Prediction of the effect of the sample biasing in scanning tunneling microscopy and of surface defects on the observed character of the dimers in the Si(001)-(2×1) surface

Self-consistent quantum-chemical cluster calculations are reported which address the controversy about the presence or absence of dimer buckling in the Si(001)-(2×{}1) reconstructed surface. The results indicate that biasing the surface, as in the scanning tunneling microscopy (STM) experiment, is likely to produce a relatively symmetric STM image even if dimers in the unbiased surface are buckled, as deduced from scattering experiments. We have also investigated a dimer close to a surface defect, such as a step, and propose a mechanism that makes the dimer there appear buckled to STM.