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We have examined the effect of synthesis conditions on α-Ga2O3 film, one of the ultra-wide bandgap semiconductors, on c-plane sapphire substrate via mist CVD process. The resultant film is dominantly composed of α-Ga2O3 phase, but a small amount of κ-Ga2O3 phase coexists when the growth temperature is higher. The source solution containing higher concentration of HCl expands the range of temperatures at which single-phase α-Ga2O3 is grown and suppresses the inclusion of κ-Ga2O3 at higher growth temperatures. Moreover, the growth with higher concentration of HCl up to 0.66 mol l−1 increases the growth rate and improves the surface roughness. Thus, HCl has a crucial role in the selective growth of α-Ga2O3 and the quality of the film. Also, some pits are observed at the surface of α-Ga2O3 and κ-Ga2O3 is precipitated inside the pit defect when the concentration of HCl is low and the growth temperature is high.

We demonstrated selective-area growth of r-SnO2 on a SiO2-masked r-TiO2 (110) substrate. The heteroepitaxy on a window started with a Volmer–Weber mode to grow islands with {100}-, {11̄0}-, and {011}-faceted sidewalls, whose growth shapes were consistent with the rutile structure’s equilibrium shape. The islands coalesced each other to make a flat (110) top surface on a striped window, and lateral overgrowth started after the complete coverage of the window. Cross-sectional transmission-electron-microscopy observation of the stripe revealed that misfit dislocations propagated perpendicularly to the facet planes by the image force effect and that the dislocation density reduced substantially in the wing regions.

ScAlMgO4 (SAM) substrates have a trigonal structure with high cleavability in the c-plane. Because this substrate can be easily cleaved, gallium oxide (Ga2O3) can be grown on the SAM substrate, and the difficulty in the heat dissipation of Ga2O3 may be solved. Therefore, in this study, we performed Ga2O3 growth on a SAM substrate using mist-chemical vapor deposition. When the growth temperature was varied at 500 °C, 600 °C, and 700 °C, ε-Ga2O3(004) grew dominantly, and the crystallinity improved with increasing temperature. Moreover, the grown ε-Ga2O3 had orthorhombic structure and formed rotational domains. Furthermore, a phase transition to β-Ga2O3 was achieved by annealing the resulting ε-Ga2O3 on the SAM substrates. We also succeeded in separating the SAM substrates on which the Ga2O3 thin films were grown.

Recently, rutile germanium dioxide (r-GeO2) has emerged as a novel ultra-wide bandgap semiconductor due to its theoretical excellent properties, that is, high thermal conductivity, ambipolar dopability, and high carrier mobility, in addition to its wide bandgap (4.44–4.68 eV). In this study, r-GeO2 thin films were grown on (001) r-TiO2 substrates by mist chemical vapor deposition. To optimize the growth conditions, we analyzed the decomposition processes of the Ge source (C6H10Ge2O7) by thermogravimetry-differential thermal analysis. It is found that GeO2 was synthesized from C6H10Ge2O7 at 553–783 °C in aqueous vapor. We accomplished fabrication of (001)-oriented r-GeO2 on r-TiO2 with a growth rate of 1.2–1.7 μm/h. On the other hand, under lower growth rate conditions (50 nm/h), the full width at half maximum of the r-GeO2 002 peak remarked a relatively small value of 560 arc sec. In addition, clear diffraction spots of r-GeO2 and r-TiO2 were observed at the r-GeO2/r-TiO2 interface, and the film was found to be significantly strained along the in-plane direction (∼2.3%) by cross-sectional transmission electron microscopy. The growth rate of ≧1 μm/h must contribute to the fabrication of thick r-GeO2 films, which can be utilized as power electronics devices with high breakdown voltage.

Recently, α-Ga2O3 has been attracting great attentions as a new wide bandgap semiconductor, however, the reason why metastable α-Ga2O3 is grown by mist chemical vapor deposition (CVD) has not been understood. In this study, in order to elucidate growth mechanism of mist CVD-grown α-Ga2O3, growth processes in the initial stage were investigated by atomic force microscopy, transmission electron microscopy, and X-ray diffraction reciprocal space mapping. We found that the characteristics of mist CVD make the relaxation mechanisms of α-Ga2O3 on (0001) sapphire different from those of molecular beam epitaxy, pulsed-laser deposition, and metalorganic chemical vapor deposition. We also proposed the growth procedure in the initial stage.