EVIDENCE FOR CHARGED DEFECTS IN CHALCOGENIDES FROM HIGH FIELD CONDUCTIVITY

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EVIDENCE FOR CHARGED DEFECTS IN CHALCOGENIDES FROM HIGH FIELD

CONDUCTIVITY

A. Feltz, H. Kahnt, F. Schirrmeister

To cite this version:

A. Feltz, H. Kahnt, F. Schirrmeister. EVIDENCE FOR CHARGED DEFECTS IN CHALCO-

GENIDES FROM HIGH FIELD CONDUCTIVITY. Journal de Physique Colloques, 1981, 42 (C4),

pp.C4-935-C4-938. �10.1051/jphyscol:19814204�. �jpa-00220832�

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EVIDENCE FOR CHARGED DEFECTS I N CHALCOGENIDES FROM H I G H F I E L D C0;iDiJCTI V I T Y

A . Feltz, H . Kahnt and F . Schirrmeister

Department of Chemistry, FrYiedrich-SchiLler-University, Jena, G. D. E.

ABSTRACT. The n o n - l i n e a r c u r r e n t v o l t a g e behaviour of amorphous Ge Se Te t h i n f i l m l a y e r s p r e p a r e d by c o o l i n g from t h e m e l t , by

4

5

thermal low t e m p e r a t u r e e v a p o r a t i o n of t h e buUc m a t e r i a l and by a p p l i c a t i o n of a f l a s h e v a p o r a t i o n t e c h n i q u e i s i n v e s t i g a t e d , i n o r - d e r t o s t u d y t h e i n f l u e n c e of t h e d e n s i t y of l o c a l i z e d i n t r i n s i c de- f e c t s t a t e s on t h e IJoolc P r e n k e l e f f e c t . The e x p e r i m e n t a l r e s u l t s a r e d i s c u s s e d i n terms of a rl~odel of s c r e e n e d c e n t r e s i n t h e c a s e of t h e f l a s h o d l a y e r s .

E J T R O D U C T I O i J

R e c e n t l y , t h e Poole ls'renlcel e f f e c t $fa4 ~ I - o w ~ t o be c o n s i s t e n t w i t h t h e i d e a s of t h e ~rlodel of chargod d a n g l i n g b o ~ ~ d s [I ,2]. The non- l i n e a r c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s of amorphous t h i n f i l m s of tlle composition Ge Se,Te p r e p a r e d by thermal vacuum e v a p o r a t i o n

4 3

could be i n t e r p r e t e d i n terms of t h e t h r e e - c t i n ~ e n s i o n a l Poole Frellliel t h e o r y Tor s i w l e c e n t r e s . The d e n s i t y of t h e s e c e n t r e s was estima- t e d t o be i n t h e o r d e r of I O ' ~ C I I I - ~ . 111 t h i s paper amorphous t h i n f i l m s of t h e same composition p r e p a r e d by f l a s h e v a p o r a t i o n a r e i n - v e s t i g a t e d , i n o r d e r t o stutly t h e i n f l u e t ~ c e of' t h e v a r i a t i o n of t h e d e f e c t c o n c e ~ l t r a t i o t l on t h e c u r r e n t - v o l t a g e bchaviour. T h e o b s e m o d f i e l d depcnclent c o n d u c t i v i t y of t h o flashed t h i n f i l m s i s d e s c r i b e d by the model of t h e Poole Freldcol e f f e c t of s c r e e n e d c e n t r e s . The d c c o n d u c t i v i t y of t h i n d i s k s of t h o bu1.c g l a s s p r e p a r e d from t h e melt

i s ~:lcasured f o r comparisorl, too.

Ti1 : O l W ' r I C A L !C:XtiiG<S

C o r ~ s i t l e r i l ~ o n l y electrons t h e i ' i o l d i n c r e a s e d c a r r i e r c i e n s i t y r e l a - terl t o t h e Poole I:rc~dccl e f f e c t cam bc d c s c r i b c d by the g e n e r a l

A:: (Y,$

) i s t h e clif i . e r e ~ ~ c o between t h e cquili'oriurn 7ern1i enorgy F , n

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814204

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C L - 9 3 6 JOURNAL DE PHYSIQUE

EF and the quasi Fermi energy BFSn describiw the non-equilibrium occupation enhanced by the electkic field. In the case of defects with negative effective correlation energy (valence Alternation pairs)

A

E was shown to be identical with the lowering of the ionization energy F,n

A E

(F,

fl)

of Poole Frenlcol centres [ 2 ] . ( F ,

fl)

implicitly depends also on tho density of charged defects, because in the case of higher defect concentrationsscroeningeffects have to be taken into consideration. Hence, the potential energy is given by

the equation

...

The screening length ro increases with decreasing density of chargod defects. Assuming random distribution of the defect centres ro is

given by n

For i'solated centres one obtains

In the case of screened centres ~ B ( F , * ) is determined fror~l equation (2) by

with rm (F',Q) being the coordinate of the maximum value of equation (2) estimated numerically. For isolated centres

6

Z(F,+) is usually determined bv

Cornbini'ng (1) and

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one obtains equation

LF)= ( 7 + ~ 5 i n h a - c 0 ~ h c i ) 17)

r i = K

n(0) u2

^I

which is identical with the expression for the Poole Frenicel effect deduced by Connell et al. [ b ] . In the case of screoned centres the field dependent carrier density is calculated from equation (1) and

( 5 ) by numerical integration. Fig. 1 shows the influe~ice of the screening length ro on the field enhanced carrier concentration.

Provided the mobility is independent of the field strength a field dependent conductivity should be observed by changing the density of

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Pig. 1 I.lodcl c a l c u l a t i o n s f o r t h e Poole F r e d c e l e f f e c t of s c r e c n e d c e n t r e s

a s u i t a b l c rr:atorial a s p r e d i c t c d above (fie;. I ) .

S X i ' 3 1 U 3 B?PT.'LL

Samples of t h e b111;c c l a s s of t h e con- p o s i t i o n GekSc ..To were prcparecl by

I,

c u t t i n g and p o l i s h i n g t h e m a t e r i a l up t o a t h i c l a ~ e s s of about 0 , l ~un. Bc- cause of t h e c h e m i c a l l y orctored n e t - work s t r u c t u r e of sucll a g l a s s formed Cram t h e t h e r m a l e q ~ ~ i l i b r i u m of t h e melt t h e i n t r i r l s i c d i s o r d e r should be l o v e r t h a n i n t h e c a s e of t h i n amorphous l a y e r s propared by vacuum e v a p o r a t i o n rihicil was a l r e a d y des- c r i b e d [5]. Ln t h e f l a s h e v a p o r a t i o n technique s m a l l ? a r t i c l e s of tfm g l a s s Irere successively evaporated a t 1000 OC a i d a t a p r o s s u r e of 1 , 3 10-3 Pa: s u b s t r a t e t e m p e r a t u r e 8

-

¹²^{O C ,} d e p o s i t i o n r a t e 2 , 5 run

.

^{s - I .} Reforo e v a p o r a t i n g t h e ,-lays a bottorrl ooninct was do- p o s i t e d upon t h c s u b s t r a t e . 111e s a n d ~ r i c h a r r a w c m e n t v a s co~npletod by d e p o s i t i o n of a t o p c o n t a c t without i n t e r r u p t i o n of t h e vacuuol p r o c e s s . The composition of t h e l a y e r s was proved by c l ~ c m i c a l an- a l y s e s . 1\11 of t h o samples showed tile i d e n t i c a l con~po s i tiorl GeqSe-Te. They were f r e e from c o n c e n t r a t i o n g r a d i e n t s [ 6 ] .

3

RESULTS A X l DISCUSSION

The f i e l c l dependent c o n d u c t i v i t y of t h e amorphous GelCSe T e 1

5

ayers

p r e p a r e d by d i f f e r e n t methods i s shorn1 i n f i g . 2. Tho experimenta.1 d a t a of t h e b u l k m a t e r i a l and of t h e f i l m s o b t a i n e d by t h e thorulal e v a p o r a t i o n technique a t lower t e m p e r a t u r e s a r c excellently d c s c r i - bed by t h e t h e o r e t i c a l c u r v e d e r i v e d r"ro~a t h e model of t h e Poole l.'rel-ucel e f f e c t of i s o l a t e d c o n t r o s correspontling t o e q u a t i o n ( 7 ) . A t room t e m p e r a t u r e and a t h i g h e r t e m p e r a t u r e s t h e obscrvcd c u r v e s of t h e f i e l d clcpendont c o n d u c t i v i t y of t h e f l a s h c d t h i n f i l m s a r e i n accordance w i t h t h e model of t h e Poole Frerixcl e f f e c t of screcned c e n t r e s ( e q u a t i o n s 1 and 5 ) assuming a s c r e e n i n g l e n g t h r of about 2,5 nn~. From e q u a t i o n ( 3 ) a d e n s i t y

&

of 2

-

⁸

.

¹⁰16 ^ctn-' i s cnlcu- l a t e d in t h e limits of an e s t i m a t e d temperature i n t e r v a l of

280

-

¹²⁷⁰K f o r t h e f r e e z i n g p o i n t i n t h e f o r m a t i o n of i n t r i n s i c d e f e c t c e n t r e s in t h e d e p o s i t i o n p r o c e s s of t h c l a y c r s by f l a s h eva- p o r a t i o n . The h i g h e r d e f e c t c o n c e n t r a t i o n i s an i n t r i n s i c c f f c c t of t h e amorphous m a t e r i a l caused by t h e p r e p a r a t i o n method and cannot

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C 4 - 9 3 3 JOURNAL DE PHYSIQUE

_~_e,~f.~e

0 P

I

.illcorporateti t i u r i r g tllc, f i l i l ! p r o c c s -

(

^{i - m x}

o/ s i n g a s i r ~ t h e c a s e 01' s p u t t c r i l z o r C'J;). ' f i e cxperi1:lontn3. r e s u l t s a r c i l l

n c c o r t l , ? ~ ~ c c 11j.ti1 tilc cloticl of '/.'.l's v ~ i t l l i l l t h e limits oC a rtrociel or^

scrcenerl Poolc l:rcri:cl a c t i v e c c n t - res. Conscqucntly

,

t h e i n t r i n s i c d e f e c t s r.rc c h a r ~ c d . J ~ l d c p c l l d c n t l y d e r c c t s t a t e s cioubiy occupiccl cult1 - - ⁴ ,5 6 7 ~ ~ n o c e u p i e u r e s u l t from tile nbscllce

'

3 : 0 . 5 F i ; ' m ] -

of p a m c ; a g t l e t i s n i r l s u c h an a120 rphous 1:ig. 2 Corn]>arisorl oi' Cc,..;e :.Tc

a n c ~ p l e s p r c p a s c d by v n k o d s r:~n t e r i a l . The I J o o l c 1 3 e r ~ ~ : c l a c t i v e 'oc;laviour of s u c h c c n t r c s r e q u i r c s inr:?cmtivcly t h e nssunlption of cloul>le excitation. li? s u c l ~ a p r o c c s s t h e U - - c c n t r c s T- o r C i a s c trallsfornccl ir:to 1)' c e ~ l t r c s 3 'C ant1 r e v e r s a l a s

3 sho'lrn in l'ig. 3 .

P ~ G . 3 i n t r a c o r w e r s i o ~ l O_i' charged d c i ' e c t s t a t e s C + ant1 T _{o r}_{C T ,} r c s p o c t i v c l y , b g doubl2 excitation

iul'-;;jz]<c,;s

[ I ] F c l t z A , , S c h i r r c ~ c i s t e r 1%

,

ll'hys. S t a t . S o l . (a)

2

(1980) 491.

[ 2 ] I<a.hnt I I . , S c i ~ i r r r a o i s t c r 10'.

,

T c l t z A , , Yhys. S t a t . S o l . ( a )

37

(1980) !< 183.

[ 3 ] ~idlllt i I . , S c h i r r n l e i s t e r i:.

,

211ys. s t a t . s o l . (a), i~r p r e s s . [ b ] Colxnell G . A. N.

,

Cnc~phauscn D. L.

,

^Paul^i!.

,

P h i l . 1 . h ~ .

2

⁽¹⁹⁷²⁾

,;:::i

.

[5]

F e l t z A .

,

Ii;ips C h .

,

. S c h i r r r ; l c i s t c r I.'.

,

Thin S o l i d Films

70

(1980) 175.

[ 6 ] I h p s C h . , S c h i r r r r ~ o i s t c r I ? , !!Cahnt I I . , F e l t z A.

,

Proc. I n t . C o d . .imorph. Scuicond 8 0 , l i i s h i n e v , USSlL, (1980) ii 38.