We have performed tight-binding calculations on a model of an amorphous silicon sample generated previously by a molecular-dynamics simulation employing the Stillinger-Weber potential. The sample consists of 588 atoms and contains a high density of floating-bond defects. Two tight-binding calculations are presented, one using the widely accepted Chadi parameters, which include only nearest-neighbor interactions, and the other using the parameters recently proposed by Allen, Broughton, and McMahan (ABM) [Phys. Rev. B 34, 859 (1986)] for a nonorthogonal basis set. Comparison of the densities of states shows similar behavior in the valence band, but the electron density near a defect is less localized with the ABM parameters. It is also found that the projected density of states on the fivefold-coordinated atoms is very close to that on the fourfold-coordinated atoms, while the projected density of states on the threefold-coordinated atoms is distinctly different and has more states in the gap.

We invert high-energy (E greater-than-or-equal-to 400 eV) photoelectron and Auger-electron interference patterns to construct 3D images of surface and interface atoms. A new scheme is introduced to correct the phase shift of the image. Image reconstruction is demonstrated for Si(111) square-root 3 x square-root 3-B, a system in which multiple-scattering effects are small and all source waves are equivalent. Using diffraction results from multiple-scattering slab calculations, we achieve a spatial resolution of 1.0-1.3 angstrom, thus qualifying the technique as a direct structural tool.

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