Solid-state reaction synthesis of superconducting refractory-metal germanides for Ge/SiGe quantum devices

Solid-phase reactions offer a route to integrate superconductors into Ge/SiGe heterostructures, but forming high-quality superconducting layers near the buried Ge quantum well remains challenging. We investigate reactions of V, Ta, and Nb with elemental Ge and with the Si 0.2 Ge 0.8 barrier of Ge/SiGe heterostructures at processing temperatures of 365-390 C. Cross-sectional electron microscopy and low-temperature electrical transport show that deposition on Ge at 390 C, in which approximately 2 nm metal increments are separated by growth stops for in-situ annealing, produces reactions throughout the deposited films and superconducting transitions at approximately 0.14 K for V-Ge, 2.1 K for Ta-Ge, and 2.8 K for Nb-Ge. The Nb-Ge layer has an out-of-plane critical field of approximately 0.8 T at 2.1 K. In the heterostructure samples, intermixing is confined to regions approximately 5-12 nm thick near the metal-SiGe interface, leaving a substantial residual metal layer. Microscopy and transport together suggest that this residual film dominates the measured superconducting response. These results establish low-temperature formation of superconducting refractory-metal-Ge layers while identifying the limited interfacial reaction as the principal obstacle to extending this approach toward the buried quantum well.