Interaction of hydrogen with substitutional and interstitial carbon defects in silicon

An ab initio cluster method is used to investigate substitutional, ${\mathrm{C}}_s,$ and interstitial, ${\mathrm{C}}_i,$ carbon defects in silicon complexed with hydrogen. We find that the binding energy of neutral H with ${\mathrm{C}}_s$ 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 ${\mathrm{H}}_2^{*}$ defect. H forms stronger bonds with interstitial ${\mathrm{C}}_i.$ In the ${\mathrm{C}}_i-\mathrm{H}$ 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 ${\mathrm{S}\mathrm{i}-(\mathrm{C}}_i-\mathrm{H})-\mathrm{S}\mathrm{i}$ defect and a $〈100〉$-oriented ${\mathrm{C}}_i-\mathrm{S}\mathrm{i}$ split interstitial with H bonded to ${\mathrm{C}}_i.$ The calculated barrier for conversion between the two stable structures is very low, $\sim 0.3$ 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 ${\mathrm{C}}_s.$ 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.