Effect of ZnO buffer layer on the quality of GaN films deposited by pulsed laser ablation

ZnO, GaN and ZnO/GaN films have been deposited on c-cut sapphire wafers by Nd:YAG pulsed laser deposition to develop preparation techniques of multi-layer devices in optoelectronics. The quality of the films was investigated as functions of deposition temperature and pressure by means of X-ray diffraction, reflection high energy electron diffraction, scanning electron microscopy and optical transmission spectroscopy. It was found that GaN films could be epitaxially grown on ZnO buffered sapphire substrates even at a low substrate temperature of 300 °C and inactivated N₂ flow, confirming the significant role of ZnO as a buffer layer. The crystallinity, surface morphology and optical transmittance of these films could be improved with increasing deposition temperature and were optimized at 800 °C. The optical band gaps of ZnO and GaN were estimated from the optical transmission spectra to be 3.31 and 3.40 eV, respectively.