Jan 1, 1983 - microscopy (TEM) and X-ray topography the defects, still present in the matrix. I - GaAs ..... LAISTER, G.M. JENKINS, P h i l . Mag. 23 ( 1 9 7 1 ) ...
ELIMINATION OF DISLOCATIONS IN GaAs SINGLE CRYSTALS M. Duseaux, C. Schiller, J. Cornier, J. Chevalier, J. Hallais
To cite this version: M. Duseaux, C. Schiller, J. Cornier, J. Chevalier, J. Hallais. ELIMINATION OF DISLOCATIONS IN GaAs SINGLE CRYSTALS. Journal de Physique Colloques, 1983, 44 (C4), pp.C4-397-C4-407. .
HAL Id: jpa-00223067 https://hal.archives-ouvertes.fr/jpa-00223067 Submitted on 1 Jan 1983
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JOURNAL DE PHYSIQUE Colloque C4, s u p p l e m e n t a u n09, T o m e 44, s e p t e m b r e 1 9 8 3
p a g e C4-397
ELIMINATION OF DISLOCATIONS I N G a A s SINGLE CRYSTALS M. ~ u s e a u x * ,C.
chiller', J . P . c o r n i e r * , J . P . c h e v a l i e r * and
J. Hallais'
' ~ a b o r a t o i r e sd1B7,ectronique e t de Physique k p p l i q u k e , 3, avenue Descartes, 54450 LimeiZ-Brivannes, France C. N . R. S., Centre d 'Etudes de Chimie M6taLLurgique, 15, rue G. llrbain, 94400 Vitry-sup-Seine, France
..
Rdsumd - P o u r p l u s i e u r s a p p l i c a t i o n s t e c h n o l o g i q u e s , d e s c r i s t a u x semic o n d u c t e u r s de g r a n d e t a i l l e s o n t demand&. D i v e r s t y p e s de d d f a u t s (ponct u e l s e t d t e n d u s ) s o n t p r k s e n t s d a n s c e s c r i s t a u x . Dans l e c a s du GaAs, on s a i t que l a p r d s e n c e d e d i s l o c a t i o n s diminue f o r t e m e n t l a d u r d e de v i e d e s l a s e r s Gal-xAlxAs/GaAs. De p l u s , d e r e c e n t e s k t u d e s m o n t r e n t que 12s prop r i d t d s k l e c t r o n i q u e s d e s c i r c u i t s i n t d g r k s a u GaAs s o n t i n f l u e n c e ' e s p a r l ' e x i s t e n c e de d i s l o c a t i o n s . L e u r prCsence d a n s l e s c r i s t a u x s y n t h C t i s k s p a r l a mgthode LEC ( L i q u i d E n c a p s u l a t e d C z o c h r a l s k i ) e s t e s s e n t i e l l e m e n t due aux c o n t r a i n t e s t h e r m i q u e s e x i s t a n t pendant l a c r o i s s a n c e . Oans c e p a p i e r , nous prksentons des r d s u l t a t s expdrimentaux montrant q u ' i l e s t p o s s i b l e de r C d u i r e f o r t e m e n t ( u n f a c t e u r 10 au m o i n s ) l a d e n s i t d d e d i s l o c a t i o n s , s o i t en d i m i n u a n t l e n i v e a u de c o n t r a i n t e s t h e r m i q u e s ( t e c h n i q u e Kyropoulos s o u s encapsulation l i q u i d e ) , s o i t en ajoutant des impuretds isodlectroniques (Indium) dans l e b a i n l i q u i d e . Abstract - Large s i z e semiconductor c r y s t a l s a r e being requested i n s e v e r a l d e v i c e - o r i e n t e d t e c h n o l o g i e s . These c r y s t a l s e x h i b i t v a r i o u s d e f e c t s ( p o i n t and e x t e n d e d d e f e c t s ) . I n d e e d , it is well-known i n t h e c a s e o f GaAs t h a t t h e p r e s e n c e o f d i s l o c a t i o n s s t r o n g l y d e c r e a s e s t h e l i f e - t i m e o f Gal-xA1xAs/ GaAs l a s e r s . Moreover, i t a p p e a r s now, t h a t t h e d i s l o c a t i o n s have a d e t r i m e n t a l e f f e c t on GaAs i n t e g r a t e d c i r c u i t s . T h e i r p r e s e n c e i n LEC ( L i q u i d E n c a p s u l a t e d C z o c h r a l s k i ) c r y s t a l s a r e m a i n l y due t o t h e t h e r m a l s t r e s s e s o c c u r r i n g d u r i n g t h e growth. We p r e s e n t i n t h i s p a p e r some e x p e r i m e n t a l r e s u l t s showing t h a t it is p o s s i b l e t o s t r o n g l y r e d u c e (by a f a c t o r 10, a t l e a s t ) t h e d i s l o c a t i o n d e n s i t y e i t h e r by d e c r e a s i n g t h e t h e r m a l s t r e s s e s l e v e l ( L i q u i d E n c a p s u l a t e d K y r o p o u l o s T e c h n i q u e ) o r by a d d i n q an i s o e l e c t r o n i c impurity (indium t o t h e melt).
INTRODUCTION S i l i c o n is t h e most used s e m i c o n d u c t o r and i t is r o u t i n e l y produced f r e e o f d i s l o c a t i o n s . Just. b e h i n d s i l i c o n , g a l l i u m a r s e n i d e (GaAs) is t h e most p o p u l a r 111-V compound. However, d i s l o c a t i o n f r e e GaAs is n o t s t i l l a v a i l a b l e on t h e market s i n c e t y p i c a l c r y s t a l s e x h i b i t . l o 4 - l o 5 d i s l o c a t i o n s p e r cm2. The i n f l u e n c e o f d i s l o c a t i o n s on e l e c t r o n i c d e v i c e s c h a r a c t e r i s t i c s is more and more s t u d i e d . E f f e c t i v e l y , P.M. P e t r o f f e t a l ( l ) have shown by a TEM s t u d y t h a t t h e d e g r a d a t i o n r a t e of d o u b l e h e t e r o s t r u c t u r e GaAlAs-GaAs l a s e r s was d u e t o t h e p r e s e n c e o f d i s l o c a t i o n s . This by n o n - r a d i a t i v e r e c o m b i n a t i o n o f m i n o r i t y c a r r i e r s on t h e f a c t is e x p l a i n e d /2,3/ d i s l o c a t i o n s . The i n f l u e n c e o f d i s l o c a t i o n s on m a j o r i t y c a r r i e r s d e v i c e s ( F i e l d E f f e c t T r a n s i s t o r s f o r example is n o t s o c l e a r . The f i r s t work made i n t h i s f i e l d is r e p o r t e d by R. Zucca /4/ who d i d n o t e s t a b l i s h c l e a r c o r r e l a t i o n between t h e d i s l o c a t i o n d e n s i t y and t h e FET f a b r i c a t i o n y i e l d . More r e c e n t l y , Y. Namishi e t a 1 / 5 , 6 / measured d r a i n - s o u r c e c u r r e n t ( I D S ) a n d t h e t h r e s h o l d v o l t a g e ( v t h ) on 20 000 FET made on a GaAs w a f e r . They found t h a t t h e most d i s l o c a t e d a r e a s c o r r e s p o n d e d t o h i g h Ids v a l u e s b u t low t h r e s h o l d v o l t a g e . They c o n c l u d e d t h a t t h e Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983447
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JOURNAL DE PHYSIQUE
e l i m i n a t i o n o f d i s l o c a t i o n s was a v e r y i m p o r t a n t problem. T h i s c o n f l i c t i n g remarks p r o b a b l y r e s u l t from s i g n i f i c a n t p r o g r e s s i n t h e c o n t r o l o f t h e d e v i c e t e c h n o l o g y which a l l o w s now, l i k e f o r t h e l a s e r s , t o p o i n t o u t t h e i n f l u e n c e o f t h e s u b s t r a t e d e f e c t s . I n t h i s p a p e r , we p r e s e n t o u r own r e s u l t s c o n c e r n i n g t h e r e d u c t i o n o f d i s l o c a t i o n d e n s i t y i n GaAs c r y s t a l s and we s h a l l s t u d y by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM) and X-ray t o p o g r a p h y t h e d e f e c t s , s t i l l p r e s e n t i n t h e m a t r i x . I
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GaAs SINGLE CRYSTAL GROWTH AND DISLOCATIONS SOURCES
The LEC ( L i q u i d E n c a p s u l a t e d C z o c h r a l s k i ) method is t y p i c a l l y u s e d t o grow GaAs s i n g l e c r y s t a l s . The growth t a k e s p l a c e i n a h i g h p r e s s u r e chamber p r e s s u r i z e d w i t h argon, t h e melt being encapsulated with l i q u i d b o r i c oxide i n o r d e r t o minimize a r s e n i c l o s s e s . The main a d v a n t a g e s o f t h i s t e c h n i q u e is t h e c i r c u l a r s h a p e o f t h e w a f e r s a n d t h e homogeneity o f i m p u r i t i e s d i s t r i b u t i o n a c r o s s a s l i c e . Depending upon t h e f i e l d o f a p p l i c a t i o n s , 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 c r y s t a l c a n be e x t r e m e l y important. One m o s t l y u s e s e i t h e r n+ doped w a f e r s ( 1 0 ~ 7 - 1 0 1 8 ~ m - ~ )o r lo8 ~ , c m ) . However, t h e d i s l o c a t i o n d e n s i t y is semi-insulating ones ( r e s i s t i v i t y r e l a t i v e l y h i g h i n s u c h i n g o t s ( l o 4 - l o 5 d i s l / c m 2 ) , and t h e i r d i s t r i b u t i o n is n o t u n i f o r m a c r o s s a s l i c e : t h e d e n s i t y is h i g h i n t h e c e n t r e and on t h e e d g e s o f t h e i n g o t , and r e a c h e s a minimum i n an i n t e r m e d i a t e r e g i o n (W-shape) /11/. T h i s f a c t is well e x p l a i n e d by t h e r m o e l a s t i c c a l c u l a t i o n s /7-lo/. E f f e c t i v e l y , t h e p r e s e n c e o f d i s l o c a t i o n s is m o s t l y d u e t o t h e r m a l s t r e s s e s , o c c u r r i n g d u r i n g t h e growth ( s e e f i g . 1 ) . Moreover, it h a s been shown t h a t t h e stress l e v e l i n c r e a s e s w i t h t h e c r y s t a l d i a m e t e r .
>
FIGURE 1 : X-ray t o p o g r a p h and i s o s t r e s s e s (kg/mm2) map o f a GaAs c r y s t a l
I n o r d e r t o p a l l i a t e t h i s phenomena, t h r e e d i f f e r e n t t e c h n i q u e s have been e x p l o r e d : i ) t o s t a r t t h e growth from a d i s l o c a t i o n - f r e e s e e d : n e c k i n g p r o c e d u r e /11-13/ i i ) t o i m a g i n e a n o t h e r c r y s t a l growth t e c h n i q u e d e c r e a s i n g t h e t h e r m a l s t r e s s e s : L i q u i d E n c a p s u l a t e d K y r o p o u l o s (LEK) method / 7 , 8 , 1 3 , 1 4 / . i i i ) t o i n t r o d u c e i m p u r i t i e s i n t h e melt i n o r d e r t o i n c r e a s e t h e e l a s t i c l i m i t o f t h e c r y s t a l /7,11,13-16/. F i r s t and s e c o n d t r a c k s have l a r g e l y been d i s c u s s e d i n p r e v i o u s p a p e r s . We s h a l l q u i c k l y h a v e an o v e r l o o k on t h e main r e s u l t s o f t h e s e s t u d i e s . B e s i d e s o u r l a s t r e s u l t s c o n c e r n i n g t h e l a s t p a r t w i l l be d e v e l o p e d .
I1
- NECKING
PROCEWRE
F i r s t l y r e p o r t e d by Dash /7/, t h i s t e c h n i q u e c o n s i s t s i n p u l l i n g a s m a l l d i a m e t e r c r y s t a l ( ( 1 mm). Due t o t h e r e d u c t i o n o f t h e d i a m e t e r , t h e d i s l o c a t i o n s coming from t h e s e e d grow o u t o f t h e c r y s t a l . The t h e r m a l s t r e s s e s b e i n g t o o s m a l l , new d i s l o c a t i o n s c a n n o t be i n t r o d u c e d . A f t e r p u l l i n g 2-3 cm, i t is p o s s i b l e t o o b t a i n a d i s l o c a t i o n - f r e e c r y s t a l a t t h e s o l i d - l i q u i d i n t e r f a c e . Then, one c a n l e t t h e c r y s t a l d i a m e t e r i n c r e a s e , and i n some m a t e r i a l s , t h e d i s l o c a t i o n s a r e n o t g e n e r a t e d i n t o t h e i n g o t s i n c e i t n e e d s a g r e a t . e r e n e r g y t o g e n e r a t e d i s l o c a t i o n s from a ' d i s l o c a t i o n - f r e e s e e d t h a n from a d i s l o c a t e d one. T h i s p r o c e d u r e h a s been s u c c e s s f u l l y a p p l i e d t o s i l i c o n f o r many y e a r s , b u t i n t h e c a s e o f GaAs, w e have shown t h a t it was n o t p o s s i b l e t o o b t a i n d i s l o c a t i o n - f r e e undoped m a t e r i a l h a v i n g a d i a m e t e r l a r g e r t h a n 15 mm /7,11-13/ w i t h t h i s t e c h n i q u e ( s e e f i g . 2 ) . Moreover, we found t h a t t h e c r i t i c a l r e s o l v e d s h e a r s t r e s s (CRSS), f o r a d i s l o c a t i o n - f r e e undoped c r y s t a l , s t r o n g l y v a r i e d ( a f a c t o r t e n a t l e a s t ) a t h i g h t e m p e r a t u r e from a c r y s t a l t o a n o t h e r . We p r o p o s e d /12/ t h a t t h e n a t u r e and t h e m o b i l i t y o f n a t i v e p o i n t d e f e c t s were t h e main f a c t o r s a l l o w i n g t h e s e v a r i a t i o n s .
FIGURE 2 : X-ray t o p o g r a p h o f a p a r t: i a l l y d i s l o c a t i o n f r e e undoped GaAs c r y s t a l . The d i a m e t e r v a r i e s i n t.he 15 - 20 mm range.
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This problem has been i n v e s t i g a t e d by transmission e l e c t r o n i c microscopy (TEM). Some small f r i n g e d d e f e c t s (0 50-100 A ) have been observed i n samples cominq f r o m d i s l o c a t i o n - f r e e areas. U n f o r t u n a t e l y , i t i s not yet p o s s i b l e t o g i v e an i n t e r p r e t a t i o n o f t h e nature of these defects. However, t h e i r c o n c e n t r a t i o n corresponds t o about 1018 at/cm3, and i t a l l o w s us t o assume t h a t t h e i r presence i s due t o n a t i v e p o i n t d e f e c t s agglomerates.
11 I
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L I Q U I D ENCAPSULATED KYROPOULOS (LEK) TECHNIQUE
I n t h e case o f t h e LEC technique, the thermoelastic c a l c u l a t i o n s /7,8/ show t h a t , the highest stresses are reached i n the c r y s t a l a t the b o r i c oxide encapsulant-argon i n t e r f a c e . I t means t h a l t h e i n g o t goes through the h i g h e s t s t r e s s l e v e l when i t leaves t h e encapsulant layer. I n order t o prevent. t h i s f a c t , a novel t.echnique has I n t h i s technique, t h e c r y s t a l growth takes p l a c e under t h e B203 been imagined /14/. l a y e r , and the i n g o t i s not p u l l e d d u r i n g the growth. Thermoelastic c a l c u l a t i o n s have a l s o been made f o r t h i s method /7,8/, and they show t h a t t h e stresses values are e f f e c t i v e l y smaller than i n the case o f the LEC geometry. F i g u r e 3a shows the i s o s t r e s s map o f a 100 mm diameter GaAs c r y s t a l d u r i n g the growth and f i g u r e 3b presents an X-ray topograph o f such an i n g o t . I f we rompare w i t h the LEC geometry, i t can be seen t h a t t h e d i s l o c a t i o n d e n s i t y has been decreased by a f a c t o r 10 ( 1 0 ~ - 1 0 4 dislocations/cm2). Moreover, the c a l c u l a t i o n s show t h a t the s t r e s s l e v e l does not depend on the diameter o f the c r y s t a l i n t h e LEK technique. E f f e c t i v e l y , our experimental r e s u l t s on 35 mm diameter LEK c r y s t a l s g i v e about the same dislocation density ( lo3-lo4 ~ m - ~ ) .
FIGURE 3 : a ) i s o s t r e s s e s (kg/mm2) map o f a 100 mn diameter GaAs c r y s t a l d u r i n g LEK growth. b ) X-ray t.opograph o f the same c r y s t a l .
So, t h i s t e c h n i q u e seems v e r y i n t e r e s t i n g s i n c e we show t h a t it was p o s s i b l e t o o b t a i n low d i s l o c a t e d GaAs s i n g l e c r y s t a l s e x h i b i t i n g l a r g e d i a m e t e r . IV
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INFLUENCE OF THE DOPANTS
I t is well-known t h a t i n t h e c a s e o f GaAs, d o p i n g w i t h g r o u p VI e l e m e n t s (S, Se, Te) s t r o n g l y d e c r e a s e s t h e d i s l o c a t i o n d e n s i t y /11/. U n f o r t u n a t e l y , t h e r e a r e two main disadvantages t o t h i s technique : 1) The c r y s t a l is h i g h l y ( 1 0 ~ ~ - 1at/cm3) 0 ~ ~ n-doped, and it is i m p o s s i b l e t o o b t a i n , by t h i s way, s e m i - i n s u l a t i n g m a t e r i a l . 2) The h i g h c o n c e n t r a t i o n o f i m p u r i t i e s which is t o i n t r o d u c e i n t o t h e c r y s t a l and t h e i r l o w l i m i t o f s o l u b i l i t y i n d u c e m i c r o p r e c i p i t a t e s i n t h e m a t r i x . I n o r d e r t o a v o i d t h e s e d i s a d v a n t a g e s , we have t r i e d t o dope t h e GaAs c r y s t a l s w i t h i s o e l e c t r o n i c i m p u r i t i e s /7,13,15,16/ g r o u p 111 and group V e l e m e n t s . The l i t e r a t u r e had a l r e a d y r e p o r t e d some r e s u l t s a b o u t t h e n i t r o g e n d o p i n g /18/ ; and t h e indium and antimony d o p i n g /19/. A s i g n i f i c a n t r e d u c t i o n o f EPD ( E t c h P i t s D e n s i t y : e have not had been o b t a i n e d by t h i s way f o r s m a l l (15-25 mm d i a m e t e r ) c r y s t a l s . W detected any d i f f e r e n c e o f t h e e l e c t . r i c a 1 p r o p e r t i e s between undoped and i s o e l e c t r o n i c doped m a t e r i a l . These r e s u l t s and t.he cornparison w i t h o t h e r d a t a p u b l i s h e d i n t h e l i t e r a t u r e /20,21/ w i l l be d i s c u s s e d e l s e w h e r e . An open q u e s t i o n c o n c e r n i n g t h e i s o e l e c t r o n i c d o p i n g was : d o e s t h e h i g h concent r a t i o n o f i m p u r i t i e s (1019 - l o z 0 at/cm3) i n t h e c r y s t a l i n d u c e m i c r o p r e c i p i t a t e s i n t h e m a t r i x , a s i n t h e c a s e o f group V I d o p i n g ? I n o r d e r t o answer t h i s q u e s t i o n , a comparison between T e l l u r i u m ( g r o u p V I ) and Indium ( g r o u p 111) doped GaAs h a s been made by TEM. Moreover, X-ray t o p o g r a p h y o f an I n doped c r y s t a l w i l l be d i s c u s s e d . IV.l.
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TEM s t u d y o f Te doped GaAs c r y s t a l (
>5
1018 at/cm3)
T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y o f Te doped GaAs s p e c i m e n s shows numerous d e f e c t s . Two c l a s s e s o f d e f e c t s a r e o b s e r v e d : on t h e o n e h a n d , t h e r e is a low d e n s i t y o f l i n e a r and p l a n a r d e f e c t s and on t h e o t h e r hand, a r e l a t i v e l y h i g h d e n s i t y of d e f e c t s , w i t h c o n t r a s t s i m i l a r t o d i s l o c a t i o n l o o p s ( s e e f i g i . r e s 4 and 6 f o r example). These h a v e s i z e s r a n i n g from a b o u t 100 fi t o 800 A i n d i a m e t e r and w i t h a d e n s i t y of about 1 0 ~ ~ - 1 0 ~ 4 These o b s e r v a t i o n s l e a d u s t o make t h e f o l l o w i n g comments. The d e n s i t y o f t h e s e l o o p s - l i k e d e f e c t s is r e l a t e d t o t h e p r e s e n c e o f l i n e a r o r p l a n a r d e f e c t s . I n both c a s e s , we n o t e t h a t t h e r e is a h i g h d e n s i t y o f v e r y s m a l l d e f e c t s ( w 1 0 0 k ) v e r y c l o s e t o t h e l i n e a r defect o r i n t h e plane of t h e stacking f a u l t . In p a r t i c u l a r , for t h e l a t t e r , t h e s m a l l d e f e c t s o c c u r m a i n l y on t h e s t a c k i n g f a u l t , w i t h v e r y few s m a l l o n e s o u t s i d e , and w i t h a normal d e n s i t y of l a r g e ( b 500 8 ) d e f e c t s i n t h e s u r r o u n d i n g m a t r i x . F o r t h e i s o l a t e d d i s l o c a t i o n , t h e o b s e r v a t i o n shows a s i m i l a r , b u t n o t i d e n t i c a l s t a t e . Here, we o b s e r v e v e r y s m a l l d e f e c t s v e r y c l o s e t o t h e d i s 600 8 ) i n l o c a t i o n l i n e ( a r r o w e d A i n f i g . 6). There a r e , however, l a r g e d e f e c t s ( a zone around t h e d i s l o c a t i o n ( d i s t a n t up t o a few t h o u s a n d a n g s t r o m s ) , and beyond t h i s , t h e r e is a d e p l e t i o n zone. A s f a r a s t h e d i s l o c a t i o n i t s e l f is c o n c e r n e d , t h e r e is a s l i g h t s u g g e s t i o n ( a r r o w e d B i n f i g . 6 ) o f p r e c i p i t a t i o n cn t h e dislocation.
A s f a r a s a l l t h e d e f e c t s w i t h d i s l o c a t i o n l o o p - l i k e c o n t r a s t a r e c o n c e r n e d , it is i m p o r t a n t t o r e a l i z e t h a t i n a l l c a s e s , t h i s c o n t r a s t is due t o s t r a i n f i e l d s , but. t h a t t h e o r i g i n o f t h e s t r a i n f i e l d may be d i v e r s e . I n d e e d , s u c h c o n t r a s t c a n be c a u s e d , n o t o n l y by s i m p l e d i s l o c a t i o n l o o p s e i t . h e r o f vacancy o r i n t e r s t i t i a l n a t u r e , but a l s o by m i s f i t s t r a i n s o f c o h e r e n t p r e c i p i t a t e s and a l s o by d i s l o c a t i o n loops surrounding incoherent precipitates. Furthermore, i f t h e difference, in s t r u c t u r e f a c t o r between p r e c i p i t a t e and m a t r i x is s l i g h t , t h e n t h e r e w i l l be l i t t l e additional contrast. I n weak-beam images ( f i g . 5 ) , t h e d e f e c t s o b s e r v e d i n t h e two-beam d a r k f i e l d imaqe ( f i g . 4 ) a r e almost always v i s i b l e , with s t r o n g c o n t r a s t . This c o n t r a s t , f o r weak-beam c o n d i t i o n s , is c a u s e d by s t r o n g d i s t o r t i o n o f t h e l a t t i c e and t h i s can
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3GURE 4 : Te doped GaAs, dark f i e l d w i t h g = ( 2 2 0 )
FIGURE 5 : Te doped GaAs, weak-beam w i t h g =
FIGURE 6 : Te doped GaAs, b r i g h t f i e l d w j t h g = (220)
.
o n l y r e a l l y be due t o d i s l o c a t i o n s , v i r t u a l l y by d e f i n i t i o n . Thus, we can s a y t h a t t h i s c o n t r a s t is c a u s e d by d i s l o c a t i o n l o o p s , a l t h o u g h t h i s n a t u r e , vacancy o r i n t e r s t i t i a l , h a s n o t b e a s c e r t a i n e d i n t h i s s t u d y , and is s t i l l a m a t t e r o f c o n t r o v e r s y i n p u b l i s h e d work /22,23/. F u r t h e r m o r e , i n t h e c a s e o f t h e l a r g e d i s l o c a t i o n l o o p s , t h e r e is an a d d i t i o n a l c o n t r a s t i n s i d e t h e l o o p i n t h e weak-beam image ( a r r o w e d P i n f i g . 5 ) . We b e l i e v e t h a t t h i s c o u l d be due t o p r e c i p i t a t e s . However, t h e p r e c i p i t a t e d e n s i t y is t o o low t o g i v e rise t o e x t r a s p o t s i n t h e d i f f r a c t i o n p a t t e r n , and hence i t is n e i t h e r p o s s i b l e t o i d e n t i f y t h e s t r u c t u r e o f such p r e c i p i t a t e o r t o produce d a r k - f i e l d images which would r e v e a l them. F i n a l l y , we n o t e t h a t t h e l a r g e s t d i s l o c a t i o n l o o p s have a t e n d e n c y t o become p r i s m a t i c , w i t h e d g e s p a r a l l e l t o (110) d i r e c t i o n / 2 4 / . On t h e o t h e r hand, i t is n o t p o s s i b l e t o s t a t e d e f i n i t e l y w h e t h e r t h e s m a l l e r d e f e c t s a r e a s s o c i a t e d w i t h p r e c i p i t a t e s o r n o t , b u t t h i s c a n be c o n s i d e r e d a s plausible.
-
TEM and X-ray t o p o g r a p h s t u d y o f a n In-doped GaAs c r y s t a l (
Lattice defects topography using MoK 61 r a d i a t i o n p o l i s h e d down t o
at/cm3)
have been d e t e c t e d by d i f f e r e n t t e c h n i q u e s . X-ray t r a n s m i s s i o n a c l a s s i c a l Lang camera ( m i c r o f o c u s 0.1 sim) h a s been performed w i t h o n w a f e r s c u t i n e a c h i n g o t t h r o u g h t h e ( 0 0 1 ) symmetry a x i s and 0.2 mm w i t h I f l o ] a n d !?I?) d i f f r a c t i o n v e c t o r s .
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The X-ray t r a n s v e r s e t o p o g r a p h p r e s e n t e d on f i g . 7 was o b t a i n e d on a ( 1 1 0 ) p l a t e c u t a l o n g t h e growth a x i s . The c r y s t a l d i a m e t e r is 20 mm Apart from l i n e a r d e f e c t s l y i n g a l o n g t h e ( 0 0 1 ) d i r e c t i o n w i t h a v e r y low d e n s i t y ( ( 5 l o Z c V 2 ) , we o b s e r v e d d e f e c t s l o c a t e d a t t h e p e r i p h e r y o f t h e c r y s t . a l w i t h i n 2 mm ; on each t y p e o f d e f e c t s , t h e c o n t r a s t o f t h e image ( b l a c k , w h i t e o r g r e y ) i s r e l a t e d t o t h e i r l o c a t i o n n e a r t h e e n t r a n c e s u r f a c e ( w h i t e o r dynamic image) o r n e a r t h e e x i t s u r f a c e ( b l a c k o r c i n e m a t i c image).
.
The d e f e c t s l i e i n j 1 1 1 3 s c a n i n g e l e c t r o n microscopy tridimensional structures p r e c i p i t a t e s . Their d e n s i t y be o b s e r v e d by TEM.
g l i d e p l a n e and a c a t h o d o l u m i n e s c e n c e image o b t a i n e d i n is shown i n f i g . 8. These c r y s t a l l o g r a p h i c d e f e c t s h a v e and a r e n o t r e l a t e d t o d i s l o c a t i o n s b u t r a t h e r t o h e r e i n t h e r a n g e o f 104 c N 2 a p p e a r s t o be t o o low t.o
FIGURE 7 : X-ray t o p o g r a p h of a In-doped GaAs c r y s t a l
(13
2 0 mm).
FIGURE 8 : Cathodoluminescence image o f t h e e d g e o f t h e In-doped GaAs c r y s t a l .
However, it was i n t e r e s t i n g t o know i f s m a l l e r m i c r o - d e f e c t s were v i s i b l e i n t h e m a t r i x . C a r e f u l 1 t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y s t u d i e s show t h a t t h e r e is no c o n t r a s t which c a n be a s s o c i a t e d t o e i t h e r d i s l o c a t i o n l o o p s o r c o h e r e n t p r e c i p i t a t e s ( s e e f i g . 9).
A t t h i s t i m e , t h e g r e a t e r low d i s l o c a t e d c r y s t a l ( l o 2 - l o 3 a b o u t 4 0 mm d i a m e t e r .
~ m - ~doped ) w i t h I n is
FIGURE 9 : In-doped GaAs, b r i g h t f i e l d w i t h g = (220)
.
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CONCLUSION I n order t o decrease t h e d i s l o c a t i o n d e n s i t y i n GaAs c r y s t a l s , we have i n v e s t i q a t e d t h r e e k i n d s o f techniques. We showed t h a t the necking procedure d i d not. allow t o o b t a i n undoped d i s l o c a t i o n - f r e e c r y s t a l s i f i t s diameter was g r e a t e r than 15-20 mm. On t h e other hand, the LEK technique has p e r m i t t e d t o synthesize low d i s l o c a t e d ( i n t h e lo3-104 cm-2 range) l a r g e c r y s t a l s ( % 10 cm diameter). This r e s u l t i s very promising since the d i s l o c a t i o n d e n s i t y i s t e n times higher i n t h e case o f t y p i c a l a v a i l a b l e GaAs. F i n a l l y , we made d i f f e r e n t s t u d i e s on i s o e l e c t r o n i c doped m a t e r i a l and we showed t h a t i t was p o s s i b l e t o o b t a i n s e m i - i n s u l a t i n g GaAs having very low d i s l o c a t i o n d e n s i t y ( i n the lo2-lo3 ~ m range). - ~ ACKNOWLEDGMENTS This work has been supported by DESTI (Contract nu 82 A 1220). We u r . P.J. Hoksnoer f o r the growth o f the In-doped c r y s t a l presented on f i g . 7.
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