Few-electron ground states of charge-tunable self-assembled quantum dots

The few-electron ground states of self-assembled InAs quantum dots are investigated using high-resolution capacitance spectroscopy in magnetic fields up to 23 T. The level structure reveals distinct shells which are labeled as s-, p-, and d-like according to their symmetry. Our measurements enable us to resolve the single-electron charging not only of the lowest (s) state with two electrons but also of the second lowest (p) state with four electrons as pronounced maxima in the capacitance spectra. Furthermore, two peaks at higher energy can be attributed to charging of the d shell with the first two electrons. We discuss the energy spectrum in terms of spatial quantization energy, Coulomb blockade, and many-particle effects. At around B=15 T we observe a magnetic-field-induced intermixing of the p and d shell. Additional fine structure in the capacitance spectra is observed and discussed both in terms of nearest-neighbor Coulomb interactions and monolayer fluctations of the dot size.