GaAs layers were grown by molecular‐beam epitaxy and by migration‐enhanced epitaxy (MEE) on pseudomorphic Si films grown on GaAs(100) substrates. The Si interlayer thickness (tSi) was varied from 0 to 3 monolayers (ML), and the effect on the GaAs growth mode was investigated by observing the behavior of the intensity of the specular spot of reflection high‐energy electron diffraction patterns. From these measurements it was concluded that the surface migration of Ga atoms is disturbed by the Si atoms on the growing surface. The disturbance increased, at the growth temperature of 520 °C, with increasing the Si interlayer thickness to the point that fortSi≥0.4 ML, the two‐dimensional (2D) growth changed to a three‐dimensional one. By increasing the growth temperature, the growth mode improved but the Si surface segregation increased, as detected by secondary ion mass spectrometry. The effect of the thermally activated Si segregation process on the GaAs growth mode is discussed. Using MEE at a growth temperature of 480 °C, the Si segregation was eliminated and a 2D growth mode was obtained.