Structure and photoluminescence of c-axis oriented Nnanocrystalline ZnO films synthesized by plasma assisted pulsed laser deposition

Nanocrystalline zinc oxide (nc-ZnO) thin films with c-axis orientation were synthesized by plasma assisted pulsed laser deposition from metallic zinc. Structural characterization and annealing effects were investigated by means of X-ray diffraction, Raman scattering and Fourier transform infrared spectroscopy. The synthesized nc-ZnO films have a wurtzite structure and high crystallographic orientation with c-axis perpendicular to the substrates and the crystallites in the as-deposited films have an average diameter of about 16 nm. At light excitation at room temperature, the nc-ZnO films emit strong UV luminescence attributed to free exciton transitions. The crystallinity of the films is improved and the ZnO crystallites grow larger during thermal annealing. The intensity and the feature of the photoluminescence from the synthesized nc-ZnO films show a strong dependence on annealing process. Several PL emission lines associated with free excitons and longitudinal optical phonon replicas of bound excitons are clearly resolved in the low-temperature photoluminescence spectrum of the annealed nc-ZnO films.