Bulk single crystals of -Ga₂O₃ and Ga-based spinels as ultra-wide bandgap transparent semiconducting oxides

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 beta-Ga2O3 and Ga-based spinels MgGa2O4, ZnGa2O4, and Zn1-xMgxGa2O4. High melting points of the materials of about 1800-1930 degrees 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 de-vices. Bulk beta-Ga2O3 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 cm(3). 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 (ne) of bulk beta-Ga2O3 crystals can be tuned within three orders of magnitude 10(16)-10(19) cm(-3) with a maximum Hall electron mobility (mu) of 160 cm(2)V(-1)s(-1), that gradually decreases with ne. In the case of the bulk Ga-based spinel crystals with no intentional doping, the maximum of ne and mu increase with decreasing the Mg content in the compound and reach values of about 10(20) cm(-3) and about 100 cm(2)V(-1)s(-1) (at ne > 10(19) cm(-3)), respectively, for pure ZnGa2O4.