Light extraction and external quantum efficiency of 235 nm far–ultraviolet‐C light‐emitting diodes on single‐crystal AlN substrates

The effect of bulk AlN single-crystal substrate thickness on the light extraction efficiency and external quantum efficiency of far ultraviolet-C light emitting diodes (far-UVC LEDs) has been investigated. AlGaN multi-quantum well LEDs designed for emission around 235 nm are grown by metal organic vapor phase epitaxy on low defect density bulk AlN substrates as well as on AlN/sapphire templates. As the AlN substrate is thinned step-by-step from 550 to 70 μm by chemo-mechanical polishing, the output power of LEDs increased from 3 μW for a thickness of 550 to 110 μW for a thickness of 70 μm (on-wafer at a dc current of 100 mA). From the Lambert–Beer law an effective absorption coefficient of (84 ± 4) cm−1 is derived for the AlN substrate. Monte Carlo ray-tracing simulations for an AlN substrate thickness of 70 μm yield an on-wafer LEE of (0.9 ± 0.1)% and the internal quantum efficiency was estimated to be (5.6 ± 1.5)%.