Early SiO2 precipitates in Si: Vacancy-oxygen versus interstitial-oxygen clusters

Oxygen precipitation in Si strongly depends on the undergoing thermal treatment. Between 350 and 450 °C thermal donor formation is activated by a 1.4-1.6 eV barrier. On the other hand, at T>500∘C, SiO2 cluster formation is limited by the interstitial oxygen (Oi) migration barrier of ∼2.5eV. Volumetric arguments imply that the formation of silica precipitates during anneals of oxygen-rich Si crystals, must be accompanied by the ejection of approximately one Si self-interstitial (Sii) per SiO2 unit that is formed. We report on ab-initio density-functional studies of small oxygen aggregates in Si, to show that the On→VOn+Sii reaction is exothermic for n⩾4. The large energy barrier required to form an intermediate Sii defect prevents the formation of VOn complexes at temperatures as low as 450 °C. Our results imply that thermal donors are not thermodynamically stable clusters, and their formation is driven by kinetics. Infra-red absorption studies can discriminate VOn and On defects. We report their local vibrational modes and compare them with the available experimental data.