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ISOTHERMAL RELAXATION CURRENTS IN ARSENIC CHALCOGENIDES
G. Adriaenssens, H. Michiel, H. Hammam
To cite this version:
G. Adriaenssens, H. Michiel, H. Hammam. ISOTHERMAL RELAXATION CURRENTS IN AR- SENIC CHALCOGENIDES. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-939-C4-942.
�10.1051/jphyscol:19814205�. �jpa-00220833�
ISOTHERMAL RELAXATION CURRENTS I N ARSENlC CHALCOGENIDES
G.J. A d r i a e n s s c n s . H. M i c h i e l and tl. Hammam
Laborato~ium voor Vas t e Stof-en Hogs DrukJysika, K. U. Lsuven, C e l e s t i jnenlaan 200 D, 8-3030 IleverLee-Leuven, Belgium.
A b s t r a c t . - The t r a n s i e n t r e s p o n s e t o s t e p v o l t a g e e x c i t a t i o n was i n v e s t i g a - t e d f o r b u l k As2S3 and As2Se3 m a t e r i a l and i n e v a p o r a t e d As2Se3 f i l m s . Au and A1 were u s e d a s e l e c t r o d e s . While i n g e n e r a l t h e r e s p o n s e proved t o be r e l a x a t i o n a l , f o r a l l As2S3 s a m p l e s and f o r t h e Au/As2Seg/Al s u b s t r a t e c e l l some a d d i t i o n a l s t r u c t u r e was o b s e r v e d which p o i n t s t o t h e t r a n s i t of a f i n i t e amount o f c h a r g e t h r o u g h t h e sample.
I n t r o d u c t i o n . - It i s a well-known f a c t t h a t when a v o l t a g e i s a p p l i e d t o one o f t h e a r s e n i c chalcogenides, p o l a r i z a t i o n o f t h e sample takes p l a c e and goes on f o r any- t h i n g f r o m minutes t o h o u r s . Hence t h e f r e q u e n t r e f e r e n c e s i n t h e l i t e r a t u r e t o e i t h e r w a i t i n g a s u f f i c i e n t l y l o n g t i m e t o a l l o w f o r s t e a d y s t a t e c o n d i t i o n s , o r measuring i m n e d i a t e l y a f t e r v o l t a g e a p p l i c a t i o n t o a v o i d d i s t o r t i o n o f t h e a p p l i e d f i e l d . S i n c e , t o o u r knowledge, t h e p o l a r i z a t i o n process i t s e l f , and t h e r e l a x a t i o n c u r r e n t s t h a t go w i t h i t , have n o t been f u l l y i n v e s t i g a t e d so f a r , t h e p r e s e n t s t u d y was undertaken.
Experimental.- As2S3 and As2Se3 b u l k m a t e r i a l was o b t a i n e d b y quenching f r o m t h e me1 t. Sanples x i t h t h i c k n e s s e x fro:: 5 3 1 ) ~ ~ t c 800un :.;ere p r e p a r e d by s l i c i n g t h e m a t e r i a l w i t h a w i r e saw and subsequent p o l i s h i n g o f t h e s u r f a c e s . We a l s o used As2S3 d i s k s purchased f r o m Servo C o r p o r a t i o n and m a t e r i a l f r o m a As2Se3 r o d k i n d l y p r o v i d e d by Dr. P. Nagels o f SCK/CEN, Mol. T h i n l a y e r s ( t y p i c a l l y 100um t o 200um) o f b u l k As2S3 and As2Se3 were p r e p a r e d by h e a t i n g i t t o t h e s o f t e n i n g p o i n t and p r e s s i n g between a l u m i n i u m and q u a r t z d i s k s . Evaporated As2Se3 samples were o b t a i - ned by open b o a t e v a p o r a t i o n of t h e b u l k m a t e r i a l o n t o e i t h e r g l a s s o r aluminium s u b s t r a t e s . F i l m s o f 20um t o 50um t h i c k were d e p o s i t e d a t about lum p e r m i n u t e . No e v a p o r a t e d As2S3 samples were examined. Except f o r some samples where t h e a l u - minium s u b s t r a t e was used as one o f t h e e l e c t r o d e s , g o l d o r a l u m i n i u m e l e c t r o d e s were e v a p o r a t e d t o form a sandwich t y p e c o n f i g u r a t i o n .
R e l a x a t i o n c u r r e n t s a f t e r s t e p v o l tage a p p l i c a t i o n were measured by way o f a B i o m a t i o n 805 t r a n s i e n t r e c o r d e r when t i m e s up t o 2 seconds were b e i n g examined;
t h e y were p l o t t e d d i r e c t l y on a c h a r t r e c o r d e r a f t e r p a s s i n g t h r o u g h a K e i t h l e y 427 c u r r e n t a m p l i f i e r f o r t i m e s between 1s and 1200s. C u r r e n t response i n t h e us-
range was b r i e f l y examined by p r e c e e d i n g t h e t r a n s i e n t r e c o r d e r w i t h a guarded c u r r e n t s e n s i n g r e s i s t o r and high-speed b u f f e r s t a g e t o e l i m i n a t e t h e i n f l u e n c e o f c a b l e and s t r a y capacitances. No s p e c i a l f e a t u r e s were observed on t h i s t i m e s c a l e . A F l u k e 415B h i g h v o l t a g e power s u p p l y w i t h r e v e r s i b l e p o l a r i t y was used as v o l t a g e source. Samples were s w i t c h e d t o e i t h e r s u p p l y v o l t a g e o r r e f e r e n c e p o t e n t i a l by means o f a mercury r e l a y . The sample t e m p e r a t u r e was h e l d c o n s t a n t w i t h i n 0.1K d u r i n g t h e r e l a x a t i o n measurements f o r temperatures between 290 K and 410 K.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814205
JOURNAL DE PHYSIQUE
RESULTS AND DISCUSSION
General behaviour.- With t h e exception o f some s p e c i f i c instances noted below, a l l samples e x h i 6 i t a f e a t u r e l e s s p o l a r i z a t i o n c u r r e n t when t h e v o l t a g e i s a p p l i e d and a d e p o l a r i z a t i o n c u r r e n t which i s n e a r l y t h e i n v e r s e o f t h e p o l a r i z a t i o n cur- r e n t when the v o l t a g e i s switched back t o zero a f t e r a p o l a r i z a t i o n time o f a t l e a s t 30 minutes.
F i g . 1 i l l u s t r a t e d t h i s behaviour a t t h r e e d i f f e r e n t temperatures f o r a 540um t h i c k As2S3,gold-electroded sample subjected t o an a p p l i e d v o l t a g e o f 1000V.
he
" n i v e r s a l " power-law dependence on time(1) o f the r e l a x a t i o n c u r r e n t s i s ob- vious from f i g . 1. A l l observed As2S3 r e l a x a t i o n c u r r e n t s a r e w e l l represented by an expression o f t h e formwhere
n = 0.75 i s e s s e n t i a l l y temperature independent, D i s t h e r m a l l y a c t i v a t e d w i t h ED = 0.3 eV,
I (-) i s t h e steady s t a t e dark c u r r e n t when p o l a r i z i n g , o r zero when d e p o l a r i - zing. As a c t i v a t i o n energy f o r t h e As2S3 dark c u r r e n t we o b t a i n Ed=1.12 eV.
While t h e same general behaviour was observed f o r t h e As2Se3 samples, values f o r D and n have n o t y e t been extracted. Here we can o n l y use the d e p o l a r i z a t i o n c u r - r e n t s f o r t h a t purpose. Since t h e steady s t a t e dark c u r r e n t i s much l a r g e r i n As2Se3 w i t h r e s p e c t t o t h e p o l a r i z a t i o n c u r r e n t than i s the case i n As2S3, tempe- r a t u r e f l u c t u a t i o n s induce v a r i a t i o n s i n I(-) which a l r e a d y a t 350 K equal t h e
~ tterm soon a f t e r t h e onset o f t h e p o l a r i z a t i o n process. - ~
Aside from t h e general agreement w i t h a t-n re l a x a t i o n behaviour, As2S3 and As2Se3 p o l a r i z a t i o n c u r r e n t s can a l s o be understood i n terms o f t h e f o r m a t i o n o f a de- p l e t i o n i a y e r a t t h e metal-semiconductor c o n t a c t such as described by f o r i n s t a n c e Matthews and Warter(2) o r Simmons and Taylor(3). The t o t a l charge t r a n s p o r t e d by t h e p o l a r i z a t i o n ( o r d e p o l a r i z a t i o n ) c u r r e n t i s found t o vary as the square r o o t of the a p p l i e d voltage, which agrees w i t h t h e n o t i o n o f a charge d e p l e t i o n region.
Fig. 1.
Polarization (full symbols) and depolarization currents (empty ones) in bulk As S
4 2 3;
E = 1.85 x 10 V / c m
ves were a l s o f i t t e d t o a model which coupled a r e l e a s e - o u t - o f - t r a p s source func- t i o n t o a d i s p e r s i v e t r a n s p o r t mechanism. I t was found t h a t o n l y a source w i t h the time dependence o f t h e a c t u a l r e l a x a t i o n c u r r e n t gave acceptable r e s u l t s , i n d i c a t i n g t h a t the observed c u r r e n t s a r e emission l i m i t e d . D e t a i l s of these and o t h e r i n v e s t i g a t i o n s i n t o the n a t u r e o f the r e l a x a t i o n c u r r e n t s and t h e d e p l e t i o n r e g i o n w i l l be presented i n a l a t e r r e p o r t .
D e v i a t i o n s from the power-law behaviour.- We observed departures from the t-"ecay on one t y p e o f As2Se3 sandwich c e l l s , and f o r sane parameter values on a l l measured As2Se3 samples. The As2Se3 case has been observed and discussed before by Abkowitz and Scher(4).
A t h i c k evaporated As2Se3 f i l m ( = 45um i n our case) on an aluminium s u b s t r a t e and w i t h an evaporated g o l d t o p e l e c t r o d e , when subjected t o s t e p v o l tage e x c i t a t i o n , e x h i b i t s t r a n s i e n t s o f the type shown i n f i g . 2 . The f i r s t t r a n s i e n t ( p o s i t i v e v o l t a g e t o the Au c o n t a c t ) i s t h e one s t u d i e d by Abkowitz and Scher. They showed t h a t the c u r r e n t maximum i s r e l a t e d t o the t r a n s i t o f a f i n i t e amount of charge through t h e sample. The g o l d e l e c t r o d e was assumed t o be the source of t h a t charge.
Our samples show t h e same f i e l d and temperature dependence o f the peak p o s i t i o n as the p u b l i s h e d r e s u l t s . As i s e v i d e n t from f i g . 2., d e p o l a r i z a t i o n c u r r e n t s d i f f e r s t r o n g l y here from the corresponding p o l a r i z a t i o n curve, and the two reverse shapes upon v o l t a g e r e v e r s a l . The f e a t u r e l e s s t r a n s i e n t f o l l o w i n g a downward v o l - tage s t e p shows a markedly f a s t e r decay than i s observed i n samples which obey t h e general r e l a x a t i o n power law.
We were n o t a b l e t o o b t a i n c a p a r a b l e r e s u l t s w i t h e i t h e r b u l k o r pressed l a y e r As2Se3 samples, o r f o r t h a t m a t t e r w i t h f i l m s which were taken o f f the s u b s t r a t e and where an evaporated g o l d o r aluminium f i l m was p u t on as back e l e c t r o d e .
I n a l l those cases we observed the general power-law decay described above.
I t i s c l e a r t h e r e f o r e t h a t a g o l d c o n t a c t by i t s e l f cannot account i o r the t r a n - s i e n t s o f f i g . 2.
D e v i a t i o n s from the power-law decay, as i l l u s t r a t e d i n f i g . 3, were observed f o r a l l As2S3 samples a t lower f i e l d s and temperatures. At room temperature the devia-
Fig. 2. Transients following switch-on and switch-off of + 80V and
-
80V to the Au contact of a 45um sample of the type shown.JOURNAL DE PHYSIQUE
F i g . 3 . R e l a x a t i o n c u r r e n t s f o r 0 . 8 m As2S3 w i t h Au/Au. Au/Al on Al/Al e l e c t r o d e s (300V a t 300 K).
t i o n s can be r e s o l v e d up t o = 10 V/cm, a t 370 K t h e y a r e o n l y seen up t o 4
= 3 x 10'~/cm. Such f i e l d and temperature dependence i s c o n s i s t e n t w i t h a t r a n s i t - t i m e phenomenon s i n c e a t h i g h e r f i e l d s o r temperatures a s h o r t e r t r a n s i t t i m e w i l l cause t h e e x t r a f e a t u r e t o be submerged i n t h e main r e l a x a t i o n peak. I t may be noted t h a t t h e same t h i n g happens t o t h e c u r r e n t maximum o f f i g . 2 . The asymmetry between p o i a r i z a t i o n and d e p o l a r i z a t i o n c u r r e n t s , and t h e r e v e r s a l w i t h v o l t a g e t h a t was observed i n t h e Au/As2Se3/Al-substrate c e l l i s a l s o found i n f i g . 3, a g a i n a c c e n t u a t i n g t h e s i m i l a r i t y o f t h e two d a t a s e t s . A s c r i b i n g t h e As2S3 f e a t u r e t o t h e t r a n s i t o f some f i n i t e amount o f charge i n analogy w i t h t h e As2Se3 r e s u l t s , one i s l e d t o h o l e t r a n s i t times i n t h e seconds range, which i s r e a s o n a b l e f o r o u r O.lmm t o 0.8nnn t h i c k samples under t h e f i e l d and temperature c o n d i t i o n s n o t e d above. I n o r d e r t o b e t t e r c h a r a c t e r i z e t h e i n d i c a t e d t r a n s i t phenomenon, i t w i l l be necessary t o s u b s t r a c t t h e superimposed r e l a x a t i o n c u r r e n t , o r t o f i n d an e x p e r i m e n t a l arangement where, analogous t o t h e As2Se3 case, a d i s t i n c t maximum can a c t u a l l y be r e s o l v e d .
Aknow1edgement.- F i n a n c i a l s u p p o r t f o r t h i s work was p r o v i d e d by t h e B e l g i a n I.I.K.W.
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2. MATTHEWS N.F.J. and WARTER P.J.Jr., Phys. Rev. 144 (1966) 610
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