Single-electron tunneling through an ultra-small metal particle

Ultrasmall ( ≲ 4nm in diameter) double-barrier tunnel junctions have been realized using a scanning-tunneling microscope, and an optimized metal particle-oxide-metallic substrate system. By measuring the electric-transport (current-voltage characteristic) of such junctions at room temperature, we have observed the Coulomb gap and the Coulomb staircase. The interpretation in terms of room temperature single-electron tunneling is supported by liquid helium temperature measurements on identical samples. The comparison of the experimental data with the conventionally used theory — orthodox model — frequently reveals two deviations: an enhanced asymptotical separation of the current-voltage characteristic, and an anomalous suppression of the first step on either side of zero voltage. These observations are tentatively attributed to the effect of slow dielectric relaxation of polarization charge induced in the tunnel oxide.