Emergence of blue emission with decreasing film thickness and grain size for ZnO grown via thermal oxidation of Zn-metal films

摘要

We have investigated the photoluminescence properties of ZnO grown on sapphire substrates via the thermal oxidation of Zn-metal films at various temperatures and thicknesses. 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 600°C, and then increase at higher temperatures. Thicker films demonstrated a modest increase in grain size as temperature increased from 300°C to 1200°C. 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 (300°C). As film thickness decreased, we observed the emergence of blue emission. The emergence of blue emission 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.