Grain size in low loss superconducting Ta thin films on c axis sapphire

Abstract

In recent years, the implementation of thin-film Ta has led to improved coherence times in superconducting circuits. Efforts to further optimize this materials set have become a focus of the subfield of materials for superconducting quantum computing. It has been previously hypothesized that grain size could be correlated with device performance. In this work, we perform a comparative grain size experiment with α-Ta on c axis sapphire. Our evaluation methods include both room-temperature chemical and structural characterization and cryogenic microwave measurements, and we report no statistical difference in device performance between smaller- and larger-grain-size devices with grain sizes of 924 and 1700 nm2, respectively. These findings suggest that grain size is not correlated with loss in the parameter regime of interest for Ta grown on c axis sapphire, narrowing the parameter space for optimization of this materials set.