Band Gap Engineering of Wurtzite-Type Narrow Band Gap Oxide Semiconductor β-CuGaO 2

• Published in: Meeting abstracts (Electrochemical Society) Vol. MA2016-02; no. 31; p. 2064
• Main Authors: Omata, Takahisa, Mizuno, Yuki, Nagatani, Hiraku, Suzuki, Issei, Kita, Masao
• Format: Journal Article
• Language: English
• Published: 01.09.2016
• ISSN: 2151-2043; 2151-2035
• DOI: 10.1149/MA2016-02/31/2064

β-CuGaO 2 is an oxide semiconductor possessing wurtzite-derived β-NaFeO 2 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-CuGaO 2 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-CuGaO 2 by alloying with β-CuAlO 2 and β-LiGaO 2 possessing wurtzite-derived β-NaFeO 2 structure in order to widen the band gap. Cu(Ga 1-x Al x )O 2 alloys were prepared by ion-exchange of Na + ions in β-Na(Ga 1-x Al x )O 2 with Cu + ions in CuCl at 250 °C for 48 h under vacuum. (Cu 1-x Li x )GaO 2 alloys were prepared by ion-exchange of Cu + ions in β-CuGaO 2 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(Ga 1-x Al x )O 2 system and in 0≤x≤1 for (Cu 1-x Li x )GaO 2 system. In β-Cu(Ga 1-x Al x )O 2 , 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-CuGaO 2 was widened up to 2.1 eV at β-Cu(Ga 0.3 Al 0.7 )O 2 ; this energy corresponds to red light. In β-(Cu 1-x Li x )GaO 2 , 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-(Cu 0.11 Li 0.89 )GaO 2 . 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-(Cu 1-x Li x )GaO 2 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-CuGaO 2 is highly controllable in the wavelength region from near-infrared to entire visible lights. Thus, it is expected that β-CuGaO 2 opens new applications of oxide semiconductors in devices working in visible region.