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V. E. ANTONOV al.: ZnSb and GaSb Bulk Amorphous Semiconductors et
phys. stat. sol. (b) 198, 497 (1996) Subject classification: 72.20; S7.13

497

Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka')

ZnSb and GaSb Bulk Amorphous Semiconductors: Transport Properties
BY V. E. ANTONOV, I. BARKALOV'), I. KOLYUBAKIN, 0. A. and

E. G. PONYATOVSKY
(Received June 14, 1996)
Temperature dependencies of the conductivity and thermopower of bulk amorphous semiconducting alloys Zn41Sb59 and Ga100-,SbZ with 47.5 < x < 55 were measured at 80 to 370 K and 120 to 370 K, respectively. The samples were prepared by solid state amorphization of the quenched high pressure phases occurring on heating at ambient pressure. The electrical properties of nonstoichiometric a-Zn41Sbsg are well described by the conventional Mott-Davis model. Those of both stoichiometric a-GaSb and nonstoichiometric a-Galoo- &ib, appear to be more unusual and require a modification of the model.

1. Introduction
Amorphous GasoSb50 and Zn41Sbsg were the first bulk amorphous semiconductors produced by a solid state reaction using spontaneous amorphization of a metastable crystalline high pressure phase [l,21. The amorphous samples obtained in this way were shown to contain no crystalline or nanocrystalline inclusions [3 to 51. The purpose of the present work was to study and compare the transport properties of stoichiometric a-Ga~oSb5~ and nonstoichiometric a-Zn41Sbsg and a-Ga100-.SbZ with 47.5 < x < 55.

2. Experiment
The initial ingots were prepared by alloying appropriate amounts of elements (each 99.999 wt% pure). The alloys were transformed to the high-pressure metallic phase, quenched to 100 K and unloaded at this temperature to retain this phase in a metastable state, and then heated at ambient pressure up to room temperature for amorphization. The details of the procedure are described elsewhere [3, 61. The X-ray examination of the samples (DRON-2.0 diffractometer, CuK, radiation) revealed no traces of crystalline phases. The dc-conductivity u and the thermoelectric power S were measured at temperatures from 80 to 370 K and 120 to 370 K, respectively.

3. Results and Discussion
Typical temperature dependencies of u and S for a-ZndlSbsg are shown in Fig. 1. The a(T) curves correspond to an activated behaviour with an activation energy E, varying
*) 142432 Chernogolovka, Moscow district, Russia. ') e-mail: barkalov@issp.ac.ru
32 physica (b) 198/1


498

V. E. ANTONOV, I. BARKALOV,I. KOLYUBAKIN, E. G. PONYATOVSKY 0. A. and
J

1100 -

- -1
7

n n

w \goo
v

n

+ 3..
m

-

--4

E _"
I
v
U

I

C

700

-

--7

b

ho

I

I

500;

lOOO/T (K')

4

6

8

'

-10

Fig. 1. Temperature dependencies of the electric conductivity u and thermopower S for amorphous
Zn41 Sb59

from sample to sample in the range 0.27 to 0.30 eV. The thermopower was positive and increased approximately linearly with the reciprocal temperature: S = (k/e) x ([Es/kT] C) with Es = (0.19 0.01) eV and C = 1 f 0.5 (Ic is the Boltzmann constant, e the elementary charge). The o(T) and S(T) data for a-Galoo-,Sb, with x = 47.5, 50, 52.5, and 55 are presented in Fig. 2 and 3. As one can see from Fig. 2, at T > 180 K all samples display activation type conductivities with E, = 0.20, 0.26, 0.28, and 0.30 eV, respectively. At lower T, the slope of lg(a) versus T-' decreases gradually. The S values of the amorphous Ga-Sb alloys were also positive. However, they varied with respect to T-' approximately linearly only at high temperatures and decreased steeply towards zero for T < 150 to 200 K (except x = 47.5). The temperature range of the activation behaviour of o(T) corresponds roughly to the region of the slow linear changes in S(l/T). Within this temperature range, the slope of the S(l/T) dependence was positive for x = 55 and negative for x 5 52.5. The electrical properties of a-Zn41SbSg are characterized by an activation type conduction with E, close to one half of the band gap of the crystalline semiconductor ZnSb. The thermopower is positive, increaseses nearly linearly with T-l, and we have Es < Eo. These results are typical for many amorphous semiconductors (7 to 91 and are well described by the conventional Mott-Davis model [7]. According to this model, the Fermi level EF is pinned close to the middle of the mobility gap, where a peak in the density of states for partly filled localized states is situated. A positive sign and a large value of S suggest that the conductivity is due to holes excited to the states near the mobility edge EV of the valence band. The relationship E, > Es indicates that the conduction is dominated by thermally activated hopping of

+

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ZnSb and GaSb Bulk Amorphous Semiconductors: Transport Properties

499

2

4

1000/T (K-')

6

8

10

12
2

Fig. 2. Temperature dependences of u for amorphous GalOO-zSbz. with and (4) 47.5

= (1) 55, (2) 52.5,

(3) 50,

1000/T (K-')
Fig. 3. Temperature dependences of S for amorphous GalOO-zSbz with z = (1) 55, (2) 52.5, (3) 50, and (4) 47.5
32"


500

V. E. ANTONOV, I. BARKALOV, I. KOLYUBAKIN, E. G. PONYATOVSKY 0. A. and

the carries between localized states of the valence band tail, with the corresponding edge EA. In this case Es = EF - EA and E, M Es W, where W is the activation energy of the hole hopping mobility. For the a-Zn41Sbsg we therefore have EF - EA M 0.19 eV and W M 0.1 eV. In contrast to non-stoichiometric a-Zn41Sb59, the transport properties of stoichiometric and nearly stoichiometric a-Ga-Sb cannot be explained on the basis of a single conductivity mechanism. Besides, since the properties of a-Galoo - zSb, alloys change gradually with x varying over the whole concentration range (Fig. 2 and 3), one needs a model attributing the concentration dependence of these properties to a gradual variation of certain model parameters. In particular, it is essential to explain the values and signs of the high temperature dS/d (1/T) for different x. Most unusual are the S(l/T) dependencies of the alloys with x 5 52.5, characterized by negative values of dS/d (l/T) remaining nearly constant over wide temperature ranges. The large values of S > k/e (nondegenerate case) combined with dS/d (1/T) < 0, observed in a-GalOo-,Sb, samples with x 5 52.5 at higher temperatures are usually described by a competition of several types of conductivity mechanisms coexisting within the same temperature interval, because hopping conductivity near the Fermi-level, the only mechanism resulting in dS/d (1/T) < 0, cannot give large values for S [7]. Within the framework of the conventional models, however, even a combination of two or more conduction mechanisms cannot yield a constant value of dS/d (1/T) over a wide temperature interval and should result in an S(l/T)dependence with a maximum. One can get a rather simple and more natural explanation of the effects observed in a-Ga-Sb if one allows for EA > EF in contrast to the usual models with EA < EF for the ptype amorphous materials. Using the standard semi-quantitative approach [7J,one may suppose that the density of states in the band tail is N(E) cx (EA- E)n. At sufficiently high temperatures, when u is mainly due to the band tail states, most holes in the tail are located near the maximum of N(E)exp ([E- &]/kT) at Em = EA - nkT. With EA < EF this is true even at low temperatures, but with EA > EF it is valid only at kT < EF -Em, that is at kT > (EA- E F ) / ( n - 1). Under these conditions (T 0: Tnexp ([EA EF W]/kT)and S M (EF- E,)/eT = k/e([EF- EA]/~T+ M k/e([E$ - PA]/kT C), where $ and n) PAcorrespond to T = 0 K. As a rule, n - 2 < C < n - 1 due to a decrease in the band gap with increasing temperature. The given formulae are common except that the difference EF - EA can have any sign. This allows for a linear variation of S(l/T) with dS/d (1/T) of either sign. C The model parameters which determine a(T) and S(T) are PF PA, and W. For the amorphous a-Galoo-zSbx alloys with x = 47.5, 50, 52.5 and 55 these are I$ - E i M -0.04, -0.04, -0.02 and f0.02 eV; C M 2.6, 4.7, 4.2, and 1.5; W M 0.24, 0.30, 0.30, and 0.28 eV, respectively. The mechanism outlined above governs the transport properties of a-Ga-Sb at temperatures higher than 150 to 200 K. With decreasing temperature, the conductivity resulting from the states near the Fermi level becomes dominating which leads to a steep decrease in S. In conclusion, the measured electric properties of a-Zn41Sb59 show that this alloy is a classical amorphous semiconductor well described by the regular Mott-Davis model throughout the temperature interval of the present study. The dominant mechanism for

+

+

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ZnSb and GaSb Bulk Amorphous Semiconductors: Transport Properties

501

the conductivity u and thermopower S is thereby a thermally activated hopping of holes excited into the tail of the valence band. The a-Ga-Sb alloys show more complex transport properties, which could be explained in the framework of a slightly modified Mott-Davis model with the assumption that the Fermi level might be positioned inside the band tail.

Acknowledgements The present work was supported by Grant No. 96-02-18545 from the Russian Foundation for Fundamental Research. One of the authors (O.I.B.) thanks the Organizing Committee of HPSP-VII and the International Science Foundation for financial support to attend the Conference.

References
111 T. R. R. MCDONALD, SARD, E. GREGORY,appl. Phys. 36,1498 (1965). R. and J. [2] I. T. BELASH E. G. PONYATOVSKY, Temp. - High Pressures 9,651 (1977). and High [3] 0. I. BARKALOV, I. KOLESNIKOV, G. PONYATOVSKY, A. E. U. DAHLBORG, DELAPLANE, R. and A. WANNBERG, J. non-crystall. Solids 176,263 (1994). 141 A. S. ARONIN,0. I. BARKALOV, E. G. PONYATOVSKY, and J. non-crystall. Solids 189, 138 (1995). [5]M. DAHLBORG, DAHLBORG, E. ANTONOV, I. BARKALOV, 1. KOLESNIKOV, U. V. 0. A. E. G. PONYATOVSKY, C. HANNON, press. and A. in [6] V. E. ANTONOV, I. BARKALOV, E. G. PONYATOVSKY, 0. and J. non-crystall. Solids 192/193, 443 (1995). [7] N. F. MOTT and E. A. DAVIS, Electron Processes in Non-Crystalline Materials, Clarendon Press, Oxford 1979. [8] A. FELTZ, Amorphe und Glasartige Anorganische Festkorper, Akademie-Verlag, Berlin 1983. [9] P. NAGELS, Amorphous Semiconductors, Ed. M. H. BRODSKY, in: Topics in Appl. Phys., Vol. 37, 2nd ed., Springer-Verlag, New York 1985.