Chemical and electronic structures of cobalt oxynitride films deposited by NH₃vs. N₂ plasma: theory vs. experiment

The chemical structures of Co oxynitrides - in particular, interactions among N and O atoms bonded to the same cobalt - are of great importance for an array of catalytic and materials applications. X-ray diffraction (XRD), core and valence band X-ray photoelectron spectroscopy (XPS) and plane wave density functional theory (DFT) calculations are used to probe chemical and electronic interactions of nitrogen-rich CoO_1-xN_x (x > 0.7) films deposited on Si(100) using NH₃ or N₂ plasma-based sputter deposition or surface nitridation. Total energy calculations indicate that the zinc blende (ZB) structure is energetically favored over the rocksalt (RS) structure for x > ∼0.2, with an energy minimum observed in the ZB structure for x∼ 0.8-0.9. This is in close agreement with XPS-derived film compositions when corrected for surface oxide/hydroxide layers. XRD data indicate that films deposited on Si(100) at room temperature display either a preferred (220) orientation or no diffraction pattern, and are consistent with either rocksalt (RS) or zinc blende (ZB) structure. Comparison between experimental and calculated X-ray excited valence band densities of states - also similar for all films synthesized herein - demonstrates a close agreement with a ZB, but not an RS structure. Core level XPS spectra exhibit systematic differences between films deposited in NH₃ vs N₂ plasma environments. Films deposited by N₂ plasma magnetron sputtering exhibit greater O content as evidenced by systematic shifts in N 1s binding energies. Excellent agreement with experiment for core level binding energies is obtained for DFT calculations based on the ZB structure, but not for the RS structure. The agreement between theory and experiment demonstrates that these N-rich Co oxynitride films exhibit the ZB structure, and forms the basis of a predictive model for understanding how N and O interactions impact the electronic, magnetic and catalytic properties of these materials.