Blue luminescent center and ultraviolet-emission dependence of ZnO films prepared by pulsed laser deposition

ZnO thin films were grown on sapphire substrates at a growth temperature of 400 degrees C by Nd:yttrium-aluminum-garnet (Nd:YAG 1064 nm) pulsed-laser deposition. The effects of oxygen pressure on the structure, stoichiometric composition and optical properties of ZnO thin films were investigated in detail. X-ray diffraction (XRD) shows that the crystal quality is highest for the ZnO film grown at a pressure of 10m Torr. Optical transmittance, electronic properties and Raman spectra show the increase in the stoichiometric composition ratio of ZnO films with increasing oxygen pressure owing to the reduction in the number of oxygen vacancies (V-O) and Zn interstitials (Zn-i). Photoluminescence (PL) spectra reveal that the UV-emission properties have greater dependence on the stoichiometry of ZnO films than on the crystal quality. A blue-light emission peak at about 465 nm (E-g = 2.66 eV) was observed in the PL spectra of ZnO thin films fabricated at various oxygen pressures. This emission peak was ascribed to the electronic transition from the donor energy level of the Zn interstitials to the acceptor energy level of the Zn vacancies (V-Zn) rather than the oxygen vacancies.