Epitaxial nitride-oxide trilayers for low-loss dielectrics in superconducting quantum circuits

Amorphous materials are a leading source of decoherence in current superconducting qubit technology, motivating the search for crystalline alternatives with intrinsically lower dielectric loss [1]. In this work, we deposit a novel epitaxial TiN/γ-Al₂O₃/TiN superconductor-insulator-superconductor (SIS) trilayer via pulsed laser deposition. Our trilayers exhibit excellent crystallinity and heteroepitaxy, along with structurally and chemically sharp interfaces. By evaluating its dielectric properties using lumped-element resonators [2], we establish the first reported dielectric loss tangent of epitaxial γ-Al₂O₃, the upper limit of which is two orders of magnitude improved over amorphous AlO_x in standard Al/AlO_x/Al SIS trilayers [3]. Furthermore, these resonators demonstrate some of the highest internal quality factors observed to date for devices with similar geometries and footprints. These findings highlight epitaxially interfaced single-crystal γ-Al₂O₃ and TiN as a compelling materials system for next-generation superconducting quantum hardware. 

[1] J. Lisenfeld et al., Nat. Commun. 6, 6182 (2015).

[2] C. R. H. McRae et al., Appl. Phys. Lett. 116, 194003 (2020).

[3] J. Zotova et al., Phys. Rev. Appl. 19, 044067 (2023).