In anisotropic PbSb2Te4 and PbSb2Te4:Cu single crystals, nine main independent components of the Hall, electrical-conductivity, thermopower, and Nernst-Ettingshausen effects and their anisotropy in the range 77–450 K have been studied. PbSb2Te4 single crystals exhibit a high hole concentration (p ≈ 3 × 1020 cm−3). Copper exhibits a donor effect and significantly (approximately by a factor of 2) reduces the hole concentration in PbSb2Te4. The temperature dependences of the kinetic coefficients, except for the Hall effect, have a form typical of the one-band model. The significant anisotropy of the Hall coefficient R 123/R 321 ≈ 2 at low temperatures corresponds to the multi-ellipsoid model of the energy spectrum of holes in PbSb2Te4. An important feature of the data on transport phenomena is the high thermopower anisotropy (ΔS ≈ 60–75 μV/K) in the mixed conductivity region caused by the mixed scattering mechanism. Data on the anisotropy of the transverse Nernst-Ettingshausen effect confirm the mixed mechanism of hole scattering; in the cleavage plane, scattering at acoustic phonons dominates, while in the trigonal axis direction, impurity scattering appears significant. Doping with copper enhances the role of impurity scattering in the direction of the trigonal axis c 3; as a result, two components of the Nernst-Ettingshausen tensor Q 321 and Q 132 in the PbSb2Te4:Cu single crystal are positive at low temperatures, whereas, in the undoped crystal, only the Q 321 component is positive.

The temperature dependences of the coefficient of the transverse Nernst-Ettingshausen effect in SnTe: In samples with different indium contents (1–16 at %) in the temperature range 100–300 K and the electrical resistivity at temperatures of 1.2–4.2 K and in magnetic fields of up to 10 kOe are investigated. The data obtained indicate the presence of resonant hole scattering into the band of quasi-local In impurity states in Sn1−x In x Te samples with In content x ≥ 0.05 and a superconducting transition with a critical temperature T c ∼ 1.5–2.2 K. The SnTe: In samples with the degree of filling of impurity states by electrons, which is close to 1/2, and the Fermi level ɛF pinned in the vicinity of the minimum energy dependence of the relaxation time τ(ɛ) are characterized by inhomogeneities of a new type, i.e., inhomogeneities of the scattering parameter r = ϖlnτ/∂lnɛ|\( \varepsilon _F \), which are induced by fluctuations of the degree of filling of quasi-local states by electrons.

On one Sb2Te3 single crystal, the temperature dependences of all three independent components of the Nernst-Ettingshausen tensor (Q ikl ) are measured in the temperature range of 85–450 K, all three components being negative. Alongside with the Nernst-Ettingshausen effect, the anisotropy of the Hall (R ikl ) and Seebeck (S ij ) coefficients and the conductivity (σ ii ) is also investigated. The carried-out analysis of the experimental data on the Nernst-Ettingshausen and Seebeck effects indicates that there is the mixed scattering mechanism with the participation of acoustic phonons and impurity ions, the relative contributions of these mechanisms varying with temperature. In the relaxation-time-tensor approximation, the values of the effective scattering parameter (r) are determined. The obtained values point to the dominant scattering at acoustic phonons in the cleavage plane and to the substantial contribution of charged ions to the scattering along the trigonal axis c 3. It is shown that it is possible to explain the major features of experimental data on the Nernst-Ettingshausen effect within the two-valence-band model with the participation of several groups of holes in the transport phenomena.

The Seebeck coefficient S was measured in a wide temperature range T <100 K in Pb 0.78 Sn 0.22 Te solid solutions doped with 3 at. % of In, with additional Cl doping of up to 3 at. %. The temperature derivative ∂| S |/∂ T changes its sign from negative to positive below 100 K. Theoretical estimations in terms of hopping conduction via highly localized indium-related states show that the transition to variable-range-hopping conduction must occur at temperatures of about 50–100 K, in agreement with the obtained experimental data.

The paper reports the study on the resistivity ρ and thermoemf S of the (Sn0.65Pb0.35)0.95Ge0.05Te solid solution layers. The dependences of ρ and S on the hole concentrations in the range 3×1019–2×1021 cm−3 exhibit jumps in the resistivity and thermoemf minima at close hole concentrations p1≈9×1019 cm−3, p2≈2.5×1020 cm−3, and p3≈4.5×1020 cm−3. The observed jumps and minima suggest a complex structure of the valence band and the presence of critical points in the energy spectrum of holes. According to the data for SnTe, the critical points in the energy spectrum at the given hole concentrations are identified as the Σ-extremum, saddle point LΣ, and Δ-extremum, respectively.

A study of the Hall and Seebeck coefficients and of resistivity has been carried out on an Sn0.62Pb0.33Ge0.05Te alloy doped by 5 and 10 at. % In. A superconducting transition with the maximum critical temperature TC∼4 K has been discovered in samples with hole concentrations p≥1×1021 cm−3. The dependence of TC on hole concentration has been established to be of a threshold nature. The onset of superconductivity is accompanied practically simultaneously by a growth of the resistivity and a sharp drop of the Seebeck coefficient. These features in the experimental data indicate the existence of a band of In resonance states within the allowed valence-band spectrum and strong resonance hole scattering to impurity states. The threshold character of the TC(p) dependence is connected with the holes filling the resonance states. A positive correlation between the resonance scattering intensity and the critical temperature is observed.

In the quaternary solid solutions (Pb z Sn 1− z ) 0.95 Ge 0.05 Te ( z =0.35 and 0.40) the effect of addition of indium (in amounts of 5–20 at. %) on the temperature dependence of the electrical conductivity σ , Hall coefficient R , Seebeck coefficient S , and Hall mobility u is investigated on samples prepared using powder technology. We found a monotonic dependence of the hole density p on the indium content N In with a tendency toward saturation at a level p max ≈3×10 21 cm −3 , an abrupt drop in the mobility in samples with p ≈ p max , and changes in the character of the temperature dependences R ( T ) and σ ( T ). We show that these peculiarities in the behavior of the kinetic coefficients can be interpreted in terms of quasilocal indium impurity states against the background of the valence band spectrum (with energy ɛ In ∼0.3 eV) and resonance hole scattering into these states.

Data on the electrical conductivity, the thermoelectric power, and the Hall and Nernst-Ettingshausen effects in the temperature range from 77 to 400 K for the a solid solution PbTe-SnTe with a high In content (3 at. %) and additional doping with chlorine and thallium are presented. Specifically, the Nernst-Ettingshausen coefficient exhibits properties which are unusual for IV–VI semiconductors: It is positive and decreases rapidly with increasing temperature. The experimental data are discussed on the basis of a model in which the main transport mechanism is hopping conduction along strongly localized electronic states of the In impurity. Conduction along delocalized states of the conduction band makes a substantial contribution to the effects observed in a transverse magnetic field. The model gives satisfactory agreement with experiment, including the sign, magnitude, and temperature dependence of the Nernst-Ettingshausen coefficient.

Self-compensation is studied in bulk samples of PbSe: (Bi, Se ex ) prepared by a metal-ceramic method. The dependence of the carrier concentration on the amount of excess selenium is investigated for various bismuth concentrations. Comparison of the experimental data with calculated data shows that the donorlike activity of bismuth accommodated in the cation sublattice is compensated by doubly ionized lead vacancies. Nonmonotonic behavior of the dependence of the carrier concentration on the selenium excess is observed in some series of samples due to the incorporation of Bi atoms into both the cation and anion sublattices.

The complex nature of the electronic energy spectrum and hopping conductivity in Pb0.78Sn0.22Te solid solutions with high indium content (3 at. %) has been investigated. The method of supplementary doping with donor (Cl) and acceptor (Tl) impurities, which do not create impurity states near the edge of the conduction band, has been used. The temperature dependence of the Seebeck coefficient S(T) in the temperature range 100-400 K for various supplementary impurity concentrations (Cl up to 3 at. %, Tl up to 2 at. %) has been measured. The results of these measurements were interpreted qualitatively. According to the expressions for the Seebeck coefficient obtained in the effective-medium approximation, the product ST depends linearly on T-2, with a slope that is proportional to the logarithmic derivative of the density of localized states with respect to energy. Estimates of the density of states show that the shape of the conduction band edge is distorted by impurity states of In, but that there is no peak in the density of states, and that the zone edge has the form of an extended ''tail'' that penetrates deeply into the band gap. (C) 1996 American Institute of Physics.

The effect of amphoteric impurity states, which are partially filled with electrons, on defect formation in narrow-gap semiconductors is investigated for the example of In impurity and PbTe. On the basis of experimental data on self-compensation in the system PbTe:In,Te-ex, the dependence of the density n(upsilon) of compensating lead vacancies on the In impurity content in the samples is calculated in a thermodynamic theory. The calculations indicate that the energy position of the amphoteric level strongly influences the character of the defect-formation process. Not only can the density of compensating defects increase (self-compensation phenomenon), but also their density can be independent and n(upsilon) can even decrease as the impurity concentration increases. (C) 1996 American Institute of Physics.

The electrical conductivity, Hall coefficient, and Seebeck coefficient in thin films of solid solutions of (Sn0.8Ge0.2)(1-x)In-x Te (x = 0-0.2) have been studied in the temperature 77-400 K. Films 200-1200 Angstrom thick were obtained by laser sputter-coating on mica substrates. These films have a Hall concentration of holes p = 2 x 10(20)-3 x 10(21) cm(-3), a mobility R sigma = 1 - 30 cm(2)/(V . sec), and a conductivity in the range rho = 2.5 x 10(-4)-3 x 10(-3) Omega . cm. All these parameters are weakly dependent on temperature. A characteristic minimum close to p similar or equal to 7 x 10(20) cm(-3) is detected on the isotherms of the dependence of the Seebeck coefficient on the hole concentration, indicating that the valence band of the solid solution has a complex structure. (C) 1996 American Institute of Physics.

Using expressions based on the effective-medium method, we invoke hopping conductivity to explain the temperature dependence of thermoelectric power in the quaternary solid solutions (Pb0.78Sn0.22)(1-y)InyTe. Our estimates of the activation energy for hopping conductivity are in order-of-magnitude agreement with values obtained previously from analysis of the electrical conductivity in these materials using the results of percolation theory. We also discuss the conditions under which our expressions can be used. (C) 1996 American Institute of Physics.

Estimates of the activation energy epsilon(3) and of the radius a of the wave functions of the impurity states have been obtained by analyzing the dependence of the electrical conductivity on the temperature and the indium concentration in the region of hopping conduction in (Pb0.78Sn0.22)(1-y) InyTe solid solutions. It is found that epsilon(3) varies from approximate to 40 meV at y = 0.03 to approximate to 10 meV at y = 0.15. The radius obtained, a approximate to 6 Angstrom, indicates that the indium impurity states are highly localized. A Mott transition to metallic conduction is observed in the solid solution at y = 0.2. (C) 1995 American Institute of Physics.