Ultrafast bulk carrier dynamics in various GaN crystals at near-infrared wavelengths under one- and two-photon absorption

Femtosecond transient absorption (TA) as a probe of ultrafast carrier dynamics was conducted at near-infrared wavelengths in a series of GaN crystals. The TA kinetics in all the GaN crystals appeared to be single exponential under one-photon (1P) excitation but biexponential under two-photon (2P) excitation, which was inconsistent with previous experimental reports and model predictions. Surface recombination and carrier diffusion could be eliminated and the TA responses were identified as phonon-assisted indirect free-carrier absorption. Modelling the bulk carrier dynamics with a simplified model revealed that, at a 1P high carrier injection level, the carrier recombination was limited by the slow capture rate of electrons via deep defects, while at a 2P low carrier injection level, the initial carrier lifetime decreased remarkably due to fast hole capturing and could be further controlled by the inherent carrier and/or dislocation concentrations.