Light extraction enhancement of AlGaN-based ultraviolet light-emitting diodes by substrate sidewall roughening

The effect of sapphire substrate's sidewall roughening on light extraction of AlGaN-based ultraviolet light-emitting diodes (UV LEDs) is investigated. Finite-difference time-domain simulation reveals that a roughening region for effective light extraction enhancement exists on the substrate's sidewall. Roughening outside the effective roughening region will result in undesirable inward photon scattering and absorption. For TM-polarization-dominated deep-UV LEDs, the effective roughening region is from the substrate's backside to the position about L/2 × tan(θc) away from the substrate's front side, where L is the side length of the LED device and θc is the critical angle of total reflection. In the LED device fabrication, picosecond laser dicing is employed to realize roughening layers on the sapphire substrate's sidewalls. The UV LEDs with three roughening layers have 13.2% higher average light output power (LOP) at 20 mA than those with two roughening layers, thanks to the larger roughening area in the ...