We report on further studies of the effects of hydrogenation of sputtered amorphous silicon barriers upon the current-voltage (I-Y) characteristics of Nb-Nb Josephson tunnel junctions. For composite trilayer barriers (a-Si/a-Si:H/a-Si) which are deposited using 8 mT of Ar, we find that there is an abrupt improvement in device chararteristics when the central hydrogenated layer is deposited using a hydrogen partial pressure which exceeds about 0.5 mT. We attribute this to the reduction in the density of localized states in the a-Si:H layer. We have observed excellent I-Y characterisitics with trilayer barrier devices whose central hydrogenated layer is only about 1/7 of the thickness of the entire barrier. This observation suggests that localized states near the geometric center of the barrier are the most significant in degrading device characteristics. Annealing experiments and published data on the diffusion of deuterium in a-Si suggest that the composite barriers will be extremely stable during processing and storage. Zero bias anomalies in device I-Y characteristics and spin density in the a-Si and a-Si:H layers have been measured. Suggestions for future experiments are made.

We have compared the tunneling characteristics of Nb-Si-Nb junctions whose amorphous silicon barriers have been sputtered in pure Ar with those sputtered Ar-H2plasmas as well as in various combinations. We observe lower subgap currents with composite barriers which comprise a central region which is hydrogenated but which is sandwiched between two thin unhydrogenated layers. The improved tunneling characteristics may be associated with the lower density of localized states in the hydrogenated silicon.

Substantial evidence has been accumulated during the course of this program which indicates that the localized states which are present in nonhydrogenated sputtered amorphous silicon barriers contribute to the larger current density observed at biases below the superconducting energy gap of the electrodes. The basic observations and the method of measuring the density of localized states at the Fermi level of the barrier materials are presented in the Appendix which is a reprint of a paper presented at the 1982 Superconductivity Conference. More recent observations and theoretical speculations are included in this report.

: Substantial evidence has been accumulated during the course of this program which indicates that the localized states which are present in nonhydrogenated sputtered amorphous silicon barriers contribute to the larger current density observed at biases below the superconducting energy gap of the electrodes. The basic observations and the method of measuring the density of localized states at the Fermi level of the barrier materials are presented in the Appendix which is a reprint of a paper presented at the 1982 Superconductivity Conference (1). More recent observations and theoretical speculations are included in this report.

We have developed an IC-compatible process for fabricating Josephson tunnel junctions, which uses dc magnetron-sputtered Nb films as both base and counterelectrodes, and rf-sputtered amorphous silicon as the tunneling barrier. Optical reflectivity measurements have been used to study the silicon barrier, and to allow precise determination of the barrier thickness. The Josephson current density varies exponentially -- over several orders of magnitude -- with the barrier thickness. The product of the critical Current and subgap resistance Vmis constant over this wide range of current density. The specific capacitance of these Junctions is ∼ 2.5 μf/cm2at a current density of a few hundred A/cm2. This is lower than the value for lead-alloy Junctions, ∼ 4.3 μf/cm2[1], and is consistent with the measured thickness and dielectric constant of the a-Si barrier. The performance of logic gates fabricated with these devices will be presented elsewhere in this conference.

A novel process for fabricating refractory superconducting tunnel junctions is described. In this method the entire superconductor-barrier-superconductor sandwich is formed before the patterning of any layer. Isolated Josephson junctions are then formed by anodizing through the upper electrode, while the devices themselves are protected by a photoresist or other insulator masks. Using this process, Nb-Si-Nb junctions have been fabricated whose critical current density varies by less than 50% over a 5 cm diam wafer. In situ measurements of Si barrier thickness by optical reflectometry indicate that device resistance varies exponentially with barrier thickness.