Plenarvorträge - DPG-Tagungen

Oberflächenphysik Dienstag
ter mobility. The results of our calculations are discussed in comparison
with experimental observations for materials like Platinum, Silver and
Copper.
O 25.2 Di 16:00 H39
Nucleation of Sub-monolayer YBa2Cu3O7−δ Films on
SrTiO3(001) Probed by X-ray Standing Waves and XPS —
•Sebastian Thiess, Tien-Lin Lee, Bruce C. C. Cowie, and Jörg
Zegenhagen — ESRF, Grenoble, France
Before epitaxial film growth commences, nucleation of complex materials
such as YBCO is necessary. The first layer then acts as a seed for the
growth of the epitaxial film. Investigating the nucleation can shed some
light on the relationship between the growth mode and the structure of
such epitaxial systems.
Scanning tunneling microscopy (STM) and spectroscopy (STS) have
suggested that submonolayers of YBCO deposited on STO(001) surfaces
first nucleate as a cubic, semiconducting phase of a mixed perovskite
composition, which in a second step transforms into the orthorhombic,
metallic YBCO phase when the film thickness exceeds one monolayer.
We obtained complementary real space information by the x-ray standing
wave technique that uses an interference field typically generated by
the superposition of two x-ray waves during Bragg reflection. By monitoring
the photoabsorption process via photoemission and fluorescence, we
measured the atomic structure of two YBCO films, 0.5 and 1.0 monolayers
thick, grown in situ onto STO(001) surfaces by PLD. Measurements
were carried out in UHV at beamline ID32 at the ESRF for a variety
of different STO(hkl) reflections at photon energies between 2.7 and 5.5
keV. Element-specific crystallograhic distributions were obtained for the
constituent atoms of the films by direct Fourier transform.
O 25.3 Di 16:15 H39
Beobachtung mesoskopischer Strukturen bei Mn-Schichten auf
Si(111) mittels STM und LEED — •K. Schwinge, J.J. Paggel
und P. Fumagalli — Institut für Experimentalphysik, Freie Universität
Berlin, Arnimallee 14, 14195 Berlin
Vorgestellt werden Strukturuntersuchungen von unterschiedlich dicken
Mn-Schichten auf Si(111)-Substraten. Die Schichten mit Dicken von
2-100 ˚A werden unter UHV-Bedingungen bei Raumtemperatur aufgedampft
und anschließend auf verschiedene Temperaturen aufgeheizt. Die
Struktur der Oberfläche wird mittels Rastertunnelmikroskopie (STM)
betrachtet. Weitere Charakterisierungen der Proben erfolgt mittels SPA-
LEED, RHEED, AES und AFM.
Beim Erwärmen der Proben bildet sich Mangansilizid mit einer Struktur
an der Oberfläche, die einer ( √ 3× √ 3)-Überstruktur auf dem Si(111)
entspricht. Sie ist bei Schichtdicken von ca. 10 ˚A mesoskopisch strukturiert
mit einer Periodizität von ca. 20 nm, was auf Verspannungen
zurückzuführen ist. Dieses ist mittels STM, AFM und auch LEED zu
beobachten. Weiterhin existieren Löcher, die tiefer sind als es der aufgedampften
Mn-Schichtdicke entspricht.
Diese Arbeit wurde unterstützt durch die Deutsche Forschungsgemeinschaft
im Rahmen des SFB 290.
O 25.4 Di 16:30 H39
Bare surface structures and quantum dots growth on the
GaAs(315)B surface — •Takayuki Suzuki, Yevgeniy Temko,
Mingchun Xu, and Karl Jacobi — Fritz-Haber-Institut der
Max-Planch-Gesellshaft, Faradayweg 4-6, 14195 Berlin
Surface structures of the GaAs(315)B surface were investigated under
Ga- and As-rich conditions. The Ga-rich surface is not flat on atomic
scale, but exhibits a very anisotropic surface morphology. Narrow stripes
of 1×1 structure extend along [12-1]. The steps between the stripes often
bunch together thus creating {101} facets. The As-rich surface is also not
flat, but facets into vicinal (5 2 11)B surfaces with steps along the [13-
1]proj. and [-321]proj.. {101} facets form on the sidewall of the steps along
the [13-1]proj.. The GaAs(315)B surface becomes flat by adsorbing InAs
with molecular beam epitaxy, and exhibits a c(2×2) reconstruction. Further
adsorption induces quantum dots (QDs) formation. The QDs shape
is given by {101}, {111}, and {2 5 11}A bounding facets. The size distribution
of the QDs is quite broad, with the length at the foot ranging from
15 to 80 nm. Many QDs exhibit signs of coalescence. Stacking-faults and
screw dislocations penetrating the QDs are directly detected with atomic
resolution. Correlation between broad size distribution and incorporation
of lattice defects are demonstrated experimentally for the first time.
O 25.5 Di 16:45 H39
Epitaxial growth of Ag on W(110) studied by Reflectance Difference
Spectroscopy — •L. D. Sun 1 , M. Hohage 1 , P. Zeppenfeld
1 , C. Deisl 2 , and E. Bertel 2 — 1 Institute of Experimental
Physics, Johannes-Kepler University Linz — 2 Institute of Physical Chemistry,
University of Innsbruck
The epitaxial growth of Ag on W(110) at room temperature has been
studied by Reflectance Difference Spectroscopy (RDS). The first two Ag
layers follow a layer-by-layer growth and show a distorted Ag(111) structure.
The anisotropic electronic transitions in the Ag film contribute substantially
to the optical anisotropy and can thus be used to monitor the
growth process. The evolution of the RDS spectrum strongly depends
on the Ag coverage. In particular, the RDS intensity at 4.7 eV increases
monotonically during the formation of the first Ag layer, whereas the
signal at 4.2 eV responds to the growth of the second layer. When the
thickness of the Ag film exceeds 3 monolayers, the RDS spectrum shows
a derivative like feature around a photon energy of 4 eV which can be
attributed to the onset of the bulk d-band transition of Ag. This observation
suggests the formation of a Ag bulk band structure at this growth
stage and thus is consistent with the growth of 3D islands on top of the
bilayer. These results are in good agreement with previous studies on
the crystallographic [1, 2] and electronic structure [3] of the Ag/W(110)
system. [1] E. Bauer et al., J. Appl. Phys. 48 (1977) 3773 [2] Y. Yang et
al., Surf. Sci. 276 (1992) 341. [3] A. Elbe et al., Surf. Sci. 397 (1998) 346
O 25.6 Di 17:00 H39
Thin TiO2 films and Au/TiO2 on Ru (0001) studied by XPS
and CO-TPD — •Zhong Zhao, H. Rauscher, and R.J. Behm —
University of Ulm
The chemical composition and adsorption behavior of welldefined
TiO2 films on Ru (0001) of up to 20 ML thickness and
Au/TiO2/Ru(0001) model catalysts are characterized by XPS and CO-
TPD. The TiO2 films were grown under UHV conditions, Au was deposited
by evaporation (Tsub =300K). XPS shows that titania grows on
Ru (0001) as either well oxidized TiO2 or reduced TiO2 (mixture of TiO2
and Ti2O3), depending on the preparation method. Neither CO2 production,
nor CO dissociation is observed when CO desorbs from surfaces on
the Au/TiO2/Ru model catalyst (dAu =2-3 nm). CO desorbs completely
at around 275 K from nanometer-sized Au clusters supported on reduced
TiO2. As the Au coverage is increased from 0.3 ML to 2 ML, the peak
temperature shifts from around 219 K to 197 K, equivalent to a decrease
in CO adsorption energy from 60 to 54 kJ/mol (v = 10 14 s −1 ). After
annealing the catalyst to 770 K, the amount of adsorbed CO decreases
significantly; corresponding STM measurements indicate a significant increase
of the Au cluster size. Neither encapsulation of the Au clusters by
reduced titanium oxides nor Au oxidation were observed after annealing.
CO adsorption on the titania film itself does not change significantly in
the presence of Au clusters.
O 25.7 Di 17:15 H39
Elementspezifische Oberflächenrekonstruktion auf Inseln bei
der surfactant-modifizierten Homoepitaxie auf Si(111):As,Sb
— •K. Schroeder 1 , A. Antons 1 , B. Voigtländer 2 , V. Cherepanov
2 und S. Blügel 1 — 1 Institut für Festkörperforschung, —
2 Institut für Schichten und Grenzflächen, Forschungszentrum Jülich, D-
52425 Jülich, Germany
Wir haben das frühe Stadium des surfactant(As, Sb)-modifizierten homoepitaktischen
Wachstums auf Si(111) mit STM und ab initio Rechnungen
untersucht. Die Ergebnisse zeigen ein unterschiedliches mikroskopisches
Verhalten für die beiden surfactants: Auf der As-bedeckten
Si(111)-Oberfläche findet man nur Inseln mit der Höhe einer Doppellage,
die die (1×1) Terrassenstruktur zeigen. Auf Sb-bedecktem Si(111) zeigen
die Inseln 2 verschiedene Oberflächenstrukturen. Am Rand der Inseln
erscheint die (1×1)-Struktur, während in der Mitte die ( √ 3 × √ 3) Terrassenstruktur
gefunden wird. Aus den Rechnungen ergibt sich, dass das
Doppellagenwachstum auf Si(111):As (1×1) schon bei Si-Clustergrößen
von 4 beginnt. Auf Si(111):Sb (1×1) sind Doppellagen energetisch instabil,
während auf Si(111):Sb ( √ 3 × √ 3) eine große kinetische Barriere
von 1 eV für die Nukleation neuer Sb-Trimere in der zweiten Lage das
Doppellagenwachstum kinetisch unterdrückt.