Résumé
Wide GaN/(AlGa)N quantum wells (QWs) host a promising realization of dipolar or indirect excitons (IX). Due to their built-in dipole moment and long lifetime, IXs offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Compared to their counterparts in GaAs-based heterostructures, IXs in nitrides are “naturally” indirect. This means, that they can be engineered and manipulated in as-grown heterostructures, even without application of an external electric bias [1, 2]. At high densities Mott transition of IXs (avalanche ionization due to screening and phase space filling) limits the formation of dense exciton fluids, and thus excitonic condensation. Mott transition of IXs has been addressed both experimentally and theoretically, but mainly in GaAs-based coupled quantum wells under electric bias. However, so far, no consensus regarding the dynamics of this transition in two-dimensional systems has been reached, neither for traditional excitons, nor for IXs.In this work we demonstrate the Mott transition from a dipolar excitonic to an electron-hole plasma state. It is demonstrated in a wide GaN/(Al,Ga)N quantum well at T = 7 K by means of spatially-resolved µPL spectroscopy [3]. Increasing optical excitation density we drive the system from the excitonic state, characterized by a diamagnetic behavior and thus a quadratic energy dependence on the magnetic field, to the unbound electron-hole state, characterized by a linear shift of the emission energy with the magnetic field, see Fig. 1. The complexity of the system requires to take into account density-dependence of both the exciton binding energy and the exciton-exciton interaction and correlation energy that are of the same order of magnitude. We estimate the carrier density at Mott transition as n_Mott=2×10^11 〖 cm〗^(-2) and address the role played by excitonic correlations in this process. Our results strongly rely on the spatial resolution of the photoluminescence and the assessment of the carrier transport. We show, that in contrast to GaAs/(Al, Ga)As systems, where transport of dipolar magnetoexcitons is strongly quenched by the magnetic field due to exciton mass enhancement, in GaN/(Al,Ga)N the band parameters are such that the IX transport is preserved up to 9 T.[1] F. Fedichkin et al, Phys. Rev. Applied, 6, 014011 (2016). [2] F. Chiaruttini et al, Nanoletters 19, 4911 (2019).[3] F. Chiaruttini et al, Phys. Rev. B 103, 045308 (2021).