The transmission modulation effect of patterns on the light extraction of ultraviolet-C light emitting diodes

The AlGaN-based ultraviolet-C light emitting diodes (UVC LEDs) exhibit low light extraction efficiency (LEE), and patterning substrate surfaces is considered an effective solution. In this work, a simplified model based on the light extraction process of typical flip-chip UVC LEDs is proposed, which accelerates the simulations and illustrates the transmission process of patterned substrates more clearly. It is found that, different from the case in InGaN-based visible LEDs, the patterns on the substrate surfaces of UVC LEDs enhance the LEE by modulating the transmittance. The effects of sub-micron patterns are also studied, and the results suggest that the effects of LEE enhancement from different-scaled patterns vary little, unless the scale is decreased below a certain threshold so that the LEE decreases significantly. The results also show that AlN substrates can exhibit a 33% relative LEE enhancement if properly patterned, in contrast to the 18% enhancement in the case of sapphire. The proposed models and the acquired conclusions should be of help in designing UVC LEDs with high efficiency, especially for those on AlN substrates.