Faceting mechanisms of GaN nanopillar under KOH wet etching

This study presents strategies for achieving GaN pillars with the desired m- (or even a-) oriented nonpolar facets through a top-down approach that combines plasma etching followed by room temperature KOH wet treatment processes. Indeed, GaN etching in KOH solution is an anisotropic process, meaning that it allows the appearance of stable facets at the macroscopic scale, while atomic processes, such as step-flow, drive the fundamental mechanisms of the wet etching at the microscopic scale. Our study highlights the key role played by both the shape (circular or hexagonal, aligned with either the m-planes or the a-planes) and the roughness of the hard mask in determining the resulting crystalline facet formation and their associated roughness. Furthermore, it underscores the importance of the GaN pillar profiles (reentrant, straight, tapered) after plasma patterning, as they strongly influence the subsequent wet etching mechanisms. Ultimately, the article demonstrates that smooth m-oriented facets can be achieved by using room temperature wet KOH (44 wt%) on slightly sloped GaN profiles after plasma etching, in conjunction with the use of hexagonal m-oriented hard masks.