Early SiO 2 precipitates in Si: Vacancy-oxygen versus interstitial-oxygen clusters

Abstract

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 , SiO 2 cluster formation is limited by the interstitial oxygen ( O i ) migration barrier of ∼ 2.5 eV . 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 ( Si i ) per SiO 2 unit that is formed. We report on ab-initio density-functional studies of small oxygen aggregates in Si, to show that the O n → VO n + Si i reaction is exothermic for n ⩾ 4 . The large energy barrier required to form an intermediate Si i defect prevents the formation of VO n 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 VO n and O n defects. We report their local vibrational modes and compare them with the available experimental data.