  Hybrid Superconductor-Semiconductor (S-Sm) materials systems are promising
candidates for quantum computing applications. Their integration into
superconducting electronics has enabled on-demand voltage tunability at
millikelvin temperatures. Ge quantum wells (Ge QWs) have been among the
semiconducting platforms interfaced with superconducting Al to realize voltage
tunable Josephson junctions. Here, we explore Nb as a superconducting material
in direct contact with Ge channels by focusing on the solid-state reactions at
the Nb/Ge interfaces. We employ Nb evaporation at cryogenic temperatures (100
K) to establish a baseline structure with atomically and chemically abrupt
Nb/Ge interfaces. By conducting systematic photoelectron spectroscopy and
transport measurements on Nb/Ge samples across varying annealing temperatures,
we elucidated the influence of Ge out-diffusion on the ultimate performance of
superconducting electronics. This study underlines the need for low-temperature
growth to minimize chemical intermixing and band bending at the Nb/Ge
interfaces.
