  We present Raman spectroscopy of the polycrystalline Si and Ge films
deposited by molecular beam deposition on a dielectric substrate. The Raman
study has been made using lasers with different wavelengths. Structural
properties of the poly-films have been studied by XRD and TEM. The Raman
spectra are characterized by appearance of the additional wide peaks around 500
cm$^{-1}$ and 290 cm$^{-1}$ in the main vibrational bands of TO(c-Si) and
TO(c-Ge) phonons, respectively. It is shown that these peaks correspond to
scattering in grain boundary area. For the poly-Si films, both a downward shift
and an asymmetrical broadening of the vibrational band of TO(c-Si) near 520
cm$^{-1}$ are observed, whereas there is only a symmetric broadening in the
spectra of poly-Ge. The Raman line shape has been modeled within the framework
of the phonon confinement theory taking into account the sizes of coherent
scattering domains obtained using XRD. The model includes a symmetrical band
broadening observed in polycrystalline films. It is shown that confinement of
phonon propagation might be in the poly-Si films. The phonon dispersion and the
density of phonon states have been simulated using density functional theory.
It has been found that phonon confinement relates to grain boundaries rather
than other extended defects such as twins (multiple twins, twin boundaries),
the appearance of which does not lead to significant changes in phonon
dispersion and density of phonon states.
