  Germanium thin films are an excellent candidate for use as a low-loss
dielectric in superconducting microwave resonators, a low-loss inter-layer
metal wiring dielectric, and passivation layers in microwave and Josephson
junction devices. In Ge/Nb structures deposited at 400 {\deg}C, we observe
intermixing over as much as 20 nm. The addition of a 10 nm Ta diffusion barrier
layer reduces the superconductor/dielectric intermixing to less than 5 nm and
enhances the structural properties of deposited a-Ge layers based on Raman
spectroscopy. Additionally, superconducting microwave resonators fabricated at
room-temperature on crystalline Ge substrates with a Ta barrier layer show
marked improvement in total and power-dependent two-level system microwave
losses.
