Mechanism of phosphorus diffusion gettering of cobalt in silicon studied by Mössbauer spectroscopy

The gettering of cobalt in silicon during phosphorus diffusion under high surface concentrations (PDG) has been investigated by Mössbauer spectroscopy between 825 and 1000 °C. It has been found that cobalt, distributed uniformly in the specimen before PDG, is concentrated within a highly phosphorus doped Si-surface layer thinner than 0.5 μm after PDG. It is also shown that PDG suppresses surface gettering and that the residual cobalt concentration in the bulk can be lower than the cobalt solubility. Mössbauer spectra of this layer show that cobalt occurs as two species there. The first of these has nearly cubic symmetry and is transformed into the second species upon annealing at 600 °C. This species is stable under the conditions of PDG. Neither the formation of phosphorus-cobalt pairs nor the presence of CoSi2 can provide a comprehensive explanation of all experimental facts.