Plenarvorträge - DPG-Tagungen

Halbleiterphysik Donnerstag
appears to be too small to be measured directly and is determined indirectly
from magneto-optical spectroscopy. Curie temperatures up to 3K
have been observed for single quantum wells with Mn content up to 4%
and with hole densities between few 10 10 and 6·10 11 cm −2 . In both cases,
the experimental Curie temperatures are in good agreement with mean
field predictions. Finally, we will conclude this presentation by showing
our recent work on MBE growth and characterization of high gap diluted
magnetic semiconductor epilayers GaMnN and ZnCoO.
HL 35.3 Do 11:15 H15
ZnO-MOVPE: Present State and Prospective Applications in
Optoelectronics and Magnetoelectronics — •Andreas Waag —
Institut für Halbleitertechnik, TU Braunschweig
Due to its band gap of 3.3 eV and exciton binding energy of 60 meV,
ZnO is a very interesting candidate for UV optoelectronics. Beyond that,
the incorporation of magnetic impurities allows to fabricate semimagnetic
semiconductors, which are transparent and ferromagnetic at room
temperature. The combination with n-type doping makes magnetic ZnO
very interesting for spinelectronics. Here, we give an overview on our activities
concerning the MOVPE growth of ZnO-related thins films and
heterostructures. After a thourough optimisation of the growth process,
ZnMgO-ZnO quantum well structures could be fabricated, indicating a
good optical quality. The incorporation of Cd, however, suffers from phase
separation, and two distinctly different Cd concentrations have been detected
in ZnCdO films. In addition, the self-organised growth of ZnO
nanorods could be demonstrated by a suitable choice of MOVPE growth
parameters. Nanorods of 30 nm diameter and a height of up to 5 µm
could be achieved. The optical quality of these nanorods is very good,
indicating that open lying surfaces play a minor role in terms of nonradiative
recombination. The incorporation of magnetic ions has been
achieved, both into normal layers as well as nanrods, and ferromagnetic
behaviour has been detected.
HL 35.4 Do 11:45 H15
Disorder effects in diluted magnetic semiconductors —
•Carsten Timm — Institut für Theoretische Physik, Freie Universität
Berlin, Arnimallee 14, 14195 Berlin
Diluted magnetic semiconductors (DMS) are promising materials for
technological applications as well as interesting from the basic-physics
point of view. In the last years it became clear that disorder plays a
crucial role in DMS due to the presence of many charged defects and
the random positions of impurity spins. In this talk the effect of disorder
on transport properties and magnetism in DMS is discussed. It is
argued that the impurity positions are partially correlated. These correlations
are crucial for the understanding of magnetic properties and
the observed metal-insulator transition. In the final part of the talk, a
calculation of the RKKY-like interaction between impurity spins is presented,
starting from a realistic band structure. This interaction is found
to be highly anisotropic, which, together with the disorder, leads to magnetic
frustration and might explain the incomplete magnetization at low
temperatures.
HL 35.5 Do 12:15 H15
Hydrogen-control of ferromagnetism in Ga1−xMnxAs thin
films — •Sebastian T. B. Goennenwein 1,2 , Hans Huebl 1 ,
Thomas A. Wassner 1 , Christoph Bihler 1 , Martin Stutzmann 1 ,
Achim Koeder 3 , Wladimir Schoch 3 , Andreas Waag 3 , and
Martin S. Brandt 1 — 1 Walter Schottky Institut, Technische Universität
München, Am Coulombwall 3, D-85748 Garching, Germany —
2 Department of Nanoscience, Delft University of Technology, Lorentzweg
1, 2628 CJ Delft, The Netherlands — 3 Abteilung Halbleiterphysik,
Universität Ulm, D-89069 Ulm, Germany
Dilute magnetic semiconductors (DMS) are investigated vigorously at
present, as these materials combine magnetic ordering with the versatile
properties of semiconductors. In the prototype DMS Ga1−xMnxAs, which
we discuss here, the ferromagnetic exchange interaction between the lo-
calized Mn magnetic moments is mediated by itinerant holes, so that the
density of both Mn moments and of holes is crucial for the magnetism.
We show that upon exposure to a remote DC deuterium plasma, the hole
density p in Ga1−xMnxAs thin films with x = 0.037 and x = 0.051 can
be reduced by several orders of magnitude. At the same time, the density
of Mn magnetic moments is not significantly affected by the plasma
treatment. Nevertheless, the ferromagnetism clearly present in the asgrown
samples has vanished after the deuteration. We analyze the effect
of the deuteration in detail, comparing the results of secondary ion mass
spectroscopy, DC magnetization, and Fourier-transform infra-red optical
experiments. We also address possible applications of this post-growth
”hydrogen-control” of the magnetic properties of Ga1−xMnxAs.
HL 35.6 Do 12:45 H15
Modified magnetic properties of ordered arrays of (II,Mn)VI
quantum wires due to reduced lateral dimensions — •P.J.
Klar 1 , L. Chen 1 , W. Heimbrodt 1 , F.J. Brieler 2 , M. Fröba 2 , T.
Kurz 3 , H.-A. Krug von Nidda 3 , and A. Loidl 3 — 1 Dept. Physics
and Material Sciences Center, Philipps-University of Marburg, Germany
— 2 Institute of Inorganic and Analytical Chemistry, Justus-Liebig University
of Gießen, Germany — 3 Dept. Physics, University of Augsburg,
Germany
We present a novel way of synthesising highly ordered arrays of
(II,Mn)VI quantum wires with lateral dimensions of 3nm to 9nm and
Mn-contents x between 0% and 100% by incorporating the semiconductor
into mesoporous SiO2 matrices. Quantum confinement increases the
band gap by about 200meV for Cd1−xMnxS and by about 150meV for
Zn1−xMnxS in the narrowest wires. A stronger p−d-hybridisation related
band-gap bowing as a function of x occurs in the (Cd,Mn)S wires compared
to bulk. The magnetic properties also change significantly. Compared
to the bulk (II,Mn)VI compounds, a reduced antiferromagnetic
coupling between the magnetic moments of the Mn 2+ ions is found. For
x ≥ 0.8, a full suppression of the paramagnetic to antiferromagnetic
phase transition of the Mn-system is observed for the 3nm wires. For
6nm and 9nm wires a phase transition occurs, but the Néel temperatures
are smaller than in bulk. The observed behavior yields information
about the length scale of the antiferromagnetic ordering. Similar effects
are also anticipated for ferromagnetic wires.
HL 35.7 Do 13:15 H15
GaN-based semiconductor compounds for spin electronics —
•Bernd Beschoten — 2. Physikalisches Institut, RWTH Aachen, Templergraben
55, 52056 Aachen
According to recent predictions of room temperature ferromagnetism
in Mn-doped GaN, the material might be promising for spintronic applications.
In the first part, the talk will focus on the material science aspects
and magnetic properties of Mn-doped GaN layers grown by molecular
beam epitaxy. Samples with low Mn content are homogeneous alloys
showing spin-glass like behavior at low temperatures, but they are NOT
ferromagnetic. Samples with higher Mn content additionally exhibit ferromagnetism
with Curie temperatures of about 750 K, which originates
from nm-scale Mn-rich clusters formed during growth.
The second part of the talk is devoted to the study of spin coherence
and spin dephasing in n-type GaN, which is another important aspect
for GaN-based spintronics. Despite densities of charged threading dislocations
of 5 ×10 8 cm −2 , this coherence studied by time-resolved Faraday
rotation yielding lifetimes of ∼ 20 ns at T = 5 K, and persists to room
temperature. The doping dependence of spin dephasing is compared to
n-GaAs, suggesting a common origin for spin relaxation in these systems.
Work done in collaboration with D.D. Awschalom, O. Brandt, L.
Däweritz, S. Dhar, A. Dohmen, K.J. Friedland, G. Güntherodt, E. Hu, E.
Johnston-Halperin, J. Keller, K.H. Ploog, and A. Trampert. Supported
by the German Federal Ministry of Education and Research (BMBF).
[1] S. Dhar et al., Appl. Phys. Lett. 82, 2077 (2003).
[2] B. Beschoten et al., Phys. Rev. B 63, 121202 (2001).