Characterization of Non-Polar ZnO Layers with Positron Annihilation Spectroscopy

We applied positron annihilation spectroscopy to study the effect of growth polarity on the vacancy defects in ZnO grown by metal-organic vapor phase deposition on sapphire. Both c-plane and a-plane ZnO layers were measured, and Zn vacancies were identified as the dominant defects detected by positrons. The results are qualitatively similar to those of earlier experiments in GaN. The Zn vacancy concentration decreases in c-plane ZnO by almost one order of magnitude (from high $10^{17} cm^{-3}$ to low $10^{17} cm^{-3}$) when the layer thickness is increased from 0.5 to 2 μm. Interestingly, in a-plane ZnO the Zn vacancy concentration is constant at a level of about $2×10^{17} cm^{-3}$ in all the samples with thicknesses varying from 0.6 to 2.4 μm. The anisotropy of the Doppler broadening of the annihilation radiation parallel and perpendicular to the hexagonal c-axis was also measured.