Photoluminescence spectroscopic and computational investigation of the origin of the visible light response of (Ga_1-xZn_x)(N _1-xO_x) photocatalyst for overall water splitting

The electronic structure of a solid solution between GaN and ZnO, GaN-rich (Ga_1-xZn_x)(N_1-xO_x), was investigated by photoluminescence spectroscopy and a plane wave based density functional method. Photoluminescence excitation spectra (PLE) of (Ga_1-xZn _x)(N_1-xO_x) photocatalysts (x = 0.05-0.11) at 20 K had PLE edges in the UV region in addition to those of undoped GaN and Zn-doped GaN. However, the absorption edges appeared in the visible region, indicating that the intrinsic band gap of GaN-rich (Ga_1-xZn _x)(N_1-xO_x) solid solutions is derived from that of the GaN component. Photoluminescence (PL) bands of (Ga_1-xZn _x)(N_1-xO_x) photocatalysts were observed at 480 and 650 nm, suggesting that the luminescence originated from electron transitions from the conduction band to Ga vacancies as native defects, or to Zn acceptor levels as impurity levels. GaN-rich (Ga_1-xZn _x)(N_1-xO_x) material containing a large amount of oxygen is likely to produce an O donor level slightly below the conduction band minimum (CBM). The Zn acceptor level is likely to be filled with electrons derived from O donor levels or thermal excitation, suggesting that the absorption in the visible light region of this material occurs via electron transitions from the Zn acceptor level to the conduction band. The electronic structure and band gap narrowing are discussed in terms of density functional theory calculations using local nonstoichiometric defect structures modeled by Zn atom replacement, O atom replacement, or Ga atom vacancies.