Low-Temperature MBE growth of superconducting Ta films on Silicon and Sapphire substrates

Recent advances in tantalum-based Transmon qubits on sapphire have demonstrated coherence times up to 0.5 milliseconds^1,2 The growth of alpha-Ta at elevated temperatures is required for the realization of desirable superconducting properties. Alternatively, superconducting tantalum qubits can be grown on Si substrates by heating the substrate, but the potential formation of silicides at the Si-Ta interface could impact qubit coherence.

Here, we demonstrate low-temperature MBE growth (<20K) to stabilize alpha-Ta independent of the substrate without using a seed layer enabling its integration with various material systems. We characterize the growth of alpha-Ta on several substrates including amorphous SiN_x, Si(111), GaAs(001), and Al₂O₃(0001). Both structural and electrical measurements indicate the successful growth of α-Ta with good superconducting properties independent of the substrate. Finally, we fabricate CPW resonator circuits and 2D Transmon qubits to compare α-Ta on Si and Sapphire substrates under the same growth conditions.

1) Place, A.P.M., et al., Nat Commun 12, 1779 (2021).

2) Wang, C., et al., npj Quantum Inf 8, 3 (2022).