Very high-current-density Nb/AlN/Nb tunnel junctions for low-noise submillimeter mixers

We have fabricated and tested submillimeter-wave superconductor-insulator-superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (J(c) approximate to 30 kA cm(-2)). The junctions have low-resistance-area products (R(N)A approximate to 5.6 Ohm mu m(2)), good subgap-to-normal resistance ratios R-sg/R-N approximate to 10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that omega RNC = 1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of T-RX = 110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits. (C) 2000 American Institute of Physics. [S0003-6951(00)04915-9].