Impact of the electronic structure on the solubility and diffusion of 3dtransition elements in silicon

The dependence of the solubilities and diffusion coefficients of Mn, Fe, and Co on doping with B, P, and As in Si have been studied by the tracer method. An enhancement of the solubility by up to 4 orders of magnitude has been observed for P concentrations greater than 5×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and temperatures between 700 °C and 850 °C. The enhancement is due to immobile substitutional 3d impurity atoms forming multiple-acceptor centers and pairs with P at those temperatures. By contrast, in intrinsic and highly-B-doped Si, impurities on interstitial sites dominate the solubility, with the exception of Mn for T<850 °C. The interstitials act as donor centers resulting in a solubility enhancement for high B dopings (T5×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$) due to the singly positive charge state and impurity-boron pairs. No evidence for a dependence of the diffusion coefficient on the charge state has been found. The analysis of the solubility on the Fermi level shows that the donor levels of interstitial Mn, Fe, and Co above 700 °C abruptly shift towards the valence band with temperature. This is the first experimental evidence that a point-defect configuration becomes unstable at high temperatures in analogy to an instability of the Si self-interstitial conjectured by Seeger and Chick [Phys. Status Solidi 29, 455 (1968)] 20 years ago. It is further proposed that the multiple-acceptor behavior of substitutional Mn, Fe, and Co is due to a strong increase in the vacancy concentration in highly-P-doped Si as compared with intrinsic Si for T<850 °C.