DETERMINATION OF SLIP PLANES IN CdxHg(1-x)Te BY ETCHING OF DISLOCATIONS INTRODUCED BY MICROHARDNESS INDENTATIONS

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Submitted on 1 Jan 1979

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DETERMINATION OF SLIP PLANES IN

CdxHg(1-x)Te BY ETCHING OF DISLOCATIONS INTRODUCED BY MICROHARDNESS

INDENTATIONS

M. Brown, A. Willoughby

To cite this version:

M. Brown, A. Willoughby. DETERMINATION OF SLIP PLANES IN CdxHg(1-x)Te BY ETCH-

ING OF DISLOCATIONS INTRODUCED BY MICROHARDNESS INDENTATIONS. Journal de

Physique Colloques, 1979, 40 (C6), pp.C6-151-C6-155. �10.1051/jphyscol:1979631�. �jpa-00219047�

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DETERMINATION OF S L I P PLANES I N CdxHg(l-x) Te BY ETCHING OF DISLOCATIONS INTRODUCED BY MICROHARDNESS INDENTATIONS

M. Brown and A.F.W. Willoughby

Engineering Materials Laboratories, The U n i v e r s i t y , Shouthampton, U. K. SO9 5NH

Rt?sumG.- L'importance du r67e du CdxHg Te (CMT), dans des detecteurs i n f r a - r o u g e s a s t i m u l @ l t i n t e r 6 t p o r t e aux p r o p r i e t 6 s mecaniqb&ixkt e l e c t r i q u e s de c e t t e substance. Tandis que l ' o n a d e j i i f a i t de longues etudes au s u j e t des p r o p r i e t e s e l e c t r i q u e s de c e t t e substance, on n ' a p u b l i e j u s - q u ' i c i que peu de renseignements s u r ces p r o p r i @ t & s mecaniques e t , en p a r t i c u l i e r , sur l e systeme des g l issements e t l e s p r o p r i e t e s des d i s l o c a t i o n s .

La p l u p a r t des attaques chimiques f a i t e s pour demontrer l a f i g u r e des d i s l o c a t i o n s du CMT ne r @ v P l e n t que des p u i t s d ' a t t a q u e sur des plans i n c l i n e s

a

moins de 20° de l ' o r i e n t a t i o n C1111. On a d e c r i t pour l e HgTe un d l a n g e chimique q u i l a i s s e v o i r des d i s l o c a t i o n s s u r t o u t e s l e s o r i e n t a t i o n s , e t nous avons demontre dans nos l a b o r a t o i r e s que ce melange e s t 6galement e f f i c a c e pour l e CMT sur l a p l u p a r t des plans ayant des i n d i c e s peu & l e v & s .

Pour e t u d i e r des roses de d i s l o c a t i o n i n d u i t e s

a

des temperatures vers 20°C par l ' i n d e n t a t i o n de micro-durete Vickers, on s ' e s t s e r v i du melange mentionne ci-dessus s u r deux faces diversement d i s - posees. On a conclu que l e s plans de glissement a c t i f s sont (111). On d e c r i t ces recherches, en l e s comparant aux recherches deja cbnnues sur l e CdTe.

Abstract.

-

The importance o f Cd H Te (CMT) f o r i n t r a - r e d d e t e c t o r a p p l i c a t i o n s has s t i m u l a t e d i n t e r e s t i n t h e mechanical and :l:&~if&al p r o p e r t i e s o f t h i s m a t e r i a l

.

Whereas considerable e f f o r t has been employed i n t h e study 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 i s m a t e r i a l , v e r y l i t t l e informa- t i o n has been published on t h e mechanical p r o p e r t i e s and, i n p a r t i c u l a r , the s l i p system and d i s l o - c a t i o n p r o p e r t i e s .

Nost r e p o r t e d d i s l o c a t i o n etches f o r CMT o n l y r e v e a l d i s l o c a t i o n e t c h - p i t s w i t h i n about 20" o f t h e I1111 o r i e n t a t i o n . A d i s l o c a t i o n etchant which r e v e a l s d i s l o c a t i o n s on a l l o r i e n t a t i o n s has been r e - p o r t e d f o r HgTe, and i t has been shown i n our l a b o r a t o r i e s t h a t t h i s i s e f f e c t i v e a l s o on most low index planes o f CMT.

D i s l o c a t i o n r o s e t t e s i n t r o d u c e d a t room temperature by Vickers microhardness i n d e n t a t i o n have been s t u d i e d u s i n g the above e t c h a n t on two d i f f e r e n t l y o r i e n t e d faces. I t i s concluded t h a t the o p e r a t i v e s l i p planes are (111). These s t u d i e s are discussed and compared w i t h r e p o r t e d work on CdTe.

1. I n t r o d u c t i o n . - The importance o f cadmium mercury t e l l u r i d e f o r i n f r a - r e d d e t e c t o r a p p l i c a t i o n s has shown the need f o r basic m a t e r i a l s c h a r a c t e r i s a t i o n t o p r o v i d e a b a s i s f o r f u r t h e r development o f devi- ces. One area where understanding i s g r e a t l y needed i s i n the f i e l d o f the mechanical p r o p e r t i e s o f t h e m a t e r i a l , and t h e p r o p e r t i e s , both dynamic and e l e c - t r o n i c , o f d i s l o c a t i o n s produced by p l a s t i c d e f o r - mation. While such s t u d i e s i n t e r n a r y a l l o y s may prove t o have s p e c i a l d i f f i c u l t i e s , they o f f e r t h e p o s s i b i l i t y o f studying e f f e c t s such as v a r y i n g t h e band gap (which i s a l t e r e d by v a r y i n g t h e Hg t o Cd r g t i o ) on 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 d i s l o c a t i o n s .

Previous work on t h i s m a t e r i a l i s very l i m i t e d , and l i t t l e i n f o r m a t i o n i s a v a i l a b l e a l s o on the pa- r e n t b i n a r y , HgTe. I n t h e o t h e r parent b i n a r y compound, CdTe, some o f the main c r y s t a l l o g r a p h i c fea- t u r e s o f s l i p and cleavage have been studied. W o l f f and Broder /1/ s t u d i e d cleavage i n s i n g l e c r y s t a l s o f CdTe and observed predominant m i c r o c l eavage a1 ong (1101 planes, w i t h cleavage a l s o along C100) planes,

and some evidence o f s l i p on the l a t t e r planes, W o l f f and Broder /1/ concluded t h a t , o f the mate- r i a l s w i t h s p h a l e r i t e s t r u c t u r e , CdTe, ZnSe and GaAs, CdTe i s t h e most i o n i c on the P h i l l i p s ' /2/ s c a l e and a l s o has the l o w e s t y i e l d s t r e n g t h . They a l s o de- t e c t e d an i n f l u e n c e o f m a t e r i a l r e s i s t i v i t y on cleavage plane, changing t o {I113 a t low r e s i s t i v i t y . Inoue e t a l . / 3 , 4 / showed, by p l a s t i c bending about a <110> bend a x i s and r e v e a l i n g d i s l o c a t i o n s by etching, t h a t t h e main s l i p planes a t 600°C were (1111. They used Warekois e t a1 . I s /5/ X-ray c a l i - b r a t i o n o f t h e p o l a r i t y o f etching, t o make conclu- sions about t h e e t c h i n g o f a and B 60" d i s l o c a t i o n s (Hol t / 6 / ) .

P a r t i c u l a r i n t e r e s t i n CdTe has f o l l o w e d t h e observation by Carlsson and A h l q u i s t /7/ and Buch and A h l q u i s t /8/ t h a t t h e y i e l d s t r e s s appears t o be a f f e c t e d both by t h e e l e c t r o n concentration and by i l l u m i n a t i o n

-

t h e l a t t e r phenomenon has been termed p h o t o - p l a s t i c i t y . Measurements o f y i e l d s t r e s s i n compression a t room temperature show t h a t t h e y i e l d

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979631

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C6-152 JOURNAL DE PHYSIQUE

s t r e s s i s a minimum (both i n t h e dark and w i t h l i g h t ) f o r h i g h r e s i s t i v i t y m a t e r i a l , and increases w i t h i n c r e a s i n g e l e c t r o n o r h o l e concentration. The e f f e c t i s n o t a simple one, however, f o r t h e c a r r i e r concentration was v a r i e d by previous annealing t o a l t e r t h e degree o f non-stoichiometry, and hence a number o f v a r i a b l e s were changing. I n a d d i t i o n , t h e y i e l d s t r e s s o f low r e s i s t i v i t y n- and p- type sam- p l e s c o u l d be increased by as much as 70% by i l l u - m i n a t i o n w i t h l i g h t o f photon energy corresponding t o the band gap energy. Swaminathan e t a l . /9/ found s i m i l a r t r e n d s using microhardness on indium-doped m a t e r i a l . They attempted t o e x p l a i n t h e e f f e c t s i n terms o f the i n t e r a c t i o n o f d i s l o c a t i o n s w i t h p o i n t defects, on t h e b a s i s o f assumed e l e c t r o n i c a c t i v i t y o f the d i s l o c a t i o n s ; b u t t h e r e i s c l e a r l y need f o r f u r t h e r work i n t h i s area.

By c o n t r a s t , v e r y 1 i t t l e i n f o r m a t i o n i s a v a i - l a b l e on t h e mechanical p r o p e r t i e s o f t h e o t h e r b i - n a r y compound, HgTe, o r the t e r n a r y CdxHg(l -x)Te.

There have been a number o f s t u d i e s o f t h e e l a s t i c behaviour o f HgTe (e.g. Cottam and Saunders / l o / ) , b u t very 1 i t t l e i s known about p l a s t i c behaviour o r the p r o p e r t i e s o f d i s l o c a t i o n s i n t h e m a t e r i a l . Ku- r i l o , S p i t o v s k i and Schneider /11/ c a r r i e d o u t com- p a r a t i v e microhardness determinations on CdTe, HgTe and C d o . ~ o H g o . ~ o T e over t h e temperature range -150°C t o +300°C and found t h a t the d i f f e r e n c e s i n hardness were most pronounced a t the higher temperatures and i n d i c a t e d t h a t t h e t e r n a r y compound was t h e hardest, w h i l e HgTe was t h e s o f t e s t . K u r i l o /12/ has a l s o r e p o r t e d t h a t t h e presence o f benzene, toluene and water decreases t h e measured microhardness o f HgTe and t h e t e r n a r y compound, b u t has no e f f e c t on CdTe.

I n view o f t h e absence o f data on t h e basic d i s l o c a t i o n p r o p e r t i e s o f CdxHg(,-,)Te, t h e present paper r e p o r t s s t u d i e s o f the r e v e l a t i o n o f d i s l o c a - t i o n s by chemical etching, and determination o f the a c t i v e s l i p planes by e t c h i n g d i s l o c a t i o n r o s e t t e s on two d i f f e r e n t l y o r i e n t e d surfaces.

2. D i s l o c a t i o n e t c h i n g i n Cd&(,-,)E.- Chemical e t c h i n g techniques have been w i d e l y used f o r revea- l i n g d i s l o c a t i o n s i n elemental /13,14/ and compound /3,4,5,15,16,17/ semiconductors. I n CdxHg(,-,]Te, however, t h e r e have been few r e p o r t s concerning d i s - l o c a t i o n s and s u b - s t r u c t u r e r e v e a l i n g etches. Parker and P i n n e l l /18/ r e p o r t e d an e t c h f o r CdxHg(l-x)Te which claimed t o reveal d i s l o c a t i o n e t c h p i t s on o r i e n t a t i o n s up t o 30' o f f the {ill}, b u t t h e grea- t e r t h e d e v i a t i o n from t h e 11111 the l e s s r e g u l a r was t h e e t c h p i t shape. For a sample whose two

e t c h i n g faces were p a r a l l e l t o I1111 planes, one f a c e showed w e l l d e f i n e d t r i a n g u l a r e t c h p i t s , whereas on t h e reverse face, o n l y an i r r e g u l a r p a t t e r n was v i s i b l e . Parker and P i n n e l l )18/ found t h a t t h e Cd content o f t h e CdxHg(,-,)Te a f f e c t e d t h e e t c h i n g time necessary t o produce e t c h p i t s as w e l l as the e t c h p i t d e f i n i t i o n . Straughan ( p r i v a t e communica- t i o n ) r e p o r t e d a s i m i l a r e t c h which revealed e t c h p i t s on one { I l l ) f a c e b u t n o t on t h e p a r a l l e l r e v e r - se face o f a CdxHg(,-,)Te s l i c e and t h i s e t c h produced p i t s on faces w i t h i n approximately 20' o f t h e { I l l } .

An

e t c h c o n t a i n i n g f e r r i c c h l o r i d e s o l u t i o n and n i t r i c a c i d (Quelch, p r i v a t e communication) was a l s o p o l a r i n nature and e t c h p i t s were produced on faces w i t h i n about 10" o f the {ill}. P o l i s a r e t a l . /19/ developed two etches f o r HgTe both o f which were p o l a r ; the f i r s t etch, " P o l i s a r Etch I", revea- l e d d i s l o c a t i o n e t c h p i t s on planes up t o about 20' from {ill), and t h e second etch, " P o l i s a r Etch 2", was claimed t o have produced d i s l o c a t i o n e t c h p i t s on a l l low index planes. P o l i s a r e t a1

.

/19/ s t a t e d t h a t t h e i r etches revealed e t c h p i t s on t h e H g { l l l } planes b u t n o t on t h e T e ( l l l 1 planes, a1 though i t was n o t c l e a r how these data were determined. Figure I shows t h e shape o f e t c h p i t s produced a f t e r e t c h i n g HgTe c r y s t a l s w i t h P o l i s a r Etches 1 and 2 /19/.

(The corner o f the t r i a n g l e near (1111 which i s li- m i t e d by the dashed l i n e shows o r i e n t a t i o n s o f p l a - nes on which e t c h p i t s a r e revealed by P o l i s a r Etch 1; t h e P o l i s a r Etch 2 r e v e a l s e t c h p i t s on =low index planes). P o l i s a r Etch 2 /19/ revealed e t c h p i t s i n HgTe on I1111 planes i n 20-30 seconds, on I110) planes i n 60-80 seconds and on { I 0 0 1 planes i n about 3 minutes.

F i g . 1 : Etch p i t shapes revealed by P o l i s a r etches 1 and 2 on planes 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 i c o r i e n - t a t i o n s i n HgTe s i n g l e c r y s t a l s . Hatched f a c e t s o f p i t s are (1111 planes and unhatched f a c e t s a r e t1001 planes. The corner o f t h e t r i a n g l e near {Ill} which i s l i m i t e d by the dashed l i n e corresponds t o t h e o r i e n - t a t i o n o f planes on which d i s l o c a t i o n e t c h p i t s a r e revealed b y P o l i s a r Etch 1. P o l i s a r Etch 2 r e v e a l s e t - ch p i t s on

all

low index planes.

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I t i s shown i n f i g u r e 1 t h a t t h e l i m i t i n g f a c e t s i n t h e e t c h p i t s a r e {1001 planes f o r e t c h i n g faces near the I1113 o r i e n t a t i o n (unhatched f a c e t s ) , { I l l } planes f o r e t c h i n g faces near 11001 o r i e n t a t i o n (hatched f a c e t s ) and a m i x t u r e o f C l O O } and (111) planes f o r o t h e r e t c h i n g face planes. As t h e r e was no e t c h r e p o r t e d which would reveal d i s l o c a t i o n e t c h p i t s on

fi

low index planes o f CdxHg( ,-,)Te and as the P o l i s a r Etch 2 /19/ appeared t o work so w e l l f o r HgTe, an attempt was made t o use t h i s e t c h f o r CdXHg(,-,)Te (where x = 0.2). The attempt proved successful and on a l l low index planes i n v e s t i g a t e d , a p a r t from the p o l a r { I l l ) planes, e t c h p i t s o f s i m i l a r shape t o those shown i n f i g u r e 1 f o r HgTe were revealed on CdxHg Te and a l s o i t was found

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t h a t s i m i l a r e t c h times were r e q u i r e d t o produce e t c h p i t s i n CdxHg Te as i n HgTe. Table I gives

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d e t a i l s o f t h e c o n s t i t u e n t s o f the above mentioned etches.

ces were then etched i n P o l i s a r Etch 2 f o r a p p r o p r i a t e times, t o reveal d i s l o c a t i o n e t c h p i t s . Each s l i c e was c a r e f u l l y a l i g n e d and attached w i t h adhesive t o a metal p l a t e w i t h a reFerence g r i d s c r i b e d on i t , so t h a t r e f e r e n c e d i r e c t i o n s on o p t i c a l micrographs could be c o r r e l a t e d w i t h t h e same reference d i r e c - t i o n s on Laue back r e f l e c t i o n X-ray photographs.

From t h e o p t i c a l and X-ray data i t was p o s s i b l e t o determine t h e s l i p t r a c e d i r e c t i o n s and thus t h e s l i p planes on t h e indented Cdo.2Hgo.eTe surfaces.

4. Results.- The Vickers Hardness Number (VHN) appea- r e d t o be independent o f i n d e n t a t i o n surface o r i e n - t a t i o n f o r t h e samples s t u d i e d i n t h i s work and t h e VHN was found t o be equal t o 43+2.

5. S l i p systems i n Cdx&t,-xl~.- S l i p planes i n semiconductors w i t h t h e diamond and s p h a l e r i t e s t r u c - t u r e s are g e n e r a l l y o f t h e C1113 f a m i l y /20/ b u t o t h e r planes, however, (most l i k e l y I1001 by analogy w i t h face centred cubic metals /2l/jmay become a c t i - ve a t h i g h e r temperatures.

Table I : D i s l o c a t i o n etches used f o r CdxHg(,-x)Te and HgTe

1 E t c omposition

Source j C ~ j ~~ o n c . : ~~ ~o n c~ . : : ~~ ~Other d~ ~ ~ i ~ e m p e r a t u r e :E t c h i n g t i m e r e q u i r e d

1 $i::t:::F:i::

of e t c h : _I _HNOl _: _HC1 :

:

of e t c h j t o produce e t c h p i t s , can be revealed

Parker e t a l .

i

CdxHg(l-x)Te

I

^60cc j ^4cc 60cc j4cc o f 5% s o l u t i o n j ~ o o m 1 5-10 minutes : Up t o about 30'

/ l a / :of Br, i n Methanol :Temperature : : o f f { I l l )

Straughan

i

CdxHg(,-x)Te

i

^60cc j ^30cc

1

^180cc

: :

^60°C

i

2 minutes : ' U p t o about 20'

( p r i v a t e : o f f (1111

conununication):

P o l i s a r E t c h 1

i

^HgTe : ^60cc I 60cc : ^60cc : 0 . 9 c c B r 2 : ^55°C j 8 seconds

:

Up t o about 20"

1191 : o f f { I l l )

1 , - , I

P o l i s a r E t c h 2

:

^HgTe I 60cc

i

^25cc

i

^90cc i 5 c c A c e t i c A c i d :Room :I111) planes, 20-30

i

^{A l l}

/19/ : ( b u t can be used I .O.lcc BP? :Temperature

:

seconds

:

s u c c e s s f u l l y f o r

:

:{110} planes, 60-80 j

1

CdxHg(,-xlTe) j ; seconds

:{100) planes, approx

:

3 minutes

i

a

- I , ,

Q u e l c h

i

CdxHg(,-x)Te :few ; - : - :30cc o f 1/10 N :Room

:

10 seconds :Up t o about l o 0

( P r i v a t e

:

^drops : : f e r r i c c h l o r i d e :Temperature

:

: o f f { I 1 1 1

Communication)

i

j s o l u t i o n

Note : A l l t h e above e t c h e s l e a v e a s t a i n on t h e s u r f a c e of t h e sample; t h e s t a i n can be removed by quickly d i p p i n g t h e sample i n v e r y

I ^-

d i l u t e ( p a l e y e l l o w c o l o u r ) 8rp/Methanol s o l u t i o n and then washinq i n Methanol.

3. I n d e n t a t i o n experiments.- Cdo.2Hgo.8Te s l i c e s were prepared f o r Vickers microhardness i n d e n t a t i o n experiments i n t h e f o l l o w i n g manner : 1. l a p w i t h 6pm g r i t Sic, 2. p o l i s h e t c h i n a 5% BrJMethanol s o l u t i o n f o r 1-2 minutes a t room temperature ( t h e s l i c e was h e l d i n a quartz basket f o r e t c h i n g and was c o n s t a n t l y a g i t a t e d t o preserve the f l a t s u r f a - ce), 3. wash i n Methanol and a i r d r y . The Cdp,2Hgo,s Te s l i c e s o f known o r i e n t a t i o n , determined by t h e Laue back r e f l e c t i o n X-ray technique, were indented a t room temperature w i t h a Vickers microhardness indenter, using a 20g l o a d f o r 30 seconds. The s l i -

I t was t h e r e f o r e expected t h a t the a c t i v e s l i p planes i n CdXHg(,-,)Te would be

Ell11

and i t w i l l be shown t h a t t h i s i s i n f a c t t h e case.

F i g u r e 2 shows d i s l o c a t i o n r o s e t t e s (revealed by e t c h i n g i n P o l i s a r Etch 2 /19/)on an i n d e n t a t i o n plane c l o s e t o {Ill}; t h e exact o r i e n t a t i o n i s shown i n f i g u r e 2c. I t can be seen i n f i g u r e 2a t h a t t h e { I l l } s l i p t r a c e d i r e c t i o n s a r e i n good agreement w i t h the d i s l o c a t i o n e t c h p i t l i n e s , whereas i n f i . gure 2b, t h e {1003 t r a c e s a r e n o t a good f i t t o t h e experimental data. The evidence f o r (1111 s l i p p l a - nes was f u r t h e r borne o u t by i n d e n t a t i o n on a d i f f e -

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C6-154 JOURNAL DE PHYSIQUE

r e n t l y o r i e n t e d p l a n e , whose o r i e n t a t i o n was near (331) as shown i n f i g u r e 3e. Again i t w i l l be seen, f r o m f i g u r e s 3a and 3b t h a t t h e C111) s l i p t r a c e d i r e c t i o n s a r e c l o s e t o t h e e x p e r i m e n t a l l y d e t e r m i - ned ones, whereas t h e C1001 t r a c e s i n f i g u r e s 3c and 3d d e v i a t e c o n s i d e r a b l y f r o m t h e e x p e r i m e n t a l e t c h - p i t t r a c e s . I t seems c l e a r , t h e r e f o r e , f r o m t h e i n d e n t a t i o n experiments, t h a t t h e o p e r a t i v e s l i p p l a n e s i n Cdo.zHgo.aTe a t room temperature a r e I1111, i n common w i t h most o t h e r s p h a l e r i t e s t r u c t u r e semi- c o n d u c t o r s . I t i s w o r t h p o i n t i n g o u t f r o m observa- t i o n o f e t c h p i t s on n e a r 1111) p l a n e s i n f i g u r e 2 , t h a t t h e e t c h p i t f a c e t s a r e most p r o b a b l y (1001 planes, as r e p o r t e d by P o l i s a r e t a l . /19/ f o r HgTe.

F i g . 2 : D i s l o c a t i o n r o s e t t e s i n C d o . z H g ~ . ~ T e revea- l e d by e t c h i n g i n P o l i s a r E t c h 2, a f t e r V i c k e r s microhardness i n d e n t a t i o n a t room temperature.

a) BB, CC, DD a r e d i r e c t i o n s o f t r a c e s o f C111) planes, b ) PP, QQ, RR a r e d i r e c t i o n s o f t r a c e s o f E l 0 0 1 planes, ( t h e l i n e X X i s a r e f e r e n c e d i r e c t i o n common t o a ) and b ) ) . c ) t h e symbol "o" shows t h e d i r e c t i o n o f t h e normal t o t h e p l a n e on w h i c h t h e i n d e n t a t i o n s were c a r r i e d o u t . (Near I1111 o r i e n t a - t i o n ) .

6 . C o n c l u d i n g remarks.- Now t h a t a r e l i a b l e d i s l o - c a t i o n e t c h i n g t e c h n i q u e has been e s t a b l i s h e d f o r Cd,Hg(,-,,Te and t h e o p e r a t i v e s l i p p l a n e s have been

mena o f c o n s i d e r a b l e p o t e n t i a l i n t e r e s t i n t e r n a r y 11-VI compounds such as CdxHg(,-x)Te. I n p a r t i c u l a r , t h e e f f e c t o f v a r y i n g t h e band gap on t h e e l e c t r o n i c b e h a v i o u r o f d i s l o c a t i o n s c o u l d be s t u d i e d . Other v a r i a b l e s o f p o s s i b l e importance a r e t h e degree o f n o n - s t o i c h i o m e t r y and t h e q u e s t i o n o f whether o r n o t p h o t o p l a s t i c i t y o c c u r s i n t h e s e m a t e r i a l s .

F i g . 3 : D i s l o c a t i o n r o s e t t e s i n C ~ O , ~ H ~ O , ~ T ~ r e - v e a l e d b y e t c h i n g i n P o l i s a r E t c h 2, a f t e r V i c k e r s microhardness i n d e n t a t i o n a t room temperature.

a ) and b ) AA, BB, CC, DD, a r e d i r e c t i o n s o f t r a c e s o f (111) planes, YY i s a r e f e r e n c e d i r e c t i o n , c ) and d) EE, FF, GG a r e d i r e c t i o n s o f t r a c e s o f C100) planes, Y Y i s a r e f e r e n c e d i r e c t i o n , e ) t h e symbol

" o n shows t h e d i r e c t i o n o f t h e normal t o t h e p l a n e

on which t h e i n d e n t a t i o n s were c a r r i e d o u t . (Near (3311 o r i e n t a t i o n ) .

Acknowledgements.

-

T h i s work has been c a r r i e d o u t w i t h t h e s u p p o r t o f t h e Procurement E x e c u t i v e , Mi- n i s t r y o f Defence and sponsored by D.C.V.D. The a u t h o r s a r e g r a t e f u l t o Dr. C.J. G i l l ham who t o o k t h e X-ray Laue photographs.

determineh, a t room t e m p e r a t u r e anyway, i t s h o u l d now be p o s s i b l e t o s t u d y o t h e r p r o p e r t i e s o f d i s l o - c a t i o n s i n d e t a i l . There a r e many d i s l o c a t i o n pheno-

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