Structure models of the “c-type” defect on si(001)

New structure models for an important type of defect, the “C-type” defect, on the Si(001) surface are proposed and their stability and their relation to scanning tunnelling microscopy images are studied by large-scale ab initio calculations, The crucial aspect of the new models is the presence of a vacancy in the second layer, in contrast to a naive picture assuming two vacancies on the surface.