Plenarvorträge - DPG-Tagungen

Halbleiterphysik Freitag
HL 50.4 Fr 11:45 H14
TE- and TM-polarization resolved spectroscopy under normal
incidence — •M. Schardt 1 , C. Dotzler 1 , S. Malzer 1 , J.
Müller 2 , G. Rurimo 2 , S. Quabis 2 , G. Leuchs 2 , and G.H. Döhler 1
— 1 Institut für Technische Physik I — 2 Institut für Optik, Universität
Erlangen-Nürnberg, Germany
Normally, optical experiments performed under normal incidence on
semiconductor structures with broken symmetry regarding the direction
perpendicular to the surface (quantum wells and quantum dots, e.g.)
yield information only about transitions involving in-plane (px- and py-
) components of the hole wave functions, because of the in-plane (TE)
polarization of the light. Transitions sensitive to the pz components are
interacting only with TM-polarized light. Recently it has been demonstrated
that a radially polarized laser beam focused through a microscope
objective with a high numerical aperture (NA ≥ 0.9) is perfectly polarized
along the optical axis in its focus. The light is mostly TM polarized
within the whole focus area. So far, experimental evidence of this feature
has been limited to an indirect proof by comparing the resulting spot size
and shape with the theoretical predictions [1]. We are now presenting an
approach allowing for a direct proof of the TM polarization. It is based
on photo current studies of heavy- and light-hole excitonic absorption in
quantum wells and self-assembled dots embedded in pin diodes. At the
same time, this approach represents a novel technique for polarization
resolved spectroscopy.
[1] S. Quabis et al., Optics Communications 179 (2000) 1-7
HL 50.5 Fr 12:00 H14
Influence of nitrogen containing barrier layers on the emission
wavelength of InAs/GaAs quantum dots — •Oliver Schumann,
Lutz Geelhaar, and Henning Riechert — Infineon Technologies
AG, Corporate Research Photonics, 81730 München
We study the effect of varying the matrix material on the emission
wavelength of InAs QDs. The addition of different combinations of nitrogen
and indium into the matrix allows to tune the confining potential of
the QDs and the local and global stress.
Samples have been grown by solid-source MBE assisted by a RF
plasma source for nitrogen incorporation. They were examined by PL
spectroscopy and TEM. We used a self-consistent Schroedinger-Poisson
solver as a simple simulation tool for the changing confinement.
While growing the QDs on a GaAsN layer almost does not affect the
PL wavelength, one can see a red shift of the emission wavelength greater
than 100 nm after introducing nitrogen in the capping layer above the
QDs. In this way an emission wavelength beyond 1.3 µm can easily be
achieved. This tremendous red shift is mainly attributed to the change
in the confining potential. By comparing QDs with quantum wells and
samples with different concentrations of indium and/or nitrogen in the
barrier layers, the role of stress with respect to the emission wavelength
will also be discussed.
HL 50.6 Fr 12:15 H14
Blinking and Bleaching of Individual Silicon Nanocrystals and
Nanocrystal Ensembles — •Jörg Martin 1,2 , Frank Cichos 1 und
Christian von Borczyskowski 2 — 1 Institut für Physik 123705, TU
Chemnitz, 09107 Chemnitz — 2 Institut für Physik 122501, TU Chemnitz,
09107 Chemnitz
We present for the first time detailed studies of individual silicon nanocrystals
by confocal microscopy. Individual nanocrystals obey narrow
emission spectra (150 meV) in the range between 500 nm and 600 nm.
The emission of single nanocrystals shows a strong intermittency (so
called blinking), which is the consequence of an electron tunneling to
surrounding trap states. The blinking process itself obeys a power law
statistics, which is equivalent to a non-stationary behavior. Indeed we
can show, that characteristic times of the blinking, especially the mean
lifetime of dark periods during the blinking are a function of the observation
time. This non-stationarity shows up as a bleaching of the ensemble,
which we can fully explain by the blinking statistics. In addition, an excitation
intensity dependence of the blinking statistics is observed. This
intensity dependence is the result of different tunneling mechanisms, such
as Auger assisted tunneling, which becomes active or inactive at different
excitation intensities. A further consequence of the charge tunneling to
trap states in the environment is a delayed luminescence of nanocrystal
ensembles, which results from the return of charges from the traps to the
nanocrystals conduction band. This delayed luminescence fits well to the
observed blinking behavior of individual nanocrystals.
HL 50.7 Fr 12:30 H14
How individual are individual CdSe nanocrystals? A single
nanocrystal study. — •Frank Cichos 1 , Abey Issac 2 , Thomas
Blaudeck 1 , and Christian von Borczyskowski 2 — 1 Institut für
Physik 123705, TU Chemnitz, 09107 Chemnitz — 2 Institut für Physik
121501, TU Chemnitz, 09107 Chemnitz
The control of the emission intermittency (”blinking”) of single semiconductor
nanocrystals is one of the challenging tasks to create nanounits
i.e. for optoelectronic applications. Such a control has to be based
on the understanding of the blinking process itself, which is currently
only poorly understood. For this purpose we study the blinking process
of individual ZnS capped CdSe nanocrystals over long time periods
to compare the behavior of different nanocrystals. While fluorescent dye
molecules commonly show a very specific blinking statistics, which differs
from molecule to molecule due to the local environment of the molecule,
all studied nanocrystals obey the same blinking statistics within the measurement
accuracy. Further, a change of the surrounding matrix does
not influence the statistics of the blinking process. Therefore, the processes
behind the blinking have to be hierarchical processes. We propose
that one of these processes, which is responsible for the dark periods in
the emission time traces of individual nanocrystals is a charge diffusion
through trap states in the direct vicinity of the nanocrystal. A more detailed
analysis of the blinking further reveals that the processes which
limit the emitting (on) and non-emitting (off) periods of the nanocrystal
are partly decoupled, since on and off periods follow different statistical
laws.
HL 50.8 Fr 12:45 H14
Femtosecond nonlinear spectroscopy of two quantum dots
coupled by dipole-dipole interaction — •Kerstin Müller 1 ,
Thomas Unold 1 , Christoph Lienau 1 , Thomas Elsaesser 1 , and
Andreas D. Wieck 2 — 1 Max-Born-Institut für Nichtlineare Optik
und Kurzzeitspektroskopie, 12489 Berlin — 2 Ruhr-Universität Bochum,
D-44870 Bochum
The transient optical nonlinearities of single semiconductor quantum
dots (QDs) are intensely investigated as QDs receive attention for implementations
of quantum logic. For realizing potentially scalable quantum
gates it is essential to demonstrate coherent interactions between different
quantum dots, e.g. via dipole-dipole coupling[1]. Here, we report the
first study of the transient optical nonlinearities of two coupled interface
quantum dots. With a 2-ps pump laser, we excite the excitonic resonance
of a first QD (QD 1) and monitor the pump-induced biexcitonic
nonlinearity of this QD with an ultrafast probe laser in a near-field spectrometer[2].
Varying the intensity of the pump laser, pronounced Rabi
oscillations are observed. We simultaneously monitor also the excitonic
nonlinearities of one neighboring QD within the 200 nm spot size of the
near-field experiment. We find a clear pump-induced spectral shift of the
exciton resonance of QD 2 due to the dipole-dipole interaction between
both QDs. The observation of Rabi oscillations on the second quantum
dot after resonant excitation of QD 1 unambiguously establishes dipoledipole
coupling between two individual quantum dots.
[1] E. Biolatti et al., Phys. Rev. Lett. 85, 5647 (2000).
[2] T. Guenther et al., Phys. Rev. Lett. 89 057401 (2002).
HL 50.9 Fr 13:00 H14
Optische Spektroskopie an selbstorganisierten InAs–Quantenpunkten
— •Stephan Lüttjohann 1 , Cedrik Meier 1 , Axel
Lorke 1 und Dirk Reuter 2 — 1 Laboratorium für Festkörperphysik,
Universität Duisburg–Essen, Lotharstraße 1, 47048 Duisburg —
2 Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstraße
150, 44780 Bochum
Die Photolumineszenz (PL) von selbstorganisierten InAs–Quantenpunkten,
eingebettet in eine Feldeffekttransistor–Struktur (MISFET),
wird in Abhängigkeit der angelegten Spannung und der Anregungsleistung
untersucht. Bei moderaten Anregungsleistungen (P ≤ 2mW/cm 2 )
können dabei gleichzeitig Kapazitäts–Spannungs-Spektren (CV) aufgenommen
und so die Anzahl der Elektronen pro Quantenpunkt bestimmt
werden. Es zeigt sich, dass mit zunehmender Anregungsleistung die CV–
Spektren zu kleineren Gatespannungen verschieben. Dies lässt sich auf eine
Löcherakkumulation an der GaAs/AlGaAs–Grenzfläche der MISFET–
Struktur zurückführen. Die bei mittlerer Anregungsleistung auftauchende
Rekombination aus p–Zuständen ist damit nicht auf Pauli–Blocking
zurückzuführen, sondern hängt mit der Verschiebung der Gatespannungsskala
zusammen, die durch die zunehmende Grenzflächenladung verursacht
wird. Darüber hinaus kann aus der gatespannungsabhängigen PL