Low barrier ZrO$_x$-based Josephson junctions

TL;DR

This paper addresses oxygen-diffusion and TLS loss concerns in Nb-based Josephson junctions with AlO_x barriers by proposing ZrO_2 as a low-barrier, oxygen-retentive tunnel oxide. The authors combine first-principles screening with a scalable top-down Nb/ZrO_x/Nb fabrication process and comprehensive structural and electronic characterization (STEM/EELS, XPS, and transport) to validate the barrier and its interfaces. Room-temperature measurements yield a relatively low barrier height (~$0.3$ eV) and a barrier width around 2.5 nm, while low-temperature data reveal sizable subgap resistance and thickness-dependent critical current. Overall, ZrO_2 barriers appear crystalline, chemically abrupt, CMOS-compatible, and scalable, offering potential advantages for merged-element transmons and large-scale superconducting electronics.

Abstract

The Josephson junction is a crucial element in superconducting devices, and niobium is a promising candidate for the superconducting material due to its large energy gap relative to aluminum. AlO$_x$ has long been regarded as the highest quality oxide tunnel barrier and is often used in niobium-based junctions. Here we propose ZrO$_x$ as an alternative tunnel barrier material for Nb electrodes. We theoretically estimate that zirconium oxide has excellent oxygen retention properties and experimentally verify that there is no significant oxygen diffusion leading to NbO$_x$ formation in the adjacent Nb electrode. We develop a top-down, subtractive fabrication process for Nb/Zr-ZrO$_x$/Nb Josephson junctions, which enables scalability and large-scale production of superconducting electronics. Using cross sectional scanning transmission electron microscopy, we experimentally find that depending on the Zr thickness, ZrO$_x$ tunnel barriers can be fully crystalline with chemically abrupt interfaces with niobium. Further analysis using electron energy loss spectroscopy reveals that ZrO$_x$ corresponds to tetragonal ZrO$_2$. Room temperature characterization of fabricated junctions using Simmons' model shows that ZrO$_2$ exhibits a low tunnel barrier height, which is promising in merged-element transmon applications. Low temperature transport measurements reveal sub-gap structure, while the low-voltage sub-gap resistance remains in the megaohm range.

Figures (9)

• Figure 1: (a) HAADF-STEM image of the Nb/ZrO$_x$-Zr/Nb films with a nominal Zr thickness of 5 nm. Magnified views of (b) the top region, showing Pt/NbO$_x$/Nb where a thin NbO$_x$ layer forms on the top Nb electrode, and (c) the top Nb/Zr–ZrO$_x$/bottom Nb region where no NbO$_x$ is observed between the top Nb electrode and the ZrO$_x$ tunnel barrier.(d) Relative atomic percentages of Nb, NbO$_x$, Zr, and ZrO$_x$ in the quadri-layer based on XPS analysis. No NbO$_x$ signal was detected. (e) Nb 3d core level XPS scan of the top Nb electrode region (pre-etch) and (f) the Zr 3d core level XPS scan of the Zr–ZrO$_x$ barrier region at the point of maximum Zr signal (after 540s of etching). The inset shows the XPS signal from Nb, indicating the absence of oxide.
• Figure 2: (a) Comparison of the EELS signal to the spectra adopted from McComb McComb1996. Colors correspond to different crystal symmetries. Here, the tetragonal stabilized zirconia shows the best match to the ZrO$_2$, used in this experiment. (b) High-resolution HAADF STEM image of the tunneling barrier that was formed from 1.5 nm of Zr prior to oxidation. Afterward, its width thickened to 2.1 nm. The top and bottom Nb electrodes are oriented at [100] zone axis. Zr atoms in this image are then overlaid with the Zr atoms in [100] ZrO$_2$ orientation. For reference, Nb atoms in electrodes are overlaid with its associated structure. The size of the scalebar in the left bottom of the image is 1 nm.
• Figure 3: JJ fabrication process and SEM/STEM images of the junction (a) Nb/Zr (5 nm)/O$_2$ 3700 Torr s/Nb quadri-layer sputtering with in-situ oxidation. (b) We define the bottom electrode by performing ICP-RIE etching using Cl$_2$,BCl$_3$,and Ar plasma to partially etch the trilayer. (c) A second ICP-RIE etching to define the cylindrical tunnel barrier. (d) SiO$_2$ spacer evaporation right after tunnel barrier etching without resist removal. (e) After lift-off of the SiO$_2$ spacer, BOE is used to etch the SiO$_2$ spacer layer. (f) The top Nb electrode is sputtered after resist removal. (g) We remove the SiO$_2$ spacer using vapor HF etching. (h) False-colored SEM image of the resultant junction. The pink region indicates the circular junction. (i) STEM cross-section edge image of the junction before SiO$_2$ scaffold removal.
• Figure 4: Room temperature I-V characteristics and tunneling parameters of junctions grown with 5 nm of Zr. (a) RA products for junctions with varying diameters, ranging from 2 $\mu$m to 9 $\mu$m. (b) Current-voltage curves from junctions with different diameters. (c) Energy diagram of of a metal-insulator-metal tunnel junction. The shaded area indicates filled electron state up to fermi-level, and when a small voltage is applied, the tunnel barrier is tilted and allows electrons to tunnel through the barrier and induces non-linear conduction. $\phi$ is tunnel barrier height and $x$ is tunnel barrier width. Extracted (d) tunnel barrier height and (e) width using Simmons' model. (f) ADF-STEM image of the Nb/Zr-ZrO$_2$/Nb trilayer. (g) STEM-EELS segmentation study of the trilayer. It reveals that 2.5 nm of 5 nm Zr was oxidized, which is consistent with the Simmons' model estimation of the tunnel barrier width.
• Figure 5: (a) Simmons' model extracted barrier height and (b) width of the junctions with varying Zr thicknesses and diameters.