Structure and UV emission of nanocrystal ZnO films by thermal oxidation of ZnS films

Abstract

ZnO films prepared by the thermal oxidation of the ZnS films through thermal evaporation are reported. The as-deposited ZnS films have transformed to ZnO films completely at 400 °C. The 400–700 °C annealed films with a preferential c-axis (0 0 2) orientation have a hexagonal wurtzite structure. The band gap of ZnO films shifts towards longer wavelength with the increase of the annealing temperature. The relationship between the band gap energy of ZnO films and the grain size is discussed. The shift of the band gap energy can be ascribed to the quantum confinement effect in nanocrystal ZnO films. The photoluminescence spectra of ZnO films show a dominant ultraviolet emission and no deep level or trap state defect emission in the green region. It confirms the absence of interstitial zinc or oxygen vacancies in ZnO films. These results indicate that ZnO film prepared by this simple thermal oxidation method is a promising candidate for optoelectronic devices and UV laser.