qPlus atomic force microscopy of the Si(100) surface: Buckled, split-off, and added dimers

Dimer configurations at the Si(100) surface have been studied with noncontact atomic force microscopy in the qPlus mode at 77 K, using both large (10 nm peak to peak) and small (0.5 nm peak to peak) oscillation amplitudes. In addition to the p(2x1), p(2x2), and c(4x2) reconstructions of the pristine surface, a variety of defect types including ad-dimers, vacancies, and split-off dimers have been imaged. Our data appear at odds with the currently accepted structural model for split-off dimers. At low oscillation amplitudes the degree of apparent dimer buckling can be tuned by varying the frequency shift set point.