Browse Prior Art Database

Minimization of Excess Sub Gap Currents in Josephson Junctions

IP.com Disclosure Number: IPCOM000052819D
Original Publication Date: 1981-Jul-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 3 page(s) / 27K

Publishing Venue

IBM

Related People

Drake, RE: AUTHOR [+3]

Abstract

Josephson junctions formed with Nb base electrodes often possess significant excess sub-gap currents, I(excess), above the theoretical quasi-particle current level. The causes of these excess currents are unknown.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 53% of the total text.

Page 1 of 3

Minimization of Excess Sub Gap Currents in Josephson Junctions

Josephson junctions formed with Nb base electrodes often possess significant excess sub-gap currents, I(excess), above the theoretical quasi- particle current level. The causes of these excess currents are unknown.

Integrated circuit fabrication of Nb-Nb oxide Josephson junctions consists of an initial deposition of a Nb base electrode. The surface of this Nb film is subsequently exposed to air, organics, and other contaminants prior to tunnel barrier formation and counterelectrode deposition. As a consequence of this exposure, a native oxide forms and impurities are dissolved at the Nb surface. The impurity-sensitive Nb surface at which the tunnel barrier is to be formed is degraded.

Minimization of excess sub-gap currents requires proper preparation of the Nb surface. The critical step necessary to minimize I(excess) involves the removal of the Nb native oxide and the additional removal of several layers of the Nb metal surface. The amount of Nb that is necessarily removed to minimize I(excess) is determined by the extent to which contaminant diffusion at the Nb surface occurs.

This surface preparation can, e.g., be accomplished by RF plasma etching. It has been found that such etching forms a NbO/NbC layer on the Nb surface. The change in I(excess) with surface RF plasma etching is a function of the RF plasma etching time, and is shown schematically below. Once I(excess) reaches a minimum, additional plasma etching under the same conditions does not significantly increase the sub-gap currents. Typical conditions for RF plasma cleaning a Nb surface at low Ar pressures (used to minimize contamination by backscattering) are 1.1 kV RF voltage, 3 mtorr Ar pressure, and 20-minute plasma etch time. Using these conditions, about 50-100 angstrom of the Nb metal surface are typically removed. Higher RF plasma etch volta...