On the basis of first-principles calculations, and an analysis of X-ray absorption measurements, an unconventional mechanism was proposed for the saturation of carriers in highly n-doped material. The mechanism involved Fermi-level pinning by a new class of defect center which contained 2 separate, but interacting, dopant atoms with no associated Si vacancies. The number of such centers increased sharply at high doping levels. A simple model could provide very good agreement with the maximum carrier concentrations which were observed in Si.

D.J.Chadi, P.H.Citrin, C.H.Park, D.L.Adler, M.A.Marcus, H.J.Gossmann: Physical Review Letters, 1997, 79[24], 4834-7