A Comparative Study of MBE-Grown GaN Films Having Predominantly Ga- or N-Polarity

Abstract

Wurtzitic GaN epilayers having both Ga and N-polarity were grown by reactive molecular beam epitaxy (MBE) using a plasma-activated nitrogen source on c-plane sapphire. The polarities were verified by convergent beam electron diffraction (CBED). High-resolution X-ray diffraction, atomic force microscopy (AFM) imaging and transmission electron microscopy (TEM) were employed to characterize the structural defects present in the films. The different topographic features of Ga and N-polarity samples and their appearance after wet etching were correlated to the measured X-ray rocking curve peak widths for both symmetric [0002] and asymmetric [1014] diffraction. For Ga-polarity samples, the [0002] diffraction is narrower than the [1014] diffraction, while for N-polarity ones the [0002] peaks are broader than [1014]. The half width of [1014] peaks for both polarity types were in the range of 5-7 arcmin indicative of, among possibly other defects, a high density of pure edge threading dislocations lying parallel to the c-axis. The 1-2 arcmin [0002] linewidths of Ga-polarity samples suggest a low density of screw dislocations, which corresponds with the TEM observations where the screw dislocation density is less than 10 7 cm -2 . In N-polarity samples, however, the [0002] diffraction peak was typically wider than 5 arcmin, suggesting either a higher density of edge dislocations and inversion domains in N-polarity samples, or the columnar structural features in AFM images, where the effective coherence length for X-ray diffraction is drastically reduced.