Structural and vibrational properties of carbon impurities in crystalline silicon

Si1-xCx alloys have been studied using self-consistent-charge density-funct ional-based right-binding calculations. The origin of experimentally observ ed carbon-induced vibrational peaks near 475, 607 and 810 cm(-1) are analys ed, based on the theoretical calculations. The stability, vibrational frequ encies, lattice relaxations. and energy gap variances of substitutional, in terstitial single-carbon and dicarbon complexes in crystalline silicon are calculated. All the impurities induce severe lattice relaxations of adjacen t Si atoms. The peak near 475 cm(-1) originates from the lattice relaxation s of Si atoms up to second-nearest neighbours from carbon impurities in all cases. The peak near 605 cm(-1) originates mainly from the midbond interst itial carbon (which is at odds with general belief) whereas the high-energy peaks near 810 cm(-1) result from the formation of the carbon complexes.