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We report the fabrication and low-frequency testing of the first edge-geometry NbN/MgO/NbN superconducting tunnel junctions. The use of an edge geometry allows very small junction areas to be obtained, while the all-NbN electrodes permit operation at 8–10 K with a potential maximum operating frequency above 1 THz. Edge definition in the base NbN film was accomplished utilizing Ar ion milling with an Al2O3 milling mask, followed by a lower energy ion cleaning step. This process has produced all-refractory-material tunnel junctions with areas as small as 0.1 μm2, resistance-area products less than 21 Ω μm2, and subgap to normal state resistance ratios larger than 18.
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Hunt et al. (1989) studied this question.
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