G. Herranz

Transition from three- to two-dimensional growth in strained SrRuO3 films on SrTiO3(001)

The morphology of strained SrRuO3 films grown on SrTiO3(001) has been investigated as a function of thickness. A transition of growth mode has been observed. At the early stages, there is a fingerlike structure originated by three-dimensional (3D) islands that nucleated along the substrate steps. Afterward, adatoms stick preferentially in the valleys of the structure and the films become progressively smoother. At a thickness above 10–20 nm, the films are extremely flat and have a self-organized structure of terraces and steps, with the growth proceeding mainly by a step flow (two-dimensional) mechanism. Relevance on film properties and possible use of the initial, nanometric, 3D structures are discussed.

Critical effects of substrate terraces and steps morphology on the growth mode of epitaxial SrRuO3 films

We report on the controlled fabrication of the arrays of self-organized fingerlike nanostructures of SrRuO3 that form on SrTiO3(001). We show that the size (width and height) of the one-dimensional nanometric units can be tuned or suppressed by using appropriate substrate vicinality θv and surface termination. Critical effects on the fingers height are observed when θv is below an angle as small as ∼0.1°. The value of θv also determines if the final two-dimensional growth is by a step flow (θv=0.5°), by layer by layer (θv=0.04°), or by coexistence of both mechanisms (θv=0.1°). The fingers are suppressed when TiO2-terminated low-miscut substrates are used, as film nucleation takes place on the terraces and then substrate steps are not active as templates for the one-dimensional structure formation. These findings shall contribute to progress toward lateral nanostructuration of complex oxide surfaces.

Self-organization in complex oxide thin films: from 2D to 0D nanostructures of SrRuO3 and CoCr2O4

We report here on the controlled fabrication of nanostructures of varied dimensionality by self-organization processes in the heteroepitaxial growth of SrRuO3 (SRO) and CoCr2O4 (CCO) films. The surface of SRO films on SrTiO3(001) substrates can show extremely smooth terraces (2D objects) separated by atomic steps, a structure of faceted islands (0D objects), a cross-hatch morphology (1D objects), an array of finger-like units (1D objects), or an array of giant bunched steps (1D objects). The surface can be tailored to a particular structure by controlling the vicinality of the substrate and the growth rate and nominal thickness of the film. In the case of CCO films, grown on (001)-oriented MgAl2O4 or MgO substrates, high aspect ratio {111}-faceted pyramids and hut clusters (0D objects), highly oriented and having a similar size, appear above a critical thickness. The size and spatial density can be tuned by varying deposition temperature, nominal thickness, and substrate. This dependence allows the fabrication of surfaces being fully faceted (2D objects), or having arrays of dislocated pyramids of up to micrometric size, or small coherently lattice strained pyramids having a nanometric size. We discuss the driving forces that originate the peculiar SRO and CCO nanostructures. The findings illustrate that the growth of complex oxides can promote a variety of novel self-organized morphologies, and suggest original strategies to fabricate templates or hybrid structures of oxides combining varied functionalities.

Controlled magnetic anisotropy of SrRuO3 thin films grown on nominally exact SrTiO3(001) substrates

Ferromagnetic SrRuO3 films with controlled in-plane magnetic anisotropy have been deposited on nominally exact (miscut <0.1°) SrTiO3(001) substrates. Films grown on as-received substrates display in-plane uniaxial magnetic anisotropy whereas films grown on treated TiO2-terminated surfaces are magnetically biaxial. It is found that the in-plane magnetic anisotropy is intimately linked to the in-plane crystallographic texture: whereas the former films are single domain, the latter are twinned. The authors show that the different textures are determined by the growth mechanisms, step flow or layer by layer, which in turn are critically determined by the substrate surface conditions.