The convergence of density-functional super-cell calculations for defect formation energies, charge transition levels, localized defect state properties and defect atomic structure and relaxation was investigated by using the As split interstitial in GaAs as an example. Super-cells which contained up to 217 atoms, and various k-space sampling

schemes, were considered. It was shown that a good description of the localized defect state dispersion and charge state transition levels required at least a 217-atom super-cell, although the defect structure and atomic relaxations could be well converged in a 65-atom cell. Formation energies were calculated for the As split interstitial, Ga vacancy and As antisite defects in GaAs; taking into account the dependence upon chemical potential and Fermi energy. It was found that the equilibrium concentrations of As interstitials would be much lower than the equilibrium concentrations of As antisites in As-rich, n-type or semi-insulating GaAs.

First-Principles Study of As Interstitials in GaAs - Convergence, Relaxation and Formation Energy. J.T.Schick, C.G.Morgan, P.Papoulias: Physical Review B, 2002, 66[19], 195302 (10pp)