We have been studying the effect of a magnetic field on the micro-photoluminescence (µ-PL) spectra of individual InAs/GaAs quantum dots (QDs). We have performed general investigations of the dependence of the exciton g-factor and diamagnetic shift on QD confinement, composition and strain. Additionally we have focused our attention on the observed redistribution between spectral lines of a single QD in increasing magnetic field. At above GaAs-barrier band gap excitation conditions the µPL-spectrum is dominated by the double negatively charged exciton carrier configuration, X2-, which decreases in intensity with increasing magnetic field in favor of the luminescence from single negatively charged, X1-, and neutral exciton, X0.
Utilizing a novel dual-laser excitation technique to compensate for internal electric fields in the sample and by studying the temperature dependence of the magnetic field-induced neutralization of the QD charge state, we have introduced a model to explain the observed effect. The magnetic field causes a slowing down of the electron drift velocity Vdr = F/B which, in turn, considerably increases the probability for electrons to be captured into localizing potentials, prohibiting capture into the QD. In the dual-laser experiments we show that by reducing the local electric field around the QD, a lower magnetic field strength is required to get full neutralization of the QD charge state compared to conventional single laser excitation. At elevated temperature, thermal ionization of the carriers out of the localized levels is predicted at which point the 5T magnetic field is not sufficient to render carrier capture at the localizing potentials and this is verified by our experiments.
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