DYNAMIC PROPERTIES OF INTERSTITIAL CARBON AND CARBON-CARBON PAIR DEFECTS IN SILICON

Interstitial carbon, C-i, defects in Si exhibit a number of unexplaine d features. The C-1 defect in the neutral charge state gives rise to t wo almost degenerate vibrational modes at 920 and 931 cm(-1) whose 2:1 absorption intensity ratio naturally suggests a trigonal defect in co nflict with uniaxial stress measurements. The dicarbon, C-s-C-i, defec t 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-s at oms. This must give rise to vibrational modes which involve the motion of both C atoms in apparent conflict with the results of photolumines cence experiments. We use an nb initio local density functional cluste r 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-i is in reasonable agreement with experiment eve n though the structure of the defect is not trigonal. We also show tha t modes in the vicinity of those detected by photoluminescence for the B form of the dicarbon center involve independent movements of the tw o C atoms. Finally, the trends in the relative energies of the A and B forms in three charge states are investigated.