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.