Interface and tunneling barrier heights of NbN/AlN/NbN tunnel junctions

The tunneling barrier height of NbN/AlN/NbN tunnel junctions was measured by investigating the barrier thickness dependence of the current density, and the junction interface was studied by cross-sectional transmission electron microscopy (TEM). We found that the current density of the junctions has two distinct types of dependency on the AlN barrier thickness, corresponding to two average barrier heights in different regions for the current density. The TEM observations showed that the junctions had a very smooth and clear electrode–barrier interface, and the crystal structures of the counterelectrode NbN films were strongly dependent on the thickness of AlN barriers. The average barrier height was estimated to be 2.35 eV in the low-Jc region, Jc<5 kA/cm2, and to be 0.88 eV in the high-Jc region, Jc>5 kA/cm2.