|Former activities performed in SFB
- Co Films on Cu(001) Surfaces:
Non-Arrhenius Growth Behavior, Reentrant Layer Growth, and Floating Cu Layers
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1. Experimental method: Grazing scattering of fast atoms
A beam of 25 keV He atoms is scattered under a polar angle of 1°-2° to the Cu(001) surface plane and an azimuthal angle of a few degrees to the  surface lattice direction. Growth-related irregularities like nucleated islands perturb the correlated scattering process (surface channeling), which results in a loss of the specular-beam intensity.
Interpretation is straightforward and based on classical mechanics computer simulations emulating trajectories of scattered atoms. The method works in real-time and real-space and yields quantitative, statistical information on the morphology of the surface of the growing film.
The specular-beam intensity is measured as function of Co coverage for
constant temperature and deposition rate F=4,5±0,3·10-3
ML/s. The chemical composition of films is deduced from electron- and
proton-induced Cu MVV (60eV) and Cu LMM (920eV) Auger-intensities.
2. N-shaped dependence of island density on growth temperature
Fig. 2.1 We observe a unique N-shaped dependence of the density
nx of nucleated islands (circles). This is at variance with
an Arrhenius-behavior, as expected from classical nucleation theory (nx%
exp (E / kT). This complex behavior is also proposed by ab-initio DFT-kMC
calculations (lines) and explained as follows (R. Pentcheva et al., Phys.
Rev. Lett., submitted):
3. Bilayer growth and reentrant layer growth
4. Floating Cu layers
Ab initio DFT calculations on the ground state of Co/Cu(001) propose a capping of Co films with Cu (R. Pentcheva and M. Scheffler, Phys. Rev. B 61(2000)2211). In order to deduce depth-dependent composition profiles, we measured Cu MVV (60 eV) and LMM (920 eV) Auger signals, induced by keV electrons at oblique incidence or 25 keV protons at grazing incidence (Fig. 4.2). Excitation by grazingly scattered protons results in a sensitivity which is restricted to the film surface (topmost layer), whereas the information depth in conventional, i.e., electron-induced Auger spectroscopy amounts to about 2ML for the MVV Auger line and 10ML for the LMM line. Assuming perfect layer-by-layer growth, we find that Co films grown at high temperatures (410 K) are capped by 2 layers of Cu (Fig. 4.3). Growth at low temperatures (140 K) results in pure Co films, i.e., interlayer mass transport between Co and Cu is suppressed.
Fig.4.1 Proton-induced Cu MVV Auger signal during deposition of 4ML Co
on Cu(001) at 410 K. After cooling down to 210 K, deposition of additional
6ML Co onto this film.
Fig. 4.2 Electron- and proton induced MVV (60eV) and LMM (920eV)
Auger signals of Cu during deposition of Co on Cu(001) at different temperatures.
Solid lines in Fig. 4.2 show calculated Auger signals for pure Co films (140 K), Co films capped with 2 layers of Cu (Fig. 4.3), and a hypothetical c(2x2) CoCu alloy throughout the film.
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