Infrared absorption measurements on n-type silicon doped with carbon and irradiated with electrons at room temperature have revealed new absorption lines at 527.4 and $748.7\hspace{0.5em}{\mathrm{cm}}^{-1},$ which originate from the same defect. The $748.7-{\mathrm{cm}}^{-1}$ line is observed only when the sample is cooled in the dark and the spectrum is measured through a low-pass filter with cutoff frequency below $6000\hspace{0.5em}{\mathrm{cm}}^{-1}.$ Light with frequency above $6000\hspace{0.5em}{\mathrm{cm}}^{-1}$ removes this line and generates the $527.4-{\mathrm{cm}}^{-1}$ line. Comparison with spectra recorded on irradiated silicon doped with ${}^{13}\mathrm{C}$ shows that the two lines represent local vibrational modes of carbon. The annealing behavior of the $748.7-{\mathrm{cm}}^{-1}$ line is identical to that of the EPR signal originating from the negative charge state of two adjacent substitutional carbon atoms $({\mathrm{C}}_s-{\mathrm{C}}_s{)}^{-}.$ The 527.4- and $748.7-{\mathrm{cm}}^{-1}$ lines are ascribed to the E modes of ${\mathrm{C}}_s-{\mathrm{C}}_s$ in the neutral and negative charge states, respectively. The structure and local vibrational modes of $({\mathrm{C}}_s-{\mathrm{C}}_s{)}^0$ and $({\mathrm{C}}_s-{\mathrm{C}}_s{)}^{-}$ have been calculated by ab initio local density functional theory. The calculated structures agree qualitatively with those obtained previously by Hartree-Fock methods, but the calculated Si-C and C-C bond lengths differ somewhat. The calculated local mode frequencies are in good agreement with those observed. The formation of ${\mathrm{C}}_s-{\mathrm{C}}_s$ has also been investigated. It is suggested that the center is formed when a vacancy is trapped by the metastable substitutional carbon-interstitial carbon center, ${\mathrm{C}}_s-{\mathrm{C}}_i.$

• Received 11 February 2000

DOI:https://doi.org/10.1103/PhysRevB.62.158