PHOTOCONDUCTIVITY PRODUCED BY POLARIZED LIGHT IN PLASTICALLY DEFORMED Ge

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Submitted on 1 Jan 1979

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PHOTOCONDUCTIVITY PRODUCED BY

POLARIZED LIGHT IN PLASTICALLY DEFORMED Ge

E. Kamieniecki

To cite this version:

E. Kamieniecki. PHOTOCONDUCTIVITY PRODUCED BY POLARIZED LIGHT IN PLAS- TICALLY DEFORMED Ge. Journal de Physique Colloques, 1979, 40 (C6), pp.C6-87-C6-89.

�10.1051/jphyscol:1979618�. �jpa-00219034�

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J O U R N A L D E PHYSIQUE CoZZoque C 6 , s u p p l h e n t au n06, tome 40, j u i n 1979, puge C6-87

PHOTOCONDUCTIVITY PRODUCED BY POLARIZED L I G H T I N PLASTICALLY DEFORMED Ge

E. Kamieniecki

I n s t i t ~ l t e o f P h y s i c s , P o l i s h Academy of S c i e n c e s , Warsaw, Poland.

Resu1ne.- 3n d i s c u t e l e spectre c a r a c t g r i s t i q u e de p h o t o c o n d u c t i v i t e , d ' a b s o r p t i o n e t de recombinaison r a d i a t i v e due aux " d i s l o c a t i o n s a 60"" dans l e Ge. On orssente un modele de deux niveaux des " d i s l o - c a t i o n s a 60"" dans l e Ge. Le niveau s u p e r i e u r A, i n t r o d u i t par l e s c o n t r a i n t e s e s t s i t u e e n v i r o n 0 , l eV au dessous de l a bande de conduction. Le niveau i n f e r i e u r I, s i t u e environ 0 , l eV au dessus de

l a bande de valence ~ e u t e t r e a t t r i b u e aux l i a i s o n s b r i s e e s des d i s l o c a t i o n s . 3n a montre que l e s densites de niveaux A e t I sont d i f f e r e n t s pour d i f f e r e n t e s regions des d i s l o c a t i o n s .

A b s t r a c t . - The c h a r a c t e r i s t i c spectra o f the p h o t o c o n d u c t i v i t y , a b s o r p t i o n and r a d i a t i v e recombination due t o "60°-dislocations" i n Ge a r e discussed. A two-level model o f t h e " 6 0 " - d i s l o c a t i o n " i n Ge i s i n f e r r e d . The upper, s t r e s s induced, A-level i s l o c a t e d about 0 . 1 eV below the conduction band edge.

The lower one, I - l e v e l , l o c a t e d about 0.1 eV above the valence band edge may be a t t r i b u t e d t o dangling d i s l o c a t i o n bonds. I t i s shown t h a t the d e n s i t i e s o f A and I s t a t e s d i f f e r f o r d i f f e r e n t regions o f d i s l o c a t i o n s .

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

-

The p h o t o c o n d u c t i v i t y represents one o f the most s e n s i t i v e experimental methods o f i n v e s t i - g a t i o n o f the e l e c t r o n i c s t a t e s i n t r o d u c e d by p l a s t i c deformation. Especial 1 y useful f o r the a n a l y s i s o f the spectrum o f such s t a t e s are the s t u d i e s o f the s p e c t r a l response of t h e e x t r i n s i c p h o t o c o n d u c t i v i t y . Over the p a s t t e n years a g r e a t number o f such spec- t r a have been r e p o r t e d f o r Ge w i t h i n t e n t i o n a l l y i n - toduced " 6 0 ° - d i s l o c a t i o n s " / I - 9 / . Nearly a l l observed p h o t o c o n d u c t i v i t y spectra reveal the same charac- t e r i s t i c s t r u c t u r e , b o t h d i t h r e s p e c t t o the amp1 i- tude o f the s i g n a l as w e l l as w i t h r e s p e c t t o the s p e c t r a l response of the r e l a x a t i o n time.

I t has been r e p o r t e d t h a t s p e c t r a o f the o p t i - cal absorption f o r n-type Ge w i t h i n t e n t i o n a l l y i n - troduced "60"-di s l o c a t i o n s " reveal a pronounced, h i g h l y d i c h r o i c peak a t 0 . 1 eV /10-12/. It was shown t h a t the d i p o l e moment o f t h e t r a n s i t i o n s r e s - p o n s i b l e f o r t h i s peak i s o r i e n t e d p a r a l l e l t o the Burgers v e c t o r o f i n t r o d u c e d " 6 0 " - d i s l o c a t i o n s " and t h a t p o l a r i z a t i o n degrees of 95% can be achieved.

From these i t was concluded t h a t t h e 0.1 eV absorp- t i o n peak i s due t o ."60°-dislocations11 s i n c e such h i g h degree o f p o l a r i z a t i o n and t h e observed d i r e c - t i o n o f t h e d i p o l e moment cannot be e x p l a i n e d by im- p u r i t y atoms, vacancies, and vacancy aggregates caught by t h e s t r e s s f i e l d surrounding t h e d i s l o c a - t i o n s /11,12/.

The r e p o r t e d r e s u l t s f o r a b s o r p t i o n suggest t h a t some c h a r a c t e r i s t i c t r a n s i t i o n s i n v o l v e d i n the p h o t o c o n d u c t i v i t y can be, a t l e a s t i n n-type Ge, iden- t i f i e d w i t h d i s l o c a t i o n s on the b a s i s o f t h e i r

d i c h r o i t i c p r o p e r t i e s .

2. C h a r a c t e r i s t i c s t r u c t u r e o f p o l a r i z a t i o n degree o f photoconductivit,y.- I n n-type c r y s t a l s a oronoun- ced ool a r i z a t i o n o f t h e p h o t o c o n d u c t i v i t y has been observed /5,8/. I n the whole i n v e s t i g a t e d s o e c t r a l range t h e t r a n s i t i o n moment i s o r i e n t e d i n t h e d i r e c - t i o n o f t h e Burgers v e c t o r as i t was already found f o r the 0.1-eV absorption peak. Figure 1 shows the s p e c t r a l dependence o f the p o l a r i z a t i o n degree o f p h o t o c o n d u c t i v i t y ,

P,

i n n-type Ge. The spectrum ex- h i b i t s t h e h i g h e s t P i n the s p e c t r a l range 0 . 1 t o 0.15 eV corresponding t o t h e p o s i t i o n o f the 0.1-eV a b s o r p t i o n peak (see curve OD i n f i g u r e 1). The c o r - responding h i g h degree o f p o l a r i z a t i o n o f t h e photo- c o n d u c t i v i t y and absorption a t photon energies near t o 0 . 1 eV suggest t h a t the same e l e c t r o n i c t r a n s i - t i o n s are i n v o l v e d i n b o t h o f them.

I

0.5

PHOTON EWERGY (eV) F i g . 1 : Spectral dependences o f the p o l a r i z a t i o n degree o f p h o t o c o n d u c t i v i t y

-

P r 5 7 , t h e a b s o r p t i m

-

OD r 1 2 and t h e r a d i a t i v e rec%mbination i n t e n s i - t y -

Rm

^{$ i l l} i n n-type Ge a t 77K.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979618

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C6-88 JOURNAL DE PHYSIQUE

An a d d i t i o n a l pronounced s t r u c t u r e i n t h e spec- t r u m o f t h e p o l a r i z a t i o n degree o f t h e p h o t o c o n d u c t i - v i t y i s t h e peak a t 0.5 eV ( s e e f i g u r e 1 ) . I t s h o u l d be n o t e d t h a t i n c r e a s e o f t h e p h o t o c o n d u c t i v i t y s i g - n a l c o r r e s p o n d i n g t o t h i s p o l a r i z a t i o n degree peak can be a l s o d i s t i n g u i s h e d i n some p r e v i o u s l y p u b l i - shed s p e c t r a f o r n - t y p e Ge /1, 3, 5, 6 , 9 / . T h i s 0.5-eV p h o t o c o n d u c t i v i t y peak c o i n c i d e s w i t h t h e r a d i a t i v e r e c o m b i n a t i o n l i n e /13,14/ ( s e e c u r v e R R I i n f i g u r e 1 ) commonly a t t r i b u t e d t o d i s l o c a t i o n s . The h a l f w i d t h o f t h i s 0.5-eV r a d i a t i v e recombina- t i o n l i n e remains c o n s t a n t i n t h e t e m p e r a t u r e r a n g e f r o m l i q u i d h e l i u m up t o about 170K /13,14/. T h i s suogest t h a t t h e 0.5-eV l i n e corresponds t o t r a n s i - t i o n s between d i s c r e t e l e v e l s i n t h e f o r b i d d e n band /13/. The c o i n c i d e n c e o f t h e r a d i a t i v e r e c o m b i n a t i o n l i n e w i t h t h e p o l a r i z a t i o n degree peak a t 0.5-eV suggests t h a t i n b o t h t h e s e e f f e c t s t r a n s i t i o n s b e t - ween t h e same d i s l o c a t i o n l e v e l s a r e i n v o l v e d /9,15/.

S i m i l a r t o n - t y p e Ge, some a b s o r p t i o n /12,19/

and p h o t o c o n d u c t i v i t y j6,8,9/ i n t h e v i c i n i t y o f 0.1 eV, as w e l l as t h e d i s t i n c t p h o t o c o n d u c t i v i t y peak a t 0.5 eV ( s e e e.g./1,6,9/)vere observed i n p- t y p e Ge. However i n c o n t r a s t t o n - t y p e Ge, no d i c h - r o i s m n e i t h e r i n a b s o r p t i o n n o r i n p h o t o c o n d u c t i v i t y has been f o u n d /8,12/.

3. T r a n s i t i o n s i n v o l v e d i n p h o t o - e f f e c t s . - The shape o f t h e 0.1-eV a b s o r p t i o n peak i n n - t y p e Ge as w e l l as t h e dependence o f t h i s peak on doping c o n c e n t r a - t i o n and t e m p e r a t u r e show t h a t b o t h t h e a b s o r p t i o n and t h e p h o t o c o n d u c t i v i t y i n v i c i n i t y o f 0 . 1 eV a r e a s s o c i ~ ~ t e d w i t h t r a n s i t i o n s f r o m t h e d i s l o c a t i o n s t a t e s (A s t a t e s i n f i g u r e 2 ) l o c a t e d a t a b o u t 0 . 1 eV below t n e c o n d u c t i o n band edge i n t o t h i s band /12, 15/.

Assuming t h a t A s t a t e s a r e i n v o l v e d i n t h e 0.5-eV r a d i a t i v e r e c o m b i n a t i o n l i n e , t h e e x i s t e n c e o f a d d i t i o n a l s t a t e s , l o c a t e d i n t h e l o w e r h a l f o f t h e band gap, about 0 . 1 eV above t h e v a l e n c e band edge ( I s t a t e s i n f i g u r e 2) can be i n f e r r e d . An i n - c r e a s e o f t h e p h o t o c o n d u c t i v i t y a t 0.5 eV r e s u l t s f r o m t w o - s t e p t r a n s i t i o n s : t h e e l e c t r o n i c t r a n s i t i o n s f r o m I to A s t a t e s ( I - A t r a n s i t i o n s i n f i a u r e 2 ) a n d t h e e x c i t a t i o n o f e l e c t r o n s f r o m A s t a t e s i n t o c o n d u c t i o n band (A-CB t r a n s i t i o n s ) i n n - t y p e c r y s t a l o r h o l e e x c i t a t i o n f r o m I s t a t e s i n t s t h e v a l e n c e band (VB- I t r a n s i t i o n s ) i n p - t y p e Ge. Such t r a n s i t i o n s f r o m A o r 1 s t a t e s i n t o t h e r e s p e c t i v e bands may o c c u r due t o thermal o r o p t i c a l e x c i t a t i o n .

S i n c e t h e p h o t o c o n d u c t i v i t y peak o c c u r r i n g i n p - t y p e Ge a t 0.5 eV behaves i s o t r o p i c a l l y i t can be i n f e r r e d t h a t b o t h I - A as w e l l as VB-I o p t i c a l t r a n - s i t i o n s a r e i s o t r o p i c . On t h e o t h e r hand, t h e d i c h - r o i c p r o p e r t i e s o f t h e 0.1-eV a b s o r p t i o n peak as w e l l as t h e i n c r e a s e o f t h e p o l a r i z a t i o n degree o f t h e p h o t o c o n d u c t i v i t y a t 0.5 eV i n n - t y p e Ge can perhaps be e x p l a i n e d assuming h i g h d i c h r o i s m o f o n l y t h e A-CB o p t i c a l t r a n s i t i o n s . The absence o f d i c h r o i s m i n p - t y p e 6e i n t h e 0.1-eV s p e c t r a l range where t h e t r a n s i t i o n s VB-I may c o n t r i b u t e t o t h e a b s o r ~ t i o n and p h o t o c o n d u c t i v i t y , shows t h a t t h e s e t r a n s i t i o n s a r e i s o t r o p i c .

The t r a n s i t i o n s i n v o l v i n g A and I d i s l o c a t i o n s t a t e s (see f i g u r e 2) e x p l a i n a l s o t h e 0.6-eV p o l a r i - z a t i o n degree peak o f t h e p h o t o c o n d u c t i v i t y o c c u r r i n g i n n - t y p e Ge (see f i g u r e 1 ) as w e l l as t h e i n c r e a s e o f t h e p h o t o c o n d u c t i v i t y observed b o t h i n n and p- t y p e Ge a t t h i s photon energy /1,6,9/

The v e r y h i g h p o l a r i z a t i o n degree and t h e d i - r e c t i o r i of t h e d i p ~ l e moment p a r a l l e l t o t h e Burgers v e c t o r o f " 6 0 " - d i s l o c a t i o n s " f o u n d a t 0 . 1 eV b o t h i n a b s o r p t i o n and p h o t o c o n d u c t i v i t y suggest t h a t a n i s o - t r o p i c A s t a t e s may be caused b y c r y s t a l d i s t o r t i o n s

E

n e a r t h e d i s l o c a t i o n l i n e /16-18, 12/. On t h e o t h e r hand, t h e l o w e r , i s o t r o p i c I s t a t e s , i n agreement

A - STATES

w i t h t h e r e c e n t t i g h t - b i n d i n g c a l c u l a t i o n s / 2 0 / , may be a t t r i b u t e d t o d a n g l i n g bonds o f " 6 0 " - d i s l o c a t i o n s " .

-0.0.v I - A 1-=a v8-A 4. Inhomogeneity o f d i s l o c a t i o n s . - F o l l o w i n g t h e i d e a

o r i g i n a l l y suggested by Shockley 1 2 1 1 , s t r o n g l y

1 - ..L---- ~ ~ ~ ~ l o c a l i z e d d a n g l i n g bond I s t a t e s f o r m a one-dimensio-

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n a l band w h i c h i s h a l f f i l l e d i n t h e n e u t r a l s t a t e .

v r l c m ~ BAND

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A d d i t i o n a l e l e c t r o n s o r h o l e s t r a p p e d i n I s t a t e s ,

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AT h+V) 0.1 0 1 0.6 1 0 1 O A

i n n and p - t y p e c r y s t a l s r e s p e c t i v e l y , r e s u l t i n an e l e c t r o s t a t i c c h a r g i n g o f t h e d i s l o c a t i o n l i n e s . S i m i - F i g . 2 : E l e c t r o n i c t r a n s i t i o n s a t 60" d i s l o c a t i o n s

i n v o l v e d i n p h o t o - e f f e c t s i n n and p - t y p e Ge. l a r l y , t h e s t r e s s i n d u c e d A s t a t e s c o u l d be p a r t i a l - l y o c c u p i e d b y e l e c t r o n s i n n - t y p e c r y s t a l a l s o r e s u l -

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E . Kamieniecki

t i n g i n an e l e c t r o s t a t i c c h a r g i n g of t h e d i s l o c a t i o n l i n e s .

S i n c e t h e t r a n s i t i o n s f r o m I t o A s t a t e s a r e i n v o l v e d i n p h o t o c o n d u c t i v i t y o c c u r r i n g a t 0.5 eV we can conclude t h a t t h e wave f u n c t i o n s o f I and A d i s - l o c a t i o n s t a t e s o v e r l a p . Thus, t h e y cannot be separa- t e d by a l a r g e d i s t a n c e as i t was suggested p r e v i o u s - l y t o e x p l a i n t h e a b s o r p t i o n 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 i n Ge /12,22/. I n such a case, c o n s i d e r i n g a homogenous d i s l o c a t i o n s t r u c t u r e , a l l t r a n s i t i o n s i n - v o l v e d i n a b s o r p t i o n and p h o t o c o n d u c t i v i t y i n n - t y p e Ge can be e x p l a i n e d o n l y assuming t h e upper (A) s t a - t e s t o be p a r t i a l l y o c c u p i e d and c o n s e q u e n t l y t h e l o w e r (I) s t a t e s f u l l y occupied.

Such a p i c t u r e c o n t r a d i c t s t h e r e s u l t s o f t h e Hal 1 e f f e c t measurements /23-25/ as w e l l as t h e r e - s u l t s o f measurements o f t h e t e m p e r a t u r e dependence o f t h e p h o t o c o n d u c t i v i t y /26,4/. F u r t h e r m o r e under t h e above assumption we would e x p e c t t h e same r e l a x a - t i o n t i m e b o t h f o r t h e p h o t o c o n d u c t i v i t y o c c u r r i n g a t 0 . 1 eV and t h a t o c c u r r i n g a t 0.5 eV and 0.6 eV.

T h i s c o n t r a d i c t s t h e e x p e r i m e n t a l r e s u l t s showing t h a t p h o t o c o n d u c t i v i t y a t 0.1 eV has much s h o r t e r r e l a x a t i o n t i m e s t h a n t h a t a p p e a r i n g a t 0.5 eV /7,9/.

Agreement w i t h t h e r e s u l t s o f t h e H a l l e f f e c t and r e l a x a t i o n t i m e o f t h e p h o t o c o n d u c t i v i t y measurements can be a c h i e v e d assuming t h a t i n d i f f e r e n t r e g i o n s o f t h e d i s l o c a t i o n ( o r f o r d i s l o c a t i o n s l o c a t e d i n d i f f e r e n t p a r t s o f t h e c r y s t a l ) t h e d e n s i t i e s of I and A s t a t e s a r e n o t c o n s t a n t . I n t h e r e g i o n s where t h e d e n s i t y o f I s t a t e s i s h i g h enough t o f i x t h e Fermi l e v e l a t t h e I l e v e l I s t a t e s a r e p a r t i a l l y occupied and t h e A s t a t e s a r e empty ( I r e g i o n s ) . On t h e o t h e r hand, i n t h e r e g i o n s where t h e d e n s i t y o f I ( o r b o t h I and A) s t a t e s i s low t h e Fermi l e v e l may be f i x e d a t t h e A 1 eve1 (A r e g i o n s )

,

and conse- q u e n t l y A s t a t e s a r e p a r t i a l l y o c c u p i e d and t h e I s t a t e s a r e f u l l y occupied. Thus i n n - t y p e c r y s t a l t h e p o t e n t i a l b a r r i e r a s s o c i a t e d w i t h I re g i o n s s h o u l d be h i g h e r t h a n t h a t a s s o c i a t e d w i t h A r e g i o n s of d i s l o c a t i o n s . Consequently, t h e r e l a x a t i o n t i m e of m a j o r i t y c a r r i e r s i n n - t y p e c r y s t a l s a s s o c i a t e d w i t h r e c o m b i n a t i o n a t I r e g i o n s w i l l be much h i g h e r t h a n t h a t a s s o c i a t e d w i t h A r e g i o n s s i n c e t h e y a r e c o n t r o l l e d b y a p o t e n t i a l b a r r i e r s a t t h e s e r e g i o n s . The h i g h v a l u e of t h e r e l a x a t i o n t i m e o f t h e photo- c o n d u c t i v i t y a t 0.5 eV and 0.6 eV shows t h a t t h e pho- t o c o n d u c t i v i t y i n t h i s s p e c t r a l r e g i o n i s m a i n l y asso- c i a t e d w i t h I r e g i o n s of d i s l o c a t i o n s . On t h e o t h e r hand, t h e s h o r t r e l a x a t i o n t i m e a t 0 . 1 eV shows t h a t i n t h i s s p e c t r a l r e g i o n t h e t r a n s i t i o n s i n A r e g i o n s

dominate.

The inhomogeneity o f t h e d i s l o c a t i o n c o r e s t r u c - t u r e may r e s u l t f r o m t h e i n t e r a c t i o n o f d i s l o c a t i o n w i t h p o i n t d e f e c t s and chemical i m p u r i t i e s 1151.

Acknowledgements.- The a u t h o r i s g r e a t l y i n d e b t e d t o Dr. W. S z k i e l k o f o r v a l u a b l e d i s c u s s i o n s .

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