Résumé
Interest in hexagonal boron nitride (hBN) is booming due to its exceptional properties and potential applications: including a wide bandgap that emits deep ultraviolet light, its 2D structure, and its excellent chemical and thermal stability. In this work, intralayer carrier diffusion and exciton-exciton annihilation in hBN are investigated by time-resolved two-color pump-probe experiments. Two-photon femtosecond excitation makes it possible to monitor the carrier relaxation dynamics in hBN under conditions of negligible surface recombination. A value of the two-photon absorption coefficient beta = 1.7 +/- 0.5 cm GW-1 is measured at 350 nm and its dependence in the 315-415 nm range. The obtained in-plane exciton diffusivity increases with excitation power due to the screening of the exciton-phonon interaction at high charge carrier densities between 1015 and 1017 cm-3. Conversely, the exciton-exciton annihilation efficiency decreases by a factor of five from 80 to 600 K. Due to these efficient Auger processes, the in-plane diffusion length in hBN is reduced from 0.24 to 0.1 mu m as the excitation density increases. The robustness of the photoluminescence intensity is mediated in the high temperature range by excitons with a binding energy of 390 meV.