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HIGH-SPEED TRANSIENT EFFECTS IN CHALCOGENIDE GLASSES
T. Shiraishi, D. Adler
To cite this version:
T. Shiraishi, D. Adler. HIGH-SPEED TRANSIENT EFFECTS IN CHALCOGENIDE GLASSES.
Journal de Physique Colloques, 1981, 42 (C4), pp.C4-579-C4-582. �10.1051/jphyscol:19814126�. �jpa-
00220744�
HIGH-SPEED TRANSIENT EFFECTS IN CHALCOGENIDE GLASSES
T . S h i r a i s h i and D. Adler
Department of EZectricaZ Engineering and Cornputer Science and Center for MateriaZs Science and Engineering, Massachusetts I n s t i t u t e
o fTechnoZogy, Cambridge,
MA02139, U.S.A.
Abstract.- High-speed t r a n s i e n t c u r r e n t s are o f t e n observed i n c h a l c o g e n i d e g l a s s e s . We e x p l a i n t h e s e e f f e c t s by invoking t h e p r e s e n c e of Valence A l t e r - n a t i o n P a i r s (VAP's). A f t e r a n e l e c t r i c f i e l d i s a p p l i e d a c r o s s t h e g l a s s , c a r r i e r s c a n t u n n e l d i r e c t l y from t h e e l e c t r o d e s o n t o t h e a p p r o p r i a t e l y charged d e f e c t c e n t e r s . T h i s l e a d s t o t h e a p p e a r a n c e o f p o t e n t i a l b a r r i e r s n e a r t h e c o n t a c t s , r e s u l t i n g i n a r a p i d decay of t h e c u r r e n t . Such a mechan- i s m c a n n o t o c c u r i n m a t e r i a l s w i t h o u t a l a r g e c o n c e n t r a t i o n o f n e g a t i v e l y c o r r e l a t e d d e f e c t s .
I n t r o d u c t i o n . - K a s t n e r e t a l . (1) showed t h a t a p a r t i c u l a r l y low-energy d e f e c t which t h e y c a l l e d a VAP, e x i s t s i n c h a l c o g e n i d e g l a s s e s . A t y p i c a l VAP c o n s i s t s o f a t h r e e - f o l d - c o o r d i n a t e d p o s i t i v e l y charged c h a l c o g e n (C3+) and a s i n g l e c o o r d i n a t e d n e g a t i v e l y c h a r g e d c h a l c o g e n (Cl-). The e f f e c t i v e c o r r e l a t i o n e n e r g y of a VAP i s n e g a t i v e , r e s u l t i n g i n a s t r o n g p i ~ i n g of t h e Fermi energy ( 2 ) . I n t h i s p a p e r , we show that t h e t r a n s i e n t c u r r e n t s observed i n e v a p o r a t e d f i l m s of Se60Te40 c a n b e e x p l a i n e d by t h e p r e s e n c e of VAP's.
Chalcogenide-Glass/Metal I n t e r f a c e s . - We assume t h a t VAP's a r e p r e s e n t i n a t y p i c a l c h a l c o g e n i d e g l a s s . We t a k e t h e energy of t h e C,,+ l e v e l a s T3 and t h a t of t h e
cl-
l e v e l as T1. For s i m p l i c i t y , we assume t h a t C O and C O have t h e same e n e r g y , s o t h a t t h e Fermi e n e r g y ,- . If,
i s pinned h a l f way aetween 'T 1 and T3. Then, we c a n e s t i m a t e t h a t a b o u t 10L4c<3 n e u t r a l c e n t e r s a r e p r e s e n t a t room t e m p e r a t u r e . I f t h i s i s t h e c a s e , v a r i a b l e - r a n g e hopping (3) c o u l d b e o b s e r v a b l e , i n p r i n c i p l e . However, i f a p o t e n t i a l b a r r i e r s u f f i c i e n t l y l a r g e t o r e t a r d t h e t u n n e l i n g of c a r - r i e r s i n t oc3+
o r Cl- s t a t e s e x i s t s n e a r t h e e l e c t r o d e s , ~ a r i a b l e ~ r a n g e hopping w i l l n o t b e observed. Thus, i t i s i m p o r t a n t t o a n a l y z e g l a s s / m e t a l i n t e r f a c e s .Let N3t and Nlt b e t h e d e n s i t i e s of C and Cl c e n t e r s , r e s p e c t i v e l y , thzit a r e 3
n e u t r a l i z e d by c a r r i e r s t u n n e l i n g from t h e e l e c t r o d e s . S u b j e c t t o t h e boundary con- d i t i o n s that N3t and Nlt v a n i s h p r i o r t o t h e t u r n i n g on of a n a p p l i e d f i e l d a t t=O, t h e r a t e e q u a t i o n s c a n b e s o l v e d t o y i e l d t $ e r e s u l t s of F i g . 1. At t h e l i m i t i n g d i s t a n c e f o r o b s e r v a b l e t u n n e l i n g xc = 1 0 0 A, N,,t r a p i d l y f a l l s o f f a f t e r a t i m e
- -
d e t e r m i n e d by t h e e l e c t r o n c a p t u r e c r o s s s e c t i o n and v e l o c i t y and t h e decay l e n g t h of t h e wave f u n c t i o n w i t h i n t h e b a r r i e r . Because of t h e v a r i a t i o n of N3t w i t h t i m e , n e g a t i v e s p a c e c h a r g e , N
-,
i s b u i l t up n e a r t h e c a t h o z e a sa r e t h e e q u i l i b r i u m f r e e e l e c t r o n and h o l e c o n c e n t r a t i o n s , r e s p e c - t i v e l y . A s i m i l a r p o s i t i v e s p a c e c h a r g e , N--, i s b u i l t up n e a r t h e anode:
The r e s u l t t h a t space-charge r e g i o n s a r e formed n e a r t h e e l e c t r o d e s b e c a u s e o f t h e t u n n e l i n g of c a r r i e r s i n t o charged d e f e c t c e n t e r s should be u n i q u e t o semiconductors
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814126
JOURNAL DE PHYSIQUE
Fig. 1: C a l c u l a t e d d i s t r i b u t i o n of N3t a s a f u n c t i o n of posi- t i o n X f o r d i f f e r e n t v a l u e s of a p p l i e d f i e l d , F.
anode
having n e g a t i v e l y c o r r e l a t e d d e f e c t s such a s VAP's. S i n c e t h e n e g a t i v e space c h a r g e i n c r e a s e s t h e p o t e n t i a l i n t h e v i c i n i t y of t h e anode, t h e space-charge l a y e r s a r e s e l f - l i m i t i n g . The r e s u l t i n g h i g h - r e s i s t a n c e r e g i o n s n e a r both e l e c t r o d e s r e t a r d f u r t h e r i n j e c t i o n once t h e width of t h e s e r e g i o n s exceed t h e c r i t i c a l d i s t a n c e f o r t u n n e l i n g . On t h e o t h e r hand, i n t h e p r o c e s s of formation of t h e p o t e n t i a l b a r r i - e r s , b u l k c a r r i e r s a r e e x t r a c t e d , forming space-charge r e g i o n s w i t h a s i g n o p p o s i t e t o t h a t of t h e p r e v i o u s l y d i s c u s s e d l a y e r s , a s shown i n Fig. 2. I n chalcogenide g l a s s e s , we. expect p
,
n"
1 0 ~ ~ c m - ~ , s o t h a t t h e s e space-charge l a y e r s a r e consider- a b l y wider t h a n t h e g o t e g t i a l b a r r i e r s n e a r t h e e l e c t r o d e s .Fig. 2. P r e d i c t e d s p a t i a l v a r i a t i o n Fig. 3. Band model f o r chalcogenide of t h e bands i n a chalcogenide g l a s s : g l a s s . The n o t a t i o n i s t h e same a s
+,-,
@, and 8 r e f e r t o C:, C;, C:, and i n Fig. 2 ( s e e t e x t ) . C:
, r e s p e c t i v e l y ( s e e t e x t ) .
Let u s c o n s i d e r a chalcogenide g l a s s w i t h t h e e q u i l i b r i u m band s t r u c t u r e shown i n Fig. 3 (dashed l i n e ) . I f a v o l t a g e i s a p y l i e d t o such a g l a s s , c e n t e r s n'ear t h e cathode a r e n e u t r a l i z e d and CO c e n t e r s n e a r t h e cathode a r e i o n i z e d by t u n n e l i n g up t o X = X
,
t h u s i n c r e a s i n g t h e n e g a t i v e space-charge i n t h a t region. 1 On t h e o t h e r hand, c ° C c e n t e r s a r e i o n i z e d and C- c e n t e r s a r e n e u t r a l i z e d n e a r t h e anode, and a s a r e s u q t an e x t r a p o s i t i v e space-charge l a y e r i s c r e a t e d up t o 1 X = Xa s shown i n Fig. 3 ( s o l i d l i n e ) . I f t h e bending n e a r t h e anode i s upward a t equi- a ' l i b r i u m , t h e o p p o s i t e r e s u l t i s o b t a i n e d . These c o n s i d e r a t i o n s a r e c o n s i s t e n t w i t h b o t h t h e a p p a r e n t l a c k of e x c e s s c a r r i e r i n j e c t i o n i n t o t h e b u l k below a c r i t i c a l a p p l i e d f i e l d and t h e e x i s t e n c e of a wide n e g a t i v e s p a c e - c h a r g e l a y e r , r e c e n t l y observed i n A U / G ~ ~ ~ T ~ ~ ~ / A U s t r u c t u r e s i n t h e u i & i t y of t h e anode ( 4 ) . However, t h e s e c o n d i t i o n s l f f e r from t h o s e i n h e t e r o j u n c t i o n t r a n s i s t o r s , such a s c-Si/chal-
f o t t h e o p p o s i t e d i r e c t i o n of t h e a p p l i e d f i e l d ; t h e r e f o r e , a n e g a t i v e space-charge l a y e r i s formed n e a r t h e anode and a p o s i t i v e space-charge l a y e r i s c r e a t e d n e a r t h e cathode. We should n o t e t h a t f o r t h i s c a s e , i n j e c t i o n of hot e l e c t r o n s and h o l e s from t h e chalcogenide g l a s s i n t o t h e - c r y s t a l l i n e semiconductor may t a k e p l a c e i n a s u f f i c i e n t l y h i g h a p p l i e d f i e l d because i n j e c t i o n of thermal e l e c t r o n s and h o l e s from t h e chalcogenide g l a s s i n t o t h e c r y s t a l l i n e semiconductor i s r e t a r d e d by t h e presence of t h e p o t e n t i a l b a r r i e r s .
High Speed T r a n s i e n t E f f e c t s . - When t h e v o l t a g e a c r o s s t h e sample is removed, t h e space-charge l a y e r s n e a r t h e e l e c t r o d e s decay r a p i d l y . This l e a d s t o a f a s t d e c r e a s e i n c u r r e n t , a s shown i n Fig. 4.
Fig. 4 . T r a n s i e n t c u r r e n t decay a t s e v e r a l temperatures.
T~~ i s t h e i n i t i a l decay time c o n s t a n t and T~~ i s t h e f i n a l decay time c o n s t a n t Fig.5. Time c o n s t a n t a s a f u n c t i o n of
temperature. The
n o t a t i o n i s t h e same a s i n Fig. 4 .
This r e l a x a t i o n can be understood a s t h e r e v e r s e p r o c e s s of t h e formation of Njt o r
Nit.
Thus, t h e r e l a x a t i o n t i m e Tr of t h e decay c u r r e n t can b e w r i t t e n a swhere nt i s t h e d e n s i t y of t u n n e l i n g e l e c t r o n s , n is t h e d e n s i t y of e l e c t r o n s on i
t h e C3 c e n t e r s , v i s t h e v e l o c i t y of t h e t u n n e l i n g e l e c t r o n s , and Cnt i s t h e i r n t
c a p t u r e c r o s s s e c t i o n .
Equation (3) shows t h a t t h e r e l a x a t i o n p r o c e s s h a s two time c o n s t a n t s and two a c t i v a t i o n e n e r g i e s ; t h i s is c o n s i s t e n t w i t h t h e experimental r e s u l t s shown i n Fig.5.
E l e c t r i c a l - c o n d u c t i v i t y s t u d i e s of evaporated Se 60Te40 f i l m s w i t h an o p t i c a l gap of 1.32eV i n d i c a t e s t h a t t h e a c t i v a t i o n energy i s about 0.35eV. However, thermoelec- tric-power measurements show t h a t t h e semiconductor is predominantly p-type. There- f o r e , we should w r i t e t h e analogue of Eq. (3) f o r h o l e s . Using Boltzmann s t a t i s t i c s , w e f i n d
1 -T1
+
E'C =
r l (0) e x p ( m a ) exp ( kT
1
(4)O p t v p t N v
But our experimental r e s u l t s i n d i c a t e 0.21eV
.ie1 = 3.61 r
lo-'
exp (1
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Comparing Eqs. ( 4 ) - ( 7 ) , we c a n s e t E
-
T = 0.21eV, and s i n c e t h e c o n d u c t i v i t y measurements show t h a t , Et-
Ev = 0 . 4 5 e ~ , we e x p e c t T1 1 = 0.13eV. Thus, C - c e n t e r s1 a r e l o c a t e d 0.13eV above t h e valence-band m o b i l i t y edge, I n a d d i t i o n , T 1
-
E v-
e f i x = 0.09eV, and t h u s e h z = 0.04eV. At F = 3 X 1 0 V/cm, we c a n e s t i m a t e 4 xa E
-
1 3 0
i.
We, t h u s , c o n c l u d e t h a t t h e high-speed t r a n s i e n t e f f e c t s o b s e r v e d i n c h a l c o - g e n i d e g l a s s e s a r i s e from t h e r e l a x a t i o n p r o c e s s of n e u t r a l C O and C O c e n t e r s t h a t3 1
a r e c r e a t e d by t h e t u n n e l i n g of c a r r i e r s from t h e e l e c t r o d e s . The d e r i v e d v a l u e o f Ef
-
T = 0.21eV i s i n good agreement w i t h t h e r e s u l t o b t a i n e d i n a w e l l - c h a r a c t e r - i z e d cAalcogenide g l a s s by Frye and Adler (5) u s f n g a n a n a l y s i s of f i e l d - e f f e c t measurements.Conclusions.- We have shown t h a t i n j e c t e d c a r r i e r s can be t r a p p e d by t h e VAP's i n c h a l c o g e n i d e g l a s s e s . These t r a p p e d c a r r i e r s t h e n c r e a t e p o t e n t i a l b a r r i e r s , which a r e s e l f - l i m i t i n g . During t h e f o r m a t i o n of t h e s e p o t e n t i a l b a r r i e r s , high-speed t r a n s i e n t c u r r e n t s s h o u l d b e o b s e r v e d . Such e f f e c t s a r e e x p e c t e d t o be u n i q u e t o s e m i c o n d u c t o r s h a v i n g n e g a t i v e l y c o r r e l a t e d d e f e c t c e n t e r s s u c h a s VAP's.
Acknowledgement.- T h i s r e s e a r c h was s u p p o r t e d by t h e M.I.T. Cabot S o l a r Energy Fund.
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37
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36
(1976) 1197.3 . MOTT, N. F., P h i l . Mag.
19
(1969) 835.4. SHTRAISHT, T . , IIDA, M - , SHINOHARA, K . , and ADLER, D., J. Non-Cryst. S o l i d s , i n p r e s s .
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