Photoluminescence dynamics due to biexcitons and exciton-exciton scattering in the layered-type semiconductor PbI2

The dynamics of photoluminescence due to biexcitons and exciton-exciton scattering (M and P emissions, respectively) has been investigated in the layered-type semiconductor PbI2 by using the optical Kerr gate method. We simultaneously observed P and M emissions under high-density excitation. The M emission emerges instantaneously, whereas the P emission shows a delayed onset whose latency increases as the excitation photon energy increases. The latency to onset indicates that the P emission takes place after the relaxation of excitons with excess energy toward the bottleneck region via exciton-longitudinal optical (LO) phonon scattering processes. Based on the time-dependent peak energy shift of the P emission and a line-shape analysis of the M emission, we evaluated the effective temperatures of both photogenerated excitonic and biexcitonic systems as well as the self-energy due to the collisions among biexcitons. We conclude that these systems are separately formed in space owing to potential fluctuations between PbI2 layers, and independently reach thermal equilibrium after ~30 ps with different cooling processes. The exciton-exciton and exciton-LO phonon scattering processes play an important role in cooling the excitonic system, whereas the biexciton-biexciton and biexciton-exciton collisions are dominant in cooling the biexcitonic system