Indirect excitons (IXs) offer an opportunity to explore dipolar bosonic fluids in semiconductors. Indeed, IXs are quasiparticles hosted by quantum heterostructures, e.g. quantum wells (QWs). Such IXs are Coulomb-bound pairs consisting of an electron and a hole individually confined at the two opposite QW interfaces. Main properties of IX fluids, such as radiative lifetimes, in-plane transport, achievable densities, interactions and possible collective behaviour are determined by IX dipole length, roughly given by the QW width, d. In contrast with GaAs-based heterostructures, GaN/(AlGa)N QWs may host IXs with relatively high binding energy even in the absence of external electric field [1]. Independent control over the IX density and temperature in electrostatic traps, consisting ofvarious metallic patterns on the sample surface, has been demonstrated [2].In this presentation we explore the effect of the dipole length on spatio-temporal dynamics of IX fluids, either confined in electrostatic traps, or expanding freely in the GaN/AlGaN QW plane, see Fig. 1. Relative importance of IX lifetime, IX-IX interaction strength and interface roughness in the quest for (i) efficient IX transport and (ii) realisation of high-density cold IX fluids is addressed. The analysis is supported by the drift-diffusion-losses model accounting for dipole length dependent interactions between IXs. The obtained results contribute to the understanding of IX fluid properties in GaN/AlGaN QWs, and could help optimisation and control of their collective behaviour.[1] F. Fedichkin et al., Phys. Rev. Applied 6, 014011 (2016)[2] F. Chiaruttini et al., Nano Letters 19, 4911 (2019)

Le laser à polaritons en géométrie de guide d’onde intègre une couche active en régime de couplage fort exciton-photon. Dans une cavité GaN horizontale courte (60µm), nous démontrons un fonctionnement en continu ou en régime de blocage de modes contrôlable, dépendant du spectre de gain, de la dispersion de vitesse de groupe et de la non-linéarité des polaritons.

Le laser à polaritons en géométrie de guide d’onde intéresse de par ses propriétés uniques résultant du couplage fort exciton-photon. Cette étude se concentre sur une cavité GaN courte de 60 µm, où le fonctionnement en continu ou en régime de blocage de mode est observé. La dépendance de ces régimes est étroitement liée au spectre de gain, à la dispersion de vitesse de groupe et à la non-linéarité des polaritons.

Les fluides quantiques d'excitons indirects sont un système bosonique intéressant en tant qu'état collectif quantique de longue durée de vie. Nous présentons les progrès récents pour leur génération, leur refroidissement et leur piégeage dans des puits quantiques GaN/(Al,Ga)N, ainsi que les premières indications de condensation dans un état collectif de forte densité, dans l'état sombre de l'exciton.

GaN/AlxGa1−xN quantum wells were grown by molecular beam epitaxy on high quality bulk (0001) GaN substrates. The quantum well thickness was set in the 6–8 nm range to favor the photoluminescence emission of indirect excitons. Indeed, such excitons are known to be spatially indirect due to the presence of the internal electric field which spatially separates the electron and hole wave functions. The growth conditions were optimized in view of minimizing the photoluminescence peak broadening. In particular, the impact of growth temperature (up to 900 °C) on the surface morphology, structural, and photoluminescence properties was studied. The diffusion of indirect excitons on the scale of tens of micrometers was measured with a micro-photoluminescence setup equipped with a spatially resolved detection. A dedicated model and its analysis allow us to extract from these measurements the exciton diffusion constant and to conclude on the optimum growth conditions for the GaN/AlxGa1−xN quantum well structures suited for studies of quantum collective effects in indirect exciton liquids.

The Ni-Au-Ga interdiffusion mechanisms taking place during rapid thermal annealing (RTA) under oxygen atmosphere of a Ni-Au/p-GaN contact are investigated by high-resolution transmission electron microscopy (HR-TEM) coupled to energy dispersive X-ray spectroscopy (EDX). It is shown that oxygen-assisted, Ni diffusion to the top surface of the metallic contact through the formation of a nickel oxide (NiOx) is accompanied by Au diffusion down to the GaN surface, and by Ga out-diffusion through the GaN/metal interface. Electrical characterizations of the contact by Transmission Line Method (TLM) show that an ohmic contact is obtained as soon as a thin, Au-Ga interfacial layer is formed, even after complete diffusion of Ni or NiOx to the top surface of the contact. Our results clarify that the presence of Ni or NiOx at the interface is not the main origin of the ohmic-like behavior in such contacts. Auto-cleaning of the interface during the interdiffusion process may play a role, but TEM-EDX analysis evidences that the creation of Ga vacancies associated to the formation of a Ga-Au interfacial layer is crucial for reducing the Schottky barrier height, and maximizing the amount of current flowing through the contact.