Relaxation Dynamics of Free Carriers

The generation of electron-hole pairs by optical absorption of photons from an excitation source like a pulsed or cw laser leads to a non-equilibrium situation. The free carriers are generated with some excess energy within the conduction and valence band, respectively. After excitation with an ultra-short pulse the carriers do not even have a thermalized distribution. The return of the semiconductor to an equilibrium state now occurs in several steps which happen on different timescales. The fastest step (hundreds of femtoseconds) is typically the formation of thermalized distributions among the electrons and holes, respectively. A common carrier temperature in the electron-hole plasma is typically achieved on a picosecond time scale. Due to the initial carrier excess energy the respective temperatures of the distributions are much higher than the lattice temperature leading to a cooling process on a hundred picosecond timescale. The lattice temperature is often not achieved in the plasma during the lifetime of the electron-hole pairs. Even a thermalized and cooled plasma is only a quasi-equilibrium situation and the carriers have to recombine o restore the equilibrium of an unexcited semiconductor. We will follow this succession of relaxation steps in this chapter and survey the involved interaction processes like carrier-carrier, intervalley, intersubband and carrier-phonon scattering as well as tunneling. We will discuss the impact of relaxation on the performance of hot-carrier solar cells and the application of carrier tunneling in electronic devices. We also present relevant experimental techniques for observation of the dynamics, namely, differential transmission and time-resolved photoluminescence spectroscopy.