EFFECT OF COMPOSITION AND STRUCTURE ON TRANSPORT PROPERTIES OF CHALCOGENIDE GLASSES

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EFFECT OF COMPOSITION AND STRUCTURE ON TRANSPORT PROPERTIES OF CHALCOGENIDE

GLASSES

A. Andriesh, V. Gerasimenko, Yu. Ivaschenko, M. Iovu, A. Mironos, V.

Smirnov, M. Chernii, S. Shutov

To cite this version:

A. Andriesh, V. Gerasimenko, Yu. Ivaschenko, M. Iovu, A. Mironos, et al.. EFFECT OF COMPO- SITION AND STRUCTURE ON TRANSPORT PROPERTIES OF CHALCOGENIDE GLASSES.

Journal de Physique Colloques, 1981, 42 (C4), pp.C4-963-C4-966. �10.1051/jphyscol:19814211�. �jpa-

00220840�

ColZoque C4, suppze'ment uu nO1O, Tome 42, octobre 1981

EFFECT OF COMPOSITION AND STRUCTURE ON TRANSPORT PROPERTIES OF CHALCOGENIDE GLASSES

A.M. Andriesh, V.S. Gerasimenko, Yu.N. Ivaschenko, M.A. lovu, :4.S. Iovu, A.V. Mironos, V.L. Smirnov, N.R. Chernii and S.D. Shutov

I n s t i Lute of Applied Physics (Kishinev, 277028/,

Engineering Physics . I n s t i t u t e (I.losccw, 11540YI, ;l. H . S . S .

A b s t r a c t . - The e f f e c t of p r e p a r a t i o n r e g i m e a n d m o l e c u l a r com- p o s i t i o n o n d i s p e r s i v e t r a n s p o r t p r o p e r t i e s of amorphous c h a l - c o g e n i d e f i l m s o f As-Se, As,S,-Sb,S, and As,S,:Sn h a s b e e n i n -

c , , L _)

v e s t i g a t e d by t i m e - o f - f l i g h t method. Hole t r a n s i t t i m e ( c o r r e - s p o n d i n g t o d r i f t m o b i l i t y ) i n t h e s y s t e m As-Se was f o u n d t o be d e p e n d e n t on p r e p a r a t i o n r e g i m e , t h i s dependence b e i n g mos- t l y e x h i b i t e d i n n o n - s t o i c h i o m e t r i c f i l m s . I n A s S Sb s y s - t e ~ n t h e t r a n s i t t i m e and i t s a c t i v a t i o n e n e r g y a?e3gra$%illy d e c r e a s e d w i t h i n c r e a s i n g of Sb2S3 c o n t e n t . The t r a n s i t t i m e of As2S3 f i l m s doped w i t h Sn up t o 0.3 a t . % i s d e c r e a s e d by as much as 3 t o 4 o r d e r s of m a g n i t u d e a t room t e m p e r a t u r e a n d i n t h e i n t e r v a l f r o m 295 t o 360 K i n c r e a s e s w i t h t e m p e r a t u r e . The r e s u l t s a r e d i s c u s s e d i n f r a m e s of a model f o r anomalous c h a r - g e c a r r i e r d r i f t c o n t r o l l e d by c a p t u r e i n a s y s t e m of e n e r g y d i s t r i b u t e d t r a p s .

It w a s e s t a b l i s h e d by r e c e n t i n v e s t i g s t i o n s t h a t p r o p e r t i e s of c h a l c o g e n i d e amorphous f i l m s a r e s t r o n g l y d e p e n d e n t on c o m p o s i t i o n a n d p r e 2 a r a t i o n r e g i m e , a s t h e s e f a c t o r s d e t e r m i n e t h e s p a t i a l and e l e c t r o n i c s t r u c t u r e o f t h e f i l m s . I n t h i s r e p o r t t h e r e s u l t s o f ex- p e r i m e n t a l s t u d y o f n o n - e q u i l i b r i u m h o l e d r i f t i n amorphous c h a l c o g e - n i d e f i l m s o f As-Se, As2S3-Sb S and A s S :Sn. T h i s s p e c i e s a r e con-

2 3 2 3

s i d e r e d a s t h e v a r i a t i o n of i t s c o r n p o s i t i o n c a u s e s d i f f e r e n t m o d i f i - c a t i o n i n t h e s t r u c t u r e .

I n t h e s y s t e m AsxSel-x (x=0.2-0.5) t h e d e v i a t i o n f r o m s t o i c h i - ometry i s f o l l o w e d by f o r m a t i o n o f a d d i t i o n a l bonds between t h e com- p o n e n t s ; t h e c h a r a c t e r o f t h e s e bonds i s s t r o n ~ l y dependent on t h e f i l m p r e p a r a t i o n r e g i m e . I n t h e s y s t e m ( A S ~ S ~ ) ~ ? S ~ ~ S ~ ) l -y (y.1 .O-0.45) t h e b a s i c t y p e o f a s t r u c t u r a l u n i t i s r e t a i n e d u n d e r v a r i a t i o n of c o m p o s i t i o n . F i n a l l y , one may e x p e c t f o u r - f o l d c o o r d i n a t i o n u n i t s t o a p p e a r when a d d i n g Sn t o A s S The f i l m s were p r e p a r e d o n g l a s s sub-

2 3 '

s t r a t e s by s t a t i o n a ~ y o r d i s c r e t e t h e r m a l d e p o s i t i o n t e c h n i q u e a s w e l l as by l a s e r p u l s e e v a p o r a t i o n o f a c h a l c o g e n i d e g l a s s t a r g e t ( l ) . The s t r u c t u r e of t h e c o n d e n s a t e was c o n t r o l l e d by long-wave i n f r a r e d s p e c t r o s c o p y .

The t r a n s p o r t i n v e s t i g a t i o n s were made by measurements o f t r a n s i t t i m e tT o f a l i g h t p u l s e e x c i t e d c h a r k e p a c k e t t r a v e l l i n g t h r o u g h a sandwich specimen o f t h i c k n e s s L i n t h e e l e c t r i c f i e l d E=-

v

( V i s t h e v o l t a g e a p p l i e d t o e l e c t r o d e s ) . A s i n most c a s e s s o c a l l &

d i s p e r s i v e t r a n s p o r t w a s o b s e r v e d ( 2 ) , t h e c u r r e n t s i g n a l i ( t ) w a s of f e a t u r e l e s s form a n d t h e t r a n s i t t i m e w ~ s d e t e r m i n e d a t t h e p o i n t o f i n t e r s e c t i o n of t h e two s t r a i g h t l i n e s , p r e s e n t i n g t h e power-law t y p e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814211

JOURNAL DE PHYSIQUE

-4

^{F i g . 1 :} The dependence

of d r i f t m o b i l i t y i n As-Se f i l m s on E/L r a t i o . 1- As2Se3

-

^t- ( 2 ) ( t h e r m a l e v a p o r a t i o n ) , 2-

I

V) As2Se5, 3- As2Se2, 4- A s Se

. -5

^{2 3'}

h 5- As2Se8 ( p u l s e e v a p o r a t i o n ) .

i

W'

F

U

-6

s i g n a l i n c o o r d i n a t e s l o g i

v e r s u s l o g

t .

The d r i f t mobi-

S

^{l i t y w a s} d e t e r m i n e d as

L/EtT. P d =

I n F i g . 1 t h e dependen-

-7

c e s of

pd

o n E/L a r e shown

f o r As-Se f i l m s o b t a i n e d by p u l s e e v a p o r a t i o n o n t h e sub- s t r a t e h e l d a t room tempera- t u r e . The f i e l d dependence of

-8

t h e d r i f t m o b i l i t y o f As2Se3

f i l m s t h e r m a l l y e v a p o r a t e d o n a h o t s u b s r a t e a t 170°C ( 2 ) i s a l s o s c h e m a t i c a l l y i n d i c a - t e d ( d a s h e d l i n e ) . It i s s e e n

-9

t h a t t h e p u l s e e v a p o r a t e d

f i l m s have t h e m o b i l i t y v a l u - e s 2 t o 4 o r d e r s of magnitude l o w e r t h a n t h a t o f t h e s t a b i -

zo ^8.0 90 10.0

l i z e d s t o i c h i o m e t r i c f i l m .

-2 '

^The t i o n o b t a i n e d by b o t h methods f i l m s o f As2Se3 composi- as w e l l as t h e f i l m o f As2SeS o r As2Se8 h a v e c o m p a t i b l e f i e l d dependences of t h e m o b i l i t y . F o r t h e AsSe f i l m t h e f i e l d dependence i s much s t r o n g e r a n d , a f t e r r e p e t i t i - o n of measurements shows i n s t a b i l i t y , I i s a p p a r e n t l y due t o pho- t o s t r u c t u r a l c h a n g e s of t h e f i l m u n d e ~ t o e x c i t a t i o n .

According t o t h e t h e o r y of t h e a l s p e r s i o n t r a n s p o r t c o n t r o l - l e d by e n e r g y d i s t r i b u t e d t r a p s ( 3 ) , t h e exponent o f t h e m o b i l i t y f i e l d dependence i s d e t e r m i n e d by t h e t r a p d i s t r i b u t i o n f u n c t i o n , w h i l e t h e m o b i l i t y v a l u e i s r e v e r s e p r o p o r t i o n a l t o i t s c o n c e n t r a - t i o n . I n t h e c a s e of e x p o n e n t i a l t r a p d i s t r i b u t i o n

n(&)=noexp(-&/c')

t h e d r i f t m o b i l i t y

p,

^AJ ( l / n o ) ( E / L ) (&o/kT)-l _{( 1} I n f r a m e s o f t h i s model t h e s i m i l a r i t y of s l o p e s of t h e l i n e s 1 , 2 , 4 a n d 5 ( F i g . 1 ) may be c o n s i d e r e d as t h e r e f l e c t i o n o f t h e s i m i l a r i t y of t h e energy d i s t r i b u t i o n of d e f e c t s i n As2Se3, As2Se5 and As2Se8, w h i l e t h e d i f f e r e n c e i n m o b i l i t y c o r r e s p o n d s t o t h e d i f f e r e n c e i n c o n c e n t r a t i o n of them. From t h e o t h e r hand, t h e d i f f e r e n t exponent

ctra of As,Sel-, films obtained by pulse evaporation for lower (solid lines) and higher (dashed lines) degree of polime- rization. Vertical marks show the absor- ption peak position for the correspon- ding glasses.

X: 1-

0.2, 2- 0.286, 3- 0,4, 4- 0.5 .

of the field dependence for AsSe films is apparently due to another kind of traps in this material. The mentioned peculiarities are correlated with the data of structural study.

According to infrared spectra the absorption bands of the films obtainea by pulse deposition are shifted to the

280 _2w

_\

200 high-enersy-side of the spectrum respec- tive to the absor~tion band of the cor- g, cm-' responding glass 17ig.2). This shift is

caused by the distortion of the basic structural unit - the pyramid ASS^^,^ due to high condensation rate

(1

O ~ m / s ) and high vapour temperature. The greatest shift

is

obeer- ved for films of x=0.286. For Se contents higher than 7% the bands corresponding to Se-Se bonds (272 l/cm) were observed in the spect- ra, while they were absent in freshly deposited films. On the cont- rary, an excess content of As (more than

40%) leads gradually to

AS-As bond formation and to rearrangement of the structure from the orpiment type to realgar type. Depending on the condensation regime only one type of bonding could be observed in these films (Fig. 2).

In the pseudobinary amorphous system As2S3- Sb2S3 there is uasieutectic interaction of components of similar structural units 74.5). With Sb2S3 loading the mobility increases (Fig.3). The grea- test variation takes place at low temperatures, where the activati- on energy of the mobility is significantly lowered. The study of the dispersive transport in (AS$-,)

0.

55 (Sb2S3)0.45 have shown (6) that the quantitative agreement of drift characteristics with expo- nential trap distribution model can be obtained providing the de- pendence of capture probability on energy is taken into account. If the capture probability c(&) increases with energy exponentially as c(&)=coexp(

E / & ' )

, then /ud is expressed as:

where (l/&eff)=(l/&o)-(l/&'), pO ^and c are mobility and life- time of carriers in conductive states. The mobility activation en- ergy &, is related to Geff by expression

The energy ~arameters of the model for (As2S3)0.55(Sb2S3)0.45 are

&,=0.037 eV: and 6

I

=0.051 eV. The experimentally observed region of

the mobilityi activation energies is 0.46 to 0.18 eV in the interval

of E/L=(4 to

40)x108

~/cm2.

JOURNAL DE PHYSIQUE

P i .

:

Temperature dependence o drift mobility in films (As2S3)y(Sb2S3)l,y

(1

to 4) and As2S3Sn0,03 (5).

y:

1-1

-0, 2-0.99, 3-0.9.4-0.55 9

l ~ , -6

v .

I

L

%

^A

similar analysis of

-7 transport in As2S3 showed that

S the drift mobility is control-

led by much deeper local cent-

5 res in interval from 0.86 to

0.70 eV. The parameters of the model for As2S3 are &=0.033 eV and &'=0.068 eV. For films of As2S3 doped with Sn the ro-

-9 om temperature drift is incre-

ased'by about three orders of magnitude. When temperature is raised up to 360 K, the mobi-

-10 lity drops d o w ~ to the value

somewhat higher than that of pure As2S3 (Fig.4). The obser-

24 2.8 3.2 ved negative activation energy is caused by the negative va-

( 1 0 3 / ~ ~ ) lue of geff, as for Sn doped As2S3 G'<

&O (

0=0.032 eV, &'=

0.024 eV).

The variation of composition slightly affects the value of

E

indicating the energy distribution of trap density, while the ca9ture probability characteristic Q ' is several times varied. The decrease of activation energy in As2S3-Sb2S3 system is also due to to strong rise 0 f 9 ~ c , as the corresponding values of Geff are in- creased. Hence the obtained results suggest that the changes of tr~nsport properties under loading of Asps3 with impurities is cau- sed mainly by changes in capture parameters.

The authors are indebted to A.I. Rudenko and V.I. Arkhipov for useful discussions.

1.

SMIRNOV

V.L. et al., in Struktura, fizi80-khimicheskie svoistva

i

primenenie nekristallicheskikh poluprovodnikov, Kishinev (1980) 55

2.

PFISTER C., SCKER

I I . ,

Adv.Phys. 27 (1978) 747

3.

RUDENXO

A.I., ARKHIPOV V.I.,

~ h i E ~ a ~ .

(1979) 465

4. POLTAVTSEV Yu.G et al., Izv. AN SSSR, ser. Neorganicheskie ma- terial~ 11

(

1975)

1 188

5. CIERNOV A.P. et al, Zhurn.neorg. khimii 22 (1977) 198

6. ARKHIPOV V.I. et al., Phys.~tat.~ol.(a) 54 (1979) 6 7