Résumé
An indirect exciton (IX) is a quasiparticle consisting of an electron and a hole spatially separated in two different planes of a quantum nanostructure, thus exhibiting a strongly dipolar character. Current research on transport properties of IXs is opening a pathway to the development of novel optoelectronic devices, which have already been demonstrated in GaAs-based heterostructures [1-3]. Applying the same ideas to IXs in wide-band gap polar quantum wells (QWs) is particularly promising because of much larger exciton binding energies and natural dipoles induced by strong built-in electric fields. We have recently studied the transport of IXs in GaN/AlGaN QWs grown on sapphire substrates, at temperatures up to 80 K [4], by mapping the micro-photoluminescence (µ-PL) signal obtained under intense, point excitation. The low-temperature PL recorded at long distances from the excitation spot (30 < r < 100µm) turned out to be a secondary PL, excited by the light emitted at the central spot, guided along the plane, due to the refractive index contrast between the layer and the substrate. At higher temperatures, this signal is rapidly quenched and the distance reached by the measurable PL is limited by recombination of excitons at non-radiative defects. Using GaN substrates instead of sapphire should both suppress the secondary emission and the nonradiative recombination, by reducing dislocation densities by 3-4 orders of magnitude. In this work, we compare exciton propagations in two GaN/Al0.19Ga0.81N QWs of identical structures, except for the substrates, respectively of GaN and sapphire. For the GaN substrate, we indeed observe the mere propagation of excitons up to 35 µm away from the excitation spot and up to 250 K (see below). We propose a drift/diffusion modelling of exciton transport, accounting for dipole-dipole repulsion in high-density regions and for disorder along the sample plane.[1]Y. Y. Kuznetsova, M. Remeika, A. A. High, A. T. Hammack, L. V. Butov, M. Hanson, and A. C. Gossard. Optics Letters 35 (10), 1587 (2010).[2]A.A. High, E.E. Novitskaya, L.V. Butov, and A.C. Gossard. Science 321, 229 (2008).[3]A.A. High, A.T. Hammack, L.V. Butov, and A.C. Gossard. Optics Letters 32, 2466 (2007).[4]F. Fedichkin, P. Andreakou, B. Jouault, M. Vladimirova, T. Guillet, C. Brimont, P. Valvin, T. Bretagnon, A. Dussaigne, N. Grandjean, P. Lefebvre. Phys. Rev. B 91, 205424 (2015).