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High quality Ta/PbBi tunnel junctions for 85-110 GHz SIS mixer experiments

By: Raisanen, A.; Prober, D.; Track, E.; Face, D.; Cui, G.-J.; Richards, P.; Crete, D.;

1987 / IEEE

Description

This item was taken from the IEEE Periodical ' High quality Ta/PbBi tunnel junctions for 85-110 GHz SIS mixer experiments ' We report on the fabrication of small area (1-6 �m2), high critical current density (103-104A/cm2) Ta/Ta oxide/ Pb0.9Bi0.1SIS tunnel junctions and six junction series arrays designed for use as broadband SIS quasiparticle mixers as 85-110 GHz. These junctions have small subgap leakage currents (1 to 5%), and a ""sharp"" current rise of width �V <30-50 �V at the sum gap voltage. An RF filter is fabricated on the substrate along with the junctions. The impedance of this filter combined with the junction capacitance is designed to provide a broadband RF match to the mixer over the entire frequency range. A sliding backshort is the only mechanical tuning element. The single junctions as well as arrays are fabricated with a window geometry on fused quartz substrates. The fused quartz substrates require the deposition of a thin amorphous Ge layer which is conducting as room temperature to avoid charging effects during the ion-beam processing steps. Preliminary measurements at 85- 100 GHz show a relatively low gain (-6.9 dB) compared to the performance expected for junctions of this quality. Model calculations suggest that the low gain arises from errors in the implementation of the RF filter design; this can be improved in future work.