Electronic states of silicon vacancy. I. Covalent states

Self-consistent generalized valence bond (GVB) and configuration-interaction calculations have been performed for a cluster model of the neutral vacancy in silicon. Three low-lying states (${}_{}{}^1{}_{}{}^{}E, {}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}, {}_{}{}^5{}_{}{}^{}A_2^{}{}_{}{}^{}$) are found to possess one electron in dangling-bond orbitals on each of the four silicon atoms around the defect. Of these three covalent states, the ${}_{}{}^1{}_{}{}^{}E$ is found to be the ground state with the excitation energies to the ${}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}$ and ${}_{}{}^5{}_{}{}^{}A_2^{}{}_{}{}^{}$ states being 0.17 and 0.60 eV, respectively. The shapes of the GVB orbitals for all three states are found to be quite similar.