Molybdenum nitride Mo2Nx films were grown on MgO(001) and on alpha-Al2O3(001) substrates by molecular beam epitaxy under nitrogen radical irradiation. X-ray photoelectron spectroscopy revealed that the composition of the film varied in the range of Mo2N1.4-Mo2N2.8 depending on the growth temperature. The deposition at 973 K gave well-crystallized films on both substrates. The high-resolution reciprocal space mapping by X-ray diffraction showed that the nitrogen-rich gamma-Mo2N crystalline phase (the composition: Mo2N1.4) was epitaxially grown on MgO at 923 K with a slight tetragonal distortion (a = 0.421 and c = 0.418 nm) to fit the MgO lattice (a = 0.421 nm). On alpha-Al2O3(0 0 1), nitrogen-rich gamma-Mo2N (Mo2N1.8) was grown at 973 K with (111) planes parallel to the substrate surface. X-ray diffraction analysis with a multi-axes diffractometer revealed that the gamma-Mo2N on alpha-Al2O3(001) had a slight rhombohedral distortion (a = 0.4173(2) and alpha = 90.46(3)degrees). Superconductivity was observed below 2.8-3 K for the films grown at 973 K on MgO and on alpha-Al2O3(001). (c) 2006 Elsevier B.V. All rights reserved.