Electrical properties of the amorphous semiconducting SeTeIn system

Journal of Non-Crystalline Solids 103 (1988) 295-299 North-Holland, Amsterdam

295

ELECTRICAL PROPERTIES OF THE AMORPHOUS SEMICONDUCTING S e - T e - l n SYSTEM A.B. GADKARI and J.K. ZOPE Department of Physics, Shivaji University, Kolhapur 416004, India Received 7 April 1987 Revised manuscript received 22 July 1987

The measurement of DC electrical conductivity and thermoelectric power of the bulk samples of chalcogenide glasses of the system SeT0-Te30, SeTo-Te30_xln x (x = 1, 3, 5, 7, 9% atomic weight) are studied. The activation energies are obtained from conductivity and thermoelectric power measurements, using Mott, Davis and Fritzsche equations. The difference of activation energies EQ = E * - Es* = 0.12 eV is explained on the basis of long range electrostatic potential fluctuations, and this difference in activation energies is in agreement with the results reported by Overhof and Beyer for the chalcogenides. And it has also been observed that addition of indium in the samples increases the electrical conductivity.

1. Introduction Although over the last decade a great deal of experimental data has been made available in the field of the transport properties of chalcogenide glasses, the nature of electronic transport still remains unclear in these materials. These solids are characterized by structureless tails of the conduction and valence band states. Several band models have been proposed to explain the electronic structure of these materials [1-4]. It has been reported in the literature that there is a difference between activation energies, calculated from electrical conductivity and thermoelectronic power [5,61. Several conduction models have been proposed to explain the electrical transport in chalcogenide glasses, largely based on the experimental observations of the p-type materials. Davis and Mott [1] proposed a singly activated conduction mechanism in the valence band. The electrical conductivity and thermoelectric power measurements of Se-Te have been reported by Mahdjuri and Zope [7,8]. The thermoelectric power in Se-Te is reported by Edmond [9]. The field dependence of electrical conductivity of Se-Te has been reported by Mehra [10]. However, the DC conductivity of the chalcogenide glass system Se-Te-Sb was in0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

vestigated as a function of temperature by Nagels and Sakai [11,12]. They showed that the conductivity was found to increase with increasing content of Sb and thus decrease in activation energy. The electrical and optical properties of As-Se-In and G e - S e - I n have been reported by Kosek and Karpova [13]. The electrical conductivity of Ag-As-Se glasses has been reported by Carcaly and Houphouest [14]. The above studies show an increase in conductivity and decrease in activation energy with increasing Sb, In content in Se-Te systems. In this paper we present results of a study of DC conductivity and thermoelectric power of bulk samples of amorphous semi-conducting SeT0-Te30, Sev0-Te30 xlnx (where x = 1, 3, 5, 7, 9% atomic weight). The difference of activation energies obtained from conductivity and thermoelectric power measurements has been discussed on the basis of long range electrostatic potential fluctuations.

2. Experimental The amorphous semiconducting compound was prepared by taking 99.99% pure elements and quantities were adjusted to the required composi-

296

A.B. Gadkari, J.K. Zope / Electricalproperties of the amorphous Se- Te-In system

tion. The silica tubes containing the contents were evacuated to 10 -6 Torr and sealed under vacuum. The glasses were prepared by heating silica tube at 850 ° C for 30 h. During heating the tubes were continuously rotated to ensure good homogeneity. The melt was suddenly quenched in ice cold water. The X-ray diffraction pattern showed broad rings. For conductivity measurements, a pellet of thickness 0.16 cm and surface area 0.78 cm 2 was prepared by applying a pressure of 2 ton for 2 min. Silver paste was used for electrical contacts. The thermoelectric power measurements were performed on 1 × 1 × 0.16 cm 3 samples held between brass rods of identical size and shape. Fine mica sheets were employed to insulate the electrical and thermocouple circuits. The temperatures of the hot junction and cold junction were recorded with the help of a chromel-alumel thermocouple. The thermo emf was recorded with the help of a dc microvoltmeter of 1 /~V resolution. The measurements were carried out for the temperature range of 270 K to 373 K. All curves were reproducible.

7'8

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8

8"2

8'4

r Te

8'6 \

8'8

u

7, t 0'9 c

I-Io ,D

9"2

9'4

Z O tO

9"6

9'8

3. Results and discussion

The experimental conductivity data of six samples of the system Se70-Te30 Sev0-Te30_xlnx (where x = 1, 3, 5, 7, 9% atomic weight) are shown in fig. 1. The conductivity is represented by Mott's equation

o(T) = 0o e x p ( - E * / k T ) ,

(1)

where E * is the activation energy and % is the conductivity pre-exponential factor. In table 1 conductivity results are presented including the activation energy, conductivity pre-exponential factor o0, room temperature conductivity o(T), energy difference EQ and factor Qo. It is observed that there is an increase in conductivity with increasing indium in the sample. The activation energy is equal to 0.37 eV for binary Sev0-Te30 and decreases to 0.30 eV for the Se70Te21In 9 sample. The approximate value of the pre-exponential factor o0 was obtained by extrapolating the curves in fig. 1 (table 1). Since the extrapolation involves a large magnitude, the pre-exponential factor is not accurate, but we get rough idea about its variation due to the addition of indium.

10

~0"2

10'4

ACTIVATION ENERGY IN *

Se7oTe30 :

x

Se70Te291nl : 0"35

0'37

a

SeToTeT,71n3 : 0"34

•

Se70Te251n5 : 0"33

=

Se70Te231n 7 : 0'31

•

Se70Te211n9 = 0'30

10'6

10.8 ,

?

3 T x103

Fig. 1. Temperature dependence of conductivity of the Se70Te30 . Se70Te30_xln x systems (where x = 1, 3, 5, 7, 9% atomic weight).

Fig. 2 shows the variation of room temperature conductivity a with indium concentration. It can be seen from fig. 2 that conductivity increases with increasing indium concentration. These glasses do not obey the lno against T -1 (K -1) relation over entire range of temperature. The amorphous S e - T e - I n system was found to obey

A.B. Gadkari, J.K. Zope / Electrical properties of the amorphous S e - Te-ln system

297

Table 1 Values of activation energy E * , E * , room temperature conductivity o$2 -1 cm -1, energy difference EQ, the factor Qo and pre-exponential factor o o for the bulk of the sample SeT0-Te30, Se7o-Te30_ x - I n x ( x = 1, 3, 5, 7, 9% atomic weight)

Glass composition

Se7o-Te3o Se7o-Te29-In 1 Se7o-Te27-In 9 SeTo-Te2s-Ins SeTo-Te23-In 7 SeT-Te21-In9

Concentrao tion x in % atomic weight

Activation energy E * in eV

0 0.l 0.3 0.5 0.7 0.9

0.37 0.35 0.34 0.32 0.31 0.30

Activation energy E * in eV 0.23 0.22 0.22 0.20 0.20 0.20

this linear relation for 273 to 350 K. However, a deviation from linearity was observed due to a change in the short range order during crystallization of the material. The variation of E * with x is shown in fig. 3. Fig. 3 shows that as the indium concentration increases the activation energy decreases. This means the conductivity of the sample

Room temperature conductivity o12 -1 cm -1 7 × 1 0 11 5 × 1 0 -11 1 × 10 11 6.5 × 10 lo 4 × 1 0 -1° 1 . 6 × 1 0 10

v~u

9"2

Preexponential factor o0f2 -1 cm -1

3.9 4.4 4.8 5.2 5.6 6

230 220 210 200 190 179

EQ = E* - E* in eV 0.14 0.13 0.12 0.12 0.11 0.10

0.38

J

-

Factor Q0

increases as the indium concentration increases and the activation energy decreases. Fig. 4 shows the variation of thermo emf with temperature difference upto 100 K. It shows that the thermo emf varies linearly with temperature difference. As we get positive thermo emf for the sample the carriers are holes and material is p-type

#o

- 9"0

Energy difference

0.37

7~ - 9./.

l

>-

> -9.6

,3 ~ - 9.8'

o

-10'2

-- I0" ~

/

/

0-36

\

~ 0-35 w a:

m z t~

O-3Z.

$ 0"33

0"32

/o

°\o\

0.31

-10.6

030 I0.8

01,

0t3 I~

0)s

0t7

CONCENTRATION

or, X

Fig. 2. Variation of room temperature conductivity ~th concentration of indium in the SeTo-Te3o_x-In x system (x = 0, 1, 3, 5, 7, 9% atomic weight).

o'.~

o!~

o!~

o'.7

o'.9

In CONCENTRATION X

Fig. 3. Dependence of electrical activation energy on concentration of indium in SeToTe3o_xln x (x = 0, 1, 3, 5, 7, 9% atomic weight).

A.B. Gadkari, J.K. Zope / Electricalproperties of the amorphous Se- Te-In system

298

to 0.2 eV for chalcogenide glasses and Si-H. They have also discussed the various transport models considering the transport properties of both chalcogenides and the hydrogenated silicon simultaneously. Q(T) is defined by the following relation:

0.8 * Se70 Te30

A/

0.7

I > E

" 5e70 Te25In5

/a/~/e/~/

0.6

/y/o//"

06

Q(T)=ln(o(T)a-lcm-1)+~s(r)

(3)

with q = - l e l for electrons and q = + ] e I for holes. Q(T) curves are generally well represented by

04.

==

EQ Q(T) = Qo kT' 0.2

0.1

i

6

i

12

i

18

214

30

36

412

48

TEMPERATURE DIFFERANCE AT('K )

Fig. 4. Variation of thermo emf with temperature difference of the Se-Te-In system with different concentrations of indium.

(4)

where Q0 values for most of the samples are close to 10. On the basis of Overhof and Beyer's finding we have plotted the graph of Q(T) against T -t (K -t) for our system as shown in fig. 6. The slope of the curves yields Qo values which are presented in table 1. Many attempts have been made to explain the non-zero value of Ee [5], our discussion is

in nature. The thermoelectric power with T -1 (K -1) is shown in fig. 5. It is observed that the thermoelectric power varies linearly in all the samples. There is a decrease in the thermoelectric power with increase in indium content in Se-Te. The thermoelectric power equation for p-type material is given by Fritzsche [15].

k(Es*+A). S= e~kT

o a • a

5e70 Te27In 3 Se7oTe2s In S SeT0 Te23In 7 Se70 Te21 In9

•

(2)

A = 1 for most of the chalcogenide glasses. The activation energy calculated from the curves, is 0.22 eV for Sev0-Te30 and decreases to 0.19 eV for SeT0-TeEl-In 9. The activation energy decreases as the percentage of indium increases in the sample. The activation energy calculated from the conductivity and thermoelectric power measurements differ by 0.14 eV. The values EQ = E * - Es* are given in table 1. Several models have been proposed to explain the features found experimentally for the difference in activation energy. For chalcogenide Nagels [16] proposed ambipolar transport which leads to a non-zero value of EQ. Overhof and Beyer [5] have reported a difference between the activation energies of the order of 0.1

SeTO Te30

x Se70 Te29Inl

o.o3 i

t tn 0.02C

0 - 0 1 ~.

.._....o = / ~e / ~ ~f""~A/~

_ /

A/a

_._~.--x /

~'-x'---'-'x

x~X~ o

.~.....~o ~

~_.~.o -.-- - ~ o ~ 0.01C

i

3'07

i

3.10

i

3.15

i

3.19

i

3"23

i

3-27

tO_3 (~1) T

Fig. 5. Dependence of thermoelectric power with temperature of SeToTe3o, Se7oTe3o_xInx (where x =1, 3, 5, 7, 9% atomic weight).

A.B. Gadkari, J.K. Zope / Electricalproperties of the amorphous Se-Te-ln system

299

EQ m a y be as large as 0.25 eV, while for u n d o p e d samples EQ -- 0.05 eV [19]. o

-

8"2

\

Se70Te29 In I

n

SeloTe27 In 3

•

Se 70Te 251n 5

o

-

8.6

-t 0-2 -10-G

Se7o Te30

x

•

5e 70Te231n7 Se70Te21In9

The authors (A.B.G.) is grateful to V.L. Mahajan and the staff of G B S B A R C , Bombay, for their help in the v a c u u m sealing of the samples. The author also expresses his sincere thanks to Dr K.S. Venkateswarlu and the staff of the Water Chemistry Division B A R C for their help in the X R D of the samples. The experimental assistance of and discussions with B.D. Murgi are gratefully acknowledged.

References

\\ ~\ \

1~03~1 T Fig. 6. Variation of Q(T) on temperature of SeToTe3o, Se70Te3o_xInx(where x = l, 3, 5, 7, 9% atomic weight). based on the model proposed b y Overhof and Beyer which accounts for the interpretation of the results for chalcogenides. The values of E O are reported in the literature [17,18]. The non-zero slope of Q ( T ) is due to the long range static potential that modulates the energy of the mobility edge in space [5]. The origin of this potential could be the electrostatic potential of the charged centres which are p r o b a b l y not more homogeneously distributed in space than a r a n d o m distribution. In this case the variation of EQ with the preparation and doping level had to be attributed to the long range potential. It has been reported that, in highly d o p e d or more disordered samples

[1] [2] [3] [4]

E.A. Davis and N.F. Mott, Phil. Mag. 22 (1970) 903. N.F. Mott, Phil. Mag. 22 (1970) 7. N.F. Mon, Phil. Mag. B26 (1970) 1015. M. Kasmer and Fritzsche, Phil. Mag, B (GB) 37 (1978) 199. [5] H. Overhof and W. Beyer, Phil. Mag. B 49 (1984) L9. [6] R. Banerjee and S. Ray, J. Non-Cryst. Solids 89 (1987) 1. [7] F. Mahdjuri, J. Appl. Phys. 8 (1975) 2248. [8] J.K. Zope, Indian J. Pure Appl. Phys. 20 (1982) 774. [9] J.T. Edmond, J. Appl. Phys. 17 (1966) 979. [10] R.M. Mehera, Radhey Sham and P.C. Mathur, Phys. Rev. 19 (1979) 6525. [11] P. Nagels, Phys. Status Sol. A 59 (1986) 505. [12] H. Sakai, K. Shimakawa, Y. Inagaki and J. Arizumi, Jap. J. Appl. Phys. 13 (1974) 500. [13] F. Kesek, Z. Cimpl, M.d. Mikhailov and E.A, Karpova, Phil. Mag. 86 (1986) 265. [14] C. Carcaly and D. Houphouest, Phil. Mag. 86 (1986) 271. [15] H. Fritzsche, J. Non-Cryst. Solids 6 (1971) 49. [16] P. Nagels, A. Calloerts and M. Denayer, Proc. Eleventh Int. Conf. on Physics of Semiconductors, ed. M. Miasek (Elsevier, Amsterdam, 1972) p. 540. [17] S. Mahadeven and K.J. Rao, J. Non-Cryst. Solids 34 (1973) 53. [18] C.H. Seager, D. Emin and R.K. Quinn, Phys. Rev. B8 (1973) 4746. [19] N.F. Mott, J. Non-Cryst. Solids 77&78 (1985) 11~.