  We report a photoluminescence (PL) spectroscopy study of charge state control
in single self-assembled InAs/GaAs quantum dots by applying electric and/or
magnetic fields at 4.2 K. Neutral and charged exciton complexes were observed
under applied bias voltages from -0.5 V to 0.5 V by controlling the carrier
tunneling. The highly negatively charged exciton emission becomes stronger with
increasing pumping power, arising from the fact that electrons have a smaller
effective mass than holes and are more easily captured by the quantum dots. The
integrated PL intensity of negatively charged excitons is affected
significantly by a magnetic field applied along the sample growth axis. This
observation is explained by a reduction in the electron drift velocity caused
by an applied magnetic field, which increases the probability of non-resonantly
excited electrons being trapped by localized potentials at the wetting layer
interface, and results in fewer electrons distributed in the quantum dots. The
hole drift velocity is also affected by the magnetic field, but it is much
weaker.
