Strategies for modifying the sidewall properties of InGaN/GaN micro-LEDs analyzed by the impact of the sidewall on charge carrier distribution and cathodoluminescence

Micro-LEDs generally suffer a loss of efficiency when they are reduced in size, as the sidewall has an increasingly greater impact on overall performance and promotes non-radiative surface recombination. However, as small micro-LEDs are in high demand due to promising applications in high-resolution displays, the influence of the sidewall must be well understood and mitigated by improvements in fabrication, e.g., directly during the etching process or by surface treatment. To improve the sidewall quality, blue InGaN/GaN micro-LEDs were defined under two different dry etching conditions by changing the plasma power and Ar/Cl₂ ratio, and their sidewalls were treated either by plasma-enhanced chemical vapor deposition of SiO_x or KOH wet chemical etching. The influence of the different surface conditions is analyzed by cathodoluminescence experiments to investigate how the distance between the excitation area and the sidewall affects the luminescence behavior. The distance dependence is significantly different for the investigated etching conditions, which is attributed to different degrees of surface passivation by hydrogen incorporated during dry etching. In addition, strong differences are observed for excitation close to or far from the sidewall, which cannot be explained only by differences in surface recombination, but can be ascribed to lateral confinement of charge carriers by the sidewall restricting spreading by diffusion. Lastly, the results strongly suggest that KOH etching not only removes the surface damaged by the etching process, but also improves the sidewall quality most likely due to the interaction of the etching by-product NH₃ with GaN.