Thermal expansion and temperature dependence of Raman modes in epitaxial layers of Ge and Ge_1-xSn_x

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 work, a temperature-dependent Raman study in epitaxial Ge and Ge_1-xSn_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 Ge_1-xSn_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· 10^6 - 4· 10^8 cm^-2) nor on alloy composition in the range 5-14 at. strain-shift coefficient for the main model of Ge and for the Ge-Ge vibrational mode of Ge_1-xSn_x is weakly dependent on temperature and is around -500 cm^-1. In Ge_1-xSn_x, the composition-shift coefficient amounts to -100 cm^-1, independent of temperature and strain.