Violet and UV luminescence emitted from ZnO thin films grown on sapphire by pulsed laser deposition

Visible violet photoluminescence (PL) has been achieved at room temperature (RT) from ZnO films grown on sapphire (001) substrate by pulsed laser deposition (PLD). Substrate temperatures of 200, 300, and 400°C have been used in an oxygen pressure of 1 mTorr during the PLD. As the oxygen pressure for the thin film deposition increases over 20 mTorr at a substrate temperature of 400°C, the violet luminescence vanishes. Instead ultra-violet (UV) and green-yellow luminescence appear. The most intense UV and green-yellow luminescence is obtained from a sample grown in an oxygen pressure of 200 mTorr at 400°C. It is concluded that the intensity of the UV luminescence strongly depends on the stoichiometry of the film as well as the crystalline quality, while the violet PL is due to a defect level in the grain boundaries of the ZnO_x crystals.