Ion Implantation into Nb/NbO/PbAuIn Josephson Tunnel Junctions
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ION IMPLANTATION JUNCTIONS
INTO Nb/NbO/PbAuIn
JOSEPHSON
TUNNEL
G. J. CLARK and S. I. RAIDER IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598 ABSTRACT Boron ions were implanted into completed planar and edge Nb/Nb oxide/PbAuln Josephson tunnel junctions to directly trim the Josephson pair currents, Io. The implantation caused an increase in Io and in the junction subgap conductance and a decrease in the junction energy gap. For a fixed junction fabrication procedure, the variations were observed to be monotonic with IB implant dose. Optimum trimming was found when the 'IBions were implanted such that the peak in the depth distribution occurred at the tunnel barrier. The implantation caused an increase in Io and in the junction subgap conductance and a decrease in the junction energy gap. The implanted junctions are stable at 80°C and under storage.
INTRODUCTION The Josephson junction, a device based on the physical phenomena of superconducting and electron tunneling offers significant potential for application in computer technology [1]. If Josephson tunnel junctions are to be used in digital integrated circuit applications, then the Josephson current, I, must be controlled within tight margins. Despite a strong dependence of Io on tunnel barrier processing parameters when fabricating Nb/Nb oxide/PbAuln tunnel junctions, good 1. control has been obtained. Only small additional adjustments in Io are generally necessary to meet design requirements. This paper addresses the possibility of providing this fine trim through the use of ion implantation. The current density, J1 , in a Josephson tunnel junction is the Josephson current, I., normalized to the junction area, A
Je.,
J, = I0 /A
•
(1)
In a Josephson tunnel junction, J, is linearly dependent on the superconducting energy gap, Eg, and is exponentially dependent on -d x (0p x m*)/, where d is the tunnel average barrier thickness, , is the average barrier height, and m* is the effective mass of the tunneling electron ie. ,
J1 = kE 1-d(4O x m*1/2
(2)
An Io trim procedure is required to alter one or more of these parameters in a reproducible, controllable manner without significantly affecting other device properties. Attempts had previously been made to trim Io by post-processing Josephson junctions by both thermal annealing [21 and by electron beam irradiation [3]. Junctions formed with Nb/Nb oxide/PbAuIn structures are more thermally stable than either Nb/Nb oxide/Nb or Pb alloy/oxide/Pb alloy junctions. Although annealing in N2 at 210'C causes the junction current densities to change by about 20% after 1 hour and by 50% after 18 hours with little change in junction subgap conductance, the low melting point Pb-alloy counterelectrode makes this an unattractive 10 trim procedure. Electron beam irradiation of Nb/oxide/Nb and Pb-alloy/oxide/Pb-alloy junctions cause current densities to increase 20 to 30% but irradiation of Nb/oxide/Pb-alloy junctions do not change more than 2 to 3% at e-beam currents of 0.1 mA and voltages to 30 keV. A
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