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We report on tunneling properties and interface structures for high-quality NbN/AlN/NbN tunnel junctions fabricated on ambient temperature MgO substrates. Junction quality and electrical parameters were systematically investigated in a very wide range for current density. The junctions show a very good junction quality with a high gap voltage, large I/sub c/R/sub N/ product, and large R/sub sg//R/sub N/ ratio as the current density varied from 100 A/cm/sup 2/ to above 100 kA/cm/sup 2/. The average barrier heights of the NbN/AlN/NbN tunnel junctions are calculated from the barrier thickness dependence of the critical current density. We found that the current density has two distinct types of dependency on the AlN barrier thickness, corresponding to two average barrier heights in different regions for the current density.