Impact of crystallographic facet of InGaN micro-LED sidewalls on electro-optical characteristics

InGaN micro-LEDs (mu LEDs) with their potential high-volume applications have attracted substantial research interest in the past years. In comparison to other III-V semiconductors, InGaN exhibits a reduced susceptibility toward non-radiative surface recombination. However, efficiency degradation becomes more prominent as dimensions shrink to a few mu m or less. Due to the high surface-to-volume ratio of the miniaturized devices, the non-radiative recombination increases and reduces the internal quantum efficiency. While many groups focus on surface passivation to mitigate surface defects, the influence of crystallographic orientation of the mu LED sidewall on the efficiency remains unexplored. This study addresses this gap by investigating the impact of crystallographic orientation of the sidewalls on the emission properties of the mu LEDs. Hexagonal and elongated mu LEDs with dimensions as small as 3.5 mu m and sidewalls with crystallographically well-defined m- and a-planes were fabricated. Electrical and optical properties were investigated using photo- and electroluminescence. External quantum efficiency (EQE) is assessed based on well-known carrier recombination models. It can be shown that mu LED performance intrinsically depends on the crystallographic orientation of the sidewalls. Comparing hexagonal mu LED structures with a-plane and m-plane sidewalls, an increase in the EQE by 33 % was observed for structures with a-plane sidewalls, accompanied by reduction in the current density of the peak EQE by a nearly two orders of magnitude compared to structures with m-plane sidewalls. By analyzing the EQE characteristics at the mu LED center and near the sidewalls, the improvements can be directly attributed to the increased radiative recombination from sidewalls with a-plane orientation.