GaAs thin films could be heteroepitaxially grown on Si(100) and Si(111) substrates at a temperature of 500 degrees C using only Ga and As ion beams with energies lower than about 50 eV. The deposition rates decreased with increasing ion energy and became zero at about 50 eV. Their surfaces were rough. A high quality GaAs film with a smooth surface was heteroepitaxially grown on the Si substrates using a mixed beam of neutral Ga and As molecular beams and Ga and As inn beams with an energy of 60 eV. However. using only neutral molecular beams resulted in the deposition of polycrystalline GaAs films with a rough surface.