  We study numerically the minimum energy path and energy barriers for
dislocation nucleation in a two-dimensional atomistic model of strained
epitaxial layers on a substrate with lattice misfit. Stress relaxation
processes from coherent to incoherent states for different transition paths are
determined using saddle point search based on a combination of repulsive
potential minimization and the Nudged Elastic Band method. The minimum energy
barrier leading to a final state with a single misfit dislocation nucleation is
determined. A strong tensile-compressive asymmetry is observed. This asymmetry
can be understood in terms of the qualitatively different transition paths for
the tensile and compressive strains.
