Optimization of step-edge substrates for high-TC superconducting devices

To pursue step-edge for high-TC superconducting grain boundary junctions or SQUIDs with high reproducibility and quality, we have developed two-step procedures of fabricating very good step-edge substrates. A protocol of precisely controlling step angles for Josephson junctions has been established, with which we can predict the step angles as well as get a better control of the fabrication process. The procedures can improve the step ramp quality substantially. There are no needles, waves, trenches, cascade, or other flaws on these surfaces. The step substrates present good uniformity with respect to any step angle. We have characterized high-TC step-edge dc SQUIDs connected in series. The I-V curves of SQUID arrays show the RSJ behavior. The enhanced modulation amplitude of 110 μV is achieved at 77 K with the step-edge dc SQUID in series. The results clearly show the high uniformity and quality of the fabricated step-edge Josephson junctions.