Reflectance anisotropy spectroscopy (RAS) has been employed to characterize the overlayer growth of GaAs onto sub to one monolayer coverages of Si δ layers deposited on the GaAs(001)‐c(4×4) surface. The low growth temperature (400 °C), required to avoid spreading of the dopant away from the δ plane, has meant that the study of a RAS feature related to the linear electro‐optic (LEO) effect is complicated by disordering at the GaAs surface. This disordering is induced not only by the growth temperature, but also by the presence of the Si δ layer itself. Variable thickness studies indicate that the LEO‐induced signal is dependent on the field profile in the surface layer. It has been observed that the intensity of the LEO feature, as a function of Si coverage, reaches a maximum at ∼0.01 ML (∼6.4×1012atoms cm−2) in agreement with previous studies of the site occupancy of Si δ layers.