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The increase of the normal-state resistance of Nb-Al/AlOx-Nb tunnel junctions by annealing at elevated temperatures is reported. Junctions with an area of 4 μm2 or smaller have been investigated. The resistance could be increased up to a factor of 5 with only a small influence on the quality of the quasiparticle characteristic. The effect can be used to adjust the resistance after fabrication. The increase of the AlOx barrier thickness has been estimated. Annealing experiments in nitrogen atmosphere and after nitridation have been carried out in order to find out from where the additional oxygene stems. The annealing properties of anodization curves have been investigated. A clear increase of the oxide peak could be observed. The results also show that the resistance is sensitive to temperature increases in the various fabrication steps.
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Lehnert et al. (1992) studied this question.
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