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 (∼6.4×1012 atoms cm−2) in agreement with previous studies of the site occupancy of Si δ layers.