Oxide semiconductors are promising “green materials” suitable for unique functional devices in the next generation. Precise surface control during the growth, similarly to other semiconductors, is a key for their property control as well as their high-quality layers and interfaces. Our recent interest has been focused on gallium oxide (Ga₂O₃), due to its wide band gap and precedent development of its β-phase bulk substrates. Here we report successful fabrication of deep ultraviolet photo-detectors with thermal treatment of the substrate surface and achievements of step-flow growths of Ga₂O₃and (AlGa)₂O₃. By applying the mist vapor-phase growth technology, which we have developed as a safe and simple growth method for oxides, the formation of “green materials” can be supported by “green technology”. Now our research stage is limited to heteroepitaxial growth on sapphire substrates, but the results indicated the growth of highly-crystalline corundum-structured α-phase Ga₂O₃with confining the lattice-mismatching-induced defects at the epilayer/substrate interface. This may lead to evolution of corundum-structured multifunctional oxide semiconductors.