  A long-standing discrepancy exists between experiments and atomistic models
concerning the critical strain needed for surface nucleation of dislocations in
silicon-germanium systems. While dislocation nucleation is readily observed in
hetero-epitaxial thin films with misfit strains less than 4%, existing
atomistic models predict that a critical strain over 7.8% is needed to overcome
the kinetic barrier for dislocation nucleation. Using zero-temperature energy
barrier calculations and finite-temperature Molecular Dynamics simulations, we
show that 3-dimensional surface features such as a sharply bent step can lower
the predicted critical nucleation strain of a shuffle-set dislocation to 6.4%,
and that of a shuffle-glide dislocation complex to 5.3%. Consistent findings
are obtained using both the Stillinger-Weber (SW) and modified embedded-atom
method (MEAM) potentials, providing support to the physical relevance of the
shuffle-glide dislocation complex, which was previously considered as an
artifact of the SW potential.
