The ion beam producing ionization of N= has been successfully utilized as the N source for nitride growth. Cubic GaN thin films are thus grown on GaAs(100) substrates. The width of the (200) X-ray diffraction peak of the epilayer is 23′ only, and the high-resolution electron energy loss spectroscopy measurements show that the optical surface phonon of cubic GaN appears at the loss energy of 82 meV.