Structure and bonding of iron-acceptor pairs in silicon

The highly mobile interstitial iron and Group III impurities (B, Al, Ga, In) form iron-acceptor pairs in silicon. Based on the migration kinetics and taking host silicon as a dielectric medium, we have simulated the pairing process in a static silicon lattice. Different from the conventional point charge ionic model, our phenomenological calculations include (i) a correction that takes into account valence electron cloud polarization which adds a short range, attractive interaction in the iron-acceptor pair bonding; and (ii) silicon lattice relaxation due to the atomic size difference which causes a local strain field. Our model explains qualitatively (i) trends among the iron-acceptor pairs revealing an increase of the electronic state hole emission energy with increasing principal quantum number of acceptor and decreasing pair separation distance; and (ii) the stable and metastable sites and configurational symmetries of the iron-acceptor pairs. The iron-acceptor pairing and bonding mechanism is also discussed.