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Документ Effect of aging on thermoelectric properties of the Bi2Te3 polycrystals and thin films(Науково-технологічний комплекс "Інститут монокристалів", 2021) Rogacheva, E. I.; Doroshenko, A. N.; Novak, K. V.; Sipatov, A. Yu.; Khramova, T. I.; Saenko, S. A.The temperature dependences (77-300 K) of the thermoelectric (TE) properties (the Seebeck coefficient S, electrical conductivity σ, Hall coefficient RH, Hall charge mobility μH>, and TE power factor P) were studied for freshly prepared and for exposed to air at room temperature during 5 years p-Bi2Te3 (60.0 at.% Te) and n-Bi2Te3 (62.8 at.% Te) polycrystals and thin films grown from them by thermal evaporation in vacuum. It was found that after aging, in the p- and n-Bi2Te3 bulk crystals and in the n-type film obtained from the n-Bi2Te3 crystal, type of conductivity is reserved but the p-type film obtained from the p-Bi2Te3 crystal, change the type of conductivity from hole to electronic. The activation energies of possible defect states were determined using the RH(T) dependences. After aging, at the temperatures close to room temperature, the p values of n-Bi2Te3 and p-Bi2Te3 polycrystals decreases by ~ 20 %, but p values of the n-type film grown from n-Bi2Te3 crystal increases by 20-30 %. In the p-type film obtained from p-Bi2Te3 polycrystal, and having changed the type of conductivity after aging, the p values exceed the p values of a film obtained from n-Bi2Te3 polycrystal by ~ 35 % at 250 K and by 25 % at 300 K, remaining at these temperatures below the p values for n-Bi2Te3 polycrystal after aging by ~ 15 %.Публікація Temperature and magnetic field dependences of thermoelectric properties of Bi1-xSbx solid solutions in the range x = 0-0.25(Elsevier Ltd, 2021) Rogacheva, E. I.; Doroshenko, A. N.; Nashchekina, O. N.Bi1–xSbx solid solutions are the best n-type thermoelectric materials for use at temperatures ≤200 K. An important parameter determining material’s figure of merit is charge carrier concentration. To determine it correctly, one should carry out measurements in a weak magnetic field. On the basis of the magnetic field dependences of the Hall coefficient and magnetoresistance, the dependences of the weak magnetic field boundary Bc on composition (x = 0–0.25) and temperature (T = 77–300 K) for polycrystalline Bi1–xSbx alloys were plotted. It was established that the Bc(x) dependences exhibit a non-monotonic behavior which is attributed to the existence of electronic phase transitions.Публікація Thickness-dependent quantum oscillations of the transport properties in bismuth selenide thin films(Elsevier, 2019) Rogacheva, E. I.; Menshikova, S. I.; Sipatov, A. Yu.; Nashchekina, O. N.The objects of the present study were thin n-Bi2Se3 films with thicknesses d = 10–100 nm, grown by thermal evaporation of n-Bi2Se3 crystals in vacuum onto heated glass substrates. The room temperature d-dependences of the Seebeck coefficient, the Hall coefficient, and the electrical conductivity of the films exhibited an oscillatory behavior, which we attribute to quantum size effects. Such interpretation of the results is supported by the fact that experimentally determined values of the oscillation period are in quite good agreement with the theoretically calculated ones. We suggest that the large amplitude and undamped character of the oscillations in the studied range of thicknesses are connected with the topologically protected gapless surface states of Bi2Se3. The observed oscillatory character of the d-dependences of the transport coefficients should be taken into account when 2D-structures are applied in nanothermoelectricity and other fields of nanoscience and nanotechnology.Публікація Effect of Deviation from Stoichiometry on Thermoelectric Properties of Bi₂Te₃ Polycrystals and Thin Films in the Temperature Range 77-300 K(Сумський державний університет, 2019) Rogacheva, E. I.; Novak, K. V.; Doroshenko, A. N.; Nashchekina, O. N.; Budnik, A. V.Bi₂Te₃ semiconductor compound and Bi₂Te₃-based solid solutions are presently among the best lowtemperature thermoelectric materials. One of the methods of controlling the conductivity type and properties of Bi₂Te₃ is changing the stoichiometry of this compound. Earlier, we have obtained the room-temperature dependences of mechanical and thermoelectric properties of Bi₂Te₃ polycrystals on the degree of deviation from stoichiometry. The goal of this work is to investigate the behavior of such dependences at other temperatures. Bismuth telluride polycrystals with compositions in the range of 59.6-67.5 at. % Te were obtained, and for all the crystals the Seebeck coefficient, the Hall coefficient, electrical conductivity and charge carrier mobility were measured in the temperature range 77-300 K. On the basis of the temperature dependences, the isotherms of kinetic coefficients were plotted. It was found that similar to the room-temperature isotherms, the isotherms at lower temperatures were non-monotonic: they exhibited inversion of the conductivity sign between 60.5 and 61.0 at. % Te and extrema near 60.0 and 63.0 at. % Te. The experimental data are interpreted taking into account changes in the band and defect structures of Bi₂Te₃ under varying stoichiometry. The obtained results make it possible to control thermoelectric properties of Bi₂Te₃ polycrystals in the temperature range 77-300 K by changing the degree of deviation from stoichiometry.Публікація Galvanomagnetic properties of polycrystalline Bi₁₋ₓSbₓ solid solutions in the concentration range x = 0-0.25(Науково-технологічний комплекс "Інститут монокристалів", 2020) Rogacheva, E. I.; Doroshenko, A. N.; Drozdova, A. A.; Nashchekina, O. N.; Men'shov, Yu. V.The dependences of the Hall coefficient, electrical conductivity, magnetoresistance, electron and hole concentration and mobility on the Bi₁₋ₓSbₓ solid solution composition in the concentration range x = 0-0.25 at 77 and 300 K in magnetic fields 1 T and 0.05 T were obtained. It was shown that all the dependences exhibit a distinct nonmonotonic oscillating behavior at both temperatures and in both magnetic fields. The presence of concentration-dependent anomalies of galvanomagnetic properties is attributed to critical phenomena accompanying the percolation-type transition from dilute to concentrated solid solutions and electronic phase transitions: a transition to a gapless state, the semimetal – semiconductor transition, and indirect – direct band gap semiconductor transition.Публікація Transport properties of the bismuth telluride thin films with different stoichiometry in the temperature range 77-300 K(Науково-технологічний комплекс "Інститут монокристалів", 2020) Rogacheva, E. I.; Novak, K. V.; Doroshenko, A. N.; Nashchekina, O. N.; Budnik, A. V.The objects of the present study are thin films with thicknesses d = 45-620 nm prepared by thermal evaporation in vacuum from a single source, using undoped p- and n-type Bi₂Te₃ polycrystals with different stoichiometry (60.0 and 62.8 at. % Te, respectively) as a charge, and subsequent condensation on glass substrates at 500 K. The temperature dependences of the Hall coefficient Rн, electrical conductivity σ, and Hall charge carrier mobility μн of thin films were obtained in the range 77-300 K. It was found that the films had the same type of conductivity as the initial polycrystals in the entire temperature range studied and, like in the initial crystals, σ and μн decreased with increasing temperature. The exponents ν in the μн(T) dependences for the bulk polycrystals were larger than those for the films and increased with increasing d. In contrast to the p-type bulk polycrystals, Rн of the p-type films decreased under increasing temperature. In the n-type Bi₂Te₃, Rн decreased with temperature for both thin films and bulk crystals, however, the character of the Rн(T) dependences for the crystals and films differed. The decrease in Rн with temperature before the range of intrinsic conductivity in all thin films is attributed to the existence of donor and acceptor defect states.Документ Temperature dependences and isotherms of galoanomagnetic properties of Bi doped PbTe crystals and thin films(Науково-технологічний комплекс "Інститут монокристалів", 2006) Rogacheva, E. I.; Lyubchenko, S. G.; Vodorez, O. S.The temperature dependences of galvanomagnetic properties (the Hall coefficient, electrical conductivity, charge carrier mobility) of (PbTe)₁₀₀₋ₓBiₓ (x = 0-1) alloys obtained by doping PbTe with elementary Bi and thin films prepared from these alloys were studied in the temperature range 80-300 K. On the basis of the temperature dependences, the isotherms of the properties on the Bi concentration, which had been observed earlier at room temperature, is preserved at lower temperatures. This confirms our earlier suggestion about the self-organization processes taking place in the defect subsystem of the crystal at certain Bi concentrations.