Step permeability effect and interlayer mass-transport in the Ge/Si(1Â 1Â 1) MBE

A simple analytical model of the mass-transport during the initial stage of Ge wetting layer formation on Si(1 1 1) is developed. The model considers growth of the multilevel Ge islands observed in recent STM studies as evolution of pyramids with two bilayer thickness. The pyramid steps flow together and, as a result, the multilevel island forms if there exists an upward transport of adatoms from the substrate surface to the top of the pyramid. The necessary condition for this is that the step at the pyramid base is permeable for the adatoms (the adatoms with greater probability climb up the step than incorporate into it). The explicit expressions for the step permeability and incorporation coefficients are obtained. It is shown that decreasing step permeability is responsible for the transition from multilevel growth mode to the layer-by-layer formation of the wetting layer which has been observed under the increasing deposition rate or/and decreasing growth temperature.