It was recalled that ductility was controlled, at the atomic level, by dislocation kink motion. The migration energy of kinks on 30 partial dislocations in Si was calculated from first principles. The electronic structure changed from semiconducting to metallic at the saddle-point configuration. The band structure energy controlled kink motion, so the valence electrons controlled the shearing motions that were associated with ductility, whereas the tensile forces that were involved in fracture depended upon ion-ion and valence forces. It was noted that, in previous work on the atomic energy barrier to crack propagation in Si, a delicate balance between the Coulomb energy and the band structure energy was found to be the controlling factor; with the Coulomb interaction providing the retarding force. Published experimental data indicated that doping had little effect upon the fracture toughness of Si. This was consistent with its small effect upon the band structure energy. Coulomb interactions played a role in the large tensile bond stretchings that were involved in fracture; as compared with the shearing motions which were involved in kink movement. The Si(111)-(2 x 1) shuffle surface reconstruction that was generated by cleavage was suggested to be governed by the Coulomb force, whereas the glide movement of dislocations was controlled by the band structure force.

Y.M.Huang, J.C.H.Spence, O.F.Sankey: Physical Review Letters, 1995, 74[17], 3392-5