Sommaire:

Due to the wurtzite crystal structure of these materials, electron and hole are separated along the QW growth axis, so that excitons can be considered as indirect excitons (IX), a family of long-living bosonic quasi-particles with dipole moment oriented along the QW growth axis.

IXs are considered as a model system for studies of collective states in quantum gases of bosons, and are also promising for the development of excitonic circuit devices.

Long lifetimes and dipole repulsion allow IXs to travel over large distances before recombination providing the opportunity to study exciton transport by optical imaging.

In this thesis we address IX transport in a set of GaN/(Al,Ga)N and ZnO/(Mg,Zn)O QWs. This choice of IXs is motivated by the high binding energy and the potential stability up to room temperature, but it presents a number of experimental challenges, including (i) the dramatic dependence of the exciton radiative lifetime on the exciton density that makes exciton density measurement very complex, (ii) the thermally activated nonradiative recombination that quenches the exciton PL at room temperature, (iii) the coexistence of photon propagation with exciton propagation along the QW plane, and the strong inhomogeneous broadening of the exciton emission due to strong built-in electric fields and the presence of both monolayer fluctuations of the QW thickness and the fluctuations of alloy composition in the barriers.

We have addressed all these issues and demonstrated exciton propagation by combining continuous wave $\mu$-photoluminescence and time-resolved spectroscopy measurements, supplemented by modelling of the exciton transport within the drift-diffusion formalism.

In the best quality GaN/(Al,Ga)N QWs grown on free-standing GaN substrates we achieved room-temperature propagation over $\sim10$~$\mu$m and up to $20$~$\mu$m at $4$~K.

Our results suggest that propagation of excitons under continuous-wave excitation is assisted by efficient screening of the in-plane disorder. Nevertheless, exciton propagation is limited by the exciton scattering on defects rather than by exciton-exciton scattering, so that improving interface quality can boost exciton transport further.

Pour plus d'informations, merci de contacter Lefebvre P.