Thin-film corundum-structured gallium oxide (α-Ga2O3) Schottky barrier diodes (SBDs) were fabricated by growing α-Ga2O3 layers on sapphire substrates by the safe, low-cost, and energy-saving MIST EPITAXY® technique, followed by lifting off the α-Ga2O3 layers from the substrates. The SBDs exhibited on-resistance and breakdown voltage of 0.1 mΩ·cm2 and 531 V (SBD1) or 0.4 mΩ·cm2 and 855 V (SBD2), respectively. These results will encourage the future evolution of low-cost and high-performance SBDs with α-Ga2O3.

The recent progress and development of corundum-structured III-oxide semiconductors are reviewed. They allow bandgap engineering from 3.7 to ∼9 eV and function engineering, leading to highly durable electronic devices and deep ultraviolet optical devices as well as multifunctional devices. Mist chemical vapor deposition can be a simple and safe growth technology and is advantageous for reducing energy and cost for the growth. This is favorable for the wide commercial use of devices at low cost. The III-oxide semiconductors are promising candidates for new devices contributing to sustainable social, economic, and technological development for the future.

We demonstrate the epitaxial lateral overgrowth of α-Ga2O3 by halide vapor phase epitaxy. We prepared patterned SiO2 masks on a (0001) α-Ga2O3/sapphire template, and then α-Ga2O3 islands were regrown selectively on the mask windows. The islands grew vertically and laterally to coalesce with each other. Facet control of the α-Ga2O3 islands was achieved by controlling the growth temperature, and inclined facets developed by decreasing the temperature. Transmission electron microscopy revealed that the crystal quality of the regrown α-Ga2O3 was improved owing to both the blocking of dislocations by the mask and the dislocation bending by the inclined facets.

Corundum-structured oxides have been attracting much attention as next-generation power device materials. A corundum-structured α-Ga 2 O 3 successfully demonstrated power device operations of Schottky barrier diodes (SBDs) with the lowest on-resistance of 0.1 mΩ cm 2 . The SBDs as a mounting device of TO220 also showed low switching-loss properties with a capacitance of 130 pF. Moreover, the thermal resistance was 13.9 °C/W, which is comparable to that of the SiC TO220 device (12.5 °C/W). On the other hand, corundum-structured α-(Rh,Ga) 2 O 3 showed p-type conductivity, which was confirmed by Hall effect measurements. The Hall coefficient, carrier density, and mobility were 8.22 cm 3 /C, 7.6 ' 10 17 /cm 3 , and 1.0 cm 2 V %1 s %1 , respectively. These values were acceptable for the p-type layer of pn diodes based on α-Ga 2 O 3 .

To obtain crack-free thick α-Ga2O3 films on sapphire substrates, effects and behaviors of buffer layers have been investigated. With the growth of an α-Ga2O3 layer, there appeared an unintentionally formed layer in the sample, which was associated with stress accumulation and could be the seed for crack generation. We obtained a thick (∼5 µm) α-Ga2O3 layer on a sapphire substrate with the insertion of α-(Al0.12Ga0.88)2O3/α-(Al0.02Ga0.98)2O3 buffer layers, and for this sample, we did not observe the intermediate layer, suggesting that the buffer layers were effective for eliminating the stress accumulation at the α-Ga2O3/sapphire interface region.