HAL Id: jpa-00220821
https://hal.archives-ouvertes.fr/jpa-00220821
Submitted on 1 Jan 1981
HAL
is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire
HAL, estdestinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
STUDIES OF CHALCOGENIDE VITREOUS SEMICONDUCTORS IN THE IOFFE
PHYSICOTECHNICAL INSTITUTE
B. Kolomiets
To cite this version:
B. Kolomiets. STUDIES OF CHALCOGENIDE VITREOUS SEMICONDUCTORS IN THE IOFFE
PHYSICOTECHNICAL INSTITUTE. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-887-C4-
897. �10.1051/jphyscol:19814194�. �jpa-00220821�
JOURNAL DE PHYSIQUE
Colloque C4, suppZBment au nOIO, Tome 42, octobre 1981 page C4-837
STUDIES OF CHALCOGENIDE VITREOUS SEMICONDUCTORS IN THE IOFFE PHYSICO- TECHNICAL INSTITUTE
A.F.' Ioffe Physical-Technical Institute, Academy 07 Sciences of the USSR, 194021, Leningrad, USSR
The broad c l a s s of semiconductors d i s c o v e r e d a t t h e I o f f e Physi- cal-Technical I n s t i t u t e i n 1955-56 fl] and termed a t p r e s e n t chalco- genide v i t r e o u s semiconductors (ChVs) has r e c e n t l y been a t t r a c t i n g e v e r i n c r e a s i n g i n t e r e s t of t h e r e s e a r c h e r s due t o many p r o p e r t i e s which a r e unusual f o r c r y s t a l l i n e semiconductors
.
During a nurnber of y e a r s , our i n t e r e s t s h e r e been p r i m a r i l y con- n e c t e d w i t h s u c h b a s i c problems a s t h e t r a n s p o r t phenomena, e l e c t r o - n i c spectrunl, t h e r o l e of i m p u r i t i e s , o p t i c a l and p h o t o e l e c t r i c phe- nomena, induced s t r u c t u r a l t r a n f ormations e t c . 12-63
.
Out of many d i r e c t i o n s pursued a t o u r I n s t i t u t e we have chosen f o r t h i s p a p e r only a few where, i n o u r o p i n i o n , t h e most i n t e r e s t i n g and novel r e s u l t s have been obtained.
1. Impurity E f f e c t on t h e E l e c t r i c a l P r o p e r t i e s of ChVS
Already t h e e a r l y experiments r e v e a l e d t h e i m p u r i t i e s t o have only a weak e f f e c t on t h e e l e c t r i c a l p r o p e r t i e s of ChVS which w
a t t r i b u t e d t o a t o t a l v a l e n c e s a t u r a t i o n of t h e i m p u r i t y atoms
7-4 .
Group 1 elements on ChVS c o n d u c t i v i t y [lo]
,
and t h e Ag i m p u r i t y was e s t a b l i s h e d t o l e a d t o a f o r m a t i o n of i m p u r i t y c e n t e r s which a f f e c t markedly t h e p o s i t i o n of t h e Fermi l e v e l 111) It w a s shown 12,13] t h a t an e f f i c i e n t doping of some ChVS of.
complex composition accompanied by a s u b s t a n t i a l change in e l e c t r i c a l parameters can be produced by high-frequency c o s p u t t e r i n g of t h e i n i - t i a l ChVS and t h e dopant. This doping t e c h n i q u e was termed modifica- t i o n . It appeared of i u p o r t a n c e t o check t h e p o s s i b i l i t y of modifying i n t h i s way v i t r e o u s a r s e n i c s e l e n i d e and s u l f i d e , i . e , t h e t y p i c a l m a t e r i a l s whose p r o p e r t i e s were s t u d i e d most thoroughly.
Our r e s u l t s p r e s e n t e d i n a s e p a r a t e r e p o r t a t t h i s Conference
$1
s u g g e s t t h a t t h e m o d i f i c a t i o n p r o c e s s achieved by high-f requency c o s p u t t e r i n g and accompanied by a c o n s i d e r a b l e change i n t h e m a t e r i a l p r o p e r t i e s o c c u r s a l s o i n f i l m s of s u c h t y p i c a l ChVS a s As2Se3 and A s S*
and o n l y t r a n s i t i o n m e t a l s w i t h u n f i l l e d d - s h e l l s r a t h e r t h a n any is t y p i c a l f o r a broad c l a s s of ChVS. A t t h e same t u n e ,o t h e r m e t a l i n t r o d u c e d a s a dopant can e f f i c i e n t l y change t h e e l e c t - r i c a l p r o p e r t i e s . This i s i l l u s t r a t e d i n Fig.1 showing t h e v a r i a t i o n i n p o s i t i o n of t h e Permi l e v e l i n v i t r e o u s A s Se f i l m s w i t h v a r i a - t i o n of c o n c e n t r a t i o n and type of a m o d i f i e r . The most e f f i c i e n t i n t h i s r e s p e c t a r e s e e n t o be N i and 1\10 c h a r a c t e r i z e d by u n f i l l e d d - s h e l l s
.
2. The Role of I m p u r i t i e s i n Luminescence
One of t h e l e a s t s t u d i e d a s p e c t s of t h e ChVS luminescence is t h e r o l e of i m p u r i t i e s which i s of i n t e r e s t from b o t h a t h e o r e t i c a l and p r a c t i c a l s t a n d p o i n t s . Indeed, an i m p u r i t y of a g i v e n charge s t a t e can m a n i f e s t i t s e l f i n d i f f e r e n t ways i n t h e ChVS network depending on t h e o r i g i n of t h e luminescence c e n t e r s .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814194
C4-888 JOURNAL DE PHYSIQUE
The a v a i l a b l e d a t a on t h e f f e c t of i m p u r i t i e s on photolumine- scence (PL) a r e c o n t r a d i c t o r y F2-151 c a l l i n g f o r f u r t h e r r e s e a r c h a l o n g t h e s e l i n e s .
We have s t u d i e d t h e e f f e c t of Grope 1 and I V i m p u r i t i e s on t h e PL of v i t r e o u s chalcogenides of a r s e n i c (As2Se3 and As2S3) and ger- manium (Ge2S3).
The imp;rities (Cu, Ag, Au, Pb, Sn) were i n t r o d u c e d d u r i n g pre- p a r a t i o n . A l l t h e PL measurements were c a r r i e d out i n t h e temperature range 4.2
-
77OK, e x c i t a t i o n produced at t h e photon energy c o r r e s - ponding t o t h e peak of t h e e x c i t a t i o n spectrum. A s t u d y was made of t h e PL e x c i t a t i o n and emission s p e c t r a , as w e l l as of t h e PL decay k i n e t i c s a t h e r s w i t c h i n g o f f t h e e x c i t a t i o n .t -
4 a t % "'
' de '5 '
i 2 ' i 6 'rb' i o ' it'
L,ha -Fig. 1. lvlodif i c a t i o n - i n d u c e d Fig.2. PL e x c i t a t i o n emission s p e c t r a F e m i l e v e l s h i f t in As2Se3 of v i t r e o u s A s S and Ge2S3 doped w i t h
f i l m s
.
i m p u r i t i e s (T 2 = 3 4.2OK).( a ) 1
-
As2S3; 2-
A s 2 3 S +0.1 at,% Cu;( b ) 1
-
Ge2S3; 2-
Ge S +0.1 a t . % Cu;2 3 3 - Ge S + 0 . 1 2 a t , % A u .
2 3
The PL e x c i t a t i o n emission s p e c t r a of v i t r e o u s A s S and Ge2S3 doped w i t h Cu and Ag a r e p r e s e n t e d i n Fig.2a. A s s e e n 3 from
Fig.2a, i n t r o d u c t i o n 0.1 a t . % Cu i n t o A s S r e s u l t s i n a s u b s t a n t i a l s h i f t of PL s p e c t r a towards low e n e r g i e s 2 ( t h e emission spectrum being s h i f t e d by 0.2 eV, and t h e e x c i t a t i o n spectrum, by 0.4 eV).
The PL i n t e n s i t y a p p e a r s t o be somewha'c reduced compared w i t h an un- doped sample.
Pig.2b d i s p l a y s d a t a on t h e e f f e c t of Cu and Au i m p u r i t i e s on t h e PL of v i t r e o u s Ge S which show t h a t while n e i t h e r Cu o r Au a f - f e c t n o t i c e a b l y t h e shape and p o s i t i o n of t h e emission and e x c i - t a t i o n s p e c t r a , t h e y r e s u l t i n an i n c r e a s e of PL i n t e n s i t y , The Au i m p u r i t y i n t r o d u c e d a t 0.12 at.% enhances t h e PL i n t e n s i t y by a f a c - t o r of 6 compared w i t h an undoped sample. Other Group I i m p u r i t i e s l i k e w i s e enhance PL i n t e n s i t y a l t h o u g h t o a l e s s e r e x t e n t . I n cont- rast, doping Ge2S3 w i t h ~ r o u p m e t a l s - ( p a r t i c u l a r l y w i t h ~ b ) d e c r e a s e s PL ( s e e Pin.3).
-
- -These e x p e r i m e n t a l d a t a permit c e r t a i n c o n c l u s i o n s t o be drawn.
None of t h e m e t a l i m p u r i t i e s i n t r o d u c e d i n t h e course of p r e p a r a t i o n i n t o v i t r e o u s a r s e n i c and germanium c h a l c o g e n i d e s c r e a t e s an a d d i t i o - n a l emission band, however t h e y e f f e c t t h e PL of undoped g l a s s e s . T h i s i m p l i e s t h a t a l t h o u g h an i m p u r i t y does n o t produce new e m i s s i o n c e n t e r s in ChVS i t changes e i t h e r t h e c o n c e n t r a t i o n of t h e c e n t e r s a l r e a d y p r e s e n t i n t h e m a t e r i a l o r t h e p r o b a b i l i t y of r a d i a t i v e t r a n - s i t i o n s due t o t h e appearence of l o c a l i z e d s t a t e s of a n o t h e r type.
The a c t u a l of t h e i m p u r i t y a c t i o n on PL i n ChVS depends apparen- t l y on t h e n a t u r e of chemical bonds between t h e atoms of t h e m a t e r i a l . Measurements showed t h e i m p u r i t y e f f e c t on PL t o be s t r o n g e r i n
germanium t h a n i n a r s e n i c chalcogenides. This cau be a t t r i b u t e d t o t h e f a c t t h a t c o v a l e n t bonds a r e more pronounced i n germanium chslco- g e n i d e s , t h e i r network being more " r i g i d n t h a n t h a t i n a r s e n i c chal- cogenides where Van d e r Waals f o r c e s p l a y an i m p o r t a n t r o l e . I n t h e l a t t e r c a s e , as is w e l l known, l o c a l rearrangement of t h e network can occur, s o t h a t t h e i m p u r i t y w i l l not cause any change of p o t e n t i a l . 3. E l e c t r o a b s o r p t i o n
It was b e l i e v e d u n t i l r e c e n t l y t h a t e l e c t r o a b s o r p t i o n (33.4) in k V S , j u s t as a b s o r p t i o n , is m a c r o i s o t r o p i c which would make a d i s - t i n c t i v e f e a t u r e of v i t r e o u s m a t e r i a l s of t h e type of A s Se
,
Se,GeSep compared w i t h t h e i r c r y s t a l l i n e analogs. Our l a t e e 3 s t u d i e s of EA i n p o l a r i z e d l i g h t have shown, however, t h a t t h e amplitude of s i g n a l depends s u b s t a n t i a l l y on t h e mutual o r i e n t a t i o n of t h e e l e c t r i c f i e l d I?. I n t h C ~VS s t u d i e d by us, da(,/ ($11
??)
is 1.5-
3 t i m e s l a r g e r t h a n ~ d , ( E l9) .
F o r b o t h components n A - F' 9w h i l e t h e degree of a n i s o t r o p y d &/A dl remains c o n s t a n t w i t h v a r y i n g f i e l d , a t l e a s t up t o l o 5 V &-I. The s p e c t r a l dependence of
a
d~
/A d1 is shown f o r a number of ChVS i n Pig.4 d i s p l a i n g a d i c r e k s e of t h e degree of a n i s o t r o p y w i t h i n c r e a s i n g photon energy d e f i c i t .Pig.3. PL i n t e n s i t y VS impu- r i t y c o n c e n t r a t i o n i n v i t - reous Ge2S3.
1
-
~ ~ ; - 2 - - Au; 3-
Pb.Pig.4. S p e c t r a l dependence -of EA a n i s o t r o p y i n v i t r e o u s AS Se
2 3' AsSe, Ge2S3 and Se a t 300°K.
We have proposed a model t a k i n g i n t o account l o c a l bond a n i s o - t r o p y t o e x p l a i n t h e o p t i c a l a n i s o t r o p y a p p e a r i n g i n ChVS under ap- p l i e d e l e c t r i c f i e l d . This model i s c o n s i d e r e d i n a s e p a r a t e r e p o r t a t t h i s Conference [18]
.
4. Charge C a r r i e r T r a n s i t Time D i s p e r s i o n
The d i s p e r s i o n e x h i b i t e d by charge c a r r i e r s i n t h e i r t r a n s p o r t i n ChVS a t t r a c t s c o n s i d e r a b l e a t t e n t i o n .
The s t o c h a s t i c d i s p e r s i o n t r a n s p o r t theory y i e l d s t h e f o l l o w i n g r e l a t i o n s h i p s f o r t h e hopping a i s t a n c e d i s t r i b u t i o n Y ( t )=cons
t
-1f+d) 9v a r i a t i o n of c u r r e n t w i t h time, and t h e dependence of d r i f t m o b i l i t y on f i e l d P and
JOURNAL DE PHYSIQUE
The parameter O( c h a r a c t e r i z e s t h e degree of d i s p e r s i o n and v a r i e s w i t h i n 0 1
.
The d i s p e r s i o n d e c r e a s e s w i t h i n c r e a s i n gA s s e e n from t h e s e e x p r e s s i o n , t h e t r a n s p o r t c h a r a c t e r i s t i c s a r e i n t e r r e l a t e d v i a parameter which, a c c o r d i n g t o t h e t h e o r y , i s t h e same f o r t t and t > t+-.
L I I Ir L'
S t u d i e s of h o l e t r a n s p o r t i n v i t r e o u s As7Se7 E9,20]
-
/ and e l e c - t r o n t r a n s p o r t i n t h e S1-xAs, system w i t h x = 0.05-
0.17 [21] car- r i e d out i n o u r l a b o r a t o r y showed t h e t r a n s p o r t parameter d i n ti- rues s h o r t e r and l o n g e r t h a n t h e t r a n s i t time t t T t o be d i f f e r e n t i n magnitude which d i s a g r e e s w i t h t h e p r e s e n t i d e a s concerning t h e d i s - p e r s i o n t r a n s p o r t . These s t u d i e s were performed a t room temperature and a t e l e c t r i c f i e l d s 104- ,
5 cm-l u s i n g t h e w e l l known d r i f t m o b i l i t y measurement t e c n i q u e .Pigs. 5 and 6 d i s p l a y c u r r e n t vs. time p l o t s f o r t h e d r i f t of ho- l e s i n As2Se3, and of e l e c t r o n s , i n Sel-xAs, (x=0.1). A n a n a l y s i s showed t h e s l o p e of t h e c u r r e n t decay r e g i o n a t t L t t r t o be small.
Pig.5. Current vs. time p r o f i l e Pig. 6. Current vs
.
time p r o f i l e f o r h o l e d r i f t i n A s Se3. f o r e l e c t r o n d r i f t i n SegOAsI0.a t V=50 ~ 0 1 % (1 )
gi*E
2 5 Volt (2). d = 1 . 1 p a t V = 18 Volt ( I ) , 9 Volt ( 2 ) and 5.5 Volt (3).D e s c r i b i n g i t by a f u n c t i o n of t h e form t-4, we o b t a i n n
'
0.2, which corresponds t o a p r a c t i c a l l y normal t r a n s p o r t w i t h d 7 0.8.An a n a l y s i s of t h e c u r r e n t decay p r o f i l e a t t t y i e l d e d cha- r a c t e r i s t i c s t y p i c a l f o r anomalous t r a n s p o r t . ~ n d e e d , % n e observed i n A s Se prolonged c u r r e n t decay corresponding t o s t r o n g d i s p e r s i o n
w i t h L 0.2
-
0.3. A s t u d y of t h e c u r r e n t p r o f i l e cor- responding t o e l e c t r o n d r i f t i n Se,-,As, a t t 7 t t r showed t h e c u r r e n t t o f o l l o w a power law behaviour f m where m v a r i e s from 2.0 t o 1.7 w i t h i n c r e a s i n g a r s e n i c c o n t e n t and p r a c t i c a l l y does n o t depend on v o l t a g e*.
*
For some samples we o b t a i n e d m > 2.0. A c o n s i d e r a t i o n of such o a s e s would be beyong t h e s c a p e of t h e p r e s e n t paper.The l a r g e e x t e n t of t h e c u r r e n t decay r e g i o n a t t z t t r and t h e t h e independence of c u r r e n t decay p r o f i l e on v o l t a g e a l s o imply t h e e x i s t e n c e of a r e l a t i v e l y s t r o n g e l e c t r o n d i s p e r s i o n which does n o t obey t h e Gaussian s t a t i s t i c s .
Thus t h e c h a r a c t e r i s t i c s o b t a i n e d a t t 4 t t r g i v e evidence f o r a close-to-normal t r a n s p o r t , while a t t 7 t t r d i s p e r s i o n t y p i c a l f o r anomalous t r a n s p o r t is observed. This can be accounted f o r by assuming d i s t r i b u t i o n of charge c a r r i e r s i n m o b i l i t y . I n t h i s c a s e , normal t r a n s p o r t w i l l t a k e p l a c e f o r charge c a r r i e r s a t t ~
,
tt h e c u r r e n t b e i n g c h a r a c t e r i z e d by a c o n s t a n t mean e f f e c t i v e cha@e c a r r i e r s having d i f f e r e n t m o b i l i t i e s . A r e l a t i o n s h i p be tween t h e charge c a r r i e r d i s t r i b u t i o n f u n c t i o n i n m o b i l i t y P(p) and t h e c u r r e n t decay p r o f i l e was e s t a b l i s h e d f o r t h i s t r a n s p o r t mechanism [22].
For a s t e pf u n c t i o n
t h e c u r r e n t was found t o be c o n s t a n t up t o t t r and t o decay by q u a d r a t i c law a f t e r ttr.
I f t h e f u n c t i o n P(J~YI ) d e c r e a s e s w i t h
y
-+0, t h e d e c r e a s e of c u r r e n t i n time a f t e r t t r w i l l be s t r o n g e r pronounced, while i f P(/) i n c r e a - s e s withc/uAO t h e c u r r e n t v a r i a t i o n w i l l be weaker. For t h e l i m i - t i n g c a s e p ( ~ *j+
00 w i t h J U - - r O one o b t a i n e s J ( t ) - t-1.A comparison of t h e o r y w i t h e x p e r i m e n t a l shows t h e e l e c t r o n d i s - t r i b u t i o n f u n c t i o n i n Sel-xAs, t o have a component which v a r i e s only weakly w i t h
p-
0. I n t h e c a s e of h o l e s , however, t h e d i s t r i - b u t i o n a p p a r e n t l y h a s an enhanced number of charge c a r r i e r s w i t h s i n a l l p.
Thus t h e above r e s u l t s imply t h a t one cannot c h a r a c t e r i z e t r a n s p o r t i n ChVS by one v a l u e of m o b i l i t y only, s o t h a t t h e observed t r a n s i t time d i s p e r s i o n may be connected w i t h a charge c a r r i e r d i s t - r i b u t i o n i n m o b i l i t y .5. S t r u c t u r a l Transformations Induced by E x t e r n a l F a c t o r s
One of t h e i n t e r e s t i n g phenomena r e v e a l e d i n r e c e n t y e a r s i n ChVS and n o t observed i n t h e i r c r y s t a l l i n e analogs a r e r e v e r s i b l e s t r u c t u r a l t r a n s f o n a t i o n s induced by e x t e r n a l f a c t o r s , most n o t a b l y by l i g h t .
S t u d i e s showed p h o t o e x c i t a t i o n i n ChVS t o produce a change n o t only of o p t i c a l but of many o t h e r p r o p e r t i e s a s w e l l , among them p h o t o e l e c t r i c , e l e c t r i c a l , c o n t a c t and physicochemical, a s shown s c h e m a t i c a l l y i n Table 1.
Of importance i s t h a t t h e s e changes a r e r e v e r s i b l e , i.e. t h e y d i s a p p e a r under h e a t i n g and may r e c o v e r completely a g a i n on i l l u m i n a - t ion.
Photoinduced o p t i c a l a n i s o t r o p y w a s observed i n a l l ChVS e x h i b i - t i n g photoinduced changes of t r a n s m i t t a n c e . A s an i l l u s t r a t i o n , Fig.7 shows s p e c t r a l c h a r a c t e r i s t i c s of photoinduced dichroism which appea- r e d i n an AsSe f i l m exposed t o p l a n e - p o l a r i z e d l i g h t from a He-Ne la- s e r ( t h e a n g l e of t u r n of t h e p l a n e of l i g h t p o l a r i z a t i o n )o is
JOURNAL DE PHYSIQUE
T a b l e 1
Photoinduced E f f e c t s i n ChVS
P r o p e r t y A f f e c t e d parameter
O p t i c a l Absorption c o e f f i c i e n t
B e f r a c t i v e i n d e x
Appearence of dichroism and b i r e f r i n g e n c e
P h o t o e l e c t r i c a l
E l e c t r i c a l Contact
P l ~ y s i c o c h e ~ a i c a l
P h o t o c o n d u c t i v i t y spectrum P h o t o c o n d u c t i v i t y amplitude Photoresponse k i n e t i c s Lux-ainpere c h a r a c t e r i s t i c D i e l e c t r i c c o n s t a n t
Charge c a r r i e r n o b i l i t y
Hagnitude and s i g n c o n t a c t photo-emf E l e c t r e t c h a r a c t e r i s t i c s
Ziicrohardness Densi-ty
S o f t e n i n g temperature
S o l u b i l i t y i n i n o r g a n i c and o r g a n i c s o e v e n t s
Others P ~ o t o e l e c t r o n emission s p e c t r a
LIossbauer s p e c t r a
p r o p o r t i o n a l t o t h e photoinduced d i c h r o i s m ) . Also s h o w a r e t h e i n i - t i a l ( T ~ ) and l i g h t - i n d u c e d s p e c t r a l c h a r a c t e r i s t i c s of dichroism and t r a n s ~ i t t a n c e a r e c o r r e l a t e d ; t h e y a r e due t o l i g h t i n t e r f e r e n c e i n t h e t h i n f i l m . The appearance of d i c h r o i s n i n ChVS i s accompanied by photoinduced b i r e f r i n g e n c e .
Thus t h e i n i t i a l l y o p t i c a l l y i s o t r o p i c ChVS f i l m s r e v e a l on il- l u m i n a t i o n w i t h p o l a r i z e d l i g h t a photoinduced a n i s o t r o p y which d i s - zppears s i n u l t a n e o u s l y with t h e d i s a p p e a r a n c e of photoinduced t r a n s - m i t t a n c e spectrum s h i f t a f t e r t h e sample anneal. The phenomenon of photoinduced a n i s o t r o p y is a t t r i b u t e d t o t h e p r e s e n c e i n ChVS f i l m s of o p t i c a l l y a n i s o t r o p i c s t r u c t u r a l elements d i s t r i b u t e d i n a random way i n s p a c e , i n p a r t i c u l a r , i t may be due t o an a n i s o t r o p y of v a r i o s atomic bonds.
It was shovm t h a t s t r u c t u r a l t r a n s f o r m a t i o n s i n ChVS van be i n - duced n o t only by l i g h t but by o t h e r e x t e r n a l f a c t o r s a s w e l l , i n p a r t i c u l a r , by exposure t o X-rays and f a s t e l e c t r o n s
.
The d i s c o v e r y of t h e photobleaching e f f e c t on f i l m s of t h e As-Se and As-S system prepared by t h e c o n v e n t i o n a l e v a p o r a t i o n technique but darkened p r e l i m i n a r i l y a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n [24]
.
A s an i l l u s t r a t i o n , we p r e s e n t i n Fig. 8 t r a n s m i s s i o n c h a r a c t e r i - s t i c s of A s Se2 f i l m s a s a f u n c t i o n of temperature. The l i n e AB shows t h e photodaJkening of t h e f i l m a t room temperature. AC and BEDC d i s - p l a y , x e s p e c t i v e l y , t h e v a r i a t i o n w i t h temperature ( a t a h e a t i n g r a t e
-
3 deg/(min) of t h e t r a n s m i t t a n c e of an exposed and unexposed a r e a s of t h e f i l m r e s p e c t i v e l y . I n t e r e s t i n g l y enough, i l l u m i n a t i o n of a p r e l i m i n a r i l y darkeyed r e g i o n a t e l e v a t e d t e m p e r a t u r e s produces blea- ching ( s e c t i o n s Y ' F and yl' F" ). Note t h a t t h e corresponding f i l m t r a n s m i t t a n c e r e a c h e s t h e same l e v e l which can be o b t a i n e d by f l l w n i - n a t i o n of unexposed f i l m a t t h e same temperature ( s e c t i o n s H P and H" P" ). Thus, t h e l i n e BEF'F"C shows t h e s t e a d y - s t a t e f i l mt r a n s m i t t a n c e r e a c h e d m d e r i l l u n ~ i n a t i o n a t d i f f e r e n t t e m p e r a t u r e s . The p h o t o b l e a c h i n g e f f e c t is s e e n t o i n c r e a s e w i t h t e m p e r a t u r e and most i m p o r t a n t l y , t o have a t e m p e r a t u r e t h r e s h o l d . The f i n a l t r a n s - m i t t a n c e l e v e l i s d e t e r m i n e d by i l l u m i n a t i o n and t e n p e r a t u r e and d o e s n o t depend on t h e i n i t i a l s t a t e of t h e f i l m .
It i s e s s e n t i a l t h a t t h e sensitivity of t h e p h o t o b l e a c h i n g pro- c e s s e x c e e d s c o n s i d e r a b l y (5-10 t i m e s f o r As2Se3 f i l m s , and 20-30 t i - Ines f o r A s S f i l m s ) t h a t of p h o t o d a r k e n i n g .
2 3
l
Fig.?. S p e c t r a l c h a r a c t e r i s t i c s of l i g h t t r a n s m i t t a n c e and p h o t o i n d u c e d-
a n i s o t r o p y i n AsSe f i l m f o r expo-60s
s u r e t i m e s 300 s ( f and TI ), 720 s (%
and T2) and 3000 s48
(
%
and T3).20
An a t t e m p t w a s made t o e x p l a i n t h e
0 r e s u l t s o b t a i n e d i n t h e f r a m e s of a con-
500 550 600 680 700 750 f i g u r a t i o n a l rnodel i n v o l v i n g two s t a b l e R(tm,- (ground and r n e t a s c a b l e ) s t a t e s of a s o n i c
u n i t a s i n CnVS between mhich t h e r i l a l and o p t i c a l t r a n s i t i o n s o c c u r ( P i g . ? ) [24]
.
The s t r u c t u r e of t h e f i l m a n d , a c c o r d i n g l y , i t s properties (op- t i c a l , physicocliemical and o t h e r s ) a r e d e t e r m i n e d , j u s s as i n s o l i d s o l u t i o n , by t h e r e l a t i v e n u n b e r of a t o m i c u n i s s r e s i d i n g i n t h e g r o - und and n m e t a s t a b l e n s t a t e s . To an a t o m i c u n i t i n the ground s t a t e c o r r e s p o n d s an e q u i l i b r i ~ u n c o n f i g u r a t i o n c o o r d i n a t e Q, and e n e r g y E I 0 , and t o t h e n e t a s t a b l e s c a x e , Q2 and E20, r e s p e c t i v e l y . O p t i c a l t r a n - s i t i o n s between t h e s t a t e s o c c u r i n a c c o r d a n c e w i t h t h e Frank- -Condon p r i n c i p l e , t h e a t o m i c c o n f i g u r a t i o n r e m a i n i n g unchanged du- r i n g t h e t r a n s i t i o n . Of p a r t i c u l a r i n t e r e s t i s t h e
c a s e where t h e e l a s t i c p r o p e r t i e s of a u n i t i n t h e ground and m e t a s t a b l e
A
s t a t e s d i f f e r c o n s i d e r a b l y which may be e x p r e s s e d v i a d i f f e t e n t s t e e p n e s s of p a r a b o l a s c h a r a c t e r i z i n g t h e s e s t a t e s (Fig.9).An a n a l y s i s o f t h e d i r e c t o p t i - c a l , r e v e r s e o p t i c a l and r e v e r s e
~ h e r m a l . t r a n s i t i o n s ( t h e d i r e c t t h e r - n a l t r a n s i t i o n b e i n g n e g l e c t e d ) y i e l d s a n e x p r e s s i o n f o r t h e s t e a d y - -st a t e p o p u l a t i o n o f t h e me t a s t a b l e s t a t e a s a f u n c t i o n of l i g h t i n t e n s i -
roc - t y I and t e m p e r a t u r e [25] .
I n t h e
e x p r e s s i o n 6
and 6;
a r e t h e d i -
Fig.8. T r a n s m i ~ t a n c e of exposed r e c t and r e v e r s e o p t i c a l t r a n s i t i o n
and unexposed p a r t s of A s Se c r o s s - s e c t i o n s , $ is a f r e q u e n c y
f i l m v s .
t e m p e r a t u r e . of t h e o r d e r of phonon f r e q u e n c y ,
i s t h e m e t a s t a b l e s t a t e w e l l d e ~ t h . t h e meaning of El ( U ) and E2
(b
) 'b e i n g c l e a r from Fig.9.
2
'
C4-894 JOURNAL DE PHYSIQUE
The e x p r e s s i o n t h u s o b t a i n e d d e s c r i b e d w e l l t h e t o t a l i t y of t h e e x p e r i m e n t a l r e s u l t s i n c l u d i n g b o t h t h e p h o t o b l e a c h i n g a t e l e v a t e d t e m p e r a t u r e s and t h e enhanced s e n s i t i v i t y t o l i g h t of t h e photoblea- c h i n g e f f e c t , a s w e l l as t h e e f f e c t of v a r i a t i o n of l i g h t i n t e n s i t y and wavelength and a number of f e a t u r e s i n t h e k i n e t i c s of p h o t o s t i - m u l a t e d p r o c e s s e s .
The c o n f i g u r a t i o n a l model p r o v i d e d a p o s s i b i l y t y n o t o n l y t o e x p l a i n t h e known f a c t s but a l s o t o p r e d i c t new phenomena. I n d e e d , i t was used t o p r e d i c t t h a t compression o f samples which i s equiva- l e n t t o h e a t i n g s h o u l d reduce p h o t o d a r k e n i n g w h i l e t h e a p p l i c a t i o n of t e n s i o n s h o u l d l e a d t o a n i n c r e a s e of photodarkening. Fig.10 shows t h e r e s u l t s o f an e x p e r i m e n t a l check on t h i s p r e d i c t i o n on A s Se
f i l m s
.
2 3Fig.9. C o n f i g u r a t i o n a l Fig. 1 0 . laximum phot o d a r k e n i n g of
diagram As2Se3 v s . a p p l i e d l o a d .
A t p r e s e n t , however, one cannot d e t e r m i n e w i t h c o n f i d e n c e t h e s t r u c t u r e of atomic u n i t s i n ChVS. One can o n l y s u g g e s t , f o r i n s t a n - c e , t h a t t h e ground s t a t e o f a u n i t i n A s Se o r AsSe f i l m s is occu- p i e d by two c o v a l e n ~ l y bonded A s atoms, w h i l e i n t h e m e t a s t a b l e s t a t e one of t h e A s atoms i s i n t h e i n t e r e s t i t i a l p o s i t i o n . iTie b e l i e - ve t h a t t h e m a j o r e f f o r t of r e s e a r c h e r s i n t h e n e a r e s t f u m r e s h o u l d be aimed a t e l u c i g a t i n g t h e n a t u r e of t h e ground and m e t a s t a b l e s t a - t e s of u n i t s i n ChVS o f d i f f e r e n t composition.
6. Low R e s i s t a n c e S t a t e i n S w i t c h i n g
The most i n t e r e s z i n g and i m p o r t a n t aspecx of t h e s w i t c h i n g e f - f e c t i s , i n o u r o p i n i o n , t h e n a t u r e of t h e low-resistance s t a t e ap- p e a r i n g a f t e r s w i t c h i n g . A h i g h c u r r e n t d e n s i i y f i l a n e n t i s formed i n t h i s c a s e . The p r o c e s s e s r e s p o n s i b l e f o r t h e f i l a m e n t f o r m a t i o n a r e a t p r e s e n t u n c l e a r , and some of i t s i m p o r t a n t c h a r a c t e r i s t i c s a r e s t i l l unknown. Of mosr i n t e r e s t h e r e i s t h e c u r r e n t f i l a m e n t tempera- t u r e . It i s t h i s t e m p e r a t u r e t h a t d e f i n e s s u c h i m p o r t a n t p r o p e r t i e s a s t h e r e v e r s l b i l i t y and s t a b i l i t y of t h e s w i t c h i n g e f f e c t .
Some a u t h o r s b e l e i v e che r e l e a s e o i J o u l e a n h e a t t o p l a y o n l y a n i n o r r o l e i n c u r r e n t filarlens; Tomlation, s o t h a t t h e rnaxitn~un ten- p e r a t u r e i n t h e c u r r e n t f i l a m e n t r e a c h e s 30-60°C a t most (26,271
.
An a n a l y s i s of e x p e r i m e n t a l d a t a c a r r i e d o u t i n o u r l a b o r a t o r y sliowed t h e c u r r e n t f i l a m e n t t e m p e r a t u r e t o a c h i e v e 500-600°K 21
.
ife b e l e i v e t h e r e s u l ~ s of a comparative s t u d y i n t o t h e r e c o v e r y of t h e h i s h - r e s i s t a n c e s t a t e s of s w i t c h e s and memory e l e m e n t s (1.a) t o be one of t h e most c o n v i n c i n g arguments f o r a s u b s t a n t i a l h e a t i n g of t h e c u r r e n t f i l a n e n t . These r e s u l t s o b t a i n e d by t h e t e c h n i q u e of r e f .
[27]
f o r a s w i t c h of Si12Ge10A~g0T148 c o m p o s i t i o n and iie of c o n p o s i t i o n and LIE of c o m p o s i t i o n Ge A s T1 a r e p r e s e n t e d i n Fi8.11.1 5 4 01
One may c l e a r l y s e e a s i m i l a r i t y i n t h e r e c o v e r y p r o c e s s e s m a n i f e s t - i n g i t s e l f i n t h e presence of a k i n k i n t h e r e s i s t a n c e R vs. time p l o t s b o t h f o r t h e s w i t c h e s and t h e memory e l e u e n t s . Since i n L'le t h e m a t e r i a l changes i t s s t a t e and t h e c u r r e n t folament is heated a t l e a s t up t o Tp, t h e s i m i l a r i t y between t h e r e s i s t a n c e r e c o v e r y pro- c e s s e s i n d i c a t e s t h a t i n s w i t c h e s t h e c u r r e n t f i l a m e n t is l i k e w i s e h e a t e d t o a h i g h temperature. An a n a l y s i s of experiixental d a t a by t h e e l e c t r o n h e a t conduction t h e o r y showed t h e f i l a m e n t cool- i n g i n s w i t c h e s t o occur w i t h a time c o n s t a n t of t h e o r d e r of a mic- rosecond from t h e maximum c u r r e n t f i l a m e n t temperature TmS 550°K.
Within t h i s t h e o r y , t h e k i n k on t h e R ( t ) curves is a s s o c i a t e d w i t h t h e h e a t i n g e f f e c t of t h e g a t i n g p u l s e . A t times i n excess of the time c o r r e s p o n d i n g t o t h e break t h i s e f f e c t becomes n e g l i g i b l e .
The l i m i t a t i o n of t h e tem- p e r a t u r e r i s e by t h e l e v e l of 550°K is a p p a r e n t l y due t o a t r a n s i t i o n t o a metal-type con- d u c t i v i t y which i s maintimed b o t h by temperature and by t h e e l e c t r i c f i e l d . I f we assume t h e presence of s u c h a t r a n s i - t i o n , t h e n from t h e h e a t balan- c e ~ t o = p c d ~ T ~ r Z o b t a i n i n g w i t h time of t h e r m a l r e l a x a t i o n
to
=$4' .
onecan d e r i v e an e x p r e s s i o n f o r t h e c u r r e n t d e n s i t y i n t h e f i - Fig. l l . R e s i s t a n c e recovery of s w i t c h lament,
m) f o r V g a t e of1.25 V o l t s ( I ) ;
;d$*%lts ( 2 ) ; 1.65 Volts ( 3 ) and which g i v e s t h e v a l u e memor element (d=2.2~rn) i n t h e memo-
j
10 A G m-2 agreeing with r y (47 and s w i t c h i n g ( 5 ) o p e r a t i n g the experrimental data [27].mode f o r V gate = 1.5 Volts. This agreement i m p l i e s t h e predominant r o l e of thermal p r o c e s s e s i n t h e c u r r e n t f i l a m e n t f o r m a t i o n . A number of phenomena of i n t e r e s t f o r a p p l i c a t i o n s a r e connected w i t h t h e development of a h i g h t e m p e r a t u r e i n t h e f i l a m e n t . Among them i s t h e r e c e n t l y des- c r i b e d l o c a l doping a t s w i t c h i n g [29] which p e r m i t s one t o o b t a i n w i t h i n t h e c u r r e n t f i l a m e n t m a t e r i a l s w i t h new p r o p e r t i e s i m p o s s i b l e t o a c h i e v e by o t h e r known t e c h n i q u e s .
The o p e r a t i o n of t h e photodoping raachanism i n ChVS which had e a r l i e r been s u g g e s t e d on t h e b a s i s of i n d i r e c t evidence (changes of t h e o p t i c a l and e l e c t r i c a l p r o p e r t i e s and of s o l u b i l i t y ) was demonst- r a t e d f o r t h e f i r s t time by d i r e c t d i f f u s i o n experiments. They y i e l - ded t h e d i f f u s i o n parameters of Ag and Au b o t h i n t h e d a r k and m d e r u l l u m i n a t i o n f o r s u c h model ChVS a s A s Se and As2Te3 2 3 [30]
.
Gubanov and Tdott s u g g e s t e d t h a t _ i m p u r i t y atoms i n ChVS always s a t u r a t e a l l t h e i r chemical bonds. Mossbauer s p e c t r o s c o p y provided f o r t h e f i r s t time e x p e r i m e n t a l evidence f o r t h i s h y p o t h e s i s Among t h e t h e o r e t i c a l s t u d i e s c a r r i e d o u t i n r e c e n t y e a r s [31,32] a t o u r
.
I n s t i t u t e one s h o u l d s p e c i a l l y mention works d e a l i n g w i t h t h e e f f e c t of e l e c t r o n - e l e c t r o n i n t e r a c t i o n on t h e major p h y s i c a l p r o p e r t i e s of d i s o r d e r e d systems w i t h l o c a l i z e d s t a t e s , i n p a r t i c u l a r ChVS. Compu- t e r s i m u l a t i o n showed t h e e x i s t e n c e of a Coulomb gap i n t h e d e n s i t y of s t a t e s of e x c i t a t i o n s d e t e r m i n i n g t h e s t a t i c hopping c o n d u c t i v i t y of t h e system [33]
.
The e f f e c t of e l e c t r o n - e l e c t r o n i n t e r a c t i o n on t h e high-frequency hopping c o n d u c t i v i t y w a s a l s o s t u d i e d . It turned o u t t h a t t h i s i n t e r a c t i o n changes s u b s t a n t i a l l y t h e f r e q u e n c y and t e m p e r a t u r e depen ence of c o n d u c t i v i t y f o r b o t h t h e zero-phonon hop- p i n g mechanism [3$ and t h e hopping mechanism 1341 a s s o c i a t e d w i t hC4-896 JOURNAL DE PHYSIQUE
electron-phonon i n t e r a c t i o n [35,36]. These s t u d i e s s t r e s s t h e imade- quacy of o n e - e l e c t r o n t h e o r i e s i n t h e d e s c r i p t i o n of d i s o r d e r e d sys- tems w i t h l o c a l i z e d e l e c t r o n s .
One s h o u l d a l s o mention i n t h i s c o n t e x t t h e o r e t i c a l s t u d i e s [37],+
[38] p r o p o s i n g a new model f o r t h e l o c a l i z e d s t a t e s i n g l a s s e s . It i s s u g g e s t e d t h a t e l e c t r o n s can become t r a p p e d n o t a t d e f e c t s but r a t h e r i n s p e c i f i c r e g i o n s of a p e r f e c t c o v a l e n t g l a s s w i t h an anomalously s m a l l l o c a l e l a s t i c i t y . This model h a s n o t y e t p r o g r e s s e d t o t h e s t a t e where i t could be f i t t e d t o experimental d a t a , however i t may b e a r f r u i t i n t h e f u t u r e .
The p r e s e n t r e p o r t c o n t a i n s o n l y t h e most i n t e r e s t i n g r e s u l t s o b t a i n e d r e c e n t l y , i t b e i n g i m p o s s i b l e t o cover i n f u l l t h e e x p e r i - m e n t a l and t h e o r e t i c a l s t u d i e s c a r r i e d o u t at t h e P h y s i c o t e c h n i c a l
I n s t i t u t e i n t h e f i e l d of ChVS. Indeed, no mention was made of s u c h i n t e r e s t i n g problems as t h e e f f e c t of a s t r o n g f i e l d on t h e e l e c t r i - c a l and p h o t o e l e c t r i c p r o p e r t i e s ; t h e temperature dependence of con- d u c t i v i t y and thermo-emf at t h e t r a n s i t i o n t o t h e l i q u i d s t a t e ; t h e e f f e c t of p r e s s u r e and p e n e t r a t i n g r a d i a t i o n ; photoinduced ESR and I R a b s o r p t i o n ; and a number of o t h e r phenomena t y p i c a l only f o r t h i s new and broad c l a s s of semiconductors.
There can be no doubt t h a t a f u r t h e r i n v e s t i g a t i o n of ChVS pro- p e r t i e s w i l l y i e l d i n t h e n e a r e s t f u t u r e a w e a l t h of new i n f o r m a t i o n of i n t e r e s t f o r t h e s o l i d s t a t e p h y s i c s and r e l e v a n t a p p l i c a t i o n s . References
1. Kolomiets B.T., Goryunova N.A. I z v .
AN
SSSR, s e r . F i z .20
(1956) 1496.2. Kolomiets B.T. Phys. S t a t . Sol. 7 (7964) 358;
7
(1964) 713.3. Kolomiets B.T. Proc. of
IX
1nterZ. Conf. on t h e Physics of semiconductors.v.2,
p.1259, Nauka, Leningrad, 1968,4, Kolomiets B.T., Lyubin V.1~1. Phys. S t a t . Sol. ( a ) 17 (1973) 11.
5. Kolomiets B.T., E l e c t r o n i c Phenomena i n on-Crystmine Semicon- d u c t o r s ( I n R u s s i a n ) , Nauka, Leningrad, 1976, p.23.
6. Kolomiets B.T. Lyubin V.IJ. lXat.Res. Bull. 1 3 (1978) 1343.
7. Kolomiets B.T., Nazarova T.F., I n : S o l i d ~ G t e Physics ( I n Rus- s i a n ) , v.11, 1959, p.22,
8. Andriesh A.LI., Kolomie ts B. T., Bazarova T.F. F i e . Tv. T e l a ,
4
(1962) 2286.
9. hlott N.F. P h i l . Mag. 19 (1969) 835.
10. El-Mosli &I. P i z . Tv. % l a 5 (1962) 1472.
11. Kolomiets B.T., Lebedev
E.A.,
Rogachev N.A. Biz. T. Poluprov.8,
(1974) 5459
12. P l a s c k R., I z u M., Sapru K., Anderson T, Ovshinsky S.R.,
F r i t z s c h e H. Amorphous & Liquid Semiconductors. Ed. by W.E.Spear (Edinburgh) 1977, p. 524.
13. S t o t z e l H., Leimer P., Kuske J., Teubner W. Amorphous semiconduc- tors-78, Pardubice, 1978, p.472.
14. Averjanov V.L., Kolomiets B,T., Lyubin V.M., Prikhodko O.Yu.
The p a p e r a t t h i s Conference.
15. Bishop S.G., Strom U., P r i e b e l e E.J. and T a y l o r P.S.
J. of Non-Cryst. Sol. 32 (1979) 359.
16. A l a s t i H., H i g a s h i ~ . ~ T a n d K a s t n e r Id. P h i l . Mag.
B39
(1979) 191.17. Weinstein B.A. P h i l . Mag. B41 (1980) 235.
18. Gelmont B.L., Kolomiets
B.K
ldazets T.F., Pavlov S.K.The p a p e r a t t h i s Conference.
19, Kolomiets B.T., Lebedev E.A., Kazakova L.P. F i z . T, Poluprov,
12
(1978) 1771.
20. Toth L., Lebedev E.A., Kazakova L.P. P h y s i c a l Phenomena i n Non-Cryst. Semicond. ( I n R u s s i a n ) , Kishinev, 1980, p. 64.
21. Lebedev E.A., Toth L. Karpova L.N. S o l i d S t a t e Commun.,
36
(1980) 139.22. Arkhi ov V.I., Lebedev E.A., Rudenko A . I . Fiz. T. Poluprov.
2
(19slB 712.
23. Zhdanov V.C., Kolomiets B.T., Lyubin V.hl., Idalinovsky V.K.
Phys. S t a t . Sol. ( a )
52
(1979) 621.24. Averyanov V.L., Kolobov A.V., Kolomiets B.T., Lyubin V.M.
Phys. S t a t . Sol. (a) 57 (1980) 81.
25. Kolobov A.V., ~ o l o m i e s B.T., Konstantinov O.V., Lyubin V.bL P r e p r i n t 676. A.P.Ioffe Physical-Technical I n s t i t u t e , 1980.
2 6 . Adler D., Henish H.K., IEott N. Rev. Mod. Phys. 5 (1978) 15.
27. P e t e r s e n K.E., Adler D. J. Appl. Phys. 50 (19797 925.
28. Kolomiets B.T., Lebedev E.A., Tsendin
~ 3 .
Pie. T. Poluprov.15
(1981) 231.
29. Kolomiets B.T., Kalmykova N.P., Lebedev E.A., Taksami I.A., Shpunt V.Kh. Fiz. 1. Poluprov., 14 (1980) 726.
30. Boltaks B.I., Dzhafarov T.D., Kudoyarova V.Kh., Kostenko
I.E.,
Imamov R.Sh., Obraztsov A.A. Piz. T. Poluprov.
12
(1979) 41.31. Gubanov. Quantum E l e c t r o n i c Theory of Amorphous Semiconductors ( I n R u s s i a n ) , Nauka, 1963, p.55.
32. S e r e g i n P.P., Sagatov M.A., Niazets T.F., V a s i l i e v L.N.
Phys. S t a t . Sol. ( a )
28
(1975) 127.33. Baranovskii S.D., Efros A.L,, Gelmont B.L., S h k l o v s k i i B . I . J. Phys. C; S o l i d S t a t e Phys. 12 (1979) 1023-1024.
34. S h k l o v s k i i B.I., E f r o s A.L. Zh3.f.P. 80, (1981) 2511.
35. E f r o s A.L. P h i l . Mag. (in p r i n t ) .
36. Baranovskii S.D. and Uzakov A.A. S o l i d S t a t e Commun., (1980) 829-832.
37. K l i n g e r M . I . , Karpov V.G. Pisma v 2h.E.T.F.
5
(1980) 1473.38. K l i n g e r NISI,, Karpov V.G., Sol. St. Comm.
