Structure of Si(001) surfaces. I. The origin of the buckling in the dimer formation on reconstructed surfaces

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Abstract
The buckling mechanism of dimers on reconstructed Si(001) surfaces was investigated by ab initio Hartree-Fock molecular-orbital calculations in the electrically neutral, negatively charged, and positively charged cases. The energy-minimized geometries of dimers depend upon their electrical charge: The most stable geometry is symmetric for the electrically neutral case, but asymmetric or buckled for the negatively and positively charged cases. These results reveal the origin of the buckling in the dimer formation to be an imbalance of the electrical charges on the Si(001) surface. The electronic structure has the remarkable feature that an unpaired electron is localized on one side of the top Si atoms in a buckled dimer. These theoretical results are found to be consistent with those of recent scanning-tunneling-microscopy experiments.