Mechanisms of silicon diffusion in erbium silicide

First-principles methods are employed to determine how Si diffuses in the layered metal silicide Er Si_2-x. Si diffusion is found to be extraordinarily anisotropic, with the migration barrier associated with Si diffusing across Er planes nearly four times larger than within the Si planes. The activation energy for in-plane diffusion of Si decreases by 20% when Er Si_2-x is grown heteroepitaxially on Si(001), as a result of the epitaxial strain. This effect is associated with how strain modifies the formation energy of the diffusing defect and changes the energy landscape of diffusion. These results are consistent with experimental observations of defect formation in heteroepitaxial Er Si_2-x films on Si(001).