Fabrication and Characterization of nanostructured Films

Thin metal films find wide application in material science and information technology. They serve as coatings or electric conductors, magnetic storage media, chemical sensors or catalysts. It is, therefore, of great importance to master and carefully control the fabrication of such films and to understand their physical and chemical properties. These properties, however, are intimately related to the structure and morphology of the films. Particular interest has recently been devoted to the fabrication of nanostructured films, i.e., structures with lateral dimensions of a few nanometers only. These nanostructures promise a miniaturization far beyond today`s standards, e.g., in electronic devices. In addition, due to the extremely small size, the physical and chemical behaviour of nanostructures can be fundamentally different from those of the bulk material. For instance, electron confinement and other "quantum-effects" can lead to new, distinct electronic, optical or magnetic properties. Similarly, finite- size effects and the enhanced density of structural imperfections can modify dramatically the adsorption process and the catalytic activity, making nanostructured metal films interesting for application as catalysts or chemical sensors. It is the aim of this proposal to characterize the atomic processes involved in the nucleation and growth of thin metal films by using well defined nanostructured template surfaces. More specifically, the substrates to be used are the "herringbone" reconstructed Au(111) surface and the Cu-O stripe phase consisting of a periodic array of alternating Cu-O and Cu stripes self-assembled upon oxygen adsorption on the clean Cu(110) surface. The nucleation and growth of various metals such as Co, Ni and Fe will be studied as a function of the growth parameters (namely surface temperature and deposition rate) and the atomic and chemical structure of the resulting nanostructured films will be characterized. The experiments will involve (i) an existing variable temperature scanning tunnelling microscope capable of providing the necessary structural and electronic information at the atomic scale. (ii) Auger Electron Spectroscopy and Thermal Desorption Spectroscopy for chemical analysis and (iii) Low Energy Electron Diffraction to characterize the growth mode and the morphology of the films.

Thin metal films, composed of several layers of a magnetic material such as cobalt or iron, deposited on a non- magnetic substrate like copper, exhibit unexpected and technologically relevant magnetic properties. For instance, magnetic multilayer devices based on the so-called `giant magneto-resistance` are currently used as position and magnetic field sensors as well as read heads of computer hard disks. The magnetic properties of such magnetic multilayer systems crucially depend on the quality and thickness of the magnetic film. Within FWF-Project P-13351 the growth of cobalt films on differently prepared copper surfaces was studied at the atomic scale using scanning tunneling microscopy (STM). In these studies, we could demonstrate that completely covering the copper surface with oxygen leads to a nearly perfect growth of atomically flat cobalt films. Previous studies had shown that on the clean copper surface and without the help of oxygen, cobalt would growth as a rough film, which turns out to deteriorate the desired magnetic properties. In the presence of oxygen, the surface tension of the cobalt film is reduced and the film completely wets the copper surface, whereas without the `surfactant` action of oxygen the cobalt forms hillocks and the film becomes rough. The oxygen, which initially is deposited on the copper substrate, `floats` to the surface of the growing cobalt film and induces an almost ideal `layer-by-layer` growth. A smaller, partial oxygen coverage on the copper surface, however, leads to a completely different growth scenario: at first a regular array of alternating stripes of oxygen covered (CuO) and copper (Cu) areas is formed spontaneously on the copper surface. This arrangement is strictly periodic and the characteristic stripe widths are in the range of a few nanometer, only. On the so prepared Cu-CuO stripe surface, cobalt preferentially grows on the CuO stripes. As a result, the thickness of a thin cobalt film is always larger above the CuO stripes than on the neighboring clean Cu stripes. Although the oxygen floats to the top of the growing cobalt film, the periodic modulation of the film thickness persists even in thicker cobalt films. The initial stripe pattern of the substrate is thus `replicated` onto the cobalt film and laterally modulated cobalt films can be fabricated in this way. Such nanostructured magnetic layers should exhibit peculiar magnetic properties which can now be investigated systematically.