Structural distortion and suppression of superconductivity in stoichiometricB1−MoNepitaxial thin films

Molybdenum nitride films with the NaCl structure B1-MoN were epitaxially grown on -Al2O3001 and MgO001 substrates at 973 K by pulsed laser deposition PLD under nitrogen radical irradiation. The highly crystalline epitaxial films enabled us to determine the three-dimensional cell parameters, which was motivated by theoretical calculations that B1-MoN, a predicted superconductor, is elastically unstable against tetragonal and trigonal distortions. On -Al2O3001, the B1-MoN phase composition, Mo1N0.98 was grown with its 111 planes parallel to the substrate surface. X-ray diffraction analysis with a multiaxes diffractometer detected only a small trigonal lattice distortion a=0.42193 nm, =89.285° with an expansion along the 111 direction perpendicular to the substrate surface. The film grown on MgO001 had the MoN1.03 composition and showed a slight tetragonal distortion a=0.4213 and c=0.424 nm due to fitting to the MgO substrate lattice a=0.4213 nm. These two stoichiometric films showed no superconductivity above 2 K. A lower nitrogen content MoN0.86 film was obtained on -Al2O3001 using a higher deposition rate. The corresponding film had a much smaller lattice constant a=0.41843 nm, and a similar distortion =89.415°. The B1-MoN0.86 film showed superconductivity with a transition temperature Tc=4.2 K. The suppression of the superconductivity of the former stoichiometric phase can be interpreted in terms of the lattice expansion.