Aging effect and origin of deep-level emission in ZnO thin film deposited by pulsed laser deposition

ZnO thin films were deposited on sapphire substrates at 400 degrees C in the pulsed laser deposition (PLD) system. Those thin films showed two emission peaks. One was near band edge emission at around 379 nm; the other was deep-level (DL) emission at around 510 nm. The aging effect on photoluminescence (PL) of the thin film was observed. It was found that the DL emission decreased with time. Post-annealing processes were carried out to find the origin of the DL emission. The thin films were annealed at 800 degrees C in N-2 or O-2 ambient gas in a rapid thermal annealing system. An atomic force microscope was used to investigate the surface morphologies of the thin films. The surface roughness of annealed thin film was much smaller than that of the as-deposited one. The transmittance of the annealed thin film decreased much compared with that of the as-deposited thin film. The DL emission of the thin film annealed in N-2 increased, and the DL emission of thin film annealed in O-2 decreased. The oxygen vacancies instead of zinc interstitials were the main reason for DL emission in ZnO thin films deposited by PLD.