DETAILED TEMPERATURE-CYCLING STUDIES OF PHOTOSTRUCTURAL CHANGE IN a-As2S3

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DETAILED TEMPERATURE-CYCLING STUDIES OF PHOTOSTRUCTURAL CHANGE IN a-As2S3

H. Hamanaka, K. Tanaka, K. Tsuji, S. Minomura

To cite this version:

H. Hamanaka, K. Tanaka, K. Tsuji, S. Minomura. DETAILED TEMPERATURE-CYCLING STUD-

IES OF PHOTOSTRUCTURAL CHANGE IN a-As2S3. Journal de Physique Colloques, 1981, 42

(C4), pp.C4-399-C4-402. �10.1051/jphyscol:1981486�. �jpa-00220943�

CoZZoque C4, suppzlment au nOIO, Tome 4 2 , octobre 1981 page C4-399

D E T A I L E D TEMPERATURE-CYCLING STUDIES OF PHOTOSTRUCTURAL CHANGE IN a-As2S3

* + **

H. Hamanaka, K. ~ a n a k a * , K. T s u j i and S. Minomura Hosei University, Koganei, Tokyo 184, Japrm

*

EZectrotechnicaZ Laboratory, S a k u r a w a , Ibaragi 305, Japan

*

^b I n s t i t u t e for Solid S t a t e Physics, Roppongi, Tokyo 106, Japan

A b s t r a c t . - D e t a i l e d q u a n t i t a t i v e a n a l y s e s of t e m p e r a t u r e depend- e n c e o f p h o t o s t r u c t u r a l change accompanying p h o t o d a r k e n i n g have been performed on As2S3 g l a s s . Main r e s u l t s we o b t a i n e d a r e ;

(1) s a t u r a t e d v a l u e s o f an o p t i c a l gap o f As2S3 g l a s s a f t e r i l l u m 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 t e n d s t o a c o n s t a n t v a l u e i n d e p e n d e n t of any p r e v i o u s h i s t o r i e s o f t h e sample s u c h a s t h e r m a l a n n e a l i n g o r l i g h t i r r a d i a t i o n , b u t ( 2 ) a change i n t h e X-ray d i f f r a c t i o n c u r v e i n d u c e d by p h o t o - e x c i t a t i o n i s somewhat d i f f e r e n t phenomenologically from t h a t by t e m p e r a t u r e e f f e c t .

I n t r o d u c t i o n . - The p h o t o d a r k e n i n g w i d e l y o b s e r v e d i n amorphous c h a l - c o g e n i d e s i s w e l l c h a r a c t e r i s e d a s a s h i f t o f t h e o p t i c a l a b s o r p t i o n edge t o lower photon e n e r g i e s accompanying l o c a l s t r u c t u r a l changer11

[21 [31. I n t h e p r e v i o u s p a p e r , we have shown t h r o u g h t h e d e t a i l e d X- r a y d i f f r a c t i o n measurements t h a t t h e d e n s i f i c a t i o n o f t h e chalcoge- n i d e g l a s s by a p p l y i n g h i g h p r e s s u r e g i v e s r i s e t o t h e s i m i l a r e f f e c t s on t h e o p t i c a l and s t r u c t u r a l p r o p e r t i e s [ 4 ] . U t s u g i and Mizushima have shown t h r o u g h t h e i r f a r - i n f r a r e d a b s o r p t i o n measurements t h a t t h e s t r u c t u r a l change o f Ge-Se g l a s s induced by p h o t o - i r r a d i a t i o n i s a t t r i b u t e d t o t h e i n c r e a s e d f l u c t u a t i o n i n Ge-Se bond a n g l e s [ 5 1 . However, a p h y s i c a l meaning of t h e c o r r e l a t i o n between t h e photodark- e n i n g and p h o t o s t r u c t u r a l change h a s n o t y e t been c l e a r up t o now.

The p u r p o s e o f t h i s p a p e r i s t o d e s c r i b e t h e d e t a i l e d a n a l y s e s of 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 d a r k e n i n g and p h o t o s t r u c t u r a l change of As2S3, and t o d i s c u s s t h e r e l a t i o n between t h e p h o t o s t r u c - t u r a l change and t h e t e m p e r a t u r e e f f e c t .

Experimental.- Amorphous As2S3 t h i n f i l m s used i n t h e o p t i c a l meas- urements were p r e p a r e d by a vacuum e v a p o r a t i o n . Powdered As2S3 g l a s s was e v a p o r a t e d from a t a n t a r u m b o a t on t o soda-lime g l a s s s u b s t r a t e a t a r a t e o f 3 A/sec i n 3 x 10-6 t o r r vacuum. A l l o f t h e a s - d e p o s i t e d f i l m s were a n n e a l e d b e f o r e o p t i c a l measurements a t 180°C f o r 30 min f o r s t a b i l i z i n g t h e f i l m s . The i l l u m i n a t i o n was p r o v i d e d by t h e combination o f t h e Xe lamp, t h e i n f r a - r e d c u t f i l t e r (IRA-25s) and t h e i n t e r f e r e n c e f i l t e r ( A = 470 nm) t h r o u g h a convex l e n s . Power o f a l i g h t i n c i d e n t upon t h e sample was k e p t a t 7.7 mW/cm2. O p t i c a l t r a n s - m i s s i o n of t h e sample a t 470 nm was r e c o r d e d on a c h a r t a s a f u n c t i o n of i l l u m i n a t i o n t i m e . X-ray d i f f r a c t i o n rneasurements of t h e sample were performed b e f o r e and a f t e r t h e i l l u m i n a t i o n a t d i f f e r e n t temper-

a t u r e s .

R e s u l t s and d i s c u s s i o n . - F i g u r e 1 shows t h e o p t i c a l t r a n s m i s s i o n p l o t t e d a g a i n s t t h e i l l u m i n a t i o n time

t

f o r d i f f e r e n t ambient temper- a t u r e s ( T ) a t which t h e i l l u m i n a t i o n s were performed. A s shown i n t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981486

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Ffg. 1 : O p t i c a l t r a n s - mission a t

X

= 470 nm a s a f u n c t i o n o f i l l u - m i n a t i o n t i m e f o r d i f - f e r e n t i l l u m i n a t i o n t e m p e r a t u r e s .

I 1 I I I I 1 I I I

5 10

T I M E (s)

xlo2

f i g u r e , o p t i c a l t r a n s m i s s i o n ( a t 470 nm) d e c r e a s e s w i t h an i n c r e a s e i n i l l u m i n a t i o n t i m e t depending on i l l u m i n a t i o n t e m p e r a t u r e T , b e i n g a t y p i c a l photodarkening. I t s h o u l d be noted t h a t a l l o f t h e o p t i c a l t r a n s m i s s i o n t e n d s t o a c o n s t a n t v a l u e ( s 2 0 % ) i n d e p e n d e n t of T.

F i g u r e 2 shows t h e s a t u r a t e d v a l u e s of t h e o p t i c a l t r a n s m i s s i o n ( a t 470 nm) b e f o r e and a f t e r t h e i l l u m i n a t i o n a s f u n c t i o n s o f T , which was o b t a i n e d by r e p l o t t i n g t h e d a t a o f F i g . 1. Open and s a l i d c i r c l e s

r e p r e s e n t t h e o p t i c a l t r a n s m i s s i o n b e f o r e ( t = 0) and a f t e r ( t = a)

t h e i l l u m i n a t i o n , r e s p e c t i v e l y . Temperature dependence of t h e o p t i c a l t r a n s m i s s i o n o f t h e a n n e a l e d sample (open c i r c l e s ) , r e f l e c t s t h e temper- a t u r e dependence o f t h e o p t i c a l gap E , ,

where E, ( 0 ) i s t h e o p t i c a l gap a t T = 0 K and y i s a c o n s t a n t , r e s p e c - t i v e l y . A l l t h e s a t u r a t e d v a l u e s o f t h e o p t i c a l t r a n s m i s s i o n a t 470nm a r e k e p t c o n s t a n t a g a i n s t a l l t h e i l l u m i n a t i o n t e m p e r a t u r e s TI which means t h a t t h e o p t i c a l gap of t h e pho.to-darkened As2S3 g l a s s always shows an i d e n t i c a l v a l u e when E, i s measured a t e a c h i l l u m i n a t i o n t e m p e r a t u r e . The i n t e r s e c t i o n o f t h e two e x t r a p o r a t e d c h a r a c t e r i s t i c l i n e s ( d e n o t e d by dashed l i n e s ) i n t h e f i g u r e i s around a t T = 393 K

(120°C).

F i g u r e 3 shows t h e o p t i c a l a b s o r p t i o n edge of As2S3 g l a s s measured a t 290 K b e f o r e and a f t e r t h e i l l u m i n a t i o n a t two d i f f e r e n t ambient t e m p e r a t u r e s . A s shown i n t h e f i g u r e , t h e a b s o r p t i o n edge i s s h i f t e d t o lower photon e n e r g i e s upon i l l u m i n a t i o n a t 77 K ( c u r v e c i n t h e f i g u r e ) compared w i t h t h a t i l l u m i n a t e d a t 290 K ( c u r v e b i n t h e f i g u r e ) . I t s h o u l d be n o t e d t h a t t h e a b s o r p t i o n edge o f t h e specimen photo-darkened a t 77 K i s s h i f t e d from t h e c u r v e c t o t h e c u r v e b by r e - i l l u m i n a t i n g t h e specimen a t 290 K , i . e . , p h o t o b l e a c h i n g , t h e d e t a i l o f which one o f t h e a u t h o r s h a s r e p o r t e d i n t h e p r e v i o u s work[61.

The above e x p e r i m e n t a l r e s u l t s i n F i g s . 2 and 3 a r e summarized i n t h e E,

-

T diagram, a s shown i n F i g . 4. I n t h e f i g u r e , e a c h v e r t i c a l arrow r e p r e s e n t s a change i n E, caused by l i g h t i r r a d i a t i o n , photo- d a r k e n i n g o r p h o t o b l e a c h i n g p r o c e s s under i s o t h e r m a l c o n d i t i o n , and o t h e r arrows i n d i c a t e t h e t e m p e r a t u r e dependence o f E, i n t h e absence of l i g h t . A s shown i n t h e f i g u r e , E, o f i l l u m i n a t e d s t a t e a t any i l l u m i n a t i o n t e m p e r a t u r e T t e n d s t o a c o n s t a n t v a l u e i n d e p e n d e n t o f t h e p r e v i o u s h i s t o r i e s , c o r r e s p o n d i n g t o t h e o p t i c a l gap a t T = 393 K

A=470nm

-

1 I I t 1 I

300 320 340 360 380

TEMPERATURE ( K )

annealed at 170°C

at 77K

A *

0 0

22 2.4 26

PHOTON ENERGY (eV) F i g . 2 : O p t i c a l t r a n s m i s s i o n ( a t 470nm) Fig. 3 : Photodarkening b e f o r e and a f t e r t h e i l l u m i n a t i o n a t (a+b o r a + c ) and photo-

d i f f e r e n t T ' s . b l e a c h i n g ( c+b)

.

(120°C). Namely,

I t i s n o t e d t h a t t h e s a t u r a t e d v a l u e E, (393) i n c r e a s e s s l o w l y towards t h e g l a s s t r a n s i t i o n t e m p e r a t u r e ( 4 5 3 K ) a s t h e power of t h e e x c i t a - t i o n l i g h t i s r a i s e d h i g h e r , a c c o r d i n g t o t h e n o n l i n e a r e f f e c t d e s c r i b e d e a r l i e r [ l ] . I n t h i s s e n s e 3 9 3 K is n o t a c h a r a c t e r i s t i c v a l u e .

I t i s q u i t e i n t e r e s t i n g t o compare eq. ( 2 ) w i t h e q . (1) from t h e v i e w p o i n t s o f t h e c o r r e l a t i o n between t h e o p t i c a l gap and t h e s t r u c - t u r a l randomness i n d u c e d by p h o t o - i r r a d i a t i o n o r t e m p e r a t u r e e f f e c t . A c o n s t a n t y i n eq. (1) i s e s s e n t i a l l y t h e t e m p e r a t u r e c o e f f i c i e n t of t h e o p t i c a l gap i n t h e o p p o s i t e s i g n , - ( a E , / a ~ )

l p ,

^and i s around 5 x 10-4 eV/K f o r a-As2S3[7]. I n many c h a l c o g e n i d e g l a s s e s , t h e major f a c t o r d e t e r m i n i n g t h e t e m p e r a t u r e c o e f f i c i e n t of E, i s (aE,/aT)

I V

because (aE,/aP)

I T

^{i s} n e g a t i v e [ 7 ] , a s i s c l e a r from t h e thermodynamic r e l a t i o n s h i p , (aE,/aT) lp = (aE,/aT)

I V -

( a / ~ ) (aE,/aP)

I T .

Namely, t h e n e g a t i v e s i g n o f (aE,/aT)

l p

i s n o t due t o a s i m p l e d i l a t a t i o n b u t i s p o s s i b l y a t t r i b u t e d t o a change i n some s o r t o f s t r u c t u r a l randomness

I I I I

a

I I I I I ^I I I

100 200 300 400

TEMPERATURE ( K )

Rig. 4 : Temperature c y c l i n g o f t h e photo- d a r k e n i n g and photo- b l e a c h i n g on t h e E,

-

^T

diagram.

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More d e t a i l e d s t r u c t u r a l a n a l y s i s o f t h e t e m p e r a t u r e e f f e c t on g l a s s n e t w o r k w i l l b e r e q u i r e d f o r f u r t h e r d i s c u s s i o n .

R e f e r e n c e s .

[ l ] TANAKA K . , J. N o n - C r y s t a l l i n e S o l i d s 35&36, (1980) 1023.

[ 2 ] HAMANAKA H . , TANAKA K . , I I Z I M A S . , S o l i d S t a t e ComUnUn.=(1977)63.

[ 3 ] TANAKA K . , Fundamental P h y s i c s o f Amorphous S e m i c o n d u c t o r s , e d . by i n a d y n a m i c a l s e n s e i n d u c e d by t h e l a t t i c e v i b r a t i o n . F o r example, t h e a v e r a g e s e p a r a t i o n <R> between c h a i n s o r l a y e r s i n c h a l c o q e n i d e g l a s s e s s h o u l d i n c r e a s e w i t h a n i n c r e a s e i n

,

-1.48 t e m p e r a t u r e b e c a u s e t h e volume expan- s i o n c o e f f i c i e n t i s l a r g e and a l w a y s p o s i t i v e . B u t a t t h e same t i m e , a s K a s t n e r h a s p o i n t e d o u t i n r e l a t i o n t o p r e s s u r e e f f e c t , < ~ 2 > , a measure

,

o f randomness, a l s o i n c r e a s e s w i t h t e m p e r a t u r e and a t some i n s t a n t R -may t h e r e f o r e b e q u i t e s m a l l , which .46

- ^?

would b e b r o a d e n i n g t h e e l e c t r o n i c

l e v e l s r e s u l t i n g i n a s h i f t o f E, t o l o w e r e n e r g y [ 7 ] .

On t h e o t h e r h a n d , p h o t o d a r k e n - i n g a l w a y s accompanies t h e i n c r e a s e i n s t r u c t u r a l randomness and t h e

1.44

l a r g e r randomness i s i n d u c e d by

E

g8.2

V a

a3

hl

8.0;

Yonezawa F.

,

( s p r i n g e r - ~ e r i a g B e r l i n ~ e i d e l b e r g NewYork 19 81) p . 1041 [ 4 ] TANAKA K . , I I Z I M A S. , A O K I K . ,MINOMURA S . , P r o c . 6 t h I n t ' l Conf.

on Amorphous and L i q u i d S e m i c o n d u c t o r s ( L e n i n g r a d 1975) p.442.

[ 5 ] UTSUGI Y . , MIZUSHIMA Y . , J. Appl. Phys. 49 (1978) 3470.

[6] TANAKA K . , S o l i d S t a t e Commun. 1 5 ( 1 9 7 4 ) 1 5 2 1 . [ 7 ] KASTNER M . , Phys. Rev. (1973T-5237.

2.3

2.4

l i g h t i r r a d i a t i o n at t h e lower i l l u - OPTICAL GAP (eV) m i n a t i o n t e m p e r a t u r e , b e i n g c l a r i f i e d

t h r o u g h t h e d e t a i l e d X-ray d i f f r a c - F i g . 5 : 20 a n d Imax/Imin ^VS. t i o n measurements [ l ]

.

E, by p h o t o s t r u c t u r a l c h a n g e

a n d t e m p e r a t u r e e f f e c t . From t h e above d i s c u s s i o n and t h e e x p e r i m e n t a l r e l a t i o n e x p r e s s e d by e q . ( 2 ) o n e i s tempted t o s p e c u l a t e t h a t t h e p h o t o - i n d u c e d s h i f t o f t h e o p t i c a l g a p i s e q u i v a l e n t t o t h a t i n d u c e d by t e m p e r a t u r e r i s e a l t h o u g h t h e r e e x i s t s a s u b s t a n t i a l d i f - f e r e n c e i n t h e n a t u r e o f <R2> between photo- and t e m p e r a t u r e e f f e c t s , i . e . , f r o z e n - i n v s . d y n a m i c a l , and t h a t t h e b o t h e f f e c t s a r e comple- mentary w i t h e a c h o t h e r a t l e a s t i n t h e t e m p e r a t u r e r a n g e i n F i g . 4 . However, t h e c o r r e l a t i o n between t h e p h o t o - a n d t e m p e r a t u r e e f f e c t s i s n o t s o s i m p l e . F i g u r e 5 shows t h e r a t i o o f t h e f i r s t d i f f u s e peak t o t h e f i r s t minimum o f t h e X-ray d i f f r a c t e d c u r v e ( I m a x / I m i n ) and t h e peak p o s i t i o n o f t h e f i r s t d i f f u s e p e a k ( 2 0 ) a s f u n c t i o n s o f t h e o p t i c a l g a p f o r p h o t o s t r u c t u r a l change and t e m p e r a t u r e e f f e c t . A s shown i n t h e f i g u r e , Imax/Iminr a measure o f a randomness, r e m a i n s c o n s t a n t f o r t h e c a s e o f t h e t e m p e r a t u r e e f f e c t (T = 290 K and 4 3 3 K )

,

which means t h a t t h e Debye-Waller t y p e f l u c t u a t i o n o f <R2> i s n o t re- f l e c t e d i n t h e X-ray d i f f u s e c u r v e i n t h i s t e m p e r a t u r e r a n g e i n s p i t e of a s u b s t a n t i a l change i n E,. On t h e o t h e r h a n d , Imax/Imin v a r i e s d r a s t i c a l l y f o r t h e c a s e o f t h e p h o t o s t r u c t u r a l c h a n g e , a s i s s e e n i n t h e f i g u r e .

1 I

~

,.

photostructural effect ~ 2 4 n x temperature effect

7:.

-

-

C-

2I ^-