(100) GaAs substrates with different misorientations were exposed to trimethyl-bismuth (TMBi) flow. The wafers were examined after exposure times of 9 and 43 min. The wafers growth was carried out at atmospheric pressure, in a metalorganic vapor phase epitaxy (MOVPE) reactor, at a temperature of 375 degrees C. The in situ spectral reflectance (SR) in the spectral range 400-1000 nm was used to monitor the deposition. The reflectivity is marked by two temporal phases, an increase followed by a slow decrease. The atomic force microscopy (AFM) measurements show two different growth modes of bismuth, big isolated islands for early exposure time and a high density of small islands for long exposure times. We also note that the islands' density and size change greatly with the misorientation of the substrate. The correlation between the results given by ex situ AFM and in situ SR is performed via theoretical simulations. We show that roughness, as well as optical and thermal effects are mainly responsible for the reflectivity behavior. We have successfully quantified the spectral evolution of the sensitivity sigma(SR) of the incident beam wavelength to the surface undulations. (C) 2012 Elsevier B.V. All rights reserved.