Peercy, eds.), p. 328. Academic Press, New York. P i a n e t t a , P., Amano, J., Woolhouse, G., and S t o l t e , C. A. (1981). Mat. Res. SOC. Symp. Proc. 1, 239. Picraux, S. T. (1973). I n " I o n I m p l a n t a t i o n i n Semiconductors and Other M a t e r i a l s " (B. L. Crowder, ed.), p. 641. Plenum Press, New York. P r i b a t , D. (1982). I n "Cohesive P r o p e r t i e s o f Semiconductors Under Laser I r r a d i a t i o n " (L. D. Laude, ed.). The Hague, The Netherlands. P r i b a t , D. , Delage, S. , Dienmegard, D. , Croset, M., Srivastava, P. C., and Bourgoin, J. C. (1982). Mat. Res. SOC. Symp. Proc. 13, 647. Rose, A., P o l l o c k , J. T. A., S c o t t , M. D., Adarns, F. M., Williams, J. S., and Lawson, E. M. (1983). Mat. Res. SOC. Symp. Proc. 13, 633. Sato, F., Sunada, T., Chikawa, J. (1982). M a t e r i a l s L e t t . 1, 111. Sealy, B. J., Surridge, R. K., Kular, S. S., and Stephens, K. G. (1978). I n "Proc. I n t . Conf. Defects and R a d i a t i o n E f f e c t s i n Semiconductors" ( I n s t . Phys. Conf. Ser. No. 46), p. 476. I n s t i t u t e o f Physics , London. Sealy, B. J. , Badawi, N. H. , Kular, S. S. , and Stephens, K. G. (1979). Am. Inst. Phys. Conf. Proc. 50, 610. Shah, N. J., McMahon, R. A., Williams, J. G. S . , and Ahmed, H. (1981). Mat. Res. SOC. Symp. Proc. 1, 201. Skolnick, M. S., C u l l i s , A. G., and Webber, H. C. (1981). Appl. Phys. L e t t . 38, 464. S t r e e t , R. A., Johnson, N. M., and Gibbons, J. F. (1979). J. Appl. Phys. 50, 8201. Sze, S. M., and I r v i n , J. C. (1965). S o l i d S t a t e E I e c t r o n . 11, 599. Thurmond, C. D., Schwartz, G. P., Kammlott, G. W., and Schwartz, B. (1980). J. Electrochem. SOC. 127, 1366. Tsu, R., Hodgson, R. T., Tan, Teh Yu, and Baglin, J. E. (1979). Phys. Rev. L e t t . 42, 1356. Uebbing, R. H. , Wagner, P. , Baumgart, H., and Queisser, H. J. (1980). Appl. Phys. L e t t . 37, 1078. White, C. W. , and Peercy, P. S. , eds. (1980). "Laser and E l e c t r o n Beam Processing o f Materials. '' Academic Press, Mew York.
CHAPTER 9 PULSED CO,
LASER ANNEALING OF SEMICONDUCTORS
R. B. James
I.
.............. . . . . . . . . . . . -. . .. .. .. .. .. .. .. .. ... ... . . . .. .. .. .. .. .. .. .. .. .. . . . . .. ... ... ... ... ... ... ... ... ... ...............
INTRODUCTION. ABSORPTION OF CO, LASER RADIATION IN SEMICONDUCTORS 111. PULSED CO2 LASER ANNEALING OF SILICON 1. Optical Properties. 2. Recrystallization 3. Redistribution o f Implanted Dopants IV. MODEL CALCULATION OF SAMPLE HEATING V. INTERACTION OF HIGH-INTENSITY PULSED CO, LASER RADIATION WITH OTHER SEMICONDUCTORS. 4. Gallium Arsenide. 5. Indium Antimonide 6 . Germanium VI. SUMMARY AND CONCLUSIONS REFERENCES. 11.
I.
Introduction Although there exists an extensive number of experimental and theoretical reports on laser annealing o f semiconductors, only a few investigations have been conducted with a laser which has a photon energy smaller than the bandgap o f the material o f interest. For photon energies less than the intrinsic absorption edge, the generation o f electron-hole pairs by single-photon absorption is not energetically allowed, and another absorption mechanism is required to optically heat the sample. In most doped semiconductors, the absorption of be1 ow-bandgap radiation is domi nated by 555
Copyngh! 0 1984 by Avddemic Press. Inc All rights of reproduction i n any form reserved. ISBN 0-12-752123-2
556
R. B. JAMES
free-carrier transitions,
although l a t t i c e a b s o r p t i o n may a l s o be
s i g n i f i c a n t f o r photon e n e r g i e s i n t h e r e s t r a l e n r e g i o n o r i n a high r e s i s t i v i t y material.
I n m a t e r i a l s where t h e a b s o r p t i o n i s
dominated by f r e e - c a r r i e r t r a n s i t i o n s , t h e p e n e t r a t i o n depth o f t h e l i g h t i s a f u n c t i o n o f t h e i n i t i a l c a r r i e r d e n s i t y i n t h e sample. Consequently,
one has some c o n t r o l over t h e energy d e p o s i t i o n by
c o n t r o l l i n g t h e c a r r i e r concentration.
An i n c r e a s e i n t h e c o n t r o l
o f the absorption c o e f f i c i e n t i s especially useful f o r applications where one would l i k e t o achieve a deeper p e n e t r a t i o n depth t h a n i s a c h i e v a b l e w i t h a l a s e r having a photon energy which exceeds t h e bandgap.
I n a d d i t i o n , one can use d i f f e r e n t i a l a b s o r p t i o n between
doped and undoped l a y e r s t o m e l t near-surface
r e g i o n s which a r e
embedded i n a s i l i c o n sample w i t h o u t m e l t i n g t h e m a t e r i a l on e i t h e r t h e f r o n t o r back sides.
T h i s m e l t i n g phenomenon, which may be
d e s i r a b l e f o r c e r t a i n device a p p l i c a t i o n s , i s l i k e l y n o t o b t a i n a b l e w i t h a v i s i b l e or u l t r a v i o l e t laser. The purpose o f t h i s c h a p t e r i s t o i n v e s t i g a t e t h e o p t i c a l h e a t i n g of semiconductors by below-bandgap l a s e r r a d i a t i o n and t o review t h e conditions required f o r laser annealing o f ion-implanted layers via free-carrier
absorption.
Most o f t h e experimental
r e p o r t s on l a s e r - i n d u c e d m e l t i n g by f r e e - c a r r i e r a b s o r p t i o n have used l a s e r s w i t h wavelengths i n t h e 9- t o 11-pm region. p r i m a r i l y due t o t h e a v a i l a b i l i t y o f CO, outputs.
This i s
l a s e r s w i t h h i g h power
For l a s e r - p r o c e s s i n g a p p l i c a t i o n s ,
t h e CO,
laser also
has s e v e r a l d i s t i n c t advantages over o t h e r l a s e r systems i n t h a t i t has a h i g h s h o t - t o - s h o t ciency,
reproducibility,
high overall e f f i -
c a p a b i l i t y t o operate a t h i g h r e p e t i t i o n r a t e s ,
r e l a t i v e l y low cost.
Several i n v e s t i g a t i o n s on l a s e r a n n e a l i n g
of i o n - i m p l a n t e d s i l i c o n have been conducted w i t h a cw CO, ( C e l l e r e t al., T s i e n e t al.,
1979; Miyao e t a l .
, 1979;
Takai e t al.,
1981) and w i t h a Q-switched, p u l s e d CO,
e t al.,
1978; C e l l e r e t al.,
e t al.,
1981; Naukkarinen e t al.,
B o t h cw and p u l s e d CO,
and a
1979; B o r o f f k a e t al.,
laser
1980; and
laser (Celler
1980; Hauck
1982; and Blomberg e t al.,
1983).
l a s e r s have been found t o be u s e f u l i n
9.
557
PULSED C 0 2 LASER ANNEALING
c e r t a i n a p p l i c a t i o n s o f l a s e r processing o f semiconductors. example,
i t has been demonstrated by Tsien e t a l .
For
(1981) t h a t cw
COP l a s e r r a d i a t i o n can provide complete sol id-phase e p i t a x i a l regrowth o f t h e damaged l a y e r o f arsenic-implanted s i l i c o n t o o b t a i n e l e c t r i c a l p r o p e r t i e s comparable t o o r b e t t e r than those achieved by furnace anneal ing.
By using a pul sed COP 1aser, complete r e -
c r y s t a l l i z a t i o n o f s i l i c o n l a y e r s which have been amorphized by i o n i m p l a n t a t i o n has been achieved i n h e a v i l y doped samples by Naukkarinen e t a1
. (1982).
From t h e r e s u l t s o f time-resolved r e f l e c t i v i t y measurements, James e t a l . (1984a) have shown t h a t f o r a 70-ns C02 l a s e r pulse, energy d e n s i t i e s o f 2.9 J / c d are r e q u i r e d t o melt samples which have been u n i f o r m l y doped w i t h boron t o a c o n c e n t r a t i o n o f 2-3 x 1019 cm-3 and implanted w i t h arsenic ions a t an energy o f 185 KeV t o a dose o f 1 0 l 6
The r e s u l t s o f secondary i o n mass spectros-
copy (SIMS) measurements on these samples showed t h a t t h e arsenic i o n s r e d i s t r i b u t e up t o a depth o f about 1 pn, w i t h t h e depth cont r o l l e d by proper choice o f t h e i m p l a n t a t i o n parameters and C02 l a s e r energy density.
Transmission e l e c t r o n microscopy (TEM)
measurements (Narayan e t a1
., 1984a) on s i m i l a r
samples show t h a t
t h e d i s l o c a t i o n s associated w i t h i m p l a n t a t i o n damage are completely removed by pulsed C02 l a s e r annealing.
I n a d d i t i o n , good r e c r y s t a l -
l i z a t i o n has a l s o been achieved i n l i g h t l y doped ion-implanted
.,
samples w i t h a pulsed C02 l a s e r (see, f o r example, Blomberg e t a1 1983 and James e t al., 1984a), although t h e beam homogeneity o f
t h e C02 l a s e r becomes an i n c r e a s i n g l y more c r i t i c a l f a c t o r as t h e i n i t i a l c a r r i e r d e n s i t y o f t h e sample i s decreased. The emphasis o f t h i s chapter w i l l be on t h e use o f pulsed C02 l a s e r s t o anneal i m p l a n t a t i o n damage i n semiconductors.
Most o f t h e
chapter w i l l concentrate on t h e l a s e r annealing o f s i l i c o n because o f t h e technological s i g n i f i c a n c e o f t h e m a t e r i a l f o r t h e f a b r i c a t i o n o f e l e c t r o n i c devices and t h e a v a i l a b i l i t y o f experimental results.
Experimental r e s u l t s w i l l a l s o be presented on t h e i n t e r -
a c t i o n o f h i g h - i n t e n s i t y l a s e r r a d i a t i o n w i t h GaAs, InSb, and Ge.
558
R. B. JAMES
The r e s u l t s o f some o f t h e experiments reviewed i n t h i s c h a p t e r a r e r e p o r t e d as f u n c t i o n s o f t h e energy d e n s i t y o f t h e CO, w h i l e o t h e r s are r e p o r t e d as f u n c t i o n s o f i n t e n s i t y .
laser,
As a r e s u l t
b o t h energy d e n s i t i e s and i n t e n s i t i e s o f t h e l a s e r pulses appear as v a r i a b l e s throughout t h e chapter.
One can o b t a i n approximate i n t e n -
s i t i e s from measurements o f t h e energy d e n s i t y by assuming a r e c t a n g u l a r o p t i c a l pulse; however, t h e pulses a r e t y p i c a l l y spiky, so t h a t t h e i n t e n s i t i e s c a l c u l a t e d by t h i s method may be c o n s i d e r a b l y d i f f e r e n t t h a n t h e peak i n t e n s i t i e s i n t h e experiment.
I n general,
t h e energy d e n s i t y i s t h e p r e f e r r e d parameter i n d e s c r i b i n g l a s e r a n n e a l i n g experiments f o r which t h e p u l s e d u r a t i o n s a r e s h o r t compared t o t h e t i m e r e q u i r e d f o r t h e heat t o be conducted out o f t h e i n t e r a c t i o n region.
Thus, t h e energy d e n s i t y o f t h e beam i s more
a p p r o p r i a t e i n a n a l y z i n g experiments which use l a s e r s having a p u l s e d u r a t i o n o f tens o f nanoseconds o r s h o r t e r .
I f t h e a b s o r p t i o n co-
e f f i c i e n t i s an e x p l i c i t f u n c t i o n o f t h e l i g h t i n t e n s i t y ,
both
energy d e n s i t y and i n t e n s i t y are i m p o r t a n t parameters, r e g a r d l e s s o f t h e pulse duration.
11.
Absorption o f CO, Laser Radiation in Semiconductors
I n most semiconductors,
free-carrier
and l a t t i c e a b s o r p t i o n
dominate t h e l i g h t a t t e n u a t i o n when t h e photon energy i s somewhat l e s s t h a n t h e i n t r i n s i c a b s o r p t i o n edge. The c o n t r i b u t i o n from free-carrier
a b s o r p t i o n depends on t h e c a r r i e r d e n s i t y and i s
u s u a l l y t h e dominant a b s o r p t i o n mechanism i n doped m a t e r i a l s .
The
l a t t i c e a b s o r p t i o n o f C02 l a s e r l i g h t occurs t h r o u g h multiphonon events and i s s i g n i f i c a n t i n m a t e r i a l s which have a l o w f r e e - c a r r i e r concentration.
For many semiconductors t h e l a t t i c e a b s o r p t i o n co-
e f f i c i e n t a t room temperature i s l e s s t h a n 1 cm-1 f o r l i g h t w i t h a wavelength near 10 pm.
T h i s h i g h transparency o f i n t r i n s i c o r
l i g h t l y doped m a t e r i a l s a l l o w s one t o use some semiconductors (such as S i , Ge, and GaAs) as window m a t e r i a l s f o r CO,
lasers.
9.
559
PULSED COz LASER ANNEALING
The p r i n c i p a l mechanism f o r absorption of COP l a s e r l i g h t by f r e e elect rons i n n-type semiconductors i n v o l v e s i n t r a c o n d u c t i o n band t r a n s i t i o n s , i n which an e l e c t r o n absorbs a photon and makes a t r a n s i t i o n t o a higher energy s t a t e w i t h i n t h e same band.
The
t r a n s i t i o n s are i n d i r e c t i n k-space and r e q u i r e an intermediary t h i r d p a r t i c l e (such as a phonon o r i m p u r i t y ) t o conserve c r y s t a l momentum.
Experimental values f o r t h e f r e e - e l e c t r o n absorption
cross sect ion a t room temperature are shown i n Table I f o r several semiconductors. The absorption o f C 0 2 l a s e r l i g h t by f r e e holes i n most p-type semiconductors i s dominated by intervalence-band t r a n s i t i o n s , where a hole occupying a s t a t e i n th e heavy- o r l i g h t - h o l e band absorbs Table I Values f o r t h e l i n e a r intraconduction-band absorption cross s e c t i o n f o r 10.6-pm l i g h t a t 300 K. The wavelength dependence o f Q f o r each mate ri a l was approximated from t h e measured r e s u l t s . e M a t eri a1 Si
Ge GaAs A1 Sb GaSb InP GaP I nAs I nSb
Carrier Concentration (1017cm- 3
0.1-3.0 0.5-5.0 1.0-5 - 0 0.4-4.0 0.5 0.4-4.0
10.0
0.3-8.0
1.0-3.0
.
a S p i t z e r and Fan (1957).
ae(h=10.6 prn) (cd)
7 x 10-17 5 x 10-17 5 x 10-17 2 x 10-16 1 x 10-16 6 x lO-l7 4 x 10-16 8 x 10-17 3 x 10-17
Siregar e t a1 (1980). Fan e t al. (1956). S p i t z e r and Whelan (1959). Turner and Reese (1960). Becker e t al. (1968). 9 Newman (1958). Spit zer e t al. (1959). Dixon (1961). J Kurnick and Powell (1959). C
1
Wavelength Dependence
Reference
560
R. B. JAMES
a photon and makes a t r a n s i t i o n t o another band w i t h i n t h e valenceband s t r u c t u r e .
For example, i n p-type Ge and InSb, absorption o f
l i g h t w i t h a wavelength near 10 pm i s p r i m a r i l y by intervalence-band t r a n s i t i o n s between s t a t e s i n t h e heavy- and l i g h t - h o l e bands.
In
semiconductors w i t h small s p i n - o r b i t s p l i t t i n g s , such as s i l i c o n , h o l e t r a n s i t i o n s are e n e r g e t i c a l l y allowed between s t a t e s i n 1) t h e heavy- and 1ight-hole bands, 2) t h e heavy- and spl it - o f f h o l e bands, and 3) t h e l i g h t - and s p l i t - o f f h o l e bands.
Since these h o l e t r a n -
s i t i o n s are d i r e c t i n k-space, t h e free-hole cross s e c t i o n a t room temperature i s u s u a l l y much l a r g e r than t h e f r e e - e l e c t r o n cross section.
(Intravalence-band h o l e t r a n s i t i o n s a l s o e x i s t i n p-type
semiconductors,
but f o r C02 l a s e r r a d i a t i o n these t r a n s i t i o n s
usual l y have a small e r probabil it y than do t h e i n t e r v a l ence-band
.
hole transitions )
Val ues f o r t h e f r e e - h o l e absorption cross
s e c t i o n a t room temperature are given i n Table I1 f o r several semiconductors. Thus, t h e l i n e a r absorption c o e f f i c i e n t a o f l i g h t w i t h a wavel e n g t h i n t h e 9- t o 11-pn r e g i o n depends on both t h e f r e e - c a r r i e r d e n s i t y and t h e type o f c a r r i e r (since a = neae + nhoh).
This
a l l o w s considerable c o n t r o l over t h e p e n e t r a t i o n depth o f t h e l i g h t , s i n c e t h e p r i n c i p a l absorption mechanism i s an e x t r i n s i c p r o p e r t y o f most doped m a t e r i a l s .
Control o f t h e l i n e a r absorption c o e f f i c i e n t
i n t h e near-surface r e g i o n i s d i f f i c u l t , and t h e r e f o r e much more 1 i m i t e d , a t l a s e r wavelengths f o r which t h e absorption i s dominated by c a r r i e r t r a n s i t i o n s across t h e valence-conduction bandgap. I n h e a v i l y doped samples (>1019~ m - ~ )t h, e l i n e a r f r e e - c a r r i e r a b s o r p t i o n c o e f f i c i e n t a t room temperature i s about 103 cm-1 o r larger.
For an absorption c o e f f i c i e n t o f 103 cm-1, t h e l i g h t pene-
t r a t i o n depth i s several microns, which i s somewhat g r e a t e r than t h a t a t t a i n a b l e using a l a s e r w i t h a photon energy above t h e bandgap. Although t h e p e n e t r a t i o n depth o f CO, l a s e r r a d i a t i o n is r a t h e r deep, one expects t h a t a t h i g h i n t e n s i t i e s t h e energy d e p o s i t i o n r a t e w i l l be s u f f i c i e n t t o m e l t t h e near-surface r e g i o n without n e c e s s a r i l y i n v o l v i n g an intensity-dependent absorption mechanism.
9.
561
PULSED COz LASER ANNEALING
Table I 1 Values f o r t h e l i n e a r f r e e - h o l e a b s o r p t i o n c r o s s s e c t i o n q, f o r l i g h t w i t h a wavelength o f 10.6 p and l i g h t w i t h a wavelength o f 9.6 pm. A l l values a r e f o r a l a t t i c e temperature o f 300 K, except f o r InSb, which i s f o r a temperature o f 20 K.
Si Ge GaAs GaSb InAs InSb A1 Sb
7 x 6.1 4.2 3.2
7 x 5.7 3.9 3.1 8.7 2.4 3.2
10-17 a x x 10-16 C x 10-l6 8.6 x 10-16 e 2.5 x 10-15 f,g 4.0 x 10-16 C
10-17 a 10-16 10-16 10-l6 10-16 10-15 10-l6
x x x x x x
b c
d
e f,g C
a Hara and N i s h i (1966). K a i s e r e t a l . (1953). B r a u n s t e i n and Kane (1962). Becker e t a l . (1961). Matossi and S t e r n (1958). K u r n i c k and Powell (1959). g Jamison and Nurmikko (1979). I n moderately o r l i g h t l y doped m a t e r i a l , t h e l i n e a r a b s o r p t i o n c o e f f i c i e n t i s much s m a l l e r and,
consequently,
the penetration
depth o f t h e l i g h t i s much l a r g e r .
I n o r d e r t o achieve successful
a n n e a l i n g o f moderately o r 1 i g h t l y doped samples, t h e f r e e - c a r r i e r d e n s i t y must be increased i n o r d e r t o i n c r e a s e t h e c o u p l i n g o f t h e C02 l a s e r l i g h t t o t h e s u b s t r a t e . The most s t r a i g h t f o r w a r d way t o i n c r e a s e t h e c a r r i e r d e n s i t y i s by h e a t i n g t h e l a t t i c e , where t h e i n c r e a s e i n t h e e l e c t r o n and h o l e d e n s i t i e s r e s u l t s from t h e temperature dependence o f t h e i n t r i n s i c c a r r i e r c o n c e n t r a t i o n . A l t e r n a t i v e l y , one can i n c r e a s e t h e c a r r i e r d e n s i t y by s i m u l t a neously i r r a d i a t i n g t h e m a t e r i a l w i t h a l a s e r having a photon energy g r e a t e r t h a n t h e bandgap (Miyao e t a1
., 1979).
E l e c t r o n - h o l e p a i r f o r m a t i o n can a l s o be achieved by a m u l t i photon a b s o r p t i o n mechanism (Yuen e t a1
.,
1980).
Multiphoton
a b s o r p t i o n has been used by Hasselbeck and Kwok (1982) t o anneal
562
R . B. JAMES
InSb w i t h a CO, laser
l a s e r by a two-photon process.
annealing
by
mu1 t i p h o t o n
and
Possibilities for
subsequent
free-carrier
a b s o r p t i o n should a l s o e x i s t f o r o t h e r narrow bandgap semiconductors,
such as InAs and a l l o y s o f Hgl-xCdxTe
( B a h i r and K a l i s h ,
1981). For s u f f i c i e n t l y h i g h CO,
laser intensities,
nonequil ib r i u m
f r e e c a r r i e r s can a l s o be generated by impact i o n i z a t i o n processes. Here, t h e l a s e r heats t h e f r e e - c a r r i e r d i s t r i b u t i o n t o t h e e x t e n t t h a t a s i g n i f i c a n t f r a c t i o n o f t h e c a r r i e r d e n s i t y has an energy g r e a t e r t h a n t h e bandgap (as measured from t h e conduction band minima f o r e l e c t r o n s and valence band maximum f o r holes).
These
h i g h l y e n e r g e t i c f r e e c a r r i e r s can r e l a x by c r e a t i n g e l e c t r o n - h o l e p a i r s through i n v e r s e Auger events.
Electron-hole p a i r formation
by impact i o n i z a t i o n has been proposed t o e x p l a i n t h e n o n l i n e a r a b s o r p t i o n o f CO,
l a s e r l i g h t i n n-type InAs, InSb, Hg0.77Cd0.23Te
(Jamison and Nurmikko,
1979),
and Ge (James and Smith,
1982a).
Approximate values f o r t h e i n t e n s i t i e s a t which c a r r i e r m u l t i p l i c a tion
by
impact
ionization
becomes
important
have
also
r e p o r t e d f o r several o t h e r semiconductors (James, 1983).
been
The Sen-
e r a t i o n o f n o n e q u i l i b r i u m e l e c t r o n - h o l e p a i r s by i m p a c t - i o n i z a t i o n events suggests t h e p o s s i b i l i t y o f l a s e r - p r o c e s s i n g free-carrier
samples v i a
a b s o r p t i o n w i t h p r a c t i c a l l y any below-bandgap
laser
r a d i a t i o n , p r o v i d e d t h e i n t e n s i t y o f t h e pulsed e x c i t a t i o n source i s s u f f i c i e n t l y g r e a t t o c r e a t e an e l e c t r o n - h o l e plasma.
111.
PULSED CO, LASER ANNEALING OF SILICON
I n t h e f a b r i c a t i o n of many e l e c t r o n i c devices, one i s r e q u i r e d t o grow both doped and s t r u c t u r e d t h i n l a y e r s o f s i l i c o n and s i l i c o n dioxide.
The a p p l i c a t i o n o f l a s e r - p r o c e s s i n g techniques t o t h e
m a n u f a c t u r i n g o f s i l i c o n devices has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n due t o t h e p o s s i b i l i t i e s o f i n c r e a s e d c o n t r o l o f j u n c t i o n depths, c a r r i e r c o n c e n t r a t i o n s i n doped l a y e r s , and c a r r i e r l i f e t i m e s .
The
c a p a b i l i t y o f c o n t r o l l i n g these c r i t i c a l parameters should r e s u l t
9.
i n improved d e v i c e performance.
I n t h i s section,
r e s u l t s are presented on t h e pulsed CO, i s organized as f o l l o w s : o f h i g h - i n t e n s i t y CO, next,
experimental
l a s e r annealing o f i o n -
i m p l a n t e d s i l i c o n t o o b t a i n device-grade m a t e r i a l .
of silicon;
563
PULSED C 0 2 LASER ANNEALING
The s e c t i o n
F i r s t , r e s u l t s a r e given f o r t h e e f f e c t
l a s e r r a d i a t i o n on t h e o p t i c a l p r o p e r t i e s
the r e c r y s t a l l i z a t i o n o f ion-implanted layers
t h a t have been melted by a CO,
l a s e r i s discussed; and f i n a l l y ,
r e s u l t s are presented f o r t h e r e d i s t r i b u t i o n o f imp1 anted dopants i n t h e near-surface region.
1.
OPTICAL PROPERTIES Time-resolved r e f l e c t i v i t y and transmi s s i v i t y measurements have
have been performed on s i l i c o n t o determine t h e onset o f n o n l i n e a r a b s o r p t i o n , t h e t h r e s h o l d f o r l a s e r - i n d u c e d m e l t i n g , and t h e m e l t durations.
R e s u l t s w i l l f i r s t be presented f o r t h e t i m e - r e s o l ved
r e f l e c t i v i t y measurements,
f o l l o w e d by t h e r e s u l t s f o r t h e time-
r e s o l v e d t r a n s m i s s i v i t y measurements. I n t h e experiments o f Naukkarinen e t a l . (1982) , a t r a n s v e r s e l y e x c i t e d atmospheric (TEA) CO,
l a s e r was used as an e x c ' i t a t i o n source.
The pulses had a d u r a t i o n o f 100 ns and maximum energy o f 2 J.
The
r e f l e c t i v i t y measurements shown i n Fig. 1 were performed as a funct i o n o f t h e l i g h t i n t e n s i t y on a l i g h t l y doped sample (5 x 1015 boron atom/cm3) and a h e a v i l y doped sample ( 5 x 1019 boron atoms/cm3).
A
photon drag d e t e c t o r and f a s t o s c i l l o s c o p e were used t o measure t h e i n t e n s i t y o f t h e r e f l e c t e d pulses.
The r e f l e c t a n c e o f t h e l i g h t l y
doped sample stayed constant f o r i n t e n s i t i e s i n t h e range o f 0.1 t o 100 MW/cm2.
I n t h e experiments, a i r breakdown would o c c a s i o n a l l y
occur near t h e focus.
I f t h e a i r breakdown occurred between t h e
sample and t h e d e t e c t o r i n t h e r e f l e c t a n c e measurements, no r e f l e c t e d p u l s e was observed.
I n t h e h e a v i l y doped sample, t h e r e f l e c t a n c e
remained constant up t o i n t e n s i t i e s i n t h e range o f 20-35 MW/cm2, t h e n i t began t o i n c r e a s e r a p i d l y from about 40-45% t o 80-100% as t h e i n t e n s i t y was f u r t h e r increased.
564
R. B. JAMES
R
0
:I , 2
4 0
0.4
1
d-\,
40 20 40 100200 I(MWcm-2)
Fig. 1 . Reflectance for S i doped with ( a ) 5 x 1 0 1 5 B a t o m s / c m 3 , ( b ) 5 x 1019
B atoms/cm3 and transmittance for Si doped with ( c ) 5 x 1015 B atoms/cm3, ( d ) l o z o B atoms/cm3 down to a 3.5-pm depth as a function o f the incident laser intensity a t 1 0 . 6 pm.
Time-resolved
[ A f t e r Naukkarinen et al.
(1982).]
r e f l e c t i v i t y measurements were
a l s o made by
Naukkarinen e t a l . (1982) f o r a sample i n which 1020 boron atoms/cm3 were d i f f u s e d t o a depth o f 3.5 crystal.
Dm i n t o a l i g h t l y doped s i l i c o n
The onset o f m e l t i n g m a n i f e s t s i t s e l f by a change i n
t h e slope o f the r e f l e c t i v i t y , since the r e f l e c t i v i t y o f molten s i l i c o n a t 10.6 MW/crn2,
prn i s c l o s e t o u n i t y .
F o r an i n t e n s i t y o f 60
the duration of the high-reflectivity
phase i s measured
t o be about 500 ns, a l t h o u g h t h e u n c e r t a i n t y i s f a i r l y l a r g e i n comparing t h e r a t i o s o f t h e i n c i d e n t and r e f l e c t e d pulses.
9.
565
PULSED COz LASER ANNEALING
A b e t t e r way t o m o n i t o r t h e h i g h - r e f l e c t i v i t y phase i s by u s i n g a probe l a s e r and measuring t h e t i m e - r e s o l v e d r e f l e c t i v i t y o f t h e probe (Auston e t al.,
1978).
The t r a n s i e n t r e f l e c t i v i t y o f a cw
633-nm HeNe l a s e r i n t h e presence o f a h i g h - i n t e n s i t y
CO, l a s e r
p u l s e has been s t u d i e d by two groups (Hasselbeck and Kwok, 1983 and James e t al.,
1984a).
R e f l e c t i o n o f t h e probe beam from t h e
s i l i c o n s u r f a c e was measured as a f u n c t i o n o f t h e pump i n t e n s i t y . The experiments by Hassel beck and Kwok (1983) were performed w i t h a s i n g l e l o n g i t u d i n a l mode TEA CO,
laser.
The c r y s t a l l i n e
samples used i n t h e s t u d i e s were u n i f o r m l y doped w i t h antimony t o a r e s i s t i v i t y o f 0.02 a-cm, which corresponds t o an e l e c t r o n conc e n t r a t i o n o f about 1.5 x l O l *
Both t h e CO,
l a s e r and t h e
u n p o l a r i z e d cw HeNe probe l a s e r were focused o n t o t h e sample. The angle o f i n c i d e n c e o f t h e probe beam was 45" and t h e pump beam was s e t a t normal incidence.
The HeNe l a s e r was focused o n t o t h e
sample and a s i l i c o n photodiode w i t h a 3-ns response t i m e was used t o monitor t h e probe beam. i n Fig.
Fig. 2.
The t i m e - r e s o l v e d r e f l e c t i v i t y i s shown
2 f o r a 100-ns l a s e r p u l s e a t an i n t e n s i t y o f 78 MW/cmZ.
Transient reflectivity o f HeNe probe laser as induced by a 100-nsec
C 0 2 laser pulse with an intensity of 78 MW/crn2. division.
Vertical scale:
Kwok ( 1 9 8 3 ) .]
Horizontal scale: 0.5 us/ 15% reflectivity/division. [ A f t e r Hasselbeck and
566
.
R. B. JAMES
The r e f l e c t i v i t y increases r a p i d l y t o a constant value where i t remains f o r an extended p e r i o d before decreasing t o i t s o r i g i n a l value.
The f l a t t o p i s measured t o have a r e f l e c t i v i t y o f 70 t
5%, which agrees w i t h t h e r e f l e c t i v i t y value f o r molten s i l i c o n a t 633 nm (Lowndes e t al.,
1982 and Kwok e t al.,
1981).
The
d u r a t i o n o f t h e h i g h - r e f l e c t i v i t y phase i s found t o decrease a t lower pump i n t e n s i t i e s . I n t h e experiments by James e t al. w i t h a pulse d u r a t i o n o f 70 ns was used.
(1984a), a TEA CO,
laser
The wavelength was con-
t r o l l e d by an i n t e r n a l g r a t i n g t h a t allowed a tunable output over t h e 9- t o ll-pm region.
For a low n i t r o g e n mix, t h e output pulse
had a maximum energy o f 5 J and was r e p r o d u c i b l e t o w i t h i n 2% from p u l s e t o pulse.
The l a s e r pulses, a f t e r impingement on a C02 l a s e r
beam i n t e g r a t o r , had a s i z e o f 12 x 12 m i n t h e t a r g e t plane and a u n i f o r m i t y o f +_lo%,according t o t h e s p e c i f i c a t i o n s o f t h e i n t e grator.
The energy d e n s i t y i n t h e t a r g e t plane o f the i n t e g r a t o r
was a d j u s t a b l e up t o about 3.1 J/cm2 f o r a wavelength o f 10.6 mn. When higher energy d e n s i t i e s were required, a germanium lens w i t h
a f o c a l l e n g t h o f 100 mn was used a t t h e t a r g e t plane o f t h e i n t e g r a t o r . The samples were u n i f o r m l y doped w i t h boron and, p r i o r t o i o n implantation, had a r e s i s t i v i t y of 0.0073 62-cm a t room temperature,
which corresponds t o a h o l e concentration o f about 2-3 x
loi9
The near-surface l a y e r o f each sample was amorphized by i m p l a n t a t i o n o f As ions a t an energy o f 180 KeV t o a dose o f 10l6
cw2.
The r e f l e c t a n c e o f t h e C02 pulse on t h e s i l i c o n sample was
measured t o be about 0.40
a t low pump i n t e n s i t i e s .
A t energy
d e n s i t i e s greater than about 2.9 J/cm2, t h e r e f l e c t a n c e o f t h e C02 r a d i a t i o n from t h e ion-implanted surface was observed t o increase i n a manner s i m i l a r t o t h a t reported by Naukkarinen e t a l . (1982) f o r h e a v i l y doped c r y s t a l l i n e s i l i c o n . Time-resolved r e f l e c t i v i t y measurements were a l s o c a r r i e d out on t h e ion-implanted samples by James e t a1
. (1984a)
t o determine t h e d u r a t i o n o f t h e h i g h - r e f l e c t i v i t y phase as a f u n c t i o n o f t h e energy d e n s i t y o f t h e Cop-laser pulse.
A cw 633-nm HeNe l a s e r was used t o measure t h e time-resolved
9.
567
PULSED CO2 LASER ANNEALING
o p t i c a l r e f l e c t i v i t y of t h e implanted surface d u r i n g and immediately a f t e r i r r a d i a t i o n w i t h a CO, l a s e r pulse. The angle o f incidence o f t h e unfocused HeNe probe l a s e r was 30" from t h e surface normal, and t h e CO, l a s e r beam impinged on t h e sample a t normal incidence.
A
s i 1i c o n avalanche photodiode and a f a s t o s c i l l oscope were used t o monitor t h e r e f l e c t i v i t y o f t h e probe beam.
Narrow-band-pass HeNe
f i l t e r s were placed d i r e c t l y i n f r o n t o f t h e photodiode t o attenuate s c a t t e r e d r a d i a t i o n from t h e s i l i c o n surface.
The observed t r a n -
s i e n t r e f l e c t i v i t y s i g n a l s were s i m i l a r i n shape t o those r e p o r t e d by Auston e t al. (1978).
For energy d e n s i t i e s exceeding t h e m e l t
threshold, t h e s i g n a l s consisted o f a f l a t t o p w i t h a d u r a t i o n t m and a decaying t a i l , f o l l o w i n g t h e h i g h - r e f l e c t i v i t y phase.
The
r e f l e c t i v i t y d u r i n g t h e h i g h - r e f l e c t i v i t y phase was approximately two times i t s i n i t i a l value ( R o ) ,
corresponding t o a r e f l e c t i v i t y
o f about 70%, which i s c o n s i s t e n t w i t h t h e measured r e f l e c t i v i t y o f molten s i l i c o n .
The r e f l e c t i v i t y was c a l i b r a t e d by using a
chopper i n t h e path o f t h e HeNe l a s e r and equating t h e signal w i t h out t h e CO, l a s e r w i t h t h e known r e f l e c t a n c e o f amorphous s i l i c o n . Values f o r t h e m e l t d u r a t i o n s were taken by adding t h e times, t m and tf, where tf i s t h e t i m e r e q u i r e d f o r t h e r e f l e c t i v i t y R t o f a l l t o a value a t which (R-Ro)
= 0.5 Ro.
The values f o r t h e m e l t
d u r a t i o n s as a f u n c t i o n o f t h e energy d e n s i t y o f t h e C02 l a s e r pulse a r e shown i n Fig. 3.
( I n t h e present case, approximate i n t e n s i t i e s
can be deduced from measurements o f t h e energy d e n s i t y by assuming a r e c t a n g u l a r pulse o f 70 ns. The pulses, however, are c l o s e r t o Gaussian w i t h about 80% of t h e energy i n a spike ( f o r a low N2 m i x ) w i t h a FWHM o f 70 ns, and t h e remainder i n a long t a i l which l a s t s f o r several hundred nanoseconds.)
The values f o r t h e m e l t dura-
t i o n s o f t h e h e a v i l y doped, ion-implanted samples are found t o be as long as 1 p s , which i s considerably l o n g e r than t h e observed m e l t d u r a t i o n s using l a s e r s w i t h a photon energy g r e a t e r than t h e bandgap (see, f o r example, Lowndes e t a1
., 1984 and Chapter 6).
These m e l t
d u r a t i o n s are s t i l l somewhat smaller than t h e melt d u r a t i o n s o f c r y s t a l 1i n e s i 1 i c o n reported by Hassel beck and Kwok (1983).
568
R. B. JAMES
1000
I
I
I
I
I
a -
- C02 CO? LASER ENERGY DENSITY
1
g
-SURFACE MELT DURATION VS
-7
i -
F & M OF PULSE = 70 nS 800 t-FWHM h = 10.6pm
-
600 8
400
2o i
rn
THRESHOLD
01.o
3.0
5.0
7.0
9.0
ENERGY DENSITY (J/cm2)
Fig. 3. Measured duration of the high-reflectivity phase versus the energy density o f the pulsed CO2 laser. The wafers were doped with boron and had a resistivity o f 0.0073 8-cm a t room temperature prior to implantation. The nearsurface region of the samples was amorphized by implantation o f 75As ions a t an energy of 180 KeV to a dose of 10l6 crne2.
The transmittance o f l i g h t l y doped p-type s i l i c o n samples has been measured as a f u n c t i o n o f t h e C02 l a s e r i n t e n s i t y .
I n the
experiments by James e t al. (1982b), a TEA l a s e r o p e r a t i n g i n t h e TEMoo mode was used. The output pulse had about 50% o f t h e energy i n t h e form o f a spike w i t h a FWHM o f 40 ns. The remainder o f t h e energy was i n a l o n g t a i l which l a s t s about 0.4 ps (depending on t h e N2 mix).
An i r i s was used t o i s o l a t e t h e c e n t r a l p o r t i o n of
t h e Gaussian beam i n order t o minimize t h e s p a t i a l v a r i a t i o n o f t h e beam.
The power d e n s i t y was c o n t r o l l e d by using c a l i b r a t e d
CaF2 attenuators, ZnSe o p t i c a l l e n s f o r focusing, and by a d j u s t i n g t h e high-voltage power supply.
The samples were coated f o r broad-
band a n t i r e f l e c t i o n a t a wavelength o f 10 pm and normal incidence. I n t h i s way, transmission data could be obtained as a f u n c t i o n o f t h e C02 l a s e r i n t e n s i t y w i t h approximately u n i f o r m s p a t i a l v a r i a t i o n
9.
569
PULSED CO;! LASER ANNEALING
and high energy r e p r o d u c i b i l i t y (1-2%) from shot t o shot.
The
maximum peak i n t e n s i t y used i n t h e experiment was i n t h e range o f 50 t o 100 MJ/cm2, a t which p o i n t a spark appears a t t h e surface o f
t h e s i l i c o n and damages t h e a n t i r e f l e c t i o n coatings.
The t r a n s -
mittance was observed t o increase s l i g h t l y f o r i n t e n s i t i e s up t o about 40 MW/cm2, which was a t t r i b u t e d t o a s t a t e - f i l l i n g e f f e c t i n t h e intervalence-band t r a n s i t i o n s (James and Smith, 1981).
For
i n t e n s i t i e s i n t h e range o f about 40 t o 70 MW/cm2, t h e t r a n s m i t tance remained a t a constant value.
A t i n t e n s i t i e s h i g h enough
f o r a spark t o appear a t t h e surface o f t h e s i l i c o n , t h e r e was a drop i n the transmittance, which was a t t r i b u t e d t o damage o f t h e a n t i r e f l e c t a n c e coatings and t o t h e generation o f an increased number o f f r e e c a r r i e r s by t h e v i s i b l e f l a s h o f l i g h t .
Similar
measurements on an n-type sample which was l i g h t l y doped w i t h antimony (8 x lOl5 antimony atoms/cm3) showed no n o t i c e a b l e change i n t h e transmittance f o r C02 l a s e r i n t e n s i t i e s l e s s than t h e damage t h r e s h o l d o f t h e a n t i r e f l e c t i o n coatings. The t r a n s m i t t a n c e o f l i g h t l y boron-doped s i l i c o n (5 x 1015 has also been i n v e s t i g a t e d by Naukkarinen e t a1
. (1982).
The t r a n s -
m i t t a n c e o f a 100-ns pulse w i t h a wavelength o f 10.6 pm was found t o remain almost constant f o r measured i n t e n s i t i e s up t o 100 MW/cm2 (Fig.
1).
When a l i g h t f l a s h would occur a t t h e surface, a drop
i n t h e transmission was observed, i n agreement w i t h t h e observation o f James e t a1
. (1982b).
.
Transmission measurements were a1 so made by Naukkari nen e t a1 t h i c k h e a v i l y boron-doped l a y e r (-102O cm-3) formed by d i f f u s i o n . For l a s e r i n t e n s i t i e s l e s s than (1982) on samples w i t h a 3 . 5 - p
about 20 W/cm2, t h e transmittance was constant, but, i n t h e range of
about 20 t o 60 MW/cm2,
(Fig.
Id).
t h e transmittance decreased r a p i d l y
The drop i n t h e t r a n s m i t t a n c e a t i n t e n s i t i e s g r e a t e r
than 20 MW/cmz i s due t o t h e heating o f t h e surface region, which causes an increase i n both t h e r e f l e c t a n c e and t h e absorption c o e f f i c i e n t o f the diffused layer.
570
R. B. JAMES
100
a
I ( R E LATlVE UNITS)
0 5 IT
0 4 IT
0 1
L
IT
0
500
0 t
Fig.
4.
Time dependence of
(ns)
( a ) the incident
laser pulse and o f the
transmitted intensity I T ( b ) with I = 8 MW/cm2, ( c ) I = 21 MW/cm2, and ( d ) I = 60 MW/cm2. The sample was doped with 1020 boron a t o m s / c m 3 t o a depth o f 3 . 5 pm. [ A f t e r Naukkarinen e t a l . ( 1 9 8 2 ) . ]
Time-resolved t r a n s m i s s i o n measurements were a1 so made same samples by Naukkarinen e t al. (1982).
o f t h e i n c i d e n t p u l s e i s shown i n Fig. 4a.
on t h e
The temporal dependence The p u l s e has a l a r g e
peak w i t h a FWHM o f approximately 100 ns f o l l o w e d by a l o n g t a i l which l a s t s about 1 p s .
The p u l s e t r a n s m i s s i o n f o r an i n c i d e n t
l a s e r i n t e n s i t y o f 8 MW/cm2 i s shown i n Fig. intensity,
4b.
A t t h i s laser
t h e t r a n s m i t t e d p u l s e has about t h e same shape as t h e
9.
571
PULSED CO2 LASER ANNEALING
i n c i d e n t pulse, and t h e a b s o r p t i o n appears t o be l i n e a r .
For an
i n c i d e n t l a s e r i n t e n s i t y o f 21 MW/cm2, t h e t r a n s m i t t e d p u l s e w i d t h begins t o s h o r t e n (Fig. 4c).
The onset o f t h e decrease i n t h e w i d t h
o f t h e peak i n t h e p u l s e shape i s c o n s i s t e n t w i t h t h e measured i n c r e a s e i n t h e r e f l e c t a n c e , as shown i n Fig. 1.
The t r a n s m i s s i o n
a t an i n c i d e n t i n t e n s i t y o f 60 MW/cm2 i s shown i n Fig.
4d,
in
which case t h e r e i s a s i g n i f i c a n t decrease i n t h e d u r a t i o n o f t h e Gaussi an-1 ike peak. I n view o f t h e o p t i c a l measurements discussed above and t h e ripple-like
f e a t u r e s t h a t appear on t h e s u r f a c e a t h i g h l i g h t
i n t e n s i t i e s (James e t al.,
1984a), t h e occurrence o f sample h e a t i n g
and subsequent me1t i n g seems t h e obvious e x p l a n a t i o n o f t h e l a s e r induced changes.
From t h e t i m e - r e s o l v e d r e f l e c t i v i t y measurements
o f James e t a l .
(1984a),
i t i s found t h a t t h e onset o f m e l t i n g
depends on t h e i n i t i a l f r e e - c a r r i e r d e n s i t y i n t h e m a t e r i a l .
A
t h r e s h o l d v a l u e a t which thermal m e l t i n g occurs i s about 3 J/cm2 f o r t h e h e a v i l y doped samples used i n t h e experiment.
The r e l a -
t i v e l y l o n g m e l t d u r a t i o n s , which have been measured by Naukkarinen e t a1
. (1982),
Hassel beck and Kwok (1983) , and James e t a1
. (1984a) ,
s t r o n g l y suggest t h a t t h e r e c r y s t a l 1 i z a t i o n times are c o n s i d e r a b l y 1 onger t h a n those observed i n laser-anneal ing experiments t h a t use l a s e r r a d i a t i o n i n t h e v i s i b l e r e g i o n o f t h e spectrum.
The measure-
ments o f Hasselbeck and Kwok (1983) on c r y s t a l l i n e s i l i c o n i r r a d i a t e d w i t h a CO, l a s e r showed a m e l t d u r a t i o n l a s t i n g almost a f a c t o r o f t e n l o n g e r than t h e d u r a t i o n s a s s o c i a t e d w i t h ruby and Nd:YAG l a s e r s . These l o n g e r r e c r y s t a l l i z a t i o n t i m e s f u r t h e r suggest t h a t t h e m e l t depths a r e somewhat g r e a t e r than had been p r e v i o u s l y achieved u s i n g a l a s e r which has a photon energy g r e a t e r t h a n t h e i n t r i n s i c a b s o r p t i o n edge. 2.
RECRYSTALLIZATION For pulsed CO,
l a s e r annealing t o be u s e f u l i n t h e processing
of e l e c t r o n i c devices, t h e removal o f damage a s s o c i a t e d with t h e i o n i m p l a n t a t i o n and an a p p r e c i a b l e a c t i v a t i o n o f t h e implanted
572
R. B. JAMES
species must occur. pulsed
Naukkarinen e t a l . (1984a),
The r e c r y s t a l l i z a t i o n o f m e l t e d l a y e r s by
CO, l a s e r s has been i n v e s t i g a t e d by C e l l e r e t a l . (1978), (1982), Blomberg e t a l .
and Narayan e t a l .
backscattering spectra (TEM),
(1984a)
,
(RBS)
(1983), James e t a l .
by t h e use o f R u t h e r f o r d
transmission
and t r a n s m i s s i o n s y n c h r o t r o n x - r a y
e l e c t r o n microscopy topographs.
Results
w i l l f i r s t be presented f o r t h e a n n e a l i n g o f l i g h t l y doped s i l i c o n samples, f o l l o w e d by t h e r e s u l t s f o r h e a v i l y doped samples. Attempts t o anneal a l i g h t l y doped sample w i t h 5 x l o i 5 boron atoms/cm3 by Naukkarinen e t a l .
(1982) were o n l y p a r t l y s u c c e s s f u l .
When t h e sample was n o t preheated, t h e b e s t c r y s t a l l i z a t i o n a t t a i n e d i s shown by c u r v e f i n Fig.
575 C,
5.
By p r e h e a t i n g t h e s u b s t r a t e t o
good r e c r y s t a l l i z a t i o n o f a l i g h t l y doped sample ( 7 x 1015
(3111-3) was achieved by Blomberg e t a l .
(1983).
The c h a n n e l i n g
3000
-
I
P
c
0
0
2000
J W
-
>
(3
5
LL W
c
+ a
. -
1000
*-.
I
CJ
ln Y V
d
a
0
m
b
80
400
120 440 C H A N N E L NUMBER
160
4 80
Fig. 5. 4He-ion backscattering spectra o f Si doped with 5 x 1019 B atoms/cm3: ( a ) random, (b) <001>-aligned incidence on a 7.5 x 15N+ ions/cm2 implanted sample, ( c ) a f t e r annealing with a 21-MW/cm2 pulse, ( d ) a f t e r a 40-MW/cm2 pulse , and ( e ) the <001>-aligned spectrum o f the unimplanted crystal. The <001>-aligned spectrum o f Si doped with 5 x 1015 B atoms/crn3 and implanted with 7.5 x 1015 15N ions/cm2 a f t e r a 150-MW/cm2 laser pulse i s given i n ( f ) . [ A f t e r Naukkarinen e t al. ( 1 9 8 2 ) . ]
9.
573
PULSED COz LASER ANNEALING
e
2E 3500 3
0 0
u
a
0
cn ..
Y 0
a
m
50
4 00 150 C H A N N E L NUMBER
200
Fig. 6. 4He+-ion backscattering spectra o f Si doped with 7 x 1 0 1 5 atoms/cm3 and implanted with 5 x 1015 50 keV 15N+ ions/cm2: ( a ) random, ( b ) shows the <001 >-aligned spectrum before, and ( c ) a f t e r a C02 laser pulse The <001>-aligned o f 70 MW/cm2 w i t h the sample preheated t o 575OC. [ A f t e r Blomberg e t al. spectrum o f the unimplanted crystal i s given i n (d). (1983).]
s p e c t r a i s shown i n Fig. 6 f o r an i n c i d e n t i n t e n s i t y o f 70 MW/cm2. The sample was i m p l a n t e d a t room temperature w i t h 5 x 1015 n i t r o g e n atoms/cm2 a t an energy o f 50 KeV.
The p r e h e a t i n g o f t h e s u b s t r a t e
was found t o be necessary t o a v o i d damaging t h e c r y s t a l s .
When
t h e p r e h e a t i n g was decreased and t h e p u l s e i n t e n s i t y increased i n o r d e r t o m e l t t h e sample, t h e sample was damaged. sequence o f t h e thermal
"run-away"
T h i s i s a con-
i n t h e a b s o r p t i o n which
is
e s p e c i a l l y d e t r i m e n t a l t o t h e a n n e a l i n g o f samples having a small l i n e a r absorption c o e f f i c i e n t .
The p r e h e a t i n g o f t h e l i g h t l y doped
sample s i g n i f i c a n t l y increases t h e d e n s i t y o f e l e c t r o n and h o l e carriers,
and t h e r e b y s i g n i f i c a n t l y increases t h e l i n e a r f r e e -
c a r r i e r absorption.
574
R. B. JAMES
Similar measurements were performed by Narayan e t al. (1984a) and James et a l . (1984a) on l i g h t l y doped s i l i c o n (111) samples which were heated t o 610°C f o r 5 minutes. The samples were implanted w i t h llB ions a t several d i f f e r e n t ion energies in order t o induce implantation damage up t o a d e p t h of about 0.9 p. The d i s l o c a t i o n loops induced by the boron implantation a r e stable against the thermal treatment a t 610°C (Gyulai and Revesz, 1979). The preheating of the ion-implanted samples increases the absorption c o e f f i c i e n t by e l e c t r i c a l l y a c t i v a t i n g a f r a c t i o n of t h e boron implants and by increasing the temperature-dependent i n t r i n s i c c a r r i e r concentration. The samples were i r r a d i a t e d with a s i n g l e pulse (A = 10.6 i.m and FWHM = 70 ns) from a COP l a s e r , and crosssection transmission electron microscopy was used t o i n v e s t i g a t e t h e melt depth i n the material and the defects in the r e c r y s t a l l i z e d layer. Figure 7 shows a cross-section micrograph of a sample i r r a d i a t e d with an energy density of 7.3 J/cm2. The complete removal o f dislocation loops in the annealed region and the abrupt
Fig. 7.
Cross-section TEM micrograph showing defect-free recrystallization
o f boron-implanted
silicon a f t e r C 0 2 laser irradiation.
lightly doped and multiply implanted with llB+ t o induce deep implantation damage.
The sample was
a t several d i f f e r e n t ion energies
The sample was heated at 6 1 0 * C for five
minutes and then irradiated with a single pulse having an energy density o f
7.3 J / c m Z .
9.
575
PULSED CO2 LASER ANNEALING
change i n d e n s i t y o f loops between t h e annealed and unannealed r e g i o n s a r e c o n s i s t e n t w i t h t h e m e l t i n g o f t h e near-surface l a y e r . M e l t depths r a n g i n g up t o about 8000 A were observed i n t h e l i g h t l y doped samples which were preheated t o i n c r e a s e t h e c o u p l i n g o f t h e CO,
laser l i g h t t o the material. Attempts t o anneal l a r g e areas o f l i g h t l y doped samples w i t h o u t
p r e h e a t i n g t h e s u b s t r a t e by James e t a l . t o be unsuccessful.
(1984a) were a l s o found
For t h e l i g h t l y doped samples,
irradiation
a t room temperature produced l o c a l i z e d s i t e s o f m e l t i n g and damage. The regions o u t s i d e o f t h e m i c r o s i t e s o f damage were n o t melted by t h e CO,
l a s e r pulse.
These small s i t e s o f l a s e r - i n d u c e d m e l t i n g
occur because o f t h e a b r u p t i n c r e a s e i n t h e a b s o r p t i o n c o e f f i c i e n t o f t h e m a t e r i a l w i t h i n c r e a s i n g temperature, which makes even small spatial
inhomogeneities i n t h e beam d e t r i m e n t a l t o t h e u n i f o r m
a n n e a l i n g o f l a r g e areas o f t h e wafer. Complete l a s e r - i n d u c e d r e c r y s t a l 1 i z a t i o n o f amorphous l a y e r s produced by i o n i m p l a n t a t i o n has been r e p o r t e d f o r l i g h t l y doped s i l i c o n w i t h o u t p r e h e a t i n g o f t h e s u b s t r a t e ( C e l l e r e t al.,
1979).
The RBS spectrum i n d i c a t e d good e p i t a x i a l regrowth o f t h e s u r f a c e l a y e r from t h e c r y s t a l l i n e s u b s t r a t e .
There appears t o be some d i s -
crepancy between t h e r e s u l t s o f C e l l e r e t a l . (1979) and t h e r e s u l t s o f Blomberg e t a l . (1983), Naukkarinen e t a l . a1
.
(1982), and James e t
(1984a) r e g a r d i n g t h e annealing o f l i g h t l y doped s i 1 i c o n w i t h o u t
t h e preheating o f the substrate. used by C e l l e r e t a l .
Unless t h e beam o f t h e CO,
laser
(1979) i s c o n s i d e r a b l y more s p a t i a l l y homo-
geneous than t h a t used by others, o r unless some a n n e a l i n g o f t h e i r samples occurred d u r i n g i m p l a n t a t i o n , a d d i t i o n a l experimental i n f o r m a t i o n w i l l be r e q u i r e d t o r e s o l v e t h i s discrepancy. I n a d d i t i o n t o removing i o n - i m p l a n t a t i o n damage, i t i s advantageous t h a t pulsed CO,
l a s e r annealing can be used t o e l e c t r i c a l l y
a c t i v a t e implanted dopants.
Van der Pauw measurements have been
performed by James e t a l . (1984a) t o determine t h e sheet r e s i s t i v i t y and s h e e t - c a r r i e r l a y e r s a f t e r CO,
c o n c e n t r a t i o n o f boronl a s e r annealing.
and a r s e n i c - i m p l a n t e d
The t h r e s h o l d energy d e n s i t y o f
R. B. JAMES
t h e l a s e r f o r complete annealing was found t o be dependent on t h e energy, species, and dose o f t h e implants. Van der Pauw measurements were made on n-type s i l i c o n wafers, 1-5 Q-Cm,
which were implanted w i t h l l B a t an energy o f 100 KeV
t o a dose o f 1 x 10l6 cm-2.
The samples were preheated t o 590°C
f o r f i v e minutes and i r r a d i a t e d w i t h a s i n g l e CO, The energy d e n s i t y o f t h e l a s e r was varied,
l a s e r pulse.
and t h e e l e c t r i c a l
a c t i v a t i o n was measured as a f u n c t i o n o f t h e energy density.
The
r e s u l t s o f t h e measurements showed t h a t f o r energy d e n s i t i e s exceeding t h e m e l t threshold, one could achieve up t o 100% a c t i v a t i o n o f t h e boron implants by pulsed CO, l a s e r annealing.
The H a l l
m o b i l i t y o f t h e samples w i t h 90-100% a c t i v a t i o n was measured t o be 30 cm2/v-s.
S i m i l a r l i g h t l y doped samples were implanted w i t h
I l B a t an energy o f 35 KeV t o a dose o f 2 x
1015 cm-2.
a c t i v a t i o n was also achieved i n these samples by pulsed anneal ing.
Complete
CO, l a s e r
The Hal 1 mobi 1 it y o f t h e samples w i t h g r e a t e r t h a n 90%
a c t i v a t i o n was measured t o be 34 cm2/v-s. P-type s i l i c o n wafers w i t h a r e s i s t i v i t y o f 5-11 62-cm were implanted w i t h 75As i o n s a t an energy o f 100 keV t o a dose o f 1 x
1016 c r 2 .
E l e c t r i c a l a c t i v a t i o n o f up t o 90% o f t h e implanted
a r s e n i c was observed i n some o f these samples, although o n l y about 75-85% o f t h e implanted arsenic was a c t i v a t e d i n most o f t h e l a s e r annealed wafers.
It i s u n c e r t a i n whether t h e s l i g h t l y lower a c t i v a t i o n o f t h e implanted arsenic i s due t o an incomplete annealing, a loss o f arsenic from t h e surface, t h e presence o f deep t r a p s which compensate t h e laser-annealed region, and/or a f r a c t i o n o f t h e arsenic atoms i n t h e r e c r y s t a l l i z e d l a y e r occupying nonsubstitutional sites.
For s i m i l a r l i g h t l y doped samples which were
implanted a t a lower dose (100 keV,
1 x 1015 cm-*),
complete
a c t i v a t i o n was achieved as a r e s u l t o f i r r a d i a t i o n w i t h l a s e r energy d e n s i t i e s g r e a t e r than about 4 J/cm*. R e s u l t s w i l l now be discussed f o r t h e annealing o f h e a v i l y doped s i l i c o n using a pulsed CO, l a s e r .
Aligned b a c k s c a t t e r i n g spectra
were taken by Naukkarinen e t a l . (1982) on samples doped w i t h boron
9.
577
PULSED COz LASER ANNEALING
t o a c o n c e n t r a t i o n of 5 x 1019 cm-3 and implanted w i t h n i t r o g e n t o a dose o f 7.5 x 1015/cm2.
The r e s u l t s f o r t h e s p e c t r a u s i n g 1.0-MeV
4He i o n s i n c i d e n t on t h e s i l i c o n wafers a r e shown i n Fig. an i n c i d e n t i n t e n s i t y o f 21 MW/cm2,
5.
At
partial recrystallization i s
observed (curve c ) , and a t an i n t e n s i t y o f 40 MW/cm2 ( c u r v e d ) , t h e amorphized l a y e r i s almost c o m p l e t e l y r e c r y s t a l l i z e d .
It was
a l s o found t h a t near t h e s u r f a c e t h e laser-annealed r e g i o n had essentially perfect crystallinity,
b u t deeper i n t h e sample t h e
dechannel ing 1eve1 i n c r e a s e d somewhat.
The same sampl es were
i m p l a n t e d t o a dose o f 1015-1016 antimony atoms/cm*, complete r e c r y s t a l l i z a t i o n was
and almost
again observed a f t e r annealing
(Fig. 8).
3000
-
Po
ORNL-DWG84-44t05
e?
In
c 3 E
- 2000 0
J
w
F
U
4000
u
u, r
i
p-k%?+P"
t
4
I-
0
U
m 0
60
90
4 20
450
480
270
300
CHANNEL N U M B E R
Fig. 8. 4He-ion backscattering spectra o f Si doped with 5 x 1019 B atoms/cm3: ( a ) random, ( b ) -alignedincidenceona 5 x 1 0 1 5 S b i o n s / c m 2 implanted sample, and ( c ) a f t e r annealing with a 4 0 - M W / c m 2 pulse. The (001 >aligned spectrum o f the unimplanted crystal i s given in ( d ) . [After Naukkarinen e t al. (1982).]
578
R. B. JAMES
Fig. 9 .
Transmission synchrotron x-ray topographs o f Si implanted with 7 . 5 x
1 0 1 5 N i o n s / c m 2 a f t e r laser annealing: ( a ) and ( b ) d o p e d w i t h 1 0 2 0 P a t o m s / c m 3 ,
{lrl}and { 3 i 1 } reflections, { 301 }
reflection.
respectively; ( c ) doped w i t h 5 ~ 1 O ~ ~ B a t o m s / c m ~ ,
[ A f t e r Naukkar inen et al
Transmission s y n c h r o t r o n x - r a y
.
.
(1982) ]
topographs o f
h e a v i l y doped
s i l i c o n samples which were i r r a d i a t e d w i t h a p u l s e d CO, shown i n Fig. 9 (Naukkarinen e t al.,
1982).
l a s e r are
F i g u r e s 9a and 9b a r e
f o r a sample doped w i t h phosphorous t o a c o n c e n t r a t i o n o f 1 0 2 0 ~ m - ~ , and Fig. 9c i s f o r a sample doped w i t h boron t o a c o n c e n t r a t i o n o f
5 x 1 0 1 9 cm-3.
The t r a c e o f t h e l a s e r p u l s e i s marked by A and t h e
p e r f e c t s i n g l e c r y s t a l i s marked by C.
The r e g i o n near t h e c e n t e r
o f t h e l a s e r spot i s almost i d e n t i c a l t o t h e p e r f e c t s i n g l e c r y s t a l . I t i s found t h a t t h e l a s e r - a n n e a l e d area i s a t a h e i g h t d i f f e r e n t
from t h e s u r r o u n d i n g i m p l a n t e d region.
Some c o n t r a c t i o n o f t h e
l a t t i c e i s expected, s i n c e t h e volume occupied by t h e i m p l a n t e d n i t r o g e n atoms i n s u b s t i t u t i o n a l s i t e s i s s m a l l e r t h a n t h e volume occupied by s i l i c o n atoms i n the l a t t i c e .
F i g u r e 9c shows a {301}
topograph o f a l a s e r - a n n e a l e d sample which was h e a v i l y doped w i t h boron.
Except f o r t h e s t r i a t i o n s
defect-free,
, the
1aser-anneal ed r e g i o n 1ooks
i n d i c a t i n g t h a t t h e r e c r y s t a l l i z e d l a y e r does n o t
c o n t a i n many a d d i t i o n a l d e f e c t s .
The KBS spectrum o f t h i s area i s
comparable t o t h a t o f t h e s u r r o u n d i n g b u l k s i l i c o n . marked by "He" i s damage caused by t h e h e l i u m i o n beam.
The r e g i o n
9.
579
PULSED CO2 LASER ANNEALING
Studies o f t h e r e c r y s t a l l i z a t i o n o f h e a v i l y doped s i l i c o n were a l s o performed by James e t a l . (1984a).
The samples were u n i f o r m l y
doped w i t h phosphorus and had a r e s i s t i v i t y a t room temperature o f 0.0026 n-cm.
The near-surface r e g i o n was i m p l a n t e d w i t h I l l 3 a t an
energy o f 185 KeV t o a dose o f 1.5
x 1016
i m p l a n t e d r e g i o n remains c r y s t a l l i n e ,
Although t h e
e l e c t r i c a l measurements on
t h e as-implanted samples show t h a t most o f t h e f r e e c a r r i e r s near t h e s u r f a c e are trapped by t h e i m p l a n t a t i o n - i n d u c e d defects. one expects t h a t much o f t h e energy i n t h e C0,
Thus,
l a s e r p u l s e w i l l be
d e p o s i t e d i n t h e l o w - r e s i s t i v i t y m a t e r i a l below t h e damaged l a y e r . The samples were annealed w i t h a pulsed CO,
l a s e r ( A = 10.6 urn,
FWHM = 70 ns) over a range o f energy d e n s i t i e s t o determine t h e
m e l t t h r e s h o l d , m e l t depth, and r e d i s t r i b u t i o n o f boron ions.
The
r e s u l t s o f t h e c r o s s - s e c t i o n TEM measurements on t h e h e a v i l y doped samples show t h a t one can m e l t r e g i o n s near t h e h i g h - t o - l o w r e s i s t i v i t y i n t e r f a c e w i t h o u t m e l t i n g t h e h i g h e r - r e s i s t i v i t y l a y e r which encapsulates t h e molten r e g i o n (Fig.
10).
Melting of the material
Fig. 10. Cross-section TEM micrograph o f a heavily doped silicon wafer which has been implanted with cm-2.
llB+ at an energy o f 185 KeV to a dose of 1 . 5 x 10l6
The sample has been irradiated a t room temperature with a pulse having
an energy density o f 6.1 J / c m 2 . i n the figure.
Some o f these samples tend to crack as shown
580
R. B. JAMES
which i s embedded i n t h e sample r e s u l t s from t h e l a r g e a b s o r p t i o n o f t h e CO, l a s e r l i g h t i n t h e l o w - r e s i s t i v i t y s u b s t r a t e , as compared t o t h e weaker a b s o r p t i o n i n t h e i m p l a n t e d r e g i o n . As t h e i n t e r f a c e r e g i o n between t h e damaged and undamaged l a y e r s m e l t s , t h e conduct i o n o f heat d r i v e s t h e m e l t f r o n t i n b o t h t h e d i r e c t i o n s o f t h e i m p l a n t e d l a y e r and t h e unimplanted s u b s t r a t e . h i g h energy d e n s i t i e s o f t h e CO,
For s u f f i c i e n t l y
l a s e r , t h e conduction o f heat
m e l t s t h e e n t i r e i m p l a n t e d region.
Since t h e a b s o r p t i o n o f t h e
l a s e r energy i s c o m p a r a t i v e l y l a r g e i n t h e undamaged s u b s t r a t e , t h e dynamics o f r e c r y s t a l l i z a t i o n may be somewhat d i f f e r e n t t h a n t h e r e c r y s t a l l i z a t i o n o f a s i m i l a r sample which has been m e l t e d w i t h a v i s i b l e laser.
I n a d d i t i o n , t h e m e l t i n g o f embedded r e g i o n s
f u r t h e r demonstrates how one can use f r e e - c a r r i e r a b s o r p t i o n t o control m a t e r i a1
t h e energy d e p o s i t i o n o f t h e l a s e r r a d i a t i o n i n t h e
.
I n many o f t h e h e a v i l y doped samples, cracks appear as a r e s u l t o f p u l s e d CO,
l a s e r a n n e a l i n g (Fig.
10).
Since t h e d e n s i t y of
b o t h heated and l i q u i d s i l i c o n i s l e s s t h a n c r y s t a l l i n e s i l i c o n a t room temperature,
t h e presence o f r a p i d h e a t i n g and subsequent
m e l t i n g o f t h e embedded l a y e r causes l a r g e s t r e s s i n t h e m a t e r i a l . The f r a c t u r i n g o f t h e sample i s a r e s u l t o f t h e l a s e r - i n d u c e d h e a t i n g o f t h e embedded l a y e r and t h e l a c k o f a f r e e s u r f a c e t o r e 1 i e v e t h e thermal s t r e s s . I n a d d i t i o n t o t h e l a s e r - i n d u c e d m e l t i n g o f t h e subsurface
l a y e r , we see from Fig. 10 t h a t m e l t i n g o f t h e s u r f a c e l a y e r a l s o occurs.
Channeling measurements by Tsien e t a l .
(1982) i n d i c a t e
t h a t t h e i m p l a n t a t i o n of boron a t 185 KeV produces much l e s s damage t o t h e t h i n c r y s t a l l i n e l a y e r a t t h e surface than t o the underlying region.
Thus,
t h e c o u p l i n g o f t h e C02 l a s e r l i g h t i s probably
somewhat l a r g e r near t h e s u r f a c e t h a n i n t h e more h e a v i l y damaged l a y e r i n t h e subsurface. If the coupling o f the l i g h t t o the i m p l a n t e d r e g i o n i s l a r g e s t near t h e surface, t h e n t h e temperature would i n c r e a s e a t a f a s t e r r a t e a t t h e s u r f a c e d u r i n g t h e t i m e which t h e l a s e r p u l s e propagates t h r o u g h t h e i m p l a n t e d r e g i o n .
As
9.
581
PULSED CO2 LASER ANNEALING
t h e temperature i n t h e surface l a y e r increases by t h e absorption o f t h e l a s e r pulse, t h e f r e e - c a r r i e r d e n s i t y also increases due t o a p a r t i a l a c t i v a t i o n o f t h e implanted dopant, which leads t o a f u r t h e r increase i n t h e absorption c o e f f i c i e n t .
An enhancement i n
t h e heating o f t h e surface, as compared t o t h e underlying region, can a l s o r e s u l t from t h e absorption o f t h e l a s e r l i g h t by t h e t h i n oxide l a y e r on t h e surface.
(For l i g h t w i t h a wavelength i n t h e
9-11-pm region, t h e l a t t i c e absorption i n s i l i c o n d i o x i d e can be as l a r g e as lo5 cm-l.)
More i n v e s t i g a t i o n s are needed t o study
t h e dynamics o f t h e energy d e p o s i t i o n and r e c r y s t a l l i z a t i o n i n both t h e melted r e g i o n a t t h e surface o f t h e sample and t h e melted r e g i o n which i s encapsulated on both sides by s o l i d m a t e r i a l .
It
should be noted t h a t t h i s me1 t i n g phenomenon o f ion-implanted s i l i c o n i s probably not p o s s i b l e t o o b t a i n w i t h a v i s i b l e o r u l t r a v i o l e t 1aser.
A s i m i l a r m e l t i n g phenomenon l i k e l y occurs f o r a h e a v i l y doped sample where t h e surface has been amorphized by i o n implantation. Since t h e absorption o f CO,
l a s e r l i g h t i n amorphous s i l i c o n i s
r e l a t i v e l y small compared t o t h e absorption i n t h e h e a v i l y doped c r y s t a l l i n e s u b s t r a t e (Brodsky e t al.,
1970), much o f t h e l a s e r
energy w i l l be deposited near t h e amorphous-crystal1 i n e i n t e r f a c e . Thus,
i n principle,
one can m e l t t h e embedded r e g i o n near t h e
i n t e r f a c e without m e l t i n g t h e e n t i r e amorphous l a y e r .
The observa-
t i o n of such m e l t i n g phenomena may r e q u i r e f a i r l y t h i c k amorphous l a y e r s due t o t h e reduced m e l t i n g temperature o f amorphous s i l i 1984 and con compared t o c r y s t a l l i n e s i l i c o n (Lowndes e t a1
.,
Wood e t al.,
1984) and t h e d i f f i c u l t y i n m a i n t a i n i n g a thermal
g r a d i e n t so t h a t t h e temperature o f t h e surface does not exceed t h e m e l t i n g temperature o f amorphous S i . c o u p l i n g o f t h e CO,
(This r e l a t i v e l y weak
l a s e r l i g h t i n t o t h e amorphous l a y e r i s i n
c o n t r a s t t o t h e strong c o u p l i n g o f v i s i b l e and u l t r a v i o l e t lasers, where one can m e l t much o f t h e amorphous l a y e r without m e l t i n g any o f t h e u n d e r l y i n g c r y s t a l l i n e substrate.)
R. B. JAMES
3.
REDISTRIBUTION
OF IMPLANTED DOPANTS
I n many device a p p l i c a t i o n s ,
one would l i k e t o c o n t r o l t h e
dopant l e v e l and p r o f i l e t o optimize t h e device properties.
Here,
we present experimental data on t h e r e d i s t r i b u t i o n o f ion-implanted species and show how dopant p r o f i l e s can be c o n t r o l l e d by varying t h e l a s e r energy density, i m p l a n t a t i o n energy, and substrate temp e r a t u r e d u r i n g l a s e r annealing.
The r e s u l t s o f SIMS measurements
w i l l f i r s t be shown f o r l i g h t l y doped s i l i c o n where the s u b s t r a t e must be heated above room temperature f o r COP l a s e r annealing, and f o l l o w e d by t h e r e s u l t s f o r h e a v i l y doped s i l i c o n where heating o f t h e s u b s t r a t e i s not r e q u i r e d f o r successful annealing. N-type
(loo ),
2-4 61-cm,
s i l i c o n samples were implanted w i t h
I 2 l S b a t an energy of 150 KeV t o a dose o f 2 x 1015 cm-*. samples were annealed w i t h a pulsed C02 l a s e r
(A
=
The
10.6 pm, FWHM
= 70 ns), and SIMS measurements were performed t o measure t h e re-
d i s t r i b u t i o n o f t h e antimony atoms (James e t a l
., 1984a).
As shown
i n Fig. 11, annealing w i t h d i f f e r e n t energy d e n s i t i e s o f t h e pulsed l a s e r provides c o n t r o l o f t h e d i f f u s i o n o f t h e antimony atoms. The f o u r curves show t h e depth p r o f i l e s o f antimony f o r : unannealed sample; d e n s i t y , EL,
(1) an
(2) a sample annealed a t a COP l a s e r energy
of 4.4 J/cm2;
(3) a laser-annealed sample a t EL = 5.9
J/cm2; (4) and a sample annealed a t EL = 8.9 J/cm2. Each wafer was heated t o 660°C f o r f i v e minutes p r i o r t o i r r a d i a t i o n w i t h t h e C02 l a s e r and was removed from t h e s u b s t r a t e heater immediately a f t e r l a s e r annealing.
As can be seen i n t h e f i g u r e , t h e antimony
atoms can r e d i s t r i b u t e t o a depth o f well over 6000 A by v a r y i n g t h e laser-energy density, thereby p r o v i d i n g a degree o f v a r i a t i o n i n t h e j u n c t i o n depth.
I n addition,
we see t h a t some o f t h e
antimony atoms segregate t o t h e surface o f t h e s i l i c o n .
Similar
segregation o f antimony has been observed i n RBS measurements o f C02 laser-annealed s i 1 i c o n by Cell e r e t a1 e t a l . (1982). o f 0.023
. (1978)
and Naukkari nen
This segregation behavior i s c o n s i s t e n t w i t h a value
f o r t h e d i s t r i b u t i o n c o e f f i c i e n t o f Sb a t t h e m e l t i n g
p o i n t o f S i ( C e l l e r e t al.,
1978; Trumbore,
1960).
Channeling
9.
583
PULSED COz LASER ANNEALING
I
I
1
ANTIMONY-121 ATOMS IN SILICON AFTER Cop LASER ANNEALING
A 0
AS IMPLANTED EL = 4.4 Jlcm2
+ EL = 5.9 J/cm2
x EL = 8.9 J/cm 2
1
3
I
0.2
‘t I
I
0.4
1
0.6
DEPTH ( p m ) Fig. 1 1 . Concentration profiles of 1 2 % b in Si before and a f t e r C02 laser annealing a t various energy densities. The four curves show the Sb profiles f o r the following samples: A, an unannealed sample; 0 , a sample annealed a t E ~ = 4 . 4 J / c m ~ ; a sampleannealedat E ~ = 5 . 9 J / c m ~a ;n d x , a sampleannealed a t 4 = 8 . 9 J / c m 2 . Each o f the samples was heated t o 66OOC for five minutes prior t o irradiation.
+,
R. B. JAMES measurements by C e l l e r e t al. (1979) on l i g h t l y doped, Sb-implanted S i showed t h a t a f t e r l a s e r i r r a d i a t i o n w i t h i n t e n s i t i e s o f 100 t o 150 MW/cm2, t h e C1001-aligned y i e l d m i n dropped t o about 5?? of t h e random value, i n d i c a t i n g e p i t a x i a l regrowth o f t h e amorphous layer.
The Sb atoms i n t h e r e c r y s t a l l i z e d l a y e r were found t o be
about 95% s u b s t i t u t i o n a l . L i g h t l y doped, p-type S i samples were a l s o implanted w i t h llB (100 keV t o 1 x 1 0 l 6 c w 2 ) ,
and concentration p r o f i l e s o f t h e
implanted boron were measured as a f u n c t i o n o f t h e energy d e n s i t y o f t h e COP laser.
Each o f t h e wafers was preheated f o r f i v e
minutes a t a temperature o f between 650 and 690°C t o increase t h e a b s o r p t i o n c o e f f i c i e n t o f t h e C02 l a s e r l i g h t i n t h e m a t e r i a l . Both t h e boron and arsenic implants were found t o d i f f u s e t o approximately u n i f o r m concentrations a f t e r l a s e r annealing and no segregation behavior was observed. 9.2 J/cm2,
For a l a s e r energy d e n s i t y o f
t h e arsenic d i f f u s e d t o a depth o f 7000 A, and boron
d i f f u s e d t o a depth o f about 1.0 p. Measurements o f dopant p r o f i l e s have also been made by Hauck e t al.
(1981) on t h e r e d i s t r i b u t i o n o f phosphorus implants i n
l i g h t l y doped s i l i c o n annealed w i t h a C02 l a s e r ( h = 10.6 p and pulse d u r a t i o n = 400 ns).
The phosphorus ions were implanted a t
an energy o f 175 KeV t o a dose o f 5 x 1015 cm-* i n a boron-doped sample w i t h a r e s i s t i v i t y o f 8 t o 10 62-cm.
As a r e s u l t o f l a s e r
annealing, t h e phosphorus ions move deeper i n t o t h e sample as shown i n Fig. 12.
Also shown i n t h e f i g u r e i s a c o n t r o l wafer which was
t h e r m a l l y annealed a t 1000°C f o r 30 minutes i n N,. Moderately doped s i l i c o n samples have a1 so been s t u d i e d by James e t al.
(1984a).
implanted w i t h llB
Antimony-doped,
0.018
Q-cm [lll] S i was
a t an energy o f 35 KeV t o a dose o f 1 x 1 0 l 6
cm-,. The r e s u l t s o f SIMS measurements f o r t h e boron p r o f i l e s are shown i n Fig. 13. The f i v e curves i n t h e f i g u r e are f o r an asimplanted sample, a sample i r r a d i a t e d a t room temperature w i t h EL = 5.8 J/cm2, a sample i r r a d i a t e d a t room temperature w i t h EL = 8.4
9.
585
PULSED CO2 LASER ANNEALING
4O 2'
c'
40"
0
Fig. 12.
0.2
0.6 DEPTH ( p m 1 0.4
Calculated !!as implanted!! impurity profile
impurity profiles a f t e r laser annealing Hauck et al.
(0)
1 .o
0.8
(0)
and the measured
and thermal annealing ( 0 ) . [ A f t e r
(1981 ) .]
J/cm2, a sample i r r a d i a t e d a t 6.3 J/cm2 w i t h t h e s u b s t r a t e heated t o 6 9 O O C f o r 5 minutes, and a sample i r r a d i a t e d w i t h f i v e shots a t 4.2
J/cm2 w i t h t h e substrate heated t o 690°C f o r 5 minutes.
The boron d i f f u s e s up t o a depth o f about 4000 A i n t h e samples due t o l a s e r annealing, and t h e r e i s no i n d i c a t i o n o f segregation o f t h e boron t o t h e surface.
The e f f e c t o f preheating t h e substrate
on t h e r e d i s t r i b u t i o n o f boron atoms can be seen from Fig.
13.
For EL = 5.8 J/cm2 and no preheating, t h e boron r e d i s t r i b u t e s up t o a depth of about 500 A , b u t when t h e substrate i s heated t o
6 9 O O C f o r f i v e minutes, t h e increased absorption causes deeper m e l t i n g t o occur,
and t h e boron r e d i s t r i b u t e s up t o a depth o f
about 3000 A a t t h e same energy d e n s i t y o f t h e l a s e r . S i m i l a r measurements were
performed on t h e antimony-doped
samples w i t h boron implanted a t an energy of 150 KeV and doses i n t h e range o f 5 x 1014 cm-2 t o 1 x 1OI6 cm-2.
The boron was found
586
R. B. JAMES
0.0
0.1
0.2
0.3
0.4
DEPTH (prn)
Fig. 13. Concentration p r o f i l e s o f llB in S i before and a f t e r CO laser 2 annealing. The five curves show the boron profiles for the following samples: + an as-implanted sample; 0 , a sample annealed a t EL = 5.8 J / c m 2 ; x, a sample annealed at EL = 8.4 J / c m 2 ; a, a sample annealed a t EL = 6.3 J / c m 2 and To = 690OC; and A, a sample annealed by five laser shots a t 4 = 4.2 J / c m 2 and the substrate heated to 690OC.
,
9.
PULSED COz LASER ANNEALING
587
t o d i f f u s e i n t h e laser-annealed samples a t approximately a u n i f o r m concentration,
where t h e maximum depth of boron d i f f u s i o n was a
f u n c t i o n o f t h e energy d e n s i t y o f t h e l a s e r .
The dopant p r o f i l e s
f o r t h e d i f f e r e n t imp1 a n t doses and l a s e r w e r g y d e n s i t i e s showed t h a t dopant p r o f i l e s can be obtained w i t h n e a r l y equal j u n c t i o n depths b u t w i t h a l a r g e range o f dopant c o n c e n t r a t i o n s by t h e use o f i o n i m p l a n t a t i o n and C02 l a s e r annealing. Depth p r o f i l e s o f i m p l a n t e d i o n s have a l s o been measured by James e t a l . (1984a) and Naukkarinen e t a l . (1982) i n s i l i c o n wafers which were h e a v i l y doped w i t h boron. I n t h e experiment o f James e t a l . (1984a), T5As i o n s were implanted a t an energy o f 180 keV t o a dose o f 1 x 1 0 l 6 r e s i s t i v i t y o f 0.0073
61-cm.
i n t o a boron-doped sample w i t h a The samples were annealed w i t h a
p u l s e d C02 l a s e r and SIMS measurements were performed t o examine t h e r e d i s t r i b u t i o n o f t h e a r s e n i c atoms.
The r e s u l t s o f t h e m a -
surements are shown i n Fig. 14 f o r several energy d e n s i t i e s o f t h e p u l s e d C02 l a s e r .
From t i m e - r e s o l v e d r e f l e c t i v i t y measurements,
t h e l a s e r t h r e s h o l d f o r m e l t i n g t h e s u r f a c e o f these samples was found t o be about 3.0 J/cm2 f o r a s u b s t r a t e temperature o f 20°C. From t h e f i g u r e we see t h a t f o r energy d e n s i t i e s up t o 8.1 J/cm2 t h e a r s e n i c i o n s d i f f u s e t o a depth o f 7000 A, and t h e r e i s no i n d i c a t i o n o f segregation o f t h e arsenic.
Some o f t h e wafers were l a s e r
annealed w i t h t h e s u b s t r a t e heated t o 690°C f o r 5 minutes.
Imne-
d i a t e l y a f t e r t h e i r r a d i a t i o n s w i t h t h e C02 l a s e r , t h e samples were removed from t h e s u b s t r a t e heater. Van der Pauw measurements on t h e as-implanted samples showed t h a t s i g n i f i c a n t thermal a c t i v a t i o n o f t h e a r s e n i c i m p l a n t s occurred d u r i n g t h e f i v e minutes i n which t h e s u b s t r a t e was a t a temperature o f 690°C.
The r e s u l t s o f t h e
SIMS measurements on t h e samples w i t h s u b s t r a t e h e a t i n g are shown i n Fig. 15.
The curves i n t h e f i g u r e show t h e As c o n c e n t r a t i o n
p r o f i l e s f o r an as-implanted wafer, a sample which has been i r r a diated at =
EL
4.1 J/cm*,
= 2.0 J/cm2, a sample which has been i r r a d i a t e d a t EL
a sample which has been i r r a d i a t e d w i t h f i v e pulses
588
R. B. JAMES
I
I
I
I
ARSENIC-75 ATOMS IN SILICON AFTER CO2 LASER ANNEALING
DEPTH ( p m ) Fig. 14. Concentration profiles o f 75As i n silicon before and a f t e r C 0 2 laser annealing at several different energy densities. Each o f the samples were heavily doped with boron and had a room temperature resistivity of 0.0073 Q-cm before implantation. The five curves show the arsenic concentrations f o r t h e following samples: 0 , an as-implanted sample; x, a sample annealed a t EL = 4.2 J /cm2; m, a sample annealed a t EL = 6.0 J /cm2; A, a sample annealed a t EL = 7.2 J / c m 2 , and a sample annealed a t EL = 8.1 J/cm2.
+,
9. ,
PULSED CO:, LASER ANNEALING
I
589
I
ARSENIC-75 ATOMS IN SILICON AFTER C02 LASER ANNEALING
Fig. 15. Concentration p r o f i l e s o f 75As i n silicon before and a f t e r Cog laser annealing. Each o f the wafers was heavily doped with boron and had a The room temperature resistivity o f 0.0073 61-cm prior t o implantation. samples were heated t o 690°C for five minutes p r i o r t o laser annealing. The five curves show the arsenic concentration which r e s u l t s from the following excitation conditions: x, an as-implanted sample; 0 , a sample annealed a t EL = 2.0 J / c m 2 ; a sample annealed a t EL = 4.1 J/cm2; A, a sample annealed w i t h five shots a t EL = 4.1 J / c m 2 ; and m, a sample annealed a t EL = 8.8 J / c m 2 .
e,
R. B. JAMES a t EL = 4.1 J/cm2,
and a sample which has been i r r a d i a t e d w i t h a
s i n g l e p u l s e a t EL = 8.8 J/cm2.
The i n c r e a s e d a b s o r p t i o n due t o
p a r t i a l a c t i v a t i o n o f t h e a r s e n i c i m p l a n t s lowers t h e m e l t t h r e s h o l d so t h a t m e l t i n g occurs a t energy d e n s i t i e s below 2.0 J/cm2.
As a
r e s u l t , one has some degree o f c o n t r o l o f t h e m e l t t h r e s h o l d o f i o n - i m p l a n t e d samples by p a r t i a l l y a c t i v a t i n g t h e imp1 anted dopants, and t h e r e b y g r e a t l y i n c r e a s i n g t h e f r e e - c a r r i e r a b s o r p t i o n i n t h e i m p l a n t e d region.
( P a r t i a l a c t i v a t i o n can be achieved by h e a t i n g
t h e s u b s t r a t e t o a p o i n t where solid-phase e p i t a x y can occur o r by a l l o w i n g some s e l f - a n n e a l i n g d u r i n g t h e i o n i m p l a n t a t i o n ) . addition,
In
we see t h a t t h e e f f e c t o f m u l t i p l e shots from t h e CO,
l a s e r on t h e r e d i s t r i b u t i o n o f a r s e n i c can be s i g n i f i c a n t (Fig. 15). The e f f e c t o f m u l t i p l e l a s e r shots on t h e maximum m e l t depth r e s u l t s f r o m t h e change i n t h e o p t i c a l p r o p e r t i e s o f t h e near-surface r e g i o n
with t h e subsequent l a s e r shots.
Once t h e near-surface r e g i o n
has m e l t e d and r e s o l i d i f i e d by t h e a b s o r p t i o n o f t h e f i r s t pulse, t h e number o f e l e c t r i c a l l y a c t i v e a r s e n i c atoms f u r t h e r increases,
As t h e c o n c e n t r a t i o n p r o f i l e o f e l e c t r i c a l l y a c t i v e i o n s changes from shot
which causes t h e f r e e - c a r r i e r a b s o r p t i o n t o a l s o increase.
t o shot, t h e r e i s a corresponding change i n b o t h t h e p e n e t r a t i o n depth and r e f l e c t a n c e o f t h e C02 l a s e r l i g h t . Depth d i s t r i b u t i o n s o f 1 5 N i n pulsed C02 l a s e r - a n n e a l e d s i l i con have been c a l c u l a t e d from t h e measured broadening o f t h e Ep = 429 KeV resonance-yield c u r v e o f t h e l 5 N (p, reaction (Naukkarinen e t al.,
1982).
The samples were u n i f o r m l y doped w i t h
boron a t a c o n c e n t r a t i o n o f 5 x 1019 B/cm3.
The annealing was done
i n an argon atmosphere w i t h a s i n g l e l a s e r p u l s e ( d u r a t i o n = 100 ns) a t an i n t e n s i t y o f about 150 MW/cm2. A f t e r l a s e r annealing t h e n i t r o g e n tends t o m i g r a t e t o t h e s u r f a c e and move o u t o f t h e sample. As t h e i n t e n s i t y i s f u r t h e r increased, t h e tendency f o r t h e n i t r o g e n t o m i g r a t e t o t h e s u r f a c e increases.
This type o f r e d i s t r i b u t i o n
o f t h e n i t r o g e n i m p l a n t s i n t h e laser-annealed r e g i o n s was a l s o observed by Blomberg e t a l . (1983) i n a l i g h t l y doped s i l i c o n wafer, which was preheated t o 575OC p r i o r t o i r r a d i a t i o n .
9. IV.
591
PULSED CO2 LASER ANNEALING
Model Calculation of Sample Heating
The exact n a t u r e o f t h e process which occurs d u r i n g pulsed l a s e r annealing
( m e l t i n g versus plasma f o r m a t i o n ) has r e c e n t l y
been a m a t t e r o f debate.
I n t h e plasma-annealing model
(van
Vechten, 1980), t h e annealing r e s u l t s f r o m t h e presence o f a dense e l e c t r o n - h o l e plasma which p e r s i s t s i n a w e l l - l o c a l i z e d r e g i o n f o r t i m e s on t h e o r d e r o f hundreds o f nanoseconds.
This high density
o f e l e c t r o n - h o l e p a i r s (-1022/~m3), which i s formed by i n t e r b a n d t r a n s i t i o n s between t h e valence and conduction bands, i s assumed t o cause t h e l a t t i c e t o become f l u i d - l i k e w i t h o u t s i g n i f i c a n t h e a t i n g o f t h e sample.
Although t h e r e appears t o be good agreement
between p r a c t i c a l l y a1 1 t h e
laser-anneal i n g
experiments
and
a
m e l t i n g model, t h e debate has c o n t i n u e d f o r several y e a r s due t o t h e d i f f i c u l t y i n q u a n t i f y i n g t h e plasma-annealing model, and t h e general acceptance t hat a dense e l e c t r o n - h o l e
plasma i s
ormed
d u r i n g t h e a b s o r p t i o n o f t h e h i g h - i n t e n s i t y l a s e r l i g h t by
nter-
band t r a n s i t i o n s .
However,
i n t h e case o f pulsed
COP 1a s e r
annealing, a dense e l e c t r o n - h o l e plasma i s n o t formed, s i n c e s n g l ephoton i n t e r b a n d t r a n s i t i o n s are not e n e r g e t i c a l l y allowed.
Even
a1 1owing f o r nonequil i b r i u m c a r r i e r s t o be generated by t h e absorpt i o n o f C02 l a s e r l i g h t , t h e d e n s i t y would be c o n s i d e r a b l y less t h a n t h e d e n s i t i e s r e q u i r e d i n a plasma-annealing model.
It i s n o t my
i n t e n t i o n t o present f u r t h e r arguments f o r a m e l t i n g - o r plasmaannealing process, b u t s u f f i c e i t t o say t h a t a plasma-annealing model seems i n a p p r o p r i a t e i n d e s c r i b i n g t h e anneal i n g o f s i 1 i c o n by a pulsed l a s e r having a wavelength w e l l below t h e i n t r i n s i c a b s o r p t i o n edge.
Furthermore, a1 1 o f t h e present experimental
o b s e r v a t i o n s on t h e annealing o f s i l i c o n wafers with a C02 l a s e r support a thermal model as t h e e x p l a n a t i o n o f t h e observed recrystal1ization.
As a consequence, t h e model c a l c u l a t i o n presented
i n t h i s s e c t i o n assumes t h a t thermal m e l t i n g i s t h e process which occurs i n t h e pulsed C02 l a s e r annealing o f s i l i c o n .
592
R. B. JAMES
I n an attempt t o understand t h e c o u p l i n g between t h e C02 l a s e r r a d i a t i o n and t h e semiconductor, i t i s o f i n t e r e s t t o d e v i s e a model f r o m which t h e computed values on o p t i c a l h e a t i n g can be compared w i t h experiment (see, f o r example, Wang e t a1.(1978), (1980a),
o r Wood and G i l e s (1981)).
c a r r i e r concentration,
n(z,t),
and t h e l i g h t i n t e n s i t y , I ( z , t ) ,
Meyer e t a l .
The equations governing t h e
t h e l a t t i c e temperature,
T(z,t),
i n t h e b u l k are g i v e n by
and
Here, a u n i f o r m l a s e r i r r a d i a t i o n i s assumed, so t h a t f o r a semii n f i n i t e sample thickness, sional.
I n Eqs. (1-3),
t h e r e l e v a n t equations a r e one dimen-
DA i s t h e ambipolar d i f f u s i o n c o e f f i c i e n t ,
g i s t h e r a t e o f e l e c t r o n - h o l e p a i r generation, n i i s t h e tempera-
ture-dependent
c a r r i e r concentration,
T ,
i s the bulk c a r r i e r
l i f e t i m e , K i s t h e thermal c o n d u c t i v i t y , p i s t h e m a t e r i a l d e n s i t y ,
C i s t h e s p e c i f i c heat, G i s t h e r a t e o f heat g e n e r a t i o n i n t h e sample, aL i s t h e a b s o r p t i o n c o e f f i c i e n t due t o t h e g e n e r a t i o n o f phonons, anI("'1) i s t h e a b s o r p t i o n c o e f f i c i e n t due t o an n-photon a b s o r p t i o n mechanism, ae i s t h e f r e e - e l e c t r o n a b s o r p t i o n c r o s s s e c t i o n , and ah i s t h e f r e e - h o l e a b s o r p t i o n cross s e c t i o n . Most o f t h e o p t i c a l and t r a n s p o r t p r o p e r t i e s depend on b o t h t h e c a r r i e r d e n s i t y and temperature and should g e n e r a l l y be taken i n t o account f o r an a c c u r a t e treatment. Equation ( 1 ) w i l l be d i f f e r e n t f o r
9.
593
PULSED CO2 LASER ANNEALING
e l e c t r o n and h o l e c a r r i e r s , except i n i n t r i n s i c m a t e r i a l where t h e e l e c t r o n and h o l e d e n s i t i e s i n e q u i l i b r i u m are t h e same. n e g l i g i b l e r a d i a t i o n losses t h e r e l e v a n t
Assuming
boundary c o n d i t i o n s
in
s o l v i n g t h e above equations f o r i n t r i n s i c m a t e r i a l a r e
K aT t G (Z=O) = 0 az s
-
n(z,t=O)
= n(z+-,t)
I(z=O,t)
= Io(t)
= "(To)
,
(4c 1
(4e)
y
and I(z +
m,t)
=
0.
(4f)
Here, To i s t h e i n i t i a l sample temperature, Gs i s t h e r a t e o f heat g e n e r a t i o n o f t h e surface, vs i s t h e s u r f a c e recombination v e l o c i t y , and I, i s t h e l i g h t i n t e n s i t y a t t h e surface. An a n a l y t i c a l s o l u t i o n t o t h e coupled s e t o f equations i s n o t possible,
and f u r t h e r
simplifications
are
required
t o obtain
reasonable estimates o f t h e o p t i c a l h e a t i n g by t h e a b s o r p t i o n o f
COE laser light.
The maximum temperature a t t h e s u r f a c e o f t h e
sample can be c a l c u l a t e d f o l l o w i n g t h e approximations made by Meyer e t a l .
(1980a).
T h i s approach c o n s i s t s o f f i r s t o b t a i n i n g
s o l u t i o n s f o r s h o r t pulses which do n o t account f o r t h e conduction o f heat.
It i s f u r t h e r assumed t h a t t h e Auger and r a d i a t i v e re-
combination times are much s h o r t e r than t h e p u l s e d u r a t i o n s , t h a t a t a p a r t i c u l a r depth,
so
t h e c a r r i e r d e n s i t y i s a t a steady
s t a t e value f o r a g i v e n l a t t i c e temperature.
Unless t h e c a r r i e r
594
R. B. JAMES
d e n s i t y i s q u i t e high, t h e recombination t i m e s i n m a t e r i a l s such as s i l i c o n and germanium can be l a r g e r t h a n t h e p u l s e d u r a t i o n i n many experiments, which would i n v a l i d a t e t h e c a l c u l a t i o n a l approach. However, i f one f u r t h e r assumes t h a t t h e a b s o r p t i o n process does n o t i n v o l v e t h e c r e a t i o n o f e l e c t r o n - h o l e p a i r s by m u l t i p h o t o n o r impact-ionization
processes,
t h e n t h e approach i s s t i l l v a l i d .
(Note t h a t t h e g e n e r a t i o n o f nonequil i b r i u m e l e c t r o n - h o l e p a i r s by t h e a b s o r p t i o n o f C02 l a s e r l i g h t has been observed i n germanium by Yuen e t a1
.,
1980 and may have been observed i n s i l i c o n by
Hasselbeck and Kwok, 1983). = 0 and T ( z ) = T(z=O),
I n i t i a l l y i t i s assumed t h a t K = an/at
so t h a t t h e equations decouple, and one
can s o l v e f o r t h e c a r r i e r d e n s i t y a t t h e s u r f a c e n(z=O,t) v a l u e o f T(z=O).
f o r each
Incorporating the effects o f c a r r i e r d i f f u s i o n
and s u r f a c e recombination i n a phenomenological way, n(z=O,t)
is
approximated by
where a i s t h e f r e e - c a r r i e r
absorption coefficient,
LA i s t h e
ambipolar d i f f u s i o n l e n g t h , and z i s t h e c a r r i e r l i f e t i m e due t o b o t h b u l k and s u r f a c e recombination (r-l = q-l Having obtained an e x p r e s s i o n f o r n(z=O,T),
+ vS/L~). Eq.
(2) can be
s o l v e d f o r a g i v e n p u l s e shape. Assuming t h a t t h e p u l s e shape i s r e c t a n g u l a r w i t h d u r a t i o n tp, one o b t a i n s from e q u a t i o n ( 2 ) (Meyer e t al.,
1980a)
where Tf i s t h e f i n a l s u r f a c e temperature a t t h e end o f t h e l a s e r pulse. The heat g e n e r a t i o n r a t e i n t h e near-surface r e g i o n can be w r i t t e n as G(z
- 0)
=
(1 -
R) Ioa ,
(7)
9.
where R i s t h e r e f l e c t i o n c o e f f i c i e n t o f t h e surface.
(7),
595
PULSED COz LASER ANNEALING
one can i n v e r t Eq.
Using Eq.
( 6 ) t o o b t a i n t h e power d e n s i t y (Io)
r e q u i r e d t o i n c r e a s e t h e s u r f a c e temperature f r o m To t o Tf as a f u n c t i o n of t h e p u l s e d u r a t i o n tp. One f i n d s
where
=v 1 -R(To)
LH
Tf
CV(T)C1
The s o l u t i o n can be g e n e r a l i z e d t o i n c o r p o r a t e thermal conduction by t h e phenomenological arguments presented by Meyer e t a l . (1980a). When f r e e - c a r r i e r a b s o r p t i o n domi nates t h e heat g e n e r a t i o n r a t e , one o b t a i n s
where LT i s t h e thermal d i f f u s i o n l e n g t h and has t h e approximate form LT(T) =
d/* [K(T)tp]’’
Equations ( 8 ) ,
(lo),
(T - To)/AT
.
(11 1
and (11) were solved by Naukkarinen e t a l .
(1982) assuming t h a t (1) t h e a b s o r p t i o n process does n o t i n v o l v e t h e c r e a t i o n o f e l e c t r o n - h o l e p a i r s and (2) t h e f r e e - c a r r i e r absorption c o e f f i c i e n t i s 6 x 0.45,
lo3
cm-I and t h e r e f l e c t i v i t y i s
where b o t h are independent o f t h e l a t t i c e temperature.
596
R. B. JAMES
4 500
MELTING POINT
444OOC
500
0
40
4
20
30 40
400
I (MWcrn-') Fig.
16.
Surface temperature ifas a function o f the incident laser
intensity I calculated for two laser pulse lengths for a sample doped with 5 x 1019 B atoms/cm3.
[ A f t e r Naukkarinen et al.
(1982).]
These values f o r the absorption c o e f f i c i e n t and r e f l e c t i v i t y a r e taken from measurements on s i l i c o n c r y s t a l s doped with about 1020 phosphorous atoms/cm3 t o a d e p t h of 3.5 @. K(T) and C(T) a r e taken from empirical expressions by Meyer e t a l . (1980b). Calculated values f o r the surface temperature ( T f ) a r e shown i n Fig. 16 f o r two rectangular pulses with durations of 50 ns and 100 ns. The c a l c u l a t i o n predicts t h a t melting of the surface l a y e r should occur a t i n t e n s i t i e s of 20 t o 30 MW/cm2, which i s in agreement with experiment. For smaller c a r r i e r d e n s i t i e s , the absorption c o e f f i c i e n t i s reduced considerably, and higher i n t e n s i t i e s are required t o melt t h e material. The surface temperature has been calculated f o r samples over a range of doping d e n s i t i e s using the method described 1982). For l i g h t l y or moderately doped above (Naukkarinen e t a1 samples, one must include t h e temperature dependences of the f r e e c a r r i e r absorption and r e f l e c t i v i t y , and Eq. (8) takes the form
.,
9.
PULSED COz LASER ANNEALING
As t h e l a t t i c e temperature increases, t h e increase i n t h e i n t r i n s i c c a r r i e r c o n c e n t r a t i o n i s given by (Meyer e t al., ni(T)
=
2.01 x 1020 (T/300
K)ls5
exp(-7020 K/T)
1980b)
.
(13)
Since t h e p r o b a b i l i t y f o r absorption o f a photon by a f r e e e l e c t r o n depends on t h e d e n s i t y o f f i n a l s t a t e s which t h e e l e c t r o n can occupy and t h e cooperation o f another p a r t i c l e t o conserve c r y s t a l momentum, t h e f r e e - c a r r i e r absorption cross s e c t i o n a1 so depends on t h e temperature.
The cross s e c t i o n i s assumed t o increase w i t h
temperature as T 3 l 2 (Smith, 1978), and i t s room temperature value has been chosen t o f i t t h e data by S p i t z e r and Fan (1957).
For
f r e e - c a r r i e r d e n s i t i e s l e s s than about 1 0 l 6 ~ m - ~l a, t t i c e absorption should also be included i n t h e expression f o r t h e absorption coefficient.
The room temperature value o f t h e multiphonon absorption
c o e f f i c i e n t a t 10.6 pm has been measured by Johnson (1959) t o be about 2 cm-'.
The l a t t i c e absorption increases w i t h temperature,
b u t since t h e temperature dependence i s considerably weaker than t h e temperature dependence o f t h e f r e e - c a r r i e r absorption, i t i s assumed t o be independent o f temperature.
Calculated values f o r
t h e surface temperature Tf are given i n Fig. 17 a t several c a r r i e r concentrations as a f u n c t i o n of t h e C02 l a s e r i n t e n s i t y f o r pulses w i t h a 100-ns d u r a t i o n (Naukkarinen e t al., 1982). As can be seen i n t h e f i g u r e , t h e r e e x i s t s a p a r t i c u l a r i n t e n s i t y a t which t h e surface temperature increases abruptly.
This thermal run away occurs
when t h e temperature i s reached a t which t h e i n t r i n s i c c a r r i e r c o n c e n t r a t i o n exceeds t h e doping density.
When t h i s temperature
t h r e s h o l d i s reached, t h e c a r r i e r concentration s t r o n g l y increases with intensity.
The abrupt increase i n t h e f r e e - c a r r i e r d e n s i t y
leads t o a l a r g e increase i n t h e f r e e - c a r r i e r
absorption and,
598
R. B. JAMES
I
++----
4500 141OOC
I
I
I
I
-
I
1
I
I
4000
500
i
I
I
I
I
I
I
I
2
5
10
20
50
400
200
500 4000 2000
I ( M W err-')
Fig. 17.
Surface temperature Tf as a function o f the incident laser inten-
sity I for d i f f e r e n t doping concentrations.
Laser pulse length tp = 100 ns.
The broken lines give T f ( l ) for n = 5 x 1015 ~ r n -and ~ n = 5 x l O l 9 ~ m - ~when , the sample i s preheated to 300OC.
consequently,
[ A f t e r Naukkarinen et a l .
t o f u r t h e r h e a t i n g o f t h e sample.
(1982).]
T h i s run-away
phenomenon i n t h e a b s o r p t i o n a l l o w s f o r m e l t i n g o f t h e s u r f a c e l a y e r f o r samples w i t h r e l a t i v e l y low doping d e n s i t i e s by o n l y small increments i n t h e energy d e n s i t y o f t h e l a s e r . F o r l i g h t l y doped samples, a c o n s i d e r a b l e i n t e n s i t y (-1 GW/cm2)
i s r e q u i r e d t o m e l t t h e s u r f a c e l a y e r (Fig. 17). v e r y a b r u p t l y as t h e i n t e n s i t y o f t h e
COP
The m e l t i n g occurs
l a s e r p u l s e i s increased.
Since i t i s d i f f i c u l t t o p r e c i s e l y c o n t r o l t h e p u l s e i n t e n s i t y , t h i s thermal run-away phenomenon i s d e t r i m e n t a l t o a t t a i n i n g complete r e c r y s t a l l i z a t i o n o f l a r g e areas w i t h o u t damaging t h e c r y s t a l s . One would l i k e t o be a b l e t o anneal l i g h t l y doped samples and a t t h e same t i m e a v o i d t h e thermal run-away i n t h e absorption.
This
can be accomplished by p r e h e a t i n g t h e sample t o a p o i n t where t h e i n t r i n s i c c a r r i e r c o n c e n t r a t i o n exceeds t h e doping d e n s i t y ,
so
9.
599
PULSED Cop LASER ANNEALING
to4
-
to3
'E 0
U
4 O2
40'
' 40
I
400
800
4200
4600
T (K) Fig. 1 8 .
Absorption coefficient
a o f Si as a function o f the temperature T [ A f t e r Blomberg et al. ( 1 9 8 3 ) .]
f o r different dopant concentrations n.
t h a t the increase i n the absorption c o e f f i c i e n t w i t h increasing l i g h t i n t e n s i t y i s n o t so abrupt.
The e f f e c t o f t h e l a t t i c e tem-
p e r a t u r e on t h e a b s o r p t i o n c o e f f i c i e n t i s shown i n Fig. 18 f o r seve r a l d i f f e r e n t doping d e n s i t i e s (Blomberg e t al., a(T) = 1.9 x 1020[cm2
K-3/21
1983).
x T 3 i 2 [ n + ni(T)]
+
9
Using Y
(14)
t h e peak s u r f a c e temperature has been c a l c u l a t e d as a f u n c t i o n o f t h e peak i n t e n s i t y by t h e method discussed above f o r a sample w i t h n = 5 x 1015 ~ m - ~ .The r e s u l t s o f t h e c a l c u l a t i o n are shown i n Fig.
19 f o r i n i t i a l s u b s t r a t e temperatures o f 300, 600, 900,
and 1200 K.
As seen i n t h e f i g u r e , p r e h e a t i n g o f t h e sample t o a
temperature o f 900 K o r l a r g e r s i g n i f i c a n t l y decreases t h e runaway b e h a v i o r i n t h e a b s o r p t i o n and a l s o decreases t h e i n t e n s i t y r e q u i r e d t o m e l t t h e surface, t h e r e b y making it e a s i e r t o achieve good r e c r y s t a l l i z a t i o n o f l i g h t l y doped s i l i c o n by t h e a b s o r p t i o n o f C02 l a s e r l i g h t .
600
R. B . JAMES
t 600
- to00 Y
Y
I-
800
600K
/
400
1
2
5
40
20
50
100 200
500 1000 2000
I (MW ern-')
Fig. 19.
Surface temperature Tf o f Si with a dopant concentration n = 5 x
1015 ~ r n - as ~ a function o f the incident laser intensity for d i f f e r e n t initial substrate temperatures To. [ A f t e r Blomberg et at. ( 1 9 8 3 ) .]
V.
Interaction o f High-Intensity Pulsed CO, Laser Radiation w i t h ether Semiconductors A t t h i s point,
t h e focus o f a t t e n t i o n w i l l be p l a c e d on t h e
i n t e r a c t i o n of h i g h - i n t e n s i t y p u l s e d C O P l a s e r l i g h t w i t h o t h e r Group I V and 111-V semiconductors.
I n s t e a d o f a t t e m p t i n g t o present
a complete study f o r several semiconductors,
the discussion w i l l
c o n c e n t r a t e on t h e a v a i l a b l e experimental
results f o r gallium
arsenide, indium antimonide, and germanium.
1.
GALLIUM ARSENIDE James e t a l . (1984b) have s t u d i e d t h e n o n l i n e a r a b s o r p t i o n and
o p t i c a l h e a t i n g o f zinc-doped g a l l i u m arsenide by p u l s e d C O P l a s e r radiation.
The dominant a b s o r p t i o n mechanism i s d i r e c t f r e e - h o l e
9.
601
PULSED CO2 LASER ANNEALING
t r a n s i t i o n s between s t a t e s i n t h e heavy- and l i g h t - h o l e bands. E x p e r i m e n t a l l y , it has been observed t h a t t h e intervalence-band a b s o r p t i o n by f r e e holes decreases with i n c r e a s i n g i n t e n s i t y due t o a s t a t e - f i l l i n g e f f e c t i n t h e resonant r e g i o n (Gibson e t al., 1972 and James e t al., i n k-space,
1984b).
Since t h e t r a n s i t i o n s a r e d i r e c t
b o t h energy and wave v e c t o r a r e conserved i n t h e
i n t e r v a l e n c e - b a n d o p t i c a l absorption.
Thus, o n l y holes i n a narrow
r e g i o n o f t h e heavy-hole band can d i r e c t l y p a r t i c i p a t e i n t h e absorption,
and t h e a b s o r p t i o n c o e f f i c i e n t i s governed by t h e
p o p u l a t i o n o f these h o l e s t a t e s .
A t low i n t e n s i t i e s , t h e popula-
t i o n o f heavy-hole s t a t e s i n t h e resonant r e g i o n i s maintained c l o s e t o t h e e q u i l i b r i u m v a l u e by t h e v a r i o u s s c a t t e r i n g processes. However, as t h e i n t e n s i t y becomes l a r g e , s c a t t e r i n g cannot m a i n t a i n t h e e q u i l i b r i u m p o p u l a t i o n o f t h e resonant heavy-hole s t a t e s , and t h e y become depleted.
As a r e s u l t , t h e a b s o r p t i o n o f C02 l a s e r
r a d i a t i o n i n p-type GaAs s a t u r a t e s a t h i g h i n t e n s i t i e s .
A theory
which has been successful i n e x p l a i n i n g most o f t h e measurements has been g i v e n by James and Smith (1980a). Transmission measurements were performed as a f u n c t i o n o f t h e C02 l a s e r i n t e n s i t y f o r a GaAs:Zn c r y s t a l with a h o l e d e n s i t y o f
1 x 1017
(James e t a l .
, 1984b). The decrease i n t h e a b s o r p t i o n
c o e f f i c i e n t w i t h i n c r e a s i n g i n t e n s i t y was found t o be reasonably w e l l s a t i s f i e d by
where a o ( w ) i s t h e a b s o r p t i o n c o e f f i c i e n t a t low i n t e n s i t y . I S ( w ) i s t h e s a t u r a t i o n i n t e n s i t y and has a v a l u e o f 20 MW/cm2 f o r l i g h t w i t h a wavelength o f 10.6 prn and room temperature c o n d i t i o n s . S i m i l a r t r a n s m i s s i o n measurements were performed on a GaAs:Zn c r y s t a l with a h o l e d e n s i t y o f 4 x l o L 7 ~ m ' ~ . Due t o l a r g e r f r e e h o l e a b s o r p t i o n i n these samples, i t was r e q u i r e d t h a t t h e wafers
602
R. B. JAMES
- 2,
where L i s the thickness of the sample. I t was found t h a t as the doping density was increased, higher i n t e n s i t i e s were required t o s a t u r a t e t h e resonant t r a n s i t i o n s . In addition, t h e onset of surface damage occurred a t lower i n t e n s i t i e s , and t h e samples often fractured due t o a shock wave r e s u l t i n g from the l a s e r radiation impinging on the samples. The decreased threshold f o r surface damage and the f r a c t u r e o f the samples g r e a t l y increased the d i f f i c u l t y in making a n accurate measurement of Is f o r t h e more heavily doped wafers. For GaAs:Zn c r y s t a l s w i t h a hole density of about 10l8 (3111-3 and l a r g e r , i t was found t h a t l a r g e areas of the surface could be melted by t h e absorption of t h e l a s e r r a d i a t i o n , i n c o n t r a s t t o the be mechanically t h i n n e d t o about 120 pm so t h a t q L
Fig. 20. SEM photograph of irradiated region where melting and thermal stresses have produced fissures in the material. The longest bar in the lower right-hand corner of the figure has a length o f 100 pn.
9.
603
PULSED C 0 2 LASER ANNEALING
r e s u l t s f o r moderately doped samples where small s i t e s o f damage would appear.
The s u r f a c e topography was s t u d i e d w i t h scanning
e l e c t r o n and Normarski i n t e r f e r e n c e microscopes.
Examination o f
t h e s u r f a c e a t t h e i n t e r a c t i o n r e g i o n showed s t r o n g evidence o f m e l t i n g and l a r g e thermal stresses.
Smooth p e r i o d i c r i p p l e s on t h e
s u r f a c e r e s u l t i n g from t h e process o f m e l t i n g and r e s o l i d i f y i n g were v i s i b l e .
A t h i g h e r energy d e n s i t i e s ,
f i s s u r e s develop on
t h e surface, which a r e o r i e n t e d along t h e c r y s t a l planes o f t h e s u b s t r a t e (Fig.
a). The
f o r m a t i o n o f t h e f i s s u r e s i s most l i k e l y
due t o t h e r e l i e f o f thermal s t r e s s e s i n t h e i n t e r a c t i o n r e g i o n f o l l o w i n g t h e a b s o r p t i o n o f t h e C02 l a s e r r a d i a t i o n . The f a c t t h a t GaAs i s a compound semiconductor a f f e c t s t h e requirements f o r successful annealing i n a v a r i e t y o f ways.
One
o f t h e most obvious i s t h a t pulsed l a s e r annealing can cause a l o s s o f s t o i c h i o m e t r y due t o t h e h i g h vapor pressure o f a r s e n i c r e l a t i v e t o gallium.
(See,
f o r example,
Badawi e t al.,
1980.)
The pulsed l a s e r annealing experiment by James e t a l . (1984b) was c a r r i e d out i n a i r a t room temperature w i t h o u t encapsulation.
A
C02 l a s e r beam i n t e g r a t o r was used i n an attempt t o remove s p a t i a l inhomogeneities i n t h e beam and t o o b t a i n as u n i f o r m annealing as possible.
The wafers were i r r a d i a t e d i n a i r , and x-ray f l u o r e s c e n c e
d a t a were taken on t h e annealed samples t o study t h e r a t i o o f Ga t o As emissions.
The r a t i o s were taken on samples which were
annealed under a v a r i e t y o f e x c i t a t i o n c o n d i t i o n s , and t h e r e s u l t s were compared t o t h e r a t i o i n t h e unannealed GaAs c r y s t a l .
The
energy o f t h e e l e c t r o n s was v a r i e d between 2, 5, 10, 15, and 30 KeV i n o r d e r t o c o n t r o l t h e e l e c t r o n p e n e t r a t i o n and, thereby, d i f f e r e n t depths i n t h e n e a r - s u r f a c e region.
probe
The r e s u l t s o f t h e
measurements showed t h a t a r s e n i c l o s s does r e s u l t from t h e m e l t i n g o f t h e s u r f a c e w i t h a C02 l a s e r p u l s e ( A = 10.6 urn, FWHM = 70 ns). The l o s s o f s t o i c h i o m e t r y i s g r e a t e s t w i t h i n a depth o f about 600 A, which i s t h e approximate p e n e t r a t i o n o f t h e 2 KeV e l e c t r o n s .
For
30 KeV e l e c t r o n s , t h e r a t i o s o f g a l l i u m t o a r s e n i c x-ray counts i n
t h e L and K s e r i e s f o r t h e annealed samples a r e w i t h i n 1-2%o f t h e
604
R. B. JAMES 4
X-RAY FLUORESCENCE DATA FROM Ga AND As ATOMS Ga
3-
As
h
t
0
.-
Y
-
v)
I - 2
z
3
0 0
1-
0
0
0.5
1
1.5
PHOTON ENERGY (KeV)
Fig. 21. Results o f x-ray fluorescence data f o r 2 keV electrons i n GaAs crystals before and a f t e r laser irradiation. The three curves show the fluorescence from the gallium and arsenic atoms in the following samples: curve ( a ) , an unirradiated sample; curve (b), a sample irradiated with a single shot a t an energy density o f 4.5 J / c m 2 ; and curve ( c ) , a sample irradiated with ten shots a t an energy density o f 5.9 J/cm2.
respective r a t i o s in the unirradiated GaAs wafers. T h e r e s u l t s o f the x-ray fluorescence data f o r 2 keV electrons are shown in Fig. 21 f o r a GaAs:Zn crystal with a hole concentration of 5.1 x 10l8 c w 3 . The three curves show the L-series x-ray fluorescence from gallium and arsenic atoms i n an unirradiated sample, a sample i r r a d i a t e d w i t h a single shot a t an energy density of 4.5 J/cm2, and a sample i r r a d i a t e d with ten shots a t an energy density of 5.9 J/cm2. The r a t i o s o f the counts corresponding t o gallium and a r s e n i c emissions a r e 1.05, 1.10, and 1.49, respectively.
9.
605
PULSED COZ LASER ANNEALING
As noted i n t h e f i g u r e , t h e s e v e r i t y o f t h e a r s e n i c l o s s depends on t h e energy d e n s i t y o f l a s e r pulse. Thus, t h e occurrence o f s u r f a c e e v a p o r a t i o n o f a r s e n i c s e t s an upper l i m i t on t h e energy d e n s i t y o f t h e C02 l a s e r f o r o p t i m a l , d e f e c t - f r e e r e c r y s t a l l i z a t i o n . T h i s ''window'' f o r pulsed C02 l a s e r annealing may be q u i t e narrow due t o t h e r e l a t i v e l y deep p e n e t r a t i o n o f COP l a s e r l i g h t i n t o t h e GaAs s u b s t r a t e , as compared t o t h e p e n e t r a t i o n o f l i g h t when t h e photon energy exceeds t h e i n t r i n s i c a b s o r p t i o n edge.
For near-
s u r f a c e r e g i o n s which have a l a r g e a b s o r p t i o n c o e f f i c i e n t a t C02 l a s e r wavelengths (i.e.,
f o r h e a v i l y doped s u b s t r a t e s and s h a l l o w
i m p l a n t s o r s h a l l o w i m p l a n t s which have been p a r t i a l l y a c t i v a t e d ) , an energy d e n s i t y "window" may e x i s t which r e s u l t s i n h i g h e l e c t r i c a l a c t i v a t i o n o f t h e i m p l a n t e d i o n s w h i l e remaining below t h e damage t h r e s h o l d due t o v a p o r i z a t i on. The v a p o r i z a t i o n o f a r s e n i c d u r i n g t h e molten phase causes a h i g h c o n c e n t r a t i o n o f a r s e n i c vacancies and/or p e n e t r a t i o n o f o t h e r atoms i n t o t h e a r s e n i c s i t e s . Experiments by James e t a l . (1984b) on GaAs c r y s t a l s i r r a d i a t e d i n a i r were designed t o study t h e p e n e t r a t i o n o f oxygen i n t o t h e sample d u r i n g t h e r a p i d s o l i d i f i c a t i o n immediately a f t e r C02 l a s e r m e l t i n g .
The r e s u l t s o f SIMS
measurements o f t h e depth p r o f i l e s o f l60 i n t h e near-surface r e g i o n a r e shown i n Fig. 22.
The GaAs:Zn c r y s t a l s used i n t h e experiment
have a f r e e - h o l e d e n s i t y o f 5.1 x
lo1*
c w 3 , and a l i n e a r a b s o r p t i o n
c o e f f i c i e n t a t room temperature o f about 2 x l o 3 cm-1.
The f o u r
curves i n t h e f i g u r e a r e f o r an u n i r r a d i a t e d GaAs c r y s t a l , a samp l e i r r a d i a t e d w i t h t e n shots a t an energy d e n s i t y o f 5.3 J/cm2 i n an a i r pressure o f 10-4-10-5 bar, a sample i r r a d i a t e d w i t h one s h o t a t an energy d e n s i t y o f 6.0 J/cm2 i n an a i r pressure o f 1 bar, and a sample i r r a d i a t e d w i t h t e n shots a t an energy d e n s i t y o f 6.7 J/cm* i n an a i r pressure of 1 bar.
I n t h e samples i r r a d i a t e d i n
a i r , oxygen i s i n c o r p o r a t e d i n t o t h e l a t t i c e t o depths comparable t o t h e depths a t which most o f t h e a r s e n i c l o s s occurs.
I n the
sample t h a t had t e n shots i n a i r , t h e oxygen p e n e t r a t e d t o a depth o f about 2000 A,
which i s c o n s i d e r a b l y deeper t h a n t h e oxygen
606
R. B . JAMES
0.1
0.0
Fig. 22.
0.2
(
SlMS measurements o f the depth p r o f i l e s o f l60i n the near-surface
region i n samples before and a f t e r pulsed C02 laser irradiation. curves shown in the figure are for the following samples: sample;
*,
0,
The four
an unirradiated
a sample irradiated with ten shots a t an energy density o f 5.3 j / c m 2
in an air pressure o f
-
bar;
A,
a sample irradiated with one shot at
an energy density o f 6.0 J / c m 2 i n an air pressure of 1 bar; and m, a sample irradiated with ten shots at an energy density o f 6.7 j / c m 2 in an air pressure
o f 1 bar.
i n c o r p o r a t i o n t h a t r e s u l t s from a s i n g l e shot i n a i r .
An i n c r e a s e
i n t h e oxygen counts i s observed t o a depth o f about 800 A i n t h e w a f e r which was i r r a d i a t e d w i t h 10 shots i n an a i r pressure o f 10-'+-10-5
bar, a l t h o u g h t h e c o n c e n t r a t i o n o f oxygen i n t h e f i r s t
200 A i s much l e s s t h a n i t was f o r t h e wafer i r r a d i a t e d w i t h a
s i n g l e shot i n one bar o f a i r . tion,
T h i s evidence o f oxygen i n c o r p o r a -
r e s u l t i n g from p u l s e d l a s e r annealing o f GaAs, p o i n t s con-
c l u s i v e l y t o t h e importance o f t h e immediate envi ronment d u r i n g t h e high-temperature c y c l e ( t h i s i s n o t t h e case f o r S i ) .
9.
607
PULSED CO2 LASER ANNEALING
It i s n o t c l e a r a t t h i s t i m e whether t h e d i f f u s i o n o f oxygen
i n t o t h e near-surface r e g i o n i s enhanced by t h e a r s e n i c vacancies t h a t are present, o r whether t h e p e n e t r a t i o n o f oxygen enhances t h e a r s e n i c loss by f o r c i n g t h e a r s e n i c toward t h e surface.
In addi-
t i on , t h e bondi ng c o n f ig u r a t i on o f t h e oxygen impuri t ies i s pres e n t l y unknown.
If t h e l o c a l i z e d o x i d e l a y e r s can be formed i n a
c o n t r o l l a b l e way, t h i s beam p r o c e s s i n g technique may be u s e f u l i n t h e f o r m a t i o n o f t h i n i n s u l a t i n g f i l m s on GaAs.
S I M S measurements were a l s o made o f t h e depth p r o f i l e s o f z i n c i n t h e COP l a s e r - i r r a d i a t e d samples. The z i n c was u n i f o r m l y doped i n t h e wafers, and a c o n s t a n t count r a t e f o r z i n c atoms was observed prior t o irradiation.
A f t e r i r r a d i a t i o n , t h e count r a t e f o r z i n c
was found t o decrease near t h e surface.
For a C02 l a s e r energy
d e n s i t y of 6.2 J/cm2, a n o t i c e a b l e decrease i n t h e z i n c counts was observed t o a depth o f about 5000 A, although t h e g r e a t e s t l o s s occurred a t depths o f 150 t o 3000 A f r o m t h e surface, where t h e z i n c counts were o n l y 6 5 7 0 % o f t h e counts i n t h e u n i r r a d i a t e d
wafer.
A t depths g r e a t e r than 5000 A, t h e count r a t e f o r z i n c atoms was uniform, as observed i n t h e wafer p r i o r t o l a s e r annealing. P r e s e n t l y , more work i s needed on t h e C02 l a s e r annealing o f GaAs t o determine i f a s u i t a b l e energy d e n s i t y "window" e x i s t s . Samples i r r a d i a t e d i n a i r are found t o have a d e f i c i e n c y o f As and marked i n c o r p o r a t i o n of oxygen,
b o t h o f which e f f e c t s cause
d e t e r i o r a t i o n o f t h e e l e c t r i c a l p r o p e r t i e s o f t h e wafers.
5.
INDIUM ANTIMONIDE Several experiments on t h e t r a n s m i s s i o n o f h i g h - i n t e n s i t y C02
l a s e r r a d i a t i o n t h r o u g h InSb have been r e p o r t e d f o r pulses o f nanosecond (Fossum e t al.,
1973b; Gibson e t a l .
, 1976;
Nee e t a1
., 1978;
and Jamison and Nurmikko, 1979) and picosecond (Schwartz e t al., 1980 and Hassel beck and Kwok, 1982) d u r a t i o n s .
These i n v e s t i g a t i o n s
have confirmed t h a t h e a t i n g and subsequent m e l t i n g o f t h e c r y s t a l can occur by f r e e - c a r r i e r absorption. The r e s u l t s o f t h e e x p e r i ments w i t h nanosecond pulses will be discussed f i r s t .
608
R. B. JAMES
n
9
Io5 L
- lo4
c u) .C 3
-
?!
Y
/o'-o-O-o
% c .c 5 lo= C
0
v)
/:
/ooO
-
.E loo t e + -
bpo /
0)
c
u)
a
lo'
0
/O
0
s
0
7 :/ -
0 0
- 0 0
Ts20K T = 77K
-0
-
1
Fig. lattice
23.
I 1111l1l
I
1
I
1
Illustration o f high-intensity
temperatures
of
20
and
77 K.
I
I
1
1
transmission [After
I
1
limit
in lnSb at
Jamison and Nurmikko
( 1 9 7 9 ) .]
E x p e r i m e n t a l l y , i t has been observed t h a t InSb e x h i b i t s a d i s t i n c t h i g h - i n t e n s i t y t r a n s m i s s i o n l i m i t f o r CO,
l a s e r l i g h t beyond
whichthetransmittancestronglydecreaseswithincreasing intensity. The onset o f t h i s l a s e r - i n d u c e d o p a c i t y occurs w e l l below t h e damage threshold o f the material.
The r e s u l t s f o r t h e t r a n s m i s s i o n o f l - n s
p u l s e s w i t h a wavelength of 10.6 urn are shown i n Fig. 23 f o r l a t t i c e temperatures o f 77 K and about 20 K (Jamison and Nurmikko, 1979). F o r a l a t t i c e temperature o f 20 K, t h e t r a n s m i t t a n c e a b r u p t l y decreases a t an i n c i d e n t i n t e n s i t y o f 2 MW/cm2.
A t h i g h e r tempera-
t u r e , t h e r e i s a much more gradual decrease i n t h e t r a n s m i t t a n c e ,
9.
609
PULSED COz LASER ANNEALING
and t h e onset of t h e n o n l i n e a r absorptTon occurs a t somewhat lower intensities.
Abrupt t r a n s m i s s i o n l i m i t s were a l s o measured by
Jamison and Nurmikko (1979) f o r Hg0.77Cd0,23Te
a t 20 K and InAs
a t 300 K f o r 10.6 pm l i g h t . S i m i l a r o b s e r v a t i o n s o f n o n l i n e a r a b s o r p t i o n i n InSb have been r e p o r t e d a t 4 K by Nee e t a l . i n t e n s i t i e s above 2 MW/cm2,
(1978) f o r 40-ns pulses and
by Fossum e t a l .
(1973) a t 77 K f o r
100-ns pulses and i n t e n s i t i e s above 1 MW/cm2, and Gibson e t a l . (1976) a t 300 MW/cm*.
K f o r 50-ns pulses and i n t e n s i t i e s g r e a t e r t h a n 1
The observed decrease i n t h e t r a n s m i t t a n c e i s a s s o c i a t e d
w i t h t h e g e n e r a t i o n o f nonequil i b r i u m e l e c t r o n - h o l e subsequent
free-carrier
absorption.
p a i r s and
The proposed mechanisms
r e s p o n s i b l e f o r t h e g e n e r a t i o n o f nonequil i b r i um c a r r i e r s have been based on t h e occurrence o f two-photon a b s o r p t i o n (Gibson e t a1
., 1976)
and impact i o n i z a t i o n events (Jamison and Nurmikko,
1980 and James,
1983), which a r e b o t h f u n c t i o n s o f t h e l a t t i c e
temperature. Picosecond pulses were used t o reduce t h e e f f e c t s o f sample h e a t i n g i n t h e t r a n s m i s s i o n measurements.
Pulses o f c o n t i n u o u s l y
v a r i a b l e d u r a t i o n between 5 and 60 ps and c o n s t a n t a m p l i t u d e were i n c i d e n t on a sample o f InSb (ne = 5 x lOI3 cm-3 a t 77 K) having a t h i c k n e s s o f 350 p (Schwartz e t a1
., 1980).
The r e s u l t s o f t h e
t r a n s m i s s i o n experiments are shown i n Fig. 24 f o r l a t t i c e temperat u r e s a t 20, 88, and 295 K.
For pulses w i t h a d u r a t i o n of l e s s
t h a n 12 ps and an i n t e n s i t y o f l e s s t h a n 30 MW/cm2, no n o n l i n e a r t r a n s m i t t a n c e was observed a t 20 and 88 K. As t h e p u l s e d u r a t i o n was i n c r e a s e d w h i l e m a i n t a i n i n g a f i x e d p u l s e i n t e n s i t y , a reduced t r a n s m i s s i o n was observed which shows t h a t t h e nonl i n e a r absorpt i o n occurs a t lower i n t e n s i t i e s f o r pulses o f l o n g e r d u r a t i o n . A t room temperature, t h e onset o f n o n l i n e a r t r a n s m i s s i o n occurred f o r pulses as s h o r t as 5 ps a t an i n t e n s i t y o f 20 MW/cm2 (Fig. 24). The experimental r e s u l t s a t 20 K were i n t e r p r e t e d i n terms o f t h e g e n e r a t i o n o f excess e l e c t r o n - h o l e events
and
subsequent
strong
p a i r s by impact i o n i z a t i o n
a b s o r p t i o n by
intervalence-band
610
R. B. JAMES
100
80
60 40
-.4-03
c
3
20
0
80
60 40
20 0
a
I-
Y I T = 295K
8oj
100
/
//
60
/
/
/
0
10
20
30 40
50
60
INCIDENT PULSE DURATION (psec)
Fig. 24. High-intensity 10.6-bm transmission o f picosecond pulses i n lnSb a t d i f f e r e n t lattice temperatures. The intensities I1 are shown in units of MW/cm2. The dashed lines depict the transmission o f 30-MW/cm2 pulses i n the absence o f nonlinearities. [ A f t e r Schwartz e t al. (1980). 1
9.
611
PULSED COz LASER ANNEALING
A t h i g h e r l a t t i c e temperatures, t h e two-photon a b s o r p t i o n process i s much more s i g n i f i c a n t due t o t h e decrease i n t h e bandgap o f InSb w i t h i n c r e a s i n g temperature.
transitions.
Ref1 e c t i v i t y measurements have been performed by Hassel beck and Kwok (1982) w i t h 75-ps COP l a s e r pulses on a sample o f uncoma t 300 K).
pensated i n t r i n s i c InSb (n = 1.2 x 10l6
The r e -
f l e c t a n c e i s shown i n Fig. 25 as a f u n c t i o n o f the. peak i n t e n s i t y incident
on t h e c r y s t a l
B r e w s t e r ' s angle.
surface,
which was
oriented a t
the
The i n t e n s i t i e s were determined by t h e f o c u s i n g
geometry o f t h e l a s e r .
A t i n t e n s i t i e s l e s s than about 0.4 GW/cm2,
t h e r e f l e c t i v i t y remains constant a t 4%.
A t an i n t e n s i t y g r e a t e r
t h a n 0.4 GW/cm2, t h e r e f l e c t i v i t y increases t o 20% and t h e n drops t o 10% a t about 1.3 GW/cm2.
The damage t h r e s h o l d o f t h e sample
was measured t o be 4.3 GW/cmz, which i s c o n s i d e r a b l y g r e a t e r t h a n t h e damage t h r e s h o l d o f 40 MW/cm* measured by Kruer e t a l . (1977)
25 20 15 0
10
5 -0 0
i
o%o
0
o.oo
0 0
I
I
I
Fig. 25. integrated reflectivity o f 7 5 9 s laser pulses a s a function o f peak intensity. A nonlinear plasma generation threshold and a melting threshold can be identified. [After Haasselbeck and Kwok ( 1 9 8 2 ) . ]
612
R. B. JAMES
f o r p u l s e s with 170-ns d u r a t i o n .
The experiment o f Hasselbeck and
Kwok (1982) was repeated several times by v a r y i n g t h e i n t e n s i t y f r o m weak t o s t r o n g t o v e r i f y t h a t no permanent damage occurred t o t h e sample d u r i n g t h e measurement. The f o l l o w i n g i n t e r p r e t a t i o n has been proposed by Hassel beck and Kwok (1982) t o d e s c r i b e t h e measured r e s u l t s .
A t an i n t e n s i t y
o f 0.4 GW/cm2, a s u f f i c i e n t l y dense e l e c t r o n - h o l e plasma was gene r a t e d i n t h e c r y s t a l t o s i g n i f i c a n t l y modify t h e r e f l e c t i v i t y o f t h e sample.
A t 1.3 GW/cm* t h e dense plasma (-10l8 f r e e c a r r i e r s / c m 3 )
absorbs enough energy t o m e l t t h e c r y s t a l , r e s u l t i n g i n a l a r g e
At
i n c r e a s e i n t h e a b s o r p t i o n and decrease i n t h e r e f l e c t i v i t y .
4.0 GW/cm2, t h e s u r f a c e i s so h o t t h a t v a p o r i z a t i o n and e l e c t r o n i o n emission occur a t t h e surface.
4.3
GW/cm2,
A t i n t e n s i t i e s g r e a t e r than
t h e s u r f a c e plasma i s s u f f i c i e n t l y dense t h a t t h e
a b s o r p t i o n o f t h e l a s e r r a d i a t i o n by t h e plasma i s dominant.
A
spark i s formed and t h e r e s u l t i n g shock wave forms a c r a t e r on t h e s u r f a c e o f t h e m o l t e n semiconductor. One o f t h e most s i g n i f i c a n t r e s u l t s o f these measurements i s t h a t t h e r e e x i s t s a range o f i n t e n s i t i e s where m e l t i n g o f t h e surf a c e l a y e r occurs w i t h o u t permanent s u r f a c e damage. The r a t i o o f t h e damage t h r e s h o l d t o t h e m e l t i n g t h r e s h o l d i s approximately t h r e e (Hasselbeck and Kwok, 1982), which i s a c o m f o r t a b l e margin f o r semiconductor processing. A t present, o n l y scanning e l e c t r o n microscopy (SEM) photographs have been used t o examine t h e r e c r y s t a l l i z e d surfaces.
Since
a r s e n i c loss i s known t o occur i n laser-annealed GaAs and t h e a n t i mony vapor pressure i s h i g h compared t o t h a t o f indium, f u r t h e r s t u d i e s should be made o f t h e p o s s i b l e departures from s t o i c h i ometry i n InSb as a r e s u l t o f pulsed l a s e r annealing. 6.
GERMANIUM The a b s o r p t i o n o f h i g h - i n t e n s i t y C02 l a s e r r a d i a t i o n by f r e e
c a r r i e r s i n Ge should be d i v i d e d i n t o two c a t e g o r i e s :
t h e absorp-
t i o n a s s o c i a t e d w i t h f r e e - h o l e c a r r i e r s and t h a t a s s o c i a t e d w i t h
9.
613
PULSED C 0 2 LASER ANNEALING
free-electron carriers.
E x p e r i m e n t a l l y , i t has been observed t h a t
t h e a b s o r p t i o n by f r e e h o l e s decreases with i n c r e a s i n g i n t e n s i t y (Gibson e t al.,
1972), and t h a t t h e a b s o r p t i o n by f r e e e l e c t r o n s
i n c r e a s e s w i t h i n c r e a s i n g i n t e n s i t y (Yuen e t al.,
1979).
Both o f
t h e s e n o n l i n e a r o p t i c a l p r o p e r t i e s have p r a c t i c a l uses t h a t have s t i m u l a t e d much o f t h e research on t h e i n t e r a c t i o n o f h i g h - i n t e n s i t y pulsed C02 l a s e r r a d i a t i o n w i t h germanium (James and Smith, 1982a).
In p-type germanium, d i r e c t f r e e - h o l e t r a n s i t i o n s between t h e heavy- and l i g h t - h o l e
bands a r e p r i m a r i l y r e s p o n s i b l e f o r t h e
a b s o r p t i o n o f l i g h t i n t h e 6- t o 25-pm r e g i o n (Kahn,
1955).
For
C02 l a s e r i n t e n s i t i e s g r e a t e r t h a n about 1 bM/cm2, t h e a b s o r p t i o n c o e f f i c i e n t due t o i n t e r v a l e n c e - b a n d t r a n s i t i o n s decrease.
i s found t o
T h i s n o n l i n e a r i t y i n t h e a b s o r p t i o n i s found t o be w e l l
2.0 -
PHIPPS AND THOMAS CARLSON et 01. B KEILMANN JAMES et ai. 0
-
A
+
Fig. 26.
295K.
Saturation intensity as a function o f the photon energy for p-Ge at
The calculated values are shown by the solid curve, and the experimental
results are from Phipps and Thomas ( 1 9 7 7 ) , Carlson et al.
( 1 9 7 6 ) , and James et al.
(1982b).
( 1 9 7 7 ) , Keilrnann
[ A f t e r James and Smith (1982b3.1
614
R. B. JAMES
s a t i s f i e d by Eq.
(15).
The measured values o f t h e s a t u r a t i o n
i n t e n s i t y I S ( w ) a r e shown i n Fig.
26 as a f u n c t i o n o f t h e photon
energy f o r doping d e n s i t i e s l e s s t h a n about 1 0 l 6 cm’3 1976; Phipps and Thomas, al.,
1982b).
1977; Carlson e t al.,
(Keilmann,
1977; and James e t
The s o l i d c u r v e on t h e f i g u r e shows t h e t h e o r e t i c a l
values o f I S ( w ) c a l c u l a t e d by James and Smith (1979).
A t i n t e n s i t i e s much g r e a t e r than I s i n l i g h t l y o r moderately doped m a t e r i a l
,
t h e t r a n s m i s s i o n model which produces Eq.
b e g i n s t o break down.
For a 1.3-ns
(15)
pulse, t h i s breakdown occurs
f o r averaged i n t e n s i t i e s g r e a t e r t h a n 200 MW/cm2 (Phi pps and Thomas, 1977).
F o r i n t e n s i t i e s g r e a t e r t h a n t h e breakdown t h r e s h o l d , t h e
c a r r i e r d e n s i t y increases a b r u p t l y . free-carrier
density
This abrupt increase i n the
has been a t t r i b u t e d t o impact i o n i z a t i o n
events by James and Smith (1982a) and t o r n u l t i p h o t o n t r a n s i t i o n s
.
by Yuen e t a1 ( 1980).
Time-resolved p h o t o c o n d u c t i v i t y measurements
have been made on n-type samples by Yuen e t a l .
(1980) and on
i n t r i n s i c samples by James ( 1 9 8 4 ~ ) . I n t h e experiments, t h e photoresponse was measured as a f u n c t i o n o f t h e l i g h t i n t e n s i t y f o r 10.6-vm r a d i a t i o n .
The r e s u l t s o f these measurements have confirmed
t h a t t h e generated c a r r i e r s a r e n o t i n e q u i l i b r i u m w i t h t h e l a t t i c e and t h e r e f o r e a r e n o t due s o l e l y t o o p t i c a l heating. Measurements o f t h e photovol t a g e were made by James ( 1 9 8 4 ~ )on i n t r i n s i c germanium c r y s t a l s w i t h a t h i c k n e s s o f 0.4 nun.
For a
70-ns pulse, t h e onset o f a p h o t o c o n d u c t i v i t y s i g n a l was found t o occur a t an energy d e n s i t y o f 1.3 Jjcmz, a f t e r t a k i n g i n t o account t h e r e f l e c t i o n l o s s a t t h e f r o n t surface. be d e t e c t e d a t lower energy d e n s i t i e s ,
Some p h o t o v o l t a g e c o u l d b u t t h e s i g n a l was much
smaller,
and t h e t r a n s m i s s i o n o f t h e l a s e r r a d i a t i o n was s t i l l
linear.
The photoresponse o f t h e c r y s t a l s was found t o decay w i t h
a t i m e c o n s t a n t o f about 40 us, which i s c o n s i s t e n t w i t h t h e e l e c t r o n - h o l e recombination r a t e i n a sample o f i n t r i n s i c germanium w i t h t h i s thickness.
I n a d d i t i o n t o t h e l a r g e peak w i t h a 4 0 - v ~
decay time, t h e r e was a l o n g t a i l o f much s m a l l e r magnitude i n t h e p h o t o v o l t a g e which l a s t e d f o r more t h a n 20 ms.
The l o n g t a i l i s
9.
615
PULSED COz LASER ANNEALING
probably due t o thermal e f f e c t s , i n which some f r e e c a r r i e r s are t h e r m a l l y generated i n t h e sample by t h e energy d e p o s i t e d by t h e l a s e r pulse.
As t h e sample s l o w l y c o o l s down, t h e r e i s a decrease
i n t h e e q u i l i b r i u m c a r r i e r c o n c e n t r a t i o n , which g i v e s r i s e t o t h e l o n g t a i l on t h e p h o t o v o l t a g e s i g n a l .
The experiments were a l s o
performed w i t h a s t r o b e l i g h t as an e x c i t a t i o n source, same e l e c t r o n - h o l e recombination r a t e was measured.
and t h e
The measured
p h o t o v o l t a g e due t o t h e s t r o b e showed no l o n g t a i l , which would be expected s i n c e t h e s t r o b e l i g h t d i d n o t heat t h e sample. When t h i s i n t e n s i t y t h r e s h o l d f o r e l e c t r o n - h o l e plasma f o r m a t i o n i s exceeded, t h e a b s o r p t i o n c o e f f i c i e n t can g r e a t l y i n c r e a s e due t o t h e subsequent f r e e - c a r r i e r t r a n s i t i o n s , and thermal energy deposit i o n near t h e s u r f a c e becomes l a r g e enough t o m e l t t h e c r y s t a l . P e r i o d i c r i p p l e f o r m a t i o n s are e a s i l y observed f o r i n t e n s i t i e s approximately t w i c e as l a r g e as t h e plasma f o r m a t i o n t h r e s h o l d . I n n-type
germanium,
t h e dominant
intraband f r e e - e l e c t r o n absorption,
a b s o r p t i o n mechanism i s
where an e l e c t r o n absorbs a
photon and i s e x c i t e d t o a s t a t e i n t h e same band.
The cross sec-
t i o n f o r t h i s process i s much s m a l l e r t h a n f o r i n t e r v a l e n c e - b a n d free-hole
transitions
(Fan e t al.,
1956), since f o r intraband
absorption, t h e c o n s e r v a t i o n o f energy and c r y s t a l momentum cannot b o t h be s a t i s f i e d w i t h o u t i n v o l v i n g a t h i r d p a r t i c l e . measurements w i t h 90-ns pulses o f 9 . 6 - p
Transmission
r a d i a t i o n have been made
on t h i c k n-type germanium c r y s t a l s by Yuen e t a l .
(1980).
Figure
27 shows t h e t r a n s m i t t e d energy E t t h r o u g h c r y s t a l s o f v a r i o u s l e n g t h s L, h e l d a t room temperature, as a f u n c t i o n o f t h e i n c i d e n t energy d e n s i t y E i o f t h e pulses. The samples were l i g h t l y doped w i t h antimony t o a r e s i s t i v i t y a t room temperature o f about 10 9cm and had an a b s o r p t i o n c o e f f i c i e n t o f about 0.02 cm’l laser radiation.
f o r C02
The energies shown i n t h e f i g u r e a r e energies
i n s i d e t h e c r y s t a l , with r e f l e c t i o n losses a l r e a d y t a k e n i n t o account.
For t h e t h i c k e r samples, t h e t r a n s m i s s i o n e x h i b i t s a
sudden t r a n s i t i o n from l i n e a r t o n o n l i n e a r absorption, q u i c k l y reaches a maximum.
For t h e 2.5-cm
and E t
c r y s t a l , t h i s maximum
616
R. B. JAMES
!.5
I
I 0 L= A L = 0 L = L=
0.6 cm 2.5 cm
0
0
0
i0 crn i5crn
0
h
N
>5
4.0
O0
-
0
u
>
o
(3 (L
AAA
A
W
z
A
A A
A
O O
0
w
0
w
c
+ -
5 z
0.5
a
OOOO
U
0
I-
IQ 0
fn
I
I
u O
0 0
I
INCIDENT ENERGY (J/crn2) Fig. 27. Transmitted 9.6-p~n CO 2 laser energy as a function of incident laser energy for n-type germanium crystals o f various lengths. [ A f t e r Yuen et al. (1980). ]
value o f t h e t r a n s m i t t e d energy d e n s i t y occurs a t E i = 1.1 J/cm2 (corresponding t o an averaged i n t e n s i t y of about 12 MW/cm2). maximum value o f E t i s higher i n t h e t h i n n e r c r y s t a l s . 0.6-cm
sample,
The
For t h e
t h e maximum transmission was not reached a t t h e
h i g h e s t i n c i d e n t energy d e n s i t y a t t a i n e d w i t h t h e experimental setup. P h o t o c o n d u c t i v i t y measurements were made by Yuen e t al. (1979) on a n-type sample doped w i t h antimony t o a r e s i s t i v i t y o f 10 Qcm a t 300
K.
For a 80-ns pulse a t a wavelength o f 9.6 pm, a photo-
v o l t a g e i s observed for i n t e n s i t i e s g r e a t e r than about 10 MW/cm2 (Fig.
28).
The p h o t o c o n d u c t i v i t y signal i s found t o be d i r e c t l y
9. I
PULSED C02 LASER ANNEALING
I
I
I
I
I
I
1.5
c c
.c
=-,
e
L
-wg 1.0 +
0
a
5 0
>
8a 0.5 I
0 0
Fig. 28. f o r a 10-cm
10 20 INCIDENT INTENSITY
30
Photovoltage as a function of incident 9.6-pm long germanium crystal.
function of time.
[ A f t e r Yuen et al.
40
I;(MW em-2) C02 laser intensity
The insert shows the photovoltage as a (1979).]
proportional t o the incident i n t e n s i t y and has a decay time of about 100 p s . The overshoot f o r times greater than 180 p s was interpreted t o r e s u l t from a decrease in t h e electron mobility due t o heating effects. The overshoot disappears a f t e r approximately 2.5 ms f o r the 10-cm long c r y s t a l s . Similar transmission measurements were a1 so performed on thinner c r y s t a l s t o i n v e s t i g a t e the onset of the nonlinear absorption and the maximum transmitted energy density a t t a i n a b l e (James, 1 9 8 4 ~ ) . The c r y s t a l s were ultrapure germani um w i t h a room temperature r e s i s t i v i t y o f 43 66-cm and a thickness of 0.4 mn. The l a s e r pulses were multimode, with 80% o f the energy in the form of a peak of 70 ns (FWHM) and the remaining 20% in a long t a i l which l a s t s f o r hundreds o f nanoseconds. An i n t e g r a t o r was used in an attempt
618
R. B. JAMES
t o s p a t i a l l y homogenize t h e beam. The onset o f t h e nonlinear transmission occurs a t an incident energy density of 1.4 J/cm2 inside the c r y s t a l s and i s more gradual than the nonlinear transmission observed by Yuen e t al. (1980) i n thick germanium c r y s t a l s . The transmitted energy density remains almost constant f o r incident energy d e n s i t i e s between 2.0 and 2.8 J/cm2. For incident energy d e n s i t i e s greater than 3.0 J/cm2, t h e r e i s a spark which appears a t the germanium surface and a sudden drop in t h e transmission. A l a r g e increase i n t h e peak of the time-resolved photoconductivity response i s also observed when a f l a s h of v i s i b l e l i g h t appears a t t h e germanium surface. R i pple-1 i ke f e a t u r e s have been observed a t s l i g h t l y higher energy d e n s i t i e s with Normarski optical and A t incident energy d e n s i t i e s scanning electron microscopes. g r e a t e r than about 9 J/cm2, cracks appear a t t h e surface which a r e oriented along crystal planes. Similar f e a t u r e s of l a s e r damage i n germanium have been observed by Willis and Emmony (1975). For heavily doped germanium c r y s t a l s , t h e l i n e a r absorption i s l a r g e enough t o d i r e c t l y heat t h e s u b s t r a t e without invoking the occurrence of c a r r i e r mu1 t i pl i c a t i on and subsequent free-carri er absorption. These intensity-dependent n o n l i n e a r i t i e s i n t h e absorpt i o n may be present, b u t t h e measurements of Yuen et a l . (1980) on n-type samples and James e t a l . (1982b) on p-type samples indic a t e t h a t t h e nonlinear absorption f o r a fixed l a s e r i n t e n s i t y is less important as the doping density i s increased. Due t o t h e s i m i l a r i t i e s of germanium with s i l i c o n , one expects t h a t t h e surface layer of heavily doped germanium can be melted w i t h a pulsed C02 l a s e r without damage t o t h e samples. Following i r r a d i a t i o n with a pulsed C02 l a s e r , r i p p l e - l i k e features appear over the interaction region a t s u f f i c i e n t l y h i g h energy d e n s i t i e s , which strongly suggests t h a t melting does occur. The threshold f o r the formation of surface r i p p l e s i s found t o be lower in samples with lower r e s i s t i v i t i e s (James, 1 9 8 4 ~ ) . This i s expected since the energy deposition i s dominated by f r e e - c a r r i e r t r a n s i t i o n s . Additional experiments should be performed on t h e pulsed C02 l a s e r
9.
619
PULSED CO2 LASER ANNEALING
annealing o f i o n - i m p l a n t e d germanium samples w i t h d i f f e r e n t f r e e c a r r i e r c o n c e n t r a t i o n s t o determine t h e energy d e n s i t i e s r e q u i r e d f o r t h e removal o f
i m p l a n t a t i o n damage and a c t i v a t i o n o f t h e
i m p l a n t e d species.
VI. The a b s o r p t i o n o f CO
Summary and Conclusions
2
l a s e r r a d i a t i o n i n most doped semiconduc-
t o r s i s dominated by f r e e - c a r r i e r t r a n s i t i o n s . high l i g h t intensities,
For s u f f i c i e n t l y
t h e energy d e p o s i t e d by t h e l a s e r p u l s e
can m e l t t h e near-surface region.
H e a v i l y doped s i l i c o n c r y s t a l s
amorphized by i o n i m p l a n t a t i o n a r e observed t o r e c r y s t a l l i z e almost c o m p l e t e l y a f t e r annealing w i t h a 100-ns p u l s e a t an i n t e n s i t y o f about 40 MW/cm2.
R e c r y s t a l l i z a t i o n o f amorphous l a y e r s has a l s o
been achieved w i t h l i g h t l y doped s i l i c o n a t comparable i n t e n s i t i e s by p r e h e a t i n g o f t h e s u b s t r a t e t o i n c r e a s e t h e a b s o r p t i o n c o e f ficient.
Channeling measurements have confirmed t h a t t h e regrowth
o f t h e amorphous l a y e r i s e p i t a x i a l t o t h e s u b s t r a t e , t h e r e i s h i g h s u b s t i t u t i o n a l i t y o f 8, As, results of
and t h a t
and Sb implants.
The
SIMS measurements show t h a t these dopants can d i f f u s e
t o depths as g r e a t as -8000 A a f t e r i r r a d i a t i o n o f t h e samples w i t h a pulsed
C O P l a s e r , which i s comparable t o o r deeper t h a n t h e depth
one can achieve w i t h a v i s i b l e o r u l t r a v i o l e t l a s e r .
TEM s t u d i e s
o f boron-implanted c r y s t a l l i n e s i l i c o n show t h a t m e l t depths i n excess o f 8000 A a r e o b t a i n a b l e w i t h o u t l a s e r - i n d u c e d d e f e c t s i n t h e annealed region.
Furthermore, by c o n t r o l 1 i n g t h e a b s o r p t i o n coef-
f i c i e n t i n d i f f e r e n t r e g i o n s near t h e surface, one can p r e f e r e n t i a l l y d e p o s i t t h e COP l a s e r energy, and thereby m e l t r e g i o n s which a r e embedded i n t h e n e a r - s u r f a c e region.
It i s u n l i k e l y t h a t t h i s
t y p e o f m e l t i n g can be o b t a i n e d u s i n g a l a s e r wavelength a t which t h e a b s o r p t i o n i s dominated by an i n t r i n s i c a b s o r p t i o n process. M e l t i n g w i t h a pulsed C02 l a s e r i s a l s o observed i n Ge and i n compound semiconductors,
such as GaAs and InSb.
For Ge and InSb,
t i m e - r e s o l v e d p h o t o c o n d u c t i v i t y measurements show t h a t n o n e q u i l i b rium electron-hole
p a i r s a r e generated p r i o r t o t h e onset o f
R. B . JAMES m e l t i n g . The mechanism r e s p o n s i b l e f o r t h e n o n l i n e a r a b s o r p t i o n i s impact i o n i z a t i o n and/or mu1t i photon events. C a r r i e r mu1t i p l i c a t i o n processes and subsequent f r e e - c a r r i e r a b s o r p t i o n suggest t h e p o s s i b i l i t y of l a s e r annealing samples which are o p t i c a l l y t h i n a t low l i g h t i n t e n s i t i e s w i t h a l a s e r having a photon energy w e l l below t h e bandgap o f t h e m a t e r i a1 , a1 though homogeneity requirements may be more s t r i n g e n t under these e x c i t a t i o n c o n d i t i o n s . I n conclusion,
one f i n d s t h a t i n t r i n s i c a b s o r p t i o n processes
a r e n o t r e q u i r e d i n t h e l a s e r a n n e a l i n g o f semiconductors.
In fact,
t h e r e may be d i s t i n c t advantages i n u s i n g a l a s e r f o r which t h e a b s o r p t i o n i s dominated by f r e e - c a r r i e r
transitions.
The two
g r e a t e s t advantages a r e t h a t one can c o n t r o l t h e a b s o r p t i o n c o e f f i c i e n t i n such a way as t o a l l o w f o r a deeper p e n e t r a t i o n o f t h e l i g h t t h a n i s a c h i e v a b l e w i t h a v i s i b l e o r u l t r a v i o l e t l a s e r , and one can p r e f e r e n t i a l l y d e p o s i t t h e l a s e r energy i n c e r t a i n l a y e r s which may be embedded i n m a t e r i a l t h a t i s r e l a t i v e l y t r a n s p a r e n t t o the laser light. free-carrier implanting.
T h i s t y p e o f h e a t i n g i s p o s s i b l e by v a r y i n g t h e
density
i n a g i v e n r e g i o n t h r o u g h doping o r i o n
F u r t h e r s t u d i e s are r e q u i r e d t o m r e f u l l y understand
t h e c r y s t a l l i n i t y o f t h e m e l t e d amorphous l a y e r s , t h e energy dens i t y "windows" f o r successful annealing, t h e s t o i c h i o m e t r y prob-
1ems t h a t occur w i t h t h e compound semiconductors , and homogeneity r e q u i r e m e n t s o f t h e CO, l a s e r beam in a c h i e v i n g u n i f o r m j u n c t i o n depths over reasonable areas.
Acknowledgments I t is a p l e a s u r e f o r me t o thank R. Jr.,
M.
I. Baskes, M. S.
Daw, and G.
commenting on t h i s manuscript. Luck,
T. K. M i l l e r , and
a r a t i o n o f t h i s chapter.
F. Wood, G. E. J e l l i s o n , J. Thomas f o r r e a d i n g and
I would a l s o l i k e t o thank J. T.
C. J. P r i c e f o r a s s i s t a n c e w i t h t h e prep-
9.
PULSED COz LASER ANNEALING
621
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CHAPTER 10
APPLICATIONS
OF
PULSED LASER PROCESSING
R. T. Young R. F. Wood
. . . . . . . . .
I. INTRODUCTION 11. EXCIMER LASERS AND EXCIMER LASER PROCESSING 1. Excimer Lasers 2. Comparison o f Annealing C h a r a c t e r i s t i c s o f Excimer and S o l i d - s t a t e Lasers 3. E f f e c t o f Pulse D u r a t i o n on Annealing 111. PHOTOVOLTAIC APPLICATIONS 4. Laser Processing and High E f f i c i e n c y Solar Cells. 5. F a b r i c a t i o n o f S o l a r C e l l s by Beam-Processing Techniques 6. I n f l u e n c e o f Dopant P r o f i l e on S u r f a c e Recombination 7. Laser-Induced Dopant D i f f u s i o n 8. Laser Damage G e t t e r i n g . 9. G r a i n Boundary Studies. 10. Summary. IV. OTHER D E V I C E APPLICATIONS 11. I m p a t t Diodes. 12. Silicon-on-Sapphire. 13. I n t e g r a t e d C i r c u i t s . V. LASER PHOTOCHEMICAL PROCESSING VI. SUBMICRON OPTICAL LITHOGRAPHY. V I I . SUMMARY AND CONCLUDING REMARKS REFERENCES
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Copyright 01984 by Academic Press, Inc. All righrs of reproduction in any form reserved.
ISBN 0-12-752123-2
626
R. T. YOUNG E T A L .
I.
Introduction
The s t u d y o f p u l s e d l a s e r p r o c e s s i n g o f semiconductors has developed s i n c e 1977 as one o f t h e most dynamic areas o f d e v i c e r e l a t e d research. twofold:
The d r i v i n g f o r c e s u n d e r l y i n g t h i s research a r e
(1) The u l t r a r a p i d m e l t i n g and s o l i d f i c a t i o n induced by
high-intensity
l a s e r pulses provide a unique t o o l t o s o l i d s t a t e
p h y s i c i s t s and m a t e r i a l s c i e n t i s t s f o r t h e s t u d y o f nonequi l i b r i u m c r y s t a l growth and t h e a s s o c i a t e d s u r f a c e m o d i f i c a t i o n phenomena. ( 2 ) The new c a p a b i l i t y o f a c h i e v i n g s u r f a c e p r o c e s s i n g w h i l e r e s t r i c -
i n g h i g h temperatures t o a r e g i o n w i t h i n a few microns o f t h e s u r face,
and f o r very b r i e f t i m e s
sec), provides s i g n i f i c a n t
advantages over c o n v e n t i o n a l p r o c e s s i n g steps i n t h e f a b r i c a t i o n o f semiconductor devices; e s p e c i a l l y i n t h e search f o r novel approaches t o t h e f o r m a t i o n o f submicron and t h r e e - d i m e n s i o n a l structures.
integrated
The fundamentals o f t h e i n t e r a c t i o n o f p u l s e d l a s e r
r a d i a t i o n w i t h semiconducting m t e r i a l s, p a r t i c u l a r l y s i 1i c o n , have been i n v e s t i g a t e d i n t e n s i v e l y ,
b o t h t h e o r e t i c a l l y and experimen-
t a l l y ; t h e s e i n v e s t i g a t i o n s and t h e i r r e s u l t s have been discussed i n d e t a i l i n t h e p r e c e d i n g c h a p t e r s o f t h i s book.
I n t h i s chapter,
we r e v i e w and d e s c r i b e t h e p o t e n t i a l a p p l i c a t i o n s o f p u l s e d l a s e r s i n t h e p r o c e s s i n g o f semiconductor devices and i n t e g r a t e d c i r c u i t s . Since space does n o t p e r m i t an e x h a u s t i v e r e v i e w o f a l l o f t h e d e v i c e - r e l a t e d work, we have chosen t o p l a c e p a r t i c u l a r emphasis on t h e most r e c e n t r e s u l t s r e l a t i n g t o t h e f a b r i c a t i o n o f p-n j u n c t i o n s o l a r c e l I s , f o r which beam-processing t e c h n i q u e s have proved t o be o u t s t a n d i n g l y s u c c e s s f u l .
Our more a b b r e v i a t e d d i s c u s s i o n s
o f o t h e r areas a r e i n t e n d e d p r i m a r i l y t o c o v e r t h e broad o u t l i n e s o f developments i n t h o s e areas.
To complement t h e s e d i s c u s s i o n s ,
t h e i n t e r e s t e d r e a d e r s h o u l d c o n s u l t r e c e n t reviews by Hess e t a l . (1983) and H i l l (1983) on t h e a p p l i c a t i o n o f beam p r o c e s s i n g t o i n t e g r a t e d c i r c u i t t e c h n o l o g y and t h e volume e d i t e d by Osgood e t a l . (1983),
which c o n t a i n s
processing.
numerous papers on l a s e r photochemical
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10. APPLICATIONS OF PULSED LASER PROCESSING
I n i t i a l i n t e r e s t has been i n t h e areas o f l a s e r a n n e a l i n g o f i o n - i m p l a n t e d l a y e r s and t h e use o f l a s e r r a d i a t i o n t o 1) induce dopant d i f f u s i o n from s o l i d , l i q u i d , and gaseous sources; 2 ) d i s s o l v e second-phase p r e c i p i t a t e s ; 3 ) produce s u p e r s a t u r a t e d a1 1oys;
4 ) form metal s i l i c i d e s ; 5 ) reduce c o n t a c t r e s i s t a n c e ; 6) promote g r a i n growth; silicon;
7 ) reduce sheet r e s i s t i v i t i e s i n p o l y c r y s t a l l i n e
8) c r y s t a l l i z e d e p o s i t e d f i l m s ;
p r o p e r t i e s i n SOS ( s i l i c o n - o n - s a p p h i r e ) back-surface damage g e t t e r i n g .
9) improve i n t e r f a c e
d e v i c e s ; and 10) induce
I r r e s p e c t i v e o f so many p o t e n t i a l
a p p l i c a t i o n s , t h e d e v i c e t h a t has been f a b r i c a t e d most s u c c e s s f u l l y by p u l s e d l a s e r a n n e a l i n g i s t h e s i l i c o n s o l a r c e l l , which i s a l a r g e - a r e a d e v i c e t h a t does n o t have a complex s t r u c t u r e .
Good
r e s u l t s have a l s o been o b t a i n e d f o r high-frequency s i l i c o n IMPATT (impact avalanche and t r a n s i t t i m e ) diodes.
For a p p l i c a t i o n s t o
t h e f a b r i c a t i o n o f more complex devices, such as MOS (metal-oxidesemiconductor) o r b i p o l a r t r a n s i s t o r s i n VLSI ( v e r y l a r g e s c a l e i n t e g r a t i o n ) and VHSIC ( v e r y h i g h speed i n t e g r a t e d c i r c u i t ) t e c h nologies, l a s e r annealing i s s t i l l i n i t s infancy. I n t h i s connection, we would l i k e t o emphasize t h a t t h e success of l a s e r p r o c e s s i n g o f devices o f t e n depends t o a g r e a t e x t e n t on t h e c h a r a c t e r i s t i c s and performance o f t h e l a s e r which i s chosen f o r the p a r t i c u l a r application.
The l a s e r s used i n d e v i c e work i n
t h e past were m o s t l y s o l i d - s t a t e l a s e r s (ruby, YAG, etc.).
These
l a s e r s have c e r t a i n drawbacks and 1 i m i t a t i o n s f o r s o p h i s t i c a t e d d e v i c e p r o c e s s i n g steps.
Foremost among t h e s e l i m i t a t i o n s i s t h e
i n h e r e n t s p a t i a l inhomogeneity o f t h e energy d e n s i t y i n t h e pulses. Also,
a diffraction-related
s t r u c t u r e i s f r e q u e n t l y observed on
s u r f a c e s due t o t h e coherent r a d i a t i o n o f s o l i d - s t a t e l a s e r s .
In
o r d e r t o reduce t h e p u l s e inhomogeneities t o a c c e p t a b l e l e v e l s , i t i s necessary t o t r a n s m i t t h e beam t h r o u g h a beam homogenizer.
The
beam homogenization techniques u s u a l l y produce e i t h e r h i g h t r a n s m i s s i o n losses o r i n t e r f e r e n c e f r i n g e s from t h e o v e r l a p p i n g beams. Furthermore, t h e homogenizers can o n l y p a r t i a l l y reduce t h e inhomog e n e i t i e s , and t h e y add t o t h e c o m p l e x i t y o f t h e processing.
628
R. T. YOUNG E T A L .
Other drawbacks of s o l i d - s t a t e l a s e r s i n c l u d e low p u l s e r e p e t i t i o n r a t e s f o r systems w i t h l a r g e diameter rods (because o f t h e heat d i s s i p a t i o n problem i n t h e i n s u l a t i n g c r y s t a l s ) and low o v e r a l l energy c o n v e r s i o n e f f i c i e n c y .
These d i f f i c u l t i e s appear t o p u t
unacceptable l i m i t a t i o n s on d e v i c e t h r o u g h p u t r a t e and c o s t f o r many a p p l i c a t i o n s . Gas l a s e r s have few o f t h e drawbacks o f s o l i d - s t a t e l a s e r s . The r e c e n t l y developed rare-gas h a l i d e excimer l a s e r s have many o f t h e c h a r a c t e r i s t i c s needed f o r e f f i c i e n t l a s e r p r o c e s s i n g o f semiconductors.
However, excimer l a s e r s w i t h s u f f i c i e n t power (e.g.
,
1.5 J / p u l s e a t 0.5 Hz) and beam u n i f o r m i t y f o r p r o c e s s i n g o f l a r g e areas, were n o t a v a i l a b l e commercially u n t i l 1981, and l a s e r s which m i g h t be viewed as t h e predecessors o f t r u e p r o d u c t i o n - t y p e l a s e r s
(1 J / p u l s e ,
100 Hz) a r e o n l y now appearing on t h e market.
Excimer
l a s e r s , i n a d d i t i o n t o t h e u n i f o r m beam and h i g h average power, can p r o v i d e many wavelengths r a n g i n g f r o m u l t r a v i o l e t ( U V ) t o vacuum U V ; t h e y a l s o have t h e advantages o f r e l a t i v e l y poor s p a t i a l and temporal coherency and good e f f i c i e n c i e s .
This type o f l a s e r not
o n l y o f f e r s t h e advantage o f b e t t e r a n n e a l i n g o f devices, b u t a l s o makes new areas o f research i n photochemical p r o c e s s i n g and i n h i g h r e s o l u t i o n o p t i c a l 1i t h o g r a p h y more promising. The remainder o f t h i s c h a p t e r i s d i v i d e d i n t o s i x sections. I n S e c t i o n 11,
t h e p r e s e n t l e v e l o f development o f UV excimer
l a s e r s , as i t r e l a t e s t o semiconductor processing, and t h e advantages o f t h e s e l a s e r s f o r such p r o c e s s i n g a r e reviewed.
I n Section
I11 , t h e l a s e r - p r o c e s s i n g technology developed f o r t h e f a b r i c a t i o n o f s i l i c o n s o l a r c e l l s i s described.
The a p p l i c a t i o n o f p u l s e d
l a s e r p r o c e s s i n g t o o t h e r devices such as IMPATT diodes, SOS s t r u c tures,
and i n t e g r a t e d c i r c u i t s i s d e s c r i b e d i n S e c t i o n I V .
The
use o f U V and I R photon-induced photochemical processes f o r f i l m d e p o s i t i o n , e t c h i n g , and doping a r e b r i e f l y discussed i n S e c t i o n V. The u t i l i z a t i o n o f t h e s h o r t wavelength and i n c o h e r e n t n a t u r e of t h e excimer l a s e r r a d i a t i o n f o r submicron o p t i c a l l i t h o g r a p h y i s i l l u s t r a t e d i n Section V I .
I n t h e l a s t s e c t i o n , we summarize t h e
629
10. APPLICATIONS OF PULSED LASER PROCESSING
c h a p t e r and p r o v i d e a few c o n c l u d i n g comments on t h e f u t u r e p r o s p e c t s o f p u l s e d l a s e r s i n semiconductor d e v i c e a p p l i c a t i o n s .
11. Rare-gas
Excimer Lasers and Excimer Laser Processing h a l i d e (RGH) excimer l a s e r s f o r m a c l a s s o f newly
developed l a s e r s which a r e capable o f e f f i c i e n t l y g e n e r a t i n g h i g h powered pulses o f r a d i a t i o n a t u l t r a v i o l e t wavelengths.
The r a p i d
advancement o f excimer l a s e r technology and t h e many unique charact e r i s t i c s a s s o c i a t e d w i t h these l a s e r s have made them very a t t r a c t i v e f o r many aspects o f semiconductor d e v i c e f a b r i c a t i o n . I n t h i s section, a b r i e f discussion o f the present s t a t e o f t h e i r development, as i t r e l a t e s t o semiconductor processing, w i l l be g i v e n and a comparison o f t h e c h a r a c t e r i s t i c s o f excimer l a s e r s w i t h those o f t h e s o l i d - s t a t e l a s e r s most commonly used i n semiconductor p r o c e s s i n g w i l l be discussed.
1.
EXCIMER LASERS The t e r m excimer was a p p a r e n t l y o r i g i n a l l y i n t r o d u c e d by Stevens
and Hutton (1960) t o r e f e r t o “ e x c i t e d dimers,” m o l e c u l a r species,
such as Xe2, A r 2 ,
which a r e c e r t a i n
and Hg2 t h a t e x i s t o n l y i n
t h e upper o r e x c i t e d s t a t e and have a r e p u l s i v e and, t h e r e f o r e , d i s s o c i a t i v e lower s t a t e .
I t was subsequently found ( B i r k , 1975;
Beens and Weller,
1975) t h a t c e r t a i n m o l e c u l a r complexes such as
KrF*, XeOH*, etc.,
a l s o e x h i b i t e d t h e same c h a r a c t e r i s t i c s and t h e s e
were r e f e r r e d t o as exciplexes.
These two classes o f molecules
provide a nearly ideal s i t u a t i o n f o r creating t h e nonequilibrium population inversion required f o r l a s e r action.
I n practice, the
d i s t i n c t i o n between e x c i t e d dimers and e x c i p l e x e s i s f r e q u e n t l y i g n o r e d and t h e y a r e commonly r e f e r r e d t o as excimers.
From t h e
mechanism and k i n e t i c s o f excimer f o r m a t i o n , i t i s p r e d i c t e d t h a t among a l l t h e excimers t h e RGH excimer l a s e r s o f f e r t h e advantages o f h i g h average power and h i g h e f f i c i e n c y ; t h e y a r e t h e most commonl y discussed excimer l a s e r s .
Since t h e demonstration o f t h e f i r s t
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R. T. YOUNG ET AL
e-beam-pumped
RGH excimer l a s e r i n 1975,
(Bran and Ewing,
1975;
S e a r l e s and H a r t , 1975) t h e p r e s e n t technology has advanced t o t h e p o i n t t h a t e-beam-pumped l a s e r s w i t h k i l o j o u l e o u t p u t energies have been c o n s t r u c t e d .
Although e-beam-pumped l a s e r s can be s c a l e d t o
h i g h p u l s e energies, t h e y a r e n o t r e l i a b l e because a t h i n , mechani c a l l y fragile,
f o i l window between t h e high-vacuum e l e c t r o n gun
chamber and t h e high-pressure gas discharge chamber i s i n v o l v e d . The downtime r e s u l t i n g from a f o i l f a i l u r e i s unacceptable f o r a p r a c t i c a l system o f h i g h average power.
On t h e o t h e r hand, excimer
l a s e r s e x c i t e d by s e l f - s u s t a i n e d e l e c t r i c discharges (Burnham e t a l . , 1976) have been s u c c e s s f u l l y developed r e c e n t l y i n t o l o n g - l i f e t i m e systems o f h i g h average power. The p h y s i c s and e n g i n e e r i n g requirements f o r making these l a s e r s re1 i a b l e , and t h e necessary c o n d i t i o n s f o r t h e homogeneous f o r m a t i o n of
p u l s e d avalanche discharges a t h i g h gas pressures have been
examined by L i n and L e v a t t e r (1979) and L e v a t t e r and L i n (1980). Based on these s t u d i e s , a 100-watt (1 J / p u l s e a t 100 Hz) RGH l a s e r w i t h e x c e l l e n t beam u n i f o r m i t y (5% v a r i a t i o n over an area 3 cm by
3 cm) and p u l s e - t o - p u l s e r e p r o d u c i b i l i t y i s now s a i d t o be a v a i l a b l e commercially. U n l i k e more c o n v e n t i o n a l poor s p a t i a l coherence.
lasers,
excimer l a s e r s have very
T h i s i s due t o t h e f a c t t h a t t h e beam i s
e x t r e m e l y rnultimode, which i s a consequence o f t h e l a r g e discharge volume and t h e s u p e r - r a d i a n t n a t u r e o f t h e l a s e r emission. result,
As a
i n t e r f e r e n c e e f f e c t s due t o l i g h t s c a t t e r i n g f r o m d u s t
p a r t i c l e s , s u r f ace i m p e r f e c t ions,
o r m a t e r i a1 inhomogenei t i e s i n
t h e n e a r - s u r f a c e r e g i o n o f t h e sample can be n e a r l y e l i m i n a t e d . The RGH l a s e r s can be operated w i t h a number o f d i f f e r e n t gas mixt u r e s , r e s u l t i n g i n d i f f e r e n t o u t p u t wavelengths.
Some o f t h e most
commonly used gases and t h e r e s u l t i n g wavelengths a r e : ArF (193 nm), K r C l (222 nm), KrF (249 nm), XeCl (308 nm), and XeF (350 nrn).
The
a v a i l a b l e wavelength range can be extended i n t o t h e v i s i b l e and i n t o t h e vacuum u l t r a v i o l e t by employing t h e RGH l a s e r as a pump
631
10. APPLICATIONS OF PULSED LASER PROCESSING l a s e r f o r n o n l i n e a r frequency c o n v e r s i o n schemes.
These charac-
t e r i s t i c s have been demonstrated r e c e n t l y t o be advantageous f o r a n n e a l i n g o f i o n - i m p l a n t a t i o n damage (Young e t a l . Lowndes e t a l .
,
, 1982a, 1983a;
1982), photochemical p r o c e s s i n g (see, e.g.,
volume e d i t e d by Osgood e t al., l i t h o g r a p h y ( J a i n e t al.,
1982).
the
1983), and high-reso1utio.n o p t i c a l
I n a d d i t i o n t o these applications,
excimer l a s e r s a r e a l s o a t t r a c t i v e t o r e s e a r c h e r s i n many areas o f fundamental s t u d i e s .
F o r example, because t h e o p t i c a l p r o p e r t i e s
of s i l i c o n a t UV wavelengths a r e v i r t u a l l y c o n s t a n t f o r t h e c r y s t a l l i n e , amorphous, and molten phases, c a l c u l a t i o n s o f energy absorption,
h e a t f l o w , and m e l t i n g i n v o l v e fewer parameters, and t h i s
makes comparisons between c a l c u l a t e d and experimental r e s u l t s more straightforward.
More s p e c i f i c a l l y , as discussed i n Chapter 3, t h e
o p t i c a l a b s o r p t i o n c o e f f i c i e n t , a, o f s i l i c o n i n e i t h e r t h e c r y s t a l l i n e o r t h e amorphous s t a t e a t UV wavelengths i s -106 cm’l
and t h e
r e f l e c t i v i t y R i s -70%; n e i t h e r q u a n t i t y depends s t r o n g l y on t h e s o l i d - l i q u i d phase change or on temperature.
I n contrast,
the
o p t i c a l p r o p e r t i e s o f s i l i c o n a t v i s i b l e wavelengths a r e s t r o n g l y temperature dependent and change d i s c o n t i n u o u s l y on m e l t i n g . 2.
COMPARISON OF ANNEALING CHARACTERISTICS
OF EXCIMER AND SOLID
STATE LASERS A comparative study of t h e a n n e a l i n g c h a r a c t e r i s t i c s o f XeCl excimer and ruby l a s e r s i n terms o f s u r f a c e morphology, dopant prof i l e r e d i s t r i b u t i o n s , and r e s i d u a l d e f e c t s has been made by Young The r e s u l t s a r e summarized i n t h e f o l l o w i n g . e t a l . (1983).
Surface morphology. An i m p o r t a n t concern i n t h e use o f p u l s e d l a s e r s i n semiconductor p r o c e s s i n g i s t h e s u r f a c e morphology a f t e r l a s e r treatment.
The p r e s e r v a t i o n o f a f l a t , f e a t u r e l e s s s u r f a c e
i s extremely i m p o r t a n t i n d e v i c e s i f m u l t i - s t e p required.
processing i s
A p a r t f r o m h o t spots and d i f f r a c t i o n - i n d u c e d l o c a l i z e d
s u r f a c e damage, (Leamy e t al.,
a p e r i o d i c s u r f a c e s t r u c t u r e has been observed 1978) f r e q u e n t l y i n ruby and Nd:YAG laser-annealed
632
R. T. YOUNG ETAL.
samples.
T h i s p e r i o d i c p a t t e r n i s t h o u g h t t o be due t o h e a t i n g
and m e l t i n g by a s t a n d i n g wave r e s u l t i n g from t h e i n t e r f e r e n c e o f t h e i n c i d e n t and t h e s c a t t e r e d wave (Oron and Sorenson,
1979).
F i g u r e s l a and l b show t y p i c a l s u r f a c e s t r u c t u r e s observed a f t e r ruby l a s e r annealing.
A beam homogenizer such as a ground g l a s s
d i f f u s e r p l a t e can e f f e c t i v e l y remove t h e major i n t e n s i t y v a r i a t i o n s . However, t h e d i f f u s e r p l a t e may produce f o c u s i n g e f f e c t s on a f i n e scale.
T h i s m i c r o f o c u s i n g can c r e a t e randomly d i s t r i b u t e d surface
r i p p l e s , as shown i n Fig. l c .
Although t h e s e r i p p l e s can be e l i m -
i n a t e d by p l a c i n g t h e sample f a r t h e r from t h e d i f f u s e r p l a t e , t h e
Fig.
1.
Surface morphology o f laser-annealed silicon surfaces.
( a ) EBlC
image showing d i f f r a c t i o n pattern and hot spots produced by a direct multimode beam o f a ruby laser (EQ = 1.6 J/crn2) ; ( b ) optical micrograph showing periodic ripple structures f r o m a direct multimode beam of a ruby laser (Er = 1.8 J /cm2) ; ( c ) optical micrograph showing randomly distributed surface ripples produced by a multimode beam o f a ruby laser transmitted through a diffuser plate
(EL = 1.8
/ c m 2 ) ; ( d ) optical micrograph showing the smooth surface a f t e r irradiation with a multimode beam o f a XeCl laser (EL = 3.5 J / c m 2 , T = 5 5 nsec).
10. APPLICATIONS OF PULSED LASER PROCESSING
633
a v a i l a b l e energy d e n s i t y f o r f e a t u r e l e s s s u r f a c e a n n e a l i n g i s r e duced t o -1.6
J/cm2.
On t h e o t h e r hand, t h e s u r f a c e morphology o f
t h e samples a f t e r XeCl excimer l a s e r a n n e a l i n g a t energy d e n s i t i e s up t o 4-5 J/cm2 (depending on t h e p u l s e d u r a t i o n t i m e ) i s smooth and f l a t (Fig. I d ) .
No unusual s u r f a c e f e a t u r e s caused by h o t spots,
d i f f r a c t i o n p a t t e r n s , o r o t h e r i n t e r f e r e n c e e f f e c t s a r e observed. To e v a l u a t e t h e u n i f o r m i t y o f t h e l a s e r beam on a m i c r o s c o p i c scale,
t h e i n t e r f a c e s between annealed and unannealed r e g i o n s i n s i l i c o n samples i m p l a n t e d w i t h boron (200 kV) were examined by t r a n s m i s s i o n e l e c t r o n microscopy (TEM).
F i g u r e 2 shows cross s e c t i o n micrographs
o f samples annealed w i t h t h e ruby l a s e r a t 2.5 J/cm* and w i t h t h e XeCl l a s e r a t 2.0 J / c d (Young e t al.,
Fig.
2.
1983a).
It i s c l e a r l y seen
TEM micrographs showing the interfaces between annealed and
unannealed regions in B-implanted
(200 k V ) Si.
634
R. T. YOUNG ETAL.
102'
AS IMPLANT 1020
"B+(35kV, l x iO'6 1019
IN Si
XeCl LASER ANNEALING 1.5 J/cm2 A
2.0 J/cm2
RUBY LASER ANNEALING
0
300
200
100
400
DEPTH (nm)
F i g . 3.
Comparison of boron implanted dopant profiles a f t e r annealing a t two
d i f f e r e n t laser energy densities with ruby and XeCl lasers.
(From Young e t a l . ,
1983a)
t h a t t h e XeCl l a s e r a n n e a l i n g r e s u l t s i n an i n t e r f a c e a t a much more u n i f o r m depth.
I n c o n t r a s t , a v a r i a t i o n i n m e l t depth as l a r g e as
-25% o v e r a 2-urn wide r e g i o n i s observed i n ruby l a s e r - a n n e a l e d
s amp 1es
.
Dopant profiles. A comparison o f l a s e r - i n d u c e d s u r f a c e m e l t i n g and dopant d i f f u s i o n i n ruby and XeCl l a s e r - a n n e a l e d samples o f llB+
(35 k V , 1 ~ 1 0 1cm-2) ~ implanted
Si was made by secondary i o n mass
spectroscopy (SIMS) p r o f i l i n g (Young e t a l . , had p u l s e d u r a t i o n t i m e s o f -25
1983a).
Both lasers
nsec, a l t h o u g h t h e p u l s e shapes
(excimer l a s e r : a p p r o x i m a t e l y t r a p e z o i d a l ; ruby l a s e r : m a t e l y Gaussian) were q u i t e d i f f e r e n t .
approxi -
F i g u r e 3 shows t h e dopant
r e d i s t r i b u t i o n i n samples annealed w i t h t h e two l a s e r s a t energy d e n s i t i e s o f 1.5 J / c d and 2.0 J/cm2.
I t i s i n t e r e s t i n g t o see t h a t
10.
635
APPLICATIONS OF PULSED LASER PROCESSING
a t t h e same energy d e n s i t y t h e r e s u l t i n g dopant p r o f i l e s a r i s i n g from t h e two l a s e r s a r e almost i d e n t i c a l .
These r e s u l t s s t r o n g l y
suggest t h a t r e g a r d l e s s o f t h e l a r g e d i f f e r e n c e s i n t h e o p t i c a l p r o p e r t i e s o f S i a t UV and v i s i b l e wavelengths,
the efficiency o f
usage o f t h e i n c i d e n t energy f o r m e l t i n g S i s u r f a c e regions t o comparable depths i s approximately t h e same f o r t h e two l a s e r s w i t h The q u a l i t y o f t h e a n n e a l i n g o f these
s i m i l a r pulse durations.
samples was subsequently examined by TEM and by van der Pauw mea-
I n a l l cases, a d i s l o c a t i o n - f r e e ,
surements.
a c t i v a t e d laser-regrown l a y e r was observed.
fully electrically Because o f t h e wide
energy window f o r excimer l a s e r annealing, deep j u n c t i o n p r o f i l e s can be r e a d i l y o b t a i n e d w i t h m u l t i p l e p u l s e s o f l a s e r r a d i a t i o n ,
4.
as i l l u s t r a t e d i n Fig.
These r e s u l t s show t h a t i n a s i l i c o n
sample a j u n c t i o n depth c l o s e t o 0.9
can be achieved w i t h 10
l a s e r p u l s e s a t 3.5 J/cm2 p e r p u l s e w i t h o u t any n o t i c e a b l e s u r f a c e 1021
I
I
I
I
I
I
I
I
600
700
000
B ( 1 0 0 k V , 1 X 1016crn-2 ) XeCl LASER ANNEALING o AS IMPLANTED
3.5 J/crn2, A
10'8
0
Fig. 4.
1 PULSE
3.5 J/cm2, 10PULSES
100
200
300
400 500 DEPTH ( n m )
900
SlMS profiles for boron in silicon a f t e r XeCl laser annealing at 3 . 5
J /cm2 with 1 and 10 pulses.
(From Young et a l . ,
1983a)
636
R. T. YOUNG E T A L .
damage.
Comparable j u n c t i o n depths c o u l d be o b t a i n e d w i t h s o l i d -
s t a t e l a s e r pulses, b u t t h e p r e v e n t i o n o f s u r f a c e damage would be e x t r e m e l y d i f f i c u l t , i f n o t impossible.
Electrically active defects.
The laser-annealed r e g i o n s a r e
d i s l o c a t i o n - f r e e under TEM o b s e r v a t i o n and have good e l e c t r i c a l p r o p e r t i e s (sheet r e s i s t i v i t y and m o b i l i t y ) under van d e r Pauw examination, b u t i t has been r e p o r t e d ( K i m e r l i n g and Benton, 1980; Mooney e t al.,
1981) t h a t h i g h c o n c e n t r a t i o n s (1013-1015 cm-3) o f
e l e c t r i c a l l y a c t i v e d e f e c t s were d e t e c t e d by DLTS (deep l e v e l t r a n s i e n t spectroscopy) i n samples i r r a d i a t e d w i t h p u l s e d ruby and Nd:YAG l a s e r s .
These d e f e c t s were thought t o be f r o z e n i n d u r i n g
t h e r a p i d quenching process.
The e x i s t e n c e of t h e s e d e f e c t s may
have a l a r g e i n f l u e n c e on d e v i c e performance. laser-annealed
samples,
i t was found
I n t h e study o f XeCl
that electrically
active
d e f e c t s a r e present a t c o n c e n t r a t i o n s much lower t h a n t h o s e r e p o r t e d f o r samples annealed w i t h s o l i d s t a t e l a s e r s (Young e t a l . ,
1983a).
F i g u r e 5 shows a t y p i c a l DLTS spectrum from S c h o t t k y diodes made on I
I
E,
I
+ 0.38eV
~~
100
150
250
200 TEMPERATURE
300
(K)
Fig. 5 . DLTS spectrum of Si-implanted ( 1 0 k V , 5 ~ 1 0 crn-2) ~ ’ p-type Si after eC\ laser annealing at 2.0 ~ / c r n 2 .
637
10. APPLICATIONS OF PULSED LASER PROCESSING S i - i m p l a n t e d (10 kV, 5x1015 cm-2),
B-doped s i l i c o n samples a f t e r
XeCl l a s e r a n n e a l i n g w i t h a 2-J/cm2,
25-nsec pulse.
A single defect
l e v e l l o c a t e d a t 0.38 eV above t h e energy Ev o f t h e valence band i s observed.
The c o n c e n t r a t i o n o f t h i s d e f e c t i s - 5 ~ 1 0 1 1 cm-3.
These r e s u l t s s t r o n g l y suggest t h a t t h e e x i s t e n c e o f e l e c t r i c a l l y a c t i v e d e f e c t s i n laser-regrown r e g i o n s i s n o t r e l a t e d s o l e l y t o t h e r a p i d quenching produced by t h e very h i g h regrowth v e l o c i t y v, as was speculated i n t h e past,
s i n c e m e l t i n g model c a l c u l a t i o n s
(see Chapter 4 ) o f t h e l a s e r - a n n e a l i n g process show t h a t t h e values o f v f o r t h e ruby and XeCl l a s e r s w i t h comparable p u l s e d u r a t i o n times are not g r e a t l y d i f f e r e n t .
The mechanism o f defect f o r m a t i o n
i n samples annealed w i t h s o l i d - s t a t e l a s e r s a p p a r e n t l y needs cons i d e r a b l e f u r t h e r study. 3.
EFFECT OF PULSE DURATION ON ANNEALING Advances i n excimer l a s e r technology i n d i c a t e t h a t , i n a d d i t i o n
t o t h e c a p a b i l i t y f o r s c a l i n g t o h i g h e r power, a l a s e r system can be designed so t h a t t h e p u l s e d u r a t i o n t i m e T~ ( o r s i m p l y
T)
can be
a d j u s t e d over a range from t e n t o several hundred nanoseconds simply Variation o f
by changing t h e r a t i o o f gas mixtures.
range w i t h s o l i d - s t a t e l a s e r s i s d i f f i c u l t ,
over t h i s
i f n o t impossible.
I n t h i s subsection, we discuss t h e e f f e c t o f t h e p u l s e d u r a t i o n on t h e a n n e a l i n g o f i o n - i m p l a n t e d s i l i c o n by comparing t h e m e l t i n g depth, c r y s t a l p e r f e c t i o n , dopant p r o f i l e s , and e l e c t r i c a l propert i e s o f samples annealed w i t h a XeCl l a s e r w i t h energy d e n s i t y E, i n t h e range o f 0.5-3.0 e t al.,
1983b).
J/cm2 and f o r
T~
o f 25 and 70 nsec, (Young
As i n d i c a t e d f r o m model c a l c u l a t i o n s (Wood and
G i l e s , 1981; Chapter 4), i t i s expected t h a t 25-nsec pulses should be more energy e f f i c i e n t i n a n n e a l i n g i o n - i m p l a n t i o n damage t h a n a r e 70-nsec pulses.
The c a l c u l a t e d r e s u l t s a r e shown i n Fig. 6.
The m e l t i n g depth as a f u n c t i o n o f l a s e r energy d e n s i t y f o r t h e two l a s e r p u l s e s as determined from TEM i s p l o t t e d i n Fig. 7. t h e o r e t i c a l and experimental
Both
d a t a show t h a t a t t h e same energy
638
R . T. YOUNG E T A L .
0.9 0.8
-
I
I
-
I
I
Fig. 6.
1
9 fnsec) 25.5 25.5 25.5 70.5 70.5 70.5
---2.0 --.-i.5 ---2.0 ---2.5
0.7 -
0.6
I
-------!.5
3.
$ -
I E~ ( J / c m 2 ) 1.0
XeCl L A S E R _ _ _ - - 25.5 - nsec --- 70.5 nsec
-
-
Calculated melt-front profiles for pulses o f various energy densities
and two different values o f the pulse duration, as indicated by the trapezoidal pulse shapes.
-
a 10,000
z
5 W 5
0 I
I
I
I
XeCl ( X = 0 . 3 0 8 p m ) o ~ = 7 0 n s o ~ = 2 5 n s
0
LL
1
5000 -
I-
n W
x’
O W
/-*
-
Fig. 7. Melting depth as a function o f laser energy density for 25- and 70-nsec laser pulses as determined from TEM.
10. APPLICATIONS OF PULSED LASER PROCESSING
639
d e n s i t y , c o n s i d e r a b l y deeper m e l t i n g i s achieved w i t h 25-nsec pulses F i g u r e 8 shows t h e e f f e c t o f
t h a n w i t h 70-nsec pulses, as expected.
p u l s e d u r a t i o n on t h e dopant p r o f i l e r e d i s t r i b u t i o n o f B-implanted (100 k V ) S i annealed w i t h EQ = 2.5
and 3.0 J/cm2;
these r e s u l t s
demonstrate t h a t s h o r t e r 1aser pulses p r o v i d e deeper dopant spreadi n g , as would be expected from t h e m e l t d u r a t i o n s . However, i t i s i n t e r e s t i n g t o see t h a t a very a b r u p t dopant p r o f i l e was o b t a i n e d on t h e sample t h a t was annealed w i t h 70-nsec pulses a t an energy d e n s i t y j u s t above t h e t h r e s h o l d f o r complete a n n e a l i n g (i.e.,
2.5 J/cm2 i n t h i s case).
observed i n a r s e n i c-imp1 anted samples.
S i m i l a r r e s u l t s were a l s o
T h i s phenomenon has n o t been
seen i n ruby o r s h o r t p u l s e (25-nsec) X e C l laser-annealed samples. The q u a l i t y of annealing, i n terms o f c r y s t a l l i n e p e r f e c t i o n o f t h e
iI
(02'
s8 u
I
I
I
I
I
I
1
B ( l 0 0 kV. I X40'6cm-Z) EXCIMER LASER ANNEALING
1O2O
t
S-IMPLANTED
toq3 0
lo'8
Fig. 8.
0
25 n see,, 2.5 J/cm'
I00
200
400 DEPTH (nm)
300
500
600
700
Comparison o f concentration profiles o f B in Si a f t e r XeCl laser
annealing a t 2 . 5 and 3.0 J / c m 2 with 25-
and 70-nsec
pulses.
640
R. T. YOUNG E T A L .
regrown l a y e r (by TEM), j u n c t i o n c h a r a c t e r i s t i c s (by dark I - V measurements), and r e s i d u a l d e f e c t s (by DLTS), i s very s i m i l a r f o r t h e two p u l s e d u r a t i o n s . From t h e s e r e s u l t s , i t can be concluded t h a t f o r a d e v i c e i n which a j u n c t i o n depth deeper t h a n 1000 A i s d e s i r e d , a l a s e r w i t h s h o r t e r p u l s e d u r a t i o n i s more energy e f f i c i e n t f o r annealing. However, l o n g e r p u l s e d u r a t i o n s may have t h e advantage o f b e t t e r c o n t r o l l i n g shallow s u r f a c e m e l t i n g (200-500 h ) and may t h e r e f o r e p r o v i d e more abrupt dopant p r o f i l e s .
Such p r o f i l e s a r e e s p e c i a l l y
c r i t i c a l f o r h i g h s w i t c h i n g speed devices t h a t r e q u i r e sharp doping changes on t h e s c a l e o f a few hundred angstroms.
I 11.
Photovol t a i c Applications
One o f t h e f i r s t , and perhaps s t i l l t h e most s u c c e s s f u l , a p p l i c a t i o n s o f p u l s e d l a s e r p r o c e s s i n g t o d a t e has been i n t h e f a b r i c a t i o n o f s i l i c o n s o l a r c e l l s (Young e t al.,
1978, 1980, 1982b).
This i s not p a r t i c u l a r l y s u r p r i s i n g since a photovoltaic c e l l i s a p-n j u n c t i o n d e v i c e t h a t does n o t have t h e complex s t r u c t u r e r e q u i r e d by most m i c r o - e l e c t r o n i c devices.
I n t h i s section, t h e
v a r i o u s l a s e r - r e l a t e d techniques which have been developed f o r s o l a r c e l l a p p l i c a t i o n s a r e discussed.
To s e t t h e stage f o r t h i s d i s c u s -
sion, we f i r s t r e v i e w some o f t h e f a c t o r s t h a t make p u l s e d l a s e r p r o c e s s i n g so s u i t a b l e f o r t h e f a b r i c a t i o n o f s o l a r c e l l s , and s k e t c h how t h e t e c h n i q u e s have e v o l v e d t o t h e e x t e n t t h a t h i g h e f f i c i e n c y s i l i c o n s o l a r c e l l s can be e a s i l y and s i m p l y f a b r i c a t e d . We t h e n p r e s e n t experimental data t o show t h a t t h e h i g h dopant conc e n t r a t i o n s achieved by i o n i m p l a n t a t i o n and l a s e r a n n e a l i n g p r o v i d e an e f f e c t i v e ' ' i n s i t u " s u r f a c e p a s s i v a t i o n t h a t suppresses s u r f a c e r e c o m b i n a t i o n and minimizes t h e e m i t t e r recombination c u r r e n t .
The
m e l t i n g o f t h e n e a r - s u r f a c e r e g i o n produced by p u l s e d l a s e r i r r a d i a t i o n o f s i l i c o n has made p o s s i b l e t h e development o f s e v e r a l
641
10. APPLICATIONS OF PULSED LASER PROCESSING p o t e n t i a l l y low-cost techniques f o r j u n c t i o n formation.
Laser-
induced s u r f a c e v a p o r i z a t i o n has been demonstrated t o be an e f f e c t i v e method o f p r o d u c i n g c o n t r o l l e d damage on t h e backside o f a c e l l blank f o r
impurity gettering.
The u l t r a r a p i d m e l t i n g and
r e c r y s t a l l i z a t i o n c h a r a c t e r i s t i c o f p u l s e d l a s e r p r o c e s s i n g have a l s o proved t o be o f c o n s i d e r a b l e i n t e r e s t i n connection w i t h fundamental s t u d i e s and m o d i f i c a t i o n s o f g r a i n boundaries, and f o r t h e f a b r i c a t i o n o f solar c e l l s from p o l y c r y s t a l l i n e silicon.
These
t o p i c s w i l l a l s o be discussed i n t h i s s e c t i o n .
4.
LASER PROCESSING AND HIGH-EFFICIENCY SOLAR CELLS It i s i n t e r e s t i n g t o c o n s i d e r b r i e f l y some o f t h e reasons why
PU
sed l a s e r a n n e a l i n g i s so s u i t a b l e f o r t h e f a b r i c a t i o n o f s o l a r
ce 1s.
These reasons become apparent ifwe compare t h e s t r u c t u r e
o f h i g h - e f f i c i e n c y c e l l s made by c o n v e n t i o n a l c e l l technology w i t h t h e s t r u c t u r e o f t h e c u r r e n t g e n e r a t i o n of h i g h - e f f i c i e n c y , l a s e r processed c e l l s .
The s o - c a l l e d " v i o l e t " c e l l technology developed
by Lindmayer and co-workers
(1972; see a l s o Hovel,
1975) uses a
low-temperature (<900°C) d i f f u s i o n process t o e l i m i n a t e t h e "dead l a y e r " and t o o b t a i n s h a l l o w j u n c t i o n depths.
It r e s u l t s i n a c e l l
w i t h e x c e l l e n t b l u e response and h i g h c u r r e n t d e n s i t y .
The dead
l a y e r i s a r e g i o n o f t h e c e l l , very near o r a t t h e surface, i n which p r e c i p i t a t e s o r aggregates o f dopants e x i s t because o f t h e h i g h c o n c e n t r a t i o n o f dopant d e p o s i t e d from t h e d i f f u s i o n source onto t h e surface. M i n o r i t y c a r r i e r s generated i n t h i s r e g i o n by t h e absorpt i o n o f l i g h t q u i c k l y recombine a t l o c a l i z e d d e f e c t s i t e s b e f o r e t h e y can reach t h e j u n c t i o n .
The r e g i o n i s t h e r e f o r e completely
"dead" as f a r as m i n o r i t y c a r r i e r c o l l e c t i o n i s concerned.
The low-
temperature d i f f u s i o n process can e f f e c t i v e l y reduce t h e dead l a y e r , b u t u n f o r t u n a t e l y , t h i s process a l s o r e s u l t s i n a r e l a t i v e l y low dopant c o n c e n t r a t i o n (1019 cm-3) a t t h e f r o n t s u r f a c e o f t h e c e l l and t h i s causes t h e sheet r e s i s t i v i t y o f t h a t s u r f a c e t o be r a t h e r h i g h (150-200
n/o). To reduce t h e power loss from s e r i e s
resistance,
642
R. T. YOUNG E T A L
t h e f r o n t surface m e t a l l i z a t i o n p a t t e r n o f the conventional shallow j u n c t i o n c e l l c o n s i s t s o f a l a r g e number o f f i n e , n a r r o w l y spaced f i n g e r s which must be a p p l i e d by p h o t o l i t h o g r a p h y , t h u s adding t o t h e c o m p l e x i t y and c o s t o f t h e c e l l s .
On t h e o t h e r hand, i n a l a s e r -
processed c e l l t h e j u n c t i o n can be made s h a l l o w w h i l e t h e dopant c o n c e n t r a t i o n a t t h e s u r f a c e i s k e p t h i g h , and hence t h e sheet r e s i s t i v i t y low.
The problem o f t h e dead l a y e r i s completely e l i m -
i n a t e d because t h e l a s e r - i n d u c e d u l t r a r a p i d r e c r y s t a l l i z a t i o n o f t h e near-surface r e g i o n p r e v e n t s t h e f o r m a t i o n o f p r e c i p i t a t e s .
Not
o n l y i s t h e s t e p o f p h o t o l i t h o g r a p h y n o t needed, t h e number o f f i n gers making up t h e f r o n t s u r f a c e m e t a l l i z a t i o n can be reduced below t h a t used f o r c o n v e n t i o n a l c e l l s .
Moreover, t h e same l o c a l i z a t i o n
and e f f i c i e n t usage o f h i g h power d e n s i t i e s i n t h e f r o n t s u r f a c e a l s o p r e v e n t s t h e s u b s t r a t e r e g i o n beyond a few microns beneath t h e s u r f a c e from b e i n g s i g n i f i c a n t l y heated.
Thus, a l l o f t h e d e l e t e r -
i o u s e f f e c t s a s s o c i a t e d w i t h high-temperature p r o c e s s i n g a r e almost completely e l i m i n a t e d .
This c h a r a c t e r i s t i c o f l a s e r processing
j u s t i f i e s t h e usage o f t h e t e r m i n o l o g y " l a s e r c o l d processing", even though t h e near-surface r e g i o n o f t h e sample i s a c t u a l l y me1 ted. The same r a p i d m e l t i n g and s o l i d i f i c a t i o n t h a t make p u l s e d l a s e r p r o c e s s i n g o f s i 1i c o n so u s e f u l f o r p h o t o v o l t a i c a p p l i c a t i o n s has n o t y e t proved s u i t a b l e f o r t h e p r o c e s s i n g o f c e l l s made from compound semiconductors such as GaAs.
Several e f f e c t s discussed i n
Chapter 8 i n d i c a t e t h a t t h e r e a r e some fundamental d i f f e r e n c e s between t h e l a s e r a n n e a l i n g o f elemental and compound semiconductors. For example, a l t h o u g h t h e n e a r - s u r f a c e r e g i o n remains molten f o r t i m e s o f t h e o r d e r o f o n l y 100 nsec, As escapes f r o m GaAs so
easily
t h a t a s i g n i f i c a n t l o s s o f As i s observed t o occur d u r i n g p u l s e d l a s e r i r r a d i a t i o n o f t h i s material.
Also, t h e v e r y r a p i d s o l i d i f i -
c a t i o n t h a t occurs p r o b a b l y causes t h e f o r m a t i o n o f a l a r g e number o f s i t e - a n t i s i t e and o t h e r t y p e s o f d e f e c t s .
Ion implantation o f a
compound semiconductor w i t h a s i n g l e dopant species causes addi -
tional
difficulties
because i t a u t o m a t i c a l l y i n t r o d u c e s a non-
s t o i c h i o m e t r y which may be i m p o s s i b l e t o remove i n t h e t i m e p e r i o d s
643
10. APPLICATIONS OF PULSED LASER PROCESSING i n v o l v e d i n p u l s e d l a s e r processing.
I t may be p o s s i b l e e v e n t u a l l y
t o s o l v e a l l o f t h e problems mentioned h e r e and t o s u c c e s s f u l l y a p p l y l a s e r p r o c e s s i n g t o GaAs and o t h e r compound semiconductors, b u t much more e x t e n s i v e research e f f o r t s a r e c e r t a i n l y needed.
5.
FABRICATION OF SOLAR CELLS BY BEAM-PROCESSING TECHNIQUES Because o f t h e aforementioned reasons, t h e a p p l i c a t i o n o f l a s e r
p r o c e s s i n g t o t h e f a b r i c a t i o n o f s i l i c o n s o l a r c e l l s has r e c e i v e d a p p r e c i a b l e a t t e n t i o n i n t h e p a s t few years.
The e a r l y work has
been c o n c e n t r a t e d on t h e o p t i m i z a t i o n o f i o n - i m p l a n t a t i o n and l a s e r a n n e a l i n g c o n d i t i o n s f o r h i g h - q u a l i t y , shallow, j u n c t i o n formation. Important parameters a r e i m p l a n t a t i o n energy and dose, choice o f l a s e r s and t h e i r mode o f o p e r a t i o n , l a s e r beam u n i f o r m i t y , and t h e e x t e n t o f p u l s e overlap.
E x t e n s i v e s t u d i e s have been performed
by Young and co-workers (Young e t al., 1983b).
1980, 1982a, 1982b, 1983a,
T h e i r work i n d i c a t e d t h a t t h e b e s t c e l l parameters f o r
n - t y p e base m a t e r i a l were o b t a i n e d f o r c e l l s w i t h j u n c t i o n depths i n t h e range from 0.1-0.2
um and sheet r e s i s t i v i t i e s o f 30-50 nD.
To s a t i s f y t h e s e c o n d i t i o n s , samples should be i m p l a n t e d a t t h e lowest p o s s i b l e energy t o a h i g h dose (-6x1015 cm-2).
Conventional
i o n i m p l a n t e r s a r e l i m i t e d by t h e e x t r e m e l y low c u r r e n t s a v a i l a b l e a t t h e low i m p l a n t a t i o n e n e r g i e s needed, implants.
For e n e r g y - e f f i c i e n t
e.g.,
5 kV f o r boron
annealing, t h e l a s e r s should be
chosen so t h a t photons a r e h i g h l y absorbed i n t h e n e a r - s u r f a c e region.
Ruby, frequency-doubled Nd:YAG,
l a s e r s a r e s u i t a b l e f o r t h i s purpose.
a l e x a n d r i t e , and excimer
F o r t h e b e s t annealing, t h e
l a s e r should have a reasonably u n i f o r m beam over as l a r g e an area as p o s s i b l e and t h e p u l s e energy d e n s i t y should be chosen so as t o m i n i m i z e t h e o v e r l a p p i n g o f pulses t h a t a r e j u s t s l i g h t l y above t h e t h r e s h o l d r e q u i r e d f o r complete annealing.
Young e t a l .
(1983a)
found t h a t t h e combination o f gas-.discharge i m p l a n t a t i o n and excimer l a s e r annealing has many advantages over c o n v e n t i o n a l mass-analyzed i o n i m p l a n t a t i o n and s o l i d - s t a t e 1982a).
l a s e r a n n e a l i n g (Young e t al.,
644
R. T. YOUNG E T A L . F i g u r e 9 i s a schematic diagram o f t h e dc glow d i s c h a r g e system
used i n t h e experiments.
I n t h e d i s c h a r g e chamber, t h e sample i s
f i x e d on an e l e c t r o d e 5.5 inches i n diameter t h a t i s surrounded by guard r i n g s t o p r o v i d e homogeneous i m p l a n t s over areas as l a r g e as 5 inches i n diameter. as dopant sources.
The gases BF, and PF, can be used d i r e c t l y
The p r i n c i p a l advantages o f gas d i s c h a r g e
i m p l a n t a t i o n over c o n v e n t i o n a l mass-separated i o n i m p l a n t a t i o n a r e
(1) t h e extreme s i m p l i c i t y o f t h e equipment and (2) t h e c a p a b i l i t y o f o b t a i n i n g a h i g h discharge c u r r e n t d e n s i t y 50-100 uA/cm2 a t low i m p l a n t a t i o n e n e r g i e s (1 k V ) , w h i l e m a i n t a i n i n g good u n i f o r m i t y o v e r l a r g e areas. U n l i k e c o n v e n t i o n a l i m p l a n t a t i o n , m u l t i p l e molecc u l a r i o n species such as BF3+, BF,+, and i m p l a n t e d by t h i s technique.
BF',
B+, etc.,
a r e generated
However, t h e experimental r e s u l t s
show no d e l e t e r i o u s e f f e c t s on c e l l performance a r i s i n g from t h e d i f f e r i n g m o l e c u l a r i o n species.
As w i t h any i m p l a n t a t i o n process,
iAS IMPLANT SYSTEM 6AS INLET
U
-SILICON ANODE
SILICON SUBSTRATE SAMPLE HOLDER
EATER SUBSTRATE EEDTHRU
THERMOCCWLE FEEDTHRU
7 I
I Fig. 9 .
VACUUM SYSTEM
I
Schematic diagram o f the dc glow discharge implantation chamber.
10. APPLICATIONS OF PULSED LASER PROCESSING
645
a n n e a l i n g i s r e q u i r e d t o remove t h e damage c r e a t e d by i o n bombardment, and t o e l e c t r i c a l l y a c t i v a t e t h e dopant ions.
Laser a n n e a l i n g
p r o v i d e s an i d e a l way t o remove t h e damage w h i l e p r e v e n t i n g t h e d i f f u s i o n o f t r a c e contaminants i n t h e source gases i n t o t h e subs t r a t e beyond t h e j u n c t i o n d e p l e t i o n region. The advantages o f excimer l a s e r s o v e r s o l i d - s t a t e l a s e r s f o r t h e a n n e a l i n g o f i o n - i m p l a n t e d s i l i c o n have been d e s c r i b e d i n t h e previous section.
A XeCl (308 nm) excimer l a s e r was used i n t h e
work r e p o r t e d by Young e t a l . (1983a).
The l a s e r p r o v i d e d a u n i f o r m
beam (-5% energy v a r i a t i o n over a 3 cm x 3 cm a r e a ) and good p u l s e to-pulse
reproducibility
(-2%).
These a r e c r u c i a l f a c t o r s i n
d e t e r m i n i n g t h e q u a l i t y o f annealing, e s p e c i a l l y when a scanned, r e p e t i t i v e l y p u l s e d mode i s used f o r l a r g e - a r e a d e v i c e annealing. The l a s e r was operated a t an energy of 1 J/pulse w i t h a p u l s e durat i o n o f 55 ns and a r o u g h l y t r a p a z o i d a l p u l s e shape (see Chapter 4). The sample was mounted on a microprocessor-controlled x-y t a b l e d u r i n g a n n e a l i n g and t h e l a s e r beam was focused t o g i v e an energy d e n s i t y o f 1.4 J/cm2.
The scan p a t t e r n o f t h e x-y t a b l e was such
t h a t t h e minimum p u l s e o v e r l a p p i n g f o r complete a n n e a l i n g was p r o vided.
For comparison s t u d i e s , a Q-switched ruby l a s e r with an
energy o u t p u t of -12 J (multimode) p e r p u l s e and a r e p e t i t i o n r a t e o f -1 pulse/min was a l s o used f o r annealing. Polished, n-type,
f l o a t - z o n e d ( F Z ) s i l i c o n wafers 2 inches i n
diameter w i t h a r e s i s i t i v i t y o f 1-3 n-cm were i m p l a n t e d w i t h boron t o a dose of 6 x 1015 cm-2 by BF3 glow d i s c h a r g e i m p l a n t a t i o n a t an energy of 1 kV, o r by c o n v e n t i o n a l mass-separated i o n implantat i o n a t 5 kV, t h e l o w e s t a v a i l a b l e energy.
The back s i d e o f each
w a f e r was made degenerate by an 75-8, l a y e r o f antimony deposited by e-beam evaporation.
The wafers were c u t i n t o s i x 1 cm x 2 cm
c e l l blanks, b o t h s i d e s o f which were t h e n annealed by l a s e r i r r a diation.
Since t h e m i n o r i t y - c a r r i e r d i f f u s i o n l e n g t h o f t h e mate-
r i a l was c l o s e t o t h e c e l l t h i c k n e s s (300 urn), t h e h e a v i l y Sb-doped l a y e r p r o b a b l y a l s o served as a back-surface f i e l d .
Half of the
samples were annealed w i t h t h e ruby l a s e r and t h e o t h e r h a l f w i t h
646
R. T. YOUNG E T k L .
t h e excimer l a s e r .
No beam homogenizer was used d u r i n g XeCl l a s e r
annealing, b u t i t was necessary t o use a ground g l a s s d i f f u s e r p l a t e w h i l e a n n e a l i n g w i t h t h e ruby l a s e r , r e s u l t i n g i n an energy l o s s o f -50%.
I n o r d e r t o e s t a b l i s h t h e optimum l a s e r a n n e a l i n g con-
d i t i o n s , a s u b s t r a t e h e a t e r which c o u l d heat t h e sample t o 500°C was used d u r i n g c e r t a i n o f t h e experiments.
Evaporated Ti-Pd-Ag
m e t a l l i z a t i o n was used t o f o r m t h e f r o n t and back contacts.
The
c o n t a c t s were s i n t e r e d f o r 2 min a t 5OO0C, t h e h i g h e s t temperature t o which t h e s u b s t r a t e s were s u b j e c t e d d u r i n g t h e e n t i r e c e l l f a b r i cation.
Two-layer a n t i r e f l e c t i o n c o a t i n g s were a p p l i e d t o t h e c e l l s
a t 250°C by e-beam e v a p o r a t i o n f r o m Ta,O,
and MgF,
sources.
All
t e s t i n g was c a r r i e d o u t w i t h t h e c e l l s h e l d a t 28°C under AM1 ( a i r mass one) i l l u m i n a t i o n . The u n i f o r m i t y o f t h e gas discharge i m p l a n t a t i o n was checked by van d e r Pauw measurements on randomly s e l e c t e d samples c u t from t h e wafers a f t e r XeCl l a s e r annealing.
SUBSTRATE TEMPERATURE ("C)
Fig.
10.
C e l l parameters as a function of substrate temperature with
E l = 1.2 J/cm2.
(Young e t a l . ,
1982b)
647
10. APPLICATIONS OF PULSED LASER PROCESSING
The r e s u l t s i n d i c a t e d t h a t t h e v a r i a t i o n o f f r e e - c a r r i e r concent r a t i o n s was n o t more t h a n 5%. I n e a r l i e r work (Young e t al.,
1982b) i n which a d i f f u s e r p l a t e
was used t o homogenize t h e energy i n t h e ruby l a s e r pulses, i t had been e s t a b l i s h e d t h a t t h e o p e n - c i r c u i t v o l t a g e (Voc) o f t h e c e l l s c o u l d be i n c r e a s e d by h e a t i n g t h e sample t o 4OOOC d u r i n g l a s e r annealing.
The behavior o f Voc and o t h e r c e l l parameters as a func-
t i o n o f s u b s t r a t e temperature Tsub found i n t h a t e a r l i e r work i s shown i n Fig.
10.;
here we a r e concerned o n l y w i t h Voc.
It was
speculated t h a t t h e improvement i n Voc was due t o t h e r e d u c t i o n o f quenched-in d e f e c t s by t h e slower r e g r o w t h v e l o c i t y v o b t a i n e d w i t h s u b s t r a t e h e a t i n g (see Chapter 4).
However, a s i m i l a r improvement
was n o t found d u r i n g XeCl l a s e r a n n e a l i n g w i t h s u b s t r a t e tempera t u r e s up t o 500°C.
M e l t i n g model c a l c u l a t i o n s (Chapter 4) show
t h a t t h e values of v o b t a i n e d w i t h ruby, Nd:YAG,
and XeCl l a s e r s
operated a t comparable energy d e n s i t i e s and p u l s e d u r a t i o n s a r e very s i m i l a r .
Furthermore, as discussed i n Sec. 11.2,
o f XeCl laser-annealed samples (Young e t al.,
DLTS s t u d i e s
1983a) i n d i c a t e d t h a t
t h e concentrations o f e l e c t r i c a l l y a c t i v e p o i n t defects i n t h e annealed r e g i o n were two t o t h r e e o r d e r s o f magnitude l o w e r t h a n t h e c o n c e n t r a t i o n s r e p o r t e d f o r samples annealed w i t h ruby o r frequency-doubled Nd:YAG l a s e r s (Kimerl i n g and Benton, 1980; Mooney e t al.,
1981).
These r e s u l t s seem t o i n d i c a t e t h a t t h e q u a l i t y of
t h e laser-regrown l a y e r i s i n f l u e n c e d more by t h e u n i f o r m i t y of t h e l a s e r beam ( o r perhaps by t h e r e l a t i o n s h i p between m e l t i n g depth and photon p e n e t r a t i o n depth), t h a n by t h e regrowth v e l o c i t i e s u s u a l l y a t t a i n e d i n d e v i c e processing.
The p h y s i c a l s i g n i f i c a n c e
o f s u b s t r a t e h e a t i n g and t h e mechanism o f d e f e c t f o r m a t i o n i n samp l e s annealed w i t h p u l s e d l a s e r s a p p a r e n t l y needs c o n s i d e r a b l e f u r t h e r study.
I n any event, e x c e l l e n t a n n e a l i n g can be o b t a i n e d
w i t h XeCl l a s e r s w i t h o u t t h e use o f beam homogenizers and s u b s t r a t e h e a t i n g , which r e p r e s e n t s a s i g n i f i c a n t advancement and s i m p l i f i c a t i o n f o r a p p l i c a t i o n s o f p u l s e d l a s e r processing.
648
R. T. YOUNG ET AL.
Under optimum l a s e r - a n n e a l i n g c o n d i t i o n s , a l l t h e c e l l s desc r i b e d i n t h e work o f Young e t a l . (1983a) had open c i r c u i t voltages i n t h e range 605-615 mV and f i l l f a c t o r s (FF) i n t h e range 0.76-0.79. However,
t h e r e were n o t i c e a b l e d i f f e r e n c e s i n t h e s h o r t c i r c u i t
currents
(Jsc).
I n general,
t h e excimer laser-annealed
cells
showed h i g h e r Jsc t h a n t h o s e annealed w i t h t h e ruby l a s e r , whereas among t h e excimer laser-annealed c e l l s t h o s e made by gas discharge implantation o f
BF, a t 1 kV had h i g h e r
u s i n g 5 kV i m p l a n t a t i o n o f llB+.
Jsc t h a n d i d those f a b r i c a t e d
These d i f f e r e n c e s can be under-
s t o o d f r o m t h e r e s u l t s o f i n t e r n a l quantum e f f i c i e n c y measurements made on c e l l s b e f o r e t h e a p p l i c a t i o n o f AR ( a n t i - r e f l e c t i o n ) coatings.
F i g u r e 11 shows t h e comparison o f t h e i n t e r n a l quantum
BF, (1 kV) and l l B + ( 5 kV) and
e f f i c i e n c y o f c e l l s implanted w i t h
t h e n annealed w i t h a XeCl l a s e r under i d e n t i c a l c o n d i t i o n s .
I n the
1 .o 0.9 >
!$ LU 0
U
0.8 0.7
U
55
0.6 0.5
Q
8 0.4 5 0.3 -I
oc
?
z
0.2 0.1
-
0 I
Fig. 1 1 .
I
l
l
l
l
l
l
l
l
l
l
l
~
l
b
Comparison o f internal quantum efficiency for cells fabricated by
BF3 ( 1 k V ) and l l B + ( 5 k V ) implantation.
~
10.
649
APPLICATIONS OF PULSED LASER PROCESSING
wavelength r e g i o n between 600 and 1100 nm, t h e response o f t h e c e l l s i s v i r t u a l l y t h e same; however, i n t h e r e g i o n around 400 nm t h e c e l l s made w i t h BF3 i m p l a n t a t i o n c o n s i s t e n t l y e x h i b i t h i g h e r response t h a n do t h e l1Bt-imp1anted
cells.
I t i s apparent from t h e above r e s u l t s t h a t t h e m o l e c u l a r i o n species, e.g-
BF3+, and o t h e r s p e c i e s t h a t may be p r e s e n t i n t h e
l a s e r - a n n e a l e d pt l a y e r formed by gaseous d i s c h a r g e i m p l a n t a t i o n do n o t degrade t h e e f f i c i e n c y o f c a r r i e r c o l l e c t i o n .
I n f a c t , one may
s p e c u l a t e t h a t t h e presence o f f l u o r i n e reduces t h e s u r f a c e s t a t e s and t h e r e f o r e c o n t r i b u t e s t o a p a r t i a l p a s s i v a t i o n o f t h e surface. Furthermore,
because o f t h e l o w e r i m p l a n t a t i o n energy and t h e
m u l t i p l e m o l e c u l a r - i o n species, t h e BF3 gaseous i m p l a n t a t i o n p r o v i d e s a non-Gaussian dopant p r o f i l e w i t h t h e peak a t t h e s u r f a c e (Wichner, 1975).
A f t e r l a s e r annealing, a more s u i t a b l e p r o f i l e f o r
s o l a r c e l l s i s o b t a i n e d t h a n can be generated f r o m t h e Gaussian p r o f i l e s t y p i c a l o f conventional i o n implantation.
A typical profile
o f t h e e l e c t r i c a l l y a c t i v e dopant i n a BF3-implanted,
XeCl l a s e r
annealed sample, determined by H a l l e f f e c t measurements i n conjunct i o n w i t h a n o d i c o x i d a t i o n and s t r i p p i n g , i s shown i n F i g . 12. high surface concentration
The
( 6 ~ 1 0 ~~0 m - ~ ) t ,h e s h a l l o w j u n c t i o n
(0.15 urn), and t h e s t r o n g b u i l t - i n e l e c t r i c a l f i e l d near t h e s u r f a c e (proportional t o the gradient o f the concentration) indicate that a n e a r l y i d e a l dopant p r o f i l e (Lindmayer and A l l i s o n ,
1972) i s
o b t a i n e d w i t h t h e beam-processing techniques d e s c r i b e d here.
Pro-
f i l e s o f t h i s t y p e , which a r e d e s i r a b l e f o r h i g h - e f f i c i e n c y c e l l s , a r e n o t e a s i l y o b t a i n e d w i t h c o n v e n t i o n a l thermal p r o c e s s i n g (Bae and D ' A i e l l o , 1977; Fossum and Burgess, 1978).
To t a k e advantage
o f t h e h i g h s u r f a c e c o n c e n t r a t i o n s and t h e r e s u l t i n g low l a t e r a l r e s i s t a n c e o f t h e l a s e r - p r o c e s s e d c e l l s , m e t a l l i z a t i o n masks; w i t h very low shadow f r a c t i o n s (-5%)
and very few f i n g e r s can be used
w i t h t h e laser-processed c e l l s . Data f o r t h e performance o f f o u r s e l e c t e d s o l a r c e l l s f a b r i c a t e d by BF, i m p l a n t a t i o n and XeCl l a s e r a n n e a l i n g a r e t a b u l a t e d i n Table I ; t h e shunt and s e r i e s r e s i s t a n c e o f t h e c e l l s a r e a l s o given
650
R. T. YOUNG E T A L
u
0
100
200
300
DEPTH (nm)
Fig. 1 2 . Dopant p r o f i l e o f a BFg-implanted,
i n t h e table.
XeCl laser-annealed S i sample.
The h i g h shunt r e s i s t a n c e and l o w s e r i e s r e s i s t a n c e
o f these c e l l s i n d i c a t e t h a t t h e e f f i c i e n c y losses from these factors are negligible.
It i s w o r t h emphasizing here t h a t t h e s e
beam-processed c e l l s a r e o f a p a r t i c u l a r l y simple s t r u c t u r e , f a b r i c a t e d w i t h o u t any complex p r o c e s s i n g steps, s u b j e c t e d t o no h i g h temperature t r e a t m e n t , and u t i l i z i n g no s u r f a c e o r edge p a s s i v a t i o n . T h i s s i t u a t i o n seems t o suggest t h a t w i t h a d d i t i o n a l o p t i m i z a t i o n s t u d i e s and improvements i n processing, and w i t h t h e i n c o r p o r a t i o n o f surface passivation,
c e l l e f f i c i e n c i e s w e l l o v e r 17% AM1 can
readily
Because o f t h e e x c e l l e n t b l u e response
be obtained.
( i n t e r n a l quantum e f f i c i e n c y o f 0.8 a t 400 nm) and low sheet r e s i s t i v i t y (-20-30
n/o) o b t a i n e d w i t h these c e l l s , i t would seem t h a t
t h i s p r o c e s s i n g technology i s s u i t a b l e n o t o n l y f o r h i g h - e f f i c i e n c y t e r r e s t r i a l c e l l s b u t a l s o f o r c o n c e n t r a t o r and space c e l l s .
10.
651
APPLICATIONS OF PULSED LASER PROCESSING
Table I Measured performance o f BF3-implanted (1 kV) XeCl laser-annealed (Ea. = 1.4 J/cm*, Tsub = 25°C) S i s o l a r c e l l s Sample No.
1 2
3 4
6.
Jsc (mA/cm2)
voc (mV)
FF
33.9 35.7 33.2 34.4
614 608 614 611
78.1 76.3 78.9 76.5
9
(%)
INFLUENCE OF THE DOPANT PROFILE
(%I 16.1 16.5 16.1 16.1
Rshunt
Rseries
( a1
(a 1
46 K 84 K 178 K 150 K
0.48 0.59 0.63 0.63
ON SURFACE RECOMBINATION
W i t h t h e r e c e n t progress i n s i l i c o n m a t e r i a l and d e v i c e p r o cessing t e c h n o l o g i e s , t h e r o l e o f s u r f a c e recombination becomes more and more c r i t i c a l i n t h e design o f h i g h - e f f i c i e n c y
cells.
Surface p a s s i v a t i o n has become an e s s e n t i a l s t e p i n t h e f a b r i c a t i o n o f very h i g h e f f i c i e n c y c e l l s . SiO,
The growth o f a t h i n (50-100 A )
l a y e r on S i s u r f a c e s a t h i g h temperatures (>8OO0C) was found
very e f f e c t i v e i n r e d u c i n g s u r f a c e recombination and i t i s now r o u t i n e l y used i n t h e f a b r i c a t i o n o f l a b o r a t o r y research-type c e l l structures.
T h i s t y p e o f s u r f a c e p a s s i v a t i o n , even though e f f e c -
t i v e , adds c o n s i d e r a b l e c o m p l e x i t y t o t h e p r o c e s s i n g because photol i t h o g r a p h y has t o be used t o c u t t h r o u g h t h e o x i d e l a y e r and d e f i n e t h e g r i d contacts.
A r e c e n t study by Cuevas e t a l .
(1984) i n d i -
cates t h a t t h e very h i g h s u r f a c e dopant c o n c e n t r a t i o n s t h a t can be achieved w i t h beam p r o c e s s i n g ( i o n i m p l a n t a t i o n and l a s e r a n n e a l i n g ) techniques p r o v i d e an " i n s i t u " s u r f a c e p a s s i v a t i o n t h a t suppresses recombination a t t h e s u r f a c e and t h u s reduces t h e e m i t t e r recombin a t i o n c u r r e n t ; t h i s w i l l be discussed next. E m i t t e r recombination c u r r e n t s were measured f o r c e l l s made from 0.25 a-cm,
n-type F Z - s i l i c o n wafers i m p l a n t e d w i t h 5 k V boron t o
doses from 2x1014 t o 1x1016 cm-2 and annealed by a ruby l a s e r .
The
r e s u l t i n g s u r f a c e c o n c e n t r a t i o n s ranged f r o m 2x1019 t o 1x1021 cm-3
652
R. T. YOUNG E T A L . I
I
I
I
1
102’
z
0
n
m 0
500
4000
4500
DEPTH
Fig.
13.
Boron concentration
2500
(density) as a function o f depth f o r five
experimental p+n silicon solar cells.
The p r o f i l e s were measured by secondary
(Cuevas e t a l . ,
ion mass spectroscopy.
2000
(s)
1984)
and t h e c o n c e n t r a t i o n p r o f i l e s were o f t h e form shown on F i g . 13. Since t h e a n n e a l i n g process does not a l t e r t h e p r o p e r t i e s o f t h e s u b s t r a t e , values o f t h e m i n o r i t y c a r r i e r ( h o l e s ) d i f f u s i o n l e n g t h Lp i n t h e base r e g i o n a r e t h e same (-200 pm) f o r d i f f e r e n t samples. The t o t a l s a t u r a t i o n c u r r e n t d e n s i t y Jo has two components, Jeo and Jbo,
r e p r e s e n t i n g t h e recombination c u r r e n t from t h e e m i t t e r and
base regions, r e s p e c t i v e l y . p l e s was about 400
pm
The base r e g i o n t h i c k n e s s o f t h e sam-
and s i n c e t h i s was g r e a t e r t h a n t h e measured
Lp, t h e long-base diode t h e o r y can be used t o c a l c u l a t e Jbo from t h e equation
653
10. APPLICATIONS OF PULSED LASER PROCESSING
I n t h i s equation, n i (= 1.25~1010 cm-3 a t 25OC) i s t h e i n t r i n s i c c a r r i e r d e n s i t y , Dp (=11 cmn/sec) i s t h e h o l e d i f f u s i o n c o e f f i c i e n t and Ndb (= 1.5~1016 Jbo = 5x10'13
(3111-3)
A/cm2.
i s t h e base donor c o n c e n t r a t i o n ; hence,
The e m i t t e r s a t u r a t i o n c u r r e n t d e n s i t y Jeo
can be o b t a i n e d by s u b t r a c t i n g Jbo from t h e measured t o t a l s a t u r a t i o n c u r r e n t d e n s i t y Jo; t h e r e s u l t s f o r p'n
j u n c t i o n s formed by
t h e f i v e i m p l a n t a t i o n doses used i n t h e s t u d y a r e given i n Table 11. The r e s u l t s i n d i c a t e t h a t Jeo decreases w i t h i n c r e a s i n g s u r f a c e dopant c o n c e n t r a t i o n N,
and s a t u r a t e s t o a low l i m i t o f about
5x10'13 A/cm* i n t h e two more h e a v i l y doped e m i t t e r s .
These r e s u l t s
a r e c o n s i s t e n t w i t h t h e measured i n c r e a s e o f Voc w i t h p r e v i o u s l y observed i n s i m i l a r p'n (Young e t al., e t al.
1982b).
Ns t h a t was
c e l l s made on 10 n-cm s u b s t r a t e s
An a n a l y t i c a l model was developed by Cuevas
(1984) t o d e s c r i b e m i n o r i t y c a r r i e r t r a n s p o r t i n shallow,
h e a v i l y doped e m i t t e r s .
Two p o s s i b l e mechanisms were suggested t o
e x p l a i n t h e b e h a v i o r o f Jeo w i t h doping d e n s i t y ; ( 1 ) a s t r o n g b u i l t i n r e t a r d i n g e l e c t r i c f i e l d i n t h e h e a v i l y doped s u r f a c e r e g i o n Table I1 Measured and c a l c u l a t e d e m i t t e r recombination c u r r e n t s a s a f u n c t i o n o f s u r f a c e dopant c o n c e n t r a t i o n . Re i s t h e sheet r e s i s t i v i t y o f t h e e m i t t e r . 2B 1
282
2B3
2B4
0 (cm-2)
2x1014
6x1014
2x1015
6x1015
1x1016
N, (cm-3)
2x1019
6x1019
2x1020
6x1020
1x1021
Re ( ~ / o )
580
191
63
24
16
5
3
2.5
1.6
1.5
7
3
1.5
0.5
0.5
SAMPLE
J eo (10-12 A/cm2) calculated
measured
28 5
654
R. T. YOUNG E T A L .
keeps t h e m i n o r i t y c a r r i e r s away from t h e s u r f a c e , a r e g i o n o f h i g h r e c o m b i n a t i o n v e l o c i t y , c o n f i n i n g them t o a moderately doped r e g i o n h a v i n g a r e l a t i v e l y l o n g l i f e t i m e and t h e r e b y p r o d u c i n g low values o f Jeo; ( 2 ) t h e m i n o r i t y c a r r i e r m o b i l i t y i s e x c e p t i o n a l l y low i n t h e h e a v i l y doped s u r f a c e l a y e r (Neugroschel and Lindholm, 1983) and hence Jeo i s suppressed.
The Jeo c a l c u l a t e d u s i n g these two
assumptions a r e a l s o g i v e n i n Table 11.
The r a t h e r l a r g e d i s c r e -
pancies between t h e c a l c u l a t e d and measured values i n t h e two most h e a v i l y doped samples may be due t o u n c e r t a i n t i e s concerning t h e e f f e c t s o f band-gap n a r r o w i n g and m i n o r i t y c a r r i e r m o b i l i t y . From t h e l i m i t e d e x p e r i m e n t a l data, i t seems t h a t t h e s u r f a c e p a s s i v a t i o n induced by heavy-doping e f f e c t s may n o t be as e f f e c t i v e as t h a t induced by a l a y e r o f t h e r m a l l y grown SiO,;
however, t h e
s i m p l i c i t y o f " i n s i t u " p a s s i v a t i o n shows promise f o r f a c i l i t a t i n g t h e p r o c e s s i n g procedures f o r t h e f a b r i c a t i o n o f h i g h - e f f i c i e n c y c e l Is.
7.
LASER-INDUCED DOPANT DIFFUSION A s a l r e a d y mentioned above, t h e n o r m a l l y s t r i n g e n t requirements
on t h e p u r i t y o f dopant sources can be s i g n i f i c a n t l y r e l a x e d when p u l s e d l a s e r s a r e used f o r j u n c t i o n f o r m a t i o n .
This very useful
c h a r a c t e r i s t i c r e s u l t s from t h e c o n d i t i o n s t h a t 1) t h e m e l t f r o n t p e n e t r a t e s o n l y a few t e n t h s o f a micron,
and 2) most o f t h e
m a t e r i a l i n t h e s u b s t r a t e r e g i o n remains a t t h e ambient temperature. A s a consequence, any source contaminants a r e unable t o d i f f u s e o u t o f t h e j u n c t i o n r e g i o n and degrade t h e m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h (MCDL) i n t h e s u b s t r a t e .
I n contrast, thermal annealing
and c o n v e n t i o n a l thermal d i f f u s i o n w i l l a1 low f a s t d i f f u s i n g i m p u r i t i e s t o m i g r a t e deep i n t o t h e s u b s t r a t e . tages,
Because o f t h e s e advan-
p u l s e d l a s e r p r o c e s s i n g can be used t o f o r m p-n j u n c t i o n s
i n a v a r i e t y o f ways o t h e r t h a n i o n i m p l a n t a t i o n , d iscus s
.
as we w i l l now
655
10. APPLICATIONS OF PULSED LASER PROCESSING a.
S o l i d Sources As discussed i n Chapter 1, p-n j u n c t i o n s can be formed i n S i by
l a s e r - i n d u c e d d i f f u s i o n o f dopant f i l m s d e p o s i t e d on t h e s u r f a c e without using ion-implantation
or t h e r m a l - d i f f u s i o n steps.
I n this
approach, a t h i n (50-100 A ) dopant f i l m i s d e p o s i t e d on t h e sample by e-beam evaporation, o r by any o t h e r t e c h n i q u e ( p a i n t i n g , sprayon, spin-on,
etc.)
t h a t y i e l d s a reasonably u n i f o r m f i l m .
After
i r r a d i a t i o n o f t h e f i l m s w i t h a p u l s e d l a s e r , source dopants a r e i n c o r p o r a t e d i n t o t h e sample and e l e c t r i c a l l y a c t i v a t e d as a consequence o f l i q u i d - p h a s e d i f f u s i o n d u r i n g l a s e r - i n d u c e d s u r f a c e melting.
I n t h i s case dopant c o n c e n t r a t i o n s may exceed t h e s o l i d
s o l u b i l i t y l i m i t i f h i g h l y c o n c e n t r a t e d dopant sources a r e used (Narayan e t al.,
1978).
Experimental r e s u l t s have shown t h a t p-n
j u n c t i o n s i l i c o n s o l a r c e l l s w i t h e f f i c i e n c i e s comparable t o i o n implanted, laser-annealed c e l l s can be f a b r i c a t e d u s i n g t h i s t e c h n i q u e (Young e t al.,
1980; Fogarassy e t al.,
1981).
Laser-induced
diffusion,
especially w i t h a s u i t a b l e low-cost f i l m deposition
technique,
c o u l d be q u i t e u s e f u l f o r t h e large-volume p r o d u c t i o n
o f s o l a r c e l l s o r o t h e r b a s i c e l e c t r o n i c s t r u c t u r e s such as j u n c t i o n s i n b i p o l a r t r a n s i s t o r s , ohmic c o n t a c t s , back s u r f a c e f i e l d s , etc.,
b.
s i n c e n e i t h e r masking n o r vacuum t e c h n o l o g y i s needed. L i q u i d and Gaseous Sources An obvious e x t e n s i o n o f t h e s t u d i e s o f l a s e r doping f r o m s o l i d
sources i s work on doping from l i q u i d and gaseous sources.
Stuck
e t a l . (1981) have shown t h a t h i g h doping c o n c e n t r a t i o n s and s a t i s f a c t o r y p-n j u n c t i o n s can be o b t a i n e d u s i n g one o r two p u l s e s o f l a s e r r a d i a t i o n i n c i d e n t on a s i l i c o n s u r f a c e i n c o n t a c t w i t h a l i q u i d c o n t a i n i n g t h e d e s i r e d dopant.
Doping d i r e c t l y f r o m t h e
gaseous s t a t e has been demonstrated by Turner e t a l .
(1981).
The
low d e n s i t y o f dopant i o n s a t t h e g a s - s o l i d i n t e r f a c e seems t o make t h i s method c o n s i d e r a b l y l e s s e f f e c t i v e t h a n l a s e r - i n d u c e d d i f f u s i o n from s o l i d and l i q u i d sources.
Indeed, Deutsch e t a l .
(1979, 1981) found t h a t t h e y had t o i r r a d i a t e t h e same s p o t on t h e
656
R. T. YOUNG ET AL
sample w i t h 25 pulses from t h e l a s e r b e f o r e s a t i s f a c t o r y doping l e v e l s c o u l d be obtained.
I n c r e a s i n g t h e p r e s s u r e o f t h e dopant
gas and t h e use o f UV l a s e r s may improve t h e doping e f f i c i e n c y and may make t h i s method of doping u s e f u l i n some instances. 8.
LASER DAMAGE GETTERING The m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h i s t h e key f a c t o r i n
d e t e r m i n i n g t h e e f f e c t s o f back s u r f a c e f i e l d s on t h e e f f i c i e n c i e s o f silicon solar cells.
Laser p r o c e s s i n g has t h e advantage o f
p r e s e r v i n g t h e MCDL i n t h e base r e g i o n o f c e l l s b u t i t does n o t improve i t .
I t i s g e n e r a l l y b e l i e v e d t h a t t h e MCDL i n s i n g l e -
crystal o r large-grain polycrystalline s i l i c o n i s l i m i t e d p r i m a r i l y by t h e presence o f t r a n s i t i o n metals and/or p o i n t d e f e c t s i n t h e as-grown m a t e r i a l (see,
f o r example, Katz e t al.,
induced damage g e t t e r i n g u s i n g a Nd:YAG, Ar-ion
l a s e r (Sandow,
1980;
ruby l a s e r (Young e t al.,
(Katz e t al.,
Hawkins and E r i k s o n ,
1982b),
1981).
Laser-
1981), an
1984),
and a
has been demonstrated t o be an
e f f e c t i v e method f o r i m p r o v i n g t h e MCDL by e l i m i n a t i n g t h e d e t r i mental e f f e c t s o f t h e s e heavy metals.
I n t h i s method, extended
d e f e c t s o f a w e l l - d e f i n e d and c o n t r o l l e d t y p e a r e c r e a t e d on t h e back s u r f a c e o f t h e sample by i n t e n s e l a s e r r a d i a t i o n , a f t e r which t h e sample i s s u b j e c t e d t o a high-temperature heat t r e a t m e n t t o generate d i s l o c a t i o n s .
These d i s l o c a t i o n s a c t as e f f e c t i v e g e t t e r -
i n g s i t e s f o r t h e heavy metal i m p u r i t i e s and, p o s s i b l y , f o r p o i n t d e f e c t s d u r i n g t h e high-temperature t r e a t m e n t .
S t r u c t u r a l charac-
t e r i s t i c s o f these extended d e f e c t s were s t u d i e d q u i t e e x t e n s i v e l y by TEM (Eggermont e t al.,
1982).
The r e s u l t s i n d i c a t e d t h a t t h e
d i s l o c a t i o n s generated by l a s e r damage a r e more s t a b l e a g a i n s t t h e r m a l t r e a t m e n t t h a n a r e t h o s e generated by mechanical damage.
An example o f t h e e f f e c t i v e n e s s o f l a s e r g e t t e r i n g as a f u n c t i o n o f l a s e r energy d e n s i t y f o r a Nd:YAG l a s e r i s demonstrated by t h e data i n Table I 1 1 which i s taken from Eggermont e t a l . (1983).
For
comparison, data on t h e g e t t e r i n g e f f i c i e n c y o f A r - i o n i m p l a n t a t i o n
657
10. APPLICATIONS OF PULSED LASER PROCESSING Table I 1 1 M i n o r i t y c a r r i e r l i f e t i m e (MCL) b e f o r e and a f t e r l a s e r , mechanical, and i o n - i m p l a n t a t i o n damage g e t t e r i n g . E Q i s t h e l a s e r energy d e n s i t y from a Nd:YAG l a s e r o p e r a t i n g a t a wavelength o f 1.06 um and a p u l s e d u r a t i o n o f 150 nsec. Laser1
El ( J/cm2
35
1
MCL~ (msec) ~
~
2.0
30
26
1.1
0.9
Mechanical
Imp1a n t a t i on2
Undamaged
0.5
2.0
0.4
~~
1) Spot s i z e 65 urn, spot spacing 150 pin, row spacing 250 pm 2 ) 7x101s argon ions/cm2 a t 140 keV 3 ) Average MCL based on c - t measurements on 40 MOS c a p a c i t o r s and mechanically damaged samples a r e a l s o shown i n t h e t a b l e .
The
r e s u l t s show t h a t t h e g e t t e r i n g e f f e c t i v e n e s s i s q u i t e s e n s i t i v e t o t h e l a s e r energy d e n s i t y and t o t h e t y p e o f damage i n t r o d u c e d i n t o t h e wafer.
The non-contact n a t u r e o f l a s e r damage g e t t e r i n g ,
t h e b e t t e r c o n t r o l o f t h e depth and amount o f damage, and t h e h i g h p r o c e s s i n g throughput (i.e.,
300 3" wafers p e r hour) have made t h e
use o f l a s e r s f o r g e t t e r i n g very a t t r a c t i v e t o t h e i n t e g r a t e d c i r c u i t industry.
T h i s same t e c h n i q u e should be u s e f u l i n t h e
f a b r i c a t i o n o f h i g h - e f f i c i e n c y c e l l s from low-cost r i b b o n and c a s t p o l y c r y s t a l l i n e s i l i c o n materials. 9.
G R A I N BOUNDARY STUDIES
The p o t e n t i a l use o f p o l y c r y s t a l l i n e
s i l i c o n f o r low-cost
t e r r e s t r i a l p h o t o v o l t a i c devices has s t i m u l a t e d numerous s t u d i e s o f g r a i n boundaries and t h e i r e l e c t r i c a l p r o p e r t i e s .
I n t h i s sub-
s e c t i o n , we w i l l d i s c u s s a few examples t h a t i l l u s t r a t e how l a s e r p r o c e s s i n g techniques can be used i n t h e s t u d y o f these p r o p e r t i e s (Wood e t a1
., 1980).
658
R. T. YOUNG ETAL.
A g r a i n boundary i s t h e i n t e r f a c e a l o n g which two c r y s t a l s o f d i f f e r e n t o r i e n t a t i o n are j o i n e d together.
This interface usually
c o n s i s t s o f o n l y a few atomic l a y e r s o f d i s o r d e r e d atoms, b u t e l a s t i c s t r a i n and e l e c t r i c f i e l d s due t o t r a p p e d charges may extend t h e g r a i n boundary e f f e c t s t o g r e a t e r d i s t a n c e s . m i s o r i e n t a t i o n angle
(e), d e n s i t y o f c o i n c i d e n c e s i t e s , and t y p e s
o f d i s l o c a t i o n s (such as edge d i s l o c a t i o n s , etc.)
Depending on t h e
screw d i s l o c a t i o n s ,
i n t h e d i s o r d e r e d region, g r a i n boundaries can be c l a s s i f i e d
as l o w - a n g l e ( 0
<
l o ) , high-angle ( 0
mixed boundaries, e t c .
>
l o o ) , twin, tilt, t w i s t ,
The e l e c t r i c a l p r o p e r t i e s , i n terms o f t h e
d e n s i t y o f d e f e c t s t a t e s and t h e e f f e c t s o f t h e d e f e c t s i t e s on c a r r i e r recombination, as w e l l as t h e tendency f o r i m p u r i t y segreg a t i o n a t t h e boundaries a r e expected t o be d i f f e r e n t a t d i f f e r e n t t y p e s o f boundaries.
W e l l - d e f i n e d " c l e a n " boundaries seldom e x i s t
i n p o l y c r y s t a l l i n e s i l i c o n grown by c o n v e n t i o n a l c r y s t a l growth techniques.
C u r r e n t research s t u d i e s e x p l o r e t h e e x t e n t t o which
v a r i o u s t y p e s o f grain-boundary r e c o m b i n a t i o n mechanisms reduce t h e photogenerated c u r r e n t and whether o r n o t an e f f e c t i v e method can be found t o " p a s s i v a t e " t h e boundaries.
To answer t h e s e q u e s t i o n s ,
s t u d i e s o f r e c o m b i n a t i o n e f f e c t s by electron-beam-induced
current
(EBIC) and scanning l a s e r spot (SLS) t e c h n i q u e s a r e f r e q u e n t l y used. These t e c h n i q u e s r e q u i r e t h e f o r m a t i o n o f a p-n j u n c t i o n and l a s e r " c o l d " p r o c e s s i n g appears t o p r o v i d e t h e i d e a l method f o r j u n c t i o n f o r m a t i o n i n t h e s e t y p e s o f s t u d i e s s i n c e contaminants a r e n o t i n t r o d u c e d i n t o t h e b u l k o f t h e samples. measurements,
By u s i n g EBIC and SLS
t h r e e t y p e s o f g r a i n boundary e f f e c t s have been
observed by Young e t a l .
( 1 9 8 2 ~ )i n l a r g e - g r a i n e d p o l y c r y s t a l l i n e
silicon.
These a r e i l l u s t r a t e d i n Fig. 14 and may be d e s c r i b e d as
follows:
(1) some boundaries a c t as r e c o m b i n a t i o n s i t e s r e d u c i n g
t h e photogenerated c u r r e n t ; ( 2 ) some show no r e c o m b i n a t i o n and do n o t a f f e c t t h e photoresponse; and ( 3 ) o t h e r s a c t u a l l y show an enhancement o f t h e photogenerated c u r r e n t .
However, i t i s reasonably
w e l l e s t a b l i s h e d t h a t i n samples which r e c e i v e a h i g h - t e m p e r a t u r e heat t r e a t m e n t , as f o r example when t h e j u n c t i o n i s formed by
10.
F i g . 14.
APPLICATIONS OF PULSED LASER PROCESSING
659
Various types of grain boundaries observed in single-pass, float-
zone, large-grained polycrystalline Si.
a ) grain boundaries show recombination;
b ) grain boundaries show enhancement of the photogenerated current; c ) grain boundaries show no recombination i n i t i a l l y ,
but; d ) they are converted to
recombination sites a f t e r heat treatment.
thermal d i f f u s i o n , t h e boundaries i n c a t e g o r i e s 2 and 3 w i l l be converted t o category 1 and become e l e c t r i c a l l y a c t i v e recombination sites.
Apparently, t h e s e phenomena a r e n o t dependent on t h e mis-
o r i e n t a t i o n angle o f t h e boundary.
S i m i l a r r e s u l t s were observed
by Turner e t a l . (1982) by u s i n g t h e SLS technique.
They r e p o r t e d
t h a t t h e enhancement e f f e c t o f t h e g r a i n boundaries was m o s t l y found f o r samples c u t from t h e m i d d l e o f a b l o c k o f Wacker c a s t polycrystalline silicon.
Since a g r a i n boundary i s i n some ways
analogous t o a surface, t h e e l e c t r i c a l p r o p e r t i e s o f a g r a i n bounda r y may be d e s c r i b e d i n terms o f an i n t e r f a c e w i t h an accumulated, depleted,
or inverted layer.
The t h r e e d i f f e r e n t t y p e s o f g r a i n
660
R. T. YOUNG ETAL.
boundary e f f e c t s c o u l d t h e n be explained, b u t t h i s can o n l y be cons i d e r e d an h y p o t h e s i s a t t h i s p o i n t . s e v e r a l q u e s t i o n s a r e needed.
Further in-depth studies o f
For i n s t a n c e , what i s t h e d r i v i n g
f o r c e t o f o r m t h e accumulated o r i n v e r t e d l a y e r , and i s t h e mechanism c h e m i c a l l y o r s t r u c t u r a l l y r e l a t e d ?
I n any case,
it i s
i m p o r t a n t t o recognize t h a t t h e r e a r e g r a i n boundaries i n t h e asgrown m a t e r i a l which do n o t a c t as recombination s i t e s , some cases, may even a c t as c u r r e n t c o l l e c t o r s .
and, i n
The l a t t e r phe-
nomenon may a r i s e from a mechanism s i m i l a r t o t h a t suggested by D i s t e f a n o and Cuomo (1977),
i.e.,
a t h i n layer o f impurity o f
o p p o s i t e t y p e t o t h e b u l k doping i s segregated t o t h e boundary i n t e r f a c e d u r i n g c r y s t a l growth.
A thorough understanding o f g r a i n
boundary s e g r e g a t i o n mechanisms and t h e d i s c o v e r y o f a way t o cont r o l t h e development o f g r a i n boundary p r o p e r t i e s d u r i n g c r y s t a l growth may p r o v i d e t h e s o l u t i o n f o r t e r r e s t r i a l p h o t o v o l t a i c a p p l i cations o f polycrystalline silicon. Pulsed l a s e r r a d i a t i o n , i n a d d i t i o n t o b e i n g used t o form j u n c t i o n s , can a l s o be used t o modify t h e m i c r o s t r u c t u r e o f t h e g r a i n boundaries, e.g.,
i n c o h e r e n t boundaries can be converted i n t o co-
h e r e n t ones by l a s e r - i n d u c e d s u r f a c e m e l t i n g (Young e t a l
., 19824).
Examples o f t h e t y p e o f r e s u l t s o b t a i n e d a r e g i v e n i n Fig.
15.
Panels 15a and 15b show a comparison o f SEM images i n t h e secondary e l e c t r o n and EBIC modes of a s e l e c t e d area o f a p o l y c r y s t a l l i n e s i l i c o n sample, a f t e r l a s e r i r r a d i a t i o n . electrically inactive
The d i f f e r e n c e s between
(B') and e l e c t r i c a l l y a c t i v e
have been examined by TEM.
( A ' ) boundaries
F i g u r e 15d i s a b r i g h t - f i e l d t r a n s -
m i s s i o n e l e c t r o n micrograph o f t h e e l e c t r i c a l l y i n a c t i v e boundary
(8'); t h e s t r u c t u r e seen i n t h e micrograph i s t y p i c a l o f a coherent boundary w i t h an a / 6 <211> t w i n vector.
On t h e o t h e r hand, t h e
e l e c t r i c a l l y a c t i v e boundary ( A ' ) d e v i a t e s from t h e i d e a l coherent morphology,
as i n d i c a t e d by t h e presence o f networks o f p a r t i a l
d i s l o c a t i o n s i n t h e TEM micrographs.
F i g u r e 15c i s a dark f i e l d
TEM micrograph ( t i l t 40") o f boundary A ' . a r e i n d i c a t e d i n t h i s micrograph.
Two d i s t i n c t r e g i o n s
The bottom r e g i o n
B, which i s
10.
661
APPLICATIONS OF PULSED LASER PROCESSING
Fig. 15. ( a ) Image o f a portion of a laser-annealed polycrystalline silicon solar c e l l taken with an electron microscope operating i n the secondary electron emission mode; ( b ) image o f the same area on the sample taken with the microscope operating i n the EBlC mode t o show the electrical response; ( c ) dark f i e l d TEM micrograph o f boundary A ' ; ( d ) bright f i e l d TEM micrograph o f
B' , the
electri-
cally inactive boundary.
seen t o c o n t a i n d i s l o c a t i o n s , i s t h o u g h t t o be r e s p o n s i b l e f o r t h e e l e c t r i c a l a c t i v i t y o f t h e boundary.
The t o p r e g i o n T, shows t h e
c h a r a c t e r i s t i c s o f a coherent boundary. the
T and
The d i f f e r e n c e between
B r e g i o n s was produced by l a s e r m e l t i n g , which e v i d e n t l y
eliminated t h e twinning dislocations.
However, t h e l a s e r r a d i a t i o n
d i d n o t a l t e r t h e m i c r o s t r u c t u r e o f the coherent t w i n boundary, as shown i n Fig. 15d.
The r e s u l t t h a t l a s e r - i n d u c e d s u r f a c e m e l t i n g
c o n v e r t s i n c o h e r e n t boundaries t o coherent ones may suggest t h a t t h e regrowth o f t h e boundary i n t h e l a s e r - m e l t e d r e g i o n has a h i g h tendency t o occur i n t h e d i r e c t i o n o f t h e low energy boundary, i.e.
,
662
R. T. YOUNG E T A L .
t h e coherent boundary.
These phenomena have been observed i n t h e
case o f regrowth o f d i s l o c a t i o n s by l a s e r m e l t i n g (Narayan e t al.,
1984). 10.
SUMMARY
Experimental r e s u l t s have demonstrated t h a t h i g h - e f f i c i e n c y p-n j u n c t i o n s o l a r c e l l s can be f a b r i c a t e d by t h e use o f l a s e r p u l s e s t o anneal i o n - i m p l a n t a t i o n damage i n s i n g l e c r y s t a l s u b s t r a t e s . Other low-cost, non-vacuum, j u n c t i o n f o r m a t i o n techniques have a l s o been developed and good r e s u l t s obtained.
Due t o t h e s i m p l i c i t y
and l o w c o s t o f t h i s t y p e o f processing, i t i s a n t i c i p a t e d t h a t t h e methods discussed here, a f t e r f u r t h e r e v o l u t i o n , may c o n t r i b u t e t o t h e development o f automated p r o c e s s i n g f o r volume p r o d u c t i o n o f solar cells.
I n a d d i t i o n t o t h e t e c h n o l o g i c a l advantages,
laser
p r o c e s s i n g has been found t o be u s e f u l i n many fundamental s t u d i e s , such as t h o s e concerned w i t h t h e e f f e c t s o f heavy doping on d e v i c e performance and t h e e l e c t r i c a l p r o p e r t i e s o f g r a i n boundaries. It was found t h a t t h e v e r y h i g h dopant d e n s i t i e s a t t h e s u r f a c e
achieved w i t h i o n i m p l a n t a t i o n and l a s e r a n n e a l i n g p r o v i d e an " i n s i t u " s u r f a c e p a s s i v a t i o n t h a t can ease t h e f a b r i c a t i o n requirements f o r s h a l l o w p-n j u n c t i o n c e l l s . The r e s u l t s from g r a i n boundary s t u d i e s i n d i c a t e t h a t t h e use o f laser r a d i a t i o n f o r junction formation i n p o l y c r y s t a l l i n e s i l i c o n
can p r o v i d e t h e f o l l o w i n g advantages: 1) C o n t r o l o f enhanced dopant d i f f u s i o n a l o n g g r a i n boundaries, which f r e q u e n t l y causes problems o f s h o r t i n g i n t h e device.
2)
P r e v e n t i o n o f g r a i n boundary t r a p p i n g o f contaminants, which
i s b e l i e v e d t o be p a r t i a l l y r e s p o n s i b l e f o r t h e development o f e l e c t r i c a l a c t i v i t y a t t h e s e boundaries.
3)
M o d i f i c a t i o n o f g r a i n boundary m i c r o s t r u c t u r e s , w i t h t h e r e s u l t t h a t e l e c t r i c a l l y a c t i v e boundaries a r e t r a n s f o r m e d i n t o e l e c t r i c a l l y i n a c t i v e ones, and/or r e d u c t i o n i n g r a i n boundary t r a p p i n g s t a t e s by 1aser-induced n e a r - s u r f a c e me1t i ng.
10.
663
APPLICATIONS OF PULSED LASER PROCESSING
These f i n d i n g s suggest t h a t l a s e r processing may have important a p p l i c a t i o n s i n t h e f a b r i c a t i o n o f s o l a r c e l l s from p o l y c r y s t a l l i n e s i l i c o n made by a v a r i e t y o f low-cost techniques.
IV.
Other Device Applications
IMPATT DIODES
10.
The IMPATT diode i s one o f t h e most powerful s o l i d - s t a t e sources o f microwave power today.
The power output and e f f i c i e n c y o f t h i s
k i n d o f device c u r r e n t l y are l i m i t e d by t h e m a t e r i a l p r e p a r a t i o n methods i n v o l v e d i n t h e device f a b r i c a t i o n .
For example, i n t h e
f a b r i c a t i o n o f high-frequency (100 GHz) p'pnn'
IMPATT diodes, pre-
c i s e c o n t r o l o f t h e f i l m thickness and dopant concentrations i n t h e two d r i f t regions (i.e.,
t h e p and n region), minimum i m p u r i t y
r e d i s t r i b u t i o n between t h e p'p
and n'n
j u n c t i o n , and a low contact
r e s i s t a n c e are r e q u i r e d i n order t o minimize t h e p a r a s i t i c r e s i s t i v e losses.
A schematic o f an i d e a l i z e d d o u b l e - d r i f t IMPATT s t r u c t u r e
and t h e associated e l e c t r i c f i e l d d i s t r i b u t i o n i s shown i n Fig. 16.
2
I
0 l-
a
a k
z 0 z
w
n
P
P+
HOLE DRIFT REGION
+ A
IND - N A I
n+
ELECTRON DRIFT
REGION
n 1
0
w
U
W
U
% D
a
E
0 l-
U w -I w I
1
Fig. 16. Schematic diagram o f an idealized double-drift IMPATT diode and the associated electric field. (Hess et a l . , 1980)
664
R. T.YOUNG E T A L .
P r e s e n t l y , t h e d r i f t r e g i o n s a r e formed e i t h e r by two-sided e p i t a x y o r s i n g l e epitaxy i n conjunction w i t h i o n implantation.
The e p i -
t a x i a l process i n c u r r e n t d e v i c e f a b r i c a t i o n i s based p r i m a r i l y on chemical vapor d e p o s i t i o n o f d i c h l o r o s i l a n e o r s i l a n e a t 1000-llOO°C under atmospheric pressure. sing,
e x t e n s i v e dopant
u s u a l l y occurs.
D u r i n g t h i s h i gh-temperature proces-
r e d i s t r i b u t i o n a t t h e growth i n t e r f a c e
Furthermore, due t o t h e r e l a t i v e l y h i g h d e p o s i t i o n
r a t e s a t t h e s e temperatures, i t i s very d i f f i c u l t t o r e p r o d u c i b l y control t h e f i l m thickness o f t h e t h i n a c t i v e layer required f o r high-frequency o p e r a t i o n ( f o r example, t h e n o r p r e g i o n i n 100 GHz IMPATT diodes i s 0.3
pm).
On t h e o t h e r hand,
progress i n t h e
development o f i o n i m p l a n t a t i o n i n t o t h i c k e p i t a x i a l l a y e r s has been hampered by t h e i n c o m p l e t e thermal annealing, w i t h t h e r e s u l t t h a t r e s i d u a l d e f e c t s remain i n t h e avalanche as w e l l as i n t h e d r i f t region.
I n t h e f i n a l d e v i c e f a b r i c a t i o n step, t h e c o n t a c t
r e s i s t a n c e o f c u r r e n t l y a v a i l a b l e devices i s l i m i t e d by t h e s o l i d s o l u b i l i t y l i m i t i n b o t h t h e n+ and p+ region. Laser-processing techniques may very we1 1 p r o v i d e an a t t r a c t i v e method o f c o m p l e t e l y o r a t l e a s t p a r t i a l l y s o l v i n g t h e aforement i o n e d problems i n so f a r as l a s e r s can be used t o (1) grow h i g h q u a l i t y e p i t a x i a l l a y e r s a t l o w s u b s t r a t e temperatures (<5OO0C) (see S e c t i o n V),
( 2 ) e f f e c t i v e l y remove i o n - i m p l a n t a t i o n damage,
and ( 3 ) i n c o r p o r a t e dopants i n t h e very t h i n s u r f a c e l a y e r a t conc e n t r a t i o n s t h a t f a r exceed t h e s o l i d s o l u b i l i t y l i m i t . Laser-annealed s i l i c o n IMPATT diodes were f a b r i c a t e d by Hess e t al.
(1980) and t h e performance compared t o t h a t o f diodes made
by c o n v e n t i o n a l furnace-processing techniques.
The rf performance
o f s e v e r a l l a s e r - and furnace-annealed IMPATT diodes i n t h e comp a r a t i v e study i s summarized i n Table I V .
The r e s u l t s c l e a r l y
i n d i c a t e t h a t t h e microwave power o u t p u t near t h e design frequency f o r cw d i o d e o p e r a t i o n i s s i g n i f i c a n t l y h i g h e r f o r laser-annealed diodes t h a n f o r diodes f a b r i c a t e d u s i n g a c o n v e n t i o n a l , furnaceanneal i n g step.
665
10. APPLICATIONS OF PULSED LASER PROCESSING Table I V . Comparison o f t h e rf performance o f l a s e r - and furnace-annealed IMPATT d i o d e s
Anneal ing Conditions Pulsed Ruby Laser 25 nsec 1.5 J/cm2
Furnace 900 "C 20 min
12.
Operating Conditions
Sample
Diode output
I(mA)
V(V)
(GHz)
P(mW)
71A-1
250 275
11.7 11.8
127 135
1.9 3.8
71A-3
200 230
10.9 11.1
133 136
2.7 3.0
698-1
250 264
10.0 10.0
123 124
0.16 0.48
698-2
300 350
9.77 9.94
121 124
0.32 0.48
698-4
365
10.0
131
0.16
SILICON-ON-SAPPHIRE S i l i c o n - o n - s a p p h i r e d e v i c e s a r e c u r r e n t l y used i n high-speed,
low-power,
radiation-hardened
integrated circuits.
Some o f t h e
major problems a s s o c i a t e d w i t h t h e SOS t e c h n o l o g y a r e t h e l a t t i c e mismatch and t h e l a r g e d i f f e r e n c e i n t h e r m a l expansion o f s i l i c o n and sapphire, w h i c h produce h i g h d e n s i t i e s o f i n t e r f a c e d e f e c t s , and l a r g e compressional s t r e s s a d j a c e n t t o t h e sapphi r e s u b s t r a t e . As a r e s u l t , t h e MOS f i e l d e f f e c t channel m o b i l i t y i s much l o w e r t h a n t h e m o b i l i t y i n b u l k S i devices.
R e c e n t l y , i t has been demon-
s t r a t e d t h a t l a s e r a n n e a l i n g o f SOS p r o v i d e s an e f f e c t i v e method t o r e l e a s e t h e s t r e s s and t o improve t h e m o b i l i t y (Gupta e t a l . ,
1981; Kobayashi, e t al.,
1982).
F i g u r e 1 7 shows t h e r e l a t i v e i m -
provement i n channel m o b i l i t y as a f u n c t i o n o f l a s e r energy d e n s i t y f o r s i n g l e and m u l t i p l e l a s e r pulses, l a s e r s (Gupta e t al.,
1981).
b o t h f o r ruby and e x c i m e r
These r e s u l t s i n d i c a t e d t h a t ( 1 ) b o t h
666
R. T. YOUNG E T A L
-
-- - 3 EXP 1 EXP
I
0
I
1
I
I
0.4 0.8 1.2 1.6 2.0 LASER ENERGY DENSITY, J E M ~
Fig. 17. Relative improvement o f the electron mobility in pulsed laser-annealed
N channel SOS transistors.
Solid lines
-
1 pulse, dashed lines
-
3 pulses.
l a s e r s can be used t o improve t h e e l e c t r o n channel m o b i l i t y (however, excimer l a s e r s a r e more e f f e c t i v e t h a n r u b y l a s e r s ) , ( 2 ) m u l t i p l e p u l s e s g i v e b e t t e r r e s u l t s t h a n 1 p u l s e , and ( 3 ) above a c e r t a i n t h r e s h o l d energy d e n s i t y , t h e m o b i l i t y s t a r t s t o decrease.
The
d e t a i l e d mechanisms f o r m o b i l i t y improvement a r e n o t y e t f u l l y understood,
but i t i s believed t h a t laser-induced
liquid-phase
e p i t a x i a l r e g r o w t h o f t h e S i f i l m near t h e v i c i n i t y o f t h e i n t e r f a c e can e f f e c t i v e l y reduce t h e d e f e c t d e n s i t y and r e l e a s e t h e s t r e s s . The c o r r e l a t i o n o f t h e m o b i l i t y improvement, s t r e s s r e l e a s e , d e f e c t r e d u c t i o n , etc.,
i s an area o f r e s e a r c h which needs e x t e n s i v e a d d i -
tional investigation.
10. APPLICATIONS OF PULSED LASER PROCESSING 13.
667
INTEGRATED CIRCUITS The t r e n d i n i n t e g r a t e d c i r c u i t development i n r e c e n t y e a r s
has been toward o b t a i n i n g lower p r o c e s s i n g temperatures w i t h t h e goals o f a c h i e v i n g s h a l l o w e r j u n c t i o n s ,
h i g h e r o p e r a t i n g speeds,
and increased p a c k i n g d e n s i t i e s (Hess e t al.,
1983; H i l l , 1983).
Many device p h y s i c i s t s a p p a r e n t l y be1 i e v e t h a t l a s e r - r e l a t e d and/or o t h e r energy beam p r o c e s s i n g techniques may h o l d t h e key t o t h e a t t a i n m e n t o f t h e s e goals. The use o f p u l s e d l a s e r s t o anneal a r s e n i c - i m p l a n t e d MOS t r a n s i s t o r s has shown encouraging r e s u l t s .
For example,
improvement
i n t h e c o n t r o l of t h r e s h o l d v o l t a g e s has been r e p o r t e d by Miyao and co-workers (1980).
These a u t h o r s demonstrated t h a t excel l e n t
c o n t r o l o f l a t e r a l dopant d i f f u s i o n d u r i n g d e v i c e f a b r i c a t i o n can be o b t a i n e d w i t h p u l s e d l a s e r - a n n e a l i n g techniques, and t h a t t h e p r o t e c t i o n o f t h e o x i d e areas from l a s e r damage can be achieved simply
by d e p o s i t i n g
a thin metallic
(reflecting)
overlayer.
The use o f l a s e r r a d i a t i o n t o promote t h e r e a c t i o n of metal f i l m s w i t h s i l i c o n s u b s t r a t e s i s another i n t e r e s t i n g area o f l a s e r processing.
P o t e n t i a l a p p l i c a t i o n s i n c l u d e t h e f o r m a t i o n o f low
r e s i s t i v i t y gate m a t e r i a l i n MOS t r a n s i s t o r s , device i n t e r c o n n e c t s , ohmic c o n t a c t s ,
etc.
As w i t h l a s e r a n n e a l i n g o f i o n - i m p l a n t e d
s i l i c o n , b o t h p u l s e d and cw l a s e r s have been used i n t h i s t y p e o f process.
I n t h e case o f p u l s e d i r r a d i a t i o n , t h e mechanism of metal
s i l i c i d e formation
i n v o l v e s m e l t i n g and i n t e r d i f f u s i o n o f t h e
c o n s t i t u e n t s i n t h e molten phase, f o l l o w e d by r a p i d s o l i d i f i c a t i o n (van Gurp e t al., von Allmen,
1979).
1979; von Allmen and Wittmer, 1979; Wittmer and S i l i c i d e s w i t h m u l t i p l e phases, many o f which
a r e thermodynamically metastable, are observed and, as a consequence o f c o n s t i t u t i o n a l supercooling, s u r f a c e morphologies w i t h c e l l u l a r s t r u c t u r e a r e formed (van Gurp e t al.,
1979).
With l a s e r annealing,
new metastable s i l i c i d e phases u n a t t a i n a b l e by thermal a n n e a l i n g can be formed r e a d i l y (Chapter 2).
Research on t h e l a s e r f o r m a t i o n
o f new s i l i c i d e s w i t h low enough sheet r e s i s t i v i t i e s t o s a t i s f y
668
R. T. YOUNG E T A L .
a new g e n e r a t i o n o f VLSI t e c h n o l o g i e s , has been a c t i v e l y conducted
i n s e v e r a l 1a b o r a t o r i e s . The r e c e n t emphasis on three-dimensional i n t e g r a t e d s t r u c t u r e s
(SOI) very Laser-induced r e c r y s t a l 1i z a t i o n o f f i n e - g r a i n e d (300-
makes l a s e r r e c r y s t a l l i z a t i o n o f s i l i c o n on i n s u l a t o r s a t t r a c t ive.
600 A ) p o l y c r y s t a l l i n e S i f i l m s d e p o s i t e d on a d i e l e c t r i c l a y e r (SiO,
o r S i 3 N 4 ) o r on g l a s s i s another area o f work b e i n g i n t e n -
s i v e l y pursued i n semiconductor r e s e a r c h l a b o r a t o r i e s .
The as-
d e p o s i t e d s i l i c o n f i l m s a r e e i t h e r continuous sheets o r i s o l a t e d i s l a n d structures.
Laser i r r a d i a t i o n i s used t o promote g r a i n
growth o r t o grow s i n g l e - c r y s t a l i s l a n d s , t h u s i m p r o v i n g t h e e l e c t r i c a l properties o f the films.
Both p u l s e d and cw l a s e r s have been
used i n t h e s e s t u d i e s (Kamins e t a l . e t al.,
1980; Lee, 1981).
, 1980a,b;
Lee e t al.,
1979; Lam
G e n e r a l l y speaking, t h e f i l m s annealed
by cw l a s e r s have l a r g e r g r a i n s i z e s t h a n t h o s e o f pulse-annealed f i l m s and t h e e l e c t r i c a l p r o p e r t i e s a r e l e s s s e n s i t i v e t o subsequent thermal treatment.
As a consequence,
cw l a s e r s o r t h e
r e c e n t l y developed r a p i d thermal - a n n e a l i n g techniques u s i n g a r c lamps o r g r a p h i t e s t r i p h e a t e r s a r e more g e n e r a l l y used f o r t h i s type o f application.
V.
Laser Photochemical Processing
Laser-induced photochemical p r o c e s s i n g f o r f i l m d e p o s i t i o n , etching,
and doping i s another r a p i d l y growing area o f research
t h a t may have a d i r e c t
impact on t h e c u r r e n t m i c r o e l e c t r o n i c
d e v i c e f a b r i c a t i o n technology i n a number o f ways.
The process
p r i m a r i l y i n v o l v e s p h o t o c h e m i c a l l y o r , sometimes i n combination, thermally i n i t i a t e d reactions i n t h e v i c i n i t y o f t h e gas-solid i n t e r f a c e by h i g h - i n t e n s i t y p u l s e d o r cw l a s e r s .
Depending on
whether t h e r e l e v a n t photo-generated fragments r e a c t w i t h o r a r e absorbed by t h e s o l i d , e t c h i n g o r d e p o s i t i o n w i l l occur.
Otherwise,
doping w i l l t a k e p l a c e i f t h e fragments d i s s o l v e on t h e molten surface.
The advantages o f t h e process a r e t h e l o w temperature
10. APPLICATIONS OF PULSED LASER PROCESSING
669
a t which t h e s u b s t r a t e can be maintained, t h e good c o n t r o l o f t h e environment which can be r e a l i z e d , and t h e c a p a b i l i t y o f d i r e c t w r i t i n g o f submicron l i n e s .
An example o f t h e s p a t i a l r e s o l u t i o n
which has been demonstrated by various l a s e r d i r e c t w r i t i n g processes i s given i n Table V ( E h r l i c h and Tsao, 1983).
This type o f
l a s e r - w r i t i n g process, which does not r e l y on photolithography may be used i n t h e r e p a i r of p h o t o l i t h o g r a p h i c masks and f o r f a b r i c a t i o n of interconnects i n customized c i r c u i t s , but a t present i s g r e a t l y l i m i t e d by t h e slow w r i t i n g rate. Several s t u d i e s i n t h e past have been concentrated on t h e understanding o f t h e molecular surface i n t e r a c t i o n s t o e x p l o r e t h e possib i l i t y o f improving t h e w r i t i n g speed.
The mechanism o f t h e nuclea-
t i o n and growth o f t h e metal l a y e r was s t u d i e d by E h r l i c h e t a l . (1982).
A two-step model was proposed i n which t h e metal deposit
i s f i r s t l o c a l i z e d by decomposition o f adsorbed organometallic molecules t o form c r i t i c a l n u c l e i , f o l l o w e d by gas-phase d e p o s i t i o n
t o form t h e bulk o f t h e deposit.
Recent experiments by Wood e t a l .
(1983) a l s o confirmed these f i n d i n g s .
These r e s u l t s suggested t h a t
t h e d e p o s i t i o n o r w r i t i n g r a t e can be increased i f t h e l a s e r wavel e n g t h could be chosen so t h a t t h e molecules w i l l have t h e l a r g e s t cross s e c t i o n f o r bond breakage and decomposition.
The excimer
l a s e r s are e s p e c i a l l y s u i t a b l e f o r t h i s purpose because o f t h e f l e x i b i l i t y i n choosing wave-lengths i n t h e UV region.
Large-area metal
Table V S p a t i a l r e s o l u t i o n o f several l a s e r - w r i t i n g processes. Process Photodeposition P y r o l y t i c deposition S i l i c o n e t c h i n g i n C1, S i l i c o n doping Polymerization
Minimum Linewidth (we11 c o n t r o l 1ed) 0.8 0.4 0.4 0.25 0.8
um um urn um pm
670
R. T. YOUNG E T A L .
such as As, Mo, W,
films,
and C r , have been d e p o s i t e d on S i w i t h
s a t i s f a c t o r y d e p o s i t i o n r a t e s (2000 A/min) and good p h y s i c a l p r o perties. Laser-induced photochemical p r o c e s s i n g has a1 so been used t o d e p o s i t d i e l e c t r i c f i l m s (Boyer e t al., Si3N4, A1,0,,
etc.,
1983).
F i l m s such as SiO,,
a l l comnonly used o p t i c a l coatings,
have been
deposited v i a t h e f o l l o w i n g reactions:
(1) SiH,
+ N20 t hv (193 nm) + SiO,
(2)
+ NH, + hv (193 nm) + Si,Ny
SiH,
+ products,
+
products, and
( 3 ) Al(CH,), t N20 t hv (193 nm) + Al,O, + products. These f i l m s e x h i b i t ow p i n h o l e d e n s i t i e s and can be d e p o s i t e d a t h i g h rates. The growth o f hydrogenated amorphous S i by l a s e r - i n d u c e d photodecomposition o f SiH4 has been r e p o r t e d by s e v e r a l groups. SiH,
Since
e x h i b i t s a s t r o n g i n f r a r e d a b s o r p t i o n band near 10.6 urn, CO,
l a s e r s can be used q u i t e e f f e c t i v e l y t o decompose SiH, deposition. purpose.
Both cw and p u l s e d CO,
and induce
l a s e r s have been used f o r t h i s
The d e p o s i t i o n has been c a r r i e d o u t w i t h l a s e r beams
e i t h e r p e r p e n d i c u l a r o r p a r a l l e l t o t h e s u b s t r a t e surface. r a t e up t o 400 A/min was r e p o r t e d by B i l e n c h i e t a l . a 100-W cw CO,
laser.
A growth
(1982) u s i n g
I n t h e case o f p u l s e d l a s e r - i n d u c e d f i l m
growth, Hanabusa and Namiki (1979) demonstrated t h a t a f i l m t h i c k ness o f 1.5 urn can be grown w i t h 100 l a s e r p u l s e s a t an energy d e n s i t y o f 0.3 J/cm2 p e r pulse.
A d e p o s i t i o n r a t e o f 4-5 pm/rnin
can be r e a l i z e d by u s i n g a t y p i c a l p u l s e d CO, l a s e r system w i t h a p u l s e r e p e t i t i o n r a t e o f 5 pulses/sec.
T h i s growth r a t e i s o r d e r s
o f magnitude f a s t e r t h a n plasma d e p o s i t i o n r a t e s .
I n b o t h cases,
s t u d i e s o f t h e o p t i c a l p r o p e r t i e s i n d i c a t e d t h a t hydrogenated a - S i was deposited.
VI.
Submicron Optical Lithography
Laser r a d i a t i o n has l o n g been t h o u g h t t o be i m p r a c t i c a l f o r high-resolution
1 i t h o g r a p h y because t h e coherent n a t u r e o f t h e
l i g h t g i v e s r i s e t o c o n s t r u c t i v e and d e s t r u c t i v e i n t e r f e r e n c e a t
671
10. APPLICATIONS OF PULSED LASER PROCESSING
Fig. 18.
Electron microscope image o f a photolithgraphic pattern developed
by XeCl laser irradiation.
The 0.5 prn lines and spaces were formed by two
pulses o f 50 mJ/crn2 for each exposure.
( J a i n e t al.,
1982)
t h e sample s u r f a c e t h a t produces a random p a t t e r n o f f l u c t u a t i n g i n t e n s i t y c a l l e d "speckle."
Very r e c e n t l y , J a i n e t a l .
IBM demonstrated t h a t h i g h - r e s o l u t i o n ,
(1982) a t
f i n e - l i n e (0.5 pm) photo-
l i t h o g r a p h i c p a t t e r n s , as shown i n F i g u r e 18, can be d e f i n e d w i t h mask exposure by UV excimer l a s e r r a d i a t i o n o f 248- and 308-nm wavelengths. free.
The images were o f h i g h q u a l i t y and t o t a l l y speckle
These f i n d i n g s a r e g e n e r a l l y regarded as a major advancement
i n deep UV l i t h o g r a p h y .
Because o f a number o f i n h e r e n t charac-
t e r i s t i c s , p a r t i c u l a r l y t h e low l e v e l o f l i g h t coherency, excimer l a s e r s appear t o have g r e a t p o t e n t i a l i n o p t i c a l l i t h o g r a p h y .
For
example, ArF (194 nm), KrF (294 nm), XeCl (308 nm), and XeF (350 nm) nm) a r e commonly used gases;
furthermore,
t h e rare-gas
halide
excimer l a s e r s can be operated w i t h a number o f d i f f e r e n t gas mixt u r e s t o o b t a i n d i f f e r e n t o u t p u t wavelengths.
I n order t o f u r t h e r
improve t h e r e s o l u t i o n o f t h e o p t i c a l l i t h o g r a p h y , t h e a v a i l a b l e wavelength range can be extended i n t o t h e vacuum u l t r a v i o l e t ( V U V )
672
R. T. YOUNG ET M .
by employing t h e RGH l a s e r as a pump l a s e r f o r n o n l i n e a r frequency conversion schemes o r by frequency d o u b l i n g o r t r i p l i n g i n s u i t a b l e gases and vapors.
The a v a i 1a b i 1 it y o f h i gh-powered 1ig h t sources
i n t h e deep and VUV wavelength r e g i o n s w i l l p e r m i t , f o r t h e f i r s t time, t h e o p t i m i z a t i o n o f t h e exposure wavelength f o r a given photor e s i s t m a t e r i a l , r a t h e r t h a n t h e r e v e r s e s i t u a t i o n e x i s t i n g now i n which a s u i t a b l e p h o t o r e s i s t must be found f o r v a r i o u s wavelengths. Other advantages o f u s i n g excimer l a s e r s t o r e p l a c e c o n v e n t i o n a l deep UV l i g h t sources such as Xe-Hg a r c o r deuterium lamps a r e (1) t h e exposure t i m e can be d r a m a t i c a l l y decreased ( f o r example,
w i t h t h e 100-W excimer l a s e r i t i s p o s s i b l e t o expose a 5" wafer i n a f r a c t i o n o f a second as compared t o several minutes r e q u i r e d w i t h c o n v e n t i o n a l lamps) and ( 2 ) because o f t h e n e a r l y instantaneous exposure time, t h e s e n s i t i v i t y requirements on p h o t o r e s i s t s a r e no l o n g e r c r i t i c a l , a l l o w i n g a more f l e x i b l e c h o i c e o f p h o t o r e s i s t s . I n p a r a l l e l w i t h J a i n ' s work on UV excimer l a s e r p h o t o l i t h o g r a p h y , S r i n i v a s a n and Wayne-Banton
(1982) a t IBM have r e c e n t l y developed
a new process f o r t h e c o n t r o l l e d e t c h i n g o f o r g a n i c polymer f i l m s
u s i n g an ArF (193 nm) excimer l a s e r .
They demonstrated t h a t t h e
193-nm r a d i a t i o n can e t c h away o r g a n i c m a t e r i a l i n a p a t t e r n d e f i n e d e n t i r e l y by t h e l a s e r beam.
The mechanism o f t h i s process, which
i s r e f e r r e d t o as " a b l a t i v e photodecomposition,"
i s believed t o
be a b s o r p t i o n of UV l i g h t a t wavelengths corresponding t o a l l o w e d electronic transitions,
f o l l o w e d by breakup o f t h e polymer chains
i n t o s m a l l e r fragments and e j e c t i o n o f t h e fragments completely out o f the films, source.
l e a v i n g a v e r t i c a l w a l l d e f i n e d by t h e l i g h t
F i g u r e 19 shows a p a t t e r n w i t h l i n e w i d t h s o f 5 urn i n a
p l a s t i c m a t e r i a l which was e t c h e d by t h i s method.
T h i s process
appears t o be a t t r a c t i v e f o r l i t h o g r a p h y s i n c e i t p r o v i d e s b o t h exposure and e t c h i n g i n a s i n g l e step, and t h e c o n v e n t i o n a l wet development process can be e l i m i n a t e d t o t a l l y .
673
10. APPLICATIONS OF PULSED LASER PROCESSING
Fig. 19.
Pattern etched in a plastic f i l m by ablative photodecomposition
using an A r F ( 1 9 3 n m ) laser. Wayne-Banton,
VII.
The line widths are 5
urn.
(Srinivasan and
1982)
SUMMARY AND CONCLUDING REMARKS I n t h i s c h a p t e r we have reviewed v a r i o u s a p p l i c a t i o n s o f p u l s e d
l a s e r p r o c e s s i n g t o semiconductor d e v i c e f a b r i c a t i o n .
We have seen
t h a t t h e development o f excimer l a s e r s appears t o have been o f part i c u l a r importance i n connection w i t h d e v i c e - r e l a t e d work, not o n l y because t h e y o p e r a t e i n t h e UV,
b u t a l s o because such l a s e r s o f
s u f f i c i e n t power t o c a r r y o u t l a r g e - a r e a a n n e a l i n g a r e now becoming available.
Excimer l a s e r a n n e a l i n g and low-cost gaseous discharge
i m p l a n t a t i o n has been demonstrated t o be an e x c e l l e n t combination f o r use i n t h e f a b r i c a t i o n o f h i g h - e f f i c i e n c y s o l a r c e l l s and i s l i k e l y t o prove u s e f u l i n o t h e r d e v i c e a p p l i c a t i o n s as w e l l .
The
f a b r i c a t i o n o f complex s t r u c t u r e s such as those i n v o l v e d i n i n t e g r a t e d c i r c u i t technology o f f e r s a more severe c h a l l enge t o l a s e r
674
R. T. YOUNG E T A L .
p r o c e s s i n g techniques, b u t i n t h i s c o n n e c t i o n we would l i k e t o p o i n t o u t t h a t much o f t h e e x p l o r a t o r y e a r l y work i n t h i s area was done w i t h s o l i d s t a t e l a s e r s , which a r e n o t v e r y w e l l s u i t e d f o r t h i s t y p e of a p p l i c a t i o n .
Many o f t h e e a r l y experiments should be r e -
peated w i t h excimer l a s e r s .
I n areas r e l a t e d t o l a s e r - i n d u c e d o r
l a s e r - a s s i s t e d photodecomposition, i t i s apparent t h a t t h e development o f u l t r a v i o l e t (excimer) l a s e r s i s t h e key t o opening up many e x c i t i n g avenues o f both fundamental and appl i e d research. Research on p u l s e d l a s e r p r o c e s s i n g o f semiconductors, fundamental s t u d i e s t o v a r i o u s d e v i c e a p p l i c a t i o n s ,
from
has been c a r -
r i e d o u t q u i t e e x t e n s i v e l y f o r about seven y e a r s a t t h i s w r i t i n g . Numerous s t u d i e s have i n d i c a t e d t h a t l a s e r p r o c e s s i n g p r o v i d e s many unique f e a t u r e s a s s o c i a t e d w i t h t h e c a p a b i l i t y o f r a p i d and w e l l l o c a l i z e d heating.
It would be s u r p r i s i n g i f l a s e r p r o c e s s i n g
i s n o t developed f o r p r o d u c t i o n t e c h n i q u e s i n t h e semiconductor i n d u s t r y i n a number o f ways.
However, i t has t o be r e a l i z e d t h a t
an i n t e g r a t e d c i r c u i t p r o d u c t i o n l i n e ,
f o r example,
consists o f
many d e d i c a t e d and complex p r o c e s s i n g s t e p s each o f which has been i n t e n s i v e l y developed over t h e p a s t decade.
I t would be very d i f -
f i c u l t i n any case f o r a new t e c h n i q u e t o overcome t h e b u i l t - i n economic and design i n e r t i a o f p r e s e n t l y o p e r a t i o n a l methods.
For
l a s e r p r o c e s s i n g t h e s i t u a t i o n i s p a r t i c u l a r l y d i f f i c u l t because of problems encountered w i t h s o l i d - s t a t e l a s e r s , such as beam non-
u n i f o r m i t y , low process throughput, low c o n v e r s i o n e f f i c i e n c y , and h i g h cost. Recent advances i n excimer l a s e r technology and t h e many a t t r a c t i v e c h a r a c t e r i s t i c s o f t h e s e l a s e r s a r e even now p r o v i d i n g new s t i m u l u s f o r i n t r o d u c i n g l a s e r p r o c e s s i n g technology i n t o t h e semiconductor i n d u s t r y . 1ow-temperature
A p p l i c a t i o n s such as a n n e a l i n g
f i l m deposition , etching,
, gettering,
doping , and submi c r o n
o p t i c a l l i t h o g r a p h y appear t o be on t h e verge o f becoming w i d e l y utilized.
From an even more o p t i m i s t i c v i e w p o i n t ,
i t i s worth
n o t i n g t h a t l a s e r manufacturers c o n s i d e r t h e p r e s e n t g e n e r a t i o n o f excimer l a s e r s as o n l y f o r e r u n n e r s o f t r u e i n d u s t r i a l l a s e r s .
10. APPLICATIONS OF PULSED LASER PROCESSING
675
Dramatic advances i n l a s e r technology can be expected over t h e n e x t decade o r two.
We a l s o t h i n k i t i m p o r t a n t t o r e a l i z e t h a t t h e
m a t e r i a l i n Chapter 9 o f t h i s book i n d i c a t e d t h a t CO,
l a s e r s may
t u r n o u t t o be f a r more u s e f u l f o r l a s e r p r o c e s s i n g t h a n was f o r merly thought possible.
CO,
When we c o n s i d e r t h a t t h e r e a r e s t i l l no
o r excimer l a s e r s a v a i l a b l e which have been designed and con-
s t r u c t e d s p e c i f i c a l l y f o r l a s e r processing, we b e l i e v e t h a t optimism f o r t h i s new f i e l d o f science and t e c h n o l o g y i s f u l l y warranted.
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Sandow, P. M. (1980). S o l i d S t a t e Technol. 23, No. 7, 74. Searles, S. K., and Hart, G. H. (1975). Appl. Phys. L e t t . 27, 243. S r i n i v a s a n , R., and Mayne-Banton, V. (1982). Appl. Phys. L e t t . 41, 576. Stevens, B., and Hutton, E. (1960). Nature 186, 1045. Stuck, R., Fogarassy, E., M u l l e r , J. C., Hodeau, M., Wattiaux, A., and S i f f e r t , P. (1981). Appl. Phys. L e t t . 38, 715. Turner, G. B., T a r r e n t , D., P o l l a c k , G., Pressley, P., and Press, R. (1981). Appl. Phys. L e t t . 39, 967. Turner, G. B., T a r r e n t , D., A l d r i c h , D., Pressley, R., and Press, R. (1982). Mat. Res. SOC. Symp. Proc. 5, 241. van Gurp, G. J., Eggermont, G. E., Tamminga, Y., Stacy, W. T., and G i j s b e r s , J. R. M. (1979). Appl. Phys. L e t t . 35, 273. von Allmen, M., and Wittmer, M. (1979). Appl. Phys. L e t t . 34, 68. Wichner, R. (1975). Unpublished. Reported a t N a t i o n a l S o l a r Photov o l t a i c Review Meeting, Los Angeles, C a l i f o r n i a , June 22-25. Wittmer, M., and von Allmen, M. (1979). J. Appl. Phys. 50, 4786. Wood, R. F., Young, R. T., Westbrook, R. D., Narayan, J., C h r i s t i e , W. H., and Cleland, J. W. (1980). S o l a r C e l l s 1, 145. Wood, R. F., and G i l e s , G. E. (1981). Phys. Rev. B 23, 2923. Wood, T. H., White, J. C., and Thacker, B. A. (1983). Mat. Res. SOC. Symp. Proc. 17, 35. Young, R. T., White, C. W., Clark, G. J., Narayan, J., Westbrook, R. D., and C h r i s t i e , W. H. (1978). I n " F i r s t European Community P h o t o v o l t a i c S o l a r Energy Conference, Proceedings o f t h e I n t e r n a t i o n a l Conference," Luxembourg, Sept. 27-30, 1977, p. 861. (Reidel, Boston). Young, R. T., Wood, R. F., Narayan, J., White, C. W., and C h r i s t i e , W. H. (1980). IEEE Trans. E l e c t r o n Devices ED-27, 807. Young, R. T., van d e r Leeden, G. A., Narayan, J., C h r i s t i e , W. H., Wood, R. F., Rothe, D. E., and L e v a t t e r , J. I. (1982a). IEEE E l e c t . Dev. L e t t . EDL-3, 280. Young, R. T., Wood, R. F., and C h r i s t i e , W. H. (1982b). J. Appl. Phys. 53, 1178. Young, R. T., van der Leeden, G. A., Wood, R. F., and Narayan, J. ( 1 9 8 2 ~ ) . I n "Proc. S i x t e e n t h I E E E P h o t o v o l t a i c S p e c i a l i s t s Conference," p. 427. Young, R. T., Narayan, J., and Chang, Y. K. (1982d). Mat. Res. SOC. Symp. Proc. 5, 111. Young, R. T., Narayan, J., C h r i s t i e , W. H., van der Leeden, G. A., L e v a t t e r , J. E., and Cheng, L. J. (1983a). S o l i d S t a t e Technol. 26, No. 11, 183. Young, R. T., Narayan, J., C h r i s t i e , W. H., van d e r Leeden, G. A., Rothe, D. E., and Sandstrom, R. L. (1983b). Mat. Res. SOC. Symp. Proc. 13, 410.
A
Annealing cw laser, 15-17 pulsed laser, 4-15 rapid thermal, 475, 668 thermal (or furnace), 5, 21, 46, 473, 664 Anti-reflection coating, 646, 648 Anti-Stokes process, 363 As in Si diffusion coefficient (liquid), 233 pulsed laser anneal, 57-59, 584, 587-590 segregation (distribution) coefficient, 63, 233 solubility limits and substitutional concentrations, 70, 285 surface properties, 45 1-453 Atomically clean surfaces, 409-422 Ge, 418 Si, 410-418 111-V compounds, 418-422 Au on GaAs, 20, 518 Auger processes, 174-176, 375, 380 screening of, 175, 177, 334 Auger electron spectroscopy (AES), 408 detection limits, 415 and surface cleaning, 408
Ablative photodecomposition, 26, 672 Absorption direct optical, 102 far-infrared, 601 intensity dependen,, 317, 334 Absorption coefficient, 99-103, 106-107, 109-112, 116, 122, 123, 170, 213, see also Optical properties free-carrier, 329, 558-562, 599 GaAs, 481 Si, 102, 110, 194 temperature dependence of 101- 107, 109-112, 358, 373 Absorption mechanisms, 169- 174 Accommodation coefficient, 258 Activation energy, 141 defects in Si, 152-156 in rate constants, 258 velocity-dependent, 274, 278-281 Alloys dilute binary, 14 subsftutional, 48 supersaturated, 8, 43, 626 surface properties, 450 two-component, 272 Amorphous Ge, thermal properties, 189- 193 Amorphous model, 196 Amorphous phase formation, 301 -306, 3x9-394 ion implantation, 46 kinetic rate theory, 304-306 orientation dependence, 301 thermodynamic interpretation, 302-304 velocity dependence, 304-306, 389-394 Amorphous Si ion implantation, 46 optical properties, see Optical properties thermal properties, 189-193, 207, 336-338 Amorphous Si, melting energy threshold, 566 surface duration, 566-568 temperature, 189, 336
B Band edge direct, 103-105 indirect, 103- 105, 109- 111 Band gap energy, 104, 111, 170 Band structure of Si, 103-105 Beam homogenization, 29, 510-512, 627, 631, 646 Beam processing ion implantation, laser annealing, 5-11, 29 e-beam annealing, 30 glow (or gas) discharge implantation, laser annealing 643-646 B in Si, 234 carrier mobility, 576 diffusion coefficient (liquid), 233 pulsed laser anneal, 55, 574-575, 579-580, 584-587
679
INDEX B in Si (continued) segregation coefficient, 63, 233 solubility limit, 70, 285 Bi in Si, 239-240 diffusion coefficient (liquid), 233 pulsed laser anneal, 60 segregation coefficient, 63, 233 solubility limits, 70, 285 Binary alloys (see alloys) Blackbody radiation, 360-362 Broken symmetry, 169 Bulk nucleation, 207-21 1, 304, 394--396
C Carbon in Si, 41 1 Capacitance, 131, 137-138 Capacitance-voltage(C-V), see Electrical measurements Carrier concentration, 8, 130, 132-135, 650 confinement, 178-179, 219-221 diffusion, 178-179. 219-221 lattice interaction, 172-178 mobility, 130, 132, 134-136, 516-518, 654 temperature, 375-377 Cell formation, 314, 354-369 Cell size, 82-83, 212-214 Cellular structure, 14, 224, 359-363 Charged particle emission, 318, 376 Chemical vapor deposition (CVD), 21 Complex dielectric function and refractive index, 98 Computer modeling, see Heat flow calculations Constitutional supercooling, 14, 83, 296 Cooling rates, 218 effect on surface, 444 cw laser annealing, 15-17 Crystal growth theory, 318-334 kinetic rate approach, 256, 257-261 nonequilibrium, 252-255 thermodynamic approach, 256, 260-261 Crystalline model, 194-196 Crystallization, see Crystal Growth Theory, Epitaxial Regrowth, Explosive Crystallization D
Damage decomposition (GaAs, compounds), 443, 602-607
ion implantaton, 4-6 laser-induced, 330, 580, 602-603 Deep level transient spectroscopy (DLTS), 147, 514, 521, 636, 640 Defects furnace annealing, 5-7, 641 pulsed laser annealing, 6, 443-446, 501-503, 636-637 at surfaces, 443 Defects in Si, 140-159 activation energy, 149-154 anneal-out temperature, 149-154 cross section, 149-154 defect profiles, 155-156 DLTS, 146-156, 636 identification, 149- 154 luminescence, 142-147 passivation, 154-155 at surface, 444 Deposited layers, Si, 21 CVD, 21 containing As, 237 e-beam, 21-22 epitaxial growth, 21 -23, 464 laser photochemical, 668-670 molecular beam epitaxy, 463-465 polycrystalline, 23, 668 recrystallization of, 237, 668 Device applications IMPAlT diodes, 663-665 integrated circuits, 667-668 silicon-on-sapphire, 665 -667 solar cells, 641-651 surface cleaning, 464 Dielectric function, 100, 115, 120, see also Optical properties Dielectric films, laser deposition of, 670 Dielectric-isolation circuits, 23 Differential absorption, Si, 579-580 Differential scanning calorimetry, 190 Diffuser plate, 632 Diffusion coefficients in liquid GaAs, 489-491 in liquid Si, 233 in solid Si, 13 Direct band gap, 104-106 Dislocations, 6, 660-661 Dissociation laser-induced photochemical, 25, 668-670 Distribution coefficient, 50-68, see Segregation coefficient Dopant diffusion laser-induced, 18-19
681
INDEX Uopant transport and redistribution, 53, 582-590 and segregation, 53-57 As in Si, 235-239 B in Si, 234 Bi, In in Si, 239-240 calculations, 51 -53, 232-241 Cu,Fe in Si, 458 Ga, Sb in Si, 241 in GaAs, 489-492 theory, 224-232 Drude model, 378, 386
E Effective mass, 127 Efficiencies of lasers, 28 of solar cells, 29, 648-651 Electrical conductance time-resolved, 338-346 Electrical measurements, 129-140 capacitance-voltage (C-V), 131, 134, 138 carrier concentration, 130, 132-135 current-voltage (I-V), 131 - 132, 138- I40 deep level transient spectroscopy, 147- 148, see DLTS Hall effect, 129-130, 134 resistivity, 130, 134, 159 Electrical properties, Si pulsed laser annealed, 576, see also Electrical measurements and GaAs, laser annealing of Electron beam annealing, 30-31 deposition of Si, 237-239 Electron beam induced current (EBIC), 658-661 Electron-electron collisions, 177 Electron-hole plasma, 167-178 formation, 103 lifetime, 173, 178, 375 observation of in Si, 383-389 effect on Raman scattering, 372 Electron paramagnetic resonance, 156-158 Electron-phonon scattering, 376 Electronic band structure see Band structure defects, see Defects and defects in Si surface structure, 433-440 Electroreflectance, 128 Ellipsometry, 101, 119-127 Energy bands in Si, 104 Energy deposition processes, see Absorption mechanisms
Energy relaxation processes, 374-389 Energy transfer carrier-lattice, 374-377 Entropy, 259-261, 442, 448 Epitaxial regrowth, 21 -23, 466 GaAs, 503-505 liquid phase, 13, 43, 45 Excimer laser annealing, 201 -206, 63 1-634 dopant profiles, 634-636 electrically active defects, 636 melt-front profiles, 204, 206 pulse duration effects, 637-640 surface morphology, 631-634 Excimer lasers, 29 characteristics, 629-63 1 and high-efficiency solar cells, 641-651 Excite-and-probe, 357-360, 377-389, 393 Explosive crystallization, 396 B Far-field conditions, 27 Fe in Si pulsed laser anneal, 74-75 segregation cells, 41 surface characterization, 457 Femtosecond measurements reflectivity, 377-380 second harmonic generation, 380-383 Float-zone refining, 14, see Zone refining Free carrier absorption, 168, 329, 334 conductance, 339 Free energy, 259-261, 448 Furnace annealing, 21, 664 Finite differences, 167
G Cia in Si, 50, 63, 241, 299-300
GaAs, laser annealing of annealing under high-pressure, 538-544 cracking of surface, 602-603 DLTS measurements, 52 1-522 dopant, redistribution, 488-492 electrically active defects, 5 15-529 encapsulant layers, 528-529 high-dose implants, 494-499 laser-induced damage, 419, 501-503, 5 10-5 12 loss of stoichiometry, 419, 505-510 melting model calculations, 477492 native oxide layer, 534 nonlinear far-infrared absorption, 601
INDEX GaAs (continued) oxygen incorporation, 530-538, 605-607 photoluminescence, 512-515 p-n junctions, 522-524 pulsed laser melting, 477-487 role of ambient atmosphere, 69-72, 539-542 substrate heating, 525-528 surface decomposition, 501-503, 603-605 surface structure, 428 time-resolvpd reflectivitv. 477-487 Gas (or glow) discharge implantation, 644 Ge impact ionization processes, 562, 614 laser annealing, 45 multiphoton absorption, 6 14 nonlinear far-infrared absorption, 613-614 optical properties, 613-618 surface cleaning, 418 surface structure, 427 thermal properties, 183 time-resolved photoconductivity, 6 14-617 Geometric surface structure, 422 Gettering implantation, 657 laser damage, 656-657 mechanical damage, 657 Glow (or gas) discharge implantation, 643-646 Grain boundary studies, 657-662 types, 658 Graphoepitaxy, 24 Green’s function, 228 Growth rates, see Melt-front velocity
H Hall effect, 129-130, 134, 496, 649 Heat absorption coefficient, 186 Heat conduction calculations, 180- 184 boundary conditions, 182-183 differential equation, 11, 181 diffusion equation, 167 finite difference form, 181 phase changes, 11, 184-185 Heat flow calculations for C 0 2 laser numerical, 592-600 role of absorption coefficients, 597-601 Heat generation function, 181 Heat generation rate, 185 Heat transfer convective, 188 radiative, 188
Heats of fusion and vaporization Si, 189 Ge, 189 GaAs, 189 Heavy-doping effects, 9 , 169 High reflectivity phase (HRP), 167
I Impact ionization, 174 Impact ionization processes, 562, 609-61 1 Impatt diodes, 663-665 Impurity diffusion, see Dopant transport and redistribution lattice location, 47-50 gettering, 656-657 segregation, see Segregation trapping, see Solute trapping In in Si cellular structure, 82-83 dopant profile, 66, 83-84, 237 pulsed laser anneal, 61 segregation coefficient, 63, 67, 280-282 concentration, 84 InP loss of P, 422, 443 surface cleaning, 418 InSb optical properties, 608-612 pulsed laser annealing, 612 Incoherent light sources, 30 Indirect band edge, 103-105, 109-1 1 I Infrared, absorption, 558-562 measurements. 127, 132 Integrated circuits, 667-668 Interface boundary condition, 293-295 multilayer, 269-270 planar, 292-293 width, 258, 279-280 Interfacial instability, 82-86, 292-301 kinetics, 263, 266 segregation coefficient, 222 stability diagrams, 273 structure, 296-299 temperature, 260 velocity, 257 undercooling, 257, 258-269 Interfacial processes diffusive, 254, 260, 281 relaxational, 254, 260, 281 Interstitial implants, 75-78, 457-463 surface characterization, 457
683
INDEX Ion channeling, 47 Ion implantation, GaAs, 484-499 antisite defects, 501 co-implantation of dopants, 33 dopant redistribution, 488-492 electrical properties, 492-501, 542 pulsed laser melting, 476-492, 504 oxygen uptake, 530-538 Ion implantation, Si, see Individual dopants in Si, e.g., B in Si surface studies, 449-463 Ion scattering and surface structure, 433 king model, 292
J Junction formation ion-implanted, laser annealed, 5-1 I , 643-65 1 laser-induced diffusion, 654-656 Junction leakage current, 9, 651-654
K Kinetic rate constants, 257 Kinetic rate theory, 257-262
Laser-induced diffusion, 18-19, 654 epitaxial growth, 21-23 grain growth, 18 photodecomposition, 670 recrystallization, 23-24 Latent heat, 258-260 Lattice strain, 79-81, 347-355, 371-372 Lattice temperature measurements excite-and-probe, 357-360 oscillations in reflectivity, 355-357 time-resolved thermal emission, 360-362 x-ray. 346-355 pulsed Raman scattering, 362-374 Liquid Ge thermal conductivity, 181 Liquid phase epitaxial regrowth, 22, 422 Liquid Si electrical conductivity, 191 optical properties, 112- 1 13 thermal properties, 181, 190-191, 202-203 Liquid-solid interface, see interface Lithography (excimer laser), 25 Local thermodynamic equilibrium, 51 Low Energy Electron Diffraction (LEED), 408, 423-433, 441, 453 time-resolved, 427
L Laser annealing CW, 15-27 pulsed, 4-15 in ultrahigh vacuum, 407 cleaning of surfaces, 406, 416 cold processing, 642 damage gettering, 656-657 glazing, 4 lithography, 25-26, 670-673 machining, 2-4 photochemical processing, 24-25, 668-670 processing, 2-4, 674 Lasers alexandrite, 27, 646 ArF, 25 Ar ion, 18 co,, 28 excimer, 29, 629-63 1 KrF, 171 Nd, 27, 646 pulsed gas, 28-29 pulsed solid state, 27-28, 627-628 ruby, 27, 646 XeCI, 29, 171, 645
M Melt depth, 340-341, 394 duration a-Si, 207-21 1, 394-395 c-Si, 332-335, 353-354 onset a-Si, 336-338 c-Si, 334-335, 353-354 threshold, 333-334, 384-385, 565-568 undercooling, 390-396 Melt-front profiles, 199-216, 222 Melt-front velocity, 13, 339-346, 476 amorphous regrowth, 390-394 influence of dopants, 344-345 Melting of embedded layers, 579-581 Melting of lattice time scale, 377-383 Melting model, 166 GaAs, 478-484 Melting model calculations amorphous model, 196-1 97, 207-212 crystalline model, 194-196 excimer lasers, 201-205 pulse shape and duration, 215-217 ruby and Nd lasers, 197-198
INDEX Metastable alloys, see Alloys, supersaturated Microstructure modifications, 660-662 Minority carrier diffusion length (MCDL), 645, 652 Minority carrier lifetime (MCL), 9, 654 Mobility, 636, 665-666, see also Carrier mobility Molecular beam epitaxy (MBE), 463-466 Monte Carlo simulation, 290 MOS transistors, 627, 667 Moving boundary problem, 183, 221 Multiphoton absorption, 561-562 Mush zones, see Slush zones
N Nanosecond measurements, see Time-resolved measurements Native oxide layer, 22, 415-418 N in Si, 572-573, 578, 590 Nonequilibrium crystal growth, 52, 252 segregation, 254 solidification, 252-253 thermodynamic processes, 252-255 Nonlinear absorption far-infrared in GaAs, 601 processes, see Absorption mechanisms Nucleation bulk, see Bulk nucleation polycrystalline Si, 394-396
0 Ohmic contacts GaAs, 20-21 Optical properties, 97- 129 amorphous Si, 113-119, 159 crystalline Si, 99-112, 101-113, 563-571 GaAs, 480-481, 601-602 Ge, 613-618 heavily doped Si, 119-127 InSb, 608-612 intensity dependent, 317, 335, 601 ion-implanted Si, 113-1 19 laser-annealed Si, 119-127 and lattice temperature measurement, 355-362 liquid Si, 112-113 temperature dependent, 101-1 12, 368-371 time resolved, 319 experimental considerations, 3 19-323 reflectivity, 329-338 transmission, 323-329
Optical prdperties, far-infrared GaAs, 601-602 Ge, 613-618 InSb, 608-612 Si, 563-571 Orientation dependence segregation coefficient, 66, 29 1 surface reordering, 423-426 Oxygen incorporation during laser annealing GaAs, 530-538, 605-607 Si, 414-417
P Phase diagram, 52, 293 Phase transitions liquid-amorphous, 389-394 Phonon emission, 174- 177 Photochemical processing, 24-25, 668-670 Photoelectron spectroscopy, 409 Photoexcitation processes, 177 Photogenerated carriers, 173 Photoluminescence, 323-327, 512-515 P in Si pulsed laser annealed, 584-585 Picosecond measurements, see Time-resolved measurements Plasma annealing model, 317, 591 electron-hole, 376, 384-390 Plasmon production, 173 p-n junction, 17, see also Junction formation grain growth, 668 nucleation and growth, 394-396 Precrystallization stage, 253 Pump-and-probe, see Excite-and-probe Proton beam annealing, 3 1
Q Quasi-stationary finite difference approximation, 229
R Raman scattering, 128 pulsed measurements, 362-374 Rate constants kinetic, 257-258 velocity-dependent, 274-28 1 Recombination electron-hole, 376 grain boundary, 658 surface, 65 1-654
INDEX Recombination rate, 170 Reflectivity, 103, 107, 108, 111, see also Optical properties temperature-induced oscillations, 355 Retrograde solubility, 71 Rutherford backscattering (RBS), 47
S Sb in Si pulsed laser annealed, 577, 582-584 segregation, 583 segregation coefficient, 63 solubility limit, 70, 283 Scanning laser spot (SLS), 657-658 Segregation kinetic rate theory, 272-292 nonequilibrium, 15, 252, 272-287 models for nonequilibrium segregation, 287-290 two component system, 272-274 Segregation coefficient, 222-223 see also Distribution coefficient calculations of, 281-283 effective, 222 equilibrium, 14, 223 interfacial, 14, 222 nonequilibrium, 28 1-283 orientational dependence, 66, 291 saturation, 290-291 velocity dependence, 273-279, 283, 344-345 Self-annealing, 3 1 Sheet resistivity, 640-641 Si. see Listings under various dopants in Si free-carrier absorption, 558-562, 599 reflectivity, 563-568 transmissivity, 568-57 1 Silicide formation, 19-20 Silicon-on-sapphire (SOS), 626, 665-667 Slush zones, 216, 482 Solar cell fabrication, 641 beam processed, 643-651 substrate heating, 646-647 violet cell, 641 Solid phase epitaxy, 15-16, 21 Solubility calculations, 283-286 equilibrium, 406 nonequilibrium, 68-73, 282 retrograde, 71 Solute trapping, 43, 44, 68, 286-287 Speckle, 25
Splat cooling, 252 Stepped surfaces, see Surface structure, vicinal Strokes process, 363 Submicron optical lithography, 670-673 Substitutional implants in Si surface characterization, 4 5 0 4 5 6 Substrate heating, 16, 212-213, 646-647 Surface cleaning Ge, 418 Group 111-Vcompounds, 418-422 Si, 410 Surface decomposition, GaAs, 444, 602-607 Surface melt duration, 329 Surface morphology and laser annealing, 630-633 Surface passivation heavy-doping effects, 654 in situ, 651 SiOz, 651 Surface recombination, 65 1-654 Surface ripples, 571, 618 Surface structure electronic, 433 GaAs, 428-430 Ge(100), 427 Ge(1 I I), 433-439 ion-implanted Si, 452-457 metastable surfaces, 430-440 Si(lOa), 423-426 Si( 1lo), 425-426 Si(l1 I), 425-426, 431-439 vicinal (stepped), 440-443
T Temperature dependence of optical properties of Si, 99-1 12 absorption coefficient, infrared, 109-1 12 visible, 99, 102, 106-107, 108, 116 band gap, Si, 107 dielectric function, 100 reflectivity, 103, 107, 108, 112 Temperature gradients (and carrier confinement), 178 Thermal gradients, 217-218, 351 Thermal conductivity a-SI, 188 GaAs, Ge, Si, 182 Thermal properties GaAs, 189 Ge, Si, 188-193 temperature-dependent, 167, 185-188 Thermal radiation, 360-362
INDEX Thermodynamic data, 188-193 Time of onset of melting, 329 Time-resolved measurements electrical conductance, 338-346, 391-392 electron diffraction, 315, 427 of lattice temperature, 317-318, 355-374 Kaman scattering, 362-374 reflectivity, 316-317, 329-338, 383-389 second harmonic generation, 380-383 thermal emission, 360-362 transmissivity, 323-329, 357-360, 383-389 x-ray diffraction, 346-355 Time-resolved reflectivity, Si, 564-568 Time-resolved transmissivity , Si, 570-57 1 Time scale, for carrier-lattice energy transfer, 374-389 intra-carrier equilibration, 375
lattice melting, 377-383 Transmission electron microscopy (TEM), 574-575, 579-560
U Undercooling, 261-269, 389-396
X X-ray diffraction time resolved, 346-355 X-ray fluorescence, GaAs, 603-604
2 Zone refining, of interstitial impurities, 44, 75-78