Effect of a mesa with different sidewall angles on the spatially resolved optical properties and light extraction efficiency for GaN-based blue and green micro-LEDs

In this paper, we studied the sidewall conditions of 28 x 52 mu m2 InGaN-based blue and green micro-LEDs with different sidewall angles and their effects on external quantum efficiency (EQE). Our findings indicate that steeper sidewall mesas can reduce non-radiative recombination and leakage current, which is beneficial for achieving high internal quantum efficiency (IQE). However, as the sidewall angle increases, the light output from the micro-LED tends to concentrate in the internal region, leading to a decrease in light extraction efficiency (LEE). Using microscopic hyperspectral imaging, we observed distinct chromaticity characteristic distributions in the internal area of mesas with different sidewall angles, compared to the entire micro-LEDs. Mesas with gentler sidewalls exhibited lower chromaticity stability. For both blue and green micro-LEDs, the optimal sidewall tilt angle was found to be 48 degrees, yielding the highest EQE. This result reflects a trade-off between LEE and IQE. Notably, the improvement in IQE for green micro-LEDs was not as significant as that for blue micro-LEDs, likely due to the higher indium content in green InGaN micro-LEDs, which results in deeper localized potential wells and a higher dislocation density. This work demonstrates the variation of IQE and LEE for blue and green micro-LEDs fabricated on mesas with different sidewall angles, aimed at achieving the highest EQE.