Effect of extended defects on phonon confinement in polycrystalline Si and Ge films

We present Raman spectroscopy of the polycrystalline Si and Ge films formed by molecular beam deposition on a dielectric substrate. Structural properties of the poly-films have been studied by XRD and TEM. The Raman spectra of poly-Si films demonstrate both a downward shift and an asymmetrical broadening of the vibrational band near 520 cm-1, while only a symmetrical broadening of the band at 300 cm-1 is observed in the spectra of poly-Ge. The Raman line shape has been modeled within the framework of phonon confinement theory, taking into account the sizes of coherent scattering domains estimated using XRD and the influence of extended defects on peak broadening. Phonon dispersion and density of phonon states have been simulated using density functional theory. It has been found that grain boundaries, rather than other extended defects such as twins (multiple twins, twin boundaries), lead to significant changes in phonon dispersion and density of phonon states.