Femtosecond time-resolved high-field transport in gallium arsenide bulk and quantum-well structures.

Abstract

Motivated by the needs of understanding the physics and advancing the technology of high-speed devices, it is the goal of this thesis to investigate experimentally the fundamental high-field transport phenomena in semiconductors on the order of a hundred femtoseconds, a time scale for the major scattering events. A series of experiments have been performed applying femtosecond optical techniques to probe transient electron transport and electric field dynamics in high field transport, in GaAs bulk and quantum well structures. I will describe experimental attempts to measure physical quantities associated with transport transients, such as electron distribution functions, electron velocity, and electron acceleration, by means of femtosecond absorption saturation, excitonic electroabsorption, and THz electromagnetic radiation, at high electric fields. Important transport phenomena such as ballistic acceleration on a time scale of 150 fs, the onset of a saturated velocity on a time scale of 1 ps, transient velocity overshoot dynamics have been observed in these experiments. Furthermore, the time-dependent electric field associated with transport has also been measured. Both space-charge screening and THz radiation contribute to the collapse of the applied electric field at high carrier densities. Under such circumstances, transport and electric field dynamics are strongly coupled. An energy overshoot is demonstrated due to the field collapse coupled with strong electron-LO phonon scattering on a time scale of 200 fs. Self-consistent transport and field considerations are required in theoretical modeling. Motivated by transport studies, a novel coherent detector for THz radiation based on excitonic electroabsorption effect has been developed. I will describe the new detector which enables us to measure THz radiation transients with significant improvement on detection speed over the conventional photoconductive sampling techniques.