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OPTICAL PROPERTIES OF II-VI COMPOUNDS WITH DISLOCATIONS
N. Klassen, Yu. Ossipyan
To cite this version:
N. Klassen, Yu. Ossipyan. OPTICAL PROPERTIES OF II-VI COMPOUNDS WITH DISLOCA- TIONS. Journal de Physique Colloques, 1979, 40 (C6), pp.C6-91-C6-94. �10.1051/jphyscol:1979619�.
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O P T I C A L PRO?ERTIES OF 1 1 - V I COMPOUNDS WITH D I S L O C A T I O N S
N.V. Klassen, Yu.A. Ossipyan.
Solid-State Physics Ins t i t u t e USSR Academy o f Sciences 142432 Chernogolovka USSR
Resume.- On e t u d i e l ' i n f l u e n c e des d i s l o c a t i o n s s u r l ' a b s o r p t i o n de l a l u m i P r e e t l a recombinaison des e l e c t r o n s e x c i t e s dans l e s u l f u r e e t l e s e l e n i u r e de cadmium, a i n s i que l a f o c a l i s a t i o n de l a l u m i P r e p a r l e s d i s l o c a t i o n s . Les r e s u l t a t s s o n t u t i 1 is e s p o u r a n a l y s e r l e s e t a t s e l e c t r o n i q u e s l o c a l i s e s e t l e s champs e l e c t r i q u e s p r o d u i t s p a r l e s d i f f e r e n t s t y p e s de d i s l o c a t i o n s .
A b s t r a c t . - The work t r e a t s t h e i n f l u e n c e o f d i s l o c a t i o n s on l i g h t a b s o r o t i o n and r e c o m b i n a t i o n o f e l e c t r o n e x c i t a t i o n s i n Cds and CdSe and, a l s o l i g h t f o c u s i n g by d i s l o c a t i o n s . The d a t a i n v o l v e d a r e used t o a n a l y s e t h e l o c a l i z e d e l e c t r o n s t a t e s and e l e c t r i c f i e l d s produced b y d i f f e r e n t d i s l o c a t i o n s
11-VI compounds w i t h t h e w u r t z i t e s t r u c t u r e ( l i k e t h e CdS and CdSe used i n o u r e x p e r i m e n t s ) a r e o f p a r t i c u l a r i n t e r e s t f o r t h e s t u d y o f d i s l o c a t i o n i n f l u e n c e on c r y s t a l e l e c t r o n s t r u c t u r e .
P l a s t i c d e f o r m a t i o n o f d i f f e r e n t l y o r i e n t e d samples can cause p r o m i n e n t d i s c r e p a n c i e s i n t h e i r d i s l o c a t i o n c o r e s t r u c t u r e /I/. L o c a l s p e c t r o s c o p y methods t o g e t h e r w i t h p o l a r i m e t r i c a n a l y s i s f o r d i s - l o c a t i o n s t r u c t u r e /2/ a l l o w s one t o r e v e a l a d i r e c t r e l a t i o n s h i p between t h i s s t r u c t u r e and o p t i c a l p r o - p e r t i e s o f t h e m i c r o s c o p i c s i t e i n v e s t i g a t e d . We have m a i n l y used two sample o r i e n t a t i o n s f o r p l a s t i c d e f o r m a t i o n . I n t h e f i r s t o r i e n t a t i o n t h e p l a s t i c f l o w proceeded a l o n g t h e b a s a l (0001) p l a n e whereas i n t h e second one t h e f l o w proceeded a l o n g (1070) p r i s m a t i c p l a n e s / I / . I n t h e f i r s t case d i s l o c a t i o n s w i t h t h e b a s a l s l i p p l a n e a r e i n t r o d u c e d and i n t h e s e c o n d - d i s l o c a t i o n s w i t h t h e p r i s m a t i c s l i p plane.
These d i s l o c a t i o n s w i l l be r e f e r r e d t o as b a s a l and p r i s m a t i c , r e s p e c t i v e l y . The basal d i s l o c a t i o n c o r e c o n t a i n s a row o f i d e n t i c a l i o n s whereas t h e p r i s - m a t i c d i s l o c a t i o n c o r e c o n t a i n s a row o f p a i r s f o r - med from o p p o s i t e l y charged i o n s / I / . Yoreover, t h e b a s a l d i s l o c a t i o n produces a s t r o n g p i e z o e l e c t r i c f i e l d w h i l s t t h e p r i s m a t i c one does n o t /3/. T h i s l e a d s one t o e x p e c t t h e occurence o f g r e a t d i s c r e - p a n c i e s i n t h e p r o p e r t i e s o f b a s a l and p r i s m a t i c d i s l o c a t e d samples. The o p p o s i t e s i g n s o f p h o t o p l a s - t i c e f f e c t f o r t h e g l i d e a l o n g b a s a l and p r i s m a t i c p l a n e s /4/ may be a good e v i d e n c e f o r t h i s expecta- t i o n .
The b a s a l d i s l o c a t i o n s i n f l u e n c e n o n r a d i a t i v e r e c o m b i n a t i o n processes f a r more s t r o n g l y t h a n t h e p r i s m a t i c ones, as p h o t o c o n d u c t i v i t y , e x c i t o n absor- p t i o n and edge luminescence experiments show /5-8/.
A f t e r i n t r o d u c i n g basal d i s l o c a t i o n s , t h e k i n e t i c s o f non-equi 1 ib r i u m c a r r i e r c o n c e n t r a t i o n decay d i f- f e r s s t r o n g l y f r o m an e x p o n e n t i a l f o r m / 6 / : a t h i g h c o n c e n t r a t i o n o f c a r r i e r s t h e r e c o m b i n a t i o n r a t e mar- k e d l y exceeds t h a t i n a n o n d i s l o c a t e d c r y s t a l b u t a t l o w c o n c e n t r a t i o n t h e r e c o m b i n a t i o n r a t e i s on t h e c o n t r a r y much l o w e r .
The d i s l o c a t i o n s o f b o t h t y p e s produce r e d i s - t r i b u t i o n o f t h e e m i s s i o n i n t e n s i t y between t h e bands o f bound e x c i t o n s /8-9/ i n f a v o u r o f t h e e x c i t o n bound t o a n e u t r a l a c c e p t o r . I n t h i s case t h e e f f e c t o f p r i m a s t i c d i s l o c a t i o n s i s s t r o n g e r : a t o r i s m a t i c d i s l o c a t i o n s t h e l i n e o f t h e e x c i t o n bound t o a neu- t r a l a c c e p t o r i s i n c r e a s e d i n a b s o l u t e v a l u e whereas a t basal d i s l o c a t i o n s i t s a b s o l u t e i n c r e a s e i s mis- s i n g / 9 / .
I n t h e e m i s s i o n spectrum o f cadmium s e l e n i d e basal d i s l o c a t i o n s produce a w i d e i n f r a r e d band po- l a r i z e d p a r a l l e l t o t h e g l i d e bands. I t s frequency i s a b o u t a h a l f o f t h a t f o r r e c o m b i n a t i o n a c r o s s t h e f o r b i d d e n gap ( F i g . 1). I n t h e p r i s m a t i c s l i p case t h i s band i s n o t observed.
I n t h e edge p a r t o f t h e e m i s s i o n spectrum o f d i s l o c a t e d cadmium s u l p h i d e a group o f r a t h e r n a r r o w n o n p o l a r i s e d bands i s observed. The bands a t 2.430 and 2.447 eV a r e d i s p l a y e d a t b o t h d i s l o c a t i o n t y p e s i d e n t i c a l l y whereas t h e i n t e r m e d i a t e band a t 2.439eV appears on1 y a t t h e b a s a l d i s l o c a t i o n s i n h i g h r e s i s - t i v i t y c r y s t a l s ( F i g . 2 ) . S i m i l a r bands have been observed i n t h e work / l o / t o o . As t h e sample tempe- r a t u r e grows t h e i n t e r m e d i a t e 1 in e d i m i n i s h e s and t h e h i g h energy l i n e s t r e n g t h e n s r e l a t i v e t h e l o w energy l i n e .
A l t h o u g h t h e 2.430 and 2.447 eV l i n e s a r e i d e n t i c a l a t b o t h d i s l o c a t i o n s t h e i r e x c i t a t i o n spec-
7
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979619
C6-92 JOURNAL DE PHYSIQUE
t r a d i f f e r remarkably. A t p r i s m a t i c d i s l o c a t i o n s a f r e e A - e x c i t o n l i n e and t h e e x c i t o n bound a t a n e u t r a l donor l i n e p r e v a i l whereas a t b a s a l d i s l o c a t i o n s t h e A - e x c i t o n l i n e i s s t r o n g l y weakened and, i n a d d i t i o n , r a t h e r a wide l o w energy t a i l appears /11/.
~nlensily of lumin (arb. wits)
F i g . 1 : The i n f l u e n c e o f b a s a l d i s l o c a t i o n s on photo- luminescence s p e c t r a o f CdSe, 77 K. D o t t e d and d o t - dashed l i n e s - nondefarmed sample, E l l C and E C, r e s p e c t i v e l y . Dashed and d o t t e d l i n e s - deformed sam- p l e ( d i s l o c a t i o n d e n s i t y ~ ~ % l O ~ c m - ~ ) , E I 1 C and E L C , r e s p e c t i v e l y .
Fig. 2 : Local cathodeluminescence s p e c t r a a t b a s a l and p r i s m a t i c d i s l o c a t i o n s , 10 K. Dot-dashed l i n e
- n o n d i f l p c a t e d r e g i o n . D o t t e d l i n e - basal d i s l o c a - t e d region. S o l i d l i n e - p r i s m a t i c d i s l o c a t e d r e g i o n .
Both basal and p r i s m a t i c d i s l o c a t i o n s produce a d d i t i o n a l o p t i c a l a b s o r p t i o n a t t h e l o w energy s i d e o f t h e a b s o r p t i o n edge b u t t h e basal a b s o r p t i o n i s much s t r o n g e r /12-13/. On t h e o t h e r hand, t h e p r i s -
m a t i c d i s l o c a t i o n s a b s o r p t i o n spectrum d i s o l a y s a s o r t o f s t r u c t u r e . A t a d i s t a n c e o f t h e o r d e r of 0.3 eV from t h e a b s o r p t i o n edge a s t e p i s seen t h a t i s t o t a l l y p o l a r i z e d p a r a l l e l t o t h e Burgers v e c t o r o f t h e d i s l o c a t i o n whereas a t s h o r t e r wavelengths t h e d i c h r o i s m weakens and r e v e r s e s i t s s i g n a t 520 nm ( F i g . 3 ) .
F i g . 3 : A b s o r p t i o n s p e c t r a o f p r i s m a t i c d i s l o c a t e d CdS. S o l i d l i n e on t h e l e f t - nondeformed sample.
S o l i d 1 in e on t h e r i g h t -deformed sample (Np108cm-2), p o l a r i s a t i o n p a r a l l e l t o t h e s l i p p l a n e s .
Dashed l i n e - deformed sample, p o l a r i s a t i o n perpen- d i c u l a r t o t h e s l i p planes.
M i c r o s c o p i c s t u d i e s o f 1 ig h t p r o p a g a t i o n
t h r o u g h d i s l o c a t e d c r y s t a l s r e v e a l s a remarkable r e -
arrangement of t h e l i g h t f l o w i n t h e v i c i n i t y o f d i s -
l o c a t i o n a r r a y s /14/. The l i g h t i s c o n c e n t r a t e d a t
one s i d e o f t h e g l i d e band whereas a t t h e o p p o s i t e
s i d e t h e i n t e n s i t y i s minimal. Hence, t h e g l i d e band
i s a prominent b o r d e r between t h e r e g i o n s o f maximal
and minimal b r i g h t n e s s e s . The w i d t h s o f these r e g i o n s
equal s e v e r a l microns ( F i g . 4 ) . Thus t h e f o c u s i n g o f
l i g h t by d i s l o c a t i o n s t a k e s p l a c e . I n t h e case t h a t
a l i g h t beam i s i n c i d e n t on t h e s l i p p l a n e i n a near-
l y p a r a l l e l f a s h i o n f r o m t h e s i d e o f a maximal b r i g h t -
ness i t does n o t c r o s s t h e band b u t changes i t s d i -
r e c t i o n and propagates a l o n g i t . So l i g h t channel1 i n g
b y d i s l o c a t i o n s i s observed. These d i s l o c a t i o n
e f f e c t s depend s t r o n g l y on t h e l i g h t p o l a r i z a t i o n
and frequency. The degree o f f o c u s i n g c o n t r a s t d e t e r -
mined as t h e r a t i o o f t h e d i f f e r e n c e between t h e
maximal and m i n i m a l i n t e n s i t i e s t o t h e i r sum f o r t h e
p o l a r i z a t i o n p e r p e n d i c u l a r t o t h e band grows monoto-
n i c a l l y as t h e wavelength approaches t h e a b s o r p t i o n
edge, whereas f o r p a r a l l e l p o l a r i z a t i o n a d i s t i n c t
minimum o f c o n t r a s t occurs a t 550 nrn ( F i g . 5).
F i g . 4 : L i g h t f o c u s i n g b y a d i s l o c a t i o n g l i d e band i n CdS. The l i g h t beam d i r e c t i o n and t h e g l i d e band a r e normal t o t h e p l a n e o f t h e micrograph. The g l i d e band r u n s f r o m t h e t o p t o t h e b o t t o m o f t h e m i c r o - graph.
F i g . 5 : S p e c t r a o f t h e degree o f f o c u s i n g c o n t r a s t a t t h e p r i s m a t i c g l i d e band, w h i c h i s shown a t f i g u r e 4. S o l i d l i n e - p o l a r i z a t i o n p a r a l l e l t o t h e g l i d e band. Dashed 1 in e - p o l a r i z a t i o n p e r p e n d i c u l a r t o t h e g l i d e band.
The d a t a p r e s e n t e d above may be accounted f o r by t h e d i s l o c a t i o n d i s t u r b a n c e o f t h e c r y s t a l e l e c - t r o n s t r u c t u r e by t h e d i s l o c a t i o n s . As was p r e d i c t e d t h e o r e t i c a l l y t h e r e a r e many d i s c r e p a n c i e s i n t h e e f f e c t s o f t h e basal and p r i s m a t i c d i s l o c a t i o n .
The a b s o r p t i o n s t e p f o r t h e p r i s m a t i c d i s l o c a - t i o n s and t h e w i d e e m i s s i o n band f o r t h e b a s a l d i s - l o c a t i o n s w h i c h i s p o l a r i z e d i n c o n s i s t e n c y w i t h t h e
a i s l o c a t i o n a n i s o t r o o y r e v e a l c l e a r l y bound e l e c t r o n s t a t e s a t b o t h t y p e s o f d i s l o c a t i o n s . The d i f f e r e n c e observed i n t h e parameters o f t h e s e s t a t e s can r e s u l t from t h e 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 c o r e s t r u c t u r e s . F o r i n s t a n c e , t h e a b s o r o t i o n steD obser- ved i n t h e ~ r i s m a t i c case may be formed b y t h e e l e c - t r o n t r a n s i t i o n i n t h e sulfur-cadmium p a i r , which i s p r e s e n t i n t h e p r i s m a t i c c o r e b u t absent i n t h e basal one. T h i s p a i r i s bound t o t h e o t h e r s b y f o u r bonds o n l y whereas i n t h e r e g u l a r l a t t i c e t h e number o f t h e s e bonds e q u a l s s i x . The energy s h i f t o f t h e t r a n - s i t i o n can be a t t r i b u t e d t o t h i s f a c t . On t h e o t h e r hand, t h e b a s a l d i s l o c a t i o n c o r e c o n t a i n s a row o f u n p a i r e d e l e c t r o n s w h i c h can f o r m a l e v e l i n t h e mid- d l e o f t h e energy gap t h u s p r o d u c i n g t h e e m i s s i o n band and t h e n o n r a d i a t i v e r e c o m b i n a t i o n observed.
The i d e n t i t y o f t h e e m i s s i o n d o u b l e t a t 21430 and 2.447 eV f o r b o t h d i s l o c a t i o n s suggests t h e p r e - sence o f s i m i l a r d i s l o c a t i o n e l e c t r o n s t a t e s i n t h e b a s a l and p r i s m a t i c cases. These s t a t e s can be orodu- ced by t h e d e f o r m a t i o n p o t e n t i a l . I t ' s w o r t h n o t i n g t h a t t h e t e m p e r a t u r e b e h a v i o r o f t h e i n t e n s i t i e s o f t h e s e bands i s n o t c o n s i s t e n t w i t h t h e c o n c l u s i o n o f t h e work / l o / assuming t h e l o w energy band t o be a yhonon r e p l i c a o f t h e h i g h energy one. The d o u b l e t i s more l i k e l y t o be a s c r i b a b l e t o t h e t r a n s i t i o n s of t h e e l e c t r o n f r o m t h e c o n d u c t i o n band and f r o m t h e d i s l o c a t i o n l e v e l below i t t o t h e d i s l o c a t i o n l e v e l above t h e valence band.
A c o n s i d e r a b l e nuinber o f d i f f e r e n c e s between t h e p r o p e r t i e s o f b a s a l and p r i s m a t i c d i s l o c a t i o n s may be accounted f o r b y t h e e l e c t r i c f i e l d w h i c h e x i s t s around t h e basal d i s l o c a t i o n . The l o w energy t a i l s t h a t o c c u r e d i n t h e a b s o r p t i o n spectrum and i n t h e d o u b l e t e x c i t a t i o n soectrum f o r t h e b a s a l case may be e x p l a i n e d b y a phonon-assisted e l e c t r o n tune- l i n g made ~ o s s i b l e by t h e s l o p e o f energy bands i n t h e e l e c t r i c f i e l d ( t h e Franz-Keldysh e f f e c t ) /15-16/.
The same f i e l d can weaken an A - e x c i t o n l i n e i n t h e e x c i t a t i o n spectrum considered. I n a d d i t i o n t o t h e h i g h f i e l d r e g i o n s , l o w f i e l d r e g i o n s n e a r t h e b a s a l d i s l o c a t i o n s m i g h t e x i s t . These r e g i o n s may e m i t a d o u b l e t i d e n t i c a l t o t h a t observed a t t h e p r i s m a t i c d i s l o c a t i o n s . Therefore, t h e s t r u c t u r e of t h e e l e c - t r i c f i e l d around t h e basal d i s l o c a t i o n s may be more compl i c a t e d t h a n t h a t p r e d i c t e d f r o m a charged 1 in e model.
I n a d d i t i o n , t h e e l e c t r i c f i e l d of b a s a l d i s -
l o c a t i o n s must s e p a r a t e n o n q u i l i b r i u m e l e c t r o n s and
h o l e s t h u s p r e v e n t i n g them f r o m recombining. The
s m a l l e r t h e c o n c e n t r a t i o n t h e h i g h e r t h e f i e l d , so
C6-94
