Low-frequency Raman scattering from silicon nanostructures

Low-frequency Raman scattering due to acoustic phonons is studied for silicon nanostructures. The lineshapes of the first-order Raman active modes exhibit asymmetry. A tail is observed toward low frequency and high frequency for the optic mode and acoustic mode, respectively. The Raman lineshapes of these modes are determined by a Gaussian envelope function convoluted with the vibrational density of states. The observed blueshift of the acoustic mode with reducing size of the nanostructures can be explained by the relaxation of the wavevector selection rule (q = 0), which is used in the phonon confinement model for positive-slope (d omega/dq > 0) phonon dispersion. Because the acoustic and optical phonon branches have high positive and moderate negative slopes, respectively, around the Gamma-point in the phonon dispersion, a larger Raman shift of the acoustic mode to a higher frequency is observed in comparison with the shift to a lower frequency of the optic mode for a given nanostructure size. (C) 2011 American Institute of Physics. [doi:10.1063/1.3633235]