Composition and strain effects on Raman vibrational modes of GeSn alloys with Sn contents up to 31 % grown by low-temperature molecular beam epitaxy

We study the behavior of Ge-Ge, Ge-Sn, and Sn-Sn vibrational modes in GeSn semiconductors with Raman Spectroscopy. Raman spectroscopy is a rapid, nanoscale spatial resolution and non-destructive approach to accurately determine the composition and strain information of GeSn, and it is thus crucial for the material investigation and device application of GeSn alloys. By using several excitation wavelengths at 532, 633 and 785 nm on a set of fully strained and fully relaxed Ge1-xSnx layers with the Sn composition in the range 2.3 % < x(Sn) < 31 %, all modes are identified and their evolution as a function of strain and Sn content is determined. The Raman shifts of all vibrational modes are found to exhibit the same function versus the composition x(Sn) and in-plane strain epsilon//, Delta omega = omega(GeSn) - omega(Ge) = ax(Sn) + b epsilon//, where a is the Sn composition factor and b is the strain shift factor. In addition, the Ge-Sn mode intensity increases with Sn content. It is discovered for the first time that the Sn composition determined from the plot of the intensity ratio of the Ge-Sn mode over the Ge-Ge mode as a function of Sn composition at 785 nm excitation agrees well with that the X-ray Diffraction (XRD) Reciprocal Space Mapping (RSM), offering a novel approach for determining Sn content by Raman spectroscopy.