Epitaxy on surfaces vicinal to Si(001). II. Growth properties of Si(001) steps.

Abstract

There is considerable interest in understanding the factors important for epitaxial growth. In particular, one would like to know how aspects of the growth kinetics affect the structures produced. To study epitaxial growth on the Si(001) surface, we have mapped out the potential-energy surfaces seen by a single silicon adatom, as it moves over the Si(001) steps. Standard molecular-dynamics methods were used. The energy surfaces identify the binding sites and give the activation energy for diffusion hops, so that likely pathways for adatom motion can be identified. We find that generally the binding at the step edges is about one-tenth of an eV more than that on the flat surface, so that at low adatom coverage, these steps are relatively weak sinks. Growth, however, does take place more readily at the step edges, because the steps act as heterogeneous nucleation sites for dimers. We have developed simple rate equations to estimate nucleation rates, accommodation coefficients, and step velocities, using the microscopic information given by the energy surfaces as input. The variation of these quantities with temperature and external driving force is explored. In agreement with experimental results, we find that growth takes place most readily at the ends of dimer rows. Furthermore, the predicted anisotropy in the growth properties of the ${\mathit{S}}_{\mathit{A}}$ and ${\mathit{S}}_{\mathit{B}}$ steps is large enough to account for the formation of dimer strings on the flat terraces. We also report on direct simulations of the impingement of atoms from an atomic beam onto the vicinal Si(001) surfaces. These simulations show the formation of kink sites at step edges and the formation of dimer strings on the terraces.