Decoupling the effects of composition and strain on the vibrational modes of GeSn semiconductors
We report on the behavior of Ge-Ge, Ge-Sn, Sn-Sn like, and disorder-activated (DA) vibrational modes in GeSn semiconductors investigated using Raman scattering spectroscopy. By using an excitation wavelength close to the E1 gap, all modes are clearly resolved and their evolution as a function of strain and Sn content is established. Previous Raman scattering studies mainly focused on the Ge-Ge peak position which is insufficient to evaluate the effects of lattice strain and Sn content. Herein to decouple the individual contributions of content and strain, the analysis was conducted on series of pseudomorphic and relaxed epitaxial layers with a Sn content in the 5-17 at.% range. The frequencies of all vibrational modes were found to display qualitatively the same behavior as a function of content and strain, viz. a linear downshift as the Sn content increases or the compressive strain relaxes. Simultaneously, the Ge-Sn and Ge-Ge peaks broaden, and the latter becomes increasingly asymmetric. The behavior of the integrated intensity, width, and asymmetry of each one of these vibrational modes was also evaluated. We found that an increase in Sn content is associated with an increase in the relative integrated intensity of Ge-Sn and DA modes. The latter also increases as the layers become more compressively strained and becomes more prominent under the x(zz)x configuration as the intensity of the adjacent LO modes decreases. The Raman mode asymmetry, coupled with the peak position, is exploited to implement an empirical approach to accurately quantify the Sn composition and lattice strain from Raman spectra.