Temperature-enhanced ultraviolet emission in ZnO thin film

We have studied the structural and optical properties of ZnO thin films prepared by thermal oxidation of ZnS films deposited by plasma assisted electron-beam evaporation on Si(1 0 0) substrates. The transformation from zinc blende ZnS to hexagonal wurtzite ZnO is confirmed by Raman and X-ray diffraction (XRD) measurement. For the sample thermally oxidized at 600 °C for 2 h, a novel UV emission peak Ix located at 3.22 eV (385 nm) has been observed. The temperature-dependent photoluminescence spectra show that the integrated intensity of Ix increases exponentially with increasing temperatures within the measuring temperature range, from 80 to 300 K, but the peak position remains nearly constant. We explain this behavior in terms of electron tunneling into the radiative recombination centers.