Small-Area Niobium/Aluminum Oxide/Niobium Junctions for SIS Mixers

Abstract

Submillimeter-wave SIS receivers require junctions with small RC products. The RC product can only be reduced by decreasing the thickness of the insulating barrier; however, this is generally accompanied by an increased subgap leakage current which adversely affects mixer performance. Thus, it is advantageous to choose a junction fabrication technology which produces inherently high quality barriers and low subgap leakage currents. Junctions fabricated from Nb/Al-Oxide/Nb trilayers (Gurvitch et al. 1983; Lichtenberger et al. 1988) are particularly well known in this regard, and have shown good performance when used in mm-wave mixers (Inatani et al. 1987; Pan et al. 1987). However, junction areas must be reduced to ≤ 1 µm² in order to meet impedance-matching requirements at submm wavelengths. Here we report Nb/Al-Oxide/Nb junctions which are among the smallest fabricated to date (see also Imamura and Hasuo 1988).

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