Role of surface step on misfit dislocation nucleation and critical thickness in semiconductor heterostructures

We discuss the role of surface step formation on misfit dislocation nucleation and critical thickness in thin film semiconductor heterostructures. On the basis of an atomistic model, it is shown that the energy change due to step formation is negative or positive depending upon the sign of the misfit. The step formation energy can even be negative for compressive misfit stress in the heterolayer, while it is definitely positive for tensile misfit stress. This conclusion is in contrast to the classical model where the step energy is always positive and independent of the sign of the misfit. The step formation energy influences the critical thickness and the energy barrier for dislocation nucleation. Using a simple atomistic simulation, we show that the critical thickness depends upon the sign of the misfit; for example, it changes from 4 nm fe films on Si(100) substrates to 6 nm for Si films on Ge(100) substrates having the same misfit.