Dynamics and Nucleation of Si Ad-dimers on the Si(100) Surface

The motion of Si ad-dimers on the Si(100) surface is studied by means of ab initio (Car-Parrinello) molecular dynamics. We observe dynamic buckling of the dimers and the rotation of a single ad-dimer on top of a substrate dimer row. A model is presented for the structure and the formation of the recently observed ``dilute'' ad-dimer rows, which explains their one-dimensional growth. We propose that these rows are intermediate structures for the strongly anisotropic epitaxial growth on Si(100).