After implantation with 12C+ at room temperature, and annealing (350C), a sharp infra-red absorption line was observed at 531/cm. When 12C+ was replaced by 13C+, the line shifted to 512/cm. The co-implantation of 12C+ and 13C+ did not give rise to additional lines. It was concluded that the 531/cm line reflected a local vibrational mode of a defect that contained a single C atom. Channelling measurements were carried out around the <100>, <110> and <111> axes of 12C+-implanted samples which had been annealed at 450C. An analysis of the data showed that some 31% of the C atoms were located at substitutional sites, while the remaining C atoms appeared to be randomly located. The population on substitutional sites, and the intensity of the 531/cm mode, exhibited identical temperature dependences. It was concluded that the 531/cm mode was the 3-dimensional T2 stretching mode of substitutional C. The effective charge of the mode was determined to be 3.4e. A first-principles local density functional cluster theory was used to calculate the structure and the local vibrational modes of substitutional C. The calculated frequencies and isotope shifts for the T2 stretching mode were in good agreement with experimental observations.

L.Hoffmann, J.C.Bach, B.B.Nielsen, P.Leary, R.Jones, S.Oberg: Physical Review B, 1997, 55[17], 11167-73