Plenarvorträge - DPG-Tagungen

Halbleiterphysik Mittwoch
This work was supported by the DFG, SFB 602 TP A7
HL 33.3 Mi 18:45 H14
Annealing of InN layers in a MOVPE reactor investigated by insitu
Spectroscopic Ellipsometry. — •Christoph Werner, Massimo
Drago, Torsten Schmidtling, Udo W. Pohl, and Wolfgang
Richter — TU-Berlin, Institut für Festkörperphysik, Hardenbergstr.
36, 10623 Berlin
Spectroscopic Ellipsometry (SE) is a powerful optical in-situ analysis
tool for growth monitoring of non-cubic materials, as already demonstrated
for GaN epitaxy [1].
Therefore we use SE in order to study annealing of epitaxial InN layers
grown under different gaseous ambients (NH3, N2 and H2) in the growth
temperature regime.
It was found that a hydrogen ambient during annealing leads to a
surface roughening. In contrast, pure nitrogen induces surface smoothening
for temperatures above 540 ◦ C, while roughening becomes dominant
at temperatures exceeding 580 ◦ C. A small addition of ammonia to the
N2 ambient, however, induces surface roughening already at 550 ◦ C. This
shows that, in contrast to GaN, ammonia is not suitable to stabilize InN
at growth temperature. This might be due to the presence of hydrogen
as a consequence of ammonia decomposition.
[1] S. Peters, T. Schmidtling, T. Trepk, U. W. Pohl, J.-T. Zettler, W.
Richter, J. Appl. Phys. 88, 4085 (2000)
HL 33.4 Mi 19:00 H14
MOVPE in-situ monitoring and analysis of substrate deoxidation
of GaSb(100) — •Kristof Möller 1 , Lars Töben 1 , Zadig
Kollonitsch 1 , Christoph Giesen 2 , Michael Heuken 2 , Frank
Willig 1 , and Thomas Hannappel 1 — 1 Hahn-Meitner-Institut, SE-
4, Glienicker Str. 100, D-14109 Berlin, Germany — 2 AIXTRON AG,
Kackertstr. 15-17, D-52072 Aachen, Germany
The crucial deoxidation of GaSb prior to epitaxy was investigated insitu
with reflectance anisotropy spectroscopy (RAS). Substrates were annealed
in an AIX-200 MOVPE reactor in the presence of triethylanti-
HL 34 Hauptvortrag Kouwenhoven
mony in H2 ambient. Deoxidation was carried out between 475 − 575 ◦ C.
RAS signals show the progress in deoxidation for different chemical pretreatments,
annealing temperatures, and annealing times. ”Epi-ready”
substrates annealed without any chemical pre-treatment showed the formation
of anisotropic, nanometer-scaled 3D-structures during deoxidation.
The anisotropic roughness was observed in-situ as an unusual increase
of the RAS signal. This enhanced RAS signal vanished after deoxidation
was completed. RAS Signals showed the absence of roughness
for chemically pre-treated substrates. The surface of the substrates was
investigated with UPS and XPS spectroscopy. The RAS-correlated information
on electronic structure and chemical surface composition is essential
for understanding and improving the deoxidation process of GaSb.
HL 33.5 Mi 19:15 H14
Exzitonenlokalisation in InGaN-Schichten — •T. Finger, J.
Christen, Th. Hempel, A. Dadgar und A. Krost — Institut für
Experimentelle Physik, Otto-von-Guericke Universität Magdeburg
Legierungsunordnung und Entmischung verursachen im ternären In-
GaN mikroskopische Kompositionsfluktuationen, die zu lokalen Potentialfluktuationen
führen. Diese Unordnungseffekte erzeugen ein charakteristisches
Temperaturverhalten der Rekombinationsenergie das von dem
üblichen Eg(T)-Verlauf (z.B. Varshni) signifikant abweicht. Mit sinkender
Temperatur T tritt zunächst eine Rotverschiebung um −σ 2 /kBT auf.
In Folge des Ausfrierens der Ladungsträger bei weiter sinkendem T erfolgt
dann eine Blauverschiebung. Es ergibt sich somit eine charakteristische
s-förmige T-Abhängigkeit der bandkantennahen Lumineszenz. Bei
Erhöhung der Anregungsleistung wird ebenfalls eine Blauverschiebung
beobachtet, die mit einem Auffüllen der statistischen Energieverteilung
erklärt werden kann. Wir haben an 100 nm dicken MOCVD-gewachsenen
InGaN-Epitaxieschichten unterschiedlichen In-Gehaltes (21% - 47%)
Temperatur- und Anregungsdichte-abhängige Lumineszenzmessungen
durchgeführt. Es traten starke Intensitätsmodulationen der Spektren
durch Fabry-Perot Interferenzen in dem Luft+InGaN/GaN (1,2 µm)+Si-
Substrat Schichtpaket auf.
Zeit: Donnerstag 09:30–10:15 Raum: H15
Hauptvortrag HL 34.1 Do 09:30 H15
Spin Qubits with Quantum Dots — •L. P. Kouwenhoven —
Technische Universiteit Delft
Qubits are 2-level systems suitable for control over the level occupation
including superpositions. In solid state, two qubit examples are the effective
2-level system in appropriately designed superconducting devices
and single-electron spin states in semiconductor quantum dots. Qubit
control is demonstrated with NMR-type pulse techniques such as Rabi
flops and spin-echo. The readout of a qubit state is an important part of
the qubit circuit since any backaction can shorten the coherence lifetime.
We discuss a recent realization of a single-shot readout of the spin of a
single electron.
HL 35 Symposium Ferromagnetische Halbleiter
Zeit: Donnerstag 10:15–13:45 Raum: H15
HL 35.1 Do 10:15 H15
Magnetic Anisotropy in Ferromagnetic III-Mn-V Semiconductors:
Issues and Observations — •Jacek K. Furdyna — University
of Notre Dame, Notre Dame, Indiana, USA
Magnetic anisotropy in III-V-based ferromagnetic (FM) semiconductor
alloys (e.g., Ga1−xMnxAs) is interesting from two points of view. First,
it can be “engineered” by imposing appropriate strain conditions on the
FM film; and second, magnetic anisotropy is expected to play an essential
role in designing spin-injection devices based on these materials.
In this paper we present the information on MA in FM semiconductors
obtained through a series of complementary methods: ferromagnetic
resonance (FMR), planar Hall effect, direct imaging of FM domains by
magneto-optical methods, and SQUID magnetization data. Special attention
will be given to the recently-observed re-orientation of the easy
axis in several of the III1−xMnxV alloys as a function of temperature
and/or illumination, and to the somewhat surprising uniaxial anisotropy
in the sample plane which some of these materials exhibit, presumably
as a consequence of surface reconstruction during the growth.
HL 35.2 Do 10:45 H15
Carrier induced ferromagnetism in p type doped II-VI heterostructures
— •David Ferrand — CEA-CNRS-UJF Joint Group,
Laboratoire de Spectrometrie Physique, BP 87 38402 Saint Martin
d’Heres, France
Carrier-induced ferromagnetism in diluted magnetic semiconductors
(D.M.S) has been reported in IV-VI bulk alloys, III-V epilayers and in
II-VI heterostructures. In comparison to III-V, the Curie temperatures
are dramatically lower in II-VI DMS, but II-VI compounds are ideal materials
for fundamental studies, in which localized spins and holes can be
introduced and controlled independently and where dimensionality effects
can be examined in modulation doped heterostructures. We will present
here an overview of our experimental studies, we have done on degenerate
Zn(1−x)MnxTe thick layers and modulation-doped Cd(1−x)MnxTe quantum
wells. Zn(1−x)MnxTe with Mn content up to several percents can be
easily doped p-type using a nitrogen cell up to 10 20 cm −3 , two orders of
magnitude larger than the density for metal-insulator transition. While
the usual antiferromagnetic behavior is observed on the susceptibility of
undoped layers, we observed (SQUID and anomalous Hall effect) positive
Curie-Weiss temperatures between 1.5 and 3K and characteristic ferromagnetic
hysteresis cycles, for layers with hole densities above a few 10 19
cm −3 . In the case of Cd(1−x)MnxTe quantum wells, the magnetic signal