Influence of Oxygen Plasma on the Growth and Stability of Epitaxial NiCo 2 O 4 Ultrathin Films on Various Substrates.

In this work, we investigated the influence of oxygen plasma on the growth of nickel cobaltite (NiCo 2 O 4 ) thin films compared to growth in a molecular oxygen atmosphere. The films were grown on MgO(001), MgAl 2 O 4 (001) and SrTiO 3 (001) substrates by oxygen plasma (atmosphere of activated oxygen)-assisted and reactive molecular beam epitaxy (molecular oxygen atmosphere). Soft X-ray photoelectron spectroscopy showed that only the use of oxygen plasma led to a spectrum characteristic of (NiCo 2 O 4 ). Low energy electron diffraction measurements were conducted to obtain information on the structure of the film surfaces. The results proved the formation of a spinel surface structure for films grown with oxygen plasma, while the formation of a rock salt structure was observed for growth with molecular oxygen. To determine the film thickness, X-ray reflectivity measurements were performed. If oxygen plasma were used to grow (NiCo 2 O 4 ) films, this would result in lower film thicknesses compared to growth using molecular oxygen although the cation flux was kept constant during deposition. Additional X-ray diffraction experiments delivered structural information about the bulk structure of the film. All films had a rock salt bulk structure after exposure to ambient conditions. Angle-resolved hard X-ray photoelectron spectroscopy revealed a homogeneous depth distribution of cations of the grown film, but no typical (NiCo 2 O 4 ) spectrum anymore. Thus, on the one hand, (NiCo 2 O 4 ) films with a spinel structure prepared using activated oxygen were not stable under ambient conditions. The structure of these films was transformed into NiCo oxide with a rock salt structure. On the other hand, it was not possible to form (NiCo 2 O 4 ) films using molecular oxygen. These films had a rock salt structure that was stable under ambient conditions.