  We used resonant laser spectroscopy of multiple confocal InGaAs quantum dots
to spatially locate charge fluctuators in the surrounding semiconductor matrix.
By mapping out the resonance condition between a narrow-band laser and the
neutral exciton transitions of individual dots in a field effect device, we
identified spectral discontinuities as arising from charging and discharging
events that take place within the volume adjacent to the quantum dots. Our
analysis suggests that residual carbon dopants are a major source of
charge-fluctuating traps in quantum dot heterostructures.
