Ab initio calculations on thermal donors in Si: an over-coordinated O atom model for the NL10 and NL8 centres

Ab initio LDF calculations are carried out on 75 atom H-terminated clusters of Si containing 4 O atoms surrounding a Si, Al or B core. The clusters represent models of NL10 and NL8 thermal defects in Si. If the Si and O atoms are arranged in the normal zig-zag sequence along (011) then there are no gap levels. Such a cluster possesses a large strain energy. An alternative arrangement of atoms gives a strain-free cluster containing over-coordinated pairs of O atoms. This is stable and possesses donor levels. A single pair of these defects gives a midgap level whereas two pairs give, in addition, a donor level in the upper half of the gap. The wavefunctions of these complexes are described and appear to be consistent with ENDOR data. The relative stability of these complexes over those with normal O bonding is greatest for Al and Si cores, less so for B and least for C. This can account for the effect of Al, B and C on the production of thermal defects. The model also implies a much greater diffusion rate for di-oxygen complexes than for a single interstitial O atom.