AC CONDUCTIVITY OF AMORPHOUS As-Se-Ag SYSTEM

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AC CONDUCTIVITY OF AMORPHOUS As-Se-Ag SYSTEM

M. Kitao, K. Hirata, S. Yamada

To cite this version:

M. Kitao, K. Hirata, S. Yamada. AC CONDUCTIVITY OF AMORPHOUS As-Se-Ag SYSTEM.

Journal de Physique Colloques, 1981, 42 (C4), pp.C4-927-C4-930. �10.1051/jphyscol:19814202�. �jpa-

00220830�

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JOURNAL DE PHYSIQUE

CoZZoque C4, supplimennt au n O I O , Torte 42, octobre 1981 page C 4 - 9 2 7

AC CONDUCTIVITY OF ArlORPHOUS A s - S e - A g SYSTEM M . K i t a o , K . H i r a t a and S. Yarnada

Research I n s t i t u t e of Electronics, Shizuoka University, Johoku 3-5-1, Hamamatsu 432, Japan.

A b s t r a c t . - A.C. c o n d u c t i v i t y aac of amorphous As40+xSe60-x and As2Se3:Ag has --

been measured. The uac o f these m a t e r i a l s v a r i e s as wS i n t h e a u d i o frequency range. A t room temperature, t h e exponent s decreases f r o m 1 t o 0.7 w i t h i n - c r e a s i n g x i n A s ~ ~ +

,

~w h i l e s i s 0.7 i r r e s p e c t i v e of Ag c o n t e n t i n S ~ ~ ~ - ~ As2Se3:Ag. W i t h d e c r e a s i n g temperature, s i n As2Se3:Ag i n c r e a s e s . D e f e c t s t a - t e s due t o Ag a d d i t i v e s a r e c o n s i d e r e d t o be d i r e c t l y concerned w i t h uac.

1

.

INTRODUCTION

The ac c o n d u c t i v i t y oac o f amorphous semiconductors n o r m a l l y e x h i b i t s a f r e - quency dependence as

oat

= - i s w i t h 0.7

5

s 5 1 . The aa, has been c o n s i d e r e d t o be caused by phonon-assisted hopping between l o c a l i z e d s t a t e s n e a r t h e Fermi l e v e l [I].

Recently, E l 1 i o t t [2,3] i n v e s t i g a t e s t h e c l a s s i c a l hopping between charged d a n g l i n g d e f e c t s c a t e s [4,5] i n chalcogenide glasses.

We have r e p o r t e d how Ag a d d i t i v e i n f l u e n c e s t h e e l e c t r i c a l and o p t i c a l p r o p e r - t i e s [6,7]. The p r e s e n t paper aims t o i n v e s t i g a t e t h e e f f e c t o f Ag a d d i t i v e i n amorphous As2Se? on ac c o n d u c t i o n . S i n c e t h e i n t r o d u c e d Ag atoms seem t o combine

-

w i t h selenium atoms, t h e b u l k As2Se3 becomes a r s e n i c r i c h . We measured aac o f a r s e n i c r i c h samples t o i n v e s t i g a t e t h e e f f e c t o f t h e d e s t r u c t i o n o f s h o r t range o r d e r due t o t h e d e v i a t i o n f r o m t h e s t o i c h i o m e t r i c c o m p o s i t i o n .

2 . EXPERIKENTAL PROCEDURE

Amorphous As2Se3:Ag was prepared by s y n t h e s i z i n g As(6N), Se(6N) and Ag(5N).

S i l v e r c o n c e n t r a t i o n i n t h e range f r o m 0.025 t o 0.5 a t % was examined. A s ~ ~ + ~ S ~ ~ ~ - ~ ( x = 0.02, 0.1, 0.2 and 0.4) samples were a l s o prepared. The samples used f o r t h e

measurements were sawn from an i n g o t i n t h e shape o f p l a t e and p o l i s h e d . The s i z e

3

o f t h e samples was t y p i c a l l y 20 x 15 x 0.3 mm3. Evaporated g o l d f i l m s were used as e l e c t r o d e s and found t o make ohmic c o n t a c t s w i t h t h e sample. A guard r i n g e l e c t r o d e was added on a s u r f a c e o f t h e samples i n o r d e r t o p r e v e n t leakage c u r r e n t . Peasure- ments o f frequency dependent c o n d u c t i v i t y were made w i t h a d i e l e c t r i c l o s s meter.

The oaC i s d e f i n e d a c c o r d i n g t o t h e r e l a t i o n : oac

-

^ow

^-

adc, where ow i s t h e t o t a l c o n d u c t i v i t y observed under ac f i e l d and odC t h e dc c o n d u c t i v i t y .

3. RESULTS AND DISCUSSION

3.1

.

A.C. C o n d u c t i v i t y o f As-Se-Ag System a t Room Temperature

The ac c o n d u c t i v i t y oa, o f amorphous A s ~ ~ + and As2Se3:Ag a t room temper- ~ S ~ ~ ~ - ~ a t u r e a r e shown i n Figs.1 and 2, r e s p e c t i v e l y . The oaC o f these amorphous m a t e r i a l s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814202

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JOURNAL DE PHYSIQUE

Ag content (at%)

A 0.5

I

6" I

^I ^I

1 lo2

1 0" 1

o6

Frequency ( Hz ) Fig.1 A.C. conductivity of As40+xSe60

-x

a t room temperature.

L I

I 02 I

o4

I

o6

Frequency ( Hz) Fig.2 A.C. conductivity of As2Se3:Ag a t room temperature.

Fig.3 Variation of the exponent s

with x i n A s ~ ~ + ~ S ~ ~ ~ - ~ and with Ag content i n As2Se3:Ag.

-

0 01 02 0.3 0.4 0.5 Ag content (at0/.)

varies a s wS in the frequency range from 100 Hz t o 500 kHz. The exponent s decreases from 1 t o 0.7 with increasing

x

in A s ~ ~ + ~ S ~ ~ ~ - ~ . In As2Sej:Ag, on the other hand, s has usually a constant value of 0.7 i r r e s p e c t i v e of Ag content, and the increment of aac due to the addition of Ag i s l i n e a r l y proportional t o Ag content i n the concentration range of more than 0.05 aYk [7]. Fig.3 shows variation of s with

x

and Ag content.

I f the randomly d i s t r i b u t e d p a i r model [I], frequency dependence of aac can be written approximately a s

where r w i s the optimum hopping distance, a the e f f e c t i v e radius of localized s t a t e ' s wave function and B ' the quantity concerned with

ro

and the c o e f f i c i e n t of relaxation time

[a].

According t o t h i s model, the exponent s decreases with

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d e c r e a s i n g rw. Since t h e v a l u e o f x r e p r e s e n t s t h e d e v i a t i o n f r o m s t o i c h i o m e t r i c c o m p o s i t i o n , t h e i n c r e a s e o f x means t h e i n c r e a s e o f t h e d e n s i t y o f l o c a l i z e d d e f e c t s t a t e s . I f t h e c a r r i e r h o p p i n g between d e f e c t s t a t e s takes p l a c e , r,,,

-

decreases w i t h i n c r e a s i n g x . I n AszSe3:Ag, s has a c o n s t a n t v a l u e . A c c o r d i n g l y ru does n o t much v a r i e d w i t h Ag c o n t e n t . I f d i s o r d e r e d s t a t e such as Ag3Se i s formed i n As7Se3:Ag sampl? [9], two d a n g l i n g bonds o f As a r e e x p e c t e d t o b e - s e t up i n t h e neighbo;rhood o f Ag2Se and c o n s t r u c t a d e f e c t p a i r . T h i s d e f e c t p a i r i s s i m i l a r t o t h e one proposed by E l l i o t t [Z]. I t i s e x p e c t e d t h a t t h e d i s t a n c e between two d e f e c t s t a t e s w i t h i n t h e p a i r i s r e s t r i c t e d i n a s m a l l range. I f t h e c a r r i e r h o p p i n g o c c u r s w i t h i n t h e d e f e c t p a i r , t h e e x p e r i m e n t a l r e s u l t s a r e qua1 i t a t i v e l y i n t e r p r e t e d , where t h e exponent s has a c o n s t a n t v a l u e and t h e magnitude o f oaC i s p r o p o r t i o n a l t o Ag c o n t e n t . A c c o r d i n g t o E l l i o t t [2,3], t h e exponent s i s r e p r e s e n t e d as

where WM i s t h e maximum b a r r i e r h e i g h t which i s t a k e n t o be equal t o t h e band gap E f o r t h e case o f c h a l c o g e n i d e g l a s s e s . However, t h e b a r r i e r h e i g h t between t h e g d e f e c t s t a t e s around Ag2Se may be s m a l l e r than Ea. I f WM i s a b o u t f o u r t h o f Eq, t h e v a l u e o f s becomes 0.7.

The above i n t e r p r e t a t i o n i s a l s o conducted f r o m t h e dependence o f t h e magnitude o f dc c o n d u c t i v i t y and i t s a c t i v a t i o n energy on x i n A s ~ ~ + and on Ag c o n t e n t ~ S ~ ~ ~ - ~ i n As2Sej:Ag, i f Ag a d d i t i v e s a r e concerned w i t h dc c o n d u c t i v i t y [ 6 ] .

3.2. Temperature Dependence o f A.C. C o n d u c t i v i t y o f As2Se3:Ag

The ac c o n d u c t i v i t y

oat

o f As2Se3:Ag was measured f r o m room t e m p e r a t u r e down t o

77 K . Temperature dependence o f t o t a l e l e c t r i c a l c o n d u c t i v i t y ow o f A s ~ ~ S ~ ~ ~ A ~ ~ . ~ i s shown i n Fig.4. As seen i n t h i s f i g u r e , t h e s l o p e o f J ~ ~ ( = ow

- ode)

v e r s u s 1/T

decreases w i t h d e c r e a s i n g temperature and w i t h i n c r e a s i n g f r e q u e n c y . As p o i n t e d o u t by E l l i o t t [3], these f a c t s suggest t h a t ac c o n d u c t i o n o f As2Se3:Ag i s caused by t h e c l a s s i c a l b a r r i e r hopping. I t i s t o be n o t e d , however, t h a t oac o f undoped AszSe3 i s independent o f t e m p e r a t u r e [ 7 ] .

Fig.4 Temperature dependence o f dc ( a d c ) and t o t a l ( o w ) c o n d u c t i v i t y o f As2Se3 c o n t a i n i n g 0.5 a t % Ag.

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C4-930 JOURNAL DE PHYSIQUE

O O O

a !

^O

r a w &

O t

-

w

Ag content (atole)

0 0

i

0.025

A 0.05

A 0.1 v 0.2

-

^r ^0.5

I I

77 100 200 300

Frequency

(Hz

) Temperature ( K )

F i g . 5 A.C. c o n d u c t i v i t y o f As2Se3 F i g . 6 V a r i a t i o n o f t h e exponent s c o n t a i n i n g 0.5 a t % Ag a t d i f f e r e n t o f As2Se3:Ag w i t h temperature.

t e m p e r a t u r e .

The aac o f As40Se60Ag0.5 a t d i f f e r e n t temperatures i s shown i n Fig.5. Fig.6 shows t e m p e r a t u r e v a r i a t i o n o f t h e exponent s f o r t h e samples w i t h v a r i o u s Ag c o n t e n t s . The v a l u e s o f s i n c r e a s e w i t h d e c r e a s i n g t e m p e r a t u r e . I f phonon-assisted h o p p i n g mechanism i s predominant, s i s p r e d i c t e d t o decrease w i t h d e c r e a s i n g temper- a t u r e

[lo].

I n c l a s s i c a l hopping mechanism, on t h e o t h e r hand, t h e t e m p e r a t u r e dependence o f s i s s a t i s f i e d w i t h e x p e r i m e n t a l r e s u l t s as seen i n e q . ( 2 ) . A t 77 K, t h e v a l u e o f s becomes more t h a n 1 . Namely s u p e r - l i n e a r f r e q u e n c y dependent conduc- t i v i t y i s observed. I n case t h a t r,,, has a s m a l l v a l u e and s t a t e s a r e p a i r i n g , i t i s p o i n t e d o u t by E l l i o t t [ll] t h a t s u p e r - l i n e a r

oat

i s produced

By i n t r o d u c i n g Ag, as mentioned above, A ~ ~ S ; may be formed. D e f e c t p a i r s around Ag2Se a r e c o n s i d e r e d t o be s a t i s f i e d w i t h above c o n d i t i o n s . I n As2Se3:Ag, t h e r e f o r e , ac c o n d u c t i o n seems t o be c o n t r i b u t e d by t h e c l a s s i c a l b a r r i e r h o p p i n g between d e f e c t s t a t e s w h i c h a r e c l o s e l y connected w i t h Ag a d d i t i v e s .

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36

(1977) 1 2 9 1 . [3] ELLIOTT S.R., P h i l o s . Kag.

37

(1978) 553.

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[5] KASTNER M. and FRITZSCHE H.

,

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[6] ISHIKAWA T., KITAO M., AKA0 H. and YAYADA S., phys. s t a t . s o l . ( a )

57

(1980) 373.

[7] KITAO M., ASAKURA N. and YAPADA S., Japan. J . Appl

.

Phys.

19

(1980) L302.

[8] KITAO M., Japan. J . Appl

.

Phys.

fl

(1972) 1472.

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[ l o ]

AUSTIN I . G . and MOTT N.F., Adv. Phys. 18 (1969) 41.

[ll] ELLIOTT S.R., S o l i d S t a t e Comm. g ( 1 9 7 8 ) 939.