We propose a single adatom exchange mechanism for surfactant-mediated epitaxial growth through first-principles calculations for the Si epitaxy on a Si(001) surface covered by an As monolayer. The As segregation is initiated by the exchange of a Si adatom with a sublayer As site, with an activation energy of about 0.1 eV which is much lower than the value of $\sim 1.0\mathrm{eV}$ for a dimer exchange. Adatom incorporation occurs with minimum surface diffusion, giving rise to a high nucleation density of two-dimensional islands, in good agreement with experiments.

• Received 28 August 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.3160