Model investigation of the Raman spectra of amorphous silicon

A model for calculating the first-order Raman spectra of amorphous silicon (a-Si) without adjustable parameters is proposed. Calculations on the original 216-atom model of a-Si, generated by the algorithm of Wooten, Winer, and Weaire (WWW) are in very good agreement with experimental spectra and give further indication that the WWW cluster is a realistic model of moderately disordered a-Si. The TA-TO assignment of the low and high frequency bands is supported by direct numerical calculations of the phase quotient and the stretching character of the vibrational modes. The calculated participation ratios and correlation lengths of the vibrational modes indicate that the high-frequency TO-like modes are strongly localized on defects. The relative intensities of the TA-, LA-, and LO-like bands depend on the intermediate-range order, while that of the TO-like band mainly on the short-range order.