Atomistics and mechanical properties of silicon

Abstract

Recently it has become possible for the first time to directly observe dislocation kink motion by electron microscopy. The method is discussed in which ab initio quantum molecular dynamics calculations in combination with these images have deepened our understanding of the atomic processes involved in both ductility and fracture in single-crystal silicon, and have allowed the controlling energy barriers to be estimated. The ab initio method avoids the need for empirical atomic potentials, on which results may otherwise sensitively depend. The electron microscope images may be used to eliminate possible defect structural models, while suggesting others. Dislocation kink formation and migration energies are measured and calculated. For silicon, unlike metals, it is found that kink mobility rather than kink formation limits dislocation velocity for given conditions of stress and temperature. Movies of kink motion have shown kinks delayed at obstacles. The fracture toughness for cracks running on (111) in silicon, the (111) shuffle and glide termination surface energies, and the surface reconstructions which cleavage generates have also been computed ab initio in good agreement with experiment. Long-range ion–ion interactions are found to be important in fracture, while shorter range valence electron forces control the shearing motions involved in dislocation kink motion and ductility. Thus the combination of in situ atomic-resolution electron microscopy and diffraction, together with ab initio calculations provide a powerful approach to understanding the structure and energetics of the atomic-scale defects which control the mechanical properties of crystalline materials. This work is a necessary preliminary to the more challenging problems of understanding fracture and plasticity at interfaces at the atomic level.