Volume 53, Nº 15 (2019)

The process of excess current carriers capture from wide-gap barrier layers into a quantum well plays a decisive role in the operation of devices based on semiconductor structures. Among these devices are lasers, light-emitting diodes, and photodetectors. The oscillating dependence of the capture rate on the quantum-well width is poorly experimentally studied. The purpose of this study is to clarify the influence of oscillations of holes capture efficiency on the temperature behavior of photoluminescence in modulation-doped n-AlGaAs/GaAs heterostructures. Temperature measurements of the integrated photoluminescence intensity of structures with different quantum-well widths corresponding to different capture rates of holes (under conditions of resonant and nonresonant capture) are carried out. It is shown that the efficiency of hole capture influences not only the integrated photoluminescence intensity, but its temperature dependence as well. In resonant structures at low temperatures (77–140 K), the photoluminescence intensity decreases with increasing temperature more sharply than that in structures with inefficient capture. Such a difference is attributed to the fact that, under resonance conditions, the capture rate of holes is so high that its change with temperature does not influence the photoluminescence intensity, and the temperature quenching of photoluminescence is defined only by a decrease in the efficiency of radiative recombination β in the quantum well. In nonresonant structures, the temperature behavior of photoluminescence depends not only on the quantum efficiency β, but also on the local hole capture rate which increases, as temperature is elevated. The results of this study are of interest for optimizing the parameters of heterostructures in designing of devices, for which the efficiency of the capture of nonequilibrium charge carriers into a quantum well plays a decisive role.

The physical and chemical properties of materials are greatly influenced by the state of their surface layers. In turn, the characteristics of the surface depend, for example, on the processes of absorption from a gas medium. In this study, by in situ measurements, the surface layers of polycrystalline silver exposed to external activated oxygen from different sources are investigated. For sources of activated oxygen, water vapor at a temperature of 1073 K and an ion source that makes it possible to obtain an oxygen ion beam with the energy 100–300 eV (5 μA/cm²) are used. It is established that, after treatment, the composition of AgO and Ag₂O in the surface layers of silver includes, apart from atomic oxygen, molecular oxygen and silver in the zero-valence state. From estimates of the ratio between the intensities of spectroscopy peaks related to silver and oxygen components, it can be concluded that there exist associative forms of oxygen in the surface layers of silver. An increase in Ag 3d_5/2–Ag 3d_3/2 spin–orbit splitting is detected, which suggests that there are oxidized silver nanoparticles on the surface.

Depending on the choice of the technology for producing CuIn_xGa_1 – xSe₂ films, a spread in the electrophysical and photoelectric parameters of photoconverters is observed, which is primarily related to the microstructure formed in the films and their phase composition. The investigation of phase-separation processes and the formation of a single-phase CuIn_xGa_1 – xSe₂ film is a key element in fabricating high-quality absorbing layers. In the study, CuIn_0.95Ga_0.05Se₂ thin films are obtained by the two-stage selenization method of previously synthesized copper−indium−gallium layers of different thickness in the temperature range of 350°C ≤ T ≤ 550°C. The surface morphology, chemical composition, and structure of the synthesized CuIn_0.95Ga_0.05Se₂ films are investigated via scanning electron microscopy, X-ray powder diffraction, and X-ray fluorescence. It is established that the synthesized films are polycrystalline and have a developed surface with an average crystallite size of 50–140 nm. On the basis of statistical analysis of the electron-microscopy data, the lowest temperature of the onset of the selenization process and the smallest required thickness of the metal layer for the formation of a continuous thin film CuIn_0.95Ga_0.05Se₂ are determined. The obtained films can be used as the active photosensitive layer in highly efficient solar-radiation converters.

The urgency of creating a mathematical model of the evaporation of amalgam components in discharge radiation sources (lamps) is due to the dependence of the parameters of the infrared (IR) radiation of special discharge sources on the vapor pressure of an emitting additive. The pressure in turn depends on the sizes and temperature of the discharge, cathode, and anode volumes. In connection with the diverse range of designs of gas-discharge lamps, a set of equations is proposed, which makes it possible to determine the vapor pressure of amalgam components taking into account the specific design of the discharge tube (the burner) and its temperature profile. The design of a discharge source of IR radiation for safety systems of aircraft, for which the calculation of cesium and mercury vapors is presented, is described. As a result of calculations, the dependences of the vapor pressure of cesium and mercury on the temperature of the discharge-tube cold point at various amalgam weights are obtained. The features of the increase in the cesium pressure on the transition from saturated vapors into unsaturated are revealed.

For silicon carbide, the problem of obtaining low-resistance ohmic contacts with improved service characteristics remains topical despite a significant body of experimental data. The influence exerted by the composition of Ni and TiAl metallization, alloying parameters, additional doping of the contact layer of silicon carbide (SiC) with N⁺ nitrogen ions, and the crystallographic Si- or C-face on the resistance of ohmic contacts  to n-6H-SiC  is examined.  It is  found  that the greatest influence on how ohmic contacts to n-6H-SiC are formed is exerted by the alloying process resulting in a decrease in the contact resistance by approximately six times. The process of additional doping with N⁺ also reduces the contact resistance by nearly a factor of four. It is found that low-resistance contacts can be obtained on both faces with approximately the same low resistance. TiAl metallization is optimal for the C-face, and Ni metallization, for the Si-face. This choice of metallization makes it possible to obtain ohmic contacts on both polar faces with approximately the same resistances on the order of 2.5 × 10^–4 Ω cm².

An alternative to the currently existing elemental basis for creating dynamic random-access memory and flash memory is memristor structures, i.e., two-electrode systems, whose operation is based on the resistance switching effect. In this paper, the results of studying the specific features of the production of memristor structures on the basis of copper sulfide as a promising material providing high-efficiency performance of the structures are reported. The process of copper sulfurization is considered. In this process, the surface region of the copper layer is transformed into sulfide, and the remaining part of the layer is used as the active electrode for the later formed memristor structure. It is shown that, as the concentrations of the initial chemical reagents is increased, the surface roughness of the sulfide layer significantly increases. The sulfide growth rate at optimal initial concentrations of the chemical reagents is ~30 nm/min. In studies of memristor structures, it is established that, as the copper sulfide thickness is increased, the ratio between the resistances in the low- and high-resistance states increases from 11.2 to 12.5. In the memristor structures formed in the study, the time of switching from the high- to low-resistance state is about 1.3 μs, whereas the time of switching from the low- to high-resistance state is 0.9 μs.

Modern microelectronic devices based on layered spin-valve structures have low power inputs, a high reliability, and a broad temperature range. Studying dynamic spin-valve modes and the possibilities of controlling these modes are of practical interest. In this work, the operating modes of a spin valve, which comprise the base for magnetoresistive random-access memory (MRAM), a binary stochastic neuron (p-bit), and various spin-transfer nanooscillators (STNOs) are considered. A mathematical model of the spin valve with longitudinal anisotropy placed into a magnetic field parallel to the anisotropy axis and perpendicular to the plane of layers is constructed. A set of equations that describe the dynamics of the magnetization vector of the free layer of the spin valve is derived. Quantitative analysis of the set of equations enables determination of the equilibrium positions of magnetization of the free layer for the spin-valve structure. The conditions for changing the type of singular points of the dynamic set of equations are found based on the bifurcation analysis of the set. Investigation into the dynamics of the magnetization vector of the free layer of a spin valve enables determination of its main operational modes as the component of the magnetoresistive random access memory, binary stochastic neuron, and spin-transfer nanooscillator, as well as the ranges of the current and magnetic field corresponding to these modes. The frequency and amplitude characteristics are calculated for spin-valve oscillators. The proposed structure with anisotropy located in a field perpendicular to the anisotropy axis is more preferable when compared to a structure with a field applied parallel to the anisotropy axis from the viewpoint of its application as the spin-transfer nanooscillator.

When developing long-life radioisotope sources of electricity for various applications (space industry, medicine, nano- and microsystem technology, cryptography, and telecommunications), an important problem lies in determining the temperature range of their reliable operation. The effect of negative and positive temperatures in the range from –60 to +60°C on the output parameters of radioisotope electricity sources based on double energy conversion is studied. It is shown that the open-circuit voltage of a radioisotope source changes four times in the indicated temperature ranges. In this case, the highest power delivered to the load is implemented at a temperature of about 0°C. The effect of temperature on all stages of energy conversion is analyzed. The investigations show that a significant decrease in the open-circuit voltage and the shape of the power-output curve are determined by two mechanisms: a decrease in the luminous intensity of radioluminescent sources of light (the temperature quenching of luminescence) and a decrease in the efficiency of photoconverters. Moreover, in the range of negative temperatures (from –60 to 0°C), the decrease in the efficiency of photoconverters is expressed only slightly, and the main contribution to the change in the output parameters of the sources is introduced by the temperature quenching of luminescence in the source of light. In the range of positive temperatures, both processes have a significant effect on reducing the output voltage and power. The developed radioisotope electrical-power sources based on double conversion can be used in electronic equipment operating at lower temperatures, for example, under Far-North conditions.