We have investigated the photoluminescence (PL) properties of ZnO grown on sapphire substrates via the thermal oxidation of Zn-metal films at various temperatures and having various thicknesses. PL spectra indicate four emission bands: excitonic ultraviolet (UV), blue, and deep-level green and yellow emission. The ratio of deep-level green emission to UV excitonic emission was observed to decrease with decreasing annealing temperature from 1200 to 300 °C, which is attributed to the generation of fewer oxygen vacancies () and interstitial oxygen ions () in the bulk. As film thickness decreased from approximately 600 to 200 nm, we observed the emergence of blue emission and a significant red shift (0.15 eV) in the bandgap. The emergence of blue emission and the corresponding decrease in emission associated with bulk defects when depletion width grows relative to the bulk suggests that the origin of the blue emission is related to the charged zinc interstitials () found within the deletion region near the interface.