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This paper explores solid-state reactions to synthesize superconducting refractory-metal germanides (V, Ta, Nb) within Ge/SiGe heterostructures for quantum device integration. By employing incremental metal deposition and in-situ annealing at 365-390 $^\circ$C, the researchers achieved superconducting transitions at 0.14 K (V-Ge), 2.1 K (Ta-Ge), and 2.8 K (Nb-Ge). While demonstrating the feasibility of low-temperature superconducting germanide formation, the study identifies limited interfacial intermixing as a key challenge for direct integration with buried quantum wells.
Superconducting Nb-Ge films with a critical field of 0.8 T form at just 390 $^\circ$C, enabling integration with Ge/SiGe heterostructures.
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 $^\circ$C. Cross-sectional electron microscopy and low-temperature electrical transport show that deposition on Ge at 390 $^\circ$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.