Plenarvorträge - DPG-Tagungen

Halbleiterphysik Montag
ture with an electron density of about 10 11 cm −2 and a high mobility of
∼ 10 6 cm 2 V −1 s −1 . A high reflectance AlGaAs/AlAs Bragg mirror below
the 2DEG together with the reflecting top layer of the sample constitute
a cavity around the optical active quantum well. Simulations of the
expected optical Faraday-signal in reflection geometry exhibit a signal enhancement
of almost one order of magnitude due to the presence of the
mirror. Using a Hall bar, we observe the integer quantum-Hall-effect and
Shubnikov-de Haas-oscillations in a magnetic field up to 8 T at a temperature
of 1.8 K. Time-resolved photoluminescence (PL) experiments using
a streak camera system yield electron and hole spin dynamics inside the
quantum well. Simultaneous PL and transport measurements at different
filling factors will be presented.
HL 12.48 Mo 16:30 Poster A
Terahertz Photoleitungs- und Transmissionsspektroskopie an
tunnelgekoppelten doppellagigen zweidimensionalen Elektronengasen
— •L. H. Böttger, S. Holland, C.-M. Hu, Ch. Heyn
und D. Heitmann — Institut für Angewandte Physik, Universität Hamburg
Es wurden spektral aufgelöste Photoleitungs- und Transmissionsmessungen
im Terahertzbereich an einem tunnelgekoppelten GaAs/AlGaAs
Doppelquantentopf durchgeführt. Auf die Probe wurde ein Mäander-
Hallbar mit einer Breite von 50µm und einer Länge von etwa 13cm
präpariert. Ein Gitterkoppler aus Gold mit einer Periode von 1µm bedeckt
den Hallbar, so dass die Goldstreifen senkrecht zum Rand über
den Kanal laufen. Der Versuchsaufbau bestand aus einem Fourierspektrometer,
das mit einem Waveguide mit einem Magnet-Kryostaten verbunden
war. Die Probe wurde in Magnetfeldern von 0 bis 12T bei einer
Temperatur von 1.8K gemessen. Es wurde sowohl in Photoleitung
als auch in Transmission die Zyklotronresonanz und ein Magnetoplasmon
über nahezu den gesamten Magnetfeldbereich beobachtet. Ein Vergleich
der Moden-Energien aus beiden Messmethoden zeigte eine gute
Übereinstimmung bei ganzzahligen Füllfaktoren (ν=2,3,...). Zwischen
den Füllfaktoren jedoch verschob sich die Zyklotronresonanz in den Photoleitungsmessungen
zu höheren Energien während die Magnetoplasmonmode
stark gedämpft wurde.
HL 12.49 Mo 16:30 Poster A
Breakdown of the quantum Hall effect in samples with intentionally
introduced defects — •Dorina Diaconescu, Sascha
Hoch, Dirk Reuter, and Andreas Wieck — Lehrstuhl für Angewandte
Festkörperphysik, Ruhr-Universität Bochum, Germany
We study the role of electron scattering on the onset of the quantum
Hall effect breakdown in 2-dimensional electron systems with intentionally
introduced defects. The defects are Ga ions or vacancies in the
crystal lattice introduced by focused ion beam (FIB) implantation. The
defects can be introduced lateral homogenously, over the whole area of
the Hall-bar, or with spatial resolution. The dependence of the breakdown
current as a function of defect density was investigated in samples
with a lateral homogenous implantation. For low implantation doses, the
critical current increases with the ion dose. Further increase of the ion
dose leads to a reduction of the critical current. The results confirm the
work of Nachtwei et al. [1], where antidot arrays have been patterned on
the GaAs/AlxGa1−xAs heterostucture. The initial increase of the critical
current with the ion dose can be explained by an increased inelastic
scattering rate at the defects. It results a more effective cooling of the
hot electrons. Higher defect densities increase the impurity-assisted inter-
Landau level transitions, and a significant decrease of the critical current
is observed.
[1] G.Nachtwei, G.Lütjering, D.Weiss, Z.H.Liu, K.von.Klitzing, and
C.T.Foxton, Phys.Rev.B.55, 6731 (1996).
HL 12.50 Mo 16:30 Poster A
Strong increase of the electron effective mass in GaAs incorporating
boron and indium — •Tino Hofmann 1 , Claas von
Middendorff 1 , Gunnar Leibiger 2 , Volker Gottschalch 2 , and
Mathias Schubert 1 — 1 Fakultät für Physik und Geowissenschaften,
Institut für Experimentelle Physik II, Universität Leipzig, Linnéstraße
5, 04103 Leipzig — 2 Fakultät für Chemie und Mineralogie, Halbleiterchemie,
Universität Leipzig, Linnéstraße 3, 04103 Leipzig
The strain-free boron- and indium-containing GaAs compounds are
promising candidates for novel III-V semiconductor solar cell absorber
materials with lattice match to GaAs, for which experimental data of the
electronic band structure are widely unknown. We employ far-infrared
magnetooptic ellipsometry to determine the electron effective mass in
non-degenerate, silicon-doped, n-type B0.03In0.06Ga0.91As with a electron
concentration ranging from 2·10 17 to ∼ 1 · 10 18 cm −3 and observe a strong
increase of the electron effective mass of 44% in B0.03In0.06Ga0.91As compared
to In0.06Ga0.94As. Further we report on the vibrational lattice mode
behavior of this quaternary alloy. For BAs, an experimentally obscure
compound, the curvature of the same conduction band extrapolates to
the free electron mass.
HL 12.51 Mo 16:30 Poster A
Optical spectroscopy of perovskitic oxynitrides — •M.
Güngerich 1 , P.J. Klar 1 , W. Heimbrodt 1 , A. Jayakar 2 ,
A. Weidenkaff 2 , S. Ebbinghaus 2 , and A. Reller 2 — 1 FB
Physik und WZMW, Philipps-Universität, Marburg — 2 Lehrstuhl für
Festkörperchemie, Universität Augsburg, Augsburg
Substitution of O by N in mixed metal oxide perovskites are possible
candidates for novel colour pigments. Several different oxynitrides with
ABO3 perovskite structure have been synthesized. Raman spectroscopy
was employed to investigate structural changes taking place when annealing
in air or nitrogen atmosphere. For samples annealed in air, Raman
signals from different types of intermediate states occur in the transition
from the oxynitrides to the corresponding oxides depending on the
type of cations in the crystal. In all the investigated compounds N2 entities
in the crystal lattices were identified by a sharp Raman feature
at 2330cm −1 after heating at temperatures ranging from 500 to 800 ◦ C.
Raman spectra of the untreated and of the partially annealed oxynitrides
show significantly broader signals than the oxides after annealing
at approx. 900 ◦ C. A possible explanation is that the crystalline order is
strongly increased after the full transition of the oxynitrides to the oxides.
Reflectance measurements of single crystals were performed to gain
insight into the correlations between band structure changes and visible
colour changes during the transition from oxides to oxynitrides.
HL 12.52 Mo 16:30 Poster A
Controlled fabrication of well-aligned single-crystalline ZnO
nanowires — •Hongjin Fan, Florian M. Kolb, Roland Scholz,
Margit Zacharias, and Ulrich Gösele — Max-Planck-Institute of
Microstructure Physics, Weinberg 2, 06120 Halle, Germany
Fabrication of ZnO nanowires with controlled morphology, size, and
orientation is essential for their applications in room-temperature UV
nanolasers. By using a vapor transport and condensation process based
on vapor-liquid-solid (VLS) mechanism, ZnO nanowire arrays were successfully
synthesized on sapphire substrates. Electron microscopy investigations
show that the nanowires are aligned mainly vertical to the
sapphire substrate, having heights up to 8 µm and diameters of 30–
150 nm. The nanowires were identified to be single-crystalline hexagonal
wurtzite ZnO and unidirectioned along 〈0001〉. They are structurally uniform
without observable dislocations or an amorphous sheath. In order
to tune the diameter and length of the nanowires, the thickness of the
catalyst Au film and growth time were varied respectively. In addition,
when the Au particles on the substrate were predefined, the position
and density of the ZnO nanowire arrays can be subsequently controlled.
Room-temperature photoluminescence measurements show a sharp UV
emission peak near the band gap of ZnO at 3.36 eV, and a very broad
green emission associated with ionized oxygen vacancy defects.
HL 12.53 Mo 16:30 Poster A
Influence of Bi doping upon the phase change characteristics
of Ge2Sb2Te5 — •Ke Wang, Daniel Wamwangi, and Matthias
Wuttig — I. Physikalisches Institut IA, RWTH Aachen, 52056 Aachen
Ge2Sb2Te5 is a material which is commonly used in rewritable optical
data storage. In this recording technology, the Ge-Sb-Te based alloys have
been successfully developed as commercial products for PD and DVD-
RAM applications due to their relatively short erasing time (30 100ns
for Ge-Sb-Te). To ensure the future success of phase change recording
the data transfer rate needs to be improved further. To achieve this goal
faster materials are required. To this end doping of Ge2Sb2Te5 has been
suggested. Here we report on the influence of Bi doping on the speed
of recrystallization (erasing) process, which is the time limiting step in
optical recording. These experiments were performed by using both a
static tester and an atomic force microscope. The evolution of bit topography
and optical reflectivity under the irradiation of the erasing laser
with different pulse duration is presented. For this Bi doped Ge2Sb2Te5
film, ultrafast erasing is demonstrated and a second amorphization phenomenon
is found during the erasing process. This behavior is related to
structural properties and the activation barrier for the structure transi-