We report photoluminescence (PL) measurements on self-assembled GaSb/GaAs quantum dots. As the laser excitation is increased from very low levels, the PL shows a strong red shift, and then a blue shift, such that it presents a U-shaped curve. Raising the temperature causes a large (<100 meV) blue shift of the PL, and shifts the minimum of the PL energy versus laser excitation curve to higher laser powers. Applying a magnetic field at lasers powers much less than1 W cm(-2) red shifts the PL energy. We explain these effects by population or depopulation of dots that are filled in the dark with holes supplied by carbon acceptors.