Anharmonicities of the C-H stretch modes in HCN, and of the passivated C acceptor in GaAs, were investigated by using  ab initio  local density functional cluster theory. The effective mass parameter for the C-H stretch mode was shown to be less than unity in HCN, and greater than unity in GaAs. The calculated anharmonic parameter for the former defect was found to be 106/cm, and was in very good agreement with experiment. In the case of the latter defect, the anharmonicity was some 50% larger; in agreement with empirical estimates. The frequencies of the fundamental transitions in both systems were shown to be very sensitive to the C-H bond length. This limited the accuracy of theoretical investigations of these high-frequency H modes. The effects of electrical anharmonicity were also considered, and it was shown that they reduced the intensity of the overtone of the C-H complex in GaAs by about 70%.

R.Jones, J.Goss, C.Ewels, S.Oberg: Physical Review B, 1994, 50[12], 8378-88