Homoepitaxial β‐Ga2O3 films grown by mist‐chemical vapor deposition technique utilizing Ni mask as a buffer interlayer are studied. The films are successfully grown on (100) and (01) oriented gallium oxide substrates upon Ni deposition with subsequent formation of Ni sub‐micron‐island mask. The thicknesses of the (100) and (01) Ni/β‐Ga2O3 films are estimated as 4 and 1.5 μm, respectively. The films grown on Ni/(01) β‐Ga2O3 wafers have relatively high crystal perfection, while films grown on Ni/() β‐Ga2O3 wafers are characterized by high growth rate. Although the proposed approach deteriorates the surface morphology and crystallinity of homoepitaxial films compared to the initial substrates, it can be applied to implement some exfoliation techniques, since the epitaxial layer has a large thickness and acceptable crystal perfection.

A thick κ-Ga2O3 layer grown by halide vapor phase epitaxy on the smooth sapphire substrate was used for the study of the recombination and structural properties of the gallium oxide κ-phase and its 2D defects. Scanning electron microscopy (SEM) revealed two distinct regions on the sample’s surface: flat areas and trigonal-like facets. Cathodoluminescence (CL) imaging revealed that facets exhibit enhanced bright contrast together with small irregular square-like shapes in the flat areas. CL spectra demonstrated noticeable differences in their shapes between flat areas and the facets: The characteristic for κ-Ga2O3 2.95 eV peak was significantly reduced for the flat part of the layer. Analysis of the structure via transmission electron microscopy (TEM) in a plan-view lamella showed that facets on the surface correspond to relatively large κ-Ga2O3 domains organized in a pseudotrigonal manner. This phenomenon should be considered as a fully-fledged pseudomorphism of κ-Ga2O3 repeating symmetry of the etched sapphire morphology. Domains are divided by the twin domain boundaries (TDBs) and have dense regular arrays of the parallel antiphase boundaries (APBs) inside them. TEM analysis of the flat areas revealed a large number of the unequally sized smaller domains containing only some separate APBs. This structure results in both a significantly lower density of APBs and a higher one of the TDBs in flat areas than in facets. Thus, it is proposed that arrays of electrically active TDBs partially quench the 2.95 eV band and lead to the electrically isolating nature of the sample in lateral directions.

Deep trap spectra and carrier diffusion lengths were measured for unintentionally doped β-Ga2O3 bulk crystals with (100) orientation. The 20-mm diameter, 15-mm length boule was pulled by the Czochralski method from gallium oxide in (010) direction. It is found that the net density of shallow donors in (100) plates cleaved from the crystal was 2.6 × 1017 cm−3, with ionization energies of 0.05 eV measured from admittance spectra. Three deep electron traps with respective ionization energies of 0.6 eV (concentration 1.1 × 1014 cm−3), 0.8 eV (concentration 3.9 × 1016 cm−3) and 1.1 eV (concentration 8.9 × 1015 cm−3) were detected by Deep Level Transient Spectroscopy. The dominant 0.8 eV trap is associated with the E2 centers due to Fe acceptors, the two other traps are the well documented E1 and E3 centers. The major deep acceptors in the lower half of the bandgap have optical ionization threshold of 2.3 eV and concentration of 4 × 1015 cm−3 and are believed to be due to the split Ga vacancies acceptors. The diffusion length of non-equilibrium charge carriers was 90 nm. The electrical properties of these (100) oriented crystals grown by Czochralski are quite similar to those synthesized by the undoped Edge-defined Film-Fed Growth technique.

Heterojunctions (HJs) of p-NiO/n-Ga2O3 were prepared by deposition of thin films of p-NiO by ion beam sputtering on bulk nominally undoped (100) oriented n-Ga2O3 samples cleaved from Czochralski-grown (010) oriented crystal. Electrical properties and deep traps spectra were studied and compared with those obtained for similar samples with Ni Schottky diodes. Characteristic features of the NiO/Ga2O3 HJs are the built-in voltage of over 2 V compared to 1 V in Ni Schottky diodes, a strong frequency dispersion of capacitance due to the presence of high density over 10(18) cm(-3) of E-c-0.16 eV traps in the thin region adjacent to the NiO/Ga2O3 interface. For heterojunction diodes the strong increase of reverse current occurs at a much higher voltage than for Schottky diodes (120 V versus 60 V).

Micron-thick layers of (Cr1-xGax)2O3 solid solutions were grown by modified mist chemical vapor deposition (mist-CVD) with three different Ga concentrations. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods were used to analyze the quality of the films. They showed good crystallinity, homogeneity and coalescence of the samples. Solid solution contents estimation was performed via applying Vegard’s law to XRD data and by its results the highest reached Cr:Ga ratio is approximately 1:1. Transmission spectra of solid solutions demonstrated blue-shift of the absorption edge with increase of the Ga contents. Optical bandgap increased from 3.06eV for undoped Cr2O3 sample to 3.73eV for the layer with the highest Ga concentration.

Представлены результаты экспериментального исследования реальной структуры тонких пленок κ-фазы оксида галлия. Методами дифракции обратно отраженных электронов в сканирующем электронном микроскопе и просвечивающей электронной микроскопии установлено, что микро-монокристаллы κ-оксида галлия состоят из совокупности трех типов поворотных доменов орторомбической симметрии, повернутых друг относительно друга на угол 120° вокруг оси роста. Монокристаллические домены характеризуются большой плотностью прямолинейных антифазных границ, формирующих при своем пересечении структуру значительной доли доменных границ.

Pt/(100) β-Ga2O3 Schottky barrier diodes were fabricated using a plate cleaved from the crystal grown by Czochralski method. Their electroconductive and photoelectric characteristics were studied. The following values were obtained: the Schottky barrier height (1.69/1.62/1.74 eV), ideality coefficient (1.09/1.14), saturation current density (9.91 × 10–15 A/cm2), diode series resistance (7.98 kΩ), and net donor concentration [(1.8–2.4) × 1018 cm–3]. The diodes demonstrate a high rectification ratio of 1010 at an applied voltage of ± 1 V and a relatively low experimental value of the leakage current density ∼10–11 A/cm2. These structures are solar-blind and also capable of operating in self-powered mode. The diodes are highly sensitive to short-wave ultraviolet radiation with a wavelength λ ≤ 265 nm. The maximum values of responsivity (20.4 A/W), external quantum efficiency (1.2 × 104%), and detectivity (9.6 × 1015 Hz0.5 × cm × W–1) of diodes were registered under exposure to irradiation at λ = 210 nm and at applied voltage of −1 V. The responsivity and external quantum efficiency values in the self-powered operation mode were 12.3 A/W and 7.2 × 103%, respectively. The diodes showed low rise and decay times in self-powered operation mode for photodiode based on Ga2O3: 14 and 30 ms, respectively.

Thick β-Ga2O3 homoepitaxial films have been grown on (2¯01) commercial substrates by mist-CVD with gallium acetylacetonate precursor for the first time. The growth rate of about 2 μm/h has been reached, which is unavailable for any other known epitaxial technique. The layer is characterized by the constant thickness and reasonable structure quality due to low stressed interface.

Gallium Oxide is a prospective ultra-wide band -gap semiconductor for high -voltage electronics. Bulk beta-Ga2O3 crystals can be utilized as substrates for device structures. The biggest challenge encountered is to grow low defect density wafers with high crystal perfection. In this work a boule with a diameter of about 20 mm and a height of 20 mm was grown by Czochralski method. The separate plates with dimensions of approx. (10 x 5 x 3) mm3 were cleaved out from the crystal along the (100) cleavage planes. XRD investigation demonstrated that the crystal is a monoclinic single -phase structure which is characterized by broad rocking curves. The etch pits density revealed by selective wet etching appeared to be high and was estimated as 2 & sdot; 107 cm - 2. The series of post-growth heat treatments was applied to eliminate these drawbacks. Annealing at 1100 degrees C during 5 h had the most effect on the crystal structure. Namely, its coherent-domain-size value increased over 400 nm, domain misorientation dropped below the arcminute. The crystal developed higher stoichiometry and higher crystalline perfection. Annealing at the same temperature, but for duration of 11 h dramatically worsen all the parameters of the crystal in combination with its fragmentation in smaller-sized domains. The etch pits density finally decreased 40 -fold and took a value of 5 & sdot; 105 cm - 2.

Record thick (up to 100 μm) epitaxial layers of a prospective metastable semiconductor Ga2O3 were grown by HVPE (Halide Vapor Phase Epitaxy) on GaN buffer layers on c-sapphire substrates. The X-ray diffraction pattern of the layers show that the structure of the layer is a pure κ(ε)-Ga2O3 without any other phases. At the same time, the organization of a domain structure was observed, which manifests itself in the form of pseudohexagonal prisms that retain the orientation of the gallium nitride sublayer. Schottky diodes with nickel contacts were fabricated and the electrical and photoelectric properties of the layers were studied. Capacitance–voltage (C–V) and frequency–capacitance (C–f) dependencies were studied, photocurrent and photocapacitance spectra were measured.

•Gallium Oxide (β-Ga2O3) crystals are utilized as substrates for high-power devices.•Thermal annealing was applied to enhance the perfection of bulk (100) β-Ga2O3 crystals.•Annealing led to increase in coherent-domain-size value and decrease in domain misorientation.•Annealing led to development a higher stoichiometry and higher crystalline perfection.•Annealing decreased etch pits density 40-fold.

The results of an experimental study of the real structure of thin films of κ-phase gallium oxide are reported. It has been established by electron backscattering diffraction in a scanning electron microscope and by transmission electron microscopy that gallium oxide single microcrystals consist of three types of rotating domains of the orthorhombic symmetry, which are rotated relative to each other around the growth axis by an angle of 120°. Single-crystal domains are characterized by a high density of straight antiphase boundaries, which, when intersecting, form a significant fraction of the domain wall structure.

The structural properties of twin domain boundaries (TDB) and antiphase boundaries (APB) in individual thin, hexagonal prismatic microcrystals of kappa-Ga2O3 grown on GaN/sapphire template with HVPE were investigated with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The microcrystals were composed from the domains with three in-plane 120 degrees rotational orientations. It was found that every individual twin domain contains a parallel array of APBs of a high density stretched in the [010] direction. APBs possess steps or interruption and can form double oppositely shifted spatially separated layers (APB dipoles). TDBs on majority of their length are incoherent being not strictly flat and serve as the border for the APBs interruptions. Panchromatic cathodoluminescence mapping of the microcrystals revealed that not all TDBs and APBs reduced its intensity. The interruptions and steps of APBs were proposed to be the main origin of the excess charged carrier recombination. A model of the atomic structure in the vicinity of the defects is proposed and the assumption about the formation of local high strain regions and dangling bonds was made.

The MSM structures based on high-quality 1.6- $\mu \text{m}$ -thick $\alpha $ -gallium oxide (Ga2O3) films grown by the halide vapor phase epitaxy with Ti/Ni interdigital contacts were developed for the detection of short-wave ultraviolet (UVC) radiation. The spectral dependences of responsivity, external quantum efficiency (EQE), and detectivity of MSM structures based on $\alpha $ -Ga2O3 were studied in the wavelength range of 205–260 nm. The responsivity, the EQE, and the detectivity are $7.19\times104$ A $\times \,\,\text{W}^{-{1}}$ , $3.79\times105$ arb.un., and $1.12\times1018$ Hz $^{\text {0.{5}}} \times $ cm $\times \,\,\text{W}^{-{1}}$ , respectively, for structures with an interelectrode distance of 30 $\mu \text{m}$ at an applied voltage of 10 V and exposure to radiation with a wavelength of 235 nm. The high values of the photoelectric characteristics were caused by the manifestation of an internal gain of the photoresponse due to the self-localization of holes in $\alpha $ -Ga2O3. The prospects of researches to develop UVC radiation detectors for wireless UVC communication were shown.

Record thick (up to 100 μm) epitaxial layers of a prospective semiconductor metastable Ga2O3 were grown by HVPE (Halide Vapor Phase Epitaxy) on GaN buffer layers on c-sapphire substrate. The X-ray diffraction spectra of the layers show that the structure of the layer is a pure k(ε)-Ga2O3 without any other phases. At the same time, the organization of the domain structure is noted, which manifests itself in the form of pseudohexagonal prisms with the inheritance of the orientation of the gallium nitride sublayer. Schottky diodes with a nickel contact were fabricated and the electrical and photoelectric properties of the layers were studied. Capacitance-voltage (C-V) and frequency-capacitance (C-f) dependencies have been studied, photocurrent and photocapacitance spectra have been measured.

The electrical conductivity of pseudohexagonal ε(κ)‐Ga 2 O 3 films under different ambient gases (H 2 , NO 2 , O 2 , and CO) is studied in a range of temperatures from 400 to 550 °C. The exposure of ε(κ)‐Ga 2 O 3 to reducing gases such as H 2 and CO results in a reversible increase in current and conductance. The exposure to the oxidizing gases such as NO 2 and O 2 has the opposite effect. The maximum response to reducing gases (H 2 and CO) is observed at 500 °C and to oxidizing gases at 550 and 450 °C for NO 2 and O 2 , respectively. The highest sensitivity to H 2 is achieved at low applied voltages (≤7.9 V). In contrast, the highest sensitivity to NO 2 is observed at high applied voltages. The response and recovery times and temporal drift of ε(κ)‐Ga 2 O 3 characteristics under different ambient are estimated. Polycrystalline ε(κ)‐Ga 2 O 3 exhibits the semiconducting mechanism of electron transport at high temperatures. A qualitative model of the gas‐sensing effect based on the modulation of electron concentration near the surface region of ε(κ)‐Ga 2 O 3 due to the chemisorption of gas molecules is described. Tin doping of ε(κ)‐Ga 2 O 3 increases the response to H 2 at the temperature range from 25 to 550 °C.

Herein, the influence of the oxygen flow on the formation of metastable polymorphs of gallium oxide (Ga 2 O 3 ) grown by halide vapor phase epitaxy (HVPE) on c‐plane patterned sapphire substrates (PSS), on gallium nitride (GaN) templates, and on m‐plane smooth sapphire substrates is reported. X‐ray diffraction, scanning electron microscopy, and cathodoluminescence are used to identify different polymorphic phases. The samples deposited on bare PSS exhibit faceted growth of the α‐Ga 2 O 3 on the cones of the sapphire substrate and the formation of the κ‐Ga 2 O 3 between the cones. In contrast, growth on GaN templates results in hexagonal columns of κ‐Ga 2 O 3 which produce a continuous smooth layer upon coalescence. The growth of Ga 2 O 3 on m‐plane sapphire substrates results in overgrown pyramids of the α‐phase. For all types of substrates, the variation of the oxygen flow affects only the thickness and coalescence of Ga 2 O 3 layers. Thus, the growth of Ga 2 O 3 metastable polymorphs is mainly influenced by the form, orientation, and symmetry of the substrate and shows a weak dependence on stoichiometry.

Dislocations introduced by Vickers tip microindentation of an a-plane free-standing semi-insulating Fe-doped GaN halide vapor phase epitaxy (HVPE) crystal were investigated by means of cathodoluminescence and scanning transmission electron microscopy techniques. Detailed combined analyses of both spectral properties and the core structure of the introduced a-screw dislocations revealed that Fe-doped GaN exhibit not only dislocation-bound emission at ∼3.35 eV of perfect a-screw dislocations previously found in such kind of samples but also luminescent bands at 3.1–3.2 and 3.3 eV due to dissociated a-screw dislocations and extended dislocation nodes previously observed only in low-resistance n-GaN. For the first time, all these luminescent bands were observed together in the same sample. Structural studies revealed the coexistence of the dislocations with the dissociated and the perfect core as well as with extended dislocation nodes, thus establishing a correlation between previously observed luminescence bands and a fine dislocation core structure.

Freshly introduced a-screw dislocations in gallium nitride are an effective source of ultraviolet radiation, characterized by intense emission of narrow luminescence doublet lines in the spectral range of 3.1-3.2 eV. Furthermore, an additional narrow spectral line with an energy of 3.3 eV has been found at the points of intersection of such dislocations, where extended dislocation nodes were formed. In this communication, we report on the spectral properties of the characteristic luminescence of such nodes, which were obtained for the (0001) gallium nitride samples with dislocations introduced by nanoindentation. The spectral position of the dislocation-related luminescence doublet experiences a redshift with increasing distance from the indentation site. It follows the spectral shift of the excitonic near-bandgap emission, associated with stress relaxation. The luminescence of the intersection points exhibits a similar tendency. At certain local positions, its doublet fine structure is observed, which has a spectral linewidth of the order of or even less than that of the exciton. In this case, the spectral splitting between components of the doublet varies irregularly depending on the position of the exciton (i.e., on the mechanical stress). We see a clear indication of quantum dot-like emission. The fine structure of the luminescence of the intersection points can be easily explained by the energy dependence of emission on their size, as well as on their density, in particular, by the formation of paired nodes, which were previously observed in experiments in a transmission electron microscope.

The growth of Ga 2 O 3 films by halide vapor phase epitaxy on plain and cone‐shaped patterned sapphire substrates (PSS) is reported. The obtained specimens are characterized by X‐ray diffraction, transmission electron microscopy, cathodoluminescence, optical transmission spectroscopy, and current–voltage measurements. Both types of Ga 2 O 3 layers are of reasonably high crystal qualities; their physical properties, however, are very different. Under the same conditions, the growth on plain substrates results in a continuous α‐Ga 2 O 3 layer, whereas the growth on PSS produces a regular array of α‐Ga 2 O 3 columns on top of the sapphire cones with the space between them filled with ε‐Ga 2 O 3 . Ga 2 O 3 films grown on plain sapphire are insulating; in contrast, Ga 2 O 3 films grown on PSS are conducting. It is found that the conductivity of Ga 2 O 3 on PSS follows the Arrhenius law with the activation energy of 0.33 eV. New luminescent bands for α‐ and ε‐phases are found. Spectral components of the defect‐related luminescence for α‐ and ε‐ phases are identified.