Dimer pairing on the C-alloyed Si(001) surface

The initial stages of carbon alloying into the Si(001) surface are studied by scanning tunneling microscopy (STM) and density functional theory. Carbo n increases the surface roughness compared to the clean surface and induces a c(4 x 4) reconstruction. To explain experimental observations, we propos e a novel surface reconstruction model that involves pairing of Si dimers m ediated by the presence of a complex of a C dimer and four nearest neighbor subsurface C atoms. The model is backed by total energy and thermal stabil ity simulations. Its calculated surface charge density agrees well with the filled state STM images.