Tunneling studies of mesoscopic all-NBN junctions

A scanning tunneling microscope (STM) 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 1-V and dI/dV-V curves to theory yields capacitances on the order of 5x10- 19 to 2 x 10- 18 F. The granular nature of the sputtered NbN greatly facilitates formation of the requisite double junction structure.