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

Abstract Molybdenum nitride Mo 2 N x films were grown on MgO(0 0 1) and on α-Al 2 O 3 (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 Mo 2 N 1.4 –Mo 2 N 2.8 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 γ-Mo 2 N crystalline phase (the composition: Mo 2 N 1.4 ) 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 α-Al 2 O 3 (0 0 1), nitrogen-rich γ-Mo 2 N (Mo 2 N 1.8 ) 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 γ-Mo 2 N on α-Al 2 O 3 (0 0 1) had a slight rhombohedral distortion ( a = 0.4173(2) and α = 90.46(3)°). Superconductivity was observed below 2.8–3 K for the films grown at 973 K on MgO and on α-Al 2 O 3 (0 0 1).