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Résumé:

Since Waltereit et al. demonstrated ten years ago the realization of polarization free (Al,Ga)N/GaN quantum wells (QW) 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, the use of non lattice-matched substrates induces strain that relaxes through the generation of dislocations or stacking faults. The dynamics of QW excitons is then dominated by their trapping on these extended defects. In this work, we have adopted a different approach consisting in the homoepitaxial growth of a 4 nm-thick non-polar Al0.06Ga0.94N/GaN single QW. We first show that the resulting heterostructure is free of stacking fault and presents a low dislocation-density that we estimate to 2.105 cm-2. From 10 to 150 K, the QW effective decay time increases from 110 to 350 ps, evidencing the absence of non-radiative recombination channel in this temperature range. The temperature dependence of exciton decay time is instead ascribed to the combined delocalization of excitons bound to interface roughness and to the thermal distribution of free excitons in k-space. From these results, we deduce an oscillator strength of 2.1013 / cm2 for QW free excitons, in agreement with values reported for thin polar (Al,Ga)N/GaN MQWs where the influence of electric fields is negligible.