We report photoluminescence (PL) spectra associated with bimodal size-distribution observed in a series of self-assembled InAs/GaAs quantum-dot (QD) heterostructures with different InAs coverages. The PL spectra exhibit a well-defined doublet-like QD peak with invariable energy positions regardless of the coverage attributed to large and small QD groups. The excitation-power dependence reveals that the high-energy peak is composed by two contributions, one from the excited state of large QDs and the other from the ground state of small QDs. The power- and the temperature-dependent plots on the integrated PL-intensity ratios show distinctive features supporting an evolution of bimodal size-distribution on the InAs coverage variation in the QD ensembles. These suggest that, in as-grown InAs QD ensembles with appropriate InAs coverage grown under an optimized condition, there can exist a specific bimodal size-distribution consisting of two groups of large and small QDs whose sizes are fixed, but whose numbers vary with the amount of InAs coverage with no rearrangement of the overall size-distribution profile. (C) 2004 Elsevier B.V. All rights reserved.