Charge carrier concentrations in linearly graded distributed polarization-doped layers from AlN to Al0.8Ga0.2N and their effect on the far-UVC LED performance

The effect of linearly graded AlN → Al0.8Ga0.2N distributed polarization doped (DPD) layers with varying thickness on the net charge carrier density, output power and external quantum efficiency (EQE) of far-ultraviolet-C light emitting diodes (far-UVC LEDs) has been investigated. Far-UVC LEDs were grown by metal organic vapor phase epitaxy (MOVPE) on AlN/sapphire templates with a linearly graded AlN → Al0.8Ga0.2N distributed polarization doped (DPD) layer with thicknesses between 25 - 150 nm. The net charge carrier density at the edge of the depletion region was determined via capacitance-voltage (CV) measurements increasing from (9.7 {plus minus} 0.54) × 1017 cm-3 for a 150 nm thick DPD layer to (2.5 {plus minus} 0.14) × 1018 cm-3 for a 50 nm thick AlN → Al0.8Ga0.2N DPD layer in excellent agreement with theoretical calculations. The average on-wafer output power (EQE) at 20 mA of the far-UVC LEDs increased from 197 µW (0.19 %) for LEDs with a 150 nm thick DPD to 314 µW (0.3 %) for LEDs with a 25 nm thick DPD. The results show that distributed polarization doping is a promising alternative to conventional Mg-doping of p-type AlGaN.