Time-dependent plasticity in silicon microbeams mediated dislocation nucleation

Understanding deformation mechanisms in silicon is critical for reliable design of miniaturized devices operating at high temper-atures. Bulk silicon is brittle, but it becomes ductile at about 540 degrees C. It creeps (deforms plastically with time) at high temperatures (similar to 800 degrees C). However, the effect of small size on ductility and creep of silicon remains elusive. Here, we report that silicon at small scales may deform plastically with time at lower temperatures (400 degrees C) above a threshold stress. We achieve this stress by bend-ing single-crystal silicon microbeams using an in situ thermome-chanical testing stage. Small size, together with bending, localize high stress near the surface of the beam close to the anchor. This localization offers flaw tolerance, allowing ductility to win over fracture. Our combined scanning, transmission electron micros-copy, and atomic force microscopy analysis reveals that as the threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high-stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time, lowering the stress while bending the beam plastically. This process con-tinues until the effective shear stress drops and dislocation activ-ities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon.