Structure and Surface Morphology of Epitaxial Ni Films on MgO(111) and (001) Substrates: Growth of High Quality Single Domain Films

Abstract

AbstractNi films were deposited by ultra high vacuum magnetron sputtering onto MgO(111) and (001) substrates kept at temperatures Ts between 20°C and 600°C. The structure and the morphology of the 100 nm thick films were analyzed using X-ray diffraction (XRD) and atomic force microscopy (AFM). On MgO(111), all Ni films have a strong [111] oriented texture with high out-of-plane as well as high in-plane orientations with an increased crystalline quality with temperature. At 300°C the Ni 111 peak width is ∼0.25° in the ω direction, showing a high crystal quality for a heteroepitaxial metal film. Pole-figure analysis shows that the films are built up of two domains for temperatures ≤200°C and at 600°C, while at 300°C and 400°C only one domain is formed. The RMS roughness, as measured with AFM, of the film increases with temperature with a local maximum between 150° to 200°C and a local minimum at 300°C which corresponds to the transition from two domains to one. On MgO(001), Ni shows three completely different textures depending on the deposition temperature. At ambient temperature a mixed (1 −4 1) and (022) texture with traces of (002) is formed. Between 100°C and 200°C Ni grows with a single domain (001) texture. At higher temperatures a highly oriented 8-fold (75-1) texture is formed. The highest crystalline quality is obtained at 100°C where the FWHM of the Ni 002 peak at 100°C is 0.24° in the ω−2θ direction and 0.18 in the ω direction which shows the good crystalline quality also in this case. Both the RMS surface rougness and the surface feature size increases with the deposition temperature but with local miminas at 100°C corresponding to the film with highest crystalline quality.The weak Ni-O interaction together with a strong metal-metal bonding between the Ni atoms results in an in plane supercell matching between the Ni film and the MgO substrates at 300-400°C for the MgO(111) substrate and 100-200°C for the MgO(001) substrate.The (7 5 −1) texture formed by Ni on MgO(001) substrates at deposition temperatures ≥300°C, places each interfacial Ni atom in the preferred position on top of an O atom on the MgO surface, and in addition, also better accommodates the mismatch between the two crystal lattices.