Si/GaAs(111) interfaces grown at 300, 573, and 773 K on both polar surfaces have been studied by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), energy-loss spectra (ELS), and work function measurements. In both polar faces Si deposits up to 8 monolayers (ML) onto a room temperature substrate result in disordered and abrupt Si interfaces. At 573 and 773 K the first Si ML grows pseudomorphically with the substrate occupying first the vacancy and defect sites of the top Ga surface layer, followed by the remaining sites. The second and subsequent monolayers have the atom density of the bulk Si crystal. A surface segregation of up to 1 ML of Ga and As atoms at 773 K occurs by diffusion through the interface of about 0.65 nm thickness. The observed decrease of the work function in the submonolayer coverage range is explained by variations in the ionization energy due to positive-outwards surface dipoles created by Si adsorption on defect sites with a charge tranfer to the As atoms of 0.12 e0 per Si atom and polarizability of 4.3×10−22 cm3 at 300 K. These values indicate the strong interaction that the Si atoms encounter at the Ga defect sites.