Based on first-principles density-functional calculations, we propose a class of nearest-neighbor donor pairs that are energetically favorable in highly $n$-type Si. These donor pairs comprise dopant atoms either fourfold coordinated at the nearest-neighbor distance or threefold coordinated through bond-breaking relaxations. For P and As dopants, the two defect states are very close in energy, less than 0.1 eV, while the threefold coordinated state is more stable by 0.24 eV for Sb dopants. The former state has a very deep donor level close to the valence band maximum, while the defect level lies deep inside the valence band for the latter. Thus, both the donor pairs are electrically inactive at very high doping levels, and they are suggested to be responsible for the observed saturation of carriers.

• Received 28 May 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.125503