Energy spectrum of charge carriers in Ag 2 Te
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TRONIC AND OPTICAL PROPERTIES OF SEMICONDUCTORS
Energy Spectrum of Charge Carriers in Ag2Te S. A. Aliyev, Z. F. Agayev^, and R. I. Selimzadeh Institute of Physics, National Academy of Sciences of Azerbaijan, pr. Dzhavida 33, AZ-1143 Baku, Azerbaijan ^e-mail: [email protected] Submitted February 2, 2008; accepted for publication March 5, 2008
Abstract—Conductivity σ(T) and Hall constant R(B, T) are studied for Ag2Te with excess 0.1% of Te. The change in the R sign from (–) to (+) is found in dependences R(B) at various temperatures. In the temperature dependences of R in a range of 1–3 kG, two extrema are found, namely, minimum at T ~ 60 and maximum at T ≈ 80 K, and at B ≥ 5 kG, the double change in sign of R from (–) to (+) and from (+) to (–) is found. Temperatures of sign inversion for R depend on the magnetic field. At B = 15 kG, the sign of R varies from (–) to (+) at T ≈ 38 K, and from (+) to (–) at T ~ 70 K. It is found approximately in the region of the change in the sign of R(T), the concentration n(T) and electrical conductivity pass through the minimum. It is established that the minima of n(T) and σ(T), extrema in R(T), and sign inversion for R(T) from (–) to (+) as well as the overestimated temperature dependence n ∝ T4 are caused by localization of conduction electrons at acceptor levels entering the conduction band of Ag2Te. The values of parameters of electrons (n, µn) and holes (p, µp) at the points of the change in the sign of R(T) from (–) to (+) and from (+) to (–) are determined. PACS numbers: 61.50.Ks, 64.70.Kb, 65.40.-b DOI: 10.1134/S1063782608120026
1. INTRODUCTION In [1–3], certain features of electrical and thermoelectric properties of p-Ag2Te with the Te excess of about 0.7 at % were discussed. From these features, passing of electrical conductivity σ(T) through a minimum at ~65 K and through a maximum at 200 K, weak dependence of R(T) in the range of 200–300 K, and passing of α(T) through a maximum at ~100 and 270 K are of special interest. The experimental data are interpreted in the context of the two-band model. Concentrations and mobilities of electrons and holes are determined. It is shown that Ag2Te possesses a very narrow band gap εg ≈ 0.03 eV, and the mentioned anomalies are associated with a small value of εg and its temperature dependence εg(T) = (0.03–7 × 10–5) eV. In the sample under consideration, concentrations at low temperatures (4.2 K) corresponded to n = 1 × 1014 cm–3 and p = 5 × 1016 cm–3, i.e., c = n/p ≈ 0.002. To reveal more concrete data on the energy spectrum of charge carriers, it was necessary to study the samples with larger n/p ratios in the magnetic fields as strong as possible. Therefore, in this study, we examined the sample n-Ag2Te with n = 6 × 1014 cm–3 and p = 5 × 1016 cm–3, so that the ratio n/p attained the values c ≈ 0.012, which is larger by a factor of six compared with the sample with excess 0.7 at % of Te. 2. EXPERIMENTAL The measurements of R(B, T) and α(T) were performed using a dc current in the range of 2–300 K in magnetic fields as high as 20
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