Institute of Advanced Data Transfer Systems

The article studies the influence of aluminum on the lattice parameters and the unit cell volume of the binary solid solution β-(AlxGa1-x)2O3. The aluminum content is determined by energy-dispersive X-ray spectroscopy. The lattice parameter values are calculated using X-ray diffractometry data. The diffraction patterns show peak broadening and shift to higher angles with increasing aluminum content. It is shown that when the aluminum fraction x changes from 0 to 0.25, the lattice parameters decrease linearly. This is consistent with Vegard's law for solid solutions. The decrease in cell volume is physically related to the fact that aluminum has a smaller ionic radius than gallium, and the Al-O bond length is shorter than Ga-O one.

Bulk crystals of β-Ga 2 O 3 were successfully grown by the edge-defined film-fed growth method. The crystalline quality of the obtained crystals was analyzed by the method of x-ray diffractometry. The full width at half maximum of the rocking curve was about 72 arcsec. The optical bandgap was determined by analyzing the optical transmission spectra and amounted to 4.7 eV. The hot disk method was used to obtain the thermal conductivity of the sample along the [001] direction in the temperature range from 30 to 120 °C. The maximum value of thermal conductivity obtained at 30 °C was 9.25 W/(m K).

The work presents the results of the experiments on annealing in air of bulk crystals of gallium oxide grown in Ar+O2 and CO2 atmospheres at a temperature of 1400 °C. The annealing time was 5 hours; the time to reach the temperature was 3.5 hours; the cooling time was 20 hours. Annealed samples show increasing of transmission in infrared area of electromagnetic spectrum and decreasing of width of X-ray rocking curve which means the reduction of the number of defects in crystals. Full width at half maximum of rocking curve for annealed samples was almost the same for both atmospheres: FWHMa = 84 arcsec for sample grown in Ar+O2 atmosphere and FWHMa = 80 arcsec for sample grown in CO2, which means that after annealing, the quality of the samples became comparable, despite the initial difference. KEYWORDS β-Ga2O3 • bulk crystal • annealing • optical spectroscopy • XRD

Bulk crystals of β-Ga 2 O 3 were successfully grown by the EFG (Stepanov) method. Analysis of the material using an X-ray diffraction showed the high crystalline quality of the obtained crystals. However, when determining the elemental composition by the Energy Dispersive X-ray Spectroscopy (EDS) method, a deviation of the crystal composition from a stoichiometric one was found and a lack of oxygen was detected. Indirectly, this indicates the non-optimal composition of the atmosphere in the growth zone. The origin of this deviation of the composition can be oxygen vacancies in the bulk of the crystal. However, the absorption spectrum does not contain peaks characteristic for oxygen vacancies in gallium oxide. An analysis of the optical transmission spectra made it possible to estimate the optical bandgap of the grown gallium oxide samples, which was 4.7 eV.

In this article, we report on fabricating thin solid films of gallium oxide by the spray-pyrolysis method. This method allows obtaining uniform thin films more easily compared with other sol-gel methods like spin-coating or dip-coating. In the experiment, sol concentrations were experimentally selected for further deposition on substrates. Morphology and chemical composition of the deposited films were studied by Scanning Electron Microscopy and Energy-Dispersive X-ray spectroscopy, respectively. The structural properties of the films were analyzed by X-ray diffraction method. The band gap of the Ga2O3 films was estimated by analyzing the optical transmission spectra and was 4.87 eV. The quality and homogeneity of the obtained coatings are evaluated.

The paper proposes a method for fabricating gallium oxide substrates from bulk β-Ga2O3 crystals by the cleavage method. Layers of β-Ga2O3, β-(AlxGa1-x)2O3 and structures of β-Ga2O3/β-(AlxGa1-x)2O3 are grown on the prepared substrates by the MOCVD method. The surface morphology of the layers and growth regimes are analyzed. The fundamental possibility of using gallium oxide substrates obtained by the cleavage method for the subsequent epitaxy is shown.

Proton irradiation of β-Ga 2 O 3 crystals has been established to lead to a significant increase in the amount of Cr 3+ ions being active in luminescence. Using angle-resolved luminescence, the features of the spectra of Cr 3+ ions were studied. The high sensitivity of photoluminescence spectra and the related selection rules to the local symmetry of Cr 3+ ions in the β-Ga 2 O 3 matrix has been found. The results obtained indicate the potential possibility of using β-Ga 2 O 3 crystals as optical dosimeters of proton irradiation.

This work is devoted to the study of the luminescence inhomogeneity nature of bulk (Gax Al 1-x) 2 O 3 samples grown by the Czochralski method. In the study of sample cleavages by the local cathodoluminescence method, regions with different luminescence were observed. To determine the cathodoluminescence contrast nature, we studied the uniformity of the aluminum distribution, the surface topography, and compared the luminescence spectra and the kinetics of emission bands for different regions of the sample. Also, to determine the luminescence bands nature, the crystal was annealed in air at 1000 • C. This made it possible to observe the change in luminescence for the same region of the sample. Based on the studies performed, it was concluded that inhomogeneous luminescence is associated with the distribution of point defects. Upon annealing in air, the transformation of nonradiative recombination centers into luminescent centers was observed.

Bulk β-Ga 2 O 3 samples grown by the Czochralski method are studied. Based on the studies of the absorbed current dynamics and the cathodoluminescence, it is shown that there is a localization of charges of both signs in the sample. The electron localization is demonstrated to lead to a substantial decrease in cathodoluminescence intensity.

A method is proposed for increasing the radiation resistance of semiconductor crystals (Al x Ga 1-x) 2 O 3 , which are promising for creating power electronics and optoelectronic devices for spacecraft performing tasks in near-earth orbits. The effect of the aluminum content on the bandgap in (Al x Ga 1-x) 2 O 3 single crystals grown from a melt by the Czochralski method is investigated. The necessary conditions for the formation of a high-quality seed and subsequent growth of single crystals are achieved by setting temperature gradients at each successive stage of growth. The results of energy-dispersive X-ray spectroscopy analysis and optical transmission spectra of (Al x Ga 1-x) 2 O 3 crystals with an aluminum content from 0 to 7.32% are presented. It is shown that an increase in the aluminum content leads to a monotonic increase in the bandgap from 4.7 eV to 5.0 eV, which provides an increase in radiation resistance.

The aluminum distribution in the surface layer of an (Al x Ga 1-x) 2 O 3 crystal grown on a sapphire seed by the Czochralski method has been studied using X-ray fluorescence and X-ray diffraction mapping and scanning electron microscopy. It is shown that the aluminum distribution correlates with a change in the physical broadening of the diffraction maxima of the (h00) plane system of the crystal and is related to a distance from the seeding zone.

Results of successful experiments on the growth of gallium oxide (β-Ga 2 O 3) crystals by the Czochralski method are reported. The influence of growth atmosphere on the crystalline perfection of obtained material has been studied. It established that high-quality transparent optical crystals can be obtained provided the atmosphere contains about 5 vol % oxygen. As-grown β-Ga 2 O 3 crystals were characterized by X-ray diffraction and optical transmission measurements.

Photocatalysts are currently widely used in various research and industrial fields, from water and air purification to solar energy, from self-cleaning surfaces to wearable biosensors and electronic devices. Among semiconductors, the growing interest is attracted to gallium oxide, especially β-Ga2O3, due to the unique physical, chemical (especially acid-resistance), physical–chemical, photochemical properties, and redox potential. These photocatalysts can be used not only as suspended mixture, but also in the form of films, nanoparticles, and nanofibers. The last one is more challenging due to the high level of surface-to-volume ratio, porousness, air and water permeability, and excellent morphological and physical–mechanical properties. Moreover, electrospinning techniques, in comparison with fabrication of nanoparticles, allow to obtain photocatalytic devices without usage of additional carrying base, which simplify the industrial process. However, to date there are limited publications related to the Ga2O3 electrospun
nanofibers. The aim of this review is to summarize current results of
gallium oxide electrospun nanofibers preparation. Special attention is given to the technological parameters of the electrospinning, the polymer solution receipts, and the properties of such nanomaterials and its potential application. Despite the limited number of publications related to Ga2O3 fibers fabrications, that data collected in this review article demonstrate great potential for practical applications.

The article presents the results of the study of the mechanical properties and defect structure of gallium oxide (Ga2O3) by using the piezoelectric composite oscillator technique. Bulk samples of the Ga2O3 beta phase in the form of single crystals and their intergrowths were obtained by growth from a melt with a shaper (Stepanov technique). The research involved studying the dependences of the longitudinal elastic modulus and the damping of elastic vibrations at a frequency of 100 kHz on the strain amplitude. Changes in the elastic and microplastic properties of the samples at different temperatures were attributed to possible relaxation phenomena in the structure of the material. Studying the defect structure in samples of pure and doped Ga2O3 is necessary to improve the technology for the production of large single crystals. The fundamental questions in this area are the influence of defects on the anisotropy of electrical conductivity, band structure, and other functional properties of the resulting semiconductor material. The purpose of this article is to establish the features of sample preparation, research, and interpretation of the results obtained by the piezoelectric composite oscillator technique for gallium oxide samples. In the studied samples, the first longitudinal vibration mode was excited, which corresponded to a length of about 27 mm and a small cross-section of the sample. The temperature dependences in the region of low and high strain amplitudes were determined separately. The crystalline quality of the prepared samples was assessed by X-ray diffraction with the analysis of the rocking curve. The value of Young's modulus obtained along the growth axis (crystalline orientation <010>) in Ga 2 O 3 crystals E≈260 GPa is in line with the results of previous studies. Relaxation peaks corresponding to various dislocation interactions were found on the temperature dependences of internal friction at a temperature of 280 K.

In this review, we consider the main gallium oxide areas of application in electronics and optoelectronics with focus on power electronics devices (rectifiers, field effect transistors), solar-blind photodetectors, luminescent devices, gas sensors, spintronic and memory devices. As an introduction, we provide the valuable data on the basic physical properties of the existing Ga2O3 polymorphic modifications. We discuss device design based on various gallium oxide crystalline forms including those exploring Ga2O3 single crystals, thick layers, thin films, nanostructures, and Ga2O3-based heterostructures. Then, the information on the parameters and characteristics of electronic and optoelectronic devices based on gallium oxide is presented. In addition, recently emerging and requiring additional research Ga2O3 application fields such as photocatalysis and thermomechanical actuating, are briefly addressed.

In this paper, we study the process of growth from the melt by Chokhralsky method of bulk β-Ga2O3 crystals. The effect of different configurations of growth zones and ambient composition on resulting bulk crystal quality has been evaluated. It is shown that the vertical gradient has a great influence on the quality of the growing crystal and the stability of growth. The optical properties of obtained β-Ga2O3 crystals were investigated. The crystallographic orientation of crystal faces and the structural quality of the samples were studied with the use of XRD.

Self-organization mechanisms promoting elimination of cracks in thick GaN layers grown on sapphire substrates are considered on the basis of the experimental results on the fabrication of the layers by Hydride Vapor-Phase Epitaxy on MOCVD-grown GaN/Al 2 O 3 templates. The obtained data support the supposition on the closure of tensile stress-related cracks via diffusion processes and demonstrate the strong contribution of bulk diffusion in addition to surface diffusion discussed earlier.

We propose a technuque of liquid-phase growth of (Al x Ga 1-x) 2 O 3 crystals with variable and controlled Al content in them. When using the Czochralski growth process Ga 2 O 3 melt was dosed by sapphire seed. By applying of the special growth zone and the regulating the process parameters, a series of crystal samples with Al content varying from 0.51 to 4.68 % at. was obtained. In addition to the standard setting of the geometry and weight of the crystals using the process parameters, with an increase of the Al content in the melt, it is possible to control the color, transparency and crystallinity of the fabricated samples.

Laser radiation coupling into a single-mode optical fiber was numerically simulated. Various options for matching the source of laser radiation with the optical fiber are considered, including direct coupling, insertion of a microlens at the end face of the optical fiber, and adding a lens between the radiation source and the optical fiber. Optimal parameters of matching elements for coupling the radiation of an edge-emitting laser diode and a vertically emitting laser into a single-mode optical fiber are determined, making it possible to obtain calculated coupling efficiencies of 65% and 100%, respectively.

This paper presents the study of oxide film heterojunction made of CuAlCrO 2 and ZnO:Al (AZO) deposited on fused quartz by sol-gel spin coating. Transmission electron microscopy (TEM) analysis has showed that the solvent, which was used to prepare the AZO sol, affects the thickness and planarity of the applied layers and the grain size of the polycrystalline film. The formation of a fine-grained structure of AZO films (grain size is up to 12 nm) leads to the formation of a smoother p-CuAlCrO 2 / n-AZO heterojunction. X-ray diffraction (XRD) analysis and TEM have revealed that the heterojunction was formed by AZO polycrystals, which orient axes [101] and [100] against the [101] and [006] directions in CuAlCrO 2 polycrystals. The optical transmission of the heterojunction in the visible range reached 70 %. The current-voltage (I-V) characteristics of p-CuAlCrO 2 / n-AZO corresponds to a diode one in the range from 4 up to 4 V. For the large values of the reverse bias, an increase in the leakage current is observed, which is associated with tunneling breakdown of heterojunction. The ideality factor of fabricated p-CuAlCrO 2 / n-AZO heterojunction is more than 2, which indicates the mechanisms of conductivity by defect-levels. In additional, it has been observed the response of the current-voltage characteristics to the green illumination (532 nm) of the diode that is associated with indirect band-gap absorption in CuAlCrO 2. The impact of the white LED illumination had no effect on the current-voltage characteristics of the diode.