In this chapter, ab initio molecular dynamics is applied to several possible reconstructions of the (001) surface of BC8 silicon. On creating a surface on bulk crystal, bonds are cut at a surface layer. Dangling bonds or localised atomic orbitals remain at the surface which can be unfavourable due the the large increase in energy that is involved.
At the surface of a solid the three dimensional periodicity of a crystal no longer exists so that the surface atoms relax from what would have been their positions in the bulk. This forms the simplest form of surface construction. Due to the nature of the highly directional covalent bonding in semiconductors this relaxation can be quite substantial. Surface atoms with dangling bonds can move closer together to form covalent bonds thus lowering their number per unit area and reducing the total energy of the system.
In an attempt to reduce the energy of the surface further, we place additional atoms on the surface in order to saturate some of the dangling bonds. One of the more common types of surface reconstructions in tetrahedrally bound semiconductors is the addition of atom pairs (dimers) to the surface. An atom in the dimer is bonded to two atoms on the surface and to the other atom of the dimer. The net effect of this is to half the number of dangling bonds per unit area of the surface and therefore reduce the surface energy. This is the method by which the (001) surface of BC8 silicon is reconstructed here.
Addition of dimers to a surface can have the effect of lowering the symmetry of the surface and increasing the size of the surface unit cell. This will also be investigated on the BC8 silicon surface, where the symmetric (1 1) dimer formation and the antisymmetric (2 1) dimers are found to give rather different surface energies.
As discussed earlier, complex crystalline forms of silicon form good models for amorphous silicon due to the short ranged disorder contained in such structures. It may be possible that surfaces of the BC8 structure also forms a reasonable model in which to study the surfaces of amorphous silicon.