Band Gap Engineering of Wurtzite-Type Narrow Band Gap Oxide Semiconductor β-CuGaO2

Abstract

β-CuGaO2 is an oxide semiconductor possessing wurtzite-derived β-NaFeO2 structure. Its band gap is 1.47 eV in near-infrared region, and the first principles calculation indicates that it is a direct band gap semiconductor. Therefore, this material is expected to be applicable to optoelectronic devices working in near-infrared region. When the band gap of β-CuGaO2 is widened into visible region, the material is expected to be applicable to visible LEDs and lasers, photocatalyst and so on. In the present study, we demonstrated band gap engineering of β-CuGaO2 by alloying with β-CuAlO2 and β-LiGaO2 possessing wurtzite-derived β-NaFeO2 structure in order to widen the band gap. Cu(Ga1-xAlx)O2 alloys were prepared by ion-exchange of Na+ ions in β-Na(Ga1-xAlx)O2 with Cu+ ions in CuCl at 250 °C for 48 h under vacuum. (Cu1-xLix)GaO2 alloys were prepared by ion-exchange of Cu+ ions in β-CuGaO2 with Li+ ions in LiCl at 300 °C for 48 h under vacuum. Wurtzite-type β-phases were obtained in 0≤x≤0.7 for Cu(Ga1-xAlx)O2 system and in 0≤x≤1 for (Cu1-xLix)GaO2 system. In β-Cu(Ga1-xAlx)O2, the optical band gap increased linearly with the increasing alloying level; no band gap bowing appeared in the present system. The band gap of β-CuGaO2 was widened up to 2.1 eV at β-Cu(Ga0.3Al0.7)O2; this energy corresponds to red light. In β-(Cu1-xLix)GaO2, the energy band gap increased almost linearly with the increasing alloying level up to x~0.9; the largest band gap in this composition range was 3.0 eV corresponding to violet light at β-(Cu0.11Li0.89)GaO2. When the alloying level increased further, the band gap steeply increased from 3.0 eV at x=0.89 to 5.6 eV at x=1. The valence band X-ray photoelectron spectra of β-(Cu1-xLix)GaO2 indicated that the steep increase in energy band gap from x~0.9 to 1 is attributed to that the Cu 3d contribution around the top of the valence band electronic states vanishes for the alloys with x>0.9. These results indicate that the energy band gap of β-CuGaO2 is highly controllable in the wavelength region from near-infrared to entire visible lights. Thus, it is expected that β-CuGaO2 opens new applications of oxide semiconductors in devices working in visible region.