Polarity determination of a-plane GaN on r-plane sapphire and its effects on lateral overgrowth and heteroepitaxy

Achieving nitride-based device structures unaffected by polarization-induced electric fields can be realized with nonpolar GaN, although polarity plays a key role in the growth. $\text{(11}\overline{2}\text{0)}$ a-plane GaN films were grown on $\text{(1}\overline{1}\text{02)}$ r-plane sapphire substrates and subsequently laterally overgrown using metalorganic chemical vapor deposition. Convergent beam electron diffraction analysis was used to determine the a-GaN polarity to explicitly define the film/substrate relationship, and subsequently to identify various growth features and surfaces observed throughout our studies of a-plane GaN. In particular, the effects of polarity on (1) lateral overgrowth from mask stripe openings aligned along $[\overline{1}{\text{100]}}_{\mathrm{GaN}}$ and (2) pit formation in heteroepitaxial films grown under nonoptimized conditions were investigated. The fundamental differences between the polar surfaces are clearly observed; analysis of the lateral epitaxial overgrowth stripes revealed that $\text{(0001)}$ surfaces grew faster than $\text{(0001̄)}$ surfaces by approximately an order of magnitude, and these stable, slow-growing $\text{(0001̄)}$ surfaces are a likely cause of pitting in a-GaN films. The growth features under investigation were imaged using scanning and transmission electron microscopy.