Том 52, № 6 (2018)

The results obtained in a study of the frequency and temperature dependences of the ac electrical conductivity of FeIn₂Se₄ single crystals are presented. It is found that the law σ ~ f^S (0.1 ≤ S ≤ 1.0) is obeyed for electrical conductivity in the 295–375 K temperature range at frequencies of 2 × 10⁴–10⁶ Hz. It shown that the frequency dependence of the conductivity in an FeIn₂Se₄ single crystal can be accounted for in terms of the multiplet model, and, consequently, the conductivity in these single crystals is characterized by the band-hopping mechanism.

Germanium, silicon, gallium arsenide, and indium antimonide n-type crystals on the metal side of the insulator–metal transition (Mott transition) are considered. In the quasi-classical approximation, the static (direct current) electrical conductivity and the drift mobility of electrons of the c band, and electrostatic fluctuations of their potential energy and the mobility edge are calculated. It is considered that a single event of the elastic Coulomb scattering of a mobile electron occurs only in a spherical region of the crystal matrix with an impurity ion at the center. The results of calculations using the proposed formulas without using fitting parameters are numerically consistent with experimental data in a wide range of concentrations of hydrogenlike donors at their weak and moderate compensation by acceptors.

The method of Mössbauer emission spectroscopy with ^119mTe and ¹¹⁹Sb parent isotopes in the radioactive-equilibrium state is used to obtain information on the localization and valence state of ^119mSn daughter atoms formed from the ¹¹⁹Sb and ^119mTe parent atoms at cation and anion sites of the lattice of PbSe and PbTe lead chalcogenides.

Polymorphic transformations in Rb_0.975Cs_0.025NO₃, Rb_0.950Cs_0.05NO₃, and Rb_0.90Cs_0.1NO₃ crystals grown by us have been studied by the X-ray diffraction method. Four different modifications are found for crystals in the range from room temperature to the melting point. The transformation temperatures and the unit-cell parameters are determined for the crystals of these modifications.

Study of structural, optical and photocatalytic properties of multilayered (1–8 layers) zinc oxide films deposited on glass substrates by sol-gel method showed, that after thermal treatment at 500°C they consist of random oriented hexagonal crystalline grains with size of 34–40 nm, forming larger particles with sizes of 100–150 nm, which do not depend on number of layers. With an increase in the number of layers, the intensity of exciton photoluminescence decreases by a factor of 10, the absorption of light in the visible and near IR ranges increases, and the efficiency of photocatalytic decomposition of the test organic dye rhodamine B increases by 10–12%. The observed changes are related to the increase in the total area of grain boundaries and to the change in the integral amount of oxygen vacancies and interstitial atoms as the number of layers increases, which makes it possible to control the properties of zinc oxide films for applications in optoelectronics, photovoltaics and photocatalysis.

The influence of Bi in GaAs barrier layers on the structural and optical properties of InAs/GaAs quantum-dot heterostructures is studied. By atomic-force microscopy and Raman spectroscopy, it is established that the introduction of Bi into GaAs to a content of up to 5 at % results in a decrease in the density of InAs quantum dots from 1.58 × 10¹⁰ to 0.93 × 10¹⁰ cm^–2. The effect is defined by a decrease in the mismatch between the crystal-lattice parameters at the InAs/GaAsBi heterointerface. In this case, an increase in the height of InAs quantum dots is detected. This increase is apparently due to intensification of the surface diffusion of In during growth at the GaAsBi surface. Analysis of the luminescence properties shows that the doping of GaAs potential barriers with Bi is accompanied by a red shift of the emission peak related to InAs quantum dots and by a decrease in the width of this peak.

The photoconductivity and its relaxation characteristics in tunneling p–i–n GaAs/AlAs heterostructures under pulsed illumination is studied. Quantum oscillations in the photoconductivity are detected depending on the bias voltage with the period independent of the light wavelength, as well as an oscillating component of the relaxation curves caused by modulation of the recombination rate at the edge of a triangular quantum well in the undoped i layer, as in the case of photoconductivity oscillations. The common nature of oscillations of the steady-state photoconductivity and relaxation curves under pulsed illumination is directly confirmed by the lack of an oscillating component in both types of dependences of some studied p–i–n heterostructures. Simultaneous suppression of the observed oscillations of dependences of both types as the temperature increases to 80 K also confirms the proposed mechanism of their formation. The dependences of these oscillations on the magnetic field and light flux power are studied. Oscillation-amplitude suppression in a magnetic field of ~2 T perpendicular to the current is caused by the effect of the Lorentz force on the ballistic motion of carriers in the triangular-quantum-well region.

The specific features of the electron spectra of II–VI and III–V semiconductor quantum dots are studied experimentally and theoretically. Analysis of the samples makes it possible to estimate the positions of the first three levels in the electron spectrum of the quantum object. Good qualitative and quantitative agreement between the experimental results and theoretical estimates is attained. It is shown that the mechanism of the experimentally observed field-emission current through a quantum dot is satisfactorily described by Morgulis–Stratton theory in experimental conditions.

The structural and electrical properties of PbS nanoparticles (40–70 nm), produced by a chemical reaction of sodium hydroxide with lead nitrate and electrophoretically deposited onto a conductive substrate, are investigated. The composition and structure of the nanoparticles are identified by X-ray analysis as pure PbS phase with a face-centered cubic lattice. Several minima, related to plasma-resonance absorption at 10–17 μm, are observed in the frustrated total internal reflection (FTIR) spectra. The layer morphology and the nanoparticle shape and sizes are determined by scanning electron and tunneling microscopy. The threedimensional topograms show that the surface fine structure is a set of faceted pyramidal spikes with a size of 5–10 nm and a density of ~400 μm^–2. An analysis of the tunneling current–voltage characteristics of individual nanospikes shows the presence of low-field emission and makes it possible to determine the barrier heights (1.6–1.8 eV), which are explained within the quantum-dot (QD) model.

The effect of the desorption of hydrogen on the mechanical characteristics and electronic structure of the armchair conformation of graphane is studied in the context of the nonorthogonal tight-binding model. It is shown that the mechanical stiffness and the Poisson ratio nonmonotonically depend on the hydrogen content and take minimum values at a hydrogen vacancy content of ~50% and ~30%, respectively. As hydrogen is desorbed, the characteristic peaks of the phonon density of states are rapidly reduced. In the initial stage of desorption, local energy levels are formed in the band gap. As the number of hydrogen vacancies is increased, these levels form an impurity band, in which the Fermi level is located.

A design for a high-sensitivity photodetector with a single layer of MoS₂ transition-metal dichalcogenide used as the basic functional element is proposed and the process of its fabrication is presented step by step. Quality evaluation and the selection of functional MoS₂ flakes is based on the results of combined optical characterization. The main operating characteristics of the fabricated device are investigated and a photosensitivity of 1.4 mA/W is demonstrated. A difference of this device in comparison with existing analogues is its high photosensitivity at low operating voltages (in the range of ±3 V).

A quantitative model for charge accumulation in an undergate dielectric during tunneling electron injection from a gate according to the Fowler–Nordheim mechanism is developed. The model takes into account electron and hole capture at hydrogen-free and hydrogen-related traps as well as the generation of surface states during the interaction of holes with hydrogen-related centers. The experimental dependences of the threshold voltage shift and gate voltage shift of n- and p-channel MOS (metal–oxide–semiconductor) transistors on the injected charge in the constant current mode are analyzed based on the model.

The effect of atomic aluminum deposited onto sapphire substrates with different nitridation levels on the quality of AlN layers grown by ammonia molecular-beam epitaxy is investigated. The nitridation of sapphire with the formation of ~1 monolayer of AlN is shown to ensure the growth of layers with a smoother surface and better crystal quality than in the case of the formation of a nitrided AlN layer with a thickness of ~2 monolayers. It is demonstrated that the change in the duration of exposure of nitrided substrates to the atomic aluminum flux does not significantly affect the parameters of subsequent AlN layers.

The fundamentals of a new technique for the cleaning and passivation of (111), (110), and (100) silicon wafer surfaces by hydride groups, which ensure a high surface purity and smoothness at the nanoscale upon long-term storage of the wafers at room temperature in air, are discussed. A new composition of the passivation solution for the long-term antioxidation protection of silicon surfaces is developed. The proposed solution is suitable for the long-term storage and repeated passivation of silicon wafers. The composition of the passivation solution and the conditions of passivation of the silicon wafers in it are described. Silicon wafers treated using the proposed technique can be used for growing epitaxial semiconductor films and different nanostructures. It is shown that only silicon surfaces prepared in this way allow SiC epitaxial films on silicon to be grown by atom substitution. The experimental dependences of the SiC and GaN film structures grown on silicon on the silicon-surface etching conditions are presented. The developed technique for silicon cleaning and passivation can both be used under laboratory conditions and easily adapted for the industrial production of silicon wafers with an oxidation-resistant surface coating.

It is shown that the method for the growth of conducting polyaniline nanotubes, based on the direct polymerization of aniline on the surface of channels in track membranes, can be used to produce nanotubes with a given conductivity. An island-type film with a channel resistance of ~10¹⁹ Ω is formed during the initial stage of polymerization (up to 2 min). As the polymerization duration increases to 3 min, the channel resistance falls by more than 10 orders of magnitude. This is attributed to the formation of a continuous film on the channel surface, i.e., a nanotube is formed. With the polymerization duration increasing further, the channel (nanotube) resistance gradually decreases to ~10¹⁹ Ω at 10 min. The conductivity of polyaniline during the formation of a hollow nanotube is estimated to be 0.01–0.04 S/cm. If the nanotube is completely filled with polyaniline, the conductivity increases to ~0.2 S/cm.