Dislocation nucleation and multiplication at crack tips in silicon

The brittle-ductile transition (BDT) has been studied in silicon single crystals of different orientations and purities. It is shown that the BDT temperature at a given loading rate can be significantly varied depending on structural parameters, especially the density of cleavage defects (steps,...). The critical BDT temperature is raised when the cleavage defect density is lowered. This is explained by observations at crack tips which prove that dislocation nucleation is highly inhomogeneous. A first preliminary attempt to identify nucleation sites by AFM is reported. In crystals containing highly perfect cleavage cracks, dislocation formation prior to fracture in mode I loading could be suppressed. In such cases, a very small number of dislocations created on purpose from remote sources, sufficed to trigger the formation of a plastic zone as soon as they touched the crack front. Experimental results are compared to theoretical models and recent numerical computations.