THEORY OF CARBON-CARBON PAIRS IN SILICON

Interstitial-substitutional carbon pairs (CiCs) in silicon display int eresting metastable behavior associated with two different structural configurations. In this work, we perform extensive ab initio calculati ons on this system. Our results show the following. (i) The metastable configuration for the neutral charge state displays C-1h symmetry and it is reminiscent of the isolated interstitial carbon configuration, i.e., a split interstitial C-Si pair with the substitutional carbon bo nded to the silicon interstitial. (ii) The ground-state configuration also has C-1h symmetry, but it consists;of a single silicon interstiti al twofold coordinated in an unusual bridge configuration between two substitutional carbon atoms. With an activation energy of 0.07 eV, thi s configuration becomes a motional-averaged state with C-3v symmetry. (iii) The ground state is lower in; energy by 0.11 eV with respect to the metastable state. The jump from one configuration to the other cor responds to a simple ''bond-switching'' mechanism with a calculated en ergy barrier of 0.13 eV. (iv) Both configurations have two electronic- states in the gap, with gap-state wave functions consistent with the l ocal bonding of the defect complex in each case. (v) Analysis of local -mode vibrations on the ground-state configuration indicates a stronge r component in one of the carbon atoms, which explains the experimenta lly observed isotope splittings. Vibrational frequencies for the metas table configuration are also predicted. All of these results are in sa tisfactory agreement with experiments. [S0163-1829(98)07236-1].