Ultrafast optical studies of electronic dynamics in semiconductors

Abstract

The dynamics of charge carriers in semiconductors are of fundamental importance for semiconductor applications. This includes studies of energy relaxation, carrier recombination, and carrier transport (both diffusive and ballistic). Due to their limited temporal resolution, electron measurement techniques cannot be used to study these processes on time scales in which the carrier-lattice system is not in equilibrium. However, in contemporary semiconductor devices with nanometer dimensions, this is the regime that is of interest. In this dissertation, ultrafast optical experimental techniques and results from various semiconductors are presented, which provide information about nonequilibrium electronic dynamics. First, a time resolved pump-probe technique is discussed, which can be used to measure carrier energy relaxation and carrier lifetime, and results are presented on reduced graphene oxide, Si/SiGe quantum wells, and single walled carbon nanotubes. Then, a spatially and temporally resolved pump-probe technique is discussed, which can be used to study carrier diffusion, and results are presented on GaAs, graphene, Si/SiGe quantum wells and single walled carbon nanotubes. Next, a quantum interference and control technique and a differential pump-probe technique that can be used to inject and detect ballistic currents are discussed along with results for the efficiency of such an injection technique and a demonstration of an AC spin polarized charge current in GaAs that was injected and detected using these techniques. Finally, a current-induced second harmonic generation technique that can be used to directly study currents is discussed, with results presented on both steady state and transient currents in GaAs.