Dynamic properties of interstitial carbon and carbon-carbon pair defects in silicon

Interstitial carbon, C{sub i}, defects in Si exhibit a number of unexplained features. The C{sub i} defect in the neutral charge state gives rise to two almost degenerate vibrational modes at 920 and 931 cm{sup {minus}1} whose 2:1 absorption intensity ratio naturally suggests a trigonal defect in conflict with uniaxial stress measurements. The dicarbon, C{sub s}-C{sub i}, defect is bistable, and the energy difference between its A and B forms is surprisingly small even though the bonding is very different. In the B form appropriate to the neutral charge state, a silicon interstitial is believed to be located near a bond-centered site between two C{sub s} atoms. This must give rise to vibrational modes which involve the motion of both C atoms in apparent conflict with the results of photoluminescence experiments. We use an {ital ab initio} local density functional cluster method, AIMPRO, to calculate the structure and vibrational modes of these defects and find that the ratio of the absorption intensities of the local modes of C{sub i} is in reasonable agreement with experiment even though the structure of the defect is not trigonal. We also show that modes in the vicinity of those detected by photoluminescence for themore » B form of the dicarbon center involve independent movements of the two C atoms. Finally, the trends in the relative energies of the A and B forms in three charge states are investigated. {copyright} {ital 1996} {ital The American Physical Society}« less