Probing III-V semiconductor heterostructures using time resolved pump-probe techniques

Abstract

In this dissertation, we applied tunable, ultra-fast lasers along with a pump-probe experimental technique to study electron and phonon dynamics. With one channel, Second Harmonic Generation can be monitored, which is a contactless and non-invasive probe that provides time resolved information on carrier dynamics and transport near interfaces. With another channel, differential reflectivity can be monitored, which gives information on coherent longitudinal acoustic phonons as they propagate into the heterostructure. We have studied ultra-fast carrier dynamics in GaAs/GaSb and GaAs/GaSb/InAs heterostructures using a pump-probe electric-field induced second harmonic (EFISH) generation technique. We observed a complicated evolution of the interfacial fields originating from the redistribution of carriers between the interfaces. The ability of the EFISH signal to monitor spatially separated regions makes pump-probe SHG a unique tool for studying relaxation and transport phenomena in multilayer semiconductor structures. We report the first studies of long-lived oscillations in femtosecond optical pump-probe measurements on GaSb/GaAs systems. The oscillations arise from a photo-generated Coherent Longitudinal Acoustic Phonon (CLAP) wave, which travels from the top surface of GaSb across the interface into the GaAs substrate, thus providing information on the optical properties of the material as a function of time/depth. Wavelength-dependent studies of the oscillations near the bandgap of GaAs indicate strong correlations to the optical properties of GaAs. We report the first use of CLAP waves to probe buried GaxIn1-xAs layers in GaAs. There are two features that can be observed when the CLAP wave enters the buried layer: (1) there is a reduction in the amplitude of the oscillations due to absorption of the probe light and (2) there is a phase change that is caused by the difference in the index of refraction and the speed of sound in GaAs and GaxIn 1-xAs. This technique is shown to be a new, non-invasive tool to measure layer thicknesses as a function of depth. A simple model has also been developed that satisfactorily characterizes our experimental results.