Raman Measurements of Optical Phonon Scattering in Sub-Micron Si_(1-x)Ge_x Films

Phonon thermal conductivity is often modeled using the phonon gas model (PGM), which assumes that all normal modes of a material can be described by plane waves (i.e. delocalized and propagating). However, as the translational symmetry of a crystal is broken through the introduction of impurities, dislocation or nanoscaling, the normal modes depart from plane wave-like nature. Allen and Feldman first developed the theory for non-plane wave modes when modeling thermal conductivity showing that with increasing frequency modes become increasingly localized. Localizing modes limits the ability of vibrations to transmit energy through the material, which leads to a reduction of overall measured thermal conductivity. In addition to changing the nature of the vibrational modes in a material, breaking the translation symmetry creates scattering sites that further reduces vibrational thermal conductivity. It is difficult to separate the convoluted effects of scattering and localization in experimental thermal conductivity measurements since most investigations employ the PGM, which only accounts for phonon scattering. The next generation of models to predict thermal transport in disordered and nanoscaled materials will hinge on a detailed understanding of both the nature and behavior of vibrations. Raman spectroscopy is uniquely equipped to
study vibrational nature (localization) and behavior (scattering) since they are
directly related to the linewidth and asymmetry of the spectral peak of the Raman active mode, respectively. Therefore, Raman spectroscopy is employed to monitor the influence of alloying, nano-scaling and crystal imperfections/dislocations on temperature dependent scattering and localization. While the information gleaned from the optical Raman modes cannot be directly related to thermal conductivities it provides insight into how various material properties influence the combined scattering and localization of vibrations. It is found that breaking the periodicity of a crystal through alloying, nanoscaling or dislocations influences scattering of vibrations more than simply through temperature independent scattering sites. Additionally, breaking translational symmetry of the crystal leads to localization of the modes which is temperature dependent.