Ab initio total-energy calculations for iron-acceptor pairs in silicon

Abstract

We present ab initio total-energy calculations for trigonal and orthorhombic iron-acceptor pairs in silicon. The total-energy calculations have been performed in the general framework of the density-functional theory treating many-particle effects in the local spin-density approximation. We use a Green's-function approach based on the linear muffin-tin orbitals theory using the atomic-sphere approximation (which unfortunately prohibits the inclusion of any lattice-relaxation effects). Our total-energy calculations lead to a model for the electronic structure of both orthorhombic and trigonal pairs that is dominated by ionic binding of the interstitial iron to the acceptor on a substitutional site and does not predict a significant covalent binding of the pair. This is in contrast to the case of ${\mathrm{Fe}}_{\mathrm{i}}$-${\mathrm{Au}}_{\mathrm{Si}}$ and ${\mathrm{Fe}}_{\mathrm{i}}$-${\mathrm{Ag}}_{\mathrm{Si}}$ pairs for which there is a strong covalent pair binding for any position of the Fermi energy. In agreement with the ionic-binding model, we do not find pairing in n-type material for any iron-acceptor pairs. We find, however, that in n-type material there is moderately strong pairing of iron with shallow donors (${\mathrm{P}}_{\mathrm{Si}}^{+}$ for example), again in agreement with an ionic model for the pair formation.