Atomistics and mechanical properties of silicon

Recently it has become possible for the first time to directly observe disl ocation kink motion by electron microscopy. The method is discussed in whic h nb initio quantum molecular dynamics calculations in combination with the se 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 avoid s 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 ki nk formation and migration energies are measured and calculated. For silico n, 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 gl ide termination surface energies, and the surface reconstructions which cle avage generates have also been computed ab initio in good agreement with ex periment. Long-range ion-ion interactions are found to be important in frac ture, while shorter range valence electron forces control the shearing moti ons 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 t he structure and energetics of the atomic-scale defects which control the m echanical properties of crystalline materials. This work is a necessary pre liminary to the more challenging problems of understanding fracture and pla sticity at interfaces at the atomic level. (C) 1999 Acta Metallurgica Inc. Published by Elsevier, Science Ltd. All rights reserved.