New Paper Online: Grain size in low loss superconducting Ta thin films on c-axis sapphire

In our newest paper, we investigate the influence of grain size in polycrystalline tantalum films on the dielectric losses of microwave resonators. In previous work, it was shown that the use of tantalum instead of niobium films enhances the coherence time of superconducting transmon qubits considerably (Link to this work Berthold Jäck was involved with: tantalum qubit paper). This performance enhancement was associated with a simpler oxide structure at the tantalum-air-interface. At the same time, insight into the influence of the bulk properties of tantalum films on dielectric losses remained missing. Because losses in niobium-based qubits correlate with grain size effects—it is believed that these losses are induced by partial oxides residing at the crystalline grain boundaries—understanding grain size effects on microwave losses in tantalum-based superconducting thin films is desirable.

To answer this question, we collaborated with Corey Rae McRae and Andras Gyenis from CU Boulder who carried out microwave loss measurements at cryogenic conditions on the tantalum thin films grown by our group. Interestingly, we find that the grain size of polycrystalline tantalum films on c-axis sapphire substrate has no influence on microwave losses. At the same time, our structural and chemical materials characterisation reveals the presence of partial tantalum oxide at the Ta2O5-tantalum interface beneath the surface oxide layer. Our results suggest that microwave losses in tantalum-based systems are dominated by surface rather than bulk losses. Hence, future efforts to further reduce microwave losses in these systems may be well invested rather in the reduction of surface losses than in the optimisation of the bulk structure. Kudos to everybody involved in this nice work.

Interested? Check out our pre-print to read more! Link to Preprint