Thermal Tuning of the Niobium-Germanium Interfaces for Hybrid Superconductor-Semiconductor Quantum Devices

Hybrid superconductor-semiconductor (S-Sm) materials systems are promising building blocks for frequency-tunable quantum devices. The resonance frequency of such devices will be defined by the voltage-tunable inductance of a hybrid S-Sm-S Josephson junction. This process relies on an effective pathway to induce superconductivity in the semiconducting channel which in turn has a strong dependence on the energy mismatch and defect populations at the S-Sm interfaces. Here, we study the influence of thermal processing on the interface conditions of Nb-Ge heterostructures. We deposited high-quality Nb thin films on Ge (001) via molecular beam epitaxy. Through rapid thermal annealing or conventional ultrahigh vacuum annealing, we tune the interface chemical and physical characteristics of the Nb-Ge interfaces. Through X-ray photoelectron spectroscopy and femtosecond ultraviolet photoelectron spectroscopy we determine the impact the thermal processes have on the band structure and chemical composition of the interface. Using atomic force and electron microscopy we determine the impact of interface reactions on the overall phase structure and morphology of the films. Those results are complemented by low-temperature measurement of the Nb transport characteristics (e.g., Tc, Bc, Ic) at various stages of the annealing cycles.