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The energetics and structure of vacancies in the second layer of GaAs(110) were examined using the ab initio total energy calculation method. Structural changes due to the presence of the vacancies are substantial. The changes are different in shape, depending on the type of vacancy and charged states. Both the Ga vacancy and the As vacancy in the second layer of the surface favor negatively charged states.

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).