Ultrafast relaxation of photoinjected nonthermal electrons in the Γ valley of GaAs studied by time- and angle-resolved photoemission spectroscopy

Abstract

We study ultrafast relaxation of nonthermal electrons confined within the \ensuremath{\Gamma} valley of GaAs, based on the momentum-resolved transient electron distribution functions determined using time- and angle-resolved photoemission spectroscopy at 293 K. To elucidate the fundamental processes that lead to the electronic quasithermalization at the nonthermal regime of relaxation, the dynamics in both $p$- and $n$-type samples of GaAs are investigated. In $p$-type GaAs, photoinjected electrons form the electron ensembles quasiequilibrated only in the momentum space within 100 fs of excitation by the Coulomb interaction among photoinjected electrons. However, the ensembles maintain strong nonthermal populations in the energy space. They are quasithermalized only after certain time delays of a few hundred femtoseconds, which depend on the excess energy of photoinjected electrons. These features in $p$-type samples are not sensitively dependent on the excitation density in the range from $1.5\ifmmode\times\else\texttimes\fi{}{10}^{16}$ to $5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$. On the other hand, in $n$-type samples, which include the cold electrons populated near the conduction-band minimum with the density of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$, photoinjected electrons are quasithermalized within 170 fs of excitation. The main interaction for the fast quasithermalization is attributed to the inelastic scattering among the cold electrons and photoinjected electrons by a dynamics screened Coulomb interaction like the electronic thermalization in metals.