Plenarvorträge - DPG-Tagungen

Halbleiterphysik Donnerstag
tical modes and the resulting emission properties in micropillars with
decreasing diameter. Calculations of the electromagnetic mode structure
are based on two alternative methods: direct solutions of the Lippmann-
Schwinger equation for the electromagnetic vector field, and a generalized
vectorial transfer matrix approach.
[1] M. Benyoucef, S.M. Ulrich, P. Michler, J. Wiersig, F. Jahnke, A.
Forchel, unpublished.
HL 36.8 Do 12:00 H17
Microphotoluminescence studies on CdSe/Zn(S,Se) quantum
dots and InGaN/GaN structures — •H. Lohmeyer, K. Sebald,
J. Gutowski, S. Einfeldt, and D. Hommel — Institut für
Festkörperphysik, Universität Bremen, Otto-Hahn-Allee, 28359 Bremen
The use of quantum dots (QD) as active medium promises laser structures
with superior properties in comparison to conventional quantumwell
lasers, especially with respect to the threshold current. A systematic
optical characterization with high spatial resolution is indispensable for
the realization of such new devices and is possible by means of microphotoluminescence
(µ-PL) measurements. We present µ-PL results for two
material systems emitting in the green and blue spectral range.
To get access to few or even single QDs, mesa structures with diameters
down to 120nm were etched out of the investigated self-organized
grown CdSe/Zn(S,Se)-QD sample. The µ-PL spectra of a 120 nm mesastructure
shows clearly separated excitonic emission lines. Additionally,
these excitonic states were characterized by µ-PL excitation spectroscopy
and time resolved measurements. As a result the spectrally broad optical
phonon replica and the first excited state of the exciton have been
identified. The recombination time of the excitonic state is determined
to 230ps, which is independent of the excitation density.
Furthermore first µ-PL results of InGaN/GaN samples with respect to
the identification of characteristic QD luminescence are discussed.
HL 36.9 Do 12:15 H17
Effects of oxidation on silicon nanocrystallites — •Luis Ramos,
Jürgen Furthmüller, and Friedhelm Bechstedt — FSU Jena -
IFTO Max-Wien-Platz, 1 D-07743 Jena Germany
The luminescence observed in porous silicon (p-Si) is known to be related
to quantum confinement of carriers in the nanostructures formed at
the surface of this material. Experimental observation confirms also that
optical properties depend on the level of oxidation in p-Si. Spherical-like
silicon nanocrystallites (NCs) have attracted special attention and several
investigations have focused in the influence of surface passivation of
Si NCs with H, with group OH, and with few Si=O bond terminations.
However, fully oxidized shells and defects in Si NCs have not been considered
so far.We apply the VASP code with PAW pseudopotentials and
DFT-LDA to study the influence of oxidation on the structural and electronic
properties of Si NCs. The absorption spectra of NCs obtained by
means of effective medium theory are compared to experimental data. To
discuss HOMO-LUMO gaps and excitonic effects we consider in addition
pair excitation energies and gaps calculated in a GW approximation.
HL 36.10 Do 12:30 H17
Dynamik von Energie- und Phasenrelaxationsprozessen in
In(Ga)As/GaAs Quantenpunkten — •Patrick Zimmer 1 , Matthias
Dworzak 1 , Harald Born 2 , Axel Hoffmann 1 , Florian
Guffarth 1 und Dieter Bimberg 1 — 1 Institut für Festkörperphysik,
Technische Universität Berlin, Hardenbergstrasse 36, 10623 Berlin —
2 Georgia State University, Department of Physics and Astronomy,
Atlanta, Georgia GA-30303
Für das Verständnis von Energie- und Phasenrelaxationsprozessen
in Quantenpunktstrukturen sind Messungen mit hoher Zeitauflösung
von zentraler Bedeutung. Hierzu bedient man sich der Zweistrahl-
Messtechnik, deren zeitliche Auflösung ausschliesslich durch die Laserpulsbreite
gegeben ist und im Bereich von wenigen Pikosekunden liegt.
Derartige Messungen an In(Ga)As-Quantenpunkten ergaben bei Anregung
oberhalb der Bandkante von GaAs eine Sättigung des Probe-Signals
in Anwesenheit des Pumpstrahls. Die beobachtete Sättigung ist abhängig
von der zeitlichen Verzögerung von Pump- und Probestrahl. Es wurde
eine Zeitkonstante von etwa 1 ns gemessen, was der strahlenden Lebensdauer
der Exzitonen in diesem System entspricht. Bei resonanter Anregung
des Systems tritt diese Zeitkonstante ebenfalls auf. Zusätzlich wird
eine deutlich kürzere Zeitkonstante im Bereich von etwa 30 ps detektiert.
Ähnliche Zeitkonstanten werden auch in anderen Experimenten beobachtet
und derzeit als Phasenrelaxationszeit der Exzitonen diskutiert.
HL 36.11 Do 12:45 H17
Resonant Raman Scattering in In(Ga)As/GaAs Quantum Dots
— •Alexander Paarmann, Florian Guffarth, Till Warming,
Axel Hoffmann, and Dieter Bimberg — TU Berlin, Institut für
Festkörperphysik, Hardenbergstr. 36, 10629 Berlin, Germany
The local vibrational properties of self-organized quantum dots (QDs)
are still a pending question. We have performed resonantly excited
Raman measurements on self-organized MOCVD and MBE grown
In(Ga)As/GaAs QD, which clearly show characteristics of local optical
phonons.
Compared to bulk LO- and TO-phonon modes an enhanced excitonphonon
coupling and a low-energy-shift when reaching resonance with
the QDs ground-state exciton energy is observed. The local modes depend
on QD shape and local strain-field. Flat and pyramidal QDs were
examined as well as multiple and single QD layers.
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.
HL 36.12 Do 13:00 H17
Time resolved spectroscopy of annealed InAs/GaAs selfassembled
quantum dots — •Cedric Bardot 1 , M. Schwab 1 , M.
Bayer 1 , D. Reuter 2 , and A. D. Wieck 2 — 1 Experimentelle Physik
II, Otto-Hahn Strasse 4, 44221 Dortmund, Germany — 2 Angewandte
Festkoerperphysik, Ruhr-Universitaet Bochum, Universitaetsstr. 150,
44780 Bochum, Germany
Understanding the optical properties of semiconductor quantum dots
is very important due to their high potential interest in pure optical
applications and for quantum computing science as well. Despite many
years of intense study several questions are still debated. Existence of a
phonon bottleneck that would slow down carrier relaxation is still unclear
for instance. Spin relaxation dynamics also needs to be clarified to
understand fully the radiative process of excitons on nanosecond scale.
Subjecting InAs/GaAs quantum dot samples to post-growth rapid
thermal anneals makes it possible to vary the dimension and the confinement
potential of quantum dots and consequently their electronic
structure. The influence of this one on optical properties can then be
analysed.
In this talk we report on measurements of the rise time and radiative
lifetime of photo-excited carriers in InAs/GaAs quantum dots for a series
of high quality annealed samples. Dependence of these two important
quantities with experimental parameters such as temperature, excitation
energy, excitation density and polarization of light is presented and interpreted.
HL 36.13 Do 13:15 H17
Quantum coherences in semiconductor quantum dots — •Hans
Christian Schneider 1 and Weng W. Chow 2 — 1 Fachbereich
Physik, TU Kaiserslautern, Postf. 3049, 67653 Kaiserslautern — 2 Sandia
National Laboratories, Albuquerque, NM 87185-0601, USA
Inversionless gain, electromagnetically induced transparency, refractive
index enhancement and group-velocity reduction are studied for semiconductor
quantum-dot structures under transient conditions. Substantial
deviations from atomic quantum coherence phenomena exist because of
many-body effects. Results on the influence of excitation-induced dephasing
in a semiconductor quantum-dots will also be presented.