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The mechanism of ultraviolet (UV) and green emission of ZnO thin films deposited on (001) sapphire substrates by pulsed laser deposition was investigated by using postannealing treatment at various annealing temperatures after deposition. Structural, electrical, and optical properties of ZnO films have been also observed. As the postannealing temperature increased, the intensity of UV (380 nm) peak and the carrier concentration were decreased while the intensity of the visible (about 490–530 nm) peak and the resistivity were increased. The role of oxygen in ZnO thin film during the annealing process was important to the change of optical properties. The mechanism of the luminescence suggested that UV luminescence of ZnO thin film was related to the transition from near band edge to valence band, and green luminescence of ZnO thin film was caused by the transition from deep donor level to valence band due to oxygen vacancies. The activation energy derived by using the variation of green emission intensity was 1.19 eV.
Kang et al. (Sun,) studied this question.