Plenarvorträge - DPG-Tagungen

Halbleiterphysik Dienstag
HL 15.13 Di 13:15 H17
Tunable mirrorless lasing based on short pitch Ferroelectric Liquid
Crystals — •W. Haase, K. Yoshino, M. Kasano, and M.
Ozaki — Technische Universitaet Darmstadt, Institut fuer Physikalische
Chemie, Petersenstr. 20, 64287 Darmstadt
The helical pitch of Ferroelectric Liquid Crystals can be tuned via
heating or via variyng the strength of the applied external electrical
field. The so called short pitch FLC-materials allowing the selective reflection
of external laser light, hence they are acting as stop band. The
HL 16 GaN: Bauelemente
applied external field can influence the length of the helical pitch by ca.
30 nm. Preparing specific mixtures is giving rise for a broad wavelength
range mirrorless lasing (1,2). Along the talk the fundamentals and the
experimental conditions will be discussed. 1)Electrically tunable waveguide
laser based on ferroelectric liquid crystals, M. Kasano, M. Ozaki, K.
Yoshino, D. Ganzke, W. Haase Appl. Phys. Lett., 82, 4026-4028 (2003). 2)
Electro-Tunable Liquid-Crystal Laser M. Ozaki, M. Kasano, T. Kitasho,
D. Ganzke, W. Haase, K. Yoshino, Adv. Mater., 15, 974-977 (2003)
Zeit: Dienstag 10:15–12:45 Raum: H13
HL 16.1 Di 10:15 H13
Pt/GaN Schottky Diodes for Hydrogen Gas Sensors — •M. Ali 1 ,
V. Cimalla 1 , V. Tilak 2 , D. Merfeld 2 , P. Sandvik 2 und O. Ambacher
1 — 1 Technical University Ilmenau, Center of Micro- and Nanotechnologies,
Gustav-Kirchhoff-Str. 1, D-98693 Ilmenau, Germany —
2 General Electrics, General Electrics Global Research Europe, Wehrle
Strasse 13, D-81675 München, Germany
The performance of Pt/GaN Schottky diodes with different thickness
of the catalytic metal were investigated as hydrogen gas detectors. The
sensors were fabricated based on epitaxial Si-doped Ga-face GaN layer
(ND = 9 × 10 16 cm −3 ) grown up to a thickness of about 3µm by metalorganic
chemical vapour deposition on c-plane sapphire substrates. The
area as well as the thickness of the Pt were varied between 250µm 2 and
1000µm 2 , 80 and 400˚A, respectively. The Pt-Schottky-diodes exhibited at
room temperature ideality factors, saturation currents and barrier heights
between 1.3 and 3.3, 6 × 10 −11 and 1.25 × 10 −7 A and 0.64 and 0.99eV,
respectively. The sensitivity to hydrogen gas was investigated in dependence
on active area, Pt thickness and the operating temperature for 1%
hydrogen in synthetic air. The change in gate voltage of the diode at a
fixed current was monitored as the diode was exposed to hydrogen gas
and for comparison to dry air in order to determine the sensor sensitivity.
We observed a significant increase of sensitivity and decrease of response
time by increasing the temperature of operation to about 350 ◦ C and by
decreasing the Pt thickness down to 80˚A.
HL 16.2 Di 10:30 H13
Modification of GaN and AlGaN Surfaces for Nano- and Picofluidic
Sensors — •G. Kittler, V. Cimalla, V. Lebedev, M.
Fischer, V. Yanev, S. Krischok, J. A. Schaefer, and O. Ambacher
— Technical University Ilmenau, Center for Micro- and Nanotechnologies,
Gustav- Kirchhoff-Str. 1, D-98693 Ilmenau
Because of strong spontaneous polarization and piezoelectric coefficients
of GaN layers and AlGaN/GaN-heterostructures free carrier concentration
profiles inside these materials are very sensitive to any manipulation
of surface charge. This physical effect can be used to develop
novel sensors for ion fluxes, gases and polar liquids. We have used
AlGaN/GaN-heterostructures with polarization induced two dimensional
electron gases in order to determine volume, pH-value, or the polarity of
water based nano- and picoliter droplets. To enable the determination of
physical and chemical properties of such droplets a precise control of the
droplet position on the active areas of the sensors is required. Firstly miscellaneous
procedures of oxidation were used to optimize the wetting of
the thin GaN-cap layers, and secondly thin structured hydrophobic layers
were investigated for this purpose. For the latter method different materials
e.g. silicon nitride, aluminum oxide, and fluorine-carbon-compounds
were examined with respect to their hydrophobic properties. Effective
procedures in controlling the wetting behavior of GaN and AlGaN surface
were proven by Atomic Force Microscopy (AFM), X-ray Photoelectron
Spectroscopy (XPS), and contact-angle-measurements and will be
presented in respect to the results obtained by electrical characterization
of the first nanofluidic sensors.
HL 16.3 Di 10:45 H13
Studies on sub-band gap absorption of AlGaN based solar-blind
photodetectors — •R. Wagner 1 , G. Cherkastinin 1 , V. Lebedev 1 ,
A. T. Winzer 2 , R. Goldhahn 2 , and O. Ambacher 1 — 1 Center of
Micro- and Nanotechnologies, TU Ilmenau, D-98684 Ilmenau, Germany
— 2 Institute of Physics, TU Ilmenau, D-98684 Ilmenau, Germany
AlGaN-based UV-detectors with high responsivities in a narrow spectral
range were grown by molecular beam epitaxy on sapphire substrates.
These devices are based on heterostructure combining three epitaxial Al-
GaN layers with different alloy composition acting as optical filter, insulator
and detector, respectively. Spectrally resolved photocurrent, photothermal
deflection spectroscopy and in situ cathodoluminescence have
been applied to obtain a sub- and near band gap absorption in AlGaN
layers to provide detailed information about defect states in a wide energy
range. By optimizing the alloy compositions and thickness of the
filter layer, a peak responsivity of up to 35 A/W was achieved over a
narrow range of wavelength with a peak position at 276 nm. The rejection
of visible light response with respect to the peak responsivity was
about 2 orders of magnitude. This type of sensors can be designed to
have a highly selective response suitable for UV flame- and bio-sensors.
However, a device performance of AlGaN structures is mainly limited by
a high density of defects responsible for reduced UV/visible contrast and
slow photoresponse. Thus, the detailed knowledge of the defect structure
is necessary for the further performance improvements.
HL 16.4 Di 11:00 H13
Facetten-Alterung von (Al,In)GaN Laserdioden — •Thomas
Schoedl 1 , Ulrich T. Schwarz 1 , Volker Kümmler 2 , Alfred
Lell 2 und Volker Härle 2 — 1 Institut für Angewandte und
Experimentelle Physik, Universität Regensburg, Universitätsstr. 31,
93053 Regensburg, Germany — 2 OSRAM Opto Semiconductors GmbH,
Wernerwerkstr. 2, 93049 Regensburg, Germany
Wir untersuchen das Alterungsverhalten von (Al,In)GaN Laserdioden
(LD) auf SiC Substraten in Abhängigkeit verschiedener Verspiegelungsarten.
So können wir zeigen, dass es zusätzlich zum Volumeneffekt auch
einen Beitrag der Laserfacetten zum Alterungsprozess gibt. Unverspiegelte
LD altern deutlich schneller als verspiegelte LD. Selbst das Aufbringen
einer λ/2 Schicht reduziert die Alterung deutlich, wobei die λ/2 Schicht
die optischen Eigenschaften der Facette nicht verändert. Um zu prüfen,
ob die Alterung lichtinduziert oder strominduziert ist, wurde eine unverspiegelte
LD an Luft gealtert. Die Stromdichte wurde so gewählt, dass
die LD am Anfang im Laserbetrieb war, nach einer Minute effektiver
Betriebszeit aber unter die Laserschwelle fällt. Dadurch ändert sich die
Photonendichte um mehrere Größenordnungen, die Alterungskurve zeigt
aber keine nennenswerte Änderung im Verlauf. Weitere Versuche mit unverspiegelten
LD in verschiedenen Atmosphären wie Ar, N2, O2 und H2
zeigen eine langsame Alterung. Das Hinzufügen von Wasserdampf lässt
die Diode wieder schnell altern. Dies lässt auf die Bildung einer Oxid-
Schicht auf der Facette schließen, die die internen Verluste vergrößert
und so zur Alterung führt.
HL 16.5 Di 11:15 H13
Biofunctionalization of AlGaN surfaces — •Barbara Baur,
Georg Steinhoff, Martin Hermann, Oliver Purrucker,
Motomu Tanaka, Martin Stutzmann, and Martin Eickhoff —
Walter Schottky Institut, TU München, D-85748 Garching
Biosensor applications of semiconductor devices for operation in aqueous
electrolyte solutions are a current topic of intense research. Wide
bandgap semiconductors, such as group III-nitrides are regarded as
promising candidates for these applications as they combine high chemical
inertness with optical transparency and low thermal noise. To combine
those materials with organic systems, a detailed understanding as well as
experimental techniques for the realization and control of the anorganic/
organic interface are a basic requirement.
We have investigated the functionalization of AlGaN surfaces by deposition
of self-assembled monolayers (SAMs) based on silanes (silanization)
for subsequent formation of lipid membranes. Silanization is performed
by a self-assembly process from an organic solution of silane molecules.