Directional Anisotropy in the Cleavage Fracture of Silicon

Total-energy pseudopotential calculations are used to study the cleavage anisotropy in silicon. It is shown that cracks propagate easily on $\{111\}$ and $\{110\}$ planes provided crack propagation proceeds in the $〈\overline{1}10〉$ 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, 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.