Blue luminescent centers and microstructural evaluation by XPS and Raman in ZnO thin films annealed in vacuum, N2 and O2

Zinc oxide (ZnO) thin films prepared on sapphire substrates by employing a pulsed laser deposition (PLD) have been annealed in vacuum, N2, and O2 ambient at annealing temperature 600 °C. X-ray diffraction (XRD) and atomic force microscopy (AFM) observation show that the annealed thin films possess (0 0 0 2) textured feature and form better crystal grains with a large size. X-ray photoelectron spectroscopy (XPS) and Raman analysis show the defects of ZnO thin films annealed in different ambient is distinctly different. The film annealed in vacuum possesses large oxygen vacancies (Vo) and Zn interstitials (Zni); while large surface defects exist in film annealed in N2 ambient. The concentrations of the intrinsic and extrinsic defects are the lowest in ZnO thin film annealed in oxygen gas. Photoluminescence (PL) spectra also reveal that the ultraviolet (UV) emission is the best for thin film annealed in O2. The blue emission (2.66 eV) is ascribed to the electronic transition from the donor energy level of Zn interstitials to acceptor energy level of Zn vacancies rather than the O vacancy.