Improvements in Optical Properties of (0001) ZnO Layers Grown on (0001) Sapphire Substrates by Halide Vapor Phase Epitaxy Using Thick Buffer Layers

The optical properties of (0001) ZnO layers grown at 1000 °C on (0001) sapphire substrates by halide vapor phase epitaxy (HVPE) were investigated by various photoluminescence (PL) measurements. A layer grown with a H 2 O/ZnCl 2 (VI/II) ratio of 20 on a 0.4-μm-thick buffer layer exhibited a significant near-band-edge (NBE) peak blueshift and degraded internal quantum efficiency ($\eta_{\text{int}}$) due to residual compressive stress. Growth with a VI/II ratio of 600 diminished the NBE peak blueshift; however, deep level emission and a reduction of PL decay time ($\tau_{\text{PL}}$) were caused by point defects generated by excess O source supply. A layer without the NBE peak blueshift and deep level emission was realized by growth with a VI/II ratio of 20 and a buffer layer of 0.8 μm. The $\eta_{\text{int}}$ and $\tau_{\text{PL}}$ for HVPE-grown layers could be improved to 4.1% and 122.8 ps by using the thick buffer layer and appropriate VI/II ratio.