Enhanced light extraction efficiency of far-ultraviolet-C LEDs by micro-LED array design

AlGaN-based far-UVC light emitting diodes (LEDs) with an emission wavelength of 233 nm were fabricated in the form of micro-LED arrays with emitter diameters ranging from 1.5 to 50 μm. The mesa was plasma etched with a sidewall angle of 45°–50°, and insulator layers made of SiNx or SiO2 were deposited. While the external quantum efficiency (EQE) of the LEDs with SiNx showed only a small dependency on the micro-LED diameter, the LEDs using SiO2 showed an increase in the peak EQE by a factor of four as compared to large area devices. This enhancement is attributed to a strong increase in the light extraction efficiency due to total internal reflection and re-direction at the inclined mesa, allowing TM-polarized light emitted in the plane of the quantum well to be extracted through the sapphire backside of the chips.