Study of the electron dynamics on semiconductor surfaces by the use of the time-resolved two-photoelectron spectroscopy with the femto-second pulse-laser.

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In this study, I constructed the experimental system consisting of a time-resolved two-photon photoelectron spectroscopy (TR2PPE) with a fsec-pulse laser, scanning tunneling microscopy (STM), and the sample treatment system. Using this system, the electron property and dynamics of the Si(001) surface is investigated, and the following results are obtained.1)The temporal evolution of electrons which temporarily occupy the unoccupied surface dangling bond state (D_<down>) and the bulk conduction band as a result of the photoexcitation is studied. The sub-ns decay of π^* (electron population at D_<down>) and C^* (electron population at the bulk conduction band near the surface) after the photo-excitation of the clean Si(001) surface. A prominent excitation-wavelength dependent decay in sub-ns temporal domain was observed for both π^* and C^*, and they are proportional with each other at temporal regime longer than 40 ps after the excitation. The results reveal a crucial role of the bulk-to-surface transition in the carrier dynamics on this surface, together with a short lifetime (<10ps) of electrons at the D_<down> state. The surface recombination velocity S is determined by comparing the solution of the diffusion equation.2)2PPE spectroscopy was utilized for investigating the electronic properties and electron dynamics on the Si(001)(2x1) surface on which the density of C-defect was examined with STM It was found that the Fermi-level pinning occurs at 〜0.8eV with respect to CBM when C-defect density is more than 0.05 monolayer on the p-type Si(001)(2x1) surface. The electron dynamics on Si(001) is affected by the C-defect on the surface. A change in the inverse of the product of the bulk-to-surface electron transition rate S and the extinction lifetime τ of the surface excited electron is examined as a function of the C-defect density, and found that it becomes twice when the C-defect density reaches to 0.1 ML on the surface.