Characterization of NbN Tunnel Junctions With Radical-Nitrided $\hbox {AlN}_{\rm x}$ Barriers

This paper reports on the electrical characteristics of NbN tunnel junctions fabricated by radical nitridation of an Al layer deposited on a base NbN layer to form the barrier. The NbN/Al-AlN_x/NbN junction characteristics were improved by using NbN films with small surface roughness and by changing nitridation conditions. The junctions showed good critical current (I_c) uniformity, low subgap leakage currents, and high critical current density (J_c) up to 15.6 kA/cm². The maximum-to-minimum spread in I_c was ± 1.5% for a series array of 200 junctions with a J_c of 4.4 kA/cm². The quality parameter (R_sg/R_n) of a single junction was 19 at this J_c value, where R_sg is the subgap resistance at 3 mV and R_n is the junction resistance at 10 mV. The junction-specific capacitance was estimated by measuring resonant steps in dc-SQUIDs and was found to be in the range of 50-80 fF/μm² for J_c in a range of 0.05-5 kA/cm². The gap voltage (V_g) was increased from 4.3 to 5.0 mV by elevating the substrate temperature during the deposition of a counter NbN layer. In addition, the features of the junctions were emphasized by comparing their characteristics with those formed with RF-plasma-biasing-nitrided AlN_x barriers.