Tunneling studies of mesoscopic all-NbN junctions

A scanning tunneling microscope was used to position a NbN tip near a NbN thin film sputtered on a Si substrate. Measurements at 4.2 K clearly show an energy gap of 5.0 mV. The Coulomb blockade of tunneling and the Coulomb staircase, formed by single-electron charging of the central electrode of a double capacitor system consisting of a substrate, a particle lodged in the oxide, and a tip, were observed at a number of points on the film. Experiments were repeated at room temperature. Fitting I-V and dI/dV-V curves to theory yields capacitances on the order of 5*10/sup 19/ to 2*10/sup -18/ F. The granular nature of the sputtered NbN greatly facilitates formation of the requisite double junction structure.<>