Ab initio calculations of the energetics of the neutral Si vacancy defect

Ab initio plane-wave pseudopotential calculations for the neutral silicon vacancy indicate a formation energy of 3.6 eV, with the surrounding lattice undergoing a tetragonal distortion with the nearby atoms forming two dimers having bond lengths 2.91 Angstrom. Close in energy is a tetrahedrally distorted structure in which the nearby atoms relax towards the vacancy by 12.6% of the bulk bond length. Additional distortions with trigonal symmetry were also investigated, but no stable structures were found. The symmetry, energetics, and geometry are found to be a sensitive function of the computational basis-set and supercell used in the plane-wave calculations.