Improving Junction Quality and AlOx Thickness Uniformity Using a Two-Step Oxidation Method

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

A two-step oxidation process for fabricating Nb/Al–AlOx/Nb superconductor–insulator–superconductor (SIS) junctions with high critical current densities was developed to improve junction quality. The process begins with oxidation at a low oxygen pressure of 0.1 mTorr, followed by exposure to a higher oxygen pressure of approximately 10 mTorr. Junctions fabricated using this method exhibit excellent performance, with a critical current density (Jc) of 13.8 kA/cm² and a quality factor (Q) of 20. Notably, even at a Jc as high as 20.5 kA/cm², the junctions maintain a Q of 12, indicating the robustness of the process at elevated current densities.

Cross-sectional STEM imaging reveals relatively uniform AlOx barrier layers with a thickness of ~0.8 nm and low thickness variation in two-step junctions. The (111)-oriented Al film is found to align with the (110) orientation of the underlying Nb layer, suggesting that the crystallographic orientation of Nb significantly influences the growth direction and texture of the Al layer.

Strain measurements from X-ray diffraction show that ~200 nm-thick Nb bottom electrodes exhibit strain values ranging from 0.71% to 1.67% in high-Jc junctions (Jc > 12 kA/cm²). In 1-step junctions, the highest Q of 11 was observed for a Nb film with a strain of 1.09%, accompanied by a Jc of 13.95 kA/cm²—demonstrating a strong dependence of Q on Nb film strain. In contrast, 2-step junctions with Nb strain values between 0.71% and 1.04% consistently showed higher Q values of 20, with Jc values around 13.7 kA/cm². These findings suggest that the two-step oxidation process can accommodate a broader range of Nb strain, thereby influencing the initial oxidation behavior and enhancing the uniformity and quality of AlOx barrier formation.

Keywords: SIS junction, High critical current density, AlOx tunnel barriers, High junction quality, AlOx thicknesses uniformity