INFRARED-ABSORPTION AND 1ST-ORDER RAMAN-SCATTERING DUE TO SINGLE AND PAIRED IMPURITY ATOMS IN DILUTE GE-SI ALLOYS

A theoretical study of the defect-induced one-phonon infrared absorption and Raman scattering due to isolated single and paired silicon atoms in a germanium lattice has been made using a Green's-function theory. The calculation considers altered radial and angular force constants between the two atoms of a pair. Numerical calculations have been performed for the impurity modes, the infrared absorption, and the first-order Raman scattering. The calculated results are in good agreement with the experimental data. The major contributions towards the infrared-absorption and the Raman scattering spectra arise from the phonon density of states of the host lattice, e.g., all the major peaks of the phonon density appear in the calculated infrared spectra. An increase of 23% in the radial force constant over that of the host Ge crystal has been observed. This Si-Si coupling in the Ge matrix is just equal to that seen in a pure Si crystal, and one finds a very interesting result: the Si-Si coupling remains the same irrespective of the matrix containing it. One peak at 200 cm-1 predicted in the calculated first-order Raman spectra has not been detected in the Raman measurements reported so far. The results stress the need of a better cluster theory on atomic scale for understanding the behavior of high-concentration alloys, rather than merely an effective-medium theory like the single-site coherent-potential-approximation theory. © 1979 The American Physical Society.