THEORY OF SUBSTITUTIONAL CARBON AND BORON IN SILICON

Local density functional theory is used to evaluate the structures, vibrational frequencies and effective charges of the substitutional impurities C and B- in silicon. The method uses a 71 atom H-terminated cluster with Gaussian basis functions. We find the B-Si length to be 2.097 angstrom and the triplet local mode at 635 cm-1, which is within 12 cm-1 of the observed one. The C-Si length and triplet mode are 2.101 angstrom and 561 cm-1 respectively. This frequency is within 50 cm-1 of the observed local mode. The effective charges on the impurities show that substitutional C leads to twice as much local mode absorption as an equal concentration of B-.