Interaction of hydrogen with substitutional and interstitial carbon defects in silicon

An ab initio cluster method is used to investigate substitutional, Cₛ, and interstitial, Cᵢ, carbon defects in silicon complexed with hydrogen. We find that the binding energy of neutral H with Cₛ is 1.01 eV, and that the defect is bistable. In the positive and neutral charge states H lies near the center of a C-Si bond, and is antibonded to C in the negative charge state. A second H atom can be trapped in a H₂* defect. H forms stronger bonds with interstitial Cᵢ. In the Cᵢ-H defect, the binding energy of H is 2.8 eV, and two low-energy structures have almost degenerate energies. These consist 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 two stable structures is very low, ~0.3 eV, implying that the defect migrates rapidly, and readily reacts with other defects or impurities present. Two possible reactions are considered: the first is with another H and the second with Cₛ. The defect is completely passivated in the former while the stable form of the latter consists of a $〈100〉$ C-C dicarbon interstitial, where one radical is passivated by H. The calculated symmetry and the local vibrational modes are in excellent agreement with those experimentally observed for the T photoluminescent center. Finally, a further reaction involving the T center and a second H atom is considered, and is found to lead to the elimination of electrical activity.