Optical spectra of single quantum dots: Influence of impurities and few-particle effects

The evolution of photoluminescence (PL) spectra of single GaAs/AlGaAs quantum dots (QD) is studied as a function of laser excitation power and temperature. At very low powers, where multi-exciton occupation of the QD can be excluded, an unexpected and pronounced spectral evolution is observed (large energy shifts and appearance of multiple emission lines). A similar evolution is observed at low excitation powers with increasing,temperature. A model, taking into account the influence of the shallow, residual impurities in the environment of each QD, explains the observed spectral evolutions in terms of photo-depletion of the QD and hopping of impurity-bound carriers back into the QD. Theoretical calculations of the PL due to N electrons + 1 hole (Ne + 1h) QD states allow us to attribute the approximate to 2 meV spaced lines in the experimental spectra to the different charge states Ne + 1h, (N - 1) e + 1h,... of the QD.