First-principles calculations of multivacancies in germanium

We carry out density-functional-theory calculations to study the stability of germanium multivacancies. We use supercells containing 216 atomic sites and simulate two configurations called the "part of hexagonal ring" (PHR) and fourfold configurations of the tri-, tetra-, and pentavacancies. We find that the fourfold configurations of the tetra- and pentavacancies are the most stable and these configurations are also the most stable in the case of silicon. However, we find that the PHR and fourfold configurations have similar energies in the case of the germanium trivacancy. These results are in contrast to those of the silicon trivacancy; the fourfold configuration has substantially lower energy than the PHR configuration. This difference between germanium and silicon is expected to originate from the fact that the four bonds in the fourfold configurations in the germanium trivacancy are weaker than those in the silicon one. By calculating dissociation energies, we find that the silicon tetravacancy is not easy to dissociate, whereas the germanium tetravacancy is not very stable compared with the silicon one. (C) 2016 The Japan Society of Applied Physics