Charging effects and the excitation spectrum of a quantum dot formed by an impurity potential

We have studied Coulomb-blockade effects through a quantum dot formed by an impurity potential near a two-dimensional electron gas (2DEG) defined at a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterojunction. We have compared the energy-level spacing within the quantum dot obtained by two methods: from the magnetic-field-induced gate-voltage shifts of the conductance peaks, and by applying a source-drain voltage across the dot. The use of the latter method in a magnetic field allows us to distinguish between charging and confinement effects, although the structure in the nonlinear excitation spectrum requires careful interpretation. As the number of electrons in the dot decreases, the barriers connecting the dot to the 2DEG thicken and the area of the dot decreases, giving rise to an increased charging energy that has been measured directly. After the final Coulomb-blockade conductance peak, we estimate that there are 20 electrons in the dot.