Electron-nuclear double resonance of interstitial positively charged iron in silicon

The positively charged state of interstitial iron in silicon was studied by means of electron-nuclear double resonance. We have found hyperfine interactions of the impurity electrons with eight shells of silicon neighbors containing 98 atoms. Because the ground state of interstitial Fe⁺ is orbitally degenerate, it has quite different features than interstitial Fe⁰, Ti⁺, and Cr⁺ in silicon. The analysis of the hyperfine interactions is much more complicated because the ground state cannot be written as a single Slater determinant. For the most important high-symmetry ligand hyperfine interactions a complete analysis with proper orbitally degenerate wave functions has been carried out. For more remote lower-symmetry neighbor sites the experimental data indicate prominence of {σ}-type ligand orbitals. It is derived that the total spin density transferred to the surrounding silicon atoms is at least 26%. Recent theoretical calculations are found to be in good agreement with this experimental result.