Electronic excitations on Si(100)(2×1)

Angle-resolved high-resolution electron-energy-loss spectroscopy has been used to study electronic excitations on both the clean and contaminated Si(100)(2×1) surfaces. Absorption edges on the clean surface were observed near 0.4 and 1.1 eV corresponding to transitions from the bulk valence band to the surface conduction band and to the bulk conduction band, respectively. The bulk-to-surface absorption edge was found to move both as a function of primary energy and angle of incidence. The resulting dispersion relationship can be explained without invoking phonon-mediated transitions. This implies that the minimum in the unoccupied surface state is at the $\Gamma$ point, that the true unit cell is larger than the (2×1), and that the reconstruction of the surface is more extensive than a simple, asymmetric dimerization of the surface atoms. One of the more interesting results of this study is the absence of observable direct transitions from the occupied to the unoccupied surface state. Transitions from the occupied surface state to the bulk conduction band are, however, observed.