Substitutional carbon in Si_1-yC_y alloys as measured with infrared absorption and Raman spectroscopy

We present a study of the infrared absorption and Raman scattering intensity of the local carbon mode in Si_1-yC_y alloys grown by direct carbon implantation followed by different recrystallization procedures. For the case of laser-induced recrystallization, the integrated infrared absorbances are found to agree with an extrapolation of the calibration curve previously determined for very low substitutional carbon concentrations in Si. We argue that this finding provides strong evidence for the achievement of nearly perfect substitutionality in laser-recrystallized films, even though their carbon concentrations are three orders of magnitude beyond the solubility limit of carbon in Si. This conclusion is found to be consistent with measurements of the intensity of defect-induced Si Raman scattering relative to the Raman intensity of the local carbon mode. The Raman intensity of the local carbon mode at 605 cm^-1 relative to the first-order Si Raman line at 521 cm^-1 provides an ideal spectroscopic tool for the determination of substitutional carbon concentrations. By correlating Raman and infrared measurements, we find that for laser excitation at 488 nm the intensity ratio is given by I₆₀₅/I₅₂₁ = (3.7±0.2)y_sub.