Niobium Hydride Evolution and Phase Equilibrium in Niobium Capping Layers

Niobium is currently being used in superconducting qubit development. Hydrogen diffuses into bulk Niobium during processing when a passivating layer is absent. Hydrogen contamination causes "hydrogen Q disease" which drastically degrades the superconducting properties of the device. This degradation has been attributed to non-superconducting hydride precipitation at cryogenic temperatures [1]. Elastic energy is expected to impact precipitate formation and equilibrium shape. The hydride stiffness tensor calculated with density functional theory (DFT) and misfit strains are combined in the free energy functional of a phase field model. The elastically soft directions of the hydride and bulk niobium are found to be of type and respectively in the niobium coordinate system. The diffusion coefficient of hydrogen in niobium at -68 °C causes nano-scale hydrides to reach equilibrium on the order of milliseconds. When in proximity of a free surface, precipitates migrate toward the free surface and coarsen while incorporated precipitates dissolve. TiN and Si are currently under consideration for capping layers. In the interest of determining possible stable intermediate phases at the interface the CALPHAD method is used to produce ternary phase diagrams for the TiN-O-Nb and Si-O-Nb systems.