High quality Nb-based superconductor-insulator-superconductor (SIS) junctions
with Al oxide (AlOxβ) tunnel barriers grown from Al overlayers are widely
reported in the literature. However, the thin barriers required for high
critical current density (Jcβ) junctions exhibit defects that result in
significant subgap leakage current that is detrimental for many applications.
High quality, high-Jcβ junctions can be realized with AlNxβ barriers, but
control of Jcβ is more difficult than with AlOxβ. It is therefore of
interest to study the growth of thin AlOxβ barriers with the ultimate goal of
achieving high quality, high-Jcβ AlOxβ junctions. In this work, 100\%\
O2β and 2\%\ O2β in Ar gas mixtures are used both statically and
dynamically to grow AlOxβ tunnel barriers over a large range of oxygen
exposures. In situ ellipsometry is used for the first time to extensively
measure AlOxβ 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 Jcβ on oxygen exposure (E) in order to
further validate the experimental setup