Kavli Affiliate: David A. Muller
| First 5 Authors: Jaehong Choi, Jaehong Choi, , ,
| Summary:
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.
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