Synthesis of transition metal based superconducting germanides for superconducting-semiconducting hybrids

Germanium quantum wells have emerged as one of the leading platforms for spin qubits. Combining these high quality semiconductors with superconductors paves the way for research in hybrid devices such as Andreev spin qubits, Kitaev chains and gate tunable superconducting qubits. Furthermore, such hybrids could allow for new ways to couple spin qubits via superconductors. For many of these applications, one requires large superconducting gaps and/or magnetic field resilience of the superconductor. In this work we study hybrid structures created by evaporating and annealing three different metals (Nb, V and Ta) on Ge substrates and on SiGe heterostructures with the goal of large superconducting gaps and field resilience. Each film is characterized by transmission electron microscopy and transport measurements. We found that each metal can fully mix with the Ge substrate to form superconducting compounds with T_C = 0.1 K for VGe, T_c = 2.1 K for TaGe and T_C = 2.85 K for NbGe. Evaporation and annealing on the SiGe heterostructures showed that each metal could diffuse up to 10 nm. We believe that these materials combined with shallower top barriers could allow for promising future hybrid devices.