Room-Temperature-Compatible α-Tantalum Growth on Niobium-Seeded Silicon for Low-Loss Superconducting Circuits

Superconducting quantum circuits demand materials with ultralow microwave losses to achieve long coherence times. Tantalum has recently emerged as a leading candidate due to its high kinetic inductance, thin native oxide, and low two-level system participation, particularly in its body-centered cubic α-phase. However, the integration of α-Ta into scalable silicon-based quantum hardware has been limited by requiring high-temperature growth or amorphous nitride seed layers, both of which introduce interfacial loss mechanisms. Here, we demonstrate the growth of high-purity α-Ta thin films on silicon substrates using 3 nm niobium seed layer, enabling epitaxial α-Ta formation at moderate (400°C) to room temperatures without compromising superconducting properties. Structural and electrical characterization reveal a residual resistivity ratio consistent with crystalline α-Ta and a superconducting transition temperature of 4.1K, confirming phase purity and low defect density. We propose a Nb-assisted nucleation mechanism, where the lattice-matched Nb seed layer templates α-Ta growth and suppresses the β-phase under moderate thermal budgets. These Nb-seeded α-Ta films provide a pathway to high-quality and manufacturable platforms for next-generation low-loss superconducting circuits.