The epitaxial growth of GaAs films on on‐axis (100) Si was studied at growth temperatures in the range 160–280 °C using ionized source beam epitaxy. Single‐crystal GaAs films could be grown at a temperature as low as 160 °C with the acceleration of a partially ionized As‐source beam, whereas at the same temperature only amorphous films were possible with neutral beams or with the ionized source beam with no acceleration. The use of an ionized As‐source beam even without beam acceleration greatly improved the surface flatness of the GaAs film, and suppressed the formation of antiphase domains. The acceleration of the ionized As beam further improved the surface quality of the film.