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Applied Physics Letters

Fabrication of ultrasmall Nb-AlOx-Nb Josephson tunnel junctions

John M. Martinis, Ronald H. Ono

Published Aug 6, 1990• 34 citations
Publisher

Abstract

We describe a fabrication process to make Nb-AlOx-Nb edge junctions with areas as small as 0.0022 μm2 and with current densities from 10 to 24 000 A/cm2. The junction conductance was low for voltages below the superconducting energy gap which indicates good quality tunnel barriers. Coulomb gap measurements obtained when the junctions were in the normal state were used to find the junction capacitance. Junction capacitance as small as 0.18 fF has been measured.

DOI
10.1063/1.103618

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For high-quality superconducting tunnel junctions (STJs), it is necessary to reduce leakage current as much as possible. We describe the fabrication of niobium STJs using the selective niobium (Nb) etching process and various ways to minimize the leakage current. The experiment shows that the leakage current mainly comes from shorts in the tunnel barrier layer rather than those around the junction edges. Through systematic analysis of the thin film stress, surface morphology and modified junction structures, we fabricate high-quality Nb STJs with a gap voltage of 2.8 mV and a leakage current at 1 mV as low as 8.1% and 0.023% at 4.2 K and 0.3 K, respectively.
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