An ab initio study of the cleavage anisotropy in silicon

Total-energy pseudopotential calculations are used to study the cleavage fracture processes in silicon. It is shown that bonds break continuously and cracks propagate easily on {111} and {110} planes provided crack propagation proceeds in the 〈1̄10〉 direction. In contrast, if the crack is driven in a 〈001〉 direction on a {110} plane the bond breaking process is discontinuous and associated with pronounced relaxations of the surrounding atoms. The discontinuous process is partly a result of some load sharing between the crack tip bond and the neighbouring bond, which results in a large lattice trapping. The different lattice trapping for different crack propagation directions can explain the experimentally observed cleavage anisotropy in silicon single crystals.