ISSN 10637834, Physics of the Solid State, 2012, Vol. 54, No. 7, pp. 1345–1347. © Pleiades Publishing, Ltd., 2012. Original Russian Text © V.V. Kaminskii, Toshihiro Kuzuya, Shinji Hirai, S.M. Solov’ev, N.V. Sharenkova, M.M. Kazanin, 2012, published in Fizika Tverdogo Tela, 2012, Vol. 54, No. 7, pp. 1269–1270.

SEMICONDUCTORS

Electrical Conductivity of SmS Polycrystals V. V. Kaminskiia, *, Toshihiro Kuzuyab, Shinji Hiraib, S. M. Solov’eva, b, N. V. Sharenkovaa, b, and M. M. Kazanina a

Ioffe PhysicalTechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia * email: [email protected] b Muroran Institute of Technology, 271 Mizumotocho, Muroran, Hokkaido, 0508585 Japan Received December 29, 2011

Abstract—The electrical conductivity of SmS polycrystals has been studied in the temperature range 300– 870 K. It has been shown that, at 300 K ≤ T ≤ 700 K, the concentration of conduction electrons is determined by electron transfer from impurity donor levels, and at T > 700 K, by that from the samarium 4f levels. DOI: 10.1134/S1063783412070190

CSR size of ~250 Å and less, electric transport is con trolled by the hopping mechanism. In our case, the CSR size is ~600 Å, and this is what accounts for the dominance of the band mechanism of electric trans port. Because the conduction electron mobility in SMS with n ~ 2 × 1019 cm–3 depends only weakly on temperature [5], one can use the Arrhenius curve to locate the positions of the donor energy levels in the band gap. The values of 0.03–0.06 eV (Fig. 1) correlates well with the depth of the impurity donor levels in SmS sin gle crystals deriving from the presence of a certain

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T, K 500 400

1000 700

250 4 8

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−4.50 InR [Ω]

The electrical properties of samarium sulfide (SmS) polycrystals at high temperatures have not been studied in detail before, with the exception of the work by Golubkov et al. [1], who obtained estimates of the thermoelectric characteristics of this material. The thermovoltaic effect revealed in SmS polycrystals [2] provides adequate reason for a comprehensive investi gation of the physical mechanisms underlying electric transport in this material. This is exactly the goal pur sued in the present study. To illustrate the reproducibility of the results obtained in our measurements, we studied two sam ples prepared by straightforward synthesis from the elementary substances (samarium and sulfur), with subsequent pressing and annealing [3]. The samples measured 6 × 7 × 10 mm and at T = 300 K had the fol lowing electrical parameters: electrical resistivities ρ = 1.1 × 10–2 and 1.6 × 10–2 Ω cm, conduction electron concentrations n = 2.11 × 1019 and 2.14 × 1019 cm–3. The Xray structural parameters of the two samples were as follows: the NaCltype crystal lattice constant was a = 5.964 ± 0.00 Å, the size of Xray coherent scat tering regions (CSR) was ~600 Å. The temperature dependences were measured on ZEM2 (ULVACRIKO Inc.) equipment by the four point probe technique in the dc mode at temperatures of 300–870 K. Figure 1 shows the temperature depen dences of the electrical resistance of the samples. They are seen to follow the same pattern in that at tempera tures below 700 K the activation energy of conduction varies within 0.03–0.06 eV, while for T > 700 K the conduction activation energy is ~0.25 eV. It was shown [4] that, in SmS polycrystals, electric transport at T > 300 K is mediated by electrons in the conduction band. At first glance, this does not seem obvious, because, for instance, in heavily defected polycrystalline SmS structures characterized by the

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2.0 2.5 103/T, K−1

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Fig. 1. Electrical resistance of two polycrystalline SmS samples as a function of the temperature (filled and open circles, respectively). The straight lines correspond to the following conduction electron activation energies: (I) 0.25, (II) 0.03 eV, (1) 0.042, (2) 0.052, (3, 5) 0.033, (4) 0.032, (6) 0.055, (7) 0.027, and (8) 0.026 eV.

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the Fermi level lying at elevated temperatures above the conduction band bottom. This may be compared with the behavior of the Fermi level in polycrystalline SmS films, where it remains close to the conduction band bottom up to T ~ 400 K [4].

Sband

E, eV

0

All this boils up to the band structure diagram of polycrystalline SmS in the region of the conduction band bottom, as is shown in Fig. 2. By and large, it is the same as in SmS single crystals. On the other hand, on single crystals, we could not observe in one experi ment such a large set of activation energies of impurity donor levels. This should possibly be attributed to a higher content of defects in polycrystals compared with single crystals (for SmS single crystals, CSR ~2500 Å, whereas for our sample it is ~600 Å).

−0.03 Ei −0.06

A remarkable feature observed in Fig. 1 at temper atures below 700 K is the presence of jumps in electri cal resistance which it undergoes with variation of temperature. They can be traced to the thermovoltaic effect. This suggestion is supported by the calculated graph in Fig. 3 which plots the temperature of the onset of generation vs. the depth of the impurity levels. The calculation was done as described in [2] by equat ing the effective impurity Bohr radius to the Debye radius of screening by conduction electrons of the electric potential generated by this impurity. As fol lows from Fig. 3, the different depths of the impurity donor levels can be related to different temperatures of voltage generation by the thermovoltaic effect. The curves were calculated for impurity donor level con centrations typical of SmS polycrystals, namely, Ni = 5 × 1020–5 × 1021 cm–3.

~ ~ −0.23

Ef

Fig. 2. Schematic diagram of the band structure of poly crystalline SmS samples in the region of the conduction band bottom. Designations: Ef is the energy of the samar ium 4f levels, Ei is the impurity level energy region, and Sband is the conduction band bottom.

2000 1 1600 2

Because for T < 700 K each of the two samples in Fig. 1 exhibits several regions with different activation energies, to each of them corresponds a jump of the thermovoltaic voltage. When the electrical resistance is measured by the fourpoint probe technique, these jumps can be perceived by the measuring equipment as a decrease or increase of the electrical resistance, depending on the direction of the electrical voltage generated by the thermovoltaic effect. Incidentally, this phenomenon will be observed always in measure ments of the electrical conductivity of polycrystalline SmS samples.

Tg, K

1200 3 800 400 0 0.030 0.035 0.040 0.045 0.050 0.055 0.060 Ei, eV Fig. 3. Calculated temperatures of the onset of generation as a function of the impurity level depth. Ni = (1) 0.5 × 1021, (2) 10 × 1021, and (3) 5 × 1021 cm–3.

ACKNOWLEDGMENTS

amount of samarium ions which are located not in the regular SmS crystal lattice sites (interstitials, sulfur sublattice vacancies). They are located 0.045 ± 0.015 eV below the conduction band [6]. The value of 0.25 eV correlates well with the depth of location of samarium 4f levels in SmS (0.23 eV) [7]. The slightly larger value should be possibly assigned to

This study was supported by the Russian Founda tion for Basic Research (project no. 110800583a) and the SMStenso Company. REFERENCES 1. A. V. Golubkov, M. M. Kazanin, V. V. Kaminskii, V. V. Sokolov, S. M. Solov’ev, and L. N. Trushnikova, Inorg. Mater. 39 (12), 1251 (2003).

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ELECTRICAL CONDUCTIVITY OF SMS POLYCRYSTALS 2. V. V. Kaminskii, V. A. Didik, M. M. Kazanin, M. V. Ro manova, and S. M. Solov’ev, Tech. Phys. Lett. 35 (11), 981 (2009). 3. A. V. Golubkov, E. V. Goncharova, V. P. Zhuze, G. M. Loginov, V. M. Sergeeva, and I. A. Smirnov, Physical Properties of Chalcogenides of RareEarth Ele ments (Nauka, Leningrad, 1973) [in Russian]. 4. L. N. Vasil’ev, V. V. Kaminskii, Yu. M. Kurapov, M. V. Romanova, and N. V. Sharenkova, Phys. Solid State 38 (3), 430 (1996).

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5. V. V. Kaminskii and M. V. Romanova, Prib. Sist. Upr., No. 8, 28 (1988). 6. A. V. Golubkov, E. V. Goncharova, V. A. Kapustin, M. A. Romanova, and I. A. Smirnov, Sov. Phys. Solid State 22 (12), 2086 (1980). 7. E. V. Shadrichev, L. S. Parfen’eva, V. I. Tamarchenko, O. S. Gryaznov, V. M. Sergeeva, and I. A. Smirnov, Sov. Phys. Solid State 18 (8), 1388 (1976).

Translated by G. Skrebtsov