Molybdenum nitride films with the $\mathrm{NaCl}$ structure $\left(B1\text{−}\mathrm{MoN}\right)$ were epitaxially grown on $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(001\right)$ and $\mathrm{MgO}\left(001\right)$ substrates at $973\mathrm{K}$ 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 $B1\text{−}\mathrm{MoN}$, a predicted superconductor, is elastically unstable against tetragonal and trigonal distortions. On $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(001\right)$, the $B1\text{−}\mathrm{MoN}$ phase (composition, ${\mathrm{Mo}}_1{\mathrm{N}}_{0.98}$) 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 [$a=0.4219\left(3\right)\mathrm{nm}$, $\alpha =89.28\left(5\right)°$] with an expansion along the [111] direction perpendicular to the substrate surface. The film grown on $\mathrm{MgO}\left(001\right)$ had the ${\mathrm{MoN}}_{1.03}$ composition and showed a slight tetragonal distortion ($a=0.4213$ and $c=0.424\mathrm{nm}$) due to fitting to the $\mathrm{MgO}$ substrate lattice $(a=0.4213\mathrm{nm})$. These two stoichiometric films showed no superconductivity above $2\mathrm{K}$. A lower nitrogen content $\left({\mathrm{MoN}}_{0.86}\right)$ film was obtained on $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(001\right)$ using a higher deposition rate. The corresponding film had a much smaller lattice constant $[a=0.4184(3\left)\mathrm{nm}\right]$, and a similar distortion $[\alpha =89.41(5)°]$. The $B1\text{−}{\mathrm{MoN}}_{0.86}$ film showed superconductivity with a transition temperature $T_c=4.2\mathrm{K}$. The suppression of the superconductivity of the former stoichiometric phase can be interpreted in terms of the lattice expansion.

• Received 10 October 2005

DOI:https://doi.org/10.1103/PhysRevB.73.052504