An  ab initio  cluster method was used to investigate substitutional and interstitial C defects which were complexed with H. It was found that the binding energy of neutral H to Cs was equal to 1.01eV, and that the defect was bistable. In the positive and neutral charge states, H lay near to the center of a C-Si bond, and was anti-bonded to C in the negative charge state. A second H atom could be trapped in an H2* defect. The H formed stronger bonds with interstitial Ci. In the case of the Ci-H defect, the binding energy of H was equal to 2.8eV, and 2 low-energy structures had almost degenerate energies. These consisted of a bond-centered Si-(Ci-H)-Si defect and a <100>-oriented Ci-Si split interstitial with H bonded to Ci. The calculated barrier for conversion between the 2 stable structures was very low (about 0.3eV); thus implying that the defect migrated rapidly, and reacted easily with other defects or impurities. Two possible reactions were considered. One involved H and the other involved Cs. The defect was completely passivated in the former case, while the stable form of the latter consisted of a <100> C-C di-carbon interstitial in which one radical was passivated by H. The calculated symmetry and the local vibrational modes were in excellent agreement with those that were experimentally observed for the T photoluminescence center. A further reaction, which involved the T center and a second H atom, was considered and was found to lead to the elimination of electrical activity. Many other possible defects could form, in theory. Among these were the reaction of migrating Ci-H with O, N, and other CH defects. It was suggested that CH formed very similar defects to those of iso-electronic impurities such as N and P. Thus, NiO2i was a candidate for the shallow thermal donor and, if N was replaced by CH, there was little change in the structural or electronic properties.

P.Leary, R.Jones, S.Oberg: Physical Review B, 1998, 57[7], 3887-99