The origin of the UV Luminescence and its Enhancement in nanocrystalline ZnO film*

ZnO is an excellent luminescent material in the UV range with a potentially wide range of applications. However, many as-grown films are observed to contain some intrinsic defects which can diminish UV-emission efficiency, limiting their practical usefulness. This study presents a route to enhance UV luminescence from ZnO sputtered films. The photoluminescence (PL) spectra of the as-grown film exhibits prominent visible PL attributed to zinc interstitial (Zn$_{\mathrm{i}})$ related defects, and a weak UV PL peak. To understand the route toward enhanced UV PL, one set of as-grown films were annealed in O$_{2}$ atmosphere and another set in Ar atmosphere. PL spectra of O$_{2}$-annealed samples revealed enhanced UV PL and elimination of the Zn$_{\mathrm{i}}$-related defect emission, however, an O$_{2}$-related defect emission was evolved. In contrast, Ar annealed films showed significantly enhanced UV emission with nearly completely quenched visible emissions. The origin of UV PL was studied by low temperature measurements which indicate that an emission related to structural defects is dominant in the UV region.