The optical properties and nanostructures of two InGaN/GaN quantum-well (QW) samples of slightly different structures are compared. In one of the samples, InN interfacial layers of a few monolayers are added to the structure between wells and barriers for improving the QW interface quality. Compared with the standard barrier-doped QW sample, the addition of the InN interfacial layers does improve the QW interface quality and hence the photon emission efficiency. The strain state analysis images show the high contrast between the clear QW interface in the sample with InN layers and the diffusive QW boundaries in the reference sample. The detection-energy-dependent photoluminescence excitation data reveal the consistent results.