AB-INITIO CALCULATIONS OF ANHARMONICITY OF THE C-H STRETCH MODE IN HCN AND GAAS

The anharmonicities of the C-H stretch modes in HCN and the passivated C acceptor in GaAs are investigated using ab initio local-density-fun ctional cluster theory. The effective-mass parameter chi for the C-H s tretch mode is shown to be less than unity in HCN, and greater than un ity for the GaAs case. The calculated anharmonic parameter for the fir st defect is found to be 106 cm(-1) and is in very good agreement with experiment. For the second defect, the anharmonicity is about 50% lar ger in agreement with empirical estimates. The frequencies of the fund amental transitions in both systems are shown to be very sensitive to the C-H length. This limits the accuracy of theoretical investigations of these high frequency H modes. Finally, the effects of electrical a nharmonicity are considered and it is shown that they reduce the inten sity of the overtone in the C-H complex in GaAs by about 70%.