Stable fourfold-coordinated vacancy clusters (Vn; n = 3 to 18) were identified in Si by using a combination of metropolis Monte Carlo, tight-binding molecular dynamics and density functional theory calculations. The calculations showed that the small vacancy defects thermodynamically favoured exclusively fourfold coordination rather than hexagonal ring-like structure formation, which had widely been adopted to explain the behavior and properties of vacancy defects. Among those examined, the fourfold V12 cluster with S22 symmetry was identified as being the most stable; yielding a formation energy of 1.16eV per vacancy. The fourfold V12 structure was about 4eV more favorable than the conventional hexagonal ring structure. It was also considered how the relative stability of the fourfold and hexagonal ring configurations would change as the cluster size increased to more than a few tens of vacancies.

Theoretical Determination of Stable Fourfold Coordinated Vacancy Clusters in Silicon. S.Lee, G.S.Hwang: Physical Review B, 2008, 78[12], 125310