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

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.