Photoluminescence excitation (PLE) studies were performed on GaAs‐Al0.25Ga0.75As quantum wells (QWs) with fractional monolayer differences. The quantized PLE peaks and their submonolayer shifts clearly show that the heterointerface of thin QWs prepared by growth‐interrupted molecular beam epitaxy has islands which extend out a lateral dimension larger than 100 A˚, but they themselves have the microroughness smaller than 30 A˚. The result of this work using exciton as the probe provides a clear evidence supporting the bimodal roughness model. ©1995 American Institute of Physics.