Kinetic Aspects of Island Nucleation Derived from Near Equilibrium Growth Experiments

Using liquid phase epitaxy from Bi solution, a by its nature a near equilibrium growth process, we study the kinetics of island formation in the heteroepitaxial system SiGe/Si(001) as dependent on growth temperature, growth rate and composition (which also determines the lattice misfit between layer and substrate). As a main result island formation can be described by classical nucleation theory, moreover, it can be described as any other crystallization process such as solid state crystallization of amorphous silicon or crystallization from a melt, provided that the limited size the islands can grow into is correctly considered. In consequence, after an incubation time period that depends on the growth temperature, islands nucleate and cover the substrate surface with time. The activation energy of island nucleation is 0.84±0.13eV. The coverage with islands depends only on the undercooling and is independent of the cooling rate in case near equilibrium growth conditions are maintained. In these cases the islands have the shape of truncated pyramids with four {111}– side facets and a base width λ that only depends on the misfit f (λ ∝ 1/f2). Deviations from the equilibrium growth stage at high growth rates (thus higher growth driving forces) result in the formation of a higher density of smaller islands with smaller facet angles. At higher growth rates, some kinetic influences begin to appear indicated by the additional appearance of shallower pyramids with four {115}– facet side faces.