Structures of thermal double donors in silicon

Accurate total-energy calculations are used to study the structures and for mation 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 metastab le TDD's form bistable negative-ii systems with the corresponding stable, e lectrically inactive staggered structures. The TDD3-TDD7 structures are fou nd to consist of four-member rings with adjacent interstitial O atoms at bo th ends. The TDD's with a central "di-Y-lid" core are found to become energ etically competitive with the four-member ring TDD's only for clusters larg er than ten O atoms.