Theory of interstitial transition atoms in GaAs

We present, for the first time, a theoretical study of 3d impurities in GaAs at the anion interstitial site. One-electron energies are obtained through a molecular-cluster model, within the multiple-scattering X{α} formalism. The results are then corrected for multiplet effects, through the Fazzio-Caldas-Zunger approach, to evaluate the location of acceptor levels in the gap as well as intra-d transition energies. Our results indicate that the trend for acceptor levels, from Co to Mn, is similar to that detected experimentally but energetically higher, confirming the assignment of the observed levels to substitutional impurities. We find also that interstitial Cu and Ni should not introduce acceptor states in the gap. All of these interstitial impurities present high-spin ground states.