Light extraction for InGaN-based green thin-film micro-LEDs through surface roughening

Indium gallium nitride (InGaN)-based micro-scale light-emitting diodes (micro-LEDs) have great potential for next-generation micro-displays. While advanced structural designs and sidewall treatments have substantially improved internal quantum efficiency (IQE) by mitigating non-radiative sidewall defects, the strong total internal reflection (TIR) in micro-LEDs remains a critical bottleneck that severely limits light extraction efficiency (LEE). Alkaline solutions have been widely employed to roughen n-GaN surfaces, which increases light scattering and effectively suppresses the TIR effect. In this work, we further investigate the performance differences of micro-LEDs with various sizes under tetramethylammonium hydroxide (TMAH) roughening on the N-polarity n-GaN surface. Green thin-film micro-LEDs grown on silicon substrates, with device sizes ranging from 54 to 14 µm, are designed and fabricated. Experimental results show that the TMAH-roughened micro-LEDs exhibit an external quantum efficiency (EQE) of 24.20% for the 54 µm device, while decreasing to 13.18% for the 14 µm device. The size-dependent EQE is demonstrated to be related to the hexagonal pyramid size variations formed by surface roughening in various-sized devices. It is observed that even under the identical roughening conditions, the average size of the hexagonal pyramids increases with micro-LED sizes. The finite difference time domain (FDTD) simulation further shows that larger hexagonal pyramids lead to a higher LEE, which is consistent with experimental results. This work emphasizes the need to consider surface morphology differences among various-sized devices when improving the LEE of micro-LEDs via surface roughening.