Résumé
Since Waltereit et al. demonstrated ten years ago the realization of polarization free (Al,Ga)N/GaN quantum wells (QWs) a lot of work has been carried out on non-polar nitride-based heterostructures. The current interest in non-polar GaN is indeed motivated by the absence of built-in electric fields, allowing the growth of wide QWs without any decrease in the overlap between electron and hole wave functions. However, even when using processing techniques such as epitaxial lateral overgrowth (ELO), non-polar GaN grown on sapphire presents high densities of extended defects. While dislocations are considered as non-radiative recombination centers, basal plane stacking faults (BSFs) are optically active and give rise to an emission centered at 3.42 eV. We present here a low-temperature time-resolved (TR) cathodoluminescence (CL) study of exciton dynamics as a function of the local BSF density in a-plane (Al,Ga)N/GaN single QWs grown by molecular beam epitaxy on a ELO-GaN template. Our TR-CL set-up has spatial and temporal resolutions of 50 nm and 50 ps, respectively, and experiments can be carried out between 25 and 300 K. We first demonstrate through CL experiments the existence of regions nearly free of any BSF as well as of regions with bundles of BSFs, indicating that the BSF distribution in the QW plane reproduces well the one of the underlying a-plane GaN template. We confirm the results obtained by Badcock et al., who demonstrated that the intersection of BSFs with the QW leads to the formation of QWRs, and we then study the local relaxation-recombination dynamics of excitons in both QW and QWRs. We show, in particular, that the dynamics of QW excitons is dominated by their capture by the BSFs and therefore exhibits a strong position dependency, explaining the large range of values reported so far for exciton radiative lifetimes in non-polar (Al,Ga)N/GaN QWs. We finally demonstrate that below 60 K, QWR excitons exhibit a zero-dimensional behavior, which we relate to their binding on localization centers such as QW-width fluctuations.