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Imbedding self-assembled lens-shaped InGaAs quantum dots in a suitably designed field-effect-type GaAs/AlAs heterostructure allows us to charge the lowest discrete quantum levels in the dots with single electrons. Because of their small diameters of about 20 nm the Coulomb charging energy is significantly smaller than the quantization energies. We extract energy spacings of about 41 meV between the s-like ground state and the first excited p-like state from capacitance as well as infrared transmission spectroscopy at low temperatures and under application of high magnetic fields.
Drexler et al. (Mon,) studied this question.