Amorphous materials are widely used in many technologically important devices and structures. In Josephson junctions, amorphous aluminium oxide film with a thickness of a few nanometers is used as tunnel barrier . The atomic structure of this thin amorphous oxide film determines the tunnelling properties of the junction. However, the disordered nature, the miniaturized dimension and the close proximity to the adjacent crystalline contacts of the oxide barrier are challenging aspects of the direct investigation of the atomic structure of the tunnel barrier layer.
In this work, we have unveiled the microscopic structure of the ultrathin (<2 nm) aluminium oxide barrier in Al/AlOx/Al Josephson junctions via transmission electron microscopy. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were used to reveal the detailed atomic structure and the chemical bonding in the junction. By combining nano-beam electron diffraction (NBED) and pair distribution function (PDF) analysis with reverse Monte Carlo (RMC) refinement  we unravelled the atomic structure of the nanosized AlOx barrier. The technique of combining NBED-PDF and RMC enabled us to treat the crystalline Al contacts and the amorphous AlOx barrier layer as integrated parts in the system and take the interfacial interaction between Al and AlOx into account while retrieving the atomic structure information.
The atomic structure of the AlOx tunnel barrier layer resembles the structure in bulk Al2O3 in terms of bond length distribution, bond angle distribution, and connectivity of the elemental structure units. However, the average coordination number of the Al atoms in the barrier is around 3.4, which is lower than that in other amorphous aluminium oxide systems . The combined NBED and RMC simulation showed that the miniaturized dimension of the oxide barrier and the presence of the interfacial interaction between crystalline Al and amorphous AlOx give rise to an oxygen deficiency at the metal/oxide interfaces in the junction. This is of importance since atomic defects such as oxygen vacancies at the interfaces are possible origins of the two-level systems that contribute to the decoherence and noise in superconducting quantum circuits based on Josephson junctions.
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To cite this abstract:Lunjie Zeng, Dung Trung Tran, Cheuk-Wai Tai, Gunnar Svensson, Eva Olsson; Atomic structure of the ultrathin amorphous aluminium oxide barrier in Al/AlOx/Al Josephson junctions. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/atomic-structure-of-the-ultrathin-amorphous-aluminium-oxide-barrier-in-alaloxal-josephson-junctions/. Accessed: April 3, 2020
EMC Abstracts - https://emc-proceedings.com/abstract/atomic-structure-of-the-ultrathin-amorphous-aluminium-oxide-barrier-in-alaloxal-josephson-junctions/