Effect of substrate nitridation and a buffer layer on the growth of a non-polar a-plane GaN epitaxial layer on an r-plane sapphire substrate by laser molecular beam epitaxy

Abstract

Non-polar a-plane GaN epitaxial layers have been successfully grown on r-plane sapphire substrates by using the laser molecular beam epitaxy (LMBE) technique through the laser ablation of a GaN solid target under radio frequency nitrogen plasma ambient at a low temperature of 600-700 °C. The role of sapphire nitridation and GaN and AlN buffers on the growth and properties of the a-plane GaN layer on r-sapphire has been investigated. Sapphire nitridation is found to produce the growth of a c-axis oriented, high-density GaN nanorod array on r-plane sapphire. However, a direct growth of GaN on r-sapphire without nitridation yields an a-plane GaN layer as confirmed with in situ reflective high energy electron diffraction, X-ray diffraction, and field-emission scanning electron microscopy observations. The introduction of low temperature (LT) GaN or high temperature (HT) AlN buffer on r-sapphire improved the optical quality of the a-plane GaN layers on r-plane sapphire with a strong band-to-band photoluminescence transition and low defect level emission at room temperature. Among the adopted approaches, HT-AlN buffer is found to be effective for improving the morphological, structural and optical properties of a-plane GaN layers grown on r-plane sapphire. A fully coalesced flat surface GaN layer is achieved under two-dimensional growth mode. The results indicate that the optimization of the pre-growth procedure is very critical to achieve a flat surface nonpolar a-plane GaN epitaxial layer on r-plane sapphire in the LMBE process.