Wachstumsarchitektur ultradünner Kobaltfilme: Struktur, Dynamik, Reaktivität

Ultrathin layers of ferromagnetic materials exhibit extraordinary magnetic properties which can also be exploited for technical applications. For an optimization of these properties and a detailed understanding of the underlaying physics, atomic control of thegrowth and structure of these systems is required. On the one hand, there is great interest in growing flat layers for the preparation of superlattices with sharp interfaces. In this context, the effect of oxygen on the growth of cobalt an the copper(110) surface has been studied. On the other hand, the fabrication of self-organized nanostructures (quantum dots or quantum wires) is desirable for the investigation of the magnetic properties of such low-dimensional systems. Therefore, the nanostructuring of ultrathin Co films on the gold(111) surface has been investigated. For the characterization of these systems, thermal energy helium atom scattering (TEAS), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM) have been used. Two new TEAS methods were developed: a refined quantitative analysis of helium interference curves delivers information about the layer distribution whereas the specular helium intensity can probe the composition of the outermost surface layer by adsorption of carbon monoxide. The growth of cobalt on the clean Cu(110) surface has been studied as a function of deposition and annealing temperature. At deposition temperatures between 100 and 350 K, only three-dimensional growth is observed. Both deposition and annealing above200 K leads to the formation of a disordered Co-Cu surface alloy due to the mach lower surface free energy of copper. The surface structure remains fcc(110) up to at least 15 monolayers (ML). The adsorption of 0.5 ML oxygen prior to Co deposition leads, at 350 K, to layer-by-layer growth of the Co film up to at least 22 ML. This is obvious from oscillations of the specular helium intensity during growth. AES experiments Show that during Co deposition the oxygen is fioating on top of the surface thereby minimizing the surface free energy and thus suppressing the Cu segregation onto the Co film. The oxygen induces a (1 x 2) reconstruction of 1 ML Co and a (3 x 1) reconstruction of thicker Co films. The reconstruction appears to stabilize the observed layer-by-layer growth. Other (n x 1) reconstructions of the 0 terminated Co film are accessible through a variation of the 0 coverage: 0 coverages above 0.5 ML can be prepared by additionally exposing the surface to oxygen both during and after the deposition process. In this way the "optimum" 0 coverage for the morphoiogical order of the films as well as for the intensity and persistente of the growth oscillations could be determined to be 2/3, corresponding to two oxygen atoms per unit cell of the (3 x 1) reconstruction. The films prepared with this method are thermally stable up to 500 K. The oxygen can be reacted away by exposure to atomic hydrogen. This leads to a clean, atomically-fiat fcc(110) Co surface that is thermally stable up to 400K. The self organisation of the Co/Au(111) nanostructure proceeds at room temperature via nucleation of bilayer Co islands at the "elbows" of the Au(111) ($\sim$ 22 x $\sqrt{3}$) "herringbone" reconstruction network. At 0.7 ML, the islands coalesce forming one-dimensional Co quantum wires which coalesce in turn at 1.6 ML. Between 3 and 5 ML, a quasi-two-dimensional growth mode is observed. Co deposition at 100 K leads to a far less regular arrangement of the Co islands. Interestingly, this arrangement can not be transformed by annealing at 300K into the one obtained by Co deposition at roorn temperature. Above 300K, gold segregates to the surface both during deposition and annealing of low-temperature grown Co films thereby forming a disordered Co-Au surface alloy which exhibits unusual adsorption properties for CO molecules. The Co quantum dot array prepared by deposition at room temperature remains stable for several hours after transfer to ambient conditions although the Co islands appear to have been oxidized. The surfacemorphology at different preparation conditions has been correlated to the dynamies of the system

Morphology of fcc Co(110) films on Cu(110)

The surface morphology of fee Co(110) films deposited on Cu(110) using oxygen as a surfactant is characterized by He atom scattering. Interference measurements reveal that the thicker Co films are quite flat and that this flatness is preserved after removal of the oxygen with atomic hydrogen. During growth of the first few layers a rough film morphology is observed which is related to the deconstruction of the Cu(110)-(2 x 1)O interface and the formation of a (1 x 2) reconstructed Co-Cu-O phase after deposition of the first Co monolayer. With increasing film thickness the measured step height changes from ( 1.28 +/- 0.02)Angstrom characteristic of the Cu(110) surface to (1.21 +/- 0.02)Angstrom attributed to the interlayer spacing of pseudomorphic fee Co(110). (C) 2000 Elsevier Science B.V. All rights reserved

How to use oxygen and atomic hydrogen to prepare atomically flat fcc Co(110) films

It is shown that atomic hydrogen from a specially designed atomic beam source is well suited for removing chemisorbed oxygen from an fcc Co(110) film that has been grown on a Cu(110) substrate using oxygen as a surfactant. Exposing the oxygen-terminated Co surface to atomic hydrogen leads to a surface reaction which destroys the (3×1) ordered-O induced surface reconstruction of the Co film. Upon annealing at 380 K, the hydrogen remaining on the O-free Co surface can be completely desorbed. With this technique, it is possible for the first time to prepare about 15 monolayers thick, atomically-flat fcc Co(110) films