Affect of film thickness on the blue photoluminescence from ZnO

Zinc oxide (ZnO) films having various thicknesses were synthesized on sapphire substrates by thermal oxidation of Zn-metallic films in air ambient. X-ray diffraction (XRD) spectra indicate that the resulting films possess a polycrystalline hexagonal wurtzite structure without preferred orientation. For films having a thickness of 200 nm, crystal grain size was observed to decrease with increasing annealing temperature up to 600C, and then increase at higher temperatures. Thicker films demonstrated a modest increase in grain size as temperature increased from 300C to 1200C. The influence of film thickness on the optical properties was investigated using room temperature photoluminescence (PL). Specifically, PL spectra indicate four emission bands: excitonic ultraviolet, blue, and deep-level green and yellow emission. The strongest UV emission and narrowest full width at half maximum (0.09 eV) was observed for films having a thickness of 200 nm and annealed at low temperature (300C). The ratio of deep-level green emission to UV excitonic emission was observed to decrease with decreasing annealing temperature, which is attributed to the generation of fewer oxygen vacancies and interstitial oxygen ions in the bulk. As film thickness decreased, 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 negatively charged Zinc interstitials found within the deletion region near the interface.