The role of oxygen vacancies in epitaxial-deposited ZnO thin films

ZnO thin films were epitaxial deposited on sapphire (0001) substrates at various temperatures by using the pulsed laser deposition (PLD) technique. An x-ray diffractometer (XRD) was used to investigate the structural properties of the thin film. It was found that all of the thin films were (0002) oriented and the intensity of (0002) peak increased with the increasing growth temperature. The ϕ-scans for the thin films indicated that the thin film grown at a temperature higher than 400 °C had an epitaxial relation with the substrate. An atomic force microscope (AFM) was used to investigate the surface morphologies of the thin films. The surface roughness and grain size of the thin films increased with increasing growth temperature. A double-beam spectrophotometer was used to measure the transmittances of the thin films. The band gap energies of the thin films were calculated by linear fitting the absorption edges for high-quality thin films. A spectrometer was used to investigate the photoluminescent (PL) properties of the thin films. It was discovered that all of the thin films showed two emissions. One was the near band edge (NBE) emission; the other was the broad deep-level (DL) emission. After checking the PL of the thin films on a different date, the aging effect of the ZnO thin film on the sapphire substrate deposited by PLD was observed. It was revealed that, the NBE emissions were enhanced and the DL emissions were decreased with time. To find the reason why the DL emission decreased with time, the as-deposited thin films were annealed at 800 °C in a N2 and O2 atmosphere for 30 min, respectively. The surface morphologies and the transmittances of the annealed thin films were investigated by the AFM and spectrophotometer. The surface roughness and the transmittance decreased much after annealing. The PL measurements for the annealed thin films indicated that, the DL emission of the thin film annealed in N2 was enhanced and that annealed in O2 was quenched. It was suggested that the oxygen vacancies, instead of zinc interstitials, played the most important role for DL emissions in ZnO thin films deposited by PLD.