Effects of annealing on the crystal structures and blue emission properties of sputtered ZnO films

ZnO films usually exhibit a complex blue emission property, but the luminescence mechanism behind the blue emission is still not clearly understood. In this study, ZnO films have been deposited on SiO2/silicon by RF magnetron sputtering. Two blue emission peaks centered at 420 and 440 nm are found for the ZnO films. The variation of the emission intensity and the peak location was investigated at different annealing temperatures. According to the calculation of the defect levels in the ZnO films, we suggest that the 440 nm (2.82 eV) emission peak corresponds to the electron transition from the interstitial Zn to the top of the valence band; the 420 nm (2.95 eV) emission peak is due to the shallow acceptor levels of vacant Zn or interstitial O. The melting point of the ZnO grains is around 700 °C which is much lower than that of bulk ZnO (1975 °C).