Dynamic Versus Static Oxidation of Nb/Al-AlOx/Nb Trilayer

TL;DR

This study compares dynamic and static oxidation methods for Nb/Al-AlOx/Nb trilayer junctions, using in situ ellipsometry to analyze growth patterns and optimize conditions for high-quality, high-Jc AlOx barriers.

Contribution

It introduces in situ ellipsometry for real-time analysis of AlOx barrier growth and compares oxidation methods to improve junction quality.

Findings

Real-time ellipsometry reveals unexpected growth patterns.

Oxygen exposure directly influences critical current density (Jc).

Dynamic oxidation can produce high-quality AlOx barriers.

Abstract

High quality Nb-based superconductor-insulator-superconductor (SIS) junctions with Al oxide (AlO$_x$) tunnel barriers grown from Al overlayers are widely reported in the literature. However, the thin barriers required for high critical current density (J$_c$) junctions exhibit defects that result in significant subgap leakage current that is detrimental for many applications. High quality, high-J$_c$ junctions can be realized with AlN$_x$ barriers, but control of J$_c$ is more difficult than with AlO$_x$. It is therefore of interest to study the growth of thin AlO$_x$ barriers with the ultimate goal of achieving high quality, high-J$_c$ AlO$_x$ junctions. In this work, 100\%\ O$_2$ and 2\%\ O$_2$ in Ar gas mixtures are used both statically and dynamically to grow AlO$_x$ tunnel barriers over a large range of oxygen exposures. In situ ellipsometry is used for the first time to extensively measure AlO$_x$ tunnel barrier growth in real time, revealing a number of unexpected patterns. Finally, a set of test junction wafers was fabricated that exhibited the well-known dependence of J$_c$ on oxygen exposure (E) in order to further validate the experimental setup.