  Asymmetric double tunnel barriers with the center electrode being a metal
cluster in the quantum regime are studied. The zero dimensionality of the
clusters used and the associated quantized energy spectra are manifest in
well-defined steps in the current voltage characteristic (IVC). Record high
current rectification ratios of 10000 for tunneling through such clusters are
demonstrated at room temperature. We are able to account for all of the
experimentally observed features by modeling our double barrier structures
using a combination of discrete states and charging effects for tunneling
through quantum dots.
