Ta-based S-I-S junctions for integrated superconducting Electronics

Abstract

In this work, we have presented S-I-S Josephson junctions with $\rmα$-Ta electrodes and a ${\rm TaO}_x$ barrier. We grew base and top $α$-Ta electrodes using seed layer engineering, showing both Nb and ${\rm TaN}_x$ are viable seed layer choices, and particularly emphasize that at room temperature a re-seed layer is required to nucleate $\rmα$-Ta for the top electrode. However, we note that the choice of seed layer can have subtle implications on the electrical tunneling behavior of the junctions. We used room-temperature probing of variable junction areas, combined with Ambegaokar-Baratoff physics, to characterize the wafer scale critical current densities. For 0.4 $μ$m$^2$ junctions, we found junction spreads as low as 2.0% across the 20 x 20 mm chip relevant for device performance. The entire process was carried out using standard industry foundry processing using 150 mm wafer tooling, which is an important step towards proving that Ta-based junctions can be implemented at scale. We have successfully fabricated and tested junctions with $\rmα$-Ta electrodes and a ${\rm TaO}_x$ insulating tunnel barrier, demonstrating that this is a material platform which merits further investigation and studies for potential application in classical and quantum superconducting technologies.