Vol 45, No 3 (2019)

The possibility of correcting the power coupling ratio of an integrated optical directional X-coupler upon excitation of a local photorefractive response in the substrate material (LiNbO₃) has been analyzed. The X-coupler operation has been numerically simulated, and the regions of its maximum sensitivity to photorefraction have been determined. The correspondence between the calculated and experimental data has been obtained. The maximum correction value of the coupling ratio is 1–2%. The photorefractive correction of the coupling ratio has been used to increase the extinction ratio of a Mach–Zehnder modulator to 47 dB.

A method has been developed for numerically estimating the recombination loss in silicon heterojunction solar cells under irradiation. The calculations are based on an analysis of the experimental short-circuit currents. The suggested model makes it possible to evaluate the degree of degradation of semiconductor structures by calculating the decrease in the bulk lifetime and in the diffusion length of carriers. The results obtained are of practical importance for examining the possibility of using this type of solar cells in space conditions.

The electronic properties of the Ba/SiC/Si(111)-8° nanointerface have been studied for the first time by photoelectron spectroscopy using synchrotron radiation. The experiments are conducted in situ in superhigh vacuum on submonolayer Ba coatings of SiC/Si(111)-8° samples grown by the method of substitution of atoms. It is found that the adsorption of Ba causes strong changes in the spectrum of the C 1s core level. It is shown that the effect is due to the formation of a new, previously unknown carbon nanostructure. It is found that the nanostructure is formed exclusively on the SiC vicinal surfaces in the presence of stabilizing adsorbed Ba metal atoms and consists of carbon rings, in which the chemical bonds are close in nature to the bonds characteristic of aromatic compounds.

The introduction of Si⁺ ions and ions of other elements into amorphous silicon dioxide during their interaction causes damage to the structural bonds, which is observed in the vibrational spectral bands. Pure SiO₂ has no optical transitions but the bands of induced point defects appear in the photoluminescence spectrum when ions/neutrons are introduced. The generation of photoluminescence-active defects by fluxes of Ar⁺ ion and thermal neutrons is compared. It is shown that the nature of damage to the structure is associated with both the specifics of the synthesis/processing of the material and the features of the interaction between the substance and ions (atomic collisions) and neutrons (collisions with atomic nuclei).

A model of interaction of gold nanoislands during chemical vapor deposition in vacuum both in the presence of charges in the vapor and after their removal has been considered. A growth stage has been revealed (the mean sediment thickness is more than 2 nm) in which the mutual attraction of gold nanoislands and a sharp increase in their sizes is observed. In this stage, the sediment with removed charges exhibits retardation of the nanoisland growth. The influence of the substrate geometric profile on the sediment morphology is demonstrated. These investigations are promising for development of methods for nanocoating segmentation and formation of ordered nanoisland ensembles.

A study of the surface topography and optical characteristics of thin AlN films used as passivating and antireflection coatings deposited on n-GaAs (100) substrates by reactive ion-plasma sputtering is reported. It was found that the process conditions affect the structure and the optical characteristics of the films, which makes it possible to obtain coatings with prescribed parameters. An analysis of the results furnished by ellipsometry and atomic-force microscopy of the surface shows that the refractive index of the films is correlated with the surface structure.

A compact pulsed plasma generator of simple design made of a coaxial cable with polyethylene insulation has been described. A plasma gun generates a cloud of carbon-hydrogen plasma with an electron density of more than 10¹³ cm^–3, including in a magnetic field produced of up to 500 G and also in the presence of the background plasma with a density of about 10¹² cm^–3, which is produced by an independent source. The high resource of the gun and stability of plasma parameters from one pulse to another allows using such a gun in laboratory experiments simulating dynamic processes in space.

Spin light-emitting diodes based on InGaAs/GaAs heterostructures with a CoPt ferromagnetic injector were fabricated. It was demonstrated that the processing of these structures in selenium vapor prior to the deposition of a CoPt contact provides an opportunity to enhance the circular polarization degree of diode emission. The observed increase in the polarization degree is attributed to the suppression of spin relaxation at the metal/semiconductor interface due to surface passivation and a reduction in the density of surface electron states as a result of processing in selenium vapor.

The results of experimental studies of the effect of the duration and current of a pulsed glow discharge on the generation of localized disturbances in the supersonic boundary layer of a flat plate at Mach numbers of the oncoming flow M = 3 and 4 have been presented. Flow pulsation measurements were performed by a hot-wire anemometer. It has been found that an increase in the duration of the impulse function leads to an increase in the amplitude and time scales of the perturbations that are generated in the boundary layer. An increase in the current of the pulse discharge leads to an increase in the amplitude of the generated pulsations. The propagation velocities in the boundary layer of localized perturbations at different Mach numbers of the oncoming flow have been estimated.

GaP/Si/Ge nanoheterostructures have been obtained using the method of pulsed laser deposition, and an energy band diagram of cascade solar cells based on these heterostructures were modeled. GaP and Ge nanolayers grown on Si substrates were studied by Raman spectroscopy and X-ray diffraction. Spectral dependences of the external quantum efficiency response of GaP/Si/Ge nanoheterostructures were determined.

Use of the method of local cathodoluminescence in Si–TiO₂ and Si–SiO₂–TiO₂ structures helps to elucidate the nature of centers influencing the operation of memristors. These measurements showed that electroforming leads to the appearance of luminescence in a 250–400 nm wavelength range in the external part of TiO₂ layer characterized by high concentration of defects. This observation leads to a conclusion that a sharp interface is formed between dielectric layers of the structure, provides estimation of the absorption coefficient of TiO₂ layer, and allows its bandgap width (~3.3 eV) to be evaluated for the oxide layers formed by the given technology.

The results of an investigation of the process of laser-induced hardening of zirconium (E110 alloy) are reported. It has been established that the laser treatment in air results in the formation of a uniformly distributed microrelief characterized by the presence of microparticles with various configurations. Overlap of the laser beam trajectories leads to the formation of a layered structure. The composition of laser-modified surface depends on the regime of processing and mostly comprises a mixture of zirconium oxides and nitrides. In addition, the formation of diffuse transition zones between the bulk metal and modified surface layer is established. The dependence of the thickness of the hardened layer on the regime of pulsed laser action is determined.

A system of spin-injection emitters employing magnetic junctions based on an array of parallel nanodimensional wires has been studied. Manufacturing technology and possible variants of the nanowire structure are discussed. Conditions of the formation of a new type of spin-injection emitters of terahertz radiation on this basis are considered. The design of the experimental setup is described, and the results of experiments are analyzed. It is established that signals in a 16–18 THz range can be generated by passing high-density current through the proposed nanowire array.

Peculiarities of the formation of multistability in a microwave generator with delayed reflection from the load have been studied. Characteristic scenarios of the appearance and evolution of multistable states are determined. The influence of nonisochronism on the scenario of transition to chaos is revealed.

Results of measurements of the coefficient of microwave reflection from a block of freshwater ice subjected to plastic deformation are presented. Strong fluctuations in the phase of reflection coefficient are revealed, with the extremum at an electromagnetic radiation wavelength of 1.35 cm. This wavelength corresponds to the region of localized plastic flow autowaves in crystalline media.

A regime of quasi-stationary microwave generation in moderately relativistic microwave oscillator of the twistron type with 50 ± 20% efficiency of electron beam power conversion into electromagnetic radiation has been obtained in experiment through optimization of the electron–wave interaction. For the selected electron beam parameters (diode voltage, 210 kV; beam current. 1.36 kA) the output microwave power at 10.63 GHz frequency was 140 ± 40 MW in a guiding magnetic field of about 1.9 T. The duration of microwave pulses was about 16 ns.

Stable nanofluids based on DG-100 grade carbon black and carbon nanotubes have been prepared, and their influence on the maximum heat-transfer capacity and thermal resistance of closed-loop two-phase thermosyphons (TPTs) intended for electronics cooling have been studied. A more than twofold increase in the critical heat flux of these TPTs as compared to those filled with water has been obtained along with a sharp decrease in their thermal resistance. It is suggested that this effect is not only due to the high thermal conductivity of the proposed nanofluids, but is also related to the formation of a specific porous structure hindering the appearance of a vapor film and enhancing the boiling process.