Nb-based superconducting tunnel junctions as submillimeter-wave direct detectors

Employing an anodization method into our fabrication process, we have successfully fabricated STJs with size of 4 (mu)m in diameter, Josephson critical current density of 1000 A/cm/sup 2/, low leakage current of less than 10 pA at 0.3 K and high fabrication yield of more than 90 percent. These STJs were applied to fabricate submillimeter-wave direct detectors using 12 linearly distributed junctions, each circular junction having a current density of 600 A/cm/sup 2/ and size of 4 (mu)m in diameter. Direct detection of submillimeter-waves was measured using a Fourier transform spectrometer, which shows a maximum sensitivity at 440 GHz and a fractional bandwidth of 10 percent.