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

TL;DR

This study investigates whether grain size affects the performance of superconducting tantalum thin films on c-axis sapphire, finding no significant correlation and thus narrowing optimization parameters for quantum computing applications.

Contribution

It provides the first comparative analysis of grain size effects on device performance in superconducting Ta films on c-axis sapphire.

Findings

No statistical difference in device performance between small and large grains.

Grain size not correlated with loss in the studied parameter regime.

Results narrow the focus for optimizing Ta thin films in quantum devices.

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 $\alpha$-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 small- and larger-grain-size devices with grain sizes of 924 nm$^2$ and 1700 nm$^2$, 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.