A Comparative Study of Niobium Superconducting Resonators on Silicon and 4H-Silicon Carbide Substrates

Energy loss from two-level systems (TLS) at metal–dielectric interfaces continues to limit coherence in superconducting quantum circuits. We investigate whether substrate selection and interface preparation can suppress TLS participation by comparing niobium (Nb) coplanar-waveguide resonators fabricated on silicon (Si) and 4H-silicon carbide (SiC). Nb films (~150 nm) are deposited by DC magnetron sputtering using a standardized baseline process. Substrate preparation spans a controlled matrix of ex-situ and in-situ cleans, including solvent sonication, O₂ plasma, piranha clean, and HF clean (dip and vapor). Resonators were fabricated by photolithography and ICP etching and measured near 10 mK via complex S₂₁ transmission. Analysis follows a delay-corrected notch-resonator circle-fit (Probst) that incorporates complex coupling and phase skew to capture line mismatch and environmental noise; we extract f₀, Q_l​, complex Q_c​, and single-photon Q_i​.

We will report a one-to-one comparison across all chips: (i) Si versus SiC under identical deposition conditions and (ii) the cleaning-sequence matrix on each substrate. The primary outcome is a ranked set of substrate/clean combinations that maximize single-photon Qi and minimize inferred TLS-related loss.