Single-electron tunneling effects in granular metal films

Cryogenic scanning tunneling microscopy is used to study local electrical-transport properties of thin granular Au/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ films in the vicinity of the percolation threshold. The current-voltage characteristics are found to vary dramatically from one tip position to another over distances of the order of a few nanometers. These characteristics often exhibit interesting Coulomb-staircase structures having unusual variations in step widths and heights due to complex tunneling paths. A triple-barrier tunnel-junction model accounts quantitatively for the experimental results.