Controlling Crystal Structure and Oxidation State in Molybdenum Nitrides through Epitaxial Stabilization

Abstract

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 (Al2O3) 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 [101̅0]MoN||[112̅0]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.