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New evidence of the paired end-bridge configuration in the room-temperature adsorption geometries of C 2 H 2 molecules on Si(0 0 1) is presented by scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. The distinct four-leaved feature occupying two adjacent Si dimer sites in the experimental empty-state STM images are well reproduced by simulations of the paired end-bridge adsorption configuration. Calculated energetics suggests that the Si(0 0 1) surface is covered by paired end-bridge structures at the saturation coverage of 1 ML, in agreement with the existing experiments.

and SB steps induces a strong buckling along the rebonded dimer row in the lower terrace. On the upper terrace of the SB step, the kink induces a local p(2×2) structure directed toward the center of the step. The strains of second-layer atoms supporting the dimers are responsible for the reconstruction near the monatomic steps on the Si(001) surface.

The structure of a clean Si(100) and a Ni-contaminated si(100) was investigated using scanning tunneling microscopy. The clean Si (100) shows the 2 × 1 reconstruction with a surface dimer vacancy density less than 2%. The major defects on the clean surface are a single dimer vacancy and the C defect. A small amount of Ni on the surface drastically changes the surface structure and produces 2 × n reconstructions. The formation of vacancy clusters is favored. A rebonded SB step is preferred on the clean Si(100) while a nonrebonded SB step with a split-off dimer is mainly observed on the Ni-contaminated Si(100) and in the vicinity of dimer vacancies of the lower terrace on the clean Si(100).

The atomic structure at monatomic-height ${\mathit{S}}_{\mathit{B}}$ steps on clean Si(001) and a Ni-contaminated Si(001) was investigated using high-resolution scanning tunneling microscopy at room temperature. Rebonded ${\mathit{S}}_{\mathit{B}}$ steps are dominant on clean Si(001). Nonrebonded ${\mathit{S}}_{\mathit{B}}$ steps with split-off dimers are favored on the Ni-contaminated Si(001) and in the vicinity of dimer vacancies on clean Si(001). The nonrebonded step with the split-off dimer is generated by the strain due to dimer vacancies. A buckled dimer was observed in the vicinity of a kink of ${\mathit{S}}_{\mathit{B}}$ step edges on clean Si(001). \textcopyright{} 1996 The American Physical Society.

Dimer-vacancy defects on clean Si(001)-2\ifmmode\times\else\texttimes\fi{}1 and Ni-contaminated Si(001)-2\ifmmode\times\else\texttimes\fi{}n surfaces are investigated by scanning tunneling microscopy (STM). The clean Si(001) surface shows the 2\ifmmode\times\else\texttimes\fi{}1 reconstruction irrespective of cooling rates faster than 150 \ifmmode^\circ\else\textdegree\fi{}C/sec. On the Si(001)-2\ifmmode\times\else\texttimes\fi{}1 surface with a surface dimer-vacancy density of 1.7%, the most abundant dimer-vacancy defect is a randomly distributed one dimer vacancy (1-DV) of the Wang-Arias-Joannopoulos model. Appreciable amounts of (1+2)-DV and 2-DV are observed. The ordered defects on the Si(001)-2\ifmmode\times\else\texttimes\fi{}n surface are mainly composed of (1+2)-DV and 2-DV. The real-space STM images reveal that the dimer adjacent to the unrebonded side of 2-DV is depressed by more than 0.5 \AA{}, representing the highly asymmetric characteristics. A small amount of Ni contamination on Si(001) drastically increases the dimer-vacancy density from below 2% to above 20%.