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.

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).

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.

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.

Basal-faceted sapphire ribbons grown using the Stepanov-LaBelle technology have a low density of surface steps. We present our results on a study of steps on the surface of a ribbon, misoriented relative to the singular face (0001) by several arc minutes. The ribbon has been characterized by means of in-line phase contrast imaging technique at Pohang Light Source, South Korea. It was shown for the first time that a step height of about 1 mu m can be determined directly from an image. The step height obtained using the phase contrast method was confirmed by atomic force microscopy measurements. We have found that the experimental contrast matches the theoretical simulations only if the calculated intensity profile has been convolved with a Gaussian function. The full width at half maximum of the Gaussian was independently got from previous measurements. We have obtained an analytical solution in the case of theoretical fully coherent phase contrast image. The inverse problem is easy to solve, since there is a direct proportionality between the contrast and the step height.

The study of thermal stresses in crystals grown from a melt is of great importance for optimizing growth regimes. The emergence of new promising materials, such as gallium oxide, requires stress calculations taking into account the anisotropy of the thermal and elastic properties of the material. A study was carried out of the influence of anisotropy on the distribution of thermoelastic stresses in thin crystalline rods of gallium oxide. Approximate formulas for the components of the stress tensor are given, obtained using the asymptotic integration of the thermoelasticity equations taking into account rectilinear anisotropy of a general form. A comparison of stress values for two growth directions was carried out. It is shown that choosing the orientation of the growth direction makes it possible to control the magnitude and distribution of thermoelastic stresses that arise in gallium oxide crystals when they are grown from a melt.

•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 method of X-ray phase contrast imaging has found wide application in coherent-synchrotron-radiation sources. In this study, this method is used in combination with X-ray diffraction topography to investigate structural defects and inhomogeneities in the volume of basal-faceted sapphire ribbons. The phase-contrast images of gas pores are analyzed in detail using computer simulations. X-ray topography methods are used to study the generation of dislocations by pores. The combination of methods provides information that is impossible to obtain using traditional optical microscopy.

A commercial epi-ready ((sic)201) beta-Ga2O3 wafer was investigated upon diamond sawing into pieces measuring 2.5 x 3 mm2. The defect structure and crystallinity in the cut samples has been studied by X-ray diffraction and a selective wet etching technique. The density of defects was estimated from the average value of etch pits calculated, including near-edge regions, and was obtained close to 109 cm-2. Blocks with lattice orientation deviated by angles of 1-3 arcmin, as well as non-stoichiometric fractions with a relative strain about (1.0-1.5) x 10-4 in the [(sic)201] direction, were found. Crystal perfection was shown to decrease significantly towards the cutting lines of the samples. To reduce the number of structural defects and increase the crystal perfection of the samples via increasing defect motion mobility, the thermal annealing was employed. Polygonization and formation of a mosaic structure coupled with dislocation wall appearance upon 3 h of annealing at 1100 degrees C was observed. The fractions characterized by non-stoichiometry phases and the block deviation disappeared. The annealing for 11 h improved the homogeneity and perfection in the crystals. The average density of the etch pits dropped down significantly to 8 x 106 cm-2.

The understanding of structural defects in basal-faceted sapphire ribbons was improved through X-ray imaging at a synchrotron source. The combination of phase contrast and X-ray diffraction makes it possible to visualize and characterize both gas voids and dislocations in the bulk of the ribbons grown by the Stepanov–LaBelle technology. Dislocations were directly related to gas voids. X-ray diffraction topography was employed to investigate the distribution, configurations, and character of the dislocations. The formation of voids of irregular shapes was detected by large-area mapping with spatial resolution in the μm range. Computer simulations of the experimental phase contrast images of microvoids were performed. The sizes of the spherical microvoids were determined. The results are discussed with reference to the available data on the emission of dislocations from the voids. The evolution of the shape, size, and arrangement of the voids during growth provides clues on the formation of block structure in basal-faceted sapphire ribbons.

Single crystalline sapphire ribbons grown by the Stepanov method exhibit rel-atively high dislocation densities and often contain slightly misoriented grains. In order to understand the formation of dislocation structures during growth, we studied neck portions cut off perpendicular to the growth axis [1010] of basal-plane-faceted ribbons. The samples have been characterized using phase-contrast and Bragg-diffraction imaging (topography) with syn-chrotron radiation. It has been found that in the growth direction from the neck towards the main body of the ribbon the dislocation density increases due to multiplication of dislocations. Combining the both imaging techniques, the dislocations were shown to be located around gas voids in sapphire crystals. Computer simulations of the phase-contrast images were carried out to obtain the correct size of the voids.

Abstract A commercial epi-ready ( ) β-Ga2O3 wafer was investigated upon diamond sawing into pieces measuring 2.5 × 3 mm2. The defect structure and crystallinity in the cut samples has been studied by X-ray diffraction and a selective wet etching technique. The density of defects was estimated from the average value of etch pits calculated, including near-edge regions, and was obtained close to 109 cm−2. Blocks with lattice orientation deviated by angles of 1−3 arcmin, as well as non-stoichiometric fractions with a relative strain about (1.0−1.5) × 10−4 in the [ ] direction, were found. Crystal perfection was shown to decrease significantly towards the cutting lines of the samples. To reduce the number of structural defects and increase the crystal perfection of the samples via increasing defect motion mobility, the thermal annealing was employed. Polygonization and formation of a mosaic structure coupled with dislocation wall appearance upon 3 h of annealing at 1100 °C was observed. The fractions characterized by non-stoichiometry phases and the block deviation disappeared. The annealing for 11 h improved the homogeneity and perfection in the crystals. The average density of the etch pits dropped down significantly to 8 × 106 cm−2.

The process of growing crystals of the NiFeGaCo alloy by the Stepanov method has been developed. It was found that the structural perfection is disturbed by the formation of dendrites, which are distributed inhomogeneously both along the length and in the cross sections of the crystals. The blocking effect of the dendrites on growth crystals of the martensite phase, which appears when the samples are cooled to the temperature of the transition of the crystal to the martensite state, is found. The elemental composition of dendritic formations was studied and it was shown that the iron content in the dendrite is approximately 30% higher, and the gallium content is 40% lower than in the matrix. Based on the modeling of heat transfer processes in the real growth zone, taking into account the experiments performed, recommendations were obtained for suppressing the formation of dendrites Keywords: Dendritic structure, shape memory alloys, Stepanov's method.

The paper deals with the measurement of residual stresses formed in shaped sapphire single crystals grown from the melt. The algorithm for calculating the normal components of the residual stress tensor from the optical conoscopic measurement data is developed for optically uniaxial crystals in the shape of wafers. Examples of measurement of residual stresses and calculation of the normal components are given for a square rod and a basal-faceted sapphire ribbon. It is shown that normal stresses are mainly concentrated in the periphery of wafers and reach 25 MPa in the rod and 70 MPa in the ribbon.

We present a Synchrotron X-ray tomography investigation of sapphire crystals of a complicated shape. The concept of crystal shaping while the growth from the melt allows avoiding tooling and associated loss of material. However, the elimination of gas bubbles and dislocations remains a severe problem. In the present communication, we show and analyze the distributions of bubbles in sapphire tubes and ribbons grown with the Stepanov technique. We approve that microtomography implemented with Synchrotron Radiation using a conventional algorithm can give good results concerning the visualization of µm-sized bubbles.

Long crystals of NiFeGaCo alloy with shape memory effect, including magnetically controlled ones, were obtained by the methods of Czochralski and Stepanov. A strong influence on the properties of crystals of dendritic formations, especially noticeable in the initial part of the crystal, has been revealed. In order to optimize the growth experiments, the heat transfer process in the thermal growth zone was simulated. It is shown that the formation of dendrites is due to a change in heat transfer during growth, which leads to an increase in the axial temperature gradient near the crystallization front as the crystal grows. This fits into the framework of the classical concepts of the transition from dendritic growth to normal growth.

The process of growing crystals of the Ni-Fe-Ga-Co alloy by the Stepanov method has been developed. It was found that the structural perfection is disturbed by the formation of dendrites, which are distributed inhomogeneously both along the length and in the cross sections of the crystals. The blocking effect on the dendrites of crystals of the martensite phase, which appears when the samples are cooled to the temperature of the transition of the crystal to the martensite state, is found. The elemental composition of dendritic formations was studied and it was shown that the iron content in the dendrite is approximately 30% higher, and the gallium content is 40% lower than in the matrix. Based on the modeling of heat transfer processes in the real growth zone, taking into account the experiments performed, recommendations were obtained for suppressing the formation of dendrites

The article deals with the measurement of residual stresses that are formed in shaped sapphire single crystals when they are grown from a melt. An algorithm is developed for calculating the normal components of the residual stress tensor from the data of optical measurements of residual stresses by the conoscopy method for optically uniaxial crystals in the form of plates. Examples of measuring residual stresses and calculating normal components for a square rod and a basal-faceted sapphire ribbon are given. It is shown that normal stresses are concentrated mainly along the periphery of the plates and reach up to 25 MPa in the rod and 70 MPa in the strip.

The paper considers the experimentally observed instability of capillary shaping during the growth of thick-walled sapphire tubes by the Stepanov method. The explanation of this phenomenon is based on the theoretical model of radiative-conductive heat transfer in a crystal. An algorithm is developed for the asymptotic expansion of the problem based on the presence of two small parameters. It is shown that the spatial density of the radiation is inhomogeneous along the cross section of the tube and is maximum near its inner walls. This leads to their overheating and the meniscus separation from the inner edges of the shaper.