X-ray diffraction investigation of hydride precipitate structure in niobium thin films*

NbH precipitates have been well known to cause “Q-disease” in accelerator cavities and have recently been discovered in Nb planar superconducting qubits. [1] The NbH precipitation upon cooling causes dislocation scarring. Near-surface defects like these are believed to negatively affect superconducting qubit performance. However, whether NbH precipitates occur only near the surface or throughout the bulk is not well understood. The sub-surface structure is difficult to probe because of high H-interstitial mobility. Time of flight, secondary mass spectrometry experiments have located hydrogen preferentially near the film surface. However, transmission electron microscopy performed on the same samples has imaged vertical grains of NbH precipitates. Both techniques are destructive. While the NbH precipitates have previously been assumed to be vertical grains [2], simulations suggest that NbH should precipitate towards free surfaces. To probe the structure non-destructively, this study performed low-temperature, high-resolution, grazing-incidence X-ray diffraction at the APS 6-ID beamline to determine the NbH precipitate structure in molecular beam epitaxial Nb(110) films on a-Al₂O₃(110). NbH domains were identified near the surface and found to be tilted at discrete low angles with respect to the Nb single-crystal axial directions.

1. J. Lee, et al., arXiv:2108.10385 (2021).

2. S. Wagner, et al., ChemPhysChem, 20 (2019) 1890-1904.