Roughening of GaN homoepitaxial surfaces due to step meandering and bunching instabilities and their suppression in hydride vapor phase epitaxy

Abstract

In this letter, we have shown that well-known surface instabilities in crystal growth, i.e., in-phase step meandering and step bunching, are the main causes of surface roughening of GaN homoepitaxial layers on vicinal +c-oriented GaN substrates. Both instabilities were effectively suppressed in hydride vapor phase epitaxy (HVPE) under appropriate conditions, which produced highly smooth as-grown surfaces suitable for reproducible device fabrication. Wavy surface morphologies, often observed in homoepitaxial GaN layers grown not only by HVPE but also by metal-organic chemical vapor deposition (MOCVD), were found to be covered by an array of meandered bunched steps consisting of m- and a-oriented sections. Because the bunched steps meandered in an in-phase fashion, facets consisting of m- and a-oriented bunched steps formed narrow macro-steps and wide macro-terraces, respectively, leading to the formation of ridges and grooves in-between the facets. Although the use of a substrate with large off-angles (≥0.4°) effectively suppressed the step meandering, i.e., wavy surface morphology, it induced a strong tendency for step bunching. Only growth on surfaces having relatively small off-angles (0.25°), under conditions providing large degrees of adatom desorption, i.e., at high temperatures or low V/III-ratios, suppressed both the meandering and bunching instabilities simultaneously and produced highly smooth as-grown surfaces. The use of the HVPE method seems better than the use of the MOCVD method in this strategy to increase the surface flatness of GaN homoepitaxial layers because it can maintain a sufficiently high growth rate even with a large degree of adatom desorption.