SINGLE-ELECTRON CHARGING OF QUANTUM-DOT ATOMS

Arrays of field-effect-confined quantum dots with diameters smaller than 100 nm have been prepared starting from AlxGa1-xAs-GaAs heterostructures. In far-infrared spectroscopy, we induce transitions between the 2-meV-separated quantum levels. We observe discrete steps in the gate-voltage dependence of the integrated absorption strength indicating directly the incremental occupation of each dot with N = 1, 2, 3, and 4 electrons. From the gate-voltage dependence, we can estimate a Coulomb charging energy of about 15 meV. On a very fine scale, we also observe a spectral fine structure for the excitation of the quantum-dot atoms.