Complex atomic-scale dynamics in grain boundaries in silicon

A combination of first-principles calculations and atomic-resolution electron microscopy have been used to determine that As segregation in Si grain boundaries occurs in the form of dimers in select atomic columns, The energy gain results from repulsion between neighbouring substitutional As atoms that achieve threefold coordination. Even more energy is gained by forming ordered chains of threefold-coordinated As atoms along the grain boundary dislocation cores, but kinetic constraints and entropic considerations limit their formation. In other work, theory predicted that segregation of vacancies causes novel reconstruction along select atomic columns, the very same columns that are readily altered by energetic electron beams used in transmission electron microscopy. The mechanisms of self-diffusion anti impurity diffusion in grain boundaries are examined and elucidated in the context of the above findings. The work was supported in part by the U.S. Office of Naval Research and the Department of Energy.