Stress-induced omega phase transition in Nb thin films for superconducting qubits

TL;DR

This study observes stress-induced omega phase formation in Nb thin films used for superconducting qubits, revealing its dependence on internal stress and grain size, which may affect superconducting properties.

Contribution

It provides the first detailed TEM evidence of omega phase in Nb thin films and links phase formation to internal stress and grain boundary density.

Findings

Omega phase size ranges from 10 to 100 nm.

Approximately 1 vol.% of Nb grains exhibit omega phase.

Omega phase is absent in large grain Nb samples.

Abstract

We report the observation of omega phase formation in Nb thin films deposited by high-power impulse magnetron sputtering (HiPIMS) for superconducting qubits using transmission electron microscopy (TEM). We hypothesize that this phase transformation to the omega phase with hexagonal structure from bcc phase as well as the formation of {111}<112> mechanical twins is induced by internal stress in the Nb thin films. In terms of lateral dimensions, the size of the omega phase of Nb range from 10 to 100 nm, which is comparable to the coherence length of Nb (~40 nm). In terms of overall volume fraction, ~1 vol.% of the Nb grains exhibit this omega phase. We also find that the omega phase in Nb is not observed in large grain Nb samples, suggesting that the phase transition can be suppressed through reducing the grain boundary density, which may serve as a source of strain and dislocations in this system. The current finding may indicate that the Nb thin film is prone to the omega phase transition due to the internal stress in the Nb thin film. We conclude by discussing effects of the omega phase on the superconducting properties of Nb thin films and discussing pathways to mitigate their formation.