Impact of Sidewall Conditions on Internal Quantum Efficiency and Light Extraction Efficiency of Micro-LEDs

The sidewall condition is a key factor determining the performance of micro-light emitting diodes (µLEDs). In this study, equilateral triangular III-nitride blue µLEDs are prepared with exclusively m-plane sidewall surfaces to confirm the impact of sidewall conditions. It is found that inductively coupled plasma-reactive ion etching (ICP-RIE) causes surface damages to the sidewall and results in rough surface morphology. As confirmed by time-resolved photoluminescence (TRPL) and X-ray photoemission spectroscopy (XPS), tetramethylammonium hydroxide (TMAH) eliminates the etching damage and flattens the sidewall surface. After ICP-RIE, 100 µm²-µLEDs yield higher external quantum efficiency (EQE) than 400 µm²-µLEDs. However, after TMAH treatment, the peak EQE of 400 µm²-µLEDs increases by ≈10% in the low current regime, whereas that of 100 µm²-µLEDs slightly decreases by ≈3%. The EQE of the 100 µm²-µLEDs decreases after TMAH treatment although the internal quantum efficiency (IQE) increases. Further, the IQE of the 100 µm²-µLEDs before and after TMAH treatment is insignificant at temperatures below 150 K, above which it becomes considerable. Based on PL, XPS, scanning transmission electron microscopy, and scanning electron microscopy results, mechanisms for the size dependence of the EQE of µLEDs are explained in terms of non-radiative recombination rate and light extraction.