Temperature dependence of vibrational modes in semiconductors depends on lattice thermal expansion and anharmonic phonon-phonon scattering. Evaluating the two contributions from experimental data is not straightforward, especially for epitaxial layers that present mechanical deformation and anisotropic lattice expansion. In this paper, a temperature-dependent Raman study in epitaxial Ge and ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ layers is presented. A model is introduced for the Raman mode energy shift as a function of temperature, comprising thermal expansion of the strained lattice and anharmonic corrections. With support of x-ray diffraction, the model is calibrated on experimental data of epitaxial Ge grown on Si and ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ grown on Ge/Si, finding that the main difference between bulk and epitaxial layers is related to the anisotropic lattice expansion. The phonon anharmonicity and other parameters do not depend on dislocation defect density (in the range $7\times {10}^6–4\times {10}^8{\mathrm{cm}}^{-2}$) nor on alloy composition in the range 5–14 at.%. The strain-shift coefficient for the main model of Ge and for the Ge-Ge vibrational mode of ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$ is weakly dependent on temperature and is around –500 ${\mathrm{cm}}^{\text{–}1}$. In ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x$, the composition-shift coefficient amounts to –100 ${\mathrm{cm}}^{\text{-1}}$, independent of temperature and strain.

• Received 17 October 2023
• Accepted 8 January 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.023801