Stress-induced structural changes in Nb thin film for superconducting qubits

We report analysis of stress-induced omega (ω) phase transition in ~170 nm thick Nb thin films on Si for superconducting qubits using transmission electron microscopy (TEM) [1]. The Nb thin film exhibits {112} <111> deformation twins on the film surface, indicating that the Nb thin film is under internal stress beyond the yield strength of Nb. Also, the formation of omega (ω) phase is associated with the {112} <111> deformation twin boundaries, which is consistent with other metals such as Ta or Ti alloys. The size of the omega (ω) phases varies from 10 to 150 nm, which is comparable to the coherence length of Nb in clean limit, and their volume fraction is ~1 vol.%. Density functional theory (DFT) study anticipated that the omega (ω) phase may have suppressed T_c (1-2 K) compared to bcc Nb (~9.2 K) [2]. We conclude by discussing how the omega (ω) phase may lead to decoherence in superconducting qubits through pair breaking.