Thermal Adatoms on Si(001)

Thermal adatoms (present on the surface at elevated temperature, in equilibrium with the step edges) are of key importance in dynamic phenomena such as step capillary wave motions, epitaxial growth, surface phase transitions, and the decay of nonequilibrium structures by surface diffusion. Here we present the first direct measurements of the thermal adatom concentration on Si(001) at elevated temperatures, from which we determine an adatom formation energy of $0.35\pm 0.05\mathrm{eV}$. Comparison with first-principles theory shows that the adatoms occur as dimers. These observations have direct implications for our understanding of surface diffusion and epitaxial growth.