Raman scattering study of GeSn under <1 0 0> and <1 1 0> uniaxial stress.

The application of strain into GeSn alloys can effectively modulate the band structures, thus creating novel electronic and photonic devices. Raman spectroscopy is a powerful tool for characterizing strain; however, the lack of Raman coefficient makes it difficult for accurate determination of strain in GeSn alloys. Here, we have investigated the Raman-strain function of Ge1-xSnx along <1 0 0> and <1 1 0> directions. GeSn nanomembranes (NMs) with different Sn compositions are transfer-printed on polyethylene terephthalate (PET) substrates. External strain is introduced by bending fixtures with different radii, leading to uniaxial tensile strain up to 0.44%. Strain analysis of flexible GeSn NMs bent along <1 0 0> and <1 1 0> directions are performed by Raman spectroscopy. The linear coefficients of Raman-strain for Ge0.96Sn0.04 are measured to be -1.81 and -2.60 cm-1, while those of Ge0.94Sn0.06 are decreased to be -2.69 and -3.82 cm-1 along <1 0 0> and <1 1 0> directions, respectively. As a result, the experimental ratio of linear coefficient (ROLC) of Ge, Ge0.96Sn0.04 and Ge0.94Sn0.06 are 1.34, 1.44 and 1.42, which agree well with theoretical ROLC values calculated by elastic compliances and phonon deformation potentials (PDPs). In addition, the compositional dependence of PDPs is analyzed qualitatively. These fundamental parameters are important in designing high performance strained GeSn electronic and photonic devices.