Alternating Bias Assisted Annealing of Amorphous Oxide Tunnel Junctions

TL;DR

This paper introduces a technique called alternating-bias assisted annealing (ABAA) to controllably increase the resistance of amorphous aluminum-oxide tunnel junctions, improving their electrical and quantum properties for electronic applications.

Contribution

The study presents a novel ABAA method for tuning tunnel junction properties, demonstrating significant resistance increases and reduced defect levels while maintaining amorphous structure.

Findings

Resistance increased by over 70% at room temperature.

Reduced junction loss by approximately 2×10⁻⁶.

Improved defect reduction and uniformity in the amorphous barrier.

Abstract

We demonstrate a transformational technique for controllably tuning the electrical properties of fabricated thermally oxidized amorphous aluminum-oxide tunnel junctions. Using conventional test equipment to apply an alternating bias to a heated tunnel barrier, giant increases in the room temperature resistance, greater than 70%, can be achieved. The rate of resistance change is shown to be strongly temperature-dependent, and is independent of junction size in the sub-micron regime. In order to measure their tunneling properties at mK temperatures, we characterized transmon qubit junctions treated with this alternating-bias assisted annealing (ABAA) technique. The measured frequencies follow the Ambegaokar-Baratoff relation between the shifted resistance and critical current. Further, these studies show a reduction of junction-contributed loss on the order of $\approx 2 \times10^{-6}$, along with a significant reduction in resonant- and off-resonant-two level system defects when compared to untreated samples. Imaging with high-resolution TEM shows that the barrier is still predominantly amorphous with a more uniform distribution of aluminum coordination across the barrier relative to untreated junctions. This new approach is expected to be widely applicable to a broad range of devices that rely on amorphous aluminum oxide, as well as the many other metal-insulator-metal structures used in modern electronics.