THE PLASTICITY OF A SEMICONDUCTING COMPOUND : CdTe, ELECTRICAL MEASUREMENT AND CHEMOMECHANICAL EFFECT

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THE PLASTICITY OF A SEMICONDUCTING COMPOUND : CdTe, ELECTRICAL MEASUREMENT

AND CHEMOMECHANICAL EFFECT

P. Haasen, H. Müller, G. Zoth

To cite this version:

P. Haasen, H. Müller, G. Zoth. THE PLASTICITY OF A SEMICONDUCTING COMPOUND : CdTe, ELECTRICAL MEASUREMENT AND CHEMOMECHANICAL EFFECT. Journal de Physique Colloques, 1983, 44 (C4), pp.C4-365-C4-373. �10.1051/jphyscol:1983444�. �jpa-00223064�

THE PLASTICITY OF A SEMICONDUCTING COMPOUND :

CdTe, ELECTRICAL MEASUREMENT AND CHEMOMECHANICAL EFFECT

P. Haasen, H. Miiller and G. ~ o t h *

z n s t i t u t filr MetaZZphysik, Universittlt Girttingen, F.R.G.

I V . PhysikaZisches I n s t i t u t and SFB 126, Universittlt Gtfttingen, F.R.G.

Resume

Des cristaux de CdTe ont &t& obtenus au laboratoire de cristallogenese de Gattingen (Kristall-Labor), e t caracterises par des mesures d ' e f f e t Hall e t thermoelectriques. Les dislocations ont e t 6 revelees par piqiires d ' a t t a q u e avant e t apres indentation r e a l i s e e dans d i f f e r e n t s e l e c t r o l y t e s polarises par rapport 1 l l & c h a n t i l l o n . L'analyse de l ' a c t i v a t i o n thermique semble indiquer qu'un mecanisme de Peierl s gouverne 1 a contrainte d' ecoulement 8 l a tempera- t u r e ambiante e t 1 plus basse temperature. Aux dislocations introduites par compression e s t associe u n niveau neutre s i t u e 1 0,3 eV au-dessus de l a bande de valence (selon l e s mesures d ' e f f e t Hall e t thermoelectrique). Dans ce materiau l a mobilite des dislocations e s t plus p e t i t e dans l e s conditions de bande plate qu'avec une charge induite sur l e s defauts dans l a couche super- f i c i e l l e du materiau.

Abstract

CdTe has been grown i n the Gottingen Kristall-Labor and characterized by Hall e f f e c t and thermoelectrical measurements. Dislocations are etch p i t t e d before and a f t e r deformation by an hardness indenter. The experiment i s per- formed i n various e l e c t r o l y t e s while a voltage i s applied w i t h respect t o the specimen. Thermal activation analysis seems t o indicate t h a t a Peierls

mechanism determines the flow s t r e s s a t and below room temperature. The d i s l o - cations introduced by compression of p-CdTe give r i s e t o s t a t e s which a r e neutral a t 0.3 eV above Ev i n the Hall e f f e c t and thermoelectrical power.

Dislocation mobility i n t h i s material i s smaller under f l a t band conditions than with an induced charge a t defects i n the surface layer of t h i s material.

1. Statement of the Problem

CdTe has the s p h a l e r i t e structure which derives from t h e diamond structure by putting the divalent atoms on one f c c s u b l a t t i c e , t h e sixvalent onto the other.

In t h i s way the l a t t i c e obtains a p o l a r i t y along < I l l > : CdTe pairs define a direc- Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983444

C4-366 JOURNAL DE PHYSIQUE

t i o n which make t h e upper and lower c r y s t a l planes o f cleaved < I l l > s l i c e s t o be occupied by d i f f e r e n t atom species. The r e l a t e d t h e o r e t i c a l problem o f t h e charge d i s t r i b u t i o n and near-surface s t r u c t u r e has been t r e a t e d by Lanoo [I]. The r e l a t e d s i t u a t i o n a t t h e end o f an e x t r a h a l f plane o f a non-screw d i s l o c a t i o n i n t h i s s t r u c t u r e i s under c a l c u l a t i o n i n T e i c h l e r ' s group. It i s now 25 years ago t h a t I have p o i n t e d o u t t h a t d i f f e r e n c e s i n p h y s i c a l p r o p e r t i e s c o u l d be expected if such compounds are p l a s t i c a l l y bent i n one d i r e c t i o n o r t h e o t h e r [21. Then edge t y p e d i s l o c a t i o n s ending w i t h I 1 o r V 1 v a l e n t atom rows are introduced i n excess which should have d i f f e r e n t e f f e c t s on t h e e l e c t r i c a l p r o p e r t i e s . I n t h e meantime t h e d i s s o c i a t i o n o f d i s l o c a t i o n s a l s o i n t h e s p h a l e r i t e s t r u c t u r e has become evidentC31

>-

and we now have t o consider a = 30' and 90' p a r t i a l d i s l o c a t i o n s as components o f t h e m a j o r i t y screw and 60' complete d i s l o c a t i o n s . They move i n t h e g l i d e s e t o f

< I l l > planes and according t o the t e r m i n a t i n g row o f atoms are c a l l e d e i t h e r Cd ( g ) o r Te ( g ) - a0 d i s l o c a t i o n s [ 4 ] . So f a r t h e b a s i c i n f o r m a t i o n on t h e p l a s t i c i t y o f semiconducting cimpounds was a v a i l a b l e a t t h e l a s t meeting on t h i s s u b j e c t a t Hiinfeld 1978. Why has i n t e r e s t concentrated s i n c e t h e n on CdTe among t h e semicon- d u c t i n g s p h a l e r i t e s as r e l a t i v e s t o the diamond s t r u c t u r e ? There a r e t e c h n o l o g i c a l a p p l i c a t i o n s , o f course, b u t on t h e s c i e n t i f i c s i d e we were i n t e r e s t e d t o extend Gwinner's e l e c t r i c a l d i s l o c a t i o n work on GaAs [51 and t o f i n d a room temperature d u c t i l e s p h a l e r i t e semiconductor t o l o o k f o r t h e chemomechanical e f f e c t . F o l l o w i n g observations on i o n i c c r y s t a l s 161 o f a surface p l a s t i c i t y depending on the c-poten- t i a l o f t h e surrounding e l e c t r o l y t e a mechanism was proposed f o r a s i m i l a r e f f e c t i n semiconductors C71. The charge on t h e d i s l o c a t i o n near a surface ( w i t h i n a Debye t h i c k n e s s ) should depend on band bending t h e r e and t h i s i n t u r n c o u l d be i n f l u e n c e d i n an e l e c t r o l y t e condensor. I f k i n k f o r m a t i o n determined d i s l o c a t i o n m o b i l i t y and t h e k i n k s were charged [81 t h e r e were w e l l d e f i n e d experimental condi- t i o n s f o r t h e chemomechanical e f f e c t t o be i n v e s t i g a t e d . The q u e s t i o n a r i s e s however whether such a room temperature p l a s t i c semiconductor, as i s necessary i n c o n t a c t w i t h t h e e l e c t r o l y t e , s t i l l deforms by t h e double k i n k mechanism o r , as i o n i c c r y s t a l s do a t t h i s temperature, by s l i p over p o i n t d e f e c t obstacles 191. These c o u l d a l s o be recharged near t h e surface and l e a d t o a changed i n t e r a c t i o n w i t h t h e charged d i s l o c a t i o n s [ l o ] . The s l i p mechanism t h e r e f o r e has t o be i n v e s t i g a t e d f i r s t and a l s o t h e e l e c t r i c a l charge o f t h e d i s l o c a t i o n s . These t o p i c s w i l l be t r e a t e d i n t h e n e x t sections.

2. M i c r o p l a s t i c i t y o f CdTe

CdTe has a mixed c o v a l e n t - i o n i c bonding w i t h an i o n i c i t y f i = 0 . 7 o n t h e P h i l l i p s s c a l e [Ill close t o t h a t o f t h e a1 k a l i h a l i d e s . S t i l l (111) a r e t h e a c t i v e s l i p planes although they expose charged s l i p steps a t t h e surface.

and by e t c h p i t t i n g i n Gattingen and by Nakagawa e t a1 [121. The l a t t e r authors conclude t h a t d i s l o c a t i o n s become immobile a f t e r t r a v e l l i n g a c e r t a i n d i s t a n c e from t h e i r sources and then a r e replaced by new ones. Nevertheless t h e i r mqvement ( a t 300 K) i s found t o be smooth and continuous; they a r e r a t h e r s t r a i g h t as expected f o r a P e i e r l s mechanism.

Macroscopic compression experiments f o r thermal a c t i v a t i o n a n a l y s i s were performed by Gutmanas e t a1 1131. I n order t o e l i m i n a t e e f f e c t s o f d i s l o c a t i o n mu1 t i p 1 ic a t i o n d u r i n g s t r e s s (Au) and temperature jumps (AT) new pulse methods were

employed using an electromagnet i n s e r i e s w i t h t h e sample and d i r e c t c u r r e n t heating o f t h e specimen. Typical experimental curves are shown i n f i g . 1. From these a c t i - v a t i o n area (Aa = volume/b) and energy AH a r e c a l c u l a t e d according t o

where M = 0.41 i s t h e Schmid f a c t o r . AT was obtained from the s t r e s s increase AoaT due t o thermal expansion alone. F i g . 2 shows t h e measured a c t i v a t i o n parameters as a f u n c t i o n o f temperature, f i g . 3 t h e y i e l d s t r e s s ( T ) temperature r e l a t i o n and

2 "

f i g . 4 t h a t o f Aa ( T ) . An a c t i v a t i o n area o f = 1 0 0 b w i t h a considerable a c t i v a t i o n b a r r i e r o f = 0.4 eV around room temperature i s t y p i c a l f o r t h e P e i e r l s mechanism.

E x t r a p o l a t i o n o f T (Aa-w), t h e o b s t a c l e p r o f i l e , f o r t h e known AH leads t o a P e i e r l s s t r e s s o f ( 2 1 f 4 ) M N / ~ ? Y dominating t h e deformation o f CdTe below 400K. The v e l o c i t y s t r e s s exponent i s obtained by these techniques t o m =

Te

Nothing much i s known on the dependence o f d i s l o c a t i o n dynamics on doping f o r t h e I I / V I compounds. This would g i v e i n f o r m a t i o n on t h e n e u t r a l p o s i t i o n o f t h e k i n k energy l e v e l s i n t h e gap as t r a c e d by the Fermi l e v e l changing w i t h doping.

Judging from t h e s i t u a t i o n i n e x t r i n s i c a l l y doped I I I / V compounds [14ITe(g) d i s l o - c a t i o n s w i l l be f a s t e r than Cd(g) d i s l o c a t i o n s o r screw d i s l o c a t i o n s . The d i s l o c a - t i o n v e l o c i t y o f t h e I I I ( g ) d i s l o c a t i o n changes r a p i d l y o n l y w i t h n-doping so t h e corresponding energy l e v e l must be near t h e conduction band. Concerning p a r t i a l s t h e 30' I I I ( g ) p a r t i a l common t o b o t h t h e slower complete d i s l o c a t i o n s may be t h e owner o f t h i s energy l e v e l . Me w i l l t u r n t o e l e c t r i c a l measurements on deformed

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CdTe i n t h e n e x t s e c t i o n . It may be noted here t h a t we expect c e r t a i n s i m i l a r i t i e s between t h e e f f e c t o f doping on d i s l o c a t i o n v e l o c i t y and t h a t o f a p o t e n t i a l a p p l i e d through an e l e c t r o l y t e near t h e surface : Both s h i f t t h e r e l a t i v e p o s i t i o n o f t h e Fermi l e v e l t o t h e band edges. The s i t u a t i o n i s l e s s c l e a r i n comparison w i t h t h e photomechanical e f f e c t : Besides changing the occupation o f d i s l o c a t i o n l e v e l s [ I 5 1 t h e photogenerated c a r r i e r s may d u r i n g t h e i r recombination t r a n s f e r momentum t o k i n k s C161 [25]. We w i l l n o t discuss t h i s e f f e c t here f u r t h e r .

3 . E l e c t r i c a l Measurements on Deformed CdTe

The f i r s t c r y s t a l s used i n our i n v e s t i g a t i o n s were obtained by t h e t r a v e l l i n g heater method. They were h i g h l y compensated and v e r y small. L a t e r b e t t e r specimens were produced i n t h e Gottingen K r i s t a l l - L a b o r b y t h e Bridgman method (H.G. B r i o n ) . Besides t h e usual measurement o f c o n d u c t i v i t y and Hal 1 e f f e c t (below room tenperature ) t h e t h e r m o e l e c t r i c power (TP) was i n v e s t i g a t e d by M i i l l e r [171. It has c e r t a i n advan- tages over the H a l l e f f e c t i f t h e specimen i s inhomogeneous ( a f t e r deformation) and i f c a r r i e r m o b i l i t i e s a r e small. Both methods a l l o w t o determine t h e p o s i t i o n o f t h e l e v e l , EF. It i s r e l a t e d t o t h e TPS by t h e w e l l known formula

f o r a p-semiconductor (negative s i g n f o r S and (EC-EF) i n s t e a d o f (EF-EV) f o r n - m a t e r i a l . Here EC, EV are conduction, valence band edges, a i s t h e exponent i n the energy dependence o f t h e r e l a x a t i o n t i m e : a = -0.5 f o r s c a t t e r i n g on a c o u s t i c a l phonons, a = 1.5 on i o n i z e d p o i n t defects. The phonon drag i s unimportant i n t h e i n v e s t i g a t e d T range.

The i n v e s t i g a t i o n was c a r r i e d o u t on undeformed p-CdTe w i t h c a r r i e r concen- t r a t i o n s between 2 . 1 0 ~ ~ and 2 . 1 0 ' ~ an-3, n-CdTe w i t h no= 3 . 1 0 ' ~ Both methods l e d t o s i m i l a r values f o r EF i f a temperature dependent a ( T ) according t o D e v l i n fl81 was used. Deformation was c a r r i e d o u t by compression t o ~ < l % i n an I n s t r o n machine a t room temperature. Etch p i t counts l e d t o N = 10 t o 6 l o 7 i n comparison w i t h No = 8.10 4 an-' before deformation. T y p i c a l r e s u l t s on p-CdTe a r e shown i n f i g . 5.

The TP i s g r e a t l y reduced and i t s T dependence found reversed. I n n-CdTe t h e r e was 12 -3 l i t t l e e f f e c t a t a1 1 o f t h e deformation i n TP except f o r a specimen w i t h no= R ' I O an which became p-type a f t e r deformation.This i n d i c a t e s t h a t deformation introduces s t a t e s i n t h e lower h a l f o f t h e gap. These must be l o c a l i z e d a t o r around d i s l o c a - t i o p s and accupy a f r a c t i o n B o f t h e specimen c r o s s s e c t i o n g i v i n g i t a c o n d u c t i v i t y

(rD. The t o t a l c o n d u c t i v i t y must be

w h i l e t h e TP w i l l be averaged i n p a r a l l e l over volume (V) and d i s l o c a t i o n (D)

Sv, Seff-and ov a r e known from experiment y i e l d i n g l a r g e values g o D =

.. .

.Q 'cm-l a s a r e a l s o derived from t h e Hall e f f e c t d a t a . Dijding and Labusch [19] g e t f o r CdS by microscopic measurements a r a d i u s r < < 3 um of t h e highly con- ducting d i s l o c a t i o n c y l i n d e r s s o t h a t from 8 = Nr2 we o b t a i n f o r our d i s l o c a t i o n d e n s i t i e s N values 6 4 .

The question a r i s e s what i s r e s p o n s i b l e f o r t h e well conducting d i s l o c a t i o n c y l i n d e r s :

a ) overlapping dangling bonds i n t h e cores of d i s l o c a t i o n s ; o r b) point d e f e c t clouds around d i s l o c a t i o n s ?

A survey 1201 of point d e f e c t l e v e l s i n CdTe does not g i v e any d e f e c t i n t h e range EV + ( 0 . 3 . . .0.4) eV t h a t we need t o explain our d a t a . On t h e o t h e r hand Oberg [21] c a l c u l a t e s f o r a d i s s o c i a t e d 60' Te(g) d i s l o c a t i o n a h a l f f u l l band a t EV + 0.5 eV, f o r t h e screw EV + 0.4 eV. Gelsdorf [221 has c h a r a c t e r i z e d by DLTS deep c e n t e r s i n n-CdTe and found only a mid-gap l i n e a t EC-0.72 eV a f t e r p l a s t i c deformation. The t r a p d e n s i t y c o r r e l a t e s well with t h e d i s l o c a t i o n d e n s i t y according t o e t c h p i t t i n g but i s a t l e a s t an o r d e r of magnitude l a r g e r ! This can only i n t e r - preted by deformation induced p o i n t d e f e c t clouds around d i s l o c a t i o n s . The same conclusion i s reached by Zoth from t h e i n t e r p r e t a t i o n of t h e strong EBIC c o n t r a s t around f r e s h d i s l o c a t i o n s i n CdTe. Unfortunateljl due t o c o n t a c t problems DLTS has not been p o s s i b l e on p-CdTe so t h a t a t t h e moment two d i f f e r e n t l e v e l s t h a t may be associated w i t h d i s l o c a t i o n s (and/or t h e i r p . d . c l o u d s ) a r e given f o r n-andp-CdTe.

From t h e velocity/doping a n a l y s i s i n t h e previous s e c t i o n t h e upper level a c t i v e i n n-CdTe could be a t t r i b u t e d t o Cd(g) d i s l o c a t i o n s while according t o Oberg [211 t h e lower one a c t i v e i n p-CdTe may belong t o t h e Te(g) d i s l o c a t i o n . This i s i n q u a l i t a - t i v e agreement with t h e i n t e r p r e t a t i o n of Hall d a t a on GaAs by Gwinner and Labusch [5] [23] and with c a l c u l a t i o n s on d i s l o c a t i o n s i n t h e samematerial by Jones e t a1 1241.

P a r t of t h e e l e c t r i c a l a c t i v i t y measured may however be due t o t h e point d e f e c t s surroundinq t h e d i s l o c a t i o n s [26,27] and i n t e r a c t i n g with them through t h e i r charges.

4 . Preliminary Results on t h e Chemomechanical E f f e c t 1281

An apparatus was b u i l t t h a t allows hardness i n d e n t a t i o n s on any point of a CdTe specimen which i s i n an e l e c t r o l y t e opposite t o a condensor p l a t e . A v o l t a g e can be applied with r e s p e c t t o t h e specimen and t h e bending of t h e energy bands near t h e s u r f a c e can be measured v i a t h e c a p a c i t y . The problem i s t o f i n d an

C4-370 JOURNAL DE PHYSIQUE

F i g . 1 : S c h e m a t i c s t r e s s t i m e c u r v e s w i t h s t r e s s jumps ( A O ) and t e m p e r a t u r e jumps ( A T ) . ( b ) shows a l s o t h e s t r e s s c h a n g e A O

due t o t h e r m a l e x p a n g J o n .

/ -

I

r . .

S O

200 100

F i g . 3 : The y i e l d s t r e s s t e m p e r a t u r e r e l a t i o n ( i n t h e d a r k )

Cl

-

- F i g . 2 : The d e p e n d e n c e

o f i v a t i o n a r e a Aa/b

.-

/

A unverfmt

o 0.2 % verformt o 0.3% r

a a / b o n t h e e f f e c t i v e s t r e s s ( T = 220

...

F i g . 5: E f f e c t o f s m a l l p l a s t i c s t r a i n s o n t h e t h e r m o e l e c t r i c p o w e r o f pi$d T e L j p = 3.5 1 0 cm

JOURNAL DE PHYSIQUE

p-CdTe air

100

L

0 1 2 3

log t [sl

.

NaCl NaOH

F i g . 6 : R o s e t t e s i z e v s t i m e o f a p p l i c a t i o n o f l o a d i n a i r a n d i n 1 m NaOH (P-Cd T e )

F i g . 7 : R o s e t t e s i z e v s . W o ly t e c o n c e n t r a t i o n

f o r t w o l o a d s ( p - C d T e )

-4 -3 -2 -1 0

log c [mole-']

e l e c t r o l y t e ( f i g . 6 ) . P l o t t e d i s the l e n g h t o f r o s e t t e arms vs. t h e i n d e n t a t i o n time. For a g i v e n t i m e a n d l o a d t h e r o s e t t e s i z e dependson t h e e l e c t r o l y t e c o n c e n t r a t i o n f i g . 7 ) . So f a r w e h a v e n o t been a b l e t o r e a l i z e t h e f l a t band bending on r o s e t t e s i z e . Perhapsthere i s no d i s l o c a t i o n l e v e l i n th e range EC - 0.leV.. .EC - 0.7eV. On t h e o t h e r hand t h e specimens i n v e s t i g a t e d so f a r were perhaps too h i g h l y doped so t h a t t h e t h i c k n e s s o f the Deb9 screening layerwas s m a l l e r t h a n t h e s i z e o f t h e d i s l o c a t i o n l o o p s u n d e r t h e r o s e t t e . F u r t h e r e x p e r i - m e n t s a r e i n p r o g r e s s . P r e v i o u s h a r d n e s s t e s t s i n a n e l e c t r o l y t e w e r e d o n e on s i l i c o n a t room temperature [291 where i t i s n o t c l e a r t o what extend p l a s t i c deformation and n o t j u s t f r a c t u r e occurred under t h e i n d e n t e r .

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