Direct observation of the thickness distribution of ultra thin AlOx barrier in Al/AlOx/Al Josephson junctions

TL;DR

This study uses atomic-resolution STEM imaging to directly observe the local thickness distribution of ultra-thin AlOx barriers in Josephson junctions, revealing a Gaussian distribution and the effects of oxidation parameters on barrier thickness.

Contribution

It provides the first direct measurement of the local barrier thickness distribution in Al/AlOx/Al Josephson junctions using advanced electron microscopy techniques.

Findings

Less than 10% of the barrier area dominates tunneling.

Barrier thickness varies from ~1 nm to ~2 nm with a Gaussian distribution.

Oxidation time increases barrier thickness more than oxygen pressure.

Abstract

We show that less than 10% of the barrier area dominates the electron tunneling in state-of-art Al/AlOx/Al Josephson junctions. They have been studied by transmission electron microscopy, specifically using atomic resolution annular dark field (ADF) scanning transmission electron microscopy (STEM) imaging. The direct observation of the local barrier thickness shows a Gaussian distribution of the barrier thickness variation along the junction, from ~1 nm to ~2 nm in the three junctions we studied. We have investigated how the thickness distribution varies with oxygen pressure (po) and oxidation time (to) and we find, in agreement with resistance measurements on similar junctions, that an increased to gives a thicker barrier than an increased po.