ULTRAFAST CARRIER AND LATTICE DYNAMICS STUDIES IN GaAs WITH INTENSE OPTICAL EXCITATION

摘要

Detailed understanding of basic scattering mechanisms of high density and highly energetic carriers (electrons and holes) are important for future semiconductor device technology. During these scattering processes carriers interact among themselves and with the lattice. Most of the scattering events occur in few tens of femtoseconds to picosecond timescale. Ultrafast lasers, with improving performance, make it possible to observe these fundamental interactions as they happen and detailed study on them becomes possible.Here we present ultrafast carrier and lattice dynamics studies in the technologically important III-V semiconductor GaAs. We performed ultrafast pump-probe spectroscopy in transient reflection and reflective electro optic sampling geometries on n-doped GaAs with 10^18-10^20 cm^-3 photoexcited electron-hole pair density. Coherent phonons and plasmons are generated in the semiconductor as a result of interaction between the photocarriers and the lattice; they modulate the dielectric properties of the sample, which can be detected by measuring transient reflectivity. By varying the excitation laser fluence, we varied the excited carrier density and measured the response of the semiconductor at different photoexcited carrier density levels. Detailed analysis of density dependent response sheds light on generation mechanism of coherent phonons and plasmons and subsequent dynamics and transport. Further analysis using a dielectric response model shows that the carrier density dependent coupled plasmon-phonon dynamics represents the hole plasma response. We also performed symmetry selective generation and detection of coherent phonons and plasmons by exciting and probing the crystal along different crystallographic directions. These results tell us about the dominating processes responsible for the generation and detection of these modes. We further analyzed the changes in the frequency response with time and probed the plasmon-phonon response at various photon energies of the exciting and probing pulse. Time dependent frequency of the coupled plasmon-phonon mode indicates transient changes in the plasma density, most likely involving transport of the interacting carriers from the observed region on the sample.