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

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.