  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.
