Characterization of room temperature-grown Tantalum superconducting films with an Aluminum buffer layer

High-quality superconducting qubits pave the way toward fault-tolerant quantum computation. Among superconducting materials, tantalum (Ta) has emerged as a promising candidate, exhibiting record coherence times exceeding 1 ms. This superior performance is attributed to Ta’s chemically robust, stoichiometric native oxide. Crucially, obtaining the body-centered cubic α-Ta phase is essential for achieving optimal superconducting properties, as opposed to the metastable β-phase. However, the high growth temperature required for α-Ta poses a challenge for large-scale integration due to substrate compatibility. In this work, we demonstrate controlled α-Ta phase formation on silicon substrates at room temperature by introducing an aluminum buffer layer. By varying the thicknesses of the Ta and Al layers, we tune the critical temperature of the Ta films from 4.2 K to 2.6 K. Interestingly, the residual resistance ratio indicates that film quality is dominated by the Al layer thickness rather than the superconducting transition temperature. AFM and SEM analyses reveal that the Ta grain size depends primarily on the Ta thickness. Microwave resonators fabricated from films with the highest and lowest critical temperatures exhibit intrinsic quality factors up to 1.8 × 10⁶ in the single-photon regime and up to 1.0 × 10⁷ in the high-photon regime. A pronounced frequency shift due to kinetic inductance is observed in the 4.2 K film, ten times larger than that of the 2.6 K film. We attribute this effect to differences in the superconducting gap, which may contribute an additional supercurrent component. These findings highlight the potential of Ta/Al hybrid films for the design and optimization of advanced superconducting quantum circuits.