High-T_cJosephson junctions fabricated by focused ion beam direct milling

Abstract

High-T c Josephson junctions were successfully fabricated by focused ion beam (FIB) direct milling. The characteristics of the junction barrier were carefully controlled by tuning the thickness of the link region. The optimal remaining thickness for the YBCO is about 70-80 nm even though the YBCO thicknesses are different. The temperature-dependence of the critical current provides good evidence of the superconductor-normal-superconductor weak link of the junctions. Although the flux-flow behavior increasingly dominates at lower temperatures, the voltage-current shows resistively shunted junction-like characteristics at temperatures close to T c . The Shapiro steps of a single junction irradiated with microwaves were measured, revealing strong Josephson effects in ion-damaged junctions. This method will be of great benefit to nanoSQUIDs for the detection of spin systems.

Investigation of the Effects of Oxygen Content in YBa2Cu3Ox on the Depth and Profile of Direct Ion Milled Trenches

“…Three-dimensional images of the first and last trenches are shown in Figure 1 (b) and (c). The wedge shaped profile of the p ≈ 0.121 trench is very similar to that obtained by [8] on an optimally doped (p ≈ 0.167) film. The next trench, with p ≈ 0.122, also exhibits a wedge shape and a nearly identical depth (≈ 300 nm), however, the width of the trench has been reduced by ≈ 1/3.…”

“…Using this process would potentially result in a low cost method for producing devices having a higher density of Josephson junctions than by direct ion milling of an optimally doped film. Use of a gold over layer in the milling process will result in a narrowing of the milled section down to the order of 20 nm -50 nm, as demonstrated in [8]. Combined with the apparent ion beam confining effects of the well ordered Cu-O chain states (p = 1/8), arrays of Josephson junctions a few tens of coherence lengths apart (ξ ∼ 10Å) could be fabricated resulting in a density approaching 10 8 /cm 2 .…”

“…Recently Wu et al [8] demonstrated that HTS Josephson junctions can reliably be fabricated by focused ion beam (FIB) milling on an optimally doped YBa 2 Cu 3 O 7−δ film (7 − δ ≈ 6.91). A gold over layer is deposited with a thickness just less than the depth to which the impacting gallium ions can penetrate.…”

“…Having a large quantity of densely packed Josephson junctions on a YBa 2 Cu 3 O ∇x film to precisely map out electronic structure and transport properties is a goal of our project. If we are to make use of the direct ion milling method of [8] to produce the desired array of Josephson junctions along the oxygen gradient, then we need to understand and control the milling process to produce uniform junctions. We find preliminary evidence here that oxygen ordering within the YBa 2 Cu 3 O 7−δ lattice significantly impacts the depth and profile of trenches produced by the ion milling process.…”

Preparation of novel HTS films and tunnel junctions for advanced C3I sensor applications

“…Recently Wu et al, 6 using an optimally doped YBa 2 Cu 3 O 7−δ film (7 − δ ≈ 6.91), demonstrated that focused ion beam (FIB) milling can be used as a reliable method to produce HTS Josephson junctions. A gold top layer is deposited on the film having a thickness just less than the depth to which impacting gallium ions can penetrate.…”