Intralayer Carrier Diffusion and Exciton‐Exciton Annihilation in Hexagonal Boron Nitride Investigated by Two‐Color Pump‐Probe Experiments

Abstract 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 β = 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 10 15 and 10 17 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 µm 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.