Temperature-dependent investigation of polarisation doping in 330 nm ultraviolet light-emitting diodes

TL;DR

This study investigates how temperature affects polarisation doping in 330 nm UV LEDs, revealing that polarisation doping offers temperature-independent carrier concentration benefits and potential for improved device performance.

Contribution

It provides the first detailed analysis of temperature-dependent effects on polarisation doping in UV LEDs, comparing different doping strategies and their impact on device efficiency and spectral quality.

Findings

Co-doped LEDs have highest electroluminescence at room temperature.

Polarisation doping results in temperature-independent carrier concentration.

Reference LEDs show increased voltage with decreasing temperature.

Abstract

Polarisation doping of Al$_x$Ga$_{1-x}$N, through grading of $x$, has realised major improvements in $p$-type conductivity in ultraviolet (UV) light-emitting diodes (LEDs) compared to conventional impurity doping. However, the exact balance between the two doping regimes to achieve the best device performance is not clear, especially as a function of operating wavelength. In this work, 330 nm LEDs with varied $p$-doping approaches were characterised as a function of temperature: Mg doped only (reference); polarisation doped and Mg doped (co-doped); and polarisation doped only. At room temperature, the co-doped LED showed the highest electroluminescence (EL) intensity, with a similar operating voltage to the reference LED. The highest hole concentration, confirmed by Hall effect measurements, as well as improved injection efficiency revealed by simulations, are credited as the main reasons for EL improvement. A parasitic near-UV luminescence tail, analogous to the "blue luminescence" in $p$-GaN, was observed in both the reference and co-doped LEDs, but was absent in the polarisation doped LED. The reference LED demonstrated the highest increase in operating voltage with decreasing temperature, while the LED with only polarisation doping showed a temperature-independent behaviour, demonstrating the benefits of a polarisation field-induced carrier concentration. Further optimisation of the compositional grading with concurrent Mg doping can potentially produce higher performance LEDs with cleaner spectra.