Direct observation of the thickness distribution of AlOx barriers in high quality Nb/Al-AlOx/Nb Josephson junctions

Hsiao-Wen Chang^1*, Tse-Jun Chen¹, Yen-Pin Chang¹, Chuang-Pin Chiu¹, Ming-Jye Wang¹

¹Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan

* Presenter:Hsiao-Wen Chang, email:hwchang@asiaa.sinica.edu.tw

Aluminum oxide (AlOx) tunnel barrier is crucial in a Nb/AlOx-Al/Nb Josephson junction. The tunneling characteristics of electrons through the barrier are primarily determined by the quality and structural properties of the AlOx insulating layer [1]. A two-step oxidation process for fabricating Nb/Al-AlOx/Nb SIS junctions with high critical current density was developed to improve the junction quality, denoted as Q = R(2mV)/Rn, where R(2mV) is the sub-gap resistance at 2 mV in I-V curve and Rn is the normal state resistance of the junction. The oxidation process begins with a low oxygen pressure of 0.1 mtorr, followed by a high oxygen pressure of approximately 10 mtorr. Junctions with a critical current density of 13.8 kA/cm² exhibit a quality factor of 20. Furthermore, junctions with a critical current density as high as 20.5 kA/cm² still show a quality factor of 12.

The AlOx thickness of the two-step junctions and regular one-step junctions was directly measured from their atomic resolution STEM images. The thickness variation in the two-step oxidation process is with a standard deviation of 0.12 nm which significantly smaller than in the one-step oxidation process, ~ 0.23 nm. This suggests that, for the two-step oxidation process, the low-pressure oxidation at the first step could improve barrier uniformity and junction quality.

Nano-pits were observed in the one-step junctions but not in the two-step junctions, resulting in a wider AlOx thickness distribution in the one-step junctions. Further analysis of STEM images shows that aluminum grows preferentially along the [111] direction on the Nb (110) plane and suggests that the orientation of Nb film can significantly influence the surface step of Al (111)-plane which is strongly correlated to the growth of the very beginning amorphous aluminum oxide.

[1] V. Somjit and B. Yildiz, “Atomic and Electronic Structure of the Al2O3/Al Interface during Oxide Propagation Probed by Ab Initio Grand Canonical Monte Carlo,” ACS Appl. Mater. Interfaces 14, pp 42613−27, 2022.

Keywords: Nb/AlOx/Nb Josephson junctions, Ultra-low oxygen pressure exposure, High current density (Jc), High junction quality (Q), AlOx barrier thicknesses uniformity