Physical Review B, 50 8378-88 (1994)

Ab Initio Calculations of Anharmonicity of
the C-H stretch mode in HCN and GaAs

R. Jones, J. Goss and C. Ewels

Department of Physics, University of Exeter, Exeter, EX4 4QL, UK.

S. Öberg

Department of Mathematics, University of Luleå, Luleå, S95187, Sweden.

Abstract

The anharmonicities of the C-H stretch modes in HCN and the passivated C acceptor in GaAs are investigated using ab initio local density functional cluster theory. The effective mass parameter, $\chi$, for the C-H stretch mode is shown to be less than unity in HCN, and greater than unity for the GaAs case. The calculated anharmonic parameter for the first 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% larger in agreement with empirical estimates. The frequencies of the fundamental 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 anharmonicity are considered and it is shown that they reduce the intensity of the overtone in the C-H complex in GaAs by about 70%.

Keywords: Anharmonicity, GaAs, HCN, Fundamental, LVM, Carbon