Thermal redistribution of valence-band holes and optical polarization in AlGaN (225–230 nm) far-UVC quantum wells on sapphire and IQE evaluation

Abstract

Accurate evaluation of internal quantum efficiency (IQE) in far-ultraviolet-C AlGaN light-emitting diodes relies on proper treatment of light extraction efficiency, which is commonly assumed to be temperature independent. Here, we show that this assumption breaks down due to the temperature dependence of band edge optical polarization in high-Al-content AlGaN multiple quantum wells (MQWs). Polarization- and temperature-dependent photoluminescence measurements are performed on 225 and 230 nm AlGaN MQWs grown on partially relaxed n-AlGaN buffers on c-sapphire. The emission is dominated by the E ∥ c-polarized component over the entire temperature range, indicating that the crystal-field split-off hole band forms the topmost valence band. At 10 K, the optical polarization degree is ρ = −0.76 for the 225 nm MQWs and ρ = −0.49 for the 230 nm MQWs, decreasing to ρ = −0.40 and −0.17, respectively, at 295 K, consistent with thermal redistribution of holes among closely spaced valence bands. Analysis based on a Boltzmann population model yields crystal-field split-off–heavy-hole energy separations of 11 and 6 meV. We demonstrate that neglecting temperature-dependent polarization leads to significant errors in IQE estimation, and here we report corrected room-temperature “estimated upper limits” IQEs of 16.7% and 11%, respectively, for 225 and 230 nm MQWs due to the active non-radiative recombination centers at 10 K.