First-principles total-energy calculations were made of trigonal and orthorhombic Fe-acceptor pairs. The total-energy calculations were performed, within the general framework of density-functional theory, by treating many-particle effects using the local spin-density approximation. A Green’s function approach was used which was based upon the linear muffin-tin orbitals theory, with an atomic-sphere approximation. This prohibited the inclusion of lattice-relaxation effects. The total-energy calculations led to a model for the electronic structure, of both orthorhombic and trigonal pairs, that was dominated by ionic binding of the interstitial Fe to the acceptor on a substitutional site. Significant covalent binding of the pair was not predicted. This contrasted with the cases of Fei-AuSi and Fei-AgSi pairs, where there was strong covalent pair binding for any Fermi energy position. In agreement with the ionic-binding model, no pairing was found in n-type material for any Fe-acceptor pairs. However, in n-type material there was a moderately strong pairing of Fe with shallow donors such as PSi+. This was also in agreement with an ionic model for pair formation.

H.Overhof, H.Weihrich: Physical Review B, 1997, 55[16], 10508-14