Hyperfine interactions in cluster models of thePbdefect center

Hyperfine interactions in the $P_b$ center (denoted schematically as ${\mathrm{Si}}_3$?Si⋅) at the Si(111)/${\mathrm{SiO}}_2$ interface have been studied with use of spin-polarized self-consistent multiple-scattering Xα calculations on ${\mathrm{Si}}_{22}$ ${\mathrm{H}}_{21}$/${\mathrm{Si}}_6$ ${\mathrm{O}}_{18}$ ${\mathrm{H}}_6$ and ${\mathrm{Si}}_{22}$ ${\mathrm{H}}_{27}$ cluster models. Our theoretical hyperfine tensor agrees very well with experiment when the trivalent atom Si’ is relaxed by a value typical of geometries found for the neutral paramagnetic charge state in semiempirical and ab initio cluster calculations. Spin-polarization effects are found to be very important for a detailed description of the $P_b$ defect, particularly with respect to the hyperfine couplings at nuclei close to the defect atom. The largest such superhyperfine interaction is produced not by the nearest-neighbor atoms as has commonly been assumed, but by three second-nearest neighbors located below Si’ in the bulk c-Si. The isotropic and anisotropic superhyperfine components and the direction of the principal axes predicted by the present calculations have been confirmed by recent ESR experiments.