Theory of carbon-carbon pairs in silicon

Interstitial-substitutional carbon pairs (CᵢCₛ) in silicon display interesting metastable behavior associated with two different structural configurations. In this work, we perform extensive ab initio calculations on this system. Our results show the following. (i) The metastable configuration for the neutral charge state displays C₁ₕ symmetry and it is reminiscent of the isolated interstitial carbon configuration, i.e., a split interstitial C-Si pair with the substitutional carbon bonded to the silicon interstitial. (ii) The ground-state configuration also has C₁ₕ symmetry, but it consists of a single silicon interstitial twofold coordinated in an unusual bridge configuration between two substitutional carbon atoms. With an activation energy of 0.07 eV, this configuration becomes a motional-averaged state with C₃ᵥ 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 corresponds to a simple ``bond-switching'' mechanism with a calculated energy barrier of 0.13 eV. (iv) Both configurations have two electronic states in the gap, with gap-state wave functions consistent with the local bonding of the defect complex in each case. (v) Analysis of local-mode vibrations on the ground-state configuration indicates a stronger component in one of the carbon atoms, which explains the experimentally observed isotope splittings. Vibrational frequencies for the metastable configuration are also predicted. All of these results are in satisfactory agreement with experiments.