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 (beta-NaFeO2-type) oxides are promising materials to expand the applicable wavelengths of these devices. In the past decade, many attractive properties of beta-NaFeO2-type oxide semiconductors have been revealed, such as the band-engineering of ZnO by alloying with beta-LiGaO2 and beta-AgGaO2, the photocatalytic activities of beta-AgGaO2 and beta-AgAlO2, and the discovery that beta-CuGaO2 is suitable for thin-film solar-cell absorbers. In this review article, we consider previous studies of beta-NaFeO2-type oxide semiconductors beta-LiGaO2, beta-AgGaO2, beta-AgAlO2, beta-CuGaO(2)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 beta-NaFeO2-type oxide semiconductors and the remaining issues that hinder the development of optoelectronic devices made from beta-NaFeO2-type oxide semiconductors.