Calculation of the spin-polarized electronic structure of an interstitial iron impurity in silicon

We apply our self-consistent, all-electron, spin-polarized Green’s-function method within an impurity-centered, dynamic basis set to study the interstitial iron impurity in silicon. We use two different formulations of the interelectron interactions: the local-spin-density (LSD) formalism and the self-interaction-corrected (SIC) local-spin-density (SIC-LSD) formalism. We find that the SIC-LSD approach is needed to obtain the correct high-spin ground state of Si:${\mathrm{Fe}}^{+}$. We propose a quantitative explanation to the observed donor ionization energy and the high-spin ground states for Si:${\mathrm{Fe}}^{+}$ within the SIC-LSD approach. For both Si:${\mathrm{Fe}}^0$ and Si:${\mathrm{Fe}}^{+}$, this approach leads to a hyperfine field, contact spin density, and ionization energy in better agreement with experiments than the simple LSD approach. The apparent dichotomy between the covalently delocalized nature of Si:Fe as suggested on the one hand by its reduced hyperfine field (relative to the free atom) and extended spin density and by the occurrence of two closely spaced, stable charge states (within 0.4 eV) and on the other hand by the atomically localized picture (suggested, for example, by the stability of a high-spin, ground-state configuration) is resolved. We find a large reduction in the hyperfine field and contact spin density due to the covalent hybridization between the impurity 3d orbitals and the tails of the delocalized ${\mathrm{sp}}^3$ hybrid orbitals of the surrounding silicon atoms. Using the calculated results, we discuss (i) the underlying mechanism for the stability and plurality of charged states, (ii) the covalent reduction in the hyperfine field, (iii) the remarkable constancy of the impurity Mössbauer isomer shift for different charged states, (iv) comparison with the multiple charged states in ionic crystals, and (v) some related speculation about the mechanism of (${\mathrm{Fe}}^{2+}$/${\mathrm{Fe}}^{3+}$) oxidation-reduction ionizations in heme proteins and electron-transporting biological systems.