DEVICE EFFECTS OF DISLOCATIONS

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DEVICE EFFECTS OF DISLOCATIONS

D. Holt

To cite this version:

D. Holt. DEVICE EFFECTS OF DISLOCATIONS. Journal de Physique Colloques, 1979, 40 (C6),

pp.C6-189-C6-199. �10.1051/jphyscol:1979639�. �jpa-00219055�

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JOURNAL DE PHYSIQUE BZZoque C6, suppZEment au n06

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tane 40, juin 1 9 7 9 , page C6

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DEVICE EFFECTS OF DISLOCATIONS

Department of MetaZZurgy and Materials Science, ImperiaZ CoZZege of Science and TechnoZogy, London SW7 2BP, U. K.

Resume.- Le r61e des d i s l o c a t i o r s dans l e s rendements ou f a i l l i t e s de deux s o r t e s de d i s p o s i t i f s e s t discute. Dans 1 'exemple des d i s p o s i t i f s

a

s i l ic i u m plans e t b i p o l a i r e s l a d e n s i t e des d i s l o c a t i o n s n ' e s t maintenant u t i l e que pour mesurer l e succes du d@veloopement de l a technologie de l e u r fabrica- t i o n . La chose frappante e s t que l ' i m p o r t a n c e des d i s l o c a t i o n s dans ces d i s p o s i t i f s e s t en elle-meme f a i b l e . E l l e s ne posent de problemes que s i e l l e s e n t r a i n e n t l a n u c l e a t i o n de o r e c i p i t e s m e t a l l i q u e s t r a v e r s a n t l e s j o n c t i o n s p-n. Les d i s l o c a t i o n s dans l e s l a s e r s

a

GaAs sont beaucoup p l u s importantes.

Les d i s l o c a t i o n s d ' e p i t a x i e e t a i e n t inherentes dans l e s j o n c t i o n s des premiers l a s e r s , oO e l l e s pro- voquaient souvent une emission f i l a m e n t a i r e . Les etudes u l t e r i e u r e s o n t p r o d u i t beaucoup d ' i n f o r m a t i o n s u r 1 a geometric des d i s l o c a t i o n s d ' e p i t a x i e e t s u r l e s mecanismes par lesquels e l l e s sont i n t r o d u i t e s dans l e s h e t e r o j o n c t i o n s . Le c h o i x des materiaux pour l a s e r s G ~ A S / G ~ ~ A ~ ~ - ~ A S de double h e t e r o s t r u c t u r e en vue de p r o d u i r e l e moins p o s s i b l e de d i s l o c a t i o n s d ' e p i t a x i e s ' e s t r e v e l @ &.re e s s e n t i e l pour a l - l o n g e r l a v i e des d i s p o s i t i f s . En o u t r e se t r o u v e n t aussi minimises de ce f a i t , l e s s i t e s de germina- t i o n des "defauts de l i g n e n o i r e " q u i e n t r a i n e n t l a degradation r a p i d e de ces lasers. L16tude de ces defauts a r e v e l e un nouveau phenomene de montee aide par 1 1 6 c l a i r e m e n t l a s e r . A i n s i a-t-on beaucoup appris s u r l e s d i s l o c a t i o n s dans c e t t e recherche o r i e n t e e vers l e s d i s p o s i t i f s , t h n d i s que l a connais- sance des d i s l o c a t i o n s e s t encore souvent importante Dour ameliorer l e s materiaux semiconducteurs e t l a technologie des d i s p o s i t i f s .

Abstract.- 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 y i e l d s and i n f a i l u r e i n two types o f device w i l l be d i s - cussed. I n t h e case o f planar, b i p o l a r s i l i c o n devices t h e d i s l o c a t i o n d e n s i t y i s now u s e f u l o n l y as an index o f the adequacy o f development o f t h e device processing technology. The sobering f a c t i s t h a t d i s l o c a t i o n s i n these devices are o f l i t t l e importance i n themselves. They cause serious problems o n l y i f they nucleate metal p r e c i p i t a t e s t h r e a d i n g t h e p-n j u n c t i o n s . D i s l o c a t i o n s i n GaAs l a s e r s are f a r more important. M i s f i t d i s l o c a t i o n s were i n h e r e n t i n t h e j u n c t i o n s o f t h e e a r l y l a s e r s where they were found t o be responsible i n many cases f o r f i l a m e n t a r y emission. Consequent s t u d i e s produced much i n f o r m a t i o n on t h e geometry o f m i s f i t d i s l o c a t i o n s and the mechanism whereby they are introduced i n h e t e r o j u n c t i o n s . The s e l e c t i o n o f .the m a t e r i a l s f o r GaAs/Ga A1 As double h e t e r o s t r u c t u r e l a s e r s t o minimize t h e m i s f i t d i s l o c a t i o n d e n s i t y proved e s s e n t i a l f o p t k x a c h i e v e m e n t o f l o n g e r device 1 ives.

As a bonus i t a l s o minimized t h e s i t e s f o r the n u c l e a t i o n o f the "dark l i n e d e f e c t s " r e s p o n s i b l e f o r r a p i d degradation o f these l a s e r s . The study o f these d e f e c t s has revealed a new phenomenon o f climb enhanced by l a s e r l i g h t . Thus much has been learned about d i s l o c a t i o n s through device-oriented r e - search and know1 edge o f d i s l o c a t i o n s i s s t i 11 o f t e n i m p o r t a n t f o r improving semiconductor m a t e r i a l s and devi ce techno1 ogy.

1. I n t r o d u c t i o n .

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I n i t i a l l y t h e r e was s t r o n g i n t e - r e s t i n d i s l o c a t i o n s among s o l i d s t a t e e l e c t r o n i c device producers. This arose from t h e f a c t t h a t one o f t h e necessary p r e c o n d i t i o n s f o r t h e p r o d u c t i o n o f

r e p r o d u c i b l e i n t r i n s i c and doped semiconductor sam- p l e s was 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 i e s by many orders o f magnitude below the l e v e l s p r e v i o u s l y a v a i l a b l e . To achieve t h i s t h e Czochralski c r y s t a l growing technique was developed and zone r e f i n i n g was invented. Only t h e r e s u l t a n t unprecedented l e - v e l s o f c r y s t a l l i n e p e r f e c t i o n , p u r i t y and u n i f o r m i - t y made p o s s i b l e the understanding o f semiconductor physics, t h e i n v e n t i o n o f t r a n s i s t o r s and the r i s e o f s o l i d s t a t e e l e c t r o n i c s . This p e r i o d coincided w i t h t h e h e r o i c age o f d i s l o c a t i o n theory and two o f t h e standard e a r l y monographs on d i s l o c a t i o n s were w r i t t e n by workers i n i n d u s t r i a l e l e c t r o n i c s labora- t o r i e s /1,2/. I n t h e e a r l y days i t was o f t e n found

t h a t the o n l y parameter t h a t c o r r e l a t e d w i t h device y i e l d s was t h e r e c i p r o c a l o f t h e d i s l o c a t i o n d e n s i t y , and no-one questioned t h e i r importance.

The m a t e r i a l s technology was l a t e r pushed t o the l i m i t so t h a t l a r g e c r y s t a l s o f Ge, S i and GaAs could be grown w i t h zero d i s l o c a t i o n d e n s i t i e s . However, i t was found t h a t t h i s m a t e r i a l was general- l y no b e t t e r f o r device p r o d u c t i o n than t h a t w i t h d i s l o c a t i o n d e n s i t i e s up t o about l o 4 c w 2 . Numerous s t u d i e s o f t h e d i s t r i b u t i o n s o f process-induced d i s - l o c a t i o n s over s l i c e s , using X-ray topography o r de- c o r a t i o n and i n f r a r e d microscopy, showed o n l y weak and unconvincing c o r r e l a t i o n s w i t h bad device charac- t e r i s t i c s . Conversely a l l t h e prooosals t o use d i s l o - cations as components o f devices, as i n t h e g r a i n boundary t r a n s i s t o r / 3 , 4 / o r f a s t s w i t c h i n g diodes using d i s l o c a t i o n s t o reduce m i n o r i t y c a r r i e r 1 if e - times /5,6/ came t o nothing. Yoreover, i t was found

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979639

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t h a t g e n e r a l l y t h e i m p o r t a n t d e f e c t s i n d e v i c e p r o - s e v e r a l p o i n t s around t h e circumference. F i g u r e 2 d u c t i o n were such r e l a t i v e l y .gross t h i n g s as p r e c i p i -

t a t e s , i m p u r i t y growth s t r i a t i o n s , and l o c a l mecha- n i c a l damage such as s c r a t c h e s o r tweezer marks /7, 8/. I n t e r e s t i n d i s l o c a t i o n s among workers i n t h e s i l i c o n d e v i c e and i n t e g r a t e d c i r c u i t i n d u s t r y f e l l t o a l o w l e v e l where i t g e n e r a l l y remains.

N e v e r t h e l e s s t h e r e i s ample evidence t h a t energy l e v e l s o r bands i n t h e f o r b i d d e n gap a r e a s s o c i a - t e d w i t h d i s l o c a t i o n s and t h a t t h e y can produce t r a p - p i n g o r r e c o m b i n a t i o n and so a f f e c t m i n o r i t y c a r r i e r 1 if e t i m e s . T h i s s h o u l d i n f l u e n c e e l e c t r i c a l and op- t i c a l e m i s s i o n p r o p e r t i e s . Thus two q u e s t i o n s must be considered. F i r s t l y , why do d i s l o c a t i o n s so o f t e n have l i t t l e o r no e f f e c t on d e v i c e s ? To answer t h i s t h e s p e c i f i c case o f p l a n a r , b i p o l a r d e v i c e s i n S i w i l l be considered. The second q u e s t i o n i s how do d i s l o c a t i o n s a f f e c t y i e l d and f a i l u r e i n new mate- r i a l s and devices and what can be l e a r n e d about d i s - l o c a t i o n s from such s t u d i e s ? Here t h e case o f GaAs

shows t h e Vopograph o v e r l a i d w i t h a t r a n s p a r e n t sheet on which each r e j e c t e d d i o d e i s marked w i t h a d o t .

1 a s e r s w i l l be considered.

F i g . 2 : The X-ray topograph o f f i g u r e 1 o v e r l a i d 2. D i s l o c a t i o n s i n s i l i c o n p-n j u n c t i o n d e v i c e s . - w i t h a p l o t r e p r e s e n t i n g a l l p h o t o t r a n s i s t o r s f o r Some o f t h e reasons f o r t h e p o o r c o r r e l a t i o n s found which BVceo< 80V a s a (After Ogden / 9 / ) ' between d i s l o c a t i o n s and d e v i c e y i e l d s can be i l l u s - S t a t i s t i c a l a n a l y s i s shows t h a t t h e r e i s a s i g n i f i - t r a t e d by means o f a r e c e n t s t u d y c a r r i e d o u t by c a n t c o r r e l a t i o n between r e g i o n s o f s l i p and h i g h

Ogden /9/ percentages o f r e j e c t e d diodes b u t i t i s f a r f r o m

b e i n g a one-to-one r e l a t i o n . One reason f o r t h i s

F i g . 1 : Montage o f t r a n s ~ n i s s i o n X-ray topographs showing a 1.1/4" d i a m e t e r Ell11 s l i c e o f s i l i c o n c o n t a i n i n g a 40 x 50 a r r a y o f p h o t o - t r a n s i s t o r s . The d i a g o n a l w h i t e l i n e s a r e s l i p bands. ( A f t e r Ogden /9/)

F i g u r e 1 i s a t r a n s m i s s i o n X-ray topograph (XRT) o f a S i s l i c e processed t o produce a l a r g e number o f p h o t o t r a n s i s t o r s . Patches o f l o c a l i z e d s l i p due t o t h e r m a l s t r e s s e s d u r i n g p r o c e s s i n g can be seen a t

p o o r c o r r e l a t i o n i s t h e a r b i t r a r y n a t u r e o f t h e q u a n t i t i e s r e l a t e d . Diodes were r e j e c t e d when BVceo

<80V. Here BVceo i s t h e back ( e m i t t e r p o s i t i v e ) v o l - t a g e between e m i t t e r and c o l l e c t o r , w i t h t h e base f l o a t i n g as i n normal o p e r a t i n g c o n d i t i o n s f o r t h i s d e v i c e , shown i n f i g u r e 3, f o r a leakage c u r r e n t o f 10pA. Thus t h e diodes marked a r e t h o s e t h a t were

" s o f t e r " o r more l e a k y t h a n a c e r t a i n a r b i t r a r y l e v e l . The presence o f d i s l o c a t i o n s was d e t e c t e d v i - s u a l l y on t h e X-ray topograph a t a l o w m a g n i f i c a t i o n . T h i s r e j e c t i o n c r i t e r i o n i s t y p i c a l o f t h e commercial s p e c i f i c a t i o n s used i n most s t u d i e s . They a r e n o t n e c e s s a r i l y a p p r o p r i a t e as i n d i c e s o f d i s l o c a t i o n e f f e c t s and t h i s f a c t o r accounts t o some e x t e n t f o r t h e p o o r c o r r e l a t i o n s found.

The p o o r c o r r e l a t i o n however does mean t h a t sometimes s l i p and d i s l o c a t i o n s a f f e c t t h e photo- t r a n s i s t o r s and sometimes t h e y do n o t . To s t u d y t h i s problem r e q u i r e s a t e c h n i q u e capable o f d i s t i n g u i - s h i n g o n l y t h e e l e c t r i c a l l y e f f e c t i v e d i s l o c a t i o n s . By means o f t h e b a r r i e r . e l e c t r o n v o l t a i c e f f e c t (BEVE) i n t h e scanning e l e c t r o n microscope

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i t i s p o s s i b l e t o d e t e c t a n l y t h o s e d i s l o c a t i o n s t h a t t h r e a d t h e l a r g e area b a s e - c o l l e c t o r P-N j u n c t i o n s

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D.B. Holt C6-191

( F i g . 3 ) and a f f e c t t h e i r charge c o l l e c t i o n e f f i c i e n - cy

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F i g . 3 : Schematic c r o s s - s e c t i o n a l diagram o f t h e s i l i c o n p h o t o t r a n s i s t o r s and t h e d e t e c t i o n system used i n t h e b a r r i e r e l e c t r o n v o l t a i c e f f e c t scanning e l e c t r o n m i c r o g r a p h o f f i g u r e 4 ( b ) . ( A f t e r H o l t and Ogden / l l / ) .

occured o n l y where t h e d i s l o c a t i o n s reached t h e t o p o f t h e s l i c e /11/. F i g u r e 3 i s n o t drawn t o s c a l e . The s l i c e was 400 um t h i c k . ( H a l f a m i l l i m e t r e i s a t y p i c a l s l i c e t h i c k n e s s . ) The a c t i v e , d i f f u s e d v o l u - me o f t h e d e v i c e s t h u s occupied o n l y t h e t o p h a l f o f one p e r c e n t o f t h e volume of t h e s l i c e . Thus i t i s g e o m e t r i c a l l y p o s s i b l e f o r numerous d i s l o c a t i o n s t o be seen by a t r a n s m i s s i o n t e c h n i q u e b u t few o r none o f them t o h i t t h e a c t i v e area o f t h e device. Conver- s e l y a orominent row o f e l e c t r i c a l l y a c t i v e s p o t s can apoear i n t h e BEVE SEX p i c t u r e , t o which l i t t l e o r n o t h i n g i n t h e XRT t r a n s m i s s i o n p i c t u r e corresponck because t h e d i s l o c a t i o n s r u n n e a r l y v e r t i c a l l y through t h e s l i c e . Thus o n l y d i s l o c a t i o n s r u n n i n g t h r o u g h t h e small a c t i v e volumes o f t h e d e v i c e s can have any e f f e c t , and t h i s i s a m a j o r f a c t o r c o n t r i - b u t i n g t o t h e p o o r c o r r e l a t i o n s found between d i s - l o c a t i o n s and y i e l d s (percentages o f s a l e a b l e d e v i c e s produced).

The t h i r d m a j o r f a c t o r produces e r r a t i c e l e c - F i g . 4 : A s i l i c o n p h o t o t r a n s i s t o r crossed b y a s l i p t r i c a l effects and so poor c o r r e l a t i o n s . This i s va- olane ( a ) Lang p r o j e c t i o n X-ray topograph. ( b ) SEN

c o n d u c t i v e mode micrograph recorded u s i n g t h e a r r a n - r i a b l e i m p u r i t y d e c o r a t i o n o f d i s l o c a t i o n s . I t was gement o f f i g u r e 3. The b a r r i e r e l e c t r o n v o l t a i c e a r l y n o t e d t h a t d i s l o c a t i o n s w h i c h threaded P-N e f f e c t c u r r e n t was used as video s i g n a l . ( c ) t h e geo-

m e t r y of t h e s l i p p l a n e c o n t a i n i n g t h e d i s l o c a t i o n s j u n c t i o n s ~ 0 r r e l a t e d w i t h . " s o f t " r e v e r s e c h a r a c t e r i s - i n t h i s device. The d i s l o c a t i o n 1 in e s have been t r a - tics and microplasma breakdown sites /12/. ~ h ~ ~ ced f r o m ( a ) and t h e e l e c t r i c a l l y a c t i v e p o i n t s f r o m

( b ) t o show t h e i r correspondence. ( d ) Secondary e l e c - was l a t e r found t o be o n l y a c o r r e l a t i o n , n o t a one- t r o n S E ? ~ micrograph showing t h e surface topography o f to-one r e l a t i o n s h i p , and Shockley /13/ suggested t h a t t h e p h o t o t r a n s i s t . o r . There a r e no s u r f a c e f e a t u r e s

c o r r e s p o n d i n g t o t h e row of s p o t s AB i n ( b ) . ( A f t e r t h e s e e f f e c t s were due t o p r e c i p i c t a t e s o c c u r r i n g a t H o l t and Ogden /11/). some d i s l o c a t i o n s , n o t t o t h e d i s l o c a t i o n s themselves.

Coinparison o f t h e d i s l o c a t i o n l i n e s i n t h e XRT ( F i g . From t h e p r a c t i c a l p o i n t o f view t h i s problem 4 ( a ) ) w i t h t h e SEY c o n t r a s t s p o t s ( F i g . 4 ( b ) ) showed was s o l v e d by s o e t z b e r g e r and Shockley /14/. They

( F i g . 4 ( c ) ) t h a t these e l e c t r i c a l l y a c t i v e ~ o i n t s c a r r i e d o u t analyses o f l a r g e numbers o f S i diodes

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annealed i n hydrogen a t 1000°C. Some batches of d i o - magnitude c a l c u l a t i o n o f t h e number o f d e f e c t s neces- des were d e l i b e r a t e l y contaminated w i t h various me- s a r y t o cause device r e j e c t i o n as i n f i g u r e 2. T h i s t a l n i t r a t e s and o t h e r s were not. .It was shown t h a t r e q u i r e d a t o t a l leakage c u r r e n t o f 10 PA ( f o r < 80V).

contamination w i t h metals l i k e Cu, Fe, Yn and Au For leakages o f 10 PA, lOnA and IOpA oer d e f e c t ( t a - r e s u l t e d i n t h e m a j o r i t y o f t h e diodes becoming b l e I ) one d i s l o c a t i o n o r s t a c k i n g f a u l t ,

l o 3

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l o 6 ,

soft. I t was shown t h a t these harmful e f f e c t s could r e s p e c t i v e l y would have t o thread the device junction.

be preuented o r reversed by ' g e t t e r i n g " . T h i s i n v o l - C l e a r l y t h i s i s an i m p o r t a n t source o f v a r i a t i o n . No ved annealing t h e m a t e r i a l w i t h a surface l a y e r of a c a l c u l a t i o n o f t h e leakage c u r r e n t t o be expected p e r g r e t t e r . Glassy l a y e r s o f m i x t u r e s of S i 0 2 w i t h P205 d i s l o c a t i o n o r f a u l t i n t h e absence o f contamination were found t o keep t h e diodes "hard", o r r e s t o r e them i s known t o t h e author. However, the minimum values t o hard c h a r a c t e r i s t i c s . I t was suggested t h a t the o f t a b l e I are so low as t o suggest t h a t d i s l o c a t i o n s e f f e c t o f metal contamination was due t o p r e c i p i t a - are r e a d i l y e l e c t r i c a l l y n e u t r a l i z e d by very small t e s t h r e a d i n g t h e j u n c t i o n and t h a t g e t t e r i n g e l i m i - concentrations o f i m p u r i t i e s . Suppose t h a t t h i s re- nated t h e p r e c i p i t a t e s . The use of phosphorus glasses q u i r e s one i m p u r i t y atom p e r i n t e r a t o m i c l e n g t h along t o prevent harmful e f f e c t s due t o Cu and Fe contami- t h e d i s l o c a t i o n t o chemically s a t i s f y t h e dangling n a t i o n has been general ever since. However, i t i s bond. F o r a d e n s i t y o f p =

l o 4

~ m - ~ i n S i t h i s r e q u i r e s o n l y i n t h e l a s t few years t h a t d i r e c t evidence has an i m p u r i t y c o n c e n t r a t i o n o f 3.7 x 10" ~ m - ~ . I n been obtained u s i n g t h e scanning e l e c t r o n microscope h e a v i l y d i s l o c a t e d , p l a s t i c a l l y deformed S i w i t h (SEY) and transmission e l e c t r o n m i c r o s c o ~ e (TEM), t o p = 10' ~ m - ~ o n l y a concentration o f 3.7 x 1015cm-3 show t h a t s o f t breakdown s i t e s do i n f a c t occur a t i s needed. Such concentrations are low compared w i t h metal p r e c i p i t a t e s occuring a t d i s l o c a t i o n s /15/ o r t h e usual doping l e v e l s i n devices. The tendency f o r a t p a r t i a l d i s l o c a t i o n s boundingstacking f a u l t s /16, such e l e c t r i c a l n e u t r a l i z a t i o n t o occur i s s t r o n g f o r , 17/. Now t h a t energy d i s p e r s i v e X-ray a n a l y s i s f a c i - i n a d d i t i o n t o t h e e l a s t i c i n t e r a c t i o n s , t h e r e are l i t i e s on T E k are becoming more common, SO too i s h i g h energy e l e c t r i c a l o r chemical i n t e r a c t i o n s b e t - d e t e c t i o n o f t h e metals t h a t cause the t r o u b l e . ween the d i s l o c a t i o n s and i m p u r i t i e s o f t h e r i g h t

An i n t e r e s t i n g f a c t i s t h a t - o f t e n s t a c k i n g f a u l t s a r e more s e r i o u s than d i s l o c a t i o n s . This has been' suggested t o be due t o t h e f a c t t h a t s t a c k i n g f a u l t s a r e more e f f e c t i v e s i n k s f o r i m p u r i t i e s and so tend t o be more r e a d i l y decorated w i t h p r e c i p i t a - tes e i t h e r a t the bounding p a r t i a l s o r throughout.

Values f o r the leakage c u r r e n t p e r s t a c k i n g f a u l t t h r e a d i n g S i p-n j u n c t i o n s o r CCD devices were t a - b u l a t e d from t h e 1 i t e r a t u r e by Ogden and W i 1 kinson /18/ as shown i n t a b l e I.

valence t o o x i d i z e o r reduce t h e dangling bonds as required. That t h i s happens has n o t been d i r e c t l y proved b u t was f r e q u e n t l y suggested i n t h e e a r l y work /22/; i s theoretica.1ly probable and i t i s supported by some experimental evidence /23/.

3. M i s f i t d i s l o c a t i o n s i n GaAs lasers.- Light-emit- t i n g devices connot be made o f s i l i c o n . Less developed m a t e r i a l s must t h e r e f o r e be employed and so d i s - l o c a t i o n s a r e s t i l l a problem. Because these more r e c e n t m a t e r i a l s and devices are more complex, new Table I : ( A f t e r Ogden and Wilkinson /18/). Leakage c u r r e n t o e r

s t a c k i n g f a u l t deduced from e l e c t r i c a l measurements and s t a c k i n g f a u l t counts.

Leakage c u r r e n t p e r

i

Reference s t a c k i n g f a u l t

15

-

⁵⁰⁰^pA ^/16/

60nA

-

⁶^pA ^/19/

7pA

-

^33pA ^/20/

% lOpA /2 I/

4

-

590pA / 18/

Year Reported

They suggested t h a t t h e f a l l i n t h e leakage c u r r e n t types o f d i s l o c a t i o n s become important and new con- p e r f a u l t as the years go by i s e s s e n t i a l l y due t o d i t i o n s are encountered y i e l d i n g new i n f o r m a t i o n , reductions i n t h e amounts of i m p u r i t i e s associated as w i l l be i l l u s t r a t e d i n t h e case o f GaAs lasers.

w i t h t h e f a u l t s . We can now make an order o f The e a r l i e s t GaAs l a s e r s were simple d i f f u s e d

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D.B. Holt C6-193

o r e p i t a x i a l p-n j u n c t i o n diodes. T h e i r most obvious t e r f a c e s /29,30/. The idea o f m i s f i t d i s l o c a t i o n s s t r u c t u r e - s e n s i t i v e p r o p e r t y was non-uniform, f i l a - arose from one o f t h e c l a s s i c a l d i s l o c a t i o n theory mentary l a s e r emission /24/ as i l l u s t r a t e d i n f i g u - c a l c u l a t i o n s by Frank and van der Merwe /31,32/ using

res 5 and 6. a model very l i k e t h a t f o r t h e P e i e r l s stress. They

were introduced i n t o semiconductor theory t o account f o r t r a p p i n g and recombination a t semiconductor hete- r o j u n c t i o n s /33/.

Much o f t h e i n t e r e s t i n m i s f i t d i s l o c a t i o n s has a r i s e n from t h e i r p o s s i b l e e f f e c t s i n semiconduc- t o r h e t e r o j u n c t i o n o r p-n j u n c t i o n devices and much of t h e experimental work on e p i t a x i a l defects, espe-

I

c i a l l y m i s f i t d i s l o c a t i o n s has been done i n indus-

0.4-rn t r i a l e l e c t r o n i c s l a b o r a t o r i e s . This has i n c l u d e d

1

the work on l a s e r s w i t h which we are concerned.

A b a l l - a n d - w i r e model a n a l y s i s was published g i v i n g t h e geometry o f t h e s i m p l e s t networks possi- b l e i n p a r a l l e l - a l i g n m e n t h e t e r o j u n c t i o n s w i t h i n t e r - Fig. 5 : E a r l y GaAs diode l a s e r s operated non-unifor- faces i n t h e t h r e e " s i n g u l a r " o r i e n t a t i o n s /34/.

mly. Only c e r t a i n f i l a m e n t s through the a c t i v e r e g i o n

emitted coherent 1 i g h t a t i s 0 1 ated l a s e r spots. Actual models were however b u i l t o n l y f o r the I1111 ( A f t e r H i l l and H o l t /25/). case. The e t c h i n g evidence o f f i g u r e 7 brought o u t

F i g . 6 : Yicrographs o f i n f r a r e d emission from a GaAs l a s e r . (a) and (b) are f r o n t and back faces o f the l a s e r showing t h e correspondence o f t h e dark i m p u r i t y growth s t r i a t i o n s . I n ( c ) i t can be seen t h a t the dark s t r i a e i n t e r s e c t the b r i g h t p-n j u n c t i o n i n p o i n t s o f minimum enission. ( A f t e r H i l l and H o l t /25/).

The e a r l i e s t studies r e p o r t e d t h a t f i l a m e n t a r y emis- s i o n c o r r e l a t e d w i t h i m p u r i t y growth s t r i a t i o n s /25/

o r when these more serious inhomogeneities were e l i - minated, w i t h m i s f i t d i s l o c a t i o n s /26,27/. M i s f i t d i s l o c a t i o n s are those t h a t occur i n i n t e r f a c e s b e t - ween two d i f f e r e n t m a t e r i a l s i n o r d e r t o accominodate a p a r t o f t h e d i f f e r e n c e i n l a t t i c e parameter ("mis- f i t " ) /28/. They are important i n t h e theory o f i n -

a feature of t h e (0011 networks which had been mis- sed i n t h e absence o f a t h r e e dimensional model ana- l y s i s . The d i s l o c a t i o n s o f t h e two orthogonal ( [ 1 1 0 1 an3 C l i 9 1 a l i g n e d ) sets are n o t s i m i l a r . Many o f t h e [ I 1 0 7 a l i g n e d d i s l o c a t i o n s etched une- venly and d i s c r e t e decorating p a r t i c l e s can be r e - solved along them, w h i l e t h e r e i s no evidence o f decoration o f t h e E l i 0 7 d i s l o c a t i o n s . The decorated

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C6-194 JOURNAL DE PHYSIQUE

d i s l o c a t i o n s c o r r e l a t e d w i t h t h e 1 a s i n g f i l a m e n t s . f a c e charges and 1 arge surface s t a t e d e n s i t i e s o f p o l a r faces. Such v i c i n a l , n o n - p o l a r s u r f a c e s a r e t h e

4

p r o b a b l e form o f b o t h { l o 0 1 f r e e s u r f a c e s and, as

we s h a l l see, i n t e r f a c e s .

a rc

A b a l l - a n d - w i r e model o f a {001) h e t e r o j u n c t i o n

i n t h e s p h a l e r i t e s t r u c t u r e was c o n s t r u c t e d b y H i l l

4 / 3 6 / a n d i s s h o w n i n f i g u r e 8 .

F i g . 7 : M i s f i t d i s l o c a t i o n s i n t h e e p i t a x i a l (001) p-n j u n c t i o n o f a GaAs l a s e r , r e v e a l e d b y e t c h i n g . ( A f t e r Abrahams and B u i o c c h i / 2 6 / ) .

The d i f f e r e n c e between t h e

C l l O l

and [ l i O I d i s l o c a t i o n s a r i s e s f r o m g e o m e t r i c a l c o n s i d e r a t i o n s t h a t a r e r e l a t e d t o t h e v i c i n a l n o n - p o l a r i t y o f { l o 0 1 s u r f a c e s i n t h e s p h a l e r i t e s t r u c t u r e . The C1111 faces of s p h a l e r i t e s t r u c t u r e m a t e r i a l s a r e w e l l known t o be p o l a r , i . e . t o c o n s i s t o f A atoms alone o r o f B atoms o n l y whereas t h e C1101 faces can r e a d i l y be shown t o be non-polar, i .e. t o c o n s i s t o f equal numbers o f A and B atoms of t h e AB compound.

A t f i r s t glance { l o o } faces, which a r e t h e square faces o f t h e f c c u n i t c e l l o f t h e s t r u c t u r e would appear t o be p o l a r . T h i s would be so i f t h e f a c e s were atomi c a l l y - f l a t and p l a n a r , i .e. " s i n g u l a r 1 1 surfaces. The { l l l l and C1101 s u r f a c e s t e n d t o be s i n g u l a r f o r e n e r g e t i c reasons, i.e. t o m i n i m i z e t h e number o f d a n g l i n g bonds p e r u n i t area. However, i n t h e case o f (0011 as Sangster / 3 5 / p o i n t e d o u t , mo- natomic s t e p s on these faces a r e p o s s i b l e as t h e y i n v o l v e no change i n t h e d a n g l i n g bond d e n s i t y . They p r o b a b l y l o w e r t h e s u r f a c e energy because such s t e p - ped, i . e . ' I u i c i n a l 1 ' f a c e s would a v o i d t h e l a r g e s u r -

F i g . 8 : B a l l - a n d - w i r e model o f a (100) h e t e r o j u n c - t i o n i n t h e s p h a l e r i t e s t r u c t u r e viewed (a) i n t h e

r

¹¹⁰⁷d i r e c t i o n and ( b ) i n t h e o r t h o g o n a l [ l l O l d i r e c t i o n . The two d i s l o c a t i o n s a r e o f t h e same po- l a r i t y (As i .e.

" 6

ll), d i f f e r e n t c o r e s t r u c t u r e and coplanar. T h a t i s t h e i n t e r f a c e i s s i n g u l a r ( A f t e r H i l l / 3 6 / ) . F o r reasons g i v e n i n t h e t e x t i t i s b e l i e v e d t h a t t h i s i s n o t t h e s i t u a t i o n i n p r a c t i c e . I n t h i s model f i v e atoms above t h e h e t e r o j u n c t i o n i n t e r f a c e J-J correspond t o f o u r below and t h e mo- d e l i s shown viewed a l o n g t h e two d i s l o c a t i o n l i n e d i r e c t i o n s i n (a) and ( b ) . I t can be seen t h a t i n o r d e r t o g e t t h e two d i s l o c a t i o n cores t o be accu- r a t e l y cop1 anar i n t h e s i n g u l a r i n t e r f a c e J-J t h e y have had t o be made w i t h cores o f d i f f e r e n t form b u t t h e same p o l a r i t y . The d o t t e d l i n e s r u n n i n g appro- x i m a t e l y v e r t i c a l l y show t h e e x t r a h a l f p l a n e s and t h e d i s t o r t i o n s i n t h e cores o f t h e m i s f i t d i s l o c a - t i o n s . One d i s l o c a t i o n w i t h l i n e d i r e c t i o n [ l l O q shown i n end-on view i n f i g u r e 8 ( a ) has t h e form o f c o r e suggested b y Abrahams and Ekstrom / 3 7 , 2 6 / . I n t h i s t h e two d a n g l i n g bonds p e r i n t e r a t o m i c d i s - tance along t h e d i s l o c a t i o n l i n e a r e connected t o two d i f f e r e n t atoms o f t h e same t y p e ( w h i t e i n t h e model, i .e. AS say).. The d i s l o c a t i o n r u n n i n g i n t h e

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L l i 0 I d i r e c t i o n normal t o t h e p l a n e of f i g u r e 8 ( b ) has one o f t h e forms o f core suggested by H o r n s t r a /38/. I n t h i s d i s l o c a t i o n b o t h d a n g l i n g bonds a r e connected t o a s i n g l e atom. T h i s i s a g a i n w h i t e so b o t h d i s l o c a t i o n s a r e 6-type, say, i n t h i s model.

However suppose t h a t t h e row o f d o u b l y bonded atoms a l o n g t h e

E l i 0 1

d i s l o c a t i o n d i f f u s e away so t h a t b o t h d i s l o c a t i o n s have t h e same c o r e form. The two s i m i l a r - c o r e - f o r m d i s l o c a t i o n s have d a n g l i n g bonds f r o m d i f f e r e n t t y p e s o f atom, i . e . t h e y a r e o f oppo- s i t e , a and B p o l a r i t y . Moreover t h e i r l i n e s pass above and below each o t h e r so t h e h e t e r o j u n c t i o n i n - t e r f a c e i s v i c i n a l , n o t s i n g u l a r as i n t h e model of f i g u r e 8. The same c o n c l u s i o n r e s u l t s i f b o t h d i s l o - c a t i o n s have t h e H o r n s t r a core-form. Thus t h e expe- r i m e n t a l evidence t h a t t h e two s e t s o f d i s l o c a t i o n s a r e d i f f e r e n t i n p h y s i c a l and chemical p r o p e r t i e s /26,27/ and t h u s presumably o f d i f f e r e n t p o l a r i t y i s evidence t h a t t h e y a r e o f t h e same c o r e f o r m ( a l - though we do n o t know which) and t h a t t h e i n t e r f a c e i s v i c i n a l and n o n - p o l a r . T h a t i s t h e r e a r e equal numbers o f As and Ga d a n g l i n g bonds i n t h e h e t e r o - j u n c t i o n m i n i m i z i n g presumably b o t h i t s energy and i t s e l e c t r i c a l and o p t i c a l e f f e c t s .

A number o f o t h e r ~ 1 1 0 1 - [ 1 i 0 ~ d i f f e r e n c e s i n t h e b e h a v i o u r o f f100) s u r f a c e s and i n t e r f a c e s have been r e p o r t e d m a i n l y b y Abrahams and h i s co- workers. These i n c l u d e assymmetries i n e t c h p i t sha- pes /39-4U/, u n i d i r e c t i o n a l c r a c k i n g /39/ and saddle shaped e l a s t i c b e n d i n g /41/. The c r a c k i n g and ben- d i n g occured i n h e t e r o s t r u c t u r e s o f t e r n a r y 111-V a l l o y s on GaAs and i t was suggested t h a t b o t h c o u l d be understood i n terms o f p o l a r d i f f e r e n c e s between t h e d i s l o c a t i o n s r u n n i n g i n [110] and [ITCJ] d i - r e c t i o n s /39-42/. There i s evidence t h a t one t y p e o f m i s f i t d i s l o c a c i o r ~ i s i n t r o d u c e d e a r l i e r and more e a s i l y t h a n t h e o t h e r /43,44/ b u t t h e mechanisms f o r t h e s e e f f e c t s a r e n o t c l e a r .

The t h e o r y o f Frank and Van d e r Merwe /28, 30- 32/ showed t h a t i t would be ( e l a s t i c a l l y ) e n e r g e t i - c a l l y f a v o u r a b l e t o i n t r o d u c e m i s f i t d i s l o c a t i o n s i n t o e p i t a x i a l i n t e r f a c e s a f t e r t h e e p i t a x i a l l a y e r exceeded a c r i t i c a l t h i c k n e s s . N e i t h e r t h i s t h e o r y n o r t h e s i m p l e geometry o f t h e b a l l -an;-wi r e models suggested how t h e m i s f i t d i s l o o a t i o n s would be i n t r o - duced. I n t h e case of t h e e p i t a x i a l growth o f one 111-V m a t e r i a l on a n o t h e r by chemical vapour deposi- t i o n subsequent TEM s t u d i e s /45/ showed t h a t t h i s was done b y t h e bending o v e r o f t h r e a d i n g d i s l o c a -

t i o n s as shown f o r t h e case o f a number o f i n t e r f a c e s i n a graded h e t e r o j u n c t i o n i n f i g u r e 9.

Fig. 9 : Formation o f m i s f i t d i s l o c a t i o n networks from t h r e a d i n g d i s l o c a t i o n s i n graded h e t e r o j u n c t i o n s . ( a ) The t h r e a d i n g d i s l o c a t i o n s have a c o n s t a n t den- s i t y pI and bend i n and o u t o f s u c c e s s i v e m i s f i t planes. ( b ) Schematic r e p r e s e n t a t i o n o f a segmented network o f m i s f i t d i s l o c a t i o n s produced i n an i n t e r - f a c e p l a n e b y t h e mechanism r e p r e s e n t e d i n (a) ( A f t e r Abrahams e t a l . / 4 5 / ) .

I n consequence t h e m i s f i t i m p 1 i e s . n o t o n l y a c e r t a i n d e n s i t y o f m i s f i t d i s l o c a t i o n s i n t h e i n t e r f a c e b u t an o f t e n l a r g e d e n s i t y o f i n c l i n e d o r t h r e a d i n g d i s - l o c a t i o n s pI w h i c h propagate t h r o u g h subsequent 1 ayers,

The m a j o r advance i n semiconductor i n j e c t i o n l a s e r s was t h e i n t r o d u c t i o n o f d o u b l e h e t e r o s t r u c t u r e (DH) l a s e r s w i t h a t l e a s t two h e t e r o j u n c t i o n s . The p r i n c i p l e i s t o grow mu1 t i l a y e r e p i t a x i a l s t r u c t u r e s o f GaxAl -xAs and GaAs. By c a r e f u l c h o i c e o f t h e va- l u e s o f x i n t h e v a r i o u s l a y e r s , a t h r e e - l a y e r 'san- dwich' i s produced. The c e n t r a l l a y e r i s t h e " a c t i v e "

one i n w h i c h h o l e - e l e c t r o n r e c o m b i n a t i o n produces l a s e r l i g h t . The l a y e r s on e i t h e r s.ide a c t , under f o r w a r d b i a s , t o i n j e c t h o l e s and e l e c t r o n s i n t o t h e a c t i v e l a y e r . The d e s i g n e r s a r t l i e s i n optimum c h o i c e o f t h e c o m ~ o s i t i o n s o f t h e t h r e e l a y e r s t o o b t a i n s i m u l t a n e o u s l y two k i n d s o f " c l o s e confine- ment" i n t h e a c t i v e l a y e r . F i r s t l y t h e changes i n band gap a r e so arranged t h a t d i s c o n t i n u i t i e s i n t h e valence and c o n d u c t i o n band edges o c c u r on t h e s i d e s o f t h e a c t i v e l a y e r away f r o m t h o s e a t w h i c h h o l e s and e l e c t r o n s r e s p e c t i v e l y a r e i n j e c t e d . These d i s -

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C6- 196 JOURNAL DE PHYSIQUE

c o n t i n u i t i e s a c t as b a r r i e r s and c o n f i n e t h e c a r r i e r s t o t h e a c t i v e l ~ y e r . Simultaneously t h e changes i n d i e l e c t r i c constant a t each s i d e o f t h e a c t i v e l a y e r a r e arranged t o c o n f i n e t h e l i g h t t o t h e a c t i v e layer.

By t h i s means t h e l i f e o f room temperature CW (con- t i n u o u s wave i . e . c o n t i n u o u s l y operating) l a s e r s have been increased from hundreds o r thousands o f hours t o several years. T h i s r e s u l t s p a r t l y from l a r g e reductions i n t h e t h r e s h o l d c u r r e n t and conse- q u e n t l y i n h e a t i n g and o t h e r h i g h power e f f e c t s . From o u r p o i n t o f view, t h e important f a c t i s tkt i t was recognised throughout t h a t i t was essentiay t o s e l e c t the m a t e r i a l s f o r a p p r o p r i a t e band gaps w h i l e ensuring approximately equal l a t t i c e parameters i n o r d e r t o minimize m i s f i t d i s l o c a t i o n d e n s i t i e s . This was done by means o f p l o t s l i k e t h a t o f f i g u r e 10.

G O S b A L S b

i n A s 6 0

-

5 . 8 -

5 . 6 -

5 . 4 -

,

^t ^I ¹

o a5 1.0 1.5 2.0 2.5

Fig. 10 : L a t t i c e constant versus band gap energy f o r a number o f 111-V compounds and t e r n a r y and quaterna- r y a l l o y s i n t e r m e d i a t e between them. L a t t i c e matching (zero m i s f i t ) combinations a r e found by drawing h o r i - z o n t a l 1 ines. ( A f t e r Kressel /42/).

M i s f i t d i s l o c a t i o n m i n i m i z a t i o n i s n o t t h e on- l y important f e a t u r e o f the m a t e r i a l s technology. Im- p r o v i n g c o n t r o l o f e p i t a x i a l growth t o produce plane, p a r a l l e l , closely-spaced h e t e r o j u n c t i o n s w i t h prede- termined abrupt composition changes and h i g h p u r i t y c o n t r o l l e d doping and s t o i c h i o m e t r y i s a1 so v i t a l However, t h e d i s l o c a t i o n t h e o r y i n p u t by Abrahams, Kressel and t h e i r colleagues was a s i g n i f i c a . n t i n - g r e d i e n t of the s c i e n t i f i c thought u n d e r l y i n g such triumphs of m a t e r i a l s technology as t h i s development o f DH l a s e r s and t h e growth o f Esaki s u p e r l a t t i c e s /46,47/.

The aim o f minimizing m i s f i t d i s l o c a t i o n s was adopted t o a v o i d t r a p p i n g o r recombination losses.

I t has been most successful i n extending device l i v e s t o u s e f u l l e v e l s f o r o p t i c a l ( f i b r e ) communications n o t o n l y by reducing t h r e s h o l d c u r r e n t s as planned, b u t , as an unexpected bonus, by m i n i m i z i n g t h e s i t e s f o r an i m p o r t a n t f a i l u r e mechanism. A r a p i d (minutes t o hours) f a i l u r e mechanism was found t o i n v o l v e t h e development o f macroscopic, i . e . light-microscope- r e s o l v a b l e "dark l i n e d e f e c t s " (DLDs) which grew through the l a s e r and quenched l a s e r emission /48/.

Transmission e l e c t r o n microscope s t u d i e s /49,50/

showed t h e DLDs t o c o i n c i d e w i t h dense arrays o f i r - r e g u l a r d i s l o c a t i o n d i p o l e s and loops as shown i n f i g u r e 11.

Fig. 11 : A montage o f b r i g h t - f i e l d TEM micrographs showing p a r t o f t h e d i s l o c a t i o n a r r a y i n / dark l i n e d e f e c t i n d i c a t e d a t A on t h e i n f r a r e d micrograph o f t h e l a s e r . ( A f t e r P e t r o f f and Hartman 1501).

These were shown t o o r i g i n a t e i n most cases from t h r e a d i n g d i s l o c a t i o n s as shown i n f i g u r e 12. The mechanism o f growth o f DLDs i s shown schematically

i n f i g u r e 13. L a t e r work e s t a b l i s h e d t h e f a c t t h a t t h e d i p o l e s were o f i n t e r s t i t i a l character /51/ and t h a t they form and grow o n l y i n t h e a c t i v e l a y e r o f t h e l a s e r , w i t h p o s s i b l y a l i t t l e o v e r l a p i n t o an adjacent c o n f i n i n g l a y e r .

The r a p i d climb i n v o l v e d i s promoted by a r e - combination-excited v i b r a t i o n a l s t a t e o f p o i n t de f e c t s which i t i s b e l i e v e d enhances t h e i r m o b i l i t y . This i s known as the phononkick mechanism 1521. Some workers b e l i e v e t h a t t h e p o i n t defects are a l s o ge- nerated by t h i s mechanism, vacancies being e m i t t e d from t h e t h r e a d i n g d i s l o c a t i o n s (P.S. Dobson, p r i - vate communication). Others b e l i e v e t h 6 t h i g h den- s i t i e s o f p o i n t d e f e c t s a r e i n i t i a l l y present due t o

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D . B . Holt C6-197

n o n - s t o i c h i o m e t r y .

F i g . 13 : Schematic diagrams showing successive s t a - ges i n t h e growth by c l i m b f r o m a t h r e a d i n g d i s l a c a - t i o n o f t h e d i o o l e s and loops c o n s t i t u t i n g a dark l i n e d e f e c t i n a double h e t e r o s t r u c t u r e l a s e r . ( A f t e r P e t r o f f and Hartman 1501). I t i s now q e n e r a l l y b e l i e - ved t h a t DLDs i n f a c t form and grow o n l y i n t h e cen- L t r a l a c t i v e l a y e r (pGaAs i n t h i s case) o f these l a -

sers.

F i g . 12 : A b r i g h t - f i e l d TEM micrograph showing t h e t h r e a d i n g d i s l o c a t i o n , marked D, a t t h e o r i g i n o f a dark l i n e d e f e c t a r r a y o f d i s l o c a t i o n s . ( A f t e r P e t r o f f and Hartman 1501).

T h i s phenomenon o f r a p i d c l i m b i n t h e presence o f i n t e n s e recombination and r a d i a t i o n remains t h e sub- j e c t o f much research. A d e f e c t s i m i l a r t o t h e DLD has r e c e n t l y been found t o o c c u r i n degraded green- e m i t t i n g Gap LEDs ( l i g h t e m i t t i n g d i o d e s ) .

1531.

4. Conclusions.- I n d i v i d u a l d i s l o c a t i o n s a r e n o t im- p o r t a n t i n S i devices. Only t h e d i s l o c a t i o n d e n s i t y i s o f some r e s i d u a l i n t e r e s t as an i n d e x o f q u a l i t y . If i t r i s e s t o o high, t h e technology needs d e t a i l e d improvements o f simple, w e l l undecstood k i n d s . D i s l o c a t i o n - r e l a t e d problems re-emerge w i t h new types o f device, e.g. CCD (charge-coupled d e v i c e s ) o r techno- l o g i e s , e. g. i o n imp1 a n t a t i o n . However, i n general t h e knowledge e x i s t s t o s o l v e them and l i t t l e t h a t i s f u n d a m e n t a l l y new i s learned.

I n t h e newer o p t o e l e c t r o n i c devices made o f compound m a t e r i a l s such as GaAs l a s e r s , much fundamental r e s e a r c h remains t o be done and q u i t e new as- p e c t s o f t h e r o l e o f d i s l o c a t ~ i o n s i n d e v i c e s a r e s t i l l t o be found. I t i s here t h a t fundamental and a p p l i e d r e s e a r c h s t i 11 come t o g e t h e r .

Acknowledgements.- Thanks a r e due t o Drs. Y.S. Abra- hams, J. Blank, C.J. B u i o c c h i , R.L. Hartman, M.J.

H i l l , H. K r e s s e l , R. Ogden, P. P e t r o f f and L.R.

Ueisberg and t h e E d i t o r s o f t h e J o u r n a l o f A p p l i e d Physics, J o u r n a l o f E l e c t r o n i c M a t e r i a l s , J o u r n a l of M a t e r i a l s Science and Journal o f S o l i d S t a t e E l e c - t r o n i c s f o r p e r m i s s i o n t o reproduce v a r i o u s f i g u r e s here, as n o t e d i n t h e c a p t i o n s .

Note added i n P r o o f

The m i s f i t d i s l o c a t i o n networks d e a l t w i t h above a r e t h o s e t h a t a r i s e by c o n s t r u c t i n g b a l l - a n d - w i r e models o f e p i t a x i a l i n t e r f a c e s f o r para1 l e l e p i t a x i a l alignements. These d i s l o c a t i o n s a r e p u r e edges w i t h t h e i r Burgers v e c t o r s i n t h e i n t e r f a c e , i . e . t h e y a r e what a r e sometimes c a l l e d " e f f i c i e n t " o r "pure" mis- f i t d i s l o c a t i o n s /a/.

The mechanism o f t h e bending o v e r o f t h r e a d i n g d i s l o c a t i o n s o f f i g u r e 9 can i t t r o d u c e d i s l o c a t i o n s o f any Burgers v e c t o r . Yany o f these w i l l have o n l y a component o f t h e i r Burgers v e c t o r i n t h e i n t e r f a c e

?lane. These a r e t h e " i n e f f i c i e n t " of "impure" mis- f i t d i s l o c a t i o n s .

TEM s t u d i e s o f m i s f i t d i s l o c a t i o n s i n h e t e r o j u n c - t i o n s between 111-V compounds have r e p o r t e d most, i f

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C6- 198 JOURNAL DE PHYSIQUE

n o t a l l , t h e . n i s f i t d i s l o c a t i o n s t o be o f t h e " i n e f - f i c i e n t ' type. On t h e o t h e r hand t h e m i s f i t d i s l o c a - t i o n s i n Ge/Si (111) i n t e r f a c e s were a l l o f t h e " e f - f i c i e n t " types p r e d i c t e d b y t h e b a l l -and-wi r e models /b,c,d,/ except t h a t t h e nodes o f t h e networks were d i s s o c i a t e d . I t i s n o t known what determines which t y p e o c c u r a l t h o u g h t h i s i s c l e a r l y a r e s u l t o f t h e ,nechanisms by w h i c h t h e y a r e i n t r o d u c e d d u r i n g growth.

/a/ Van d e r Yervie, J.H., C.R.C. C r i t . Revs. S o l . S t a t e and Mat. S c i . , - 7 (1978) 209-231.

/b/ C u l l i s , A.S. and Booker, G.R., Proc. 2 5 t h Anniv.

.'4eet. EtIAG, Cambridge, ( I n s t . Phys: London and B r i s t o l ) 1971, p.320.

/ c / C u l l i s , A.G. and Booker, 3 . R.

,

Proc. 5 t h Eur.

Conf. E l e c t r o n 'rlicrosc.

,

Yanchester ( I n s t . Phys:

L.ondon and B r i s t o l ) 1972, p. 532.

/d/ Cul l i s , A.G., J . Y i c r o s c . - 98 (1973) 191-195.

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