High T c superconducting B1 phase MoN films prepared by low-energy ion-assisted deposition

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Abstract
Metastable B1 phase MoN was synthesized by low‐energy ion‐assisted thin‐film growth techniques at deposition temperatures TD =400, 500, and 700 °C. The films were condensed under the action of mixed Ar+ and N2+ ion species at an ion to condensing atom arrival rate ratio of 1.0. For each deposition temperature, films were prepared at various ion energies in the range 2–200 eV. The presence of chemically reactive N2+ ions and the use of low ion energies allow both structural and chemical selectivity, facilitate increased incorporation of nitrogen, and promote ordering of the defect structure of cubic MoN. The films were characterized by x‐ray diffraction, by measurements of superconducting temperature Tc and by temperature‐dependent resistivity ρ(T). Collectively the film properties strongly depend on ion energy and deposition temperature. The composition, lattice parameter, crystalline preferred orientation, and grain size can be controlled by varying ion energy. The resistivity shows an anomalous temperature dependence that is typical of disordered metals, i.e., electron localization governs electrical conduction with the temperature coefficient of resistivity changing sign according to the Mooij correlation. At optimum deposition conditions the resistivity is metallike (phonon contribution ρthermal >0), Tc=6.2 K, the resistivity ratio RR=1.1, and ρ300 K =120 μΩ cm. Annealing of films at 700 °C causes an improvement in properties with Tc≂12 K, RR=1.7, and ρthermal =60–80 μΩ cm. In films with large thermal resistivities we observe a ρ∼T2 behavior at low temperatures and saturation of resistivity at high temperatures (ρsat =200–280 μΩ cm) which suggest that MoN with the B1 crystal structure is a strongly coupled and disordered high Tc superconductor.