Dark conductivity below and above Tg in a-Se

Journal of Non-Crystalline Solids 77 & 78 (1985) 1237-1240 North-Holland, Amsterdam

1237

DARK CONDUCTIVITY BELOW AND ABOVE Tg IN a-Se J.P. LARMAGNAC, D. CARLES AND G. LEFRANCOIS LECAP, Facult~ des Sciences de Rouen, B.P. 67 76130 MONT SAINT AIGNAN FRANCE The temperature dependance of the dark conductivity of amorphous selenium thin f i l m s is studied around the glass t r a n s i t i o n region (Tg). The observed increase of the a c t i v a t i o n energy above Tg, is associated with the process of generation, in thermal e q u i l i b r i u m , of charged defects. I. INTRODUCTION The vitreous nature of amorphous selenium leads to well-known changes in i t s physical properties in the glass temperature region (Tg). Above Tg the material is in quasi thermodynamic e q u i l i b r i u m . On the contrary, below Tg, the frozen-in metastable state is characterized by a departure from thermodynamic e q u i l i b r i u m , which depends on the conditions of preparation and ageing of the samples. The e f f e c t of glass t r a n s i t i o n on d i f f e r e n t physical properties of amorphous selenium, such as volume, v i s c o s i t y , mechanical properties, heat capacity, has been often studied, but the e f f e c t on e l e c t r o n i c conductivity is less known. This paper presents results on the temperature dependance of dark conductivity in amorphous selenium thin f i l m s around the glass t r a n s i t i o n region. 2. EXPERIMENTAL PROCEDURE The a-Se samples were planar thin f i l m s (~1~m t h i c k ) , prepared by thermal evaporation under high vaccuum conditions (2.10 -9 Torr) on a substrate held at 293 K. Conductivity was measured in s i t u , at increasing temperature (0.33 K min-1). With the used heating rate and without exceeding 333 K, no sign of c r y s t a l l i z a t i o n is noticed : by fast cooling down to 290 K, the same level of c o n d u c t i v i t y as observed before the heating process, is restored. 3. RESULTS The Figure shows the v a r i a t i o n s of the logarithm of i n t e n s i t y versus r e c i procal temperature, under two d i f f e r e n t conditions of e l e c t r i c f i e l d . We may notice that the experimental curves are made up of two s t r a i g h t lines which define d i f f e r e n t a c t i v a t i o n energies above and below Tg. The marked break is situated in the glass t r a n s i t i o n region, such as i t is valued by thermodynamic measurements I 0022-3093/85/$03.30 © Elsevier SciencePub~shem B.V. (No~h-Hdland PhyficsPublishing Division)

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J.P. Larmagnac et al.

LOG I

I

/ Dark conductivity below and above Tg in a-Se

,

!

!

- - 2 7

--,,.,...

-,>-.,._._

- -30

._ii

3

3il

3i2

FIGURE : T e m p ~ a t u r e

3.=3

, O~/T

dependance of dark c~ent

a - elect~c

field

= 5 . 1 0 5 V.m - I

b - ~ect~c

field

= 5 . 1 0 6 V.m - I

These results are summarized in the following table :

electric field

Ea(T
Ea(T>Tg)

T break

5.105 V.m"I

0.81 eV

1.31 eV

315 K

5.106 V.m-I

0.70 eV

1.15 eV

319 K

We may underline that the a c t i v a t i o n energy measured f o r T > Tg is greater than half the energy gap ( ~ 2 , 1 e V i n a-Se) 2 Recently Kastner et a l . have observed an analog break by measuring the thermal dependance of the capture rate of recombination centers in t r a n s i e n t conductivity measurements on amorphous chalcogenides 3

Z P Larmagnacetal./Darkconductivi~ belowandaboveTgina-Se

1239

5. DISCUSSION

Dark conductZon b d o w Tg In amorphous chalcogenides the m a j o r i t y c a r r i e r s (holes) are created by thermal e x c i t a t i o n of electrons located in the top of the valence band, i n t o two steps : f i r s t ,

+

t r a n s i t i o n to the l e v e l s associated with C3 defects (which

become C~), then, t r a n s i t i o n to the Co3 l e v e l s (which become t i o n ) 4,

Ci a f t e r r e l a x a -

The energy to create one hole in the valence band is the average value of the energies involved in the two kinds of thermal t r a n s i t i o n s . This leads to pin the Fermi level near the middle of the gap of chalcogenides. The Fermi l e v e l of a-Se l i e s 0.13 eV below the mid gap 5. Then we can value the departure between the Fermi level and the valence band edge (E F - EV) to 0.92 eV. Due to space charged e f f e c t s , e l e c t r i c f i e l d reduces somewhat t h i s energy. Our r e s u l t s agree with t h i s model.

Dark comduct~on above Tg Defects states in the gap play an important part in the conduction process. Above Tg, in the supercooled l i q u i d s t a t e , the density of charged defects, in thermal e q u i l i b r i u m , is governed by the reaction : 0

2 +

2 C2 ÷

C3

+

Ci

+ C~ and C2 o From the mass action law, the densities N N_ and No of C3, 6 +' :

are r e l a t e d by the equation

N+ = N_ = No eEVAP/2 kT but, in the v i t r e o u s s t a t e , these d e n s i t i e s are frozen in at the temperature 6 :

Tg

N÷ = N_ = No e-EvAP/2kTg The c a r r i e r s generation is c o n t r o l l e d by the density of the acceptor l e v e l s N+, which is thermally a c t i v a t e d only above Tg. Then the increase of the a c t i vation energy of c o n d u c t i v i t y , observed above Tg, can be likened to EVAP/2. From our experimental r e s u l t s we deduce : EVAP ~ 0.95 eV This r e s u l t c o n s t i t u t e s on e q u i l i b r i u m measurement of energy of defects generation in amorphous selenium.

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J.P. Larmagnac et aL / Dark conductivity below and above Tg in a-Se

REFERENCES I) J.P. Larmagnac, J. Grenet and P. Michon, J. Non-Cryst. Solids, 45 (1981) 157 2) A. E. Owen and W. E. Spear, Phys. Chem. Glasses, 17 (1976) 175. 3) M. A. Kastner, T. Thio and D. Monroe, J. Non-Cryst. Solids, 66 (1984) 309. 4) E. A. Davis, Amorphous semiconductors, edited by M. H. Brodsky (Springer Verlag, Berlin, 1979), p. 41. 5) N. F. Mott and E. A. Davis, Electronic processes in non-crystalline materials (Clarendon Press, Oxford, 1979). 6) M. A. Kastner and H. Fritzsche, Phil. Mag. B 37 (1978) 199.