Preparation and structural characterization of sputtered CoO, NiO, and Ni_0.5Co_0.5O thin epitaxial films

Single phase CoO, NiO, and Ni_0.5Co_0.5O epitaxial films have been prepared by reactive sputtering onto 〈0001〉 α−Al₂O₃ substrates maintained at 373 K. Epitaxy was confirmed by x-ray diffraction (XRD) and high resolution electron microscopy (HREM) techniques. XRD experiments indicate that these monoxide films are cubic and contain rotation twins with the twin axis parallel to 〈111〉. Lattice parameters for the CoO and NiO films are 0.4254 ± 0.0001 nm and 0.4173 ± 0.0006 nm, respectively, and agree with published values for the corresponding bulk oxides. The lattice parameter 0.4220 ± 0.0001 nm for the Ni_0.5Co_0.5O film lies between those of CoO and NiO and suggests that the mixed oxide film is compositionally homogeneous. Cross-sectional HREM images of the Ni_0.5Co_0.5O specimen show Σ3(12) twin boundaries perpendicular to the oxide-substrate interface. The twin regions are approximately 30 nm in size and are uniformly distributed throughout the film. The epitaxial orientation of the monoxide films with respect to the substrate can be summarized by the relationships [111] monoxide // [0001] α−Al₂O₃, [10] monoxide // [100] α−Al₂O₃, and [11] monoxide // [110] α−Al₂O₃.