Effects of AlN Buffer Layers on the Structural and the Optical Properties of GaN Epilayers Grown on Al2O3 Substrates by using Plasma-assisted Molecular Beam Epitaxy

GaN epilayers on AlN buffer layers with various thicknesses were grown on sapphire substrates by using plasma-assisted molecular-beam epitaxy. The GaN epilayer with an AlN buffer layer was much smaller than the GaN epilayer without an AlN buffer layer. The crystal quality of the GaN active layer was improved by utilizing an AlN layer, which acted as a nucleation layer. The reduced defect density promoted GaN coalition. The double-crystal rocking curves and the photoluminescence spectra showed that the GaN epilayer grown on a 4-nm AlN buffer layer had the best quality among the several kinds of samples. The photoluminescence intensity of the GaN epilayer which is related to the density of the crystal defects was lower when an AlN buffer layer was used the thin AlN nucleation layer protected against stain propagation. These results indicate that GaN epilayers grown on AlN buffer layers hold promise for applications in short-wavelength optoelectronic devices.