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We achieved the successful fabrication of Sn-doped α-Ga 2 O 3 thin films with higher electron mobility and wider conductivity controls by improving the crystal quality. α-Ga 2 O 3 films showed n-type conductivity with a maximum electron mobility of 24 cm 2 V %1 s %1 . The carrier concentration was successfully controlled in the range of 10 17 -10 19 cm %3 . Crystal defects such as dislocations severely compensate the free carriers in α-Ga 2 O 3 films and restrict the mobility at low carrier concentrations. Therefore, to achieve further conductivity control and higher mobility, improving the crystallinity of α-Ga 2 O 3 films is necessary.

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.

Following the previous achievement of highly crystalline α-Ga2O3 thin films on c-plane sapphire, the growth of corundum-structured α-(Al x Ga1-x )2O3 was examined aiming at the future application of α-(Al x Ga1-x )2O3/Ga2O3 heterostructures to power devices and other functional devices. The results show the control of x and band gap up to 0.81 and 7.8 eV, respectively, maintaining the dominant corundum structure. The transmission electron microscope observation suggested the formation of the crystallographically good interface of α-(Al x Ga1-x )2O3/Ga2O3 without the severe generation of threading dislocation lines from the interface.

Highly crystalline corundum-structured -(Ga 1Àx Fe x ) 2 O 3 alloy thin films were fabricated on c-plane sapphire substrates by using a mist chemical vapor deposition method. The full-widths at half maximum of X-ray diffraction rocking curves were smaller than 100 arcsec for the entire range of x from 0 to 1. Optical band gaps were artificially tuned to a value between those of -Ga 2 O 3 and -Fe 2 O 3 , that is, 2.2 and 5.3 eV with changing the Fe content x in the films. Magnetic measurements revealed ferromagnetic properties of a -(Ga 1Àx Fe x ) 2 O 3 (x ¼ 0:24) thin film at 110 K.

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 .

Corundum-structured -Ga 2 O 3 epitaxial thin films were grown on c-plane -Al 2 O 3 (sapphire) substrates by a mist chemical vapor deposition method. To reveal the defect structures, the -Ga 2 O 3 film was observed by high-resolution transmission electron microscopy (TEM). We found that the -Ga 2 O 3 thin film was in-plane compressive stressed from the -Al 2 O 3 substrate. Although misfit dislocations were periodically generated at the -Ga 2 O 3 /-Al 2 O 3 interface owing to the large lattice mismatches between -Ga 2 O 3 and -Al 2 O 3 , 3.54% (c-axis) and 4.81% (a-axis), most of the misfit dislocations did not thread through the layer. An extra-half plane was f 2110g consisting only of Ga. Screw dislocations were not confirmed, i.e., the density was under 10 7 cm À2 . The threading dislocation density was 7 Â 10 10 cm À2 .

Efforts have been made to reduce the density of defects in corundum-structured α-Ga2O3 thin films on sapphire substrates by applying quasi-graded α-(Al x Ga1− x )2O3 buffer layers. Transmission electron microscopy images revealed that most strains were located in the α-(Al x Ga1− x )2O3 buffer layers, and that the total density of dislocations in the α-Ga2O3 thin films was successfully decreased by more than one order of magnitude compared with that without buffer layers, that is, the screw and edge dislocation densities were about 3 × 108 and 6 × 108 cm−2, respectively.