Solid solubility and diffusion of boron in single-crystalline cobalt disilicide

Abstract

The temperature dependence of the solid solubility [CsCoSi2(B)] and the lattice diffusion coefficient [DCoSi2(B)] of boron in single-crystalline cobalt disilicide (CoSi2) has been investigated between 450 and 1000 °C. Both CsMCoSi2(B) and DCoSi2(B) are found to be considerably higher than the corresponding quantities in silicon. Using a thermodynamical interpretation, the experimental data show that boron-doped CoSi2 can be described as a regular solution in the dilute limit with an enthalpy of solution of ∼−0.4 eV. The experimental data and theoretical estimates of the excess enthalpy of solution indicate a weak interaction of boron with the silicon and cobalt atoms in CoSi2 suggesting that the boron atoms occupy sites in the CoSi2 lattice with a small contribution to the Gibbs energy of the solution phase. The diffusion data yield a high mobility of the boron atoms with an activation energy of ∼2.0 eV for the lattice diffusion coefficient which is ∼1.0 eV lower than that reported for the self-diffusion of Si(Ge) and Co in CoSi2. Boron is a small atom which can occupy interstitial sites in the relatively opened CaF2 structure of CoSi2, and it is argued that the diffusion of boron may occur via a mixed process where interstitial/substitutional interchange takes place, including trapping and detrapping of the boron atoms.