Ultraviolet emission and microstructural evolution in pulsed-laser-deposited ZnO films

ZnO thin film has been deposited on a sapphire (001) at a temperature of 400 °C using a pulsed laser deposition with oxygen pressures of 1, 20, 50, 200, 300, and 500 mTorr. As the oxygen pressure for the thin film deposition increases, the grain size of the films increases up to a certain point, but then the size decreases over 50 mTorr as measured by x-ray diffractometry and scanning electron microscopy. In contrast, the intensity of UV photoluminescence continuously increases with the oxygen pressure. This is probably because the stoichiometry of oxygen-deficient ZnO film is improved by increasing oxygen pressure. It is concluded that there exist two different mechanisms for ZnO formation according to applied oxygen pressures, and that the UV Iuminescence intensity strongly depends on the stoichiometry in the ZnO film rather than the grain size of the thin film.