The initial growth of Si on the Si(001) surface was investigated by studying the stability of various Si ad-clusters and by calculating energy barriers for some fundamental processes by using a first-principles method. A perpendicular Si ad-dimer on top of a substrate dimer row was easily formed and was the most stable of dimers; in accord with experiment. The dimer could be a diffusing unit at high temperatures, but could not be a nucleus for dimer row growth at lower temperatures. On the other hand, a quasi-stable parallel dimer in a trough attracted monomers and could be the nucleus of a diluted-dimer row.
T.Yamasaki, T.Uda, K.Terakura: Physical Review Letters, 1996, 76, 2949-52