Spontaneous and stimulated luminescence in highly excited InGaN/GaN multiple quantum wells grown by MOCVD over sapphire and bulk GaN substrates is reported. By using high-power laser pulses of 20-ps duration, free-carrier densities as high as 10(19) cm(-3) were achieved, which resulted in almost screened built-in field. Therefore, dynamics of spontaneous and stimulated emission due solely to band potential fluctuations were revealed. In comparison with the heteroepitaxial counterparts, structures grown over bulk GaN exhibited strong stimulated emission within the first 100ps of relaxation, a reduced intensity of spontaneous emission on the later stage of relaxation, and domination of delocalized carriers in the spontaneous spectra. The obtained results imply a significant reduction of band potential fluctuations in the InGaN/GaN structures grown over bulk GaN. (c) 2005 Elsevier B.V. All rights reserved.