Deep levels due to isolated single and pair vacancies in C, Si and Ge

In the second-neighbour tight-binding framework and using the well-known k-space Green function approach, the authors have examined the electronic structure of an unreconstructed isolated vacancy (with Td symmetry) and the method is extended to explain qualitatively the divacancy levels (with D3d symmetry) in elemental semiconductors. The calculated results have been discussed and compared with the existing theoretical data.