  The role of a simple surface defect, such as a step, for relaxing the stress
applied to a semiconductor, has been investigated by means of large scale first
principles calculations. Our results indicate that the step is the privileged
site for initiating plasticity, with the formation and glide of 60$^\circ$
dislocations for both tensile and compressive deformations. We have also
examined the effect of surface and step termination on the plastic mechanisms.
