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Applied Physics Letters

Specific capacitance of Nb/AlOx/Nb Josephson junctions with critical current densities in the range of 0.1–18 kA/cm2

Masaaki Maezawa, Masahiro Aoyagi, Hiroshi Nakagawa, I. Kurosawa, S. Takada

Published Apr 17, 1995• 66 citations
Publisher

Abstract

The specific capacitance Cs of Nb/AlOx/Nb Josephson tunnel junction has been measured by means of the superconducting quantum interference device (SQUID) resonance technique. We have investigated the junctions with critical current densities Jc in the range of 0.1–18 kA/cm2 and found that 1/Cs linearly depends on a logarithm of Jc. This suggests that the barrier thickness is moderately uniform in the junction area and increases continuously during growth. The results also show that increasing Jc reduces time constants of the junction.

DOI
10.1063/1.113927

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The specific capacitance Cs of Nb/AlOx/Nb Josephson tunnel junction has been measured by means of the superconducting quantum interference device (SQUID) resonance technique. We have investigated the junctions with critical current densities Jc in the range of 0.1–18 kA/cm2 and found that 1/Cs linearly depends on a logarithm of Jc. This suggests that the barrier thickness is moderately uniform in the junction area and increases continuously during growth. The results also show that increasing Jc reduces time constants of the junction.
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Abstract—The parameters of the tunneling barrier of Nb/Al–AlOx/Nb Josephson junctions were estimated in a wide range of current densities using the Simmons method. It is shown that with an increase in resistivity RnS from 60 to 2100 Ω μm2 (a reduction in the density of the tunneling current of junction J from 3.5 to 0.1 kA/cm2), the height of the tunneling barrier increases from 0.85 to 1.18 eV and its width increases from 10.7 to 13.3 Å. The experimentally observed linear dependence of the tunneling barrier parameters on RnS made it possible to estimate the capacitance of junctions in the range of RnS 10–30 Ω μm2 required to create low-noise submillimeter-range receivers.
Nb/Al-AlOx/Nb tunnel junctions with controllable critical current density JC are fabricated using the standard selective Nb etching process. Tunnel barriers are formed in different oxygen exposure conditions (oxygen pressure P and oxidation time t), giving rise to JC ranging from 100 A/cm2 to above 2000 A/cm2. JC shows a familiar linear dependence on P × t in logarithmic scales. We calculate the energy levels of the phase- and flux-type qubits using the achievable junction parameters and show that the fabricated Nb/Al-AlOx/Nb tunnel junctions can be used conveniently for quantum computation applications in the future.
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