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 2 O 3 ) but cubic Mo 2 N on (001) SrTiO 3 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 [101̅0] MoN ||[112̅0] Al2O3 for MoN on c-cut Al 2 O 3 and (001) Mo2N ||(001) STO and [111] Mo2N ||[111] STO for Mo 2 N on SrTiO 3 . 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 Mo 2 N.