TLS loss in room-temperature-nucleated alpha-Ta resonators*

Like Niobium, Tantalum has demonstrated utility as a metallization layer within microfabricated superconducting quantum information processors. But, unlike Nb, high temperatures must be used to ensure the formation of the bcc (alpha) phase of the pure metal. It is known that a thin Nb layer can nucleate the growth of alpha-Ta at room temperature, but the precise qualities of nucleated films remain unclear. We perform a side-by-side comparison of nucleated and non-nucleated films patterned into resonators. We observe moderately greater TLS densities in nucleated Ta films as compared to pure Ta, consistent with higher defect densities evident in electron microscopy, but we suggest that this difference may be tolerable for substrates or processes that cannot withstand high temperatures.