Engineering Niobium-Germanium Interfaces for Voltage-Tunable Quantum Devices*

Voltage-tunable hybrid superconductor-semiconductor Josephson junctions have recently emerged as promising building blocks for low-loss frequency-tunable quantum devices such as qubits, couplers, and magnetic flux sensors. The realization of hybrid devices in group IV semiconductors is of particular interest due to higher scalability and low dielectric loss at microwave frequencies. However, inducing superconductivity in Si and Ge via proximity effect has been proven to be challenging because of large interfacial energy barriers and defect densities. Here, we utilize molecular beam epitaxy to engineer the energy bands at Nb-Ge interfaces. By creating a gradient in Nb:Ga ratio throughout the superconducting layers, we create smooth potential gradients at the interfaces. Various thermal cycling schemes under vacuum and in inert atmospheres are used for tuning the interface structures. Using transmission electron microscopy we determine the competing secondary phases that may form in the stacks. This is complemented by cryogenic magneto-transport measurements on the resulting Nb/Ge heterostructures where critical physical parameters including the induced gap size, the critical magnetic field, and the normal coherence length for the proximitized phases are determined.