NbTiN/AlN/NbTiN SIS Junctions Realized by Reactive Bias Target Ion Beam Deposition

Abstract

The current state-of-the-art approach for superconductor–insulator–superconductor (SIS) junction fabric-ation is based on magnetron sputtering and the Gurvitch Al overlayer trilayer process, where an Al overlayer is deposited onto the Nb base electrode in order to subsequently grow a critical $\sim$ 1-nm AlO $_{x}$ or AlN tunnel barrier. While the switch from AlO $_x$ to AlN barriers has significantly increased the achievable critical current density, and significant research has been performed to understand and optimize inductively coupled plasma AlN growth, the use of Nb electrodes provides an upper limit to the frequency range for low-noise operation when employed in terahertz (THz) mixer applications and has led to the study and use of alternative materials with higher transition temperatures ( $T_{C}$ ) such as NbN and NbTiN. The Nb electrodes also impose stringent cooling requirements; replacement of both electrodes with higher $T_{C}$ materials can increase the required device operating temperature to 10 K, thus reducing the power consumption of the refrigeration system, a building block for the realization of more energy efficient superconducting computing systems, such as those based on single-flux-quantum logic. In this work, we have departed from conventional SIS material growth techniques through the use of an alternative material deposition technology—reactive bias target ion beam deposition (RBTIBD)—that offers unique capabilities to tailor materials and interfaces. Using RBTIBD technology, we have realized the first ever NbTiN/AlN/NbTiN SIS junctions with highest yet reported sum-gap voltages exceeding 5.0 mV, potentially extending the theoretical limit of low-loss SIS mixing applications beyond 1.2 THz and relaxing the cooling requirements for superconducting device applications.