Stress-induced structural changes in superconducting Nb thin films

We report on the analysis of stress-induced structural changes and the formation of an omega (.) phase in polycrystalline Nb thin films deposited on Si by high-power impulse magnetron sputtering (HiPIMS) for superconducting qubits using x-ray diffraction (XRD), transmission electron microscopy (TEM), and density-functional theory (DFT). XRD analysis indicates that internal stresses in the Nb thin films lead to the formation of {112}< 111 > deformation twins and TEM analysis shows that omega phases nucleate at some of the twin boundaries in the Nb thin films. The size of. phases ranges from 10 to 100 nm, which is comparable to the coherence length of Nb (approximate to 40 nm), and approximate to 1% volume fraction of Nb grains exhibit this omega phase. The details of the formation mechanism and superconducting properties of the. phases are investigated by DFT and potential roles of the omega phase in Nb as a source of decoherence in superconducting qubits are also discussed.