Direct Measurement of Microwave Loss in Nb Films for Superconducting Qubits

The supplementary material provides detailed descriptions of the experimental procedures and characterization techniques used in this study. It includes information on Nb film deposition (S1), STEM measurements (S2), XPS analysis (S3), ToF-SIMS data (S4 and S5), XRD (S6), Williamson-Hall fitting calculations (S7), and details of the DR measurement setup (S8). The supplementary figures (FIG. S1-S7) illustrate the microstructure, chemical composition, and crystallographic properties of the Nb films, as well as the methodology for assessing their microwave loss characteristics.