GaAs epitaxial films grown on Si substrates by molecular beam epitaxy were investigated by photoluminescence (PL) and Raman spectroscopy measurements. The GaAs layer was irradiated in situ with a 50 keV electron beam during growth. The PL spectrum of the electron-beam-irradiated GaAs/Si consisted of four well-resolved peaks, and the peak related to the intrinsic transitions between the conduction band and the heavy-hole is only 2.8 meV, which is much smaller than the value obtained from the unirradiated GaAs/Si. The ratios of the transverse-optical phonon and the longitudinal-optical phonon of the Raman spectra for the electron-beam-irradiated GaAs/Si is much smaller than that obtained from the unirradiated GaAs/Si. These results indicate that the crystallinity of the GaAs/Si is improved by irradiation with an electron beam during growth, and the improvement of the crystallinity of the GaAs bilayer might originate from the annihilation of dislocations by accelerating the formation of a single domain during the growth.