Local vibrational modes of two neighboring substitutional carbon atoms in silicon

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 cm − 1 , which originate from the same defect. The 7 4 8 . 7 − 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 cm − 1 . Light with frequency above 6000 cm − 1 removes this line and generates the 5 2 7 . 4 − cm − 1 line. Comparison with spectra recorded on irradiated silicon doped with 13 C shows that the two lines represent local vibrational modes of carbon. The annealing behavior of the 7 4 8 . 7 − cm − 1 line is identical to that of the EPR signal originating from the negative charge state of two adjacent substitutional carbon atoms ( C s − C s ) − . The 527.4- and 7 4 8 . 7 − cm − 1 lines are ascribed to the E modes of C s − C s in the neutral and negative charge states, respectively. The structure and local vibrational modes of ( C s − C s ) 0 and ( C s − 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 C s − 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, C s − C i .