Quantum-dot electron occupancy controlled by a charged scanning probe

A 1D channel, defined in a GaAs/AlGaAs heterostructure, has been used to detect the oscillations in charge as single electrons tunnel into or out of a group of quantum dots about $1\mu \mathrm{m}$ from the channel. The quantum dots were formed in the highly doped donor layer of the same device due to intrinsic donor disorder. Electron occupancy of the quantum dots was controlled by the position and bias of a charged probe. High-resolution images of channel conductance revealed sets of concentric oscillations, or halos, each set being centered on a quantum dot. Uneven spacing was observed between the halos implying that the dot electron energy-level spacings were not constant. This is characteristic of a dot occupied by a small number of electrons. Changes in halo spacing at intersections between different sets of halos suggest that there was electrostatic interaction between the quantum dots.