Verlag des Forschungszentrums Jülich
JUEL-3614
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 350K, only three-dimensional growth is observed. Both
deposition and annealing above 200K leads to the formation of a disordered
Co-Cu surface alloy due to the much lower surface free energy of copper. The
surface structure remains fcc(110) up to at least 15 monolayers (ML).
The adsorption of 0.5ML oxygen prior to Co deposition leads, at 350K, to
layer-by-layer growth of the Co film up to at least 22ML. This is obvious
from oscillations of the specular helium intensity during growth. AES
experiments show that during Co deposition the oxygen is floating 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 (1x2) reconstruction
of 1ML Co and a (3x1) reconstruction of thicker Co films. The
reconstruction appears to stabilize the observed layer-by-layer growth. Other
(nx1) reconstructions of the O terminated Co film are accessible
through a variation of the O coverage: O coverages above 0.5ML can be
prepared by additionally exposing the surface to oxygen both
during and after the deposition process. In this way the
"optimum" O coverage for the morphological order of the films as well as for
the intensity and persistence of the growth oscillations could be determined to
be 2/3, corresponding to two oxygen atoms per unit cell of the (3x1)
reconstruction. The films prepared with this method are thermally stable up to
500K. The oxygen can be reacted away by exposure to atomic hydrogen. This
leads to a clean, atomically-flat 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) (~22 x [Wurzel aus]3) "herringbone" reconstruction network. At 0.7ML, the
islands coalesce forming one-dimensional Co quantum wires which coalesce in
turn at 1.6ML. Between 3 and 5ML, a quasi-two-dimensional growth mode is
observed. Co deposition at 100K 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 room 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
surface morphology at different preparation conditions has been correlated to
the dynamics of the system.
Tölkes, Christian
Wachstumsarchitektur ultradünner Kobaltfilme: Struktur, Dynamik, Reaktivität
150 S., 1998
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 the growth 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 on 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.
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