Isomer-shift of interstitial and substitutional iron in silicon and germanium

We describe results from in-beam Mossbauer implantation measurements for57Fe inn-type Si, inp-type Si, and in Ge. Ab initio calculations for the contact densities from density functional theory in the local-density approximation are presented and their results are employed to locate the type of position of Fe in the lattice of the host. Thus the different spectral features corresponding to interstitial and substitutional Fe in Si and Ge are unambiguously identified.