Surface passivation of GaN micropillar structures through KOH wet etch

Surface properties are important for structures such as micropillars and nanowires, which are critical for emerging devices including μLEDs, nano-lasers, and vertical power transistors due to increased surface to volume ratios. Fabrication of III-Nitride micropillars can be realized through a top-down approach, where structures are defined through lithography and reactive ion etching (RIE). While effective at forming these micropillar structures, RIE etching leaves behind roughened, non-vertical sidewalls. This surface damage increases non-radiative recombination, forms current leakage paths, and can severely degrade device performance. However, damage can be removed through a follow-up wet etch in potassium hydroxide (KOH) solution. KOH acts as a crystallographic etchant, preferentially exposing vertical <1-100> m-planes, producing smooth, vertical sidewalls. Here, we investigate KOH wet etch passivation for 2.5 μm diameter top-down fabricated GaN micropillars utilizing different temperatures and solution concentrations, and the effects of a Ni etch mask present during wet etching. We observed an average etch rate of 11.67 nm/min for micropillars etched in 60% AZ400k solution compared to 9.44 nm/min for micropillars etched in 20% AZ400k solution, both at a temperature of 80°C. At a constant 40% AZ400k concentration, an average etch rate of 14.39 nm/min for micropillars etched at 90°C are observed compared to 9.89 nm/min for micropillars etched at 70°C. Micropillars with a Ni etch mask present during KOH etching have an average etch rate of 9.46 nm/min compared to 12.83 nm/min for those without a Ni mask. The effects of KOH etching work to further optimize the performance of GaN-based micropillar and nanowire devices.