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The growth of bulkx β-Ga 2 O 3 single crystals by the Czochralski method is reported and discussed in terms of crucial growth conditions and correlated with basic electrical and optical properties of the obtained crystals. β-Ga 2 O 3 crystals have a tendency to a spiral formation due to free carrier absorption in the near infrared (NIR) wavelength range, which hampers radiative heat transfer through the growing crystal. Moderate or low free electron concentrations (<5×10 17 cm −3 ) lead to cylindrical crystals with a high crystallized fraction ( g ≥0.5). The use of a CO 2 -containing growth atmosphere provides oxygen partial pressures between 0.8 and 4.4×10 −2 bar that is sufficient to obtain cylindrical and semiconducting crystals. Doping with Sn increases the free electron concentration in the crystals to high values (~10 19 cm −3 ) that lead to an immediate spiral formation, while doping with Mg (>6 wt ppm) provides insulating crystals with reduced probability of the spiral formation. The estimated Mg equilibrium segregation coefficient across the liquid–solid interface is 0.10–0.12. Annealing of undoped crystals in an oxidizing atmosphere at temperatures ≥1200 °C for 20 h decreases the bulk free electron concentration by about one order of magnitude, while the crystal surface becomes insulating. However, Mg:β-Ga 2 O 3 crystals are insensitive to annealing in both oxygen- and hydrogen-containing atmospheres. The transmittance spectra showed a steep absorption edge at 260 nm and virtually full transparency in the visible and NIR wavelength range for low and moderate free electron concentrations. We also demonstrated the possibility of growing 2 in. diameter β-Ga 2 O 3 single crystals by the Czochralski method. The good crystal quality is evidenced by rocking curve FWHM values of below 50". We noted that most dislocations propagate parallel to (100) plane. Further, we also provide thermal properties of the crystals as a function of temperature. [ABSTRACT FROM AUTHOR]

In the course of development of transparent semiconducting oxides (TSOs) we compare the growth and basic physical properties bulk single crystals of ultra-wide bandgap (UWBG) TSOs, namely β-Ga 2 O 3 and Ga-based spinels MgGa 2 O 4 , ZnGa 2 O 4 , and Zn 1-x Mg x Ga 2 O 4. High melting points of the materials of about 1800 -1930 °C and their thermal instability, including incongruent decomposition of Ga-based spinels, require additional tools to obtain large crystal volume of high structural quality that can be used for electronic and optoelectronic devices. Bulk β-Ga 2 O 3 single crystals were grown by the Czochralski method with a diameter up to 2 inch, while the Ga-based spinel single crystals either by the Czochralski, Kyropoulos-like, or vertical gradient freeze / Bridgman methods with a volume of several to over a dozen cm3. The UWBG TSOs discussed here have optical bandgaps of about 4.6 - 5 eV and great transparency in the UV / visible spectrum. The materials can be obtained as electrical insulators, n -type semiconductors, or n -type degenerate semiconductors. The free electron concentration (n e) of bulk β-Ga 2 O 3 crystals can be tuned within three orders of magnitude 1016 - 1019 cm−3 with a maximum Hall electron mobility (μ) of 160 cm2V−1s−1, that gradually decreases with n e. In the case of the bulk Ga-based spinel crystals with no intentional doping, the maximum of n e and μ increase with decreasing the Mg content in the compound and reach values of about 1020 cm−3 and about 100 cm2V−1s−1 (at n e > 1019 cm−3), respectively, for pure ZnGa 2 O 4. [ABSTRACT FROM AUTHOR]