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With the increasing annealing temperature in ultrahigh vacuum, hydrogen-terminated Si(001) surface exhibit a (1×2) reconstruction around 500 °C, c(4×4) in the narrow range of 580–750 °C, and a weak SiC transmission pattern above 750 °C. The simultaneous disappearance of c(4×4) and the appearance of the SiC pattern suggest that the c(4×4) reconstruction is associated with carbon contamination. The carbon concentration of 2.5×1018 atoms/cm3 is insufficient for carbon atoms to be a component of the c(4×4) structure, and therefore the carbon must be having another effect. A consequence of this conclusion is that carbon is not responsible for C-type defects.

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Local structures of dilute isovalent and heterovalent impurity atoms in Si crystal ͑Si:X, XϭGa, Ge, As͒ have been studied by fluorescence x-ray absorption fine structure. The distortion of local lattice around the impurity atoms was evaluated from the Si-X bond length determined by extended x-ray absorption fine structure. The results demonstrate that the local lattice deformation is strongly dependent on the electronic configuration of impurity atoms, i.e., we find an anomalous expansion (0.09Ϯ0.01 Å) along the ͓111͔ direction for donor ͑As͒ atoms but much smaller magnitude (0.03Ϯ0.01 Å) for isovalent ͑Ge͒ atoms and acceptor ͑Ga͒ atoms. The results suggest that the local lattice distortions are strongly affected by the Coulomb interactions between the localized charge, which piles up to screen the ion core and the bond charge, and the ion-core repulsion. Absence of anomaly in case of negatively charged Ga atoms suggests that the former mechanism is a dominant factor for anomalous lattice expansion.