Origin of hydrogen-inclusion-induced critical current deviation in Nb/AlOx/Al/Nb Josephson junctions

Abstract

We investigated the mechanisms that change the critical current density (Jc) of Nb/AlOx/Al/Nb Josephson junctions due to the inclusion of hydrogen in the Nb electrodes. Our investigations were performed according to three aspects: the superconductivity change, the change in thickness of the barrier layer, and the change in the barrier height due to the electronic effect. The results are as follows: (a) the hydrogen-inclusion-accompanied changes in the superconductivity parameters, such as the junction gap voltage, were much less than those of the critical current density, (b) the effect of hydrogen inclusion on Jc varied depending on the electrodes, i.e., the upper electrode above the barrier layer was the most affected, (c) the junctions with increased Ics due to hydrogen exclusion showed the identical amount of decrease in the junction resistance measured at room temperature, and (d) the hydrogen exclusion from the junction electrodes had no influence on the Nb/Al/AlOx/Al/Nb junctions, which had an extra Al layer. Based on these results we conclude that the Jc change is mainly caused by the change in junction resistance. A one order of magnitude smaller effect is caused by the superconductivity change. We believe the Jc change is caused by a Nb work function increase due to the hydrogen inclusion, resulting in an increase in barrier height.

Diffusion Stop-Layers for Superconducting Integrated Circuits and Qubits With Nb-Based Josephson Junctions

“…[12]- [14]. Therefore, the presence of hydrogen in Nb base electrode has no effect on the tunnel barrier height and hence has much less effect on the critical current density of Josephson junctions than hydrogen dissolved in Nb counter electrode, as was explained in our work [4] (see also [6] and our work [15] in the present issue).…”

“…Most importantly, it increases the work function of hydrogen-contaminated niobium, Nb(H), with respect to the clean Nb [10], [11]. As a result, the average height of the tunnel barrier in Nb/Al/AlO x /Nb junctions and the barrier asymmetry become dependent on the c H in the CE near the tunnel barrier, as was proposed in our work [4] (see also [6]). As was also emphasized in our previous works, Nb base electrode in real Josephson junctions is coated by a thin Al layer which is only partially consumed by the oxidation forming AlO x tunnel barrier.…”

“…The best results were obtained with aluminum DSLs which completely eliminated effects of BE connection to M0 layer studied in [1]- [3] and effects of CE to Ti/Au contact pad connections studied in [4]- [6], [15]. With molybdenum DSLs we found a substantial reduction in the value of the second effect but it was not completely eliminated, apparently due to a higher diffusivity and solubility of H in molybdenum than in aluminum.…”

“…Long-term changes in I c were first found in Nb circuits with Pd coating [5], [6] and were suggested to be caused by hydrogen absorption and desorption. It is well known that bulk Nb and Nb films can dissolve large amounts of hydrogen at room temperature, up to c H~ 50 atomic percent, where c H is the hydrogen content (H/Nb ratio).…”

“…The described above dependences of Josephson junction properties on the junction's environment and circuit patterns have been explained as resulting from hydrogen contamination of Nb circuit layers during wafer processing, with its subsequent migration towards or away from the AlO x tunnel barrier during the manufacturing cycle and later on upon its completion [3]- [6]. Long-term changes in I c were first found in Nb circuits with Pd coating [5], [6] and were suggested to be caused by hydrogen absorption and desorption.…”