Pairing and hyperfine interactions of Cd-P complexes in silicon

Abstract

Theoretical calculations of formation energies, binding energies of Cd-P pairs in Si, and hyperfine parameters at the Cd site are performed. The ab initio all electron full-potential linear muffin-tin orbital method based on smooth Hankel functions, local-density-functional theory, and the supercell technique are utilized. Cd-P pairs with different numbers of P atoms, in several charge states, are studied. In each complex, the phosphorus atom is placed at the substitutional site while Cd is localized at the substitutional or interstitial sites. We find that substitutionals ${\mathrm{CdP}}^{0,\ensuremath{-}1}$ and ${\mathrm{CdP}}_{2}^{0}$ are stable complexes with binding energies around 0.7--0.8 eV. The calculated electric-field gradients of these complexes at the Cd site give quadrupolar coupling constants in agreement with previous theoretical and experimental assessments. However, Cd at interstitial positions weakly binds to P for n-type samples, and gives hyperfine parameters much lower than previous tentative experimental assignations. We find the cluster ${\mathrm{CdP}}_{3},$ being as stable as CdP and ${\mathrm{CdP}}_{2},$ can explain the origin of the third quadrupolar coupling constant found at a high P level doping. These facts suggest that a segregation process of P atoms around Cd could take place. Calculated charge states, transition state levels, and equilibrium concentrations are in accord with experimental measurements.