Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
Nb2O3 structure, a surprising result because Nb2O3 is not known as bulk
material. It can be concluded that our growth method is not restricted
to the growth of vanadiumoxide.
[1] H. Niehus et al., Phys. Stat. Sol. a 187 (2001) 151
O 5.6 Mo 12:30 H44
Growth of a GaSe half-sheet on Si(111) — •Bengt Jaeckel,
Rainer Fritsche, Andreas Klein, and Wolfram Jaegermann
— Darmstadt University of Technology, Surface Science Division, Department
of Materials Science, Petersenstr. 23, 64287 Darmstadt
The evolution of the Si(111):GaSe surface termination is investigated
with scanning tunneling microscopy (STM), low energy electron diffraction
(LEED) and SXPS. GaSe is evaporated from an effusion cell filled
with GaSe compound as source material onto Si(111)-(7×7) surfaces held
at 540 ◦ C substrate temperature. Although both Gallium and Selenium
are exposed during the formation process, the surface passes through different
reconstructions, which are known for Ga-terminated Si(111) surfaces.
The passivated surface is characterized by a three-fold symmetry
of the LEED-pattern, a hexagonal periodicity of the lattice imaged with
atomic resolved STM and flat-band conditions measured with SXPS.
In the beginning a √ 3 × √ 3 R30 ◦ - Ga surface is formed, followed by a
6.3 × 6.3 and a 6.3 √ 3 × 6.3 √ 3 - Ga surface, which corresponds to one
third, 0.7 and 1 monolayer of Gallium. No Si(111)-(1 × 1):GaSe terminated
areas are observed below one third monolayer coverage.
O 5.7 Mo 12:45 H44
STM Studies of ultrathin SixGe1−x films grown on Si(111)
by Chemical Vapour Deposition — •Selvi Gopalakrishnan, H.
Rauscher, and R.J. Behm — Abt. Oberflächenchemie und Katalyse,
Universität Ulm, 89069 Ulm
The influence of terrace width, temperature and gas composition on
the morphology and composition of epitaxial SixGe1−x films grown on
Si(111) by chemical vapour deposition (CVD) was investigated by STM
and XPS. Si2H6 and GeH4 were used as the precursor gases. Investigations
were carried out with two different GeH4:Si2H6 gas ratios, (a) a 7:3
ratio with an effective Ge:Si ratio of about 1:1 and (b) a 9:1 ratio where
the effective ratio of Ge:Si is about 4:1. SiGe layers were deposited at
a constant pressure of 2.5×10 −6 mbar on wafers with two different terrace
widths (average terrace widths of 2000 A and up to 6000 A) and at
deposition temperatures of 750 K and 850 K. At 750 K, hydrogen is completely
desorbed from the Ge surface and at 850 K the rate of hydrogen
desorption from the Si surface is a maximum. The large terrace widths
allow the study of the SiGe CVD process, in particular the nucleation
behaviour, at low pressures and higher temperatures and derive data on
the mobility of the mobile Si and Ge containing species.
O 5.8 Mo 13:00 H44
Vergleich der ab-initio berechneten elektronischen Struktur verschiedener
Stapelfolgen von Co (0001) — •Lilli Sacharow 1 ,
Stefan Heinze 1 , Gustav Bihlmayer 2 , Stefan Blügel 2 , Jens
Wiebe 1 , Markus Morgenstern 1 und Roland Wiesendanger 1 —
1 Institut für Angewandte Physik, Universität Hamburg, Jungiusstrasse
11, D-20355 Hamburg; — 2 Institut für Festkörperforschung, Forschungszentrum
Jülich, D-52425 Jülich
In dieser Arbeit wurden DFT-LDA Berechnungen der elektronischen
Struktur eines 11-Lagen Co-Films in verschiedener
Stapelfolge durchgeführt. Insbesondere wurden im Hinblick auf
Rastertunnelspektroskopie-Messungen der reine hcp-Film (...ABAB) und
der hcp-Film mit einem Stapelfehler an der Oberfläche (...ABAC) untersucht.
Die Zustandsdichte im Vakuum, bei Abständen zur Oberfläche
unter 5 ˚A, ist in beiden Fällen dominiert von einem Minoritätspeak
0.6 eV unterhalb der Fermikante. Bei der Oberfläche mit stapelfehler
ist dieser ca. 20% grösser als bei reinem hcp-Film. Die Verteilung der
Zustände in der Brillouin Zone zeigt, dass er hauptsächlich von einem
Bandminimum zwischen Γ-Punkt und dem Brillouinzonenrand getragen
wird, während der Γ-Punkt selbst nur mit ca 5% beiträgt.
O 6 Struktur und Dynamik reiner Oberflächen
Zeit: Montag 11:15–13:15 Raum: H45
O 6.1 Mo 11:15 H45
Co Subsurface Segregation in CoAl(111) — •Lutz Hammer,
Volker Blum, Christian Schmidt, Ole Wieckhorst, Wolfgang
Meier, Stefan Müller, and Klaus Heinz — Lehrstuhl
für Festköerperphysik, Universität Erlangen-Nürnberg, Staudtstr. 7,
D-91058 Erlangen
The geometrical and chemical structure of the CoAl(111) surface is
investigated by quantitative low-energy electron diffraction (LEED) and
calculations applying density functional theory (DFT). It is found that
the surface assumes an Al-Co-Co-Co layer stacking within the top few
layers and the usual alternating B2 stacking below. The topmost layers
thus form a unit cell of the bcc-based D03 crystal structure (i.e. a A3B
superlattice of bcc(111) atomic planes), despite of the fact that the bulk
phase diagram of CoAl shows no D03 phase. Its stability at the surface is
due to a slight Co excess of the nominally stoichiometric sample, which
is incorporated in the bulk by forming Co antisite defects. These defects
are enriched in undercoordinated surface sites, there replacing Al atoms
which assume fully coordinated sites in the bulk, in effect lowering the
total energy. The process competes with the energetically favourable Al
termination of the surface, so that the Co antisites concentrate in the
still undercoordinated third layer rather than in the top layer, leading to
the observed termination.
O 6.2 Mo 11:30 H45
Multilayer relaxations of the Cu(11{2n+1}) surfaces calculated
with the FLAPW method — •Juarez L. F. Da Silva, Kurt
Schroeder, and Stefan Blügel — Institut für Festkörperforschung,
Forschungszentrum Jülich, D-52425 Jülich, Germany
We report a systematic first-principles study of the multilayer relaxation
of the stepped Cu(11{2n + 1}) surfaces, for 2n + 1 = 3, 5, 7, and
9. Our calculations are based on density-functional theory, employing
the all-electron full-potential linearized augmented plane wave (FLAPW)
method. We found that Cu(113), Cu(115), Cu(117), and Cu(119) have
a multilayer relaxation sequence represented by, − + − · · ·, − − + − · · ·,
− − −+− · · ·, and − − − −+−· · ·, respectively, where − and + indicate
an interlayer distance contraction and expansion, respectively. Thus, our
FLAPW calculations confirm the trend obtained by low-energy electron
diffraction (LEED) intensity analysis for Cu(113), Cu(115), and Cu(117),
i.e., in a stepped metal surface with n + 1 atom rows in the terraces the
first n interlayer spacings contract, the n + 1 interlayer spacing expands.
However, for Cu(115), our result is in disagreement with those obtained
by surface x-ray diffraction, which obtained −+−−· · ·. Furthermore, we
found in general a good agreement with the magnitude of the interlayer
relaxations and by LEED.
O 6.3 Mo 11:45 H45
X-ray reflectivity study of the aqueous solution-gas interface —
•Michael Paulus, Michael Sprung, Christian Gutt, and Metin
Tolan — Experimentelle Physik I, Universität Dortmund, 44221 Dortmund
Interfacial properties of aqueous salt solutions are important in many
biological and geological systems.e.g. for the reactivity of sea salt aerosols
in the marine boundary layer.
In the past a large number of theoretical work has been performed to
study the liquid - gas interface of aqueous alkali halide electrolyte solutions.
Molecular dynamics simulations revealed a partial ion enrichement
of alkali halides solution close to the liquid-gas interface. This finding is
in contrast to expectations based on classical electrostatic calculations.
It has been reported that this effect is most prominent if sodium iodide is
used as the electrolyte. The expected thickness of the ion enriched region
is of the order of 1nm.
We report on x-ray reflectivity experiments of aqueous solutions of
sodium iodine of different concentrations. The experiments provided for
the first time a detailed picture of the electron density profile of aqueous
salt solutions close to the liquid-gas interface. In contrast to the results
of molecular dynamic simulations, we observed for all investigated concentrations
a water enriched region of thickness 1-2 nm at the surface.