Development of New
Josephson Junctions for Rapid Single Flux Quantum (RSFQ) Devices
at University of California at Berkeley
and Arizona State University
New Josephson device configurations are presently under intense investigation to find the optimum device for application in the Rapid Single Flux Quantum ( RSFQ) Josephson junction (JJ) logic family. Ideally, the Josephson junctions would have nonhysteretic current-voltage characteristics. (Fig. 1) Today's superconducting niobium integrated circuit (SC IC) fabrication technology is well established, reliable and reproducible. But it depends on Nb/AlOx/Nb tunnel junctions with strongly hysteretic current-voltage characteristics; the junctions must be shunted with a resistor to make the characteristic nonhysteretic. (Fig. 2) The added resistor takes considerable room on the chip and introduces parasitic inductances that degrade performance.
One type of junction under study in a number of laboratories modifies the tunnel
junction by replacing the AlOx barrier by an AlOx-Al-AlOx layered structure.
They are referred to as SINIS junctions and the current-voltage characteristic
is nonhysteretic. A second criterion for desirable junctions is that the product
of the critical current times the normal-state resistance IcRn be as high as possible, at least 0.5 mV for high-speed circuits. In most cases
the values being obtained are up to about 0.3 mV; work is continuing to raise
this value.
An alternate approach under study is to use a normal conductor for the barrier in a so-called SNS structure. Junctions of this kind have been made with a wide variety of materials. But usual normal conductors have too high a carrier density with the result that the IcRn products are very low. In a collaboration between the group of professor Nathan Newman of Arizona State University and that of Professor Ted Van Duzer of the University of California at Berkeley, NbN/TaN/NbN junctions on an MgO substrate with IcRn products over 1 mV were demonstrated. Most recently, this device has been modified and formed on a silicon substrate with the aim of making it a "drop-in" replacement for the present resistor-shunted tunnel junction for use in 4 K Nb integrated circuits. The structure is now Nb/NbTiN/TaN/NbYiN/Nb and is made on an oxidized silicon chip. Very preliminary results show an IcRn product over 2 mV. The next step is to determine if the properties are repeatable over a large number of junctions on a chip and from run-to-run. If good uniformity is achieved, demonstration RSFQ logic circuits will be made for operation at ultra-high speeds.