Ultrafast carrier dynamics in Si and on Si surfaces studied by femtosecond time-resolved two-photon photoemission spectroscopy

We review our recent results on ultrafast dynamics of photogenerated electrons in Si and at Si(001)-(2x1) surfaces studied by femtosecond time-resolved two-photon photoemission spectroscopy. The photoemissin from the conduction band minimum (CBM) in Si, emitted via an inverse LEED state promoted by the surface photoeffect, provides a powerful tool to study the hot-electron dynamics in the bulk conduction band of Si. The relaxation in the X valley has been characterized with a fast formation of quasi-equilibrated hot electron system near the CBM and the energy relaxation process specified with the time constant of 240 fs (at 296 K), which is not dependent on the electron excess energy initially given to the electrons. The bulk conduction electrons are transferred into the surface un-occupied state on the Si(001)-(2x1) surface with respective contributions; the hot electrons are less effective in the transition. Ultrafast density loss process of conduction electrons is induced in 1 ps of excitation near the surface, which is specific to the relaxing electrons with higher energy and higher T*. The dynamical electron-hole recombination mechanism via a surface deep localized state has been proposed.