Ge thin-films with tantalum diffusion-barriers for use in Nb-based superconductor technology

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.