It was noted that the aggregation of O impurities during thermal annealing gave rise to so-called thermal double-donor defects in Si and Ge. A microscopic structure for the core of these defects was proposed on the basis of first-principles calculations. In the case of Si, the core contained three O atoms, exhibited bistability and had a total energy which was appreciably lower than that of 3 isolated O interstitials. Impurities such as B, Al, N and H were found to bind strongly to the defect and to transform it into a shallow single donor.

D.J.Chadi: Physical Review Letters, 1996, 77[5], 861-4