Résumé
Long-lived indirect excitons, artificially produced by applying a bias to arsenide double quantum wells (DQWs), are currently the subject of intense investigations. The separation of electron and hole wave functions across the central barrier indeed yields a situation where the recombination time can be made much larger than other relaxation and interaction times between the excitons, to be considered, here, as composite bosonic particles. For example, exciton rings have been observed away from the excitation spot and their formation mechanism and instabilities were described and explained [1]. The current trend of research is the spatial manipulation of such artificial dipolar particles [2], the formation of condensates of them using traps [3] and the study of their coherence properties [4]. Due to the presence of strong internal electric fields, group-III nitride polar single quantum wells naturally induce long-lived excitons. The latter have, in addition, much larger binding energies, smaller Bohr radii and larger total effective masses than their counterparts in arsenide DQWs. Therefore, they appear to be good candidates for the study of dense gases of dipolar exciton, with expected higher temperature of quantum degeneracy. In this context, we present spatially-resolved and time-resolved photoluminescence experiments on GaN/(Al,Ga)N QWs, showing exciton radiative lifetimes of tens of microseconds. We study in detail the dynamics and range of their relaxation/diffusion along the growth plane, as a function of temperature. We discuss the results in terms of the potentiality of these systems for the creation and manipulation of dipolar exciton condensates. [1] L.V. Butov et al. Phys. Rev. Lett. 92, 117404 (2004). [2] A.G. Winbow, Phys. Rev Lett. 106, 196806 (2011). [3] A.T. Hammack et al., Phys. Rev. Lett. 96, 227402 (2006). [4] A.A. High et al., Nature 483, 584 (2012).