Heteroepitaxy of nitrogen-polar, nonpolar, and semipolar gallium nitride by MOCVD

Since the early breakthroughs of two-step GaN growth and Mg-acceptor activation by Prof. Akasaki in the 1980s and Dr. Nakamura in the 1990s, nearly all the works related to GaN-based materials and devices were performed on Ga-polar (0001) c-plane. In spite of its popularity and technological dominance, Ga-polar c-plane orientation has fundamental limitations, including the well-known quantum confined Stark effect (QCSE) and the difficulty in micro-fabrication due to its chemical inertness. In the recent years, there has been increasing interest in exploring other crystallographic orientations for high brightness light-emitting diodes, enhancement mode transistors, and novel bio/chemical sensors, to name a few possibilities. This dissertation presents our investigations on the heteroepitaxy of N-polar c-plane (0001&barbelow), nonpolar a-plane (112&barbelow0) and m-plane (101&barbelow0), as well as semipolar (112&barbelow2) GaN by metalorganic chemical vapor deposition (MOCVD).To bypass the conventional knob-turning exercise for optimizing GaN heteroepitaxy process for each orientation, we constructed the first kinetic Wulff plots (growth rate polar plots) through differential selective area growth. Insights from the kinetic Wulff plots were used to explain complex phenomena in nonpolar GaN growth, including island formation, surface pits, and surface striations. Based on the kinetic Wulff plots, we designed and carried out a two-step growth of nonpolar a-plane (112&barbelow0) GaN on r-plane sapphire. By correlating the morphological evolution with the microstructure of a-plane GaN, we proposed a model for the reduction of basal-plane stacking faults (BSFs) and associated partial dislocations (PDs). For the growth of nonpolar m-plane (101&barbelow0) GaN on m-plane SiC, we demonstrated an effective way (Al composition graded AlGaN layers) for reducing the BSF density. The possible mechanisms for the formation of BSFs in nonpolar and semipolar GaN were summarized and discussed. Mirror-like semipolar (112&barbelow2) GaN with improved quality has been consistently attained on m-plane sapphire through a two-step growth scheme. And the preliminary results of semipolar (112&barbelow2) InGaN green quantum wells were very encouraging. We have achieved atomically smooth N-polar (0001&barbelow) GaN with crystalline quality comparable to that of Ga-polar c-plane GaN by adopting an appropriate nitridation for slightly offcut sapphire substrates, and investigated the challenges in growing high quality N-polar InGaN.