KOH Wet Etching Mechanisms of III-N Nanopillars: Impact of Temperature and Concentration

Abstract

Combining dry and wet etching processes is a common technique to pattern III-N semiconductors integrated in optoelectronic devices with a controlled final shape and high-quality crystallographic facets. However, the wet mechanisms driving the final pattern morphology have never been deeply studied. In this work, we investigate the mechanisms involved during KOH wet etching applied on AlN and GaN pillars previously obtained by Cl₂ plasma etching with a hexagonal hard mask, whose edges are oriented with an a or m nonpolar III-N crystallographic orientation. These pillars are intended to serve as the first building blocks for core-shell ultraviolet light-emitting diodes (UV LED), for which the quality of the patterning of the III-N core pillar plays a key role in the subsequent quantum well regrowth. We discuss the impact of the KOH concentration (5 wt % vs 44 wt %), the solution temperature (from room temperature to 80 °C), and the hard mask orientation on the etching kinetics, the etch propagation mechanisms, the crystallographic plane stability, and roughness formation. Our results show how the stability of the crystallographic c- and m-planes and the vulnerability of specific kink sites determine the progression of the KOH wet etching process and the formation of roughness depending on the wet etching conditions and the shape of the hard mask used. With this understanding, we developed a two-step process combining dry and wet etching capable of fabricating high-aspect ratio AlN and GaN nanopillars with the desired smooth and anisotropic m-oriented sidewalls.

Keywords: AlN; GaN; KOH wet etching; crystal orientation-dependent process; roughness formation.