STEP-INDUCED STATE OF A SI(001) RECONSTRUCTED SURFACE

We calculate the electronic structure of a Si(001) stepped surface, applying the DV-Xalpha-LCAO method in the density functional approach. For the surface, a slab model which consists of 4 Si-layers, including two different types of single-layer steps is applied. The dimers of the outermost Si atoms are assumed to take an asymmetric geometry. A localized state is found at the dimer on the lower terrace just below the S(A) step. We also simulate scanning tunneling microscopy (STM) images and scanning tunneling spectroscopy (STS) spectra in the first-principles approach. In the STS spectra, the localized state at the S(A) step gives a sharp peak, such as observed in the experiment by Iwawaki et al. [J. Vac. Sci. Technol. B 9 (1991) 711]. However, the up- and down-atoms of the dimer at the step edge of the upper terrace do not show any significant difference in the spectra. In the STM images, the dimer seems buckled no more than the geometry. Furthermore, to clarify the origin of the localized state, we analyze a phenomenological tight-binding model for the pi-band of dimer rows. One of the pi-states localizes to the dimer site just below the S(A) step, due to a potential induced at the step edge.