Oral Presentation
Novel Fabrication Process for High Quality and High Jc Nb/Al–AlOx/Nb Josephson Junctions
Presenter: Yen-Pin Chang (ASIAA)
The critical current density, Jc, of Nb/AlOx-Al/Nb junctions is mainly controlled by the oxygen exposure (EO2= PO2 ·t) during aluminum (Al) oxidation process, where PO2 is the oxygen partial pressure and t is the oxidation time. Empirical results reported a universal Jc-EO2 relation of Jc~EO2^-α with an exponent α value of 0.5 in the low Jc regime and 1.6 in the high Jc regime [1]. It is well known that the junction quality of SIS junction becomes worse as its critical current density gets higher, i.e. the thickness of the AlOx barrier layer is thinner. The defects in the tunnel barrier might become an important issue.
We have developed a novel 2-step Al oxidation process, instead of 1-step oxidation exposure used in the regular oxidation process, for fabricating Nb/AlOx-Al/Nb junctions with high critical current density (>10 kA/cm^2). Our 2-step oxidation process consists of an initial Al oxidation with a low oxygen partial pressure (~0.1mTorr) for 60 minutes and a follow-up oxidation process with a higher oxygen partial pressure (~12 mTorr) for a designed time. The Nb/AlOx-Al/Nb with 2-step oxidation process show a much smaller exponent α value, 0.32 instead of 1.6. The junction quality (Rg/Rn) is much enhanced, where Rg is the sub-gap resistance at 2 mV and Rn is the normal state resistance. Our 2-step oxidation process provides a possible solution of fabricating high quality Nb/AlOx-Al/Nb junctions with a Jc higher than 10 kA/cm^2. Furthermore, Cai et al. reported that the oxide layer (AlOx) grows fast at the beginning of oxidation process but reaches a saturated thickness gradually [2]. The saturated thickness of AlOx depends on the oxygen pressure only. Their result might provide a clue to prove the batch-to-batch reproducibility.
[1] S. K. Tolpygo et al., "Properties of Unshunted and Resistively Shunted Nb/AlOx-Al/Nb Josephson Junctions With Critical Current Densities from 0.1 mA/μm2 to 1 mA/μm2," IEEE Trans. Appl. Supercond., vol. 27, Issue 4, pp. 26-30 (2017).
[2] Na Cai and Guangwen Zhou, "Effect of oxygen gas pressure on the kinetics of alumina film growth during the oxidation of Al(111) at room temperature," Phys. Rev. B, 125445 (2011).
asiaa.sinica.edu.tw