Subpicosecond Raman Spectroscopy of Electron — LO Phonon Dynamics in GaAs

Abstract

Highly excited (“hot”) conduction band electrons in GaAs lose their energy primarily by the emission of zone center LO phonons[1]. This process is of interest because of the high-speed devices fabricated from GaAs. We have measured with sub-picosecond time resolution the spontaneous Raman spectrum of optically-pumped GaAs. When the optically-injected carrier density is less than 10¹⁷ cm⁻³, we temporally resolve the growth of the optically-induced nonequilibrium LO phonon population and show that the electron phonon scattering time is about 165 femtoseconds. This is the first direct measure of the scattering rate. Previous work[2] at 77K and 3 psec time resolution has measured the lifetime of these phonons to be 7 psec. In addition to studying the dynamics of LO phonon emission, we have used sub-picosecond Raman scattering at higher injected carrier densities to study the dynamics of the screening of thermal LO phonons by a non-equilibrium electron plasma. We find that the screening of the phonon by the plasma is not efficient until the carriers have relaxed to the band edges, which takes about 3 psec.