Controlling the thickness of Josephson tunnel barriers with atomic layer deposition

TL;DR

This study investigates the use of Atomic Layer Deposition to precisely control the thickness of Josephson tunnel barriers, revealing challenges with interfacial layer formation that impact qubit applications.

Contribution

It provides experimental and modeling insights into ALD-grown Josephson barriers, highlighting the potential and limitations of using ALD for ultrathin, uniform barriers in quantum devices.

Findings

ALD can produce ultrathin, controllable tunnel barriers.

An interfacial layer up to 2 nm forms, affecting barrier quality.

Aluminum may not be ideal as a wetting layer due to IL formation.

Abstract

Atomic Layer Deposition (ALD) is a promising technique for producing Josephson junctions (JJs) with lower defect densities for qubit applications. A key problem with using ALD for JJs is the interfacial layer (IL) that develops underneath the tunnel barrier. An IL up to 2 nm forms between ALD Al2O3 and Al. However, the IL thickness is unknown for ALD films less 1 nm. In this work, Nb-Al-ALD-Al2O3-Nb trilayers with tunnel barriers from 0.6 - 1.6 nm were grown in situ. Nb-Al-AlOx-Nb JJs with thermally oxidized tunnel barrier were produced for reference. RN was obtained using a four-point method at 300 K. JC, and its dependence on barrier thickness, was calculated from the Ambegaokar-Baratoff formula. The Al surface was modeled using ab initio molecular dynamics to study the nucleation of Al2O3 on Al. Current voltage characteristics were taken at 4 K to corroborate the room temperature measurements. Together, these results suggest that ALD may be used to grow an ultrathin, uniform tunnel barrier with controllable tunnel resistance and JC, but a thin IL develops during the nucleation stage of ALD growth that may disqualify Al as a suitable wetting layer for ALD JJ based qubits.