Core structure of thermal donors in silicon

Aggregation of oxygen impurities as a result of thermal annealing gives rise to ''thermal'' double donor defects in Si and Ge. A microscopic structure for the core of these defects is proposed on the basis of first-principles calculations. In Si, the core contains three oxygen atoms,exhibits bistability, and has a total energy appreciably lower than three isolated oxygen interstitials. Impurities such as B, Al, N, and H are found to bind strongly to the defect and to transform it into a shallow single donor.