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We have studied the effects of Si doping on the recombination dynamics and exciton localization in modulation-doped GaN/Al0.07Ga0.93N multiple-quantum-well structures by means of photoluminescence (PL) and time-resolved PL measurements. The PL peak position shows a blue shift as the Si doping in the barriers is increased (up to 4.2×1019 cm-3). For even higher doping levels a red shift of the PL emission is observed. The decay time of an undoped sample shows nonexponential behavior, while the Si doped samples show mono-exponential behavior. Surprisingly, the PL decay time at 2 K is found to be nearly constant for all doping levels, in these samples.