Positive binding energy of a biexciton confined in a localization center formed in a single InxGa1-xN/GaN quantum disk

We report microphotoluminescence spectroscopy performed on individual and ensemble InGaN/GaN quantum disks (Q-disks). The typical spectrum of a single Q-disk exhibited the contribution of localization centers (LCs) formed in the InGaN active layer of the Q-disks, characterized by sharp lines appearing on the low energy side of the spectra. In addition, a broader emission peak identified as the luminescence of the quasi-two-dimensional (2D) InGaN active layer surrounding the LCs appears systematically at higher energy. Time-resolved photoluminescence experiment performed on single Q-disks exhibited the excitonic transfer, from the 2D InGaN active layer to LCs, at the submicroscopic scale. Excitation power dependence studies and linear polarization analysis allowed us to identify a biexciton complex confined in a LC in a single Q-disk with a surprising positive binding energy of 13 meV. The absence of screening effect by increasing the excitation power density and the fast excitonic radiative lifetime of a few hundred picoseconds that we measured on several individual Q-disks indicate that the absence of internal electric field in the structure can explain the observed positive biexciton binding energy.