Negative surface energy change associated with step formation caused by misfit dislocation nucleation in semiconductor heterostructures

We have calculated the surface energy change due to step formation caused by misfit dislocation nucleation in thin-film semiconductor heterostructures. It is found that the surface energy change is negative for compressive misfit stress in the heteroepitaxial film, while it is positive for tensile misfit stress. This conclusion is in contrast to the classical model where the step formation energy is always positive and independent of the sign of the misfit. The calculated energy change is qualitatively explained by counting the number of dangling bonds on the surface. Using atomistic simulations, we have calculated the critical thickness of dislocation nucleation taking into account the surface energy change, and found that it varied from 4 nm for Ge films on Si(001) substrates to 6 nm for Si films on Ge(001) substrates having the same misfit.