Controlling crystal structure and oxidation state in molybdenum nitrides through epitaxial stabilization

Epitaxial molybdenum nitride films with different crystal structures and chemical compositions were successfully synthesized by a chemical solution deposition technique. Hexagonal MoN was stabilized on c-cut sapphire (Al ₂O₃) but cubic Mo₂N on (001) SrTiO₃ even though exactly the same Mo-polymer precursor solution and the processing parameters (such as the annealing temperature and environment) were used. Both X-ray diffraction and high-resolution transmission electron microscopy confirmed the growth of epitaxial molybdenum nitride films with an epitaxial relationship between the film and the substrate as (0001)_MoN||(0001) _Al2O3 and [101̄0]_MoN||[112̄0]_Al2O3 for MoN on c-cut Al₂O₃ and (001)_Mo2N||(001) _STO and [111]_Mo2N||[111]_STO for Mo₂N on SrTiO₃. The formation of epitaxial molybdenum nitride films with different oxidation states resulted in very different electrical properties: a superconducting transition temperature of 12 K for MoN, whereas it was 4.5 K for Mo₂N.