β-Ga2O3 is expected to help realize high-performance, energy-saving power devices because it has a wider bandgap (range between 4.5 and 4.9 eV [,,]) than SiC or GaN. The development of β-Ga2O3-based Schottky barrier diodes [,,,] and field-effect transistors [,,,] for power electronics is progressing rapidly. Furthermore, because, like Si, β-Ga2O3 can be grown from a melt, theoretically, it should have the same low-cost and high-quality characteristics as Si. However, at present, the most widely used growth method for oxide single crystals requires precious-metal crucibles [,,,]. In the case of high-melting-point oxides, the use of expensive iridium crucibles is a bottleneck, resulting in high production costs for single-crystal substrates. Another issue is that crystal growth is limited by the low oxygen partial pressure applied to suppress the oxidation of the iridium crucible, leading to the decomposition and volatilization of gallium oxide and resulting in a large number of crystal defects. The use of iridium crucibles is not ideal, especially for β-Ga2O3 growth, because iridium chemically attacks the gallium oxide melt and results in the decomposition of gallium oxide at high temperatures with the release of oxygen.
In an effort to overcome the problem of decomposition and volatilization in low-oxygen partial-pressure atmospheres, Hoshikawa et al. contributed to the development of a technology for the growth of β-Ga2O3 bulk single crystals using the Bridgman method with platinum-based alloys [,]. Recently, we successfully grew Gd–Al–Ga crystals (melting point: ~1820 ℃) using the OCCC method and avoiding the use of a precious-metal crucible []. The OCCC method combines two conventional and well-known methods: the cold crucible method [,,,,,] for the heat source and the Czochralski (CZ) method for crystal growth.
In our experiments, a water-cooled Cu basket was filled with the raw material, and only the central portion of the body of material was melted using a high-frequency generator heating device. The sintered surrounding material close to the Cu basket was not melted, and hence melt leakage from the basket was prevented. Subsequently, as in the CZ method, the seed crystal was placed in contact with the melt, necking occurred, and the crystal was pulled up and simultaneously rotated, enabling bulk single-crystal growth. Because there is no oxidation of the crucible in this method, there are no restrictions on the atmosphere during growth, and it is possible to grow the melt by using a 100% oxygen atmosphere, which is expected to significantly reduce the number of oxygen defects in the grown crystals. A similar technique was successfully applied for the growth of α-Al2O3 and SrTiO3 single crystals using a high-frequency (3–10 MHz) vacuum tube generator as a power source [,]. In this work, bulk β-Ga2O3 crystals were grown using the OCCC technique with a SiC transistor generator as the power source. To the best of our knowledge, this is the first study reported in the literature on the application of this new single-crystal growth method to β-Ga2O3.