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

Abstract

The effect of linearly graded AlN → Al0.8Ga0.2N distributed polarization-doped (DPD) layers with varying thicknesses 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 on AlN/sapphire templates with a linearly graded AlN → Al0.8Ga0.2N DPD layer with thicknesses between 25 and 150 nm. The net charge carrier density at the edge of the depletion region was determined via capacitance–voltage measurements increasing from (9.7 ± 0.5) × 1017 cm−3 for a 150 nm thick DPD layer to (2.50 ± 0.14) × 1018 cm−3 for a 50 nm thick AlN → Al0.8Ga0.2N DPD layer, which is 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 layer to 314 μW (0.3%) for LEDs with a 25 nm thick DPD layer. The results show that distributed polarization doping is a promising alternative to conventional Mg doping of p-type AlGaN.