Subpicosecond thermalization of carriers in polar GaN/AlN quantum dots

We investigate at room temperature the thermalization and recombination dynamics of near-resonantly photogenerated carriers in extit{c}-plane GaN/AlN wurtzite quantum dots (QDs) using time-resolved photoluminescence and non-degenerate pump--probe differential transmission. Owing to a continuous gradient in dot height, the QD ensemble exhibits a broad emission range from 3.5 to 4.5~eV. Time-resolved photoluminescence reveals two distinct recombination lifetimes associated with two families of QDs with different morphologies, whose emission energies are governed by the interplay between the quantum-confined Stark effect and lateral confinement. Pump--probe measurements evidence an ultrafast carrier thermalization with subpicosecond rise times on the order of 200~fs, significantly shorter than in other epitaxial III--V QD systems. At longer delays, the relaxation dynamics is non-exponential and strongly dependent on both carrier density and probe energy. It is reproduced by a phenomenological model based on multi-carrier scattering processes, whose efficiency progressively vanishes as the probe energy approaches the luminescence maximum, where interband recombination dominates.