卷 51, 编号 6 (2017)

The electrical resistivity and thermopower of cobalt monosilicide (CoSi) and dilute alloys of CoSi with iron at temperatures from 2 to 370 K are experimentally investigated. CoSi is a semimetal and considered to be a promising thermoelectric material. It crystallizes into the cubic structure without an inversion center. This feature suggests the existence of topologically nontrivial electronic states and makes CoSi a candidate for the Weyl semimetal class. The main goal of the study is to find experimental confirmation of this assignment. It is shown that the experimental temperature dependences of the electrical resistivity and thermopower of CoSi and Co_1–xFe_xSi (x = 0.04) at low temperatures cannot be interpreted within the standard theory of conductivity in metals and may be related to topological features of the electronic structure of this compound.

The results of calculations of lattice thermal conductivity in bismuth telluride are presented and its decrease in a nanostructured material due to boundary scattering is estimated. Calculation is carried out using the lattice-dynamics method with the real phonon spectrum and the phonon–phonon interaction taken into account. Its results well agree with experimental data for the crystalline material. The estimates for the nanostructured material give a decrease in the thermal conductivity by 30% for grain sizes of 20 nm. Calculations by this method are compared with those obtained using approximate methods of description of the spectrum and scattering processes. It is shown that the least difference in the estimates (about 10%) can be obtained when using the approximation of a constant matrix element of phonon–phonon scattering with correction of the frequency dependence for acoustic modes.

The results of studying the galvanomagnetic properties of Bi₈₅Sb₁₅ thin films on substrates with various thermal-expansion coefficients are presented. The significant effect of the difference between the thermal expansions of film and substrate materials on the galvanomagnetic properties of the films is revealed. An analysis of the film properties within the two-band model shows that the difference in the properties of films on different substrates is associated with changes in the charge-carrier concentration and mobility. It is shown that a decrease in the thermal-expansion coefficient of the substrate material results in a decrease in the carrier concentration in the film composition under study, which indicates a significant difference in changes in the film band structure in comparison with changes occurring with increasing antimony concentration.

Nanopowders of Bi₂Te₃ and R_0.1Bi_1.9Te₃ (where R = Er, Tm, Yb, Lu) are obtained by microwave solvothermal synthesis. The powder-like materials are compacted by cold isostatic compression followed by annealing in argon. The influence of the doping agent on the structure and characteristics of the derived materials are investigated. It is demonstrated that the introduction of rare-earth elements (2 at %) into the bismuth-telluride lattice leads to a decrease in the electrical resistivity and to an increase in the Seebeck coefficient. The best thermoelectric properties are obtained for the sample of bismuth telluride doped with thulium.

A brief overview of the current state of research on Fe-based semiconducting Heusler alloys is given. The most significant achievement in this area is the increase of thermoelectric figure of merit to ZT > 1 in the p-type Fe(V,Nb)Sb-based compounds. Besides these compounds, growing attention has been paid in recent years to the study of promising thermoelectric materials based on Fe₂TiZ (Z = Al, Si, Sn) Heusler alloys and the investigation of multifunctional Fe₂MnZ (Z = Al, Si) compounds, which may be of interest as thermoelectric materials as well as magnetic semiconductors with a high Curie temperature.

The temperature dependences of the conductivity and thermoelectric power for a series of samples W_1–xNb_xS₂, W_1–xNb_xSe₂, WS_2–ySe_y, W_1–xNb_xS_2–ySe_y are studied at low temperatures. It is found that the cation substitution of W atoms with Nb leads to an increase in the conductivity and a decrease in the thermoelectric power. The anion substitution of S with Se atoms results in a simultaneous increase in the conductivity and thermoelectric power. The highest power factor among the samples studied is inherent to W_0.8Nb_0.2Se₂.

The high-temperature X-ray diffraction technique is used to study AgCuSe_0.5(S,Te)_0.5 crystals. It is shown that, at room temperature, the AgCuSe_0.5S_0.5 crystal is composed of Cu_1.96S and AgCuSe phases. At a temperature of 695 K, these phases transform into a single face-centered cubic (fcc) phase. The transformation is reversible. The AgCuSe_0.5Te_0.5 composition consists of three phases, specifically, Cu₂Te, AgCuSe, and a cubic phase. At 444 K, both orthorhombic phases simultaneously transform into a diamond-like cubic phase. In this transformation, the cubic phase plays the role of a seed. From the temperature dependence of the lattice parameters, the thermal-expansion coefficients of the phases involved in both compositions are calculated for the main crystallographic directions.

Iron ions with energies of 90 and 250 keV and a dose of 10¹⁶ cm^–2 are implanted into a silicon single crystal with the (110) orientation. The method of Rutherford backscattering in combination with channeling is used to study the distribution profiles of the introduced impurity and also the profiles of the distribution of radiation-induced defects in the crystal lattice. Experimental data are compared with the results of simulation performed using the TRIM software package. It is shown that, at an energy of 4.6 keV/nucleon, the average projected ranges coincide; however, at an energy of 1.6 keV/nucleon, the difference amounts to 35%. In addition, it is shown that the calculation incorrectly takes into account the dose dependence at energies of 1.6–4.6 keV/nucleon.

The conductivity and photoconductivity of ZnSe crystals doped with transition elements are studied. It is shown that the doping of ZnSe crystals with 3d impurity elements is not accompanied by the appearance of electrically active levels of these impurities. At the same time, the introduction of these impurities into the cation sublattice brings about the formation of electrically active intrinsic defects. It is established that ZnSe crystals doped with Ti, V, Cr, Fe, Co, or Ni exhibit high-temperature impurity photoconductivity. Photoconductivity mechanisms in the crystals are proposed. From the position of the first ionization photoconductivity band, the energies of ground states of 3d²⁺ ions in ZnSe crystals are determined.

The electron-transport and optical properties of heterostructures with a surface InGaAs/InAlAs quantum well in the cases of inverted δ doping with Si atoms (below the quantum well) and of standard δ doping (above the quantum well) are compared. It is shown that, in the case of inverted doping, the two-dimensional electron density in the quantum well is increased in comparison with the case of the standard arrangement of the doping layer at identical compositions and thicknesses of other heterostructure layers. The experimentally observed features of low-temperature electron transport (Shubnikov–de Haas oscillations, Hall effect) and the photoluminescence spectra of heterostructures are interpreted by simulating the band structure.

Magnetotransport in a two-dimensional two-component system consisting of electrons and holes with the same concentrations is studied. Balance equations to describe charge carrier and heat transfer are derived from the classical kinetic equation. The charge-carrier density and temperature distributions and electric-current densities are calculated by solving the balance equations for a long strip sample. In a sufficiently high magnetic field, regions of increased and decreased charge-carrier density, temperature, and fluxes are formed near the sample edges. This leads to nontrivial positive magnetoresistance.

X-ray diffraction and atomic-force microscopy are used to study the structure and surface morphology of Se₉₅Te₅ chalcogenide glassy semiconductor films and the influence exerted on these films by doping with samarium. The parameters of the first sharp diffraction peak (FSDP), observed in X-ray diffraction patterns, are used to determine the numerical values of the local structure parameters, such as the “quasiperiod” of density fluctuations, correlation length [size of medium-range-order (MRO) regions], and nanovoid diameters. In addition, the numerical values of the roughness-amplitude parameters are determined. It is found that the disorder in the atomic structure and the irregularities on the surface of the films under study become more pronounced with increasing percentage content of the samarium impurity.

Ab initio calculations of the electronic spectrum, deformation electron density, and phonon frequencies at the center of the Brillouin zone of the CdAs₂ tetragonal compound are carried out in the context of the density-functional method. It is established that the crystal is an indirect-gap semiconductor with a band gap of ~1 eV, which is in good agreement with well-known optical and electrical experimental data. The features of chemical bonding in the crystal are studied. The features are defined by the fact that As atoms form spiral chins of covalent As–As bonds, whereas Cd–As bonds are ionic-covalent. The ab initio calculated phonon frequencies are compared with the frequencies calculated in the Keating model and with experimental data; the partial contributions of Cd and As atoms are analyzed.

A p-CuI/n-ZnO barrier structure is investigated as a promising base diode structure for a semitransparent near-ultraviolet detector. We analyze the crystal structure and electrical and optical properties of zinc-oxide nanoarrays electrodeposited in the pulsed mode and copper-iodide films formed by the successive ionic layer adsorption and reaction (SILAR) method, which were used as the basis for an n-ZnO/p-CuI barrier heterostructure sensitive to ultraviolet radiation in the spectral range of 365–370 nm. Using the I–V characteristics, a shunting resistance of R_sh · S_c = 879 Ω cm², a series resistance of R_s · S_c = 8.5 Ω cm², a diode rectification factor of K = 17.6, a rectifying p–n-junction barrier height of Φ = 1.1 eV, and a diode ideality factor of η = 2.4 are established. It is demonstrated that at low forward biases (0 V < ^U < 0.15 V), the effects of charge-carrier recombination and tunneling are equal. As the bias increases above 0.15 V, the tunneling–recombination transport mechanism starts working. The diode saturation current J₀ is found to be 6.4 × 10^–6 mA cm^–2 for recombination and tunneling charge-carrier transport and 2.7 × 10^–3 mA cm^–2 for tunneling–recombination charge-carrier transport.

A new method for triggering reversely switched dynistors (RSDs) into submicrosecond modes with high current-rise rates being switched at a substantially lower primary ignition threshold is proposed and studied numerically in detail by computer simulation methods. The numerical problem considers all the relevant physical laws for the spatially distributed and discrete elements of RSD switching unit, including the nonlocal isochronous interaction between the working regions of power RSDs or the photodiode control switches and external-circuit components. The simulation results confirm the possibility of reaching, in actual practice for RSDs, current-rise rates up to dJ/dt = 3 × 10¹⁰ A cm^–2 s^–1 in circuits based on lowpower semiconductor control switches at primary ignition levels relative to a reversely injected charge, which has a density as low as 1–2 μC/cm². These parameters have been considered previously as only a theoretical limit, unachievable in the submicrosecond range for real thyristor switches.

It is experimentally shown that the process of the solid-phase synthesis of copper silicide is sensitive to magnetic fields. Upon low-temperature vacuum annealing of a Cu/Si(100) thin-film structure, a static magnetic field with moderate strength stimulates the alloying of silicon in copper and suppresses synthesis of the Cu³Si phase.

A method for the separation of thin ITO (indium-tin-oxide) films from a silicon substrate by pulsed laser irradiation is studied. The method enables the detachment of films with thicknesses of 360 nm and more without their destruction. The separation process consists in successive irradiation of the surface with single microsecond laser pulses at a wavelength of 650 nm. Upon being detached from silicon substrates, the films produced by high-frequency magnetron sputtering have a transmittance of 65% in the visible spectral range and a resistivity of ~1.2 kΩ/□. The thermal stresses appearing in thin ITO films and leading to their detachment are estimated.