Origin of carrier localization in AlGaN-based quantum well structures and implications for efficiency droop

Abstract

We investigate carrier localization in Al-rich AlGaN/AlN quantum well (QW) structures. Low temperature time-resolved photoluminescence (PL) experiments reveal a strong variation of the carrier decay times with detection photon energy, suggesting a strong impact of carrier localization, which is found to depend primarily on the QW width. In combination with time-integrated PL measurements and numerical band structure calculations, we are able to provide conclusive evidence that the localization strength in AlGaN-based QW structures is directly coupled to the oscillator strength, providing an explanation for its strong dependence on the QW width. This is further supported by the observation of a strong polarization field dependency of the carrier localization, which excludes excitons and may be explained by the accumulation of electrons close to the QW interface, while holes are independently localized across the QW. We complete our discussion by proposing a model to explain the well-known phenomenon of efficiency droop in accordance with our findings, suggesting delocalization-induced Auger recombination as the responsible loss channel.