A detailed investigation of plastic relaxation onset in heteroepitaxial SiGe islands on Si͑001͒ is presented. The strain field induced by a straight misfit-dislocation segment is modeled by finite-element-method ͑FEM͒ calculations in three dimensions, fully taking into account the interaction with the multifaceted free surfaces of realistic islands. The total elastic energies before and after the placement of a 60°dislocation segment in the most favorable position are therefore evaluated by a

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... ll FEM approach, for different island sizes and compositions. The critical volumes with composition for inserting the dislocation are finally obtained and successfully compared with the data in a report by Marzegalli et al. ͓Phys. Rev. Lett. 99, 235505 ͑2007͔͒, where experimental values are compared to a simpler approach.