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.
We review desorption and structural changes on Si(001)-(2x1) surfaces induced by nanosecond laser irradiation with fluences well below thresholds of melting and ablation. Atomic imaging of the irradiated surface by scanning tunneling microscopy (STM) has shown that bond breaking takes place at intrinsic lattice sites and at atomic sites neighboring vacancies, leading to newly generation of vacancies and sequential growth of vacancy clusters. The bond breaking selectively removes outermost Si-dimers, exposing 1x1 like new phase. Repeated irradiations with a fixed fluence enlarge the new phase region up to 80% of the total surface area with a constant. The major products by the bond breaking are Si atoms emitting with a fluence-independent translational energy distribution, indicating strongly that the bond breaking is a purely electronic. Both efficiencies of Si-desorption and vacancy formation follow a common superlinear function of excitation intensity and show strong photon energy dependence with a prominent peak at 2.7 eV. The electronic bond breaking is shown to originate from nonlinear localization of excited species in surface electronic states.
We review the bond breaking and structural changes on clean surfaces of Si(111)-(7X7) and of InP(110)-(1X1) induced by ns- and fs-laser irradiation with fluences below thresholds of melting and ablation. Atomic imaging of the irradiated surface by scanning tunneling microscopy (STM) has shown that the bond breaking of adatoms of Si(111)- (7X7) is induced by an electronic process to form adatom vacancies mostly at individual adatom sites. Si atoms in the electronic ground state are desorbed with a peak translational energy of 0.06 eV, as a direct consequence of the bond breaking. On the other hand, STM images of the irradiated InP(110)-(1X1) surfaces have revealed the preferential removal of the top-most P atoms, with significant formation yields of vacancy strings consisting of several adjacent vacancies on the quasi-one dimensional P rows. The isolated In vacancies are also formed, but with a much smaller yield. For both surfaces, bond breaking takes place at intrinsic sites of the surface structures, and the efficiency is strongly site-sensitive, resonantly wavelength-dependent, and highly super-linear with respect to the excitation intensity. The electronic bond breaking is shown originate from non-linear localization of excited species in surface electronic states.
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