Microscopic structure of the NL10 heat-treatment center in silicon: Study by electron-nuclear double resonance

The defect center giving rise to the Si-NL10 EPR spectrum has been investigated by the electron-nuclear double-resonance (ENDOR) technique. Three separate experiments have been performed: oxygen ENDOR of ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ nuclei, aluminum ENDOR of ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, and field-stepped ENDOR. As a result a microscopic model for the Si-NL10 center can be proposed. According to that model the Si-NL10 center is an oxygen cluster which may also incorporate an acceptor. The center grows during annealing at about 470 °C and develops several species by subsequent addition of oxygen atoms along a [01¯1] direction. In the studied sample the basic Si-NL10 center species was of orthorhombic 2mm ($C_{2v}$) symmetry type and contained two oxygen atoms and an aluminum atom. More tentatively, we propose that a vacancy exists on the twofold axis of the defect. The next species to develop upon annealing had the symmetry lowered to monoclinic m type ($C_{1h}$) and contained most probably three oxygen atoms. The lowering of the symmetry cannot be resolved in EPR which retains the overall orthorhombic character. Oxygen atoms incorporated in the center take the usual puckered bonded interstitial position with the bond angle of about 114°. The structure of the Si-NL10 center is planar in the (011) plane, while at the same time that crystallographic plane appears as forbidden in regard to the spin distribution. On the basis of the constructed model, two possible interpretations of the Si-NL10 center are proposed; one of them identifies it as an acceptor [(TD${)}^{\mathrm{-}}$] state of the silicon thermal donor (TD).