We have systematically studied the influence of Si doping on the optical characteristics of InGaN/GaN multiple quantum wells (MQWs) using photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy combined with studies of optically pumped stimulated emission and structural properties from these materials. The MQWs were grown on 1.8-μm-thick GaN layers on c-plane sapphire films by metalorganic chemical vapor deposition. The structures consisted of 12 MQWs with 3-nm-thick InGaN wells, 4.5-nm-thick GaN barriers, and a 0.1 -μm-thick Al0.07Ga0.93N capping layer. The Si doping level in the GaN barriers was varied from 1 × 1017 to 3 × 1019 cm3. PL and PLE measurements show a decrease in the Stokes shift with increasing Si doping concentration. The 10 K radiative recombination lifetime was observed to decrease with increasing Si doping concentration (n), from ∼ 30 ns (for n < 1 × 1017 cm-3) to ∼4 ns (for n = 3 x 1019 cm-3). To elucidate whether non-radiative recombination processes affect the measured lifetime, the temperature-dependence of the measured lifetime was investigated. The reduced Stokes shift, the decrease in radiative recombination lifetime, and the increase in structural and interface quality with increasing Si doping indicate that the incorporation of Si in the GaN barriers results in a decrease in carrier localization at potential fluctuations in the InGaN active regions and the interfaces.