Multinary wurtzite-type oxide semiconductors: Present status and perspectives

Oxide-based optoelectronic devices have been limited in applicable wavelength to the near-UV region because there are few viable binary wurtzite-type oxides, but ternary wurtzite-type (β-NaFeO₂-type) oxides are promising materials to expand the applicable wavelengths of these devices. In the past decade, many attractive properties of β-NaFeO₂-type oxide semiconductors have been revealed, such as the band-engineering of ZnO by alloying with β-LiGaO₂ and β-AgGaO₂, the photocatalytic activities of β-AgGaO₂ and β-AgAlO₂, and the discovery that β-CuGaO₂ is suitable for thin-film solar-cell absorbers. In this review article, we consider previous studies of β-NaFeO₂-type oxide semiconductors - β-LiGaO₂, β-AgGaO₂, β-AgAlO₂, β-CuGaO₂ - and their alloys with ZnO, and discuss their structural features, optical and electrical properties, and the relationship between their crystal structures and electronic band structures. We describe the outlook of β-NaFeO₂-type oxide semiconductors and the remaining issues that hinder the development of optoelectronic devices made from β-NaFeO₂-type oxide semiconductors.