N-polar GaN evolution on nominally on-axis c-plane sapphire by MOCVD Part-II: Microstructural investigation

Abstract

The present work is part-II of a study that addresses in detail the microstructural features of nitrogen-polar gallium nitride (N-polar GaN) deposited on c-plane sapphire wafers under the metalorganic chemical vapor deposition conditions described in the part-I work. The growth process was interrupted at various stages and characterized the depositions using transmission electron microscopy. The plan view diffraction study in combination with the Moiré fringe analysis revealed the presence of rotational domains of 4° in the low-temperature GaN nucleation layer. A crystallographic basis for the occurrence of these rotational domains is proposed based on the arrangement of surface atoms in the underlying sapphire substrate. Pure a dislocations are found to accommodate the twist between the rotational domains. In addition, Ga-polar inversion domains are observed in the matrix of N-polar GaN, and they originate at atomic level steps induced by the initial thermal cleaning and nitridation process of sapphire wafers at 1100°C. The rotational domains, the accommodating a dislocations and the inversion domains are replicated into the N-polar GaN epilayers.