Superconducting molybdenum nitride epitaxial thin films deposited on MgO and a-Al2O3 substrates by molecular beam epitaxy

Molybdenum nitride Mo2Nx films were grown on MgO(0 0 1) and on a-Al2O3(0 0 1) 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 g-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 a-Al2O3(0 0 1), nitrogen-rich g-Mo2N (Mo2N1.8) was grown at 973 K with (1 1 1) planes parallel to the substrate surface. X-ray diffraction analysis with a multi-axes diffractometer revealed that the g-Mo2N on a-Al2O3(0 0 1) had a slight rhombohedral distortion (a = 0.4173(2) and a = 90.46(3)8). Superconductivity was observed below 2.8–3 K for the films grown at 973 K on MgO and on a-Al2O3(0 0 1).