Low-temperature scanning-tunneling-microscopy observations of the Si(001) surface with a low surface-defect density

We have settled a controversy that surface dimers in the Si(001) surface are intrinsically symmetric or asymmetric. Atom-resolved scanning-tunneling-microscopy (STM) images of a low-defect-density ({~}1%) surface at 144 K give definite evidence for the asymmetric-dimer model. We have identified a local influence of surface defects on buckling of dimers at low temperatures, which was not clarified in previous STM observations at 120 K.