Based on first-principles calculations and analysis of x-ray absorption measurements, an unconventional mechanism is proposed for the saturation of carriers in highly $n$-doped Si. The mechanism is Fermi-level pinning from a new class of defect centers containing two separated but interacting dopant atoms with no associated Si vacancies. The number of such centers increases sharply at high doping levels. A simple model provides very good agreement with the maximum carrier concentrations observed in Si.

• Received 24 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.4834