Plenarvorträge - DPG-Tagungen

Halbleiterphysik Freitag
die Energiedifferenz zwischen X 2− – und X 3− –Rekombination zu 2,6 meV
bestimmt werden.
HL 50.10 Fr 13:15 H14
Wavelength selective carrier storage in self-organized quantum
dots — •Till Warming 1 , F. Guffarth 1 , R. Heitz 1 , M. Geller 1 ,
P. Brunkov 2 , V.M. Ustinov 2 , and D. Bimberg 1 — 1 Institut für
Festkörperphysik, Technische Universität Berlin, — 2 A.F.Ioffe Physico-
Technical Institute RAS, 194021 St-Petersburg, Russia
Wavelength selective carrier storage in self-organized InAs/GaAs quantum
dots (QDs) is investigated. The small homogeneous broadening of
HL 51 Si/Ge
the exciton transition of a single quantum dot is in high contrast to the
large inhomogeneous broadening of the ground state transition energy
of an ensemble of self-organized QDs. This benefit enables in addition
to the spatially addressing wavelength parallel data storage in future
memory devices. Here we analyze the data storage process in two-color
photocurrent experiments and investigate the dependencies of the electric
field, the temperature and the laser power. This work was funded by the
Nanomat project of the European Commission Growth Programme, contract
number G5RD-CT-2001- 00545, Intas project 2001-774, and SFB
296 of DFG.
Zeit: Freitag 11:30–12:15 Raum: H17
HL 51.1 Fr 11:30 H17
Selective wet chemical etching of Ge islands grown on Si(001)
— •Georgios Katsaros, Carlos Manzano, Giovanni Costantini,
Alexander Bittner, Ulrich Denker, Mathieu Stoffel,
Oliver Schmidt, and Klaus Kern — Max Planck Institut für
Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany
Although Ge quantum dots on Si(001) have been thoroughly studied,
relatively little is known about their composition which, on the other
hand, is a very important factor in determining their optical and electronic
properties. We tackle this problem by means of selective chemical
etching in which Ge is removed preferentially over Si (or vice versa). By
quantitatively knowing the selectivity of the etchant, the atomic force
microscopy images of the structures that remain after the etching of a
quantum dot sample can be interpreted in terms of a spatially resolved
map of the quantum dot composition. The results obtained by applying
this technique to pyramid and dome islands show that the composition
of both types of dots is highly anisotropic. This can be ascribed to the
kinetic pathway that is responsible for the dot formation, growth and
transformation (pyramid-to- dome transition). For every possible application
in a device the Ge quantum dots have to be overgrown with Si. By
applying selective etching of Si over Ge we were able to remove the Si cap
and to get information on the intermixing and the shape changes that
take place during capping as well as on the composition of the remaining
dots. Finally, using the quantum dots as an etching-resistant ”mask” we
could produce interesting free- standing Ge-rich nanostructures.
HL 51.2 Fr 11:45 H17
Metal-Induced Crystallization of Silicon-Germanium Alloys —
•Mario Gjukic, Michael Buschbeck, Robert Lechner, Jens
Lübke, and Martin Stutzmann — Walter Schottky Institut, Technische
Universität München, Am Coulombwall 3, D-85747 Garching
The fabrication of large-grained polycrystalline silicon and silicongermanium
(poly-SiGe) thin films is increasingly important for largearea
electronic devices such as flat panel displays or thin film solar
cells. Fundamental for both applications is the use of cheap and transparent
substrates, such as glass, requiring low temperature processing
steps (T < 500 ◦ C). One method which is known to lead to poly-Si layers
with good structural and electronic properties is the ALuminum-
Induced Layer Exchange (ALILE) process. Here, a bilayer structure of
HL 52 SiC II
aluminum (Al) and amorphous silicon (a-Si) is deposited on a glass substrate
and annealed below the eutectic temperature of the binary Al-Si
system (T < 577 ◦ C). If the layers are separated by a thin oxide film, for
example aluminum oxide or silicon oxide, the annealing process results
in a complete layer exchange (Al and Si change positions) and the formation
of a coherent poly-Si film. Here, we report the successful realization
of poly-SiGe layers over the entire composition range on glass substrates
via the ALILE process. We have investigated if the ALILE process is
also applicable to binary semiconductors such as SiGe or if significant
phase segregation occurs. In addition, optical and electronic properties
of the resulting poly-SiGe layers have been studied. The possibility to
use the obtained alloy films as seed layers for thin film solar cells will be
discussed.
HL 51.3 Fr 12:00 H17
Thermal Conductivity of Isotopically Enriched 28 Si: Revisited
— •R. K. Kremer 1 , M. Cardona 1 , H.-J. Pohl 2 , G. G. Devyatych
3 , and P. G. Sennikov 3 — 1 MPI für Festkörperforschung,
Heisenbergstr. 1, D-70569 Stuttgart, Germany — 2 VITCON Projectconsult
GmbH, Otto-Schott-Str. 13, D-07745 Jena, — 3 Institute of Chemistry
of Highly-Pure Substances, Russian Academy
The thermal conductivity of isotopically enriched 28 Si recently has attracted
particular attention because of a claim of a 60% higher roomtemperature
thermal conductivity of 28 Si as compared to that of Si with
a natural isotope mixture nat Si [1]. It was argued, however, that this result
cannot be reconciled with theoretical estimates which give, at most,
a 20% increase. Because of the potential technological importance of a
significantly larger thermal conductivity of isotopically pure samples of
Si we have, with a steady-state heat-flow technique, redetermined the
thermal conductivity of the previously measured samples and new single
crystal samples of 28 Si and nat Si between 10K and 320K. To estimate
and reduce the disturbing influence of thermal radiation losses at elevated
temperature we have particularly taken care to utilize samples with identical
geometrical dimensions. Close to room-temperature we consistently
find an increase in the thermal conductivity of 28 Si with respect of that
of nat Si of about 10 ± 2 %, whereas the values of this enhancement below
100K are close to that reported in ref. [1].
[1] T. Ruf et al., Solid State Commun., 115, 243 (2000).
Zeit: Freitag 12:15–13:00 Raum: H17
HL 52.1 Fr 12:15 H17
Wird Coimplantation von Donatoren (P, N) mit Si oder C in
4H-SiC dominiert vom Site-competition-Effekt oder der Erzeugung
elektrisch neutraler Defekte? — •Frank Schmid und Gerhard
Pensl — Institut für Angewandte Physik, Universität Erlangen-
Nürnberg
Durch vergleichende Hall-Effekt Messungen an Phosphor
(P)/Stickstoff (N) implantierten und Kohlenstoff (C) oder Silizium (Si)
coimplantierten p-Typ 4H-SiC Epischichten wurde das Ausheilverhalten
von P- bzw. N-Donatoren untersucht. Bei den P- und C/P-implantierten
Proben konnte eine identische elektrische Aktivierung der P Atome
festgestellt werden, während sich bei der Si/P-implantierten Probe die
Konzentration der P-Donatoren um 40% verringerte. Damit ist gezeigt,
dass das Ausheilverhalten der P-implantierten Proben durch den Sitecompetition-Effekt
dominiert wird. Im Vergleich zur N-implantierten
Probe, zeigt die C/N-implantierte Probe eine Reduktion (40%) der elektrisch
aktiven N Atome, was möglicherweise auf den Site-competition-
Effekt zurückgeführt werden kann. Jedoch zeigt die Si/N-implantierte
Probe eindeutig eine starke Verringerung der freien Elektronenkonzentration
(75%) und der Konzentration der Kompensation (20%). Als Ursache
wird die Bildung von thermisch stabilen und elektrisch neutralen
VSi(N)4-Defektkomplexen vorgeschlagen.