Arsenic deactivation in Si: Electronic structure and charge states of vacancy-impurity clusters

Various vacancy-impurity complexes that are believed to account for the deactivation of As in highly n-doped silicon are investigated using the density functional theory. The electronic structure and defect levels of these impurities are determined, both of which are crucial for the understanding and modeling of the deactivation process. We find that the As₁V complex can trap up to two conduction electrons at high n doping. Both As₂V and As₃V bind a single extra electron at elevated Fermi levels, which makes these complexes very efficient at deactivating arsenic. We relate the impurity levels to the ones found in the isolated vacancy and find that, in general, all As dopants tend to bind their fifth valence electron much more strongly than in the fourfold coordination, when placed next to a lattice vacancy. Therefore, the donor level of As in such configurations is shifted deep into the band gap or even below. For the isolated Si lattice vacancy, our calculations predict that at very high n doping, it might adopt a charge state -4 and thus act itself as an effective electron trap center. In addition, we observe that AsₙV₂ clusters $(n=2,3)$ containing a divacancy may also play an important role in the deactivation process of As at lower annealing temperatures.