Realizing Josephson Junctions on Epitaxial Nitride Trilayers

Expanding the design space while reducing loss mechanisms is a major challenge in Josephson Junctions (JJs) and quantum circuits. Epitaxial trilayer stacks of NbTiN/AlN/NbTiN, where AlN serves as the insulating barrier layer of the JJ is a promising replacement for Al/AlOx/Al devices in quantum circuits. Alloyed NbTiN possesses a Tc near 17 K, allowing for far higher operating temperatures compared to aluminum, and an oxygen-free highly-crystalline heterostructure, where precise thickness and compositional control is enabled by plasma-assisted molecular beam epitaxy (PAMBE). However, realizing a JJ on this heterostructure presents unique fabrication challenges given the vertical trilayer geometry. We present two general approaches to trilayer nitride JJ design with the micron-scale active areas contacted by air bridges vs. leads on an insulating oxide encapsulation. Details of device fabrication and cryogenic current-voltage characterization will be discussed.