Defect species in Ga-doped ZnO films characterized by photoluminescence

Photoluminescence (PL) spectra of Ga-doped ZnO (GZO) films were investigated with reference to those of undoped ZnO films to elucidate the effect of high-level Ga3+ doping. A transition from ZnO-like to GZO-like spectra occurred at a Ga content around 2 at. %. The room temperature PL spectra of sufficiently oxidized GZO films exhibited band edge and violet components, while emissions at wavelengths longer than 480 nm were sharply cut off. The close resemblance of the spectral shapes of the GZO and Zn-rich ZnO films indicated disordering of the ZnO lattice by excess Ga dopants. Deposition under a reducing atmosphere at 100 and 200 °C produced oxygen-deficient GZO films with additional emission signals corresponding to oxygen vacancy (VO) defects between 480 and 600 nm. For GZO films with Ga content larger than 4 at. %, increasing the deposition temperature above 400 °C or postannealing at 500 °C smeared out deep-level emission signals, suppressed the near-band edge emission, and deactivated the donor role of Ga3+. These changes can be ascribed to outdiffusion of Ga3+ from the cation sites and rearrangement of the ZnO crystal network. Argon plasma treatment of GZO films generated a VO-related emission band through preferential sputtering of oxygen atoms. Hydrogen donors trapped at the resulting VOs would be the origin of a slight increase in carrier concentration, by 1 × 1020 cm−3.