Exploring the impacts of u-GaN surface roughness for the optoelectronic performance of mini-LEDs

In this study, 300 × 300 µm² mini-LEDs were fabricated on the patterned sapphire substrate and lifted off by using the diode-pumped solid-state laser. Then, the surface of u-GaN was flattened and roughened (FR) by using the ICP and chemical etching. By adjusting the processing time, three types of blue and green mini-LEDs with different u-GaN roughness surfaces were fabricated. The scanning electron microscope (SEM) and atomic force microscopy (AFM) technologies were employed to measure the roughness of the u-GaN surface of these mini-LEDs. The measured optoelectronic performance of these devices shows that there would be higher light extraction efficiency and external quantum efficiency (EQE) in the mini-LEDs with a rougher u-GaN surface. To further investigate the physical mechanisms of mini-LEDs with varying roughness of u-GaN surface, the luminance distribution and light pattern of these mini-LEDs were measured. The results of junction temperature rising curves of these mini-LEDs suggest that the higher roughness surface could improve the thermal performance of devices. Moreover, it should be noted that there would be a marginal effect for the u-GaN flattening and roughening process for enhancing the performance of mini-LEDs; thus, the cost and optimization of performance should be balanced during the manufacturing procedures.