Atomistic study of the effect of crack tip ledges on the nucleation of dislocations in silicon single crystals at elevated temperature

We model a single crystal of silicon approaching dimensions of 20 nm x 20 nm x 10 nm with up to 200,000 atoms based on the ReaxFF first principles based reactive force field, applied here to examine the influence of crack tip heterogeneities on the nucleation of dislocations from crack tips as the temperature is increased well beyond 1000 K. Our study aims at generating insight into mechanisms of the brittle-to-ductile transition (BDT) observed in silicon, which is known to change the behavior of silicon crystals from predominantly brittle at low temperatures to ductile at higher temperatures, likely involving a competition between brittle bond-breaking and emission of dislocations. We first examine the crystallographic orientations most widely known to exhibit fracture, for example failure on the {1 1 1} and {1 1 0} planes. We find that even at very high temperatures considered here (around 1500 K), the material behaved in a completely brittle manner for systems with cracks oriented in the {1 1 1} and {1 1 0} planes. In contrast, when the crack plane is changed to the {1 0 0} orientation, the formation of complete dislocation loops is observed. The results reported in our paper is the first direct observation of dislocation loops in silicon driven by an increase in the temperature, and extends earlier studies that were focused solely on extremely thin quasi-two-dimensional models. We demonstrate here that the formation of these loops can be linked to the geometrical ledges at the crack tip with favorable orientations for slip on the {1 1 1} planes. The detailed mechanisms leading to the nucleation of dislocations involve changes at the crack tip with bond rotations and formation of ledges, and we find that the emergence of dislocations causes the crack to arrest, but re-initiation takes place and no conclusive nucleation of dislocations and complete arrest of the crack is observed. The thickness effect is discussed together with other sources for formation of ledges along the crack front. Our findings provide important insight towards the development of models for the brittle-ductile-transition in silicon and potentially other materials, and emphasize on the significance of thickness effects in simulations of fracture in crystal slabs. (C) 2011 Elsevier B.V. All rights reserved.