  We report on the behavior of Ge-Ge, Ge-Sn, Sn-Sn like and disorder-activated
vibrational modes in GeSn semiconductors investigated using Raman scattering
spectroscopy. By using an excitation wavelength close to E1 gap, all modes are
clearly resolved and their evolution as a function of strain and Sn content is
established. In order to decouple the individual contribution of content and
strain, the analysis was conducted on series of pseudomorphic and relaxed
epitaxial layers with a Sn content in the 5-17at.% range. All vibrational modes
were found to display the same qualitative behavior as a function of content
and strain, viz. a linear downshift as the Sn content increases or the
compressive strain relaxes. Simultaneously, Ge-Sn and Ge-Ge peaks broaden, and
the latter becomes increasingly asymmetric. This asymmetry, coupled with the
peak position, is exploited in an empirical method to accurately quantify the
Sn composition and lattice strain from Raman spectra.
