  Quantum dots exhibit reproducible conductance fluctuations at low
temperatures due to electron quantum interference. The sensitivity of these
fluctuations to the underlying disorder potential has only recently been fully
realized. We exploit this sensitivity to obtain a novel tool for better
understanding the role that background impurities play in the electrical
properties of high-mobility AlGaAs/GaAs heterostructures and nanoscale devices.
In particular, we report the remarkable ability to first alter the disorder
potential in an undoped AlGaAs/GaAs heterostructure by optical illumination and
then reset it back to its initial configuration by room temperature thermal
cycling in the dark. We attribute this behavior to a mixture of C background
impurities acting as shallow acceptors and deep trapping by Si impurities. This
"alter and reset" capability, not possible in modulation-doped
heterostructures, offers an exciting route to studying how scattering from even
small densities of charged impurities influences the properties of nanoscale
semiconductor devices.
