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We present a model to describe the Reflectance-difference (RD) spectra of InGaAs grown on GaAs (001) at T=500 o C, in the energy range from 2.3-3.5 eV. The model assumes the presence of an orthorhombic strain in the InGaAs epilayer that accounts for an anisotropic process of nucleation of InGaAs islands. We show that the developed model leads to accurate fits to the experimental RD spectra of InGaAs/GaAs for epilayer thickness both below and above of the critical thickness for the 2D-3D growth-mode transition. From the fitting of the theoretical model to the experimental RD line shapes we obtain quantitative information on changes in surface stoichiometry and morphology at the onset of the 2D-3D transition. Our results demonstrate that the RD line shape of InGaAs/GaAs in the 2.3-3.5 eV energy range can be explained entirely in terms of surface-modified InGaAs E 1 and E 1 +∆ 1 bulk transitions with no need to invoke surface states.

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