  We have measured the oscillator strength and quantum efficiency of excitons
confined in large InGaAs quantum dots by recording the spontaneous emission
decay rate while systematically varying the distance between the quantum dots
and a semiconductor-air interface. The size of the quantum dots is measured by
in-plane transmission electron microscopy and we find average in-plane
diameters of 40 nm. We have calculated the oscillator strength of excitons of
that size and predict a very large oscillator strength due to Coulomb effects.
This is in stark contrast to the measured oscillator strength, which turns out
to be much below the upper limit imposed by the strong confinement model. We
attribute these findings to exciton localization in local potential minima
arising from alloy intermixing inside the quantum dots.
