TIME-RESOLVED PHONON SPECTROSCOPY OF AMORPHOUS As2S3

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TIME-RESOLVED PHONON SPECTROSCOPY OF AMORPHOUS As2S3

U. Strom, P. Klein, K. Weiser, S. Wolf

To cite this version:

U. Strom, P. Klein, K. Weiser, S. Wolf. TIME-RESOLVED PHONON SPECTROSCOPY OF AMORPHOUS As2S3. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-30-C6-32.

�10.1051/jphyscol:1981607�. �jpa-00221156�

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CoZZoque C6, suppZ6ment au n012, Tome 4 2 , dgcembre 1981 page C6-30

TIME-RESOLVED PHONON SPECTROSCOPY OF AMORPHOUS As2S3

U . Strom, P.B. K l e i n , K. Weiser and S.A. Wolf

NavaZ Research Laboratory, Washington, D.C. 20375, U.S.A.

Abstract.- Phonon p u l s e s which were o p t i c a l l y induced i n t h i n evaporated f i l m s of amorphous A s S3 have been s t u d i e d w i t h a superconducting NbN bolometer. The observed b a l l i s g i c and d i f f u s i v e h e a t p r o p a g a t i o n p r o v i d e s a measure of t h e phonon mean f r e e p a t h (m.f.p.). The r e s u l t s a r e r e l a t e d t o t h e m i c r o s c o p i c s t r u c t u r a l o r d e r of t h e c h a l c o g e n i d e g l a s s a s evidenced from p r e v i o u s o p t i c a l and s t r u c t u r a l s t u d i e s .

The p r e s e n t experiment examines phonons which are induced o p t i c a l l y i n t h i n evaporated f i l m s of t h e c h a l c o g e n i d e g l a s s As2S3. The g l a s s f i l m s a r e d e p o s i t e d on one s i d e of a 1 mm t h i c k s a p p h i r e d i s k . A NbN s u p e r c o n d u c t i n g bolometer1 (1 mm x

1 mm) d e p o s i t e d on t h e o t h e r s i d e of t h e d i s k s e r v e s a s a phonon d e t e c t o r . The time r e s o l u t i o n of t h e e x p e r i m e n t a l system was determined by measuring t h e b a l l i s t i c phonon response induced o p t i c a l l y i n a 2002 t h i c k NbN m e t a l l i c f i l m . The bolometer r e s p o n s e f o r an 8 n s e c i n c i d e n t o p t i c a l p u l s e of A = 5796g w i t h - t o t a l energy of 3.5x10-~ j o u l e / p u l s e is shown i n Fig. 1, F u r t h e r d e t a i l s of t h e bolometer can b e found i n Ref. 1.

Fig. 1. Bolometer r e s p o n s e f o r o p t i c a l l y heated

w NbN metal f i l m . Spike n e a r t = O due t o d i r e c t

I bolometer h e a t i n g by p a r t i a l l y t r a n s m i t t e d l a s e r p u l s e . Bolom e r r e s p o n s e time e s t i m a t e d t o b e l e s s t h a n LO-" s e c ( s e e Ref. 1 ) . Delayed p u l s e s 0 50 loo I5O 200 250 r e p r e s e n t b a l l i s t i c TA and LA phonons p r o p a g a t i n g

TIME ( 1 0 - ~ s e c ) through 1 mm t h i c k s a p p h i r e .

The bolometer r e s p o n s e f o r a photo-excited AszS3 f i l m of t h i c k n e s s d-1.8 pm is shown i n Figs. 2 and 3, (Note: t h e f i l m d e p o s i t i o n c o n d i t i o n s were n e a r l y i d e n t i c a l t o t b o s e used i n p r e v i o u s o p t i c a l Z and s t r u c t u r a l s t u d i e s e 3 ) For l i g h t w i t h wave- l e n g t h A = 4882g we f i n d t h a t t h e product of a b s o r p t i o n c o e f f i c i e n t a and f i l m t h i c k - n e s s d is ad-0.7. T h i s v a l u e of a d t y p i f i e s t h e g l a s s a f t e r photodarkening,4*5 i.e. t h e t h e r m a l l y r e v e r s i b l e photo-induced band edge s h i f t which i s on t h e o r d e r of 0.1 eV toward t h e red. None of t h e phonon parameters d i s c u s s e d i n t h i s paper were a f f e c t e d by r e v e r s i b l e photodarkenfng. A s s e e n i n Figs. 2 and 3, h e a t p r o p a g a t i o n i n amorphous As2S3 a t T = 11.5K i s predominantly d i f f u s i v e . The b a l l i s t i c components a t 0.1 and 0 . 2 ~ s e c a r e n o t due t o phonon p r o p a g a t i o n a c r o s s t h e e n t i r e width of t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981607

Fig. 2. Bolometer r e s p o n s e f o r o p t i c a l l y Fig. 3. Long t i m e b o l o m e t e r r e s p o n s e h e a t e d As S film. B a l l i s t i c p u l s e s as due t o d i f f u s i v e phonon t r a n s p o r t i n i n F i g . 1: 3 ~ e a k a t 0.35 & s e c due t o As2S3.

r e f l e c t i o n from bolometer e l e c t r o d e .

f i l m , b u t a r e due t o phonon g e n e r a t i o n by t h e p a r t i a l l y t r a n s m i t t e d l i g h t i n t h e r e g i o n of t h e g l a s s f i l m c l o s e s t t o t h e s a p p h i r e s u b s t r a t e . The r a t i o of t h e i n t e - g r a t e d b a l l i s t i c t o d i f f u s i v e phonon s i g n a l s y i e l d s t h e phonon m.f.p..i i n amorphous A s p s 3 . The n m b e r of b a l l i s t i c phonona i g - a ~ e - * , w h e r e a s t h e number o f d i f f u s i v e phonons is g i v e n by ID

-

5 1

l i g h t , a

-

.

I n Fig. 4 v a r i o u s measurements of A f o r g l a s s y As S and S i 0 2 a r e compared. The 2 3

l o w e r two c u r v e s r e p r e s e n t A ( T ) = ~ K / C V ~ , where t h e thermal c o n d u c t i v i t y K , s p e c i f i c h e a t C and Debye sound v e l o c i t y vD we o b t a i n e d from low t e m p e r a t u r e t h e r m a l measure- m e n t ~ . ~ ' ~ The upper c u r v e i n Fig. 4 i s t h e phonon m,f,p, a t T-lK a s a f u n c t i o n of phonon f r e q u e n c y fg, measured f o r S i 0 2 b y D i e t s c h e and IZinder8 u s i n g monochromatic phonon g e n e r a t i o n t e c h n i q u e s . Temperature and f r e q u e n c y s c a l e s a r e r e l a t e d by hf =

D 3kT. A s s e e n i n Fig. 4, A o b t a i n e d from t h e r m a l measurements i s g e n e r a l l y s m a l l e r by a f a c t o r of 3-5 t h a n A o b t a i n e d w i t h t h e o t h e r two methods shown i n Fig. 4. The c a u s e f o r t h i s d i f f e r e n c e c a n n o t be a d d r e s s e d h e r e . We w i l l r a t h e r s p e c u l a t e on t h e n a t u r e of t h e s c a t t e r i n g which g i v e s rise t o t h e ~ - f - ~ " f r e q u e n c y dependence observed between fD=lOO and 300 GHz i n S i 0 2 , w i t h which o u r r e s u l t i n As2S3 a t 11.5K, o r a t t h e e q u i v a l e n t phonon e n e r g y of -700 GHz, i s c o n s i s t e n t .

The s t r o n g f r e q u e n c y dependent s c a t t e r i n g above fD-100 GHz h a s been a t t r i b u t e d v a r i o u s l y t o s t r u c t u r a l i r r e g u l a r i t i e s , phonons, lo o r t u n n e l i n g modes.' None o f

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f, ( GHz) ^{Fig. 4.} Phonon mean f r e e p a t h i n g l a s s y Si02, 100 loo0 As S and an e v a p o r a t e d As S

2 3 f i l m .

1

t h e s e can be c o m p l e t e l y excluded. The importance E of c o n s i d e r i n g t h e l o c a l g l a s s s t r u c t u r e i s

U s t r e s s e d h e r e . There e x i s t s c o n s i d e r a b l e evidence lo-' f o r remnants of " c r y s t a l - l i k e " o r medium-range

o r d e r i n t h e g l a s s . S p e c i f i c e v i d e n c e f o r medium- range o r d e r i n As2S3 was o b t a i n e d from two-phonon v i b r a t i o n a l l o and n u c l e a r quadrupole resonance s t u d i e s . l 1 * l 2 A d d i t i o n a l i n f o r m a t i o n comes from f a r i n f r a r e d a b s o r p t i o n measurements i n g l a s s e s , 1 3 where c o r r e l a t i o n l e n g t h s e-102 f o r As2S3 and -2012 lo-7. f o r S i 0 2 c a n be d e f i n e d . Such c o r r e l a t i o n l e n g t h s

1 2 5 ¹⁰ 20 r e p r e s e n t a n average s p a c i n g between s t a t i c o r TEMPERATURE ( K dynamic c h a r g e s i n t h e g l a s s and can be a d i r e c t consequence of medium-range o r d e r . R e c e n t l y p h i l l i p s 1 4 h a s f u r t h e r advanced t h e con- c e p t of medium-range o r d e r w i t h s p e c i f i c t o p o l o g i c a l p r o p o s a l s , Ribbon-like poly- m e r i c s t r u c t u r e s ( r a f t s ) proposed f o r As2S3 do indeed e x h i b i t a dominant width of

-102 which is comparable t o l2. Phonon s c a t t e r i n g would d e v i a t e from Rayleigh-type s c a t t e r i n g 1 5 ( - i 4 f o r s p h e r e s ) f o r phonon f r e q u e n c i e s f 2 vD/2r(. For 1=1012 and vD=1.7 x l ~ ~ c m / s e c - ~ t h i s y i e l d s f ,300 GHz. More d e t a i l e d e s t i m a t e s w i l l be compli- c a t e d by i n e l a s t i c p r o c e s s e s 8 a s w e l l a s t h e high a n i s o t r o p y of t h e s c a t t e r i n g s t r u c - t u r e s .

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