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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%.

Atomic structures of Ga and As atoms on GaAs(110) were examined employing a first-principles pseudopotential method. Both Ga and As atoms reside in the center of a triangle consisting of a surface Ga and two surface As atoms in the single-atom chemisorbed state. Adsorption energies for Ga and As atoms are 3.1 and 3.5 eV, respectively. Energy barrier heights of Ga and As atoms for the migration along the path through the interstitial channel were found to be 0.6 and 1.0 eV, respectively. Simulations on the deposition of two atoms reveal that a pair formation is stable against separate single-atom chemisorptions.