Structures of thermal double donors in silicon

Accurate total-energy calculations are used to study the structures and formation energies of oxygen chains as models for thermal double donors (TDD's) in Si. We find that the first three TDD's (TDD0-TDD2) consist of one four-member ring, with one or two adjacent interstitial O atoms. These metastable TDD's form bistable negative-U systems with the corresponding stable, electrically inactive staggered structures. The TDD3-TDD7 structures are found to consist of four-member rings with adjacent interstitial O atoms at both ends. The TDD's with a central "di-Y-lid" core are found to become energetically competitive with the four-member ring TDD's only for clusters larger than ten O atoms.