Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
I bzw. Typ II-Stufen [1] verglichen werden kann.
Neben Tripelstufen werden auch Makrostufen gefunden. Diese Makrostufen
verlaufen ebenfalls in -Richtungen. Makrostufen, die aus
Tripelstufen des Typs I entstanden sind, sind erheblich steiler als solche,
die von Stufen des Typs II herrühren. Dies lässt sich durch eine attraktive
Wechselwirkung zwischen Tripelstufen des Typs I bei Stufenabständen <
1 nm erklären [1].
[1] Puchin et al., J. Phys.: Cond. Matt. 13 (2001) 2081
O 14.40 Mo 18:00 Bereich C
Au/Si(111): Local ordering of the (5 × 2) reconstruction — •C.
Seifert, F.-J. Meyer-zu Heringdorf, and M. Horn-von Hoegen
— Inst. für Laser- und Plasmaphysik, Universität Duisburg-Essen
(Campus Essen)
At Au coverages ΘAu < 1/3ML and substrate temperatures above
300 ◦ C, Si(111) surfaces exhibit a striped (5×2) reconstruction, consisting
of three different domains with an angle of 60 ◦ between them. Upon deposition
of Au, each of the domains forms needles that grow predominantly
in the 2-fold direction of the reconstruction. The resulting anisotropy is
so dominating, that if one domain encounters another one with a different
rotational orientation, the needle-like domain keeps growing, and can
erode the other domains (1) or even completely intersect them (2). Here
we discuss a combined High Resolution Low Energy Electron Diffraction
(SPA-LEED) and Scanning Tunneling Microscopy (STM) experiment to
understand the details of the (5 × 2) reconstruction. The (5 × 2) LEED
pattern can be understood as a (5 × 1) pattern with elongated (5 × 2)
spots that form stripes located at 50% Brillouin-Zone (BZ). The detailed
shape of the stripes depends on the sample preparation temperature.
Room temperature STM measurements combined with fourier analysis
and simple simulations of the adatom arrangement indicate a local ordering
of the gold adatoms.
[1] T. Hasegawa, S. Hosaka, S. Hosoki Surf. Sc. 357-358 (1996), 858-862
[2] F.-J. Meyer zu Heringdorf, R.M. Tromp, unpublished
O 14.41 Mo 18:00 Bereich C
GIXRD-measurements and -simulation of CaF2 on Si(111)
— •Andreas Gerdes 1 , Carsten Deiter 1 , Eddy Patrick
Rugeramigabo 1 , and Joachim Wollschläger 2 — 1 Institut für
Festkörperphysik, Universität Hannover, Appelstr. 2 — 2 Institut für
physikalische Chemie und Festkörperphysik, Universität Bremen, im
Nord-Westen-Bremens NW2
Due to the small lattice mismatch at room temperature CaF2 is a
good candidate to study growth processes on Si(111). Further the system
semiconductor/insulator promises interesting features for electronic
devices on the nanoscale. We grew CaF2 films of different thickness at
T = 500 ◦ C - 600 ◦ C and investigated them by means of GIXRD. Intensity
oscillations due to the finite CaF2 films thickness show that the films
are very homogeneous.
O 14.42 Mo 18:00 Bereich C
GIXRD and AFM Investigations of CaF2/Si Multilayers —
•Carsten Deiter 1 , Andreas Gerdes 1 , Eddy Patrick Rugeramigabo
1 , Joachim Wollschläger 2 , Cunrang Wang 3 , Bernd
Müller 3 , and Karl Hofmann 3 — 1 Institut für Festkörperphysik,
Universität Hannover, D-30167 Hannover — 2 Institut für Angewandte
und Physikalische Chemie, Universität Bremen, D-28359 Bremen
— 3 Institut für Halbleiterbauelemente und Werkstoffe, Universität
Hannover, D-30167 Hannover
Semiconductor/insulator multilayers with few nanometer thick films
offer the opportunity to fabricate nanoelectronic devices with very interesting
features. Due to small lattice mismatch the system CaF2/Si(111)
is a good combination for growth of films with high morphological and
electronical quality.
We have investigated Si/CaF2/Si(111) structures by means of GIXRD
and AFM. Si films (thickness 4nm) and clusters are deposited on CaF2
(thickness 3nm) at temperatures between 800K and 1000K with and
whithout deposition of additional Sb as a surfactant.
O 14.43 Mo 18:00 Bereich C
Growth and morphology of Co films on Pd(111) — •M. Przybylski,
M. Wasniowska, W. Wulfhekel, and J. Kirschner —
Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle
Ultrathin Co films were grown on an atomically clean and flat surfaces
of Pd(111). Scanning Tunneling Microscopy (STM) shows that at 300K
the growth starts by a formation of double-layer patches of a hexagonal
shape. The Moiré pattern due to the 8.7% lattice mismatch of Co and
Pd was clearly seen. At 170K, mobility of Co atoms is strongly reduced
resulting in a more dense packing of the double-layer patches. At the coverage
of ≈2ML of Co grown at 170K almost the whole surface (95%) is
covered with the double-layer patches, in comparison to only 70% if the
growth proceeds at 300K. In the latter case higher layers nucleate when
the patches approach the size of 50nm. After annealing at 420K, the STM
image shows drastically changed morphology of the film. A monolayer of
Co appears which is continuous and covers 98% of the sample area. This
is associated with changes of the LEED pattern: after annealing only
single spots of hexagonal symmetry, typical for Pd(111), are seen with
no superstructure spots, which were clearly visible before annealing.
O 14.44 Mo 18:00 Bereich C
X-ray study of the surface dynamics at the charge density
wave transition in NbSe2 — •Bridget Murphy 1 , Jochim Stettner
1 , Martin Müller 1 , Ingo Grotkopp 1 , Peter Boesecke 2 ,
Till H. Metzger 2 , Michael Krisch 2 , Herwig Requardt 2 , and
Werner Press 1,3 — 1 Institut für Experimentelle und Angewandte
Physik, Christian-Albrechts-Universität Kiel, Leibnizstraße 19, 24098
Kiel — 2 ESRF, BP 220, 38043 Grenoble Cedex 9, France — 3 Institut
Laue-Langevin, BP 156, 38043 Grenoble Cedex 9, France
2H-NbSe2 is a van der Waals bonded layered structure and displays a
Peierls distortion resulting in a charge density wave transition at 33K.
We investigated this transition using grazing incidence X-ray diffraction
and inelastic X-ray scattering. The elastically scattered intensity of the
satellite reflection, which is proportional to the square of the amplitude
of the modulation, as well as the respective diffuse scattering have been
measured for different incidence angles in order to carry out a direct comparison
between the surface and bulk behaviour. We found that the temperature
dependence of the surface charge density wave satellite clearly
varies from that in the bulk and that the surface transition appears to be
continuous. Moreover, the surface CDW transition occurs 1.4 K above
that of the bulk. We conclude that we likely observe the unusual case
of a ’surface transition’. In order to improve the understanding of the
dynamics phonon dispersion curves at room temperature and around the
phase transition have been determined by inelastic X-ray scattering.
O 14.45 Mo 18:00 Bereich C
Die Allotropie von Antimon und die Ostwaldsche Stufenregel —
•B. Stegemann 1,2 , T. M. Bernhardt 2 , B. Kaiser 1 und K. Rademann
1 — 1 Institut für Chemie, Humboldt-Universität zu Berlin, Brook-
Taylor-Str. 2, D-12489 Berlin — 2 Institut für Experimentalphysik, Freie
Universität Berlin, Arnimallee 14, D-14195 Berlin
Das Element Antimon existiert in mehreren festen Modifikationen (Allotropie).
Eine bisher unbekannte, feste Antimonmodifikation, bestehend
aus periodisch angeordneten, undissoziierten Sb4-Clustern, konnte durch
kontrollierte Abscheidung von Sb4-Clustern aus der Gasphase auf geeigneten
Substraten erzeugt und mittels Rastertunnelmikroskopie nachgewiesen
werden. Diese Modifikation weist einen metastabilen Charakter
auf und wandelt sich bei Erreichen einer kritischen Größe in die amorphe
Phase um. In Abhängigkeit von der Bedeckung und der Temperatur
führt dann ein weiterer Phasenübergang zur Ausbildung der energetisch
günstigsten, kristallinen Antimonmodifikation.
Die stufenweisen Umwandlungen dieser allotropen Modifikationen sind
irreversibel (monotrop) und können mit der Ostwaldschen Stufenregel
erklärt werden, gemäß der ein in mehreren energetischen Zuständen vorkommendes
chemisches System nicht direkt in den thermodynamisch stabilen
Gleichgewichtszustand übergeht, sondern stufenweise über metastabile
Zwischenzustände dorthin gelangt.
O 14.46 Mo 18:00 Bereich C
Orientational and positional phase transitions in geometric confined
systems of alkanes - a numerical study — •Frank Oliver
Pfeiffer und Heiko Rieger — Theoretische Physik, Universität des
Saarlandes, 66041 Saarbrücken, Deutschland
A classical phenomenological atom-atom potential is used to study alkane
chains in a single 2D hexagonal pore via the Monte Carlo method.
We find different phases characterized by orientational and positional order
and discuss the influence of (1) the surface structure of the pore wall
and (2) the pore diameter on the correlation functions. Our calculations
for the confinement are compared to the bulk scenario and to recent
experiments on n-alkanes in nano-pores by Huber and Knorr.