Exciton Interactions and Dynamics

Excitons are a stable species in many semiconductors with a sizeable excitonic binding energy at least at low temperatures. These excitons have scattering interactions among themselves, with free carriers or with phonons. Such interaction processes determine the fate of the excitons starting from their initial formation from free electrons and holes until ionization due to thermal dissociation by phonons or screening at high carrier/exciton densities. Scattering is further essential for relaxation processes within an exciton population like thermalization and cooling. It can also assist recombination of the excitons leading to pronounced additional features in optical spectra. The energy relaxation of excitons is typically accompanied by ballistic or diffusive spatial relaxation in case of a spatially inhomogeneous generation. Fluctuating potentials in disordered semiconductors like alloys or in quantum wells can localize excitons which then relax below a mobility edge via hopping processes. This chapter ends with the discussion of the recombination of excitons and its temporal dynamics. Photoluminescence is one of the central tools to access the named relaxation processes. This method—in particular the time-resolved version, e.g., using a streak camera—and the analysis of the resulting spectra will be described in large detail. But we also highlight other experimental techniques like optical pump/THz probe, transient absorption microscopy and laser-induced gratings. We restrict the discussion of exciton dynamics here on materials where the quasi-particles have at least one motional degree of freedom. The dynamics of excitons in quantum dots will be presented in Chap.  8 .