When an external parameter such as an electric field or an optical generation rate is changed as a function of time, carriers in the semiconductor respond on this disturbance by a redistribution controlled by relaxation times. Relaxation proceeds by elastic or inelastic scattering with carriers, phonons, defects, or spin momenta, and respective time constants range from femtoseconds to years. Relaxation of injected carriers is given by the carrier lifetime and related to their diffusion or drift length. Nonthermal excess energy of hot carriers is transferred to the lattice mostly by optical phonons. At high carrier density also plasmons, and at high carrier-generation rates and low lattice temperature, condensation into electron-hole droplets with evaporation into excitons are involved. Optical phonons, excited by fast carriers or by an IR light pulse, relax their momenta by elastic scattering with phonons in the same branch, or by a decay into acoustical phonons. Relaxation of excitons created by nonresonant optical excitation proceeds by inelastic scattering, eventually yielding radiative recombination for momenta near the zone center. The rise time in the luminescence after pulse excitation is controlled by the balance to uncorrelated electron-hole pairs. Resonantly excited excitons show a fast rise in the coherent regime and an exponential decay with an observed time constant depending on excitation density. Carrier spin and orbital momenta are coherently aligned by excitation with polarized light. The subsequent relaxation can be detected by the degree of polarization of the radiative recombination. Holes in semiconductors with 1 degenerate valence bands at the zone center have short spin-relaxation times in the sub-ps range; lifting this degeneracy slows relaxation down. Electrons have usually longer spin-relaxation times, limited by various mechanisms. In an exciton with weak electron-hole interaction the spin-relaxation time of the sequential spin flip of electron and hole is given by the slower particle, while at stronger interaction the faster simultaneous spin flip occurs.
Böer, K. W., & Pohl, U. W. (2017). Dynamic Processes. In Semiconductor Physics (pp. 1–56). Springer International Publishing. https://doi.org/10.1007/978-3-319-06540-3_32-1