It has been confirmed that Si, Sn, and Ge are effective n-type dopants for β-Ga2O3. This letter shows that Nb doping is also a viable method for controlling the electrical resistivity and carrier density of β-Ga2O3, corresponding to the theoretical calculations about the doping of Ga2O3 with Nb [H. Peelaers and C. G. Van de Walle, Phys. Rev. B 94(19), 4 (2016)]. β-Ga2O3 single crystals with different Nb concentrations were grown by the optical floating zone method. The electrical resistivity can be varied from 3.6 × 102 Ω cm to 5.5 × 10−3 Ω cm by increasing the Nb doping concentration, and the related free carrier concentration increases from 9.55 × 1016 cm−3 to 1.8 × 1019 cm−3. The transmittance spectra and photoluminescence spectra were measured to systematically study the optical properties of Nb-doped β-Ga2O3 single crystals. The strong absorption near the IR region in the crystals is related to the increase in conductive electrons, and the decrease in blue luminescence intensity indicates a decrease in the VO concentration induced by increasing the carrier concentration.

The rapid development of bulk β-Ga2O3 crystals has attracted much attention to their use as ultra-wide bandgap materials for next-generation power devices owing to its large bandgap (~ 4.9 eV) and large breakdown electric field of about 8 MV/cm. Low cost and high quality of large β-Ga2O3 single-crystal substrates can be attained by melting growth techniques widely used in the industry. In this paper, we first present an overview of the properties of β-Ga2O3 crystals in bulk form. We then describe the various methods for producing bulk β-Ga2O3 crystals and their applications. Finally, we will present a future perspective of the research in the area in the area of single crystal growth.

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