Impact of etching conditions on the sidewall quality of InGaN/GaN micro-LEDs investigated by cathodoluminescence imaging

As micro-LEDs are further miniaturized for applications in high-resolution displays, size is becoming increasingly important for overall efficiency. To achieve high internal quantum efficiency, non-radiative recombination processes at the sidewall must be well understood and adequately mitigated. For this purpose, blue InGaN/GaN micro-LEDs were defined under two different dry etching conditions by changing the plasma power and Ar/Cl2 ratio, resulting in LEDs that were either stronger physically etched or stronger chemically etched. The LEDs were investigated using monochromatic cathodoluminescence imaging and time-resolved cathodoluminescence to determine how the etching conditions affect the detrimental influence of the sidewall on the quantum efficiency. A significant reduction of cathodoluminescence intensity and lifetime of the stronger physically etched structure from the bulk material toward the sidewall is observed, which is caused by an increase in the non-radiative recombination activity. Conversely, the stronger chemical etching conditions do not lead to a considerable degradation of the luminescence properties near the sidewall, which is attributed to the passivation of near-surface point defects with hydrogen during the etching process. This is corroborated by the intentional depassivation of such defects by low-energy electron beam irradiation or thermal annealing in an inert gas atmosphere leading to a similar efficiency deterioration, as well as by the recovery of efficiency by hydrogen plasma treatment. Overall, the results demonstrate that hydrogen can very effectively restore the efficiency of micro-LEDs at the sidewall. Therefore, the possible influence of hydrogen on the point defects at the sidewall should be considered in future studies investigating sidewall treatments.