Том 53, № 6 (2019)

The effect of the geometric shape of the samples on the uncertainty in measurements of the thermal conductivity coefficient of materials by the laser-flash method is investigated. A mathematical model simulating the process of measuring the thermal conductivity of samples made of graphite, Mg₂Si_0.4Sn_0.6, and bismuth telluride by the laser-flash method is created in the Comsol Multiphysics software. Cylindrical samples with plane-parallel sides, samples shaped as a truncated cylinder, as well as parallelepiped-shaped samples with a square base are investigated. It is shown that the uncertainty in the measurements does not exceed 2% for the samples with plane-parallel sides and sizes up to 12.7 mm. For the samples shaped as a truncated cylinder with a diameter of 3 mm and at an inclination angle of φ = 1.5°, the measurement uncertainty does not exceed 3%. With increasing sample diameter and angle φ, the measurement uncertainty increases significantly.

The Seebeck coefficient S and electrical conductivity σ of bismuth-telluride-based alloys with substituents in the Bi and Te sublattices are studied at pressures up to 12 GPa at room temperature. It is shown that the electrical conductivity increases with pressure and, despite a decrease in the Seebeck coefficient, the power factor S²σ increases in p-Bi_0.5Sb_1.5Te₃ and n-Bi₂Te_1.65Se_0.65S_0.7 alloys. A maximum increase in the power factor is observed for n-Bi₂Te_1.65Se_0.65S_0.7 in the pressure range of 3–4 GPa corresponding to the electronic topological phase transition. The studied alloys are used in modeling the thermoelectric module with adjustable mechanical stress applied to thermoelements.

Nanocomposite thermoelectrics based on Bi_0.45Sb_1.55Te_2.985 solid solution of p-type conductivity are fabricated by the hot pressing of nanopowders of this solid solution with the addition of SiO₂ microparticles. Investigations of the thermoelectric properties show that the thermoelectric power of the nanocomposites increases in a wide temperature range of 80–420 K, while the thermal conductivity considerably decreases at 80–320 K, which, despite a decrease in the electrical conductivity, leads to an increase in the thermoelectric efficiency in the nanostructured material without the SiO₂ addition by almost 50% (at 300 K). When adding SiO₂, the efficiency decreases. The initial thermoelectric fabricated without nanostructuring, in which the maximal thermoelectric figure of merit ZT = 1 at 390 K, is most efficient at temperatures above 350 K.

The possibility of fabricating thermoelectric coolers for operating temperatures below 90 K is considered. It is impossible to use the standard thermoelectric-cell scheme consisting of two semiconductor legs of n-type and p-type conductivity connected into an in-series electrical circuit for these temperatures. There is only one effective n-type thermoelectric material based on Bi–Sb solid solutions for the region of cryogenic temperatures. For this case, thermoelectric cells with a thermoelectric n-type leg and passive leg based on a high-temperature superconductor (HTSC) are investigated. The design of a thermoelectric cooler (module) consisting of thermoelectric n-type legs (extruded Bi_0.91Sb_0.09 crystals) and passive HTSC-based legs (YBa₂Cu₃O_{7 –x}) is proposed. A magnetic field is used to increase the thermoelectric figure of merit ZT of the module. The maximal temperature drop between the hot and cold sides ΔT > 13.5 K and maximal cooling capacity Q_c > 0.36 W are attained for the cryogenic module at a hot side temperature of T_h = 80 K, current consumption of I = 6.4 A, and voltage of U = 0.10 V.

The structure, composition, and thermoelectric properties of cobalt monosilicide obtained by crystallization from a supersaturated solution–melt in tin are studied. A technique for the synthesis and directional solidification of microcrystalline and bulk textured materials in a single technological cycle is developed.

The results of experimental investigation into Fe₂Ti_{1 –x}V_xSn alloys (x = 0, 0.06, 0.15, and 0.2) are presented. It is established from the temperature dependences of the electrical conductivity, Seebeck coefficient, and thermal conductivity that the studied compositions exhibit the transport properties typical of semiconductors, while the partial substitution of titanium atoms by vanadium atoms leads to a change in the conductivity from p-type to n-type; the undoped Fe₂TiSn sample possesses the best thermoelectric characteristics.

The properties of Co–Si thin films grown by the thermal sintering of Co and Si layers are studied. Co and Si layers are produced by chemical vapor deposition. To form cobalt silicide, the obtained two-layer structure is annealed at a temperature of 760 K for 12 h. The thermoelectric properties of the film structure are studied in the temperature range of 300–800 K. The temperature dependences of the thermoelectric power and resistivity, as well as structural data, indicate the formation of a multilayer structure containing layers with excess silicon and cobalt.

This work reports on the epitaxial-film growth and characterization of a new wide-gap semiconductor α-Ga₂O₃. Layers are deposited by chloride vapor phase epitaxy on sapphire substrates with a basal orientation. The thickness of the layers of the investigated samples is from 0.5 μm to a value of 10 μm, the latter being record-breaking for today. The structural and optical properties of the obtained samples are studied. It is shown that all samples are structurally uniform, single phase, and have an R\(\bar {3}\)c corundum-like structure similar to that of the sapphire used as the substrate. It is shown that the full-width at half-maximum of the rocking curves for the (0006) reflection of α-Ga₂O₃ changes with the layer thickness and approaches 240 arcsec for the thickest layers. Both thin and thick layers are transparent in the visible and UV (ultraviolet) spectral range up to an absorption edge at 5.2 eV.

The cathodoluminescence and absorption of plastically deformed ZnS(O) single crystals are investigated in the light of band-anticrossing theory. The difference in the oxygen content in the surface layer and in the sample bulk is found using a scanning electron microscope and according to cathodoluminescence data. This fact explains the specifics of the spectral position of the fundamental absorption edge and exciton spectra. The shift dynamics of the bands of self-activated luminescence on deep A centers (SA luminescence) during deformation recrystallization with an increase in the dissolved oxygen concentration is presented. Restriction of the spectral range of the appearance of self-activated luminescence at shallow levels—“edge” luminescence—is established. The nature of the emission bands in the wavelength ranges of 336–350 and 364–390 nm is established. These results refine the energy model of ZnS(O) crystals and can be useful in the case of the practical use of their structure-sensitive properties.

By means of in situ ultrahigh vacuum reflection electron microscopy, the nucleation of vacancy islands on wide terraces of the Si(100) surface is investigated. The temperature dependence of the displacement of a vacancy island nucleation center is determined in the process of heating a sample with a dc electric current. On the basis of a theoretical model, the effective electric charge of addimers is estimated in the direction across dimer rows of the surface. The effective charge has a positive sign and does not exceed 15 units of the elementary charge in the temperature range of 1020–1120°C.

The current–voltage characteristics of n⁺-GaAs/n⁰-GaAs/N⁰-AlGaAs/N⁺-AlGaAs/n⁺-GaAs isotype heterostructures and n⁺-GaAs/n⁰-GaAs/n⁺-GaAs homostructures are studied. It is shown that, for a heterostructure under reverse bias providing the injection of electrons from n⁰-GaAs into N⁰-AlGaAs, the maximum operating voltage reaches a value of 48 V at a thickness of the N⁰-AlGaAs layer of 1.0 μm, and the current–voltage characteristic has no region of negative differential resistance. The operation of a homostructure is accompanied by a transition to the negative-differential-resistance region at a voltage of 10 V. Theoretical analysis in terms of the energy-balance model demonstrated that the reverse-biased isotype heterostructure has no negative-differential-resistance region because, in this case, the field domain does not collapse in contrast to what occurs in homostructures.

The introduction of CsPbBr₃ nanocrystals into a porous ZnO matrix leads to the appearance of a photoconductivity response in the visible spectral range. Measurement of the spectral dependences of the absorption, photoluminescence (PL), and photoconductivity showed that the shape and position of the PL peak are associated with defects in CsPbBr₃. Analysis of the photoconductivity relaxation kinetics shows the possibility of accelerating the recombination of photoexcited charge carriers by more than an order of magnitude. The mechanisms responsible for this effect are discussed.

Asymmetrical double InAs/InAsSb/InAsSbP heterostructures are grown by metalorganic vapor phase epitaxy. Two types (A and B) of light-emitting diodes with wavelengths of 4.1 and 4.7 μm at the emission-spectrum maximum are formed from these heterostructures. The room-temperature I–V and electroluminescence characteristics of the light-emitting diodes are investigated. The emission powers of light-emitting diodes A and B in the quasi-continuous mode (at a frequency of 512 Hz) at a current of 250 mA are 24 and 15 μW, respectively. In the pulsed mode (at a frequency of 512 Hz and a pulse length of 1 μs), the emission powers of light-emitting diodes A and B at a current of 2.1 A reach 158 and 76 μW, respectively. The developed light-emitting diodes can be used as high-efficiency emission sources in optical absorption sensors for detecting carbon dioxide and monoxide gases in the atmosphere.

The currents flowing in metal–CaF₂–n-Si and metal–SiO₂–CaF₂–n-Si structures with the same (about 1.5 nm) fluoride thickness are compared in the reverse-bias mode. It is revealed that the current in the case of a two-layer dielectric can be notably higher within a certain voltage range. Such unexpected behavior is associated with the coexistence of both electron and hole components of the current as well as with the configuration of the SiO₂–CaF₂ barrier through which tunneling occurs. The results of measurements and explanatory simulation data are presented.

For the first time, comprehensive comparative investigations of ultraviolet photodetectors with Cr Schottky barriers formed on 4H-SiC epitaxial layers are carried out by the X-ray and optical methods before and after irradiation with 15-MeV protons with fluences in the range of (1–4) × 10¹² cm^–2. When increasing the fluence of proton irradiation, the formation of localized regions with negative deformation is observed along with the unperturbed silicon-carbide matrix. Agreement between the X-ray and optical studies is obtained, which makes it possible to explain the features of the spectral changes in the photosensitivity of detectors in the range of 200–400 nm with an increase in the fluence of proton irradiation. The ultraviolet Cr/4H-SiC photodetectors withstand irradiation by 15-MeV protons with a fluence of 4 × 10¹² cm^–2 virtually without any changes in the photosensitivity due to the gettering of simple defects by cluster and amorphous formations, which lead to partial structural improvement of the irradiated material.

Highly adhesive tin-monoselenide (SnSe) layers (200 ± 10) nm thick are grown by hydrochemical deposition from a trilonate reactive mixture. X-ray diffraction shows that the synthesized films crystallize in the orthorhombic system (space group Pnma). The significant oxygen content in the film surface layers is explained by the partial oxidation of samples with SnO₂ phase formation. The results of ion etching to a depth of 18 nm show a sharp decrease in the oxygen content over thickness and real correspondence to the SnSe elemental composition. The band gap determined by optical studies for direct transitions is 1.69 eV. The synthesized SnSe layers exhibit p-type conductivity, which is characteristic of this material.