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

Abstract

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-AlNx/NbN junction characteristics were improved by using NbN films with small surface roughness and by changing nitridation conditions. The junctions showed good critical current (Ic) uniformity, low subgap leakage currents, and high critical current density (Jc) up to 15.6 kA/cm2. The maximum-to-minimum spread in Ic was ± 1.5% for a series array of 200 junctions with a Jc of 4.4 kA/cm2. The quality parameter (Rsg/Rn) of a single junction was 19 at this Jc value, where Rsg is the subgap resistance at 3 mV and Rn 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 Jc in a range of 0.05-5 kA/cm2. The gap voltage (Vg) 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 AlNx barriers.