Interface Roughness, Carrier Localization, and Wave Function Overlap in c-Plane (In, Ga) N/GaN Quantum Wells: Interplay of Well Width, Alloy Microstructure, Structural Inhomogeneities, and Coulomb Effects

In this work, we present a detailed analysis of the interplay of Coulomb effects and different mechanisms that can lead to carrier-localization effects in c-plane (In, Ga) N/GaN quantum wells. As mechanisms for carrier localization, we consider here effects introduced by random alloy fluctuations as well as structural inhomogeneities such as well-width fluctuations. Special attention is paid to the impact of the well width on the results. All calculations have been carried out in the framework of atomistic tight-binding theory. Our theoretical investigations show that independent of the well widths studied here, carrier-localization effects due to built-in fields, well-width fluctuations, and random-alloy fluctuations dominate over Coulomb effects in terms of charge-density redistributions. However, the situation is less clear cut when the well-width fluctuations are absent. For a large well width (approximately >2.5 nm), charge-density redistributions are possible, but the electronic and optical properties are basically dominated by the out-of-plane carrier separation originating from the electrostatic built-in field. The situation changes for lower well widths (<2.5 nm), where the Coulomb effect can lead to significant charge-density redistributions and, thus, might compensate for a large fraction of the spatial in-plane wave-function separation observed in a single-particle picture. Given that this in-plane separation has been regarded as one of the main drivers behind the green gap problem, our calculations indicate that radiative recombination rates might significantly benefit from a reduced quantum-well-barrier-interface roughness.