FARADAY ROTATION IN a-As2Se3

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FARADAY ROTATION IN a-As2Se3

B. Vanhuyse, P. van den Keybus, W. Grevendonk

To cite this version:

B. Vanhuyse, P. van den Keybus, W. Grevendonk. FARADAY ROTATION IN a-As2Se3. Journal de

Physique Colloques, 1981, 42 (C4), pp.C4-923-C4-926. �10.1051/jphyscol:19814201�. �jpa-00220829�

JOURNAL DE P H Y S I Q U E

Coiloque C4, supplhnent au n020, Tome 42, octobre 1981 page C4-923

FARADAY ROTATION I N a-As2Se3

B. Vanhuyse, P. Van den Keybus and W. Grevendonk

Laboratorivm voor Vaste Stof-en Hoge Drukfysika, l<athoZieke U n i v e r s i t e i t Leuven, CeZestijnenZaan 200 D, 3-3030 Leuven, Belgiwn

A b s t r a c t . - Measurements o f F a r a d a y R o t a t i o n (FR) i n a n o r p h o u s As2Se3 a s a f u n c t i o n of p h o t o n e n e r g y a r e r e p o r t e d . I n t h e b u l k m a t e r i a l t h e t e m p e r a t u r e d e p e n d e n c e o f t h e FR a n d t h e d e r i v e d e n e r g y g a p EER i s i n v e s t i g a t e d i n t h e r a n g e 3-300 K . F o r EER a t e m p e r a t u r e z o e f f i c i e n t o f 2.4 x eV/K i s f o u n d . I n amgrphous f i l m s t h e FR c o u l d b e measured t h r o u g h t h e e n e r q y r a n g e w h e r e t h e s i g n o f t h e FP.-angle i s r e v e r s e d . The p h o t o n e n e r g y c o r r e s - p o n d i n g t o z e r o FR i s i n a g r e e m e n t w i t h t h e g a p v a l u e c a l c u l a t e d f r o m t h e m e a s u r e m e n t s i n b u l k m a t e r i a l , t a k i n g i n t o a c c o u n t t h e e x p e c t e d d i f f e r e n c e s . between b u l k and f i l m .

I n t r o d u c t i o n . - I n a p r e v i o u s p a p e r ( l ) , we r e p o r t e d m e a s u r e m e n t s o f FR i n b u l k a-As,S,. A n a l y s i s w i t h t h e f o r m u l a e o f K o l o d z i e j c z a k , Lax

L J

and N i s h i n a ( K L N ) ( 2 ) , and o f X o r t and S c h e r (MS) ( 3 ) r e v e a l e d a n e n e r g y g a p Eg FR

,

l a r g e r t h a n t h e g a p E o P T t d e t e r m i n e d w i t h o t h e r expe-

T

r i m e n t s a s o p t i c a l 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 m e a s u r e m e n t s . I t w a s s u g g e s t e d t h a t FR i s i n s e n s i t i v e t o e l e c t r o n - p h o n o n i n t e r a c - t i o n s and t h a t t h e t r a n s i t i o n s i n v o l v e d a r e

"

d i r e c t - l i k e " .

A s a-As2Se3 i s a g l a s s , which r e s e m b l e s a-As2S3 i n many p r o p e r - t i e s , we i n v e s t i g a t e d i f t h i s s u g g e s t i o n i s s u s t a i n e d b y s i m i l a r mea- s u r e m e n t s on t h i s n a t e r i a l . A l s o t h e t e m p e r a t u r e d e p e n d e n c e o f FR i s i n v e s t i g a t e d a s it c a n r e v e a l some a s p e c t s of t h e n a t u r e o f t h e gaF.

W e a l s o r e p o r t FR-measurements t h r o u g h t h e a b s o r p t i o n e d g e i n t h i n a-As2Se3 f i l m s i n o r d e r t o o b t a i n a d i r e c t d e t e m i n a t i o n on t h e e n e r g y g a p . The e x p e r i m e n t a l method was e s s e n t i a l l y t h e s a n e a s h a s been d e s c r i b e d i n r e f . 1.

E x p e r i m e n t s on b u l k m a t e r i a l . - a-As2Se3 was p r e p a r e d by a u e n c h i n g from t h e m e l t . A f t e r c u t t i n g and p o l i s h i n g , t h e s a m p l e t h i c k n e s s was 0 . 3 3 mm. Measurements w e r e p e r f o r m e d i n a m a g n e t i c f i e l d o f 3 . 5 7 T a n d f i t t e d t o t h e KLN- f o r m u l a f o r d i r e c t t r a n s i t i o n s i n c r y s t a l l i n e m a t e r i a l ( 2 ) a n d t o t h e MS- f o r m u l a f o r amorphous s e m i c o n d u c t o r s ( 3 ) . B o t h d i s p e r s i o n f o r m u l a e r e v e a l e d t h e same e n e r g y g a p E FR

g

The r e s u l t s are shown i n f i g u r e 1. The v a l u e s EFR a r e s i g n i f i c a n t l y C;

l a r g e r t h a n t h e EoPT o b t a i n e d b y o p t i c a l a b s o r p t i o n and p h o t o c o n d u c - q

t i v i t y m e a s u r e m e n i s , a s was n o t e d a l r e a d y f o r a-As2S3.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814201

C4-924 JOURNAL DE PHYSIQUE

Fig. 1. Ter,Iperature deprdence of

$ :

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

roPT

t h e F R a r i g l e i t s e l i i s CJ

s t ill i n c r e a s i n g monotonously. The EFR v a l u e s v s . t e m p e r a t u r e T c a n 9

b e l i n e a r l y f i t t e d :

where EFR i s g i v e n i n e V and T i n k e l v i n . The t e m p e r a t u r e c o e f f i c i e n t 4

i s q u i t e d i f f e r e n t from v a l u e s f o u n d n e a r t h e o p t i c a l gaF by e l e c t r o - aE

a b s o r p t i o n m e a s u r e m e n t s ( 4 ) where

3 a

T =

-

^{5 .} eV/K, d i s p e r s i o n :

-

7 x eV/K ( 5 ) and o p t i c a l a b s o r p t i o n m e a s u r e m e n t s : - 5 . 6 x l 0 - ~ eV/K ( 6 ) . A s i m i l a r s i t u a t i o n i s e n c o u n t e r e d i n a - A s 2 S g

,

trhere we f o u n d

a = -

aE

a

T 1 . 5 x e V / K ( 1 ) compared t o 3 . 0 5 x e V / K from op- t i c a l a b s o r p t i o n ( 7 ) a n d ( 3

2

2 ) x e V / K f r o n d i s p e r s i o n ( 5 )

.

T h e s e v a l u e s s h o u l d n o t n e c e s s a r i l y c o n t r a d i c t e a c h o t h e r . I f , f o r F R , a n o t h e r e n e r g y - g a p i s i n v o l v e d , t h e n a l s o 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 o f t h e e n e r g y - g a p c a n b e d i f f e r e n t . I t i s i n t e r e s t i n g t o compare t h e e n e r g y - g a p s and t h e i r t e m p e r a t u r e - d e p e n d e n c e s f o r se- v e r a l m a t e r i a l s ( t a b l e 1)

.

One s h o u l d n o t e t h a t , f o r d i r e c t g a p s e m i c o n d u c t o r s , t h e EFR i s g v i r t u a l l y t h e same a s E O ' ~ . The s m a l l d i f f e r e n c e r e s u l t s f r o m t h e

g

c o n t r i b u t i o n o f l o w e r v a l e n c e b a n d s i n t h e c a s e o f F R - m e a s u r e m e n t s ( l 4 b

Table 1. hergy-gaps (roan t a n m a t u r e ) and their tanperawe-coeff i c i e n t s f a r several materials; (p) denotes present mrl:.

a-As2S3 a-As2Se3 c-S i c-GaAs cCaSb C-InAs c-InSb

F o r i n d i r e c t g a p s e m i c o n d u c t o r s , t h e d i f f e r e n c e s c a n b e much l a r g e r , t h e o p t i c a l a b s o r p t i o n m e a s u r e m e n t s y i e l d i n g t h e i n d i r e c t g a p . The t a b l e s u g g e s t s t h a t e l e c t r o n - p h o n o n i n t e r a c t i o n s a r e l e s s i m ~ o r t a n t f o r F.R., e v e n f o r amorrhous s e m i c o n d u c t o r s .

E x p e r i m e n t s o n e v a p o r a t e d f i l m s . - The f i l m s o f a-As2Se3 were ? r e p a r e d by e v a p o r a t i o n of t h e s a n e b u l k m a t e r i a l on b o r o s i l i c a t e g l a s s s u b - s t r a t e s . T h i c k n e s s e s w e r e a b o u t 1 un. To r e d u c e s t a t i s t i c a l e r r o r s , d u e t o t h e i r v e r y h i g h . a b s o r p t i o n , t h e f i l m s were m e a s u r e d s e v e r a l t i m e s a n d t h e r e s u l t s a d d e d u p . The r e s u l t i n g b e h a v i o u r of t h e FR i s shown i n f i g u r e 2. A s i g n r e v e r s a l a t 471 nm c o r r e s p o n d s t o a n e n e r g y o f 2.63 eV. T h e o r e t i c a l l y , t h e s i g n r e v e r s a l o c c u r s a t a n

E O(ev) ~ g 2.60 (8) 1.90 (8) 1.107 (9) 1.35 (11) 0.67 (11) 0.36 (11) 0.165 (11)

e n e r g y , l o w e r t h a n t h e g a p e n e r g y , by a s m a l l amount ^%

r,

h T b e i n g t h e r e l a x a t i o n t i l e . The o b s e r v e d s m a l l d i f f e r e n c e i n 'EZR f o r f i l m

a

E O ~

+ ^{e v ~ )}

-3.05 (7)

-5.6 (6)

-2.3 (9)

-5 (11)

-3.5 (11)

-2.8 (11)

-2.8 (11)

2

a n d b u l k m a t e r i a l c a n b e e x p l a i n e d by a d i f f e r e n c e i n c o m p o s i t i o n a n d s t r u c t u r e o f b o t h s a m p l e s .

I n t h i s c o n t e x t , t h e m e a s u r e m e n t s of t h e r e f r a c t i v e i n d e x i n e v a p o r a t e d f i l n s o f a-As40Se60 by d e N e u f v i l l e e t a l . ( 1 5 ) a r e i n - t e r e s t i n g , a s t h e r e i s a s t r o n g c o r r e l a t i o n b e t w e e n t h i s q u a n t i t y a n d FR ( 1 ) . They a n a l y s e d t h e i r r e s u l t s i n t e r m s o f t h e f o r n u l a o f Wemple and D i Domenico ( 1 6 ) and found a n o s c i l l a t o r e n e r g y Eosc =

1

- -

3.99 eV. A s Wemple and D i Domenico p o i n t e d o u t , t h i s p a r a m e t e r i s a~~

1 0 e

( 1 ) - 1 . 5 (1)

-2.4 (p)

-1.4 (10)

-5.4 (12)

-3.6 (13)

-2.2 (12)

-2.2 (12)

PO

_g

3.1 2.35 (p) 3.3 (10) 1.41 (12) 0.74 (13) 0.36 (12) 0.18 (12)

c o r r e l a t e d t o t h e l o w e s t d i r e c t bandgap e n e r g y E~ by t h e r e l a t i o n Eosc = 1 . 5 E~ The v a l u e f o r Eosc, f o u n d by d e N e u f v i l l e e t 4 d l . , s o

4 - D

r e s u l t s i n a n Ea of 2.66 eV, which i s v e r y c l o s e t o c u r

"

z e r o t r a n s i - t i o n " e n e r g y o f 2 . 6 3 eV.

1

. l

gap nondirect nondirect i n d i r e c t d i r e c t d i r e c t d i r e c t d i r e c t

C4-926 JOURNAL DE PHYSIQUE

Fig. 2. Sign r e v e r s a l of t h e FR in a-As2Se3 f i l m . C o n c l u s i o n

B o t h t h e v a l u e s o f E~~ a n d 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 s u g g e s t 4

t h a t F R i n a m o r p h o u s c h a i c o g e n i d e s e m i c o n d u c t o r s i s a s s o c i a t e d w i t h d i r e c t - l i k e t r a n s i t i o n s . I t i s c o n f i r m e d d i r e c t l y b y F.R. m e a s u r e - m e n t s i n t h i n f i l m s t h a t E~~ i s c o n s i d e r a b l y l a r g e r t h a n t h e

OPT ⁹

u s u a l l y a c c e p t e d E

4 .

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