Dependence of the vibrational spectra of amorphous silicon on the defect concentration and ring distribution

The Raman spectra of nine 216-atom models of amorphous silicon (a-Si) are c alculated using the bond polarizability approximation of Raman scattering. These a-Si models, generated by the activation relaxation technique, have d ifferent concentrations of coordination defects, ring statistics and local strain distributions, which cause changes in the vibrational density of sta tes and the Raman scattering. Analysis of the vibrational modes indicates t hat an increase in the number of coordination defects leads to an increase in the high-frequency localization and to mixing of the TA modes with other high-frequency modes. Calculation of partial Raman spectra indicates that five-coordinated Si atoms enhance the high-frequency part of the LO Raman p eak at about 400 cm(-1) and lead to characteristic band at about 600 cm(-1) on the high-frequency side of the TO Raman peak. For their part, the three -coordinated Si atoms contribute to the low-frequency part of the LO peak. A weak correlation between the number of four-membered rings and the intens ity of the LO Raman peak is also established although there is no correlati on between the number of three- and four-membered rings and the total strai n energy.