Local vibrational modes of two neighboring substitutional carbon atoms in silicon

Infrared absorption measurements on n-type silicon doped with carbon and ir radiated with electrons at room temperature have revealed new absorption li nes at 527.4 and 748.7 cm(-1), which originate from the same defect. The 74 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 be low 6000 cm(-1). Light with frequency above 6000 cm(-1) removes this line a nd generates the 527.4-cm(-1) line. Comparison with spectra recorded on irr adiated cm silicon doped with C-13 shows that the two lines represent local vibrational modes of carbon. The annealing behavior of the 748.7-cm(-1) li ne is identical to that of the EPR signal originating from the negative cha rge state of two adjacent substitutional carbon atoms (C-s-C-s)(-). The 527 .4- and 748.7-cm(-1) lines are ascribed to the E modes of C-s-C-s in the ne utral and negative charge states, respectively. The structure and local vib rational 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 qua litatively 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 o f C-s-C-s has also been investigated. It is suggested that the center is fo rmed when a vacancy is trapped by the metastable substitutional carbon-inte rstitial carbon center, C-s-C-i.