PHOTOPLASTIC EFFECT IN CdTe

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PHOTOPLASTIC EFFECT IN CdTe

E. Gutmanas, P. Haasen

To cite this version:

E. Gutmanas, P. Haasen. PHOTOPLASTIC EFFECT IN CdTe. Journal de Physique Colloques,

1979, 40 (C6), pp.C6-169-C6-172. �10.1051/jphyscol:1979634�. �jpa-00219050�

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JOURNAL DE PHYSIQUE Colloque C6, suppldment au n06, tome 40, juin 1979, page (26-169

PHOTOPLASTIC EFFECT I N CdTe

E.Y. ~utmanas" and P. ~aasen'"

'

Department of Materials Engineering, Techion-Israel I n s t i t u t e of l'e'echnology, Haifa, I s r a e l .

* *

I n s t i t u t fiir Metallphysik der Universitiit Gdttingen, F. R. G.

R6sume.- L ' i n f l u e n c e de l a d6formation p l a s t i q u e , des d i f f 6 r e n t s t r a i t e m e n t s thermiques, e t du dopage sur l e s parametres de l ' e f f e t p h o t o p l a s t i q u e (PPE) o n t St6 e t u d i e e s p o u r l e CdTe. Un modele de l ' e f f e t PPE du CdTe e s t d i s c u t 6 .

Abstract.- The i n f l u e n c e o f p l a s t i c deformation, o f v a r i o u s heat treatments and o f doping on t h e pho- t o p l a s t i c e f f e c t (PPE) i n CdTe was studied. A model o f t h e PPE i n CdTe i s discussed.

1. I n t r o d u c t i o n . - A p h o t o p l a s t i c e f f e c t (PPE) o r a and on s i n g l e c r y s t a l s prepared from CdTe, grown i n change o f f l o w s t r e s s , o, by i l l u m i n a t i o n was obser- t h e G6ttingen C r y s t a l Laboratory (p-type, pe=450cm, ved i n a number of 11-IV compounds /1-5/. U s u a l l y a up=80cm2/V.s) henceforth c a l l e d CdTe-G. Doped samples p o s i t i v e PPE o r an increase, +Aoph, by i l l u m i n a t i o n were prepared by annealing o f CdTe-G a t 780°C (20 i s observed. A negative PPE, -Auph, was observed i n hours) i n sealed q u a r t z tubes t o g e t h e r w i t h NaCl In-doped h i g h r e s i s t i v i t y CdTe p o l y c r y s t a l s /5/. powder, henceforth c a l l e d CdTe-G(C1). Other samples Recently n e g a t i v e and p o s i t i v e PPE were observed on r a p i d l y cooled i n a i r ("quenched") were before annea- t h e same CdTe samples depending on p l a s t i c s t r a i n , l e d i n sealed quartz tubes ( t o g e t h e r w i t h CdTe powder).

E, i l l u m i n a t i o n , I , and heat treatment /6/. I n mo- The mechanical measurements were conducted i n com- d e l s proposed t o e x p l a i n a p o s i t i v e PPE i t i s u s u a l - p r e s s i o n on an I n s t r o n t e s t i n g machine. The cross l y assumed t h a t t r a p p i n g o f l i g h t - i n d u c e d c a r r i e r s - head speed used was S=5xl0-~mm/min. t h a t provided a e l e c t r o n s o r holes-by charged p o i n t defects r e s u l t s s t r a i n r a t e ~ l O - ~ / s .

i n an increased i n t e r a c t i o n ( e l a s t i c o r e l e c t r i c a l P o s i t i v e , +Aoph, and negative, -Aoph, PPE were o r both) w i t h charged d i s l o c a t i o n s /1,2,7/. The measured according t o f i g u r e 1. The r a t i o o f the dependence o f t h e d i s l o c a t i o n s v e l o c i t y i n a semi- p l a s t i c s t r a i n r a t e s i n the n o n i l l u m i n a t e d

Ld

conductor on doping was explained i n /8/ by an elec- (d-darkness) and i l l u m i n a t e d

i1

( 1 - l i g h t ) s t a t e s a t t r o s t a t i c c o n t r i b u t i o n t o t h e k i n k f o r m a t i o n energy, t h e i n s t a n t o f s w i t c h i n g on o r s w i t c h i n g o f f t h e t h a t i s decreased i f t h e l i n e charge o f t h e d i s l o - l i g h t were c a l c u l a t e d measuring t h e slope changes o f c a t i o n i s increased and v i c e versa. A change o f t h e t h e s t r e s s - s t r a i n curve / 6 , 9 /

l i n e charge o f t h e d i s l o c a t i o n by l i g h t - i n d u c e d kd &e-6d - - - -

c a r r i e r s may r e s u l t i n a n e g a t i v e as w e l l as p o s i - (11

t i v e PPE. I n /6/ models f o r t h e PPE i n CdTe are where o d i s t h e slope o f the s t r e s s - s t r a i n curve suggested f o r b o t h cases : 1) when t h e d i s l o c a t i o n ₌

$

in the darkness and bl is the value of in motion i s c o n t r o l l e d by l o c a l i n t e r a c t i o n s w i t h t h e l i g h t ( a t t h e p o i n t s "on" and " o f f " i n f i g u r e 1).

p o i n t defects i l l u m i n a t i o n may r e s u l t i n changes i n s t r u c t u r e o f these centers; 2) when t h e d i s l o c a t i o n

ie

elastic ^is ^theregion slope of the stress-strain ( E = ~ ) at the same S.

. ^.

curve can be in expres- the motion i s c o n t r o l l e d by formation o f double k i n k s sed a l s o as

i l l u m i n a t i o n may r e s u l t i n changes o f the l i n e char-

6

⁼pbV, ( 2 )

ge o f a d i s l o c a t i o n . where p i s t h e d e n s i t y of mobile d i s l o c a t i o n , b i s This report presents new ^Of PPE t h e Burgers v e c t o r and V i s the average v e l o c i t y o f s t u d i e s i n CdTe. A pronounced i n f l u e n c e o f c h l o r i n e dislocations. Measuring the slopes changes of doping on t h e PPE was observed. More d e t a i l s f o r

the o - t curve a t the i n s t a n t s "on" and " o f f " , there- t h e d i s c u s s i o n o f t h e PPE i n CdTe were obtained.

f o r e measures t h e f l u x e s pV o f moving d i s l o c a t i o n s 2. Experimental d e t a i l s . - The experiments were p e r f o r - in the illuminated and non-il sta- med on p o l y c r y s t a l s w i t h l a r g e ( s e v e r a l mm) g r a i n s i z e tes. Since at the instants

and and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979634

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C6- 1 7 0 J O U R N U DE PHYSIQUE

are t h e same i n both s t a t e s we can consider pl=pd t o be a good approximation. Them from ( 1 ) and ( 2 ) i t f o l l o w s

i . e . one can o b t a i n d a t a on a change i n d i s l o c a t i o n v e l o c i t y by i l l u m i n a t i o n .

For i l l u m i n a t i o n the same o p t i c a l systems as i n /6/ were used.

F i g . 1 : Schematic drawing o f p o s i t i v e , +Aoph, and negative, -Ao h, p h o t o p l a s t i c e f f e c t s . 5,: 6 , and bd a r e e x p e r i k e n t a l parameters f o r d e t e r m ~ n a t ~ o n o f kd/tl r a t i o according t o expression ( 1 ) .

3. Results and discussion.- Figure 2 represents t h e s p e c t r a l dependence o f t h e r a t i o . td/E1 f o r CdTe-G and CdTe-G(C1) samples. A peak a t the wave l e n g t h h=0.85pm, corresponding t o t h e i n s t r i n s i c a b s o r p t i o n edge, can be seen on b o t h curves ( t h i s peak i s w e l l pronounced a l s o i n CdTe-G samples r a p i d l y cooled from 900°C). As can be seen, t h e peak a t

X

= 0.93 pm t h a t was weak i n CdTe-G samples i s w e l l pronounced i n CdTe-G(C1) and even h i g h e r than t h e peak X = 0.85 pm.

F i g . 2 : S p e c t r a l dependence o f t / L f o r CdTe-G (curve 1 ) and f o r CdTe-G(C1) ( c u r i e

1).

In

/6/ i t was noted t h a t according t o t h e band s t r u c t u r e o f CdTe (see f i g u r e 3

-

data from review papers by Kr6ger

/ l o /

and Wald /11/ t h e peak a t

X = 0.93 um corresponds t o a t r a n s i t i o n from a l o c a l l e v e l o f t h e ( v - - . x + ) - complex (X+ = C1- ,BrTe,

Cd Te

InEd, AIEd..

.

^{. ) ,} t h a t i s a h o l e t r a p . I t was assumed /6/ t h a t l i g h t - i n d u c e d t r a n s i t i o n s from ( v - - - x + ) - t o t h e conduction band, p r o v i d i n g a h o l e trapped by t h e Cd complex, can r e s u l t i n a s t r o n g e r 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 the center.

CONDUCTION BAND

VALENCE BAND

F i g . 3 : Some l o c a l i z e d l e v e l s w i t h i n t h e energy gap o f CdTe /10,11/.

Also an increase i n the energy o f double k i n k forma- t i o n by a decrease o f d i s l o c a t i o n s charge on d i s l o c a - t i o n s i n t h e g l i d e s e t r e s u l t s i n a p o s i t i v e PPE. The data o f f i g u r e 2 c o n f i r m t h i s assumption. As can be seen from f i g u r e 4, t h e p o s i t i v e PPE i n CdTe-G(C1) samples i s much more pronounced w i t h i n c r e a s i n g p l a s t i c deformation than i n CdTe-G samples. I n /6/ a t r a n s i - t i o n from p o s i t i v e t o n e g a t i v e PPE w i t h E and w i t h an increase o f i l l u m i n a t i o n , I, was observed. Such a behaviour was explained by recombination processes o f f r e e e l e c t r o n s w i t h trapped holes. F i g u r e 5 represents the dependence o f kd/E1 ( f o r t h e p o s i t i v e PPE) and B1/Ed ( f o r t h e n e g a t i v e PPE) on I obtained on CdTe-G ( c u r v e 1) ( ~ 4 % ) and CdTe-G(C1) (curve 2) ( ~ 8 % ) sam- p l e s . As can be seen, t h e r e i s an increase o f PPE and ^I no t r a n s i t i o n from p o s i t i v e PPE t o n e g a t i v e w i t h I i n CdTe-G(C1) samples even a t l a r g e r ^E. This can be t h e r e s u l t o f a h i g h c o n c e n t r a t i o n o f ( y i i - ~ l + ) - cen- t e r s compared'with t h e c o n c e n t r a t i o n o f f r e e elec- t r o n s induced by i l l u m i n a t i o n .

F i g u r e 6 represents t h e dependence o f t h e kd/Cl r a t i o on t h e temperature T from which CdTe-G

q '

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E .Y. Gutmanas and P

.

^Haasen C 6 - 1 7 1 samples were r a p i d l y c o o l e d (q-"quenched"). A de-

c r e a s e i n PPE a t Tq>5000C i s observed f o r b o t h wave l e n g t h s , b e i n g s h a r p e r f o r X=0.85pm. T h i s c o r r e l a - t e s w i t h /12/ a c c o r d i n g t o which complexes as ( v C ~ . C ~ + ) - a r e s t a b l e up t o r e l a t i v e l y h i g h tempera- t u r e s (>700°C when t h e donor c o n c e n t r a t i o n i s h i g h ) . The decrease o f

id/El

w i t h i n c r e a s i n g T i s apparent-

q

l y a r e s u l t o f an i n c r e a s e i n t h e c o n c e n t r a t i o n o f r e c o m b i n a t i o n c e n t e r s .

F i g . 4 : The dependence o f r a t i o on t h e s t r e s s o n o r m a l i z e d on t h e y i e l d stPess o f o r CdTe-G, A= 0.85pm ( c u r v e 1 ) and f o r ~ d ~ e - ~ f ~ l ) , X=0.93um ( c u r v e 2 ) , 1=4.5xlO-~W/cm~.

F i g . 5 : The dependence o f

; /El

( 6 /id) r a t i o on i l l u m i n a t i o n , I, f o r CdTe-G ?curve

1)

and f o r CdTe-G ( C l ) ( c u r v e 2). X=0.8 e 1.2pm.

d i s l o c a t i o n s must be s t r o n ? , s i n c e such a d e f e c t i s asymmetric /13,14/. No marked d i f f e r e n c e i n y i e l d s t r e s s , ay, f o r CdTe-G and CdTe-G(C1) was observed i n t h e n o n i l l u m i n a t e d s t a t e . On t h e o t h e r hand a s t r o n g i n f l u e n c e o f such d e f e c t s on V and ay can be expected i n models o f d i s l o c a t i o n m o t i o n i n a c r y s - t a l w i t h p o i n t d e f e c t s o f an e s t i m a t e d c o n c e n t r a t i o n NC1=5x10'7cm-3 ( t h a t corresponds t o 235ppm o f p i n n i n g c e n t e r s i n CdTe). T h i s i n d i c a t e s t h a t presumably t h e change i n t h e l i n e charge o f moving d i s l o c a t i o n s un- d e r i l l u m i n a t i o n r e s u l t s i n a change o f d o u b l e k i n k f o r m a t i o n energy /8/ p r o v i d i n g a p o s i t i v e ( o r nega- t i v e ) PPE i n CdTe. I n /15/ a n o t i c e a b l e i n f l u e n c e o f charged d e f e c t s o f such c o n c e n t r a t i o n s on t h e a c t i - v a t i o n energy o f d i s l o c a t i o n m o t i o n by t h e d o u b l e k i n k mechanism was r e p o r t e d f o r InSb and GaAs. F u r - t h e r measurements o f t h e ( v - - Cl+)-complex concen-

Cd

t r a t i o n i n CdTe-G and i n CdTe-G(C1) a f t e r v a r i o u s h e a t t r e a t m e n t s s h o u l d be c a r r i e d o u t .

I t s h o u l d be n o t e d t h a t t h e c r i t i c a l r e s o l v e d shear s t r e s s o f CdTe-G s i n g l e c r y s t a l s 7s'~0.3kp/mm2 (r3N/mm2), i s c o n s i d e r a b l y l o w e r t h a n T, o b t a i n e d f o r CdTe i n / 3 / .

F i g . 6 : The dependence o f /: ) on T t h e temperature f o r m which r a p i d g o o f i n g ( " & e n c h i l i " ) o f CdTe-G samples was performed f o r X = 0.85um ( c u r - ve l ) and

X

= 0.93pm ( c u r v e 2 ) .

Acknowledgements.

-

The a u t h o r s w i s h t o thank Dip1

.

Eng. N. T r a v i t z k y f o r h e l p i n experiments, Dr. P l i t t f o r p r o v i d i n g specimens, t h e DFG f o r f i n a n c i a l sup- p o r t .

I n /6/ i t was n o t e d t h a t i f t h e complex

( v c ~ . c ~ + ) - ( w i t h o r w i t h o u t h o l e t r a p p e d ) i s i n t e r a c - t i n g w i t h t h e d i s l o c a t i o n , and i f m a p , where a i s

P P e i e r l s s t r e s s , t h e i n t e r a c t i o n o f t h i s d e f e c t w i t h

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JOURNAL DE PHYSIQUE

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