Kinetic stability of missing-dimer and single-atom defects on Si(100)

Scanning tunneling microscopy shows that the Si(100) surface has a large number of missing-dimer vacancies and almost no missing-atom vacancies. In this paper we provide a kinetic explanation for these observations. We use the Stillinger-Weber potential to calculate the energy barriers opposing various atomic and dimer displacements along the surface. These calculations indicate that vacancy formation by thermal fluctuations is a very slow process. If a single-atom vacancy is formed the atom that used to be dimerized with the missing atom will leave the dimer row rapidly; the missing-atom vacancy is thus transformed into a missing-dimer vacancy. The chance that two adsorbed atoms will fill a missing-dimer vacancy is very low: the first atom that fills the vacancy leaves it quickly and its lifetime at the vacancy site is shorter than the time in which the second atom joins it; moreover, vacancy filling competes for adsorbed single atoms with capture by steps and islands and with dimer formation on top of the surface.