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)%.