Low resonator losses are hard to achieve in all classes of materials that have been considered for plasmonics applications. In this framework, we are investigating the dielectric function of heavily-doped germanium films epitaxially grown on silicon wafers, a material suitable for fabricating plasmonic resonators up to mid-infrared frequencies hence useful for molecular sensing. We determine the frequency-dependent scattering rate from infrared spectroscopy data. We derive the free carrier concentration from the Hall coefficient and the donor density by secondary ion mass spectroscopy. We are also working on the calculation of first-principle scattering cross-sections.