We have systematically studied the optical properties of InGaN/GaN multiple quantum wells (MQWs) at 10 K under different excitation conditions using photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. We found that the PL emission consists of a strong main peak at 2.80 eV and a much weaker and broader secondary peak at similar to 2.25 eV. We observed that the peak position blueshifts and the spectral width narrows for the main peak when the excitation energies are varied from 3.81 eV (above the band gap of the AlGaN capping layer) to 2.99 eV the band (below gap of the GaN barrier layers). The intensity ratio of the main peak to the secondary peak also varied with excitation energy. The two observed emission peaks originate from different layers of the MQWs. Time-integrated and time-resolved PL revealed that the InGaN-related spontaneous emission processes are strongly affected by inhomogeneity and carrier localization in the MQWs. From these studies under varying excitation energies, we conclude that interface-related defects and roughness may play an important role in the InGaN-related emission mechanism during the carrier transfer between different layers of the MQWs. (C) 1998 American Institute of Physics. [S0003-6951(98)00648-2].