TRANSPORT PROPERTIES OF AMORPHOUS As2Se3

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TRANSPORT PROPERTIES OF AMORPHOUS As2Se3

A. Sharp, J. Marshall, H. Fortuna

To cite this version:

A. Sharp, J. Marshall, H. Fortuna. TRANSPORT PROPERTIES OF AMORPHOUS As2Se3. Journal

de Physique Colloques, 1981, 42 (C4), pp.C4-159-C4-162. �10.1051/jphyscol:1981432�. �jpa-00220888�

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JOURNAL DE PHYSIQUE

CoZZoque

C4,

supplgment au nOIO, Yome

42,

octobre 1981

TRANSPORT PROPERTIES OF AMORPHOUS As2Se3

A.C. Sharp, J . M . Marshall and H.S. Fortuna

Department

o f

Physics, Dundee College o f Technology, Dundee

DDZ ZHG,

ScotZand,

U. K.

A b s t r a c t : - Measurements of t h e m o b i l i t y and degree of t r a n s i t p u l s e d i s p e r s i o n a s a f u n c t i o n of t e m p e r a t u r e between 190K and 360K a r e r e p o r t e d f o r f i l m s of AszSea p r e p a r e d by quenching from t h e m e l t , r . f . s p u t t e r i n g , and thermal e v a p o r a t i o n . The d i s p e r s i o n parameter

a

e x h i b i t s a marked temperature dependence f o r v i t r e o u s f i l m s . An a n a l y s i s o f cL i n d i c a t e s t h a t t h e m o b i l i t y - c o n t r o l l i n g t r a p s a r e d i s t r i b u t e d over an energy range o f about 0.07eV. The d.c. e l e c t r i c a l c o n d u c t i v i t y of t h e specimens h a s a l s o been measured o v e r t h e same temperature range, and a computer a n a l y s i s of t h e d a t a s u g g e s t s t h e e x i s t e n c e of more t h a n one e x p o n e n t i a l component.

These r e s u l t s a r e i n t e r p r e t e d v i a a model i n which t h e h o l e c a r r i e r s i n t e r a c t with t h r e e d i s t i n c t s e t s of t r a p s c e n t r e d on e n e r g i e s 0 . 3 , 0.4 and 0.6eV above t h e valence band m o b i l i t y edge.

I n t r o d u c t i o n : - Measurements of t h e m o b i l i t y of h o l e c a r r i e r s i n AszSes using t i m e of f l i g h t t e c h n i q u e s a r e complicated by a s i g n i f i c a n t s p r e a d i n g of t h e c a r r i e r p a c k e t a s it moves through t h e sample (1). Scher and Montroll (21, whose work r e p r e s e n t s t h e f i r s t d e t a i l e d a t t e m p t t o a n a l y s e d i s p e r s i v e t r a n s p o r t , showed t h a t even i f t h e c u r r e n t t r a n s i t p u l s e appears f e a t u r e l e s s when d i s p l a y e d on l i n e a r a x e s , a break o r d i s c o n t i n u i t y o f g r a d i e n t should s t i l l b e observed i f t h e same p u l s e i s d i s p l a y e d on l o g a r i t h m i c axes. I n t h i s c a s e t h e break corresponds t o t h e t r a n s i t time of t h e f a s t e s t few p e r c e n t of c a r r i e r s . T h e i r t h e o r y p r e d i c t e d t h a t t h e g r a d i e n t s of such l o g ( I ) - l o g ( t ) graphs b e f o r e and a f t e r t h e break should be

(1-a) and ( l + a ) r e s p e c t i v e l y , where a i s a measure of t h e degree o f d i s p e r s i o n of t h e c a r r i e r packet. This t h e o r y was a p p l i e d t o t r a n s i t p u l s e s i n AszSe3 by P f i s t e r and Scher

( 1 1 ,

who observed t h e degree of d i s p e r s i o n t o b e independent of temperature and a p p l i e d e l e c t r i c f i e l d . P a r a d o x i c a l l y , Kolomiets, Lebedev and Kasakova ( 3 ) and Marshall and Sharp ( 4 ) have observed a t e m p e r a t u r e dependence of a f o r v i t r e o u s samples. I t t h u s seemed d e s i r a b l e t o make a d e t a i l e d comparison of t h e p r o p e r t i e s of As2Ses p r e p a r e d by d i f f e r e n t methods. Although such

specimens may have b r o a d l y s i m i l a r p r o p e r t i e s , t h e r e w i l l i n e v i t a b l y b e s t r u c t u r a l d i f f e r e n c e s which may a l t e r t h e d e n s i t y and n a t u r e of d e f e c t s i t e s and t h u s l e a d t o changes i n t h e l o c a l i s e d s t a t e d i s t r i b u t i o n w i t h i n t h e m o b i l i t y gap ( 8 ) .

Experimental D e t a i l s : - Thin samples of v i t r e o u s AszSe3 ( t y p i c a l l y 30 - 100 microns t h i c k ) were made from t h e b u l k g l a s s by a t e c h n i q u e of compression between mica s h e e t s ( 5 ) . Evaporated samples were made u s i n g open b o a t e v a p o r a t i o n o f t h e bulk g l a s s o n t o aluminium f o i l which was subsequently removed by f l e x i n g . The r . f . s p u t t e r e d samples were a l s o d e p o s i t e d o n t o aluminium f o i l by b i a s s p u t t e r i n g o f a bulk g l a s s t a r g e t i n an argon atmosphere. An argon p r e s s u r s of 15 microns Hg, b i a s v o l t a g e of 800V, and a power d e n s i t y n o t exceeding 6Wcm r e s u l t e d i n a d e p o s i t i o n r a t e of about 2 microns p e r hour. Sample composition was m o n i t o r e d using X-ray f l u o r e s c e n c e t e c h n i q u e s : from a s t o i c h i o m e t r i c b u l k g l a s s (38.8% A s by w e i g h t ) , r . f s p u t t e r e d s a n p l e s proved t o b e s l i g h t l y Se r i c h (37.4% A s ) , w h i l s t evaporated samples were s l i g h t l y A s r i c h (39.9% A s ) . A l l specimens were equipped w i t h s e m i t r a n s p a r e n t g o l d e l e c t r o d e s evaporated o n t o t h e t o p and b o t t a n s u r f a c e s t o form a 'sandwich-cell' c o n f i g u r a t i o n .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981432

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C4- 160 JOURNAL DE PHYSIQIJE

The time o f f l i g h t method f o r measuring d r i f t m o b i l i t i e s has been d e s c r i b e d i n d e t a i l elsewhere

(1,s).

I n t h i s work, excess c a r r i e r s were g e n e r a t e d n e a r t h e t o p e l e c t r o d e by e i t h e r a s h o r t p u l s e of 8keV e l e c t r o n s o r a f l a s h o f s t r o n g l y absorbed l i g h t . The c u r r e n t t r a n s i e n t s produced by t h e d r i f t o f h o l e s through t h e sample m d e r t h e i n f l u e n c e o f an a p p l i e d e l e c t r i c f i e l d were c a p t u r e d using a Datalab DL905 t r a n s i e n t r e c o r d e r , and t h e n t r a n s f e r r e d t o a computer f o r s t o r a g e and a n a l y s i s . Dark

conductivitymeasurements were performed on t h e s a n e samples i n t h e same v a c u m system. The s a n p l e s were allowed t o come i n t o e q u i l i b r i m w i t h t h e a p p l i e d f i e l d f o r 24 hours b e f o r e c o n d u c t i v i t y measurements were made.

R e s u l t s and Discussion:- Figure 1 shows t y p i c a l c u r r e n t t r a n s i e n t s i n v i t r e o u s As S e 3 a t 3OOK and 268K ( t h e upper p a r t shows t h e p u l s e s d i s p l a y e d on l i n e a r a x e s ,

while

t h e lower p a r t shows t h e sane p u l s e s drawn on l o g a r i t h m i c a x e s ) . Both p u l s e s were o b t a i n e d using an e l e c t r i c f i e l d o f 2.1 x

l o 5

V / c m . Changing t h e a p p l i e d f i e l d d i d n o t s i g n i f i c a n t l y a f f e c t t h e shape of a p u l s e a t a g i v e n temperature, b u t t h e p u l s e s becane much more d i s p e r s i v e a s t h e temperature was lowered. I n c o n t r a s t , t h e degree o f d i s p e r s i o n observed i n e v a p o r a t e d f i l m s depended much l e s s on temperature.

Figure 2 shows t h e v a l u e o f t h e d i s p e r s i o n parameter a i c a l c u l a t e d from t h e i n i t i a l s l o p e o f t h e log (I) - l o g ( t ) graphs a s a f u n c t i o n o f temperature f o r both v i t r e o u s and e v a p o r a t e d samples. Although a small i n c r e a s e i n a i w i t h temperature was observed f o r evaporated specimens, i t i s almost c o n s t a n t w i t h i n experimental e r r o r . The v a l u e o f

a

c a l c u l a t e d from t h e f i n a l s l o p e o f t h e l o g - 1 q graphs was very s e n s i t i v e t o t h e c h o i c e o f c u r r e n t b a s e l i n e , and consequently extremely d i f f i c u l t t o measure a c c u r a t e l y , although

af

appeared t o

decrease

with i n c r e a s i n g temperature f o r v i t r e o u s s a n p l e s .

The h o l e (and e l e c t r o n ) c u r r e n t t r a n s i e n t s observed i n r . f . s p u t t e r e d samples d e p o s i t e d a t room temperature d i d n o t e x h i b i t a d i s c o n t i n u i t y of g r a d i e n t , b u t appeared t o decay e x p o n e n t i a l l y down t o t h e n o i s e l e v e l . Specimens d e p o s i t e d a t h i g h e r temperatures (near t h e g l a s s t r a n s i t i o n ) a r e c u r r e n t l y being i n v e s t i g a t e d . Marshall ( 6 ) , using Monte C a r l o computer s i m u l a t i o n , has analysed t h e v a l u e of a t o

TIME ( t )

LOG (1) LOG (1)

evaporated x

300 340

F i g u r e 2

.-

Temperature dependence o f a i f o r v i t - reous and evaporated As2Se3

F i g u r e 1. - T r a n s i t p u l s e s a t 300K and 268K d i s p l a y e d on l i n e a r and l o g a r i t h n i c axes.

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be e x p e c t e d f o r a t r a p - l i m i t e d t r a n s p o r t m e c h a n i s n where t h e t r a p s have a Gaussian energy d i s t r i b u t i o n . Our d a t a i n d i c a t e t h a t a s p r e a d o f t r a p e n e r g i e s of about 3kT o r 0.07

+

0.02 eV would b e n e c e s s a r y t o account f o r t h e d i s p e r s i o n observed i n our e v a p o r a t e d specimens using t h i s model. The almost l i n e a r v a r i a t i o n of

a

observed f o r v i t r e o u s As2Se3 samples i s t h e s t r o n g e s t temperature dependence t o be e x p e c t e d , and h a s been shown t o be c o n s i s t e n t w i t h extended s t a t e conduction w i t h m u l t i p l e - t r a p p i n g

i n an e x p o n e n t i a l d i s t r i b u t i o n o f t r a p p i n g s t a t e s (1,111. Using t h i s a n a l y s i s t h e width of t h e t r a p energy d i s t r i b u t i o n i n v i t r e o u s s a n p l e s would b e 0 . 0 3 - 3.04 eV.

The d r i f t m o b i l i t y ( c a l c u l a t e d from t h e break i n l o g

-

log g r a p h s ) and d.c. conduct- i v i t y a s a f u n c t i o n o f temperature f o r a t y p i c a l v i t r e o u s sample 58 microns t h i c k a r e d i s p l a y e d i n f i g u r e s 3 and 4 r e s p e c t i v e l y . The d r i f t m o b i l i t y a c t i v a t i o n energy

(AE,,) i s f i e l d dependent (71, and when c o r r e c t e d t o z e r o f i e l d i s 0.62 f 0.02 eV above 300K, f a l l i n g t o about 0 . 5 eV between 300K and 270K. An a b r u p t charge i n AE,, t o about 0.15 f 0.05eV was observed below 265K, a l t h o q h t h e r e a r e n o t enough d a t a p o i n t s below 265K t o be c e r t a i n about t h e a c t i v a t i o n energy i n this regime. The d r i f t m o b i l i t y i n evaporated f i l m s has been observed down t o 190K, and f o r t h e s e samples AzU d e c r e a s e s from 0.65 f 0.03eV t o 0.45

+

0.04eV below 290K ( a t z e r o f i e l d ) .

The d.c. c o n d u c t i v i t y r e s u l t s have been analysed by c a n p u t e r using a prcgram develop- ed by Provencher (101, which i s designed t o a n a l y s e d a t a canposed o f random n o i s e p l u s an unknown c o n s t a n t b a s e l i n e p l u s a sun o f e x p o n e n t i a l decay f u n c t i o n s . The a n a l y s i s o f v i t r e o u s A s Se d a t a i n d i c a t e d t h e presence o f two e x p o n e n t i a l components w i t h a c t i v a t i o n e n e r g i e s 2 of 0.91

+

0.OleV and 0.51 f 0.06eV ( c o r r e c t e d t o z e r o f i e l d ) , and t r e a t e d t h e low temperature d a t a a s a b a s e l i n e . However, when a n a l y s i n g o n l y t h e low temperature d a t a 2 e x p o n e n t i a l canponents were a g a i n r e s o l v e d with a c t i v a t i o n e n e r g i e s o f 0.5 2 O.leV and 0 . 3 ? 0.leV. The dashed l i n e s i n f i g u r e 4 correspond t o t h e t h r e e i n d i v i d u a l e x p o n e n t i a l components w h i l e t h e s o l i d l i n e i s t h e sun of t h e s e , and shows a g o o d f i t t o t h e experimental p o i n t s .

I n evaporated As2Se3 A E ~ d e c r e a s e s from 0.90eV t o 0.7eV below 290K, and t o 0.6eV below 250K, while AE,, d e c r e a s e s f r a n 0.65eV t o 0.44eV below 290K. For r . f . s p u t t e r e d samples d e c r e a s e s from 0.91 t o 0.75eV below 280K.

F i g u r e 3.

-

Temperature dependence

o f d r i f t m o b i l i t y i n v i t r e o u s F i g u r e 4.- Temperature dependence of A s Se

2 3 t h e dark c o n d u c t i v i t y (sane s a n p l e a s

Fig. 3 ) .

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C 4 - 162 JOURNAL DE PHYSIQUE

The evidence f o r a t l e a s t t h r e e s e t s o f h o l e t r a p s canes from TSC and t r a n s i e n t p h o t o c o n d u c t i v i t y n easurements a s w e l l a s f r a n t r a n s p o r t p r o p e r t i e s ( 9 )

.

^Three

d i s t i n c t l e v e l s 0.63eV, 0.43eV and 0.3eV above t h e valence band m o b i l i t y e Q e have been observed, with t r a p d e n s i t i e s o f No. 6 3 ^Q^, 3 x 1 0 l ~ c m - ~ , N O . 4 3 ".' 3 x 1 0 l ' a n - ~ and N O . 3

<

l ~ l ~ c r n - ~ . To a n a l y s e our d a t a using t h i s model we m u s t assume t h a t t h e c a r r i e r s r e s p o n s i b l e f o r t h e d r i f t m o b i l i t y and c o n d u c t i v i t y move i n t h e same cond- u c t i o n p a t h . The s i m i l a r f i e l d dependence o f t h e m o b i l i t y and c o n d u c t i v i t y , and t h e o b s e r v a t i o n o f changes i n AE,, and AE, a t roughly t h e s a n e temperatures

suggest

t h a t t h i s assumption i s reasonable. I f our h i g h temperature r e s u l t s a r e a n a l y s e d i n terms o f t r a p l i m i t e d band motion, 0 0 and Po e x t r a p o l a t e d from f i g u r e s 3 and 4 i n d i c a t e a valence band m o b i l i t y e Q e d e n s i t y o f s t a t e s o f 2.8 x i f we t a k e y t o b e 5

cm2/v/s

i n t h e random phase regime. This i n t u r n i m p l i e s N 0 . 6 3 = 2.1 x 1016cm-3 f o r t h e m o b i l i t y c o n t r o l l i n g t r a p s .

A s t h e t e n p e r a t u r e drops from 300K t o 270K, our r e s u l t s a r e c o n s i s t e n t w i t h amove- ment o f t h e conduction p a t h up through l o c a l i s e d band t a i l s t a t e s 0 . 1 - 0.15eV i n width. An a n a l y s i s o f t h e f i e l d dependence of AE, i n t h i s regime i n terms o f t h e lowerin? o_f3a t r a p p i n g energy b a r r i e r (7) i n d i c a t e s a band t a i l d e n s i t y o f s t a t e s o f 8 x 10' cm

.

Below 270K t h e r e i s a marked charge i n A E ~ and AE,, s u g g e s t i n g t h e o n s e t o f hopping through a t r a p l e v e l 0.42eV above t h e valence band m o b i l i t y edge.

One would e x p e c t such hopping t o predominate when t h e a v e r q e time f o r thermal r e l e a s e t o t h e band becanes g r e a t e r t h a n t h e ~ c p e c t e d hopping time between s i t e s , i . e .

vP)ph1

e x p { 6 / k ~ } >

\hh-'

e x p { 2 ~ / ~ ~ } exp{w/k~}

which y i e l d s N o . I, 3 = 1 0 ' 8 ~ - 3 i f we t a k e w =0.02eV (1/3 r d o f t h e t r a p l e v e l w i d t h ) , and Ro t o b e 1.1m. A s i m i l a r a n a l y s i s can b e performed t o s e e a t what temperature one would e x p e c t hopping i n t h e 0.3eV l e v e l and 0.63eV l e v e l t o occur. The f a c t t h a t ho p i n g through t h e 0.3eV l e v e l i s n o t observed s e t s an upper l i m i t o f about 1 0 ~ ~ a n - ' on t h e t r a p d e n s i t y i n t h i s l e v e l , because a t h i g h e r d e n s i t i e s t h e cond- u c t i o n p a t h would pause i n t h i s l e v e l and n o t move s t r a i g h t t o t h e 0.43eV l e v e l . The a n a l y s i s f o r t h e l e v e l 0.63eV above t h e band y i e l d s an expected temperature o f 115K f o r t h e o n s e t o f hopping througn t h i s l e v e l which i s much lower t h a n t h e 230K observed i n f i g u r e 4. T h i s i n d i c a t e s e i t h e r Ro should be g r e a t e r f o r t h e s e t r a p s

(about 2nm), o r N o . 6 3 should b e n e a r e r

lo1

7cm- 3 .

Acknow1eaements.- We would l i k e t o thank D r S.W. Provencher f o r t h e c a n p u t e r program used i n t h i s s t u d y , and D r P.G. LeComber f o r u s e f u l d i s c u s s i o n s . References.

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747 (1978).

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2772 (1976).

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2362 (1977).