Abstract
Optical studies of semiconductor quantum dots (QDs) have been a subject of very intensive recent investigations. It has been experimentally and theoretically established that the number of carriers which occupy a photoexcited QD greatly affect its photoluminescence (PL) spectrum.1-3 In spite of its neutral nature, optical spectroscopy has very recently proved to be a useful means for investigating and preparing charged QD systems.4,5 We report here on continuous wave (cw) and pulsed optical PL spectroscopy of single self-assembled QDs (SAQDs) embedded within a mixed type quantum well (QW) structure. This specific design, which facilitates charge separation by optical means, is used here to tune the charge state of the QD under study. We compare the PL emission spectra for a single In(Ga)As/GaAs SAQD occupied with an increasing number of neutral multiexcitons (Fig la), and the emission of the same type of dots embedded in a GaAs/AlAs mixed type structure (Fig lb). Using time-resolved spectroscopy and a comparison between cw and pulse-excited PL spectra6'' we are able to accurately determine the charge state of the single QDs at a given excitation intensity level. We determine the collective carrier state from which each spectral line originates. In Fig 2 we compare the excitation intensity dependence of different emission peaks from both neutral and charged dots, for cw and pulse excitation modes. For pulsed excitation, the lines resulting from recombination of a negatively charged excitons show a behavior which is typical to cw excitation mode of neutral QDs, while spectral lines which originate from neutral charge state behave differently.
© 2001 Optical Society of America
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