Substitutional carbon in germanium

Carbon impurities implanted into single-crystalline germanium are studied with infrared absorption spec troscopy and ion channeling. After implantation of C-12(+) room temperature and subsequent annealing at 350 degrees C, a sharp infrared absorption line is observed at 531 cm(-1). When C-12(+) is substituted by C-13(+), the Line shifts down in frequency to 512 cm(-1) and co-implantation of C-12(+) and C-13(+) does not give rise to additional lines. Therefore, the 531-cm(-1) line represents a local vibrational mode of a defect containing a single carbon atom. Channeling measurements are carried out around the [100], [110], and [111] axes in C-12(+)-implanted samples annealed at 450 degrees C. The analysis of the data shows that 31+/-3% of the carbon atoms are located at substitutional sites, while the remaining carbon atoms appear to be located randomly. The population of the substitutional site and the intensity of the 531-cm(-1) mode have identical temperature dependencies. It is concluded that the 531-cm(-1) mode is the three-dimensional T-2 stretch mode of substitutional carbon. The effective charge of the mode is determined to be (3.4+/-0.5)e. Ab initio local density functional cluster theory is applied to calculate the structure and the local vibrational modes of substitutional carbon in germanium. The calculated frequencies and isotope shifts for the T-2 stretch mode are in good agreement with the observations.