Plenarvorträge - DPG-Tagungen

Halbleiterphysik Dienstag
During the silanization process, hydroxyl groups which are formed on
oxidic surfaces and act as covalent coupling sites for alkylsilane have
to be provided. Thermally and chemically oxidized AlGaN surfaces are
compared with respect to their efficiency in the silanization process. We
have applied X-ray photoelectron spectroscopy (XPS) and contact angle
measurements to characterize the SAM formation and to investigate the
properties (coverage, hydrophobicity) of the functionalized layer.
HL 16.6 Di 11:30 H13
Improvement of GaN Schottky Diodes by Thermal Oxidation
— •Olaf Weidemann 1 , Eva Monroy 2 , Eike Hahn 3 , Martin Hermann
1 , Edgar Zaus 1 , Martin Stutzmann 1 , and Martin Eickhoff
1 — 1 Walter Schottky Institut, TU München, 85748 Garching —
2 CEA-CNRS-UJF Nanophysique et Semiconducteurs, CEA-Grenoble,
38054 Grenoble, France — 3 Laboratorium für Elektronenmikroskopie,
Universität Karlsruhe, 76128 Karlsruhe
GaN Schottky contacts have gathered increasing interest for the application
as UV detectors and, more recently, as high temperature gas
sensitive devices and GasFETs with catalytic Schottky contacts. However,
Schottky contacts on GaN suffer from high reverse currents due to
the presence of threading dislocations. To improve the device characteristics
and to guarantee reproducible electronic properties, a passivation
of these defects is desirable.
We have investigated the effect of thermal oxidation on the electronic
characteristics of Schottky contacts on heteroepitaxial n- and p-type GaN
layers grown by plasma assisted molecular beam epitaxy. Thermal oxidation
was found to lead to significant device improvement, notably a
decrease in reverse leakage current and an enhanced rectification ratio,
which has been quantified by IV- and CV-measurements. Analysis of the
oxidized layers by TEM and AFM revealed that preferential oxidation
at crystal defects is the dominating mechanism for the observed device
improvement.
HL 16.7 Di 11:45 H13
AlGaN/GaN Field Effect Transistors for Chemical and Biological
Sensor Applications — •Georg Steinhoff, Barbara
Baur, Martin Hermann, Martin Stutzmann, and Martin Eickhoff
— Walter Schottky Institut, Technische Universität München, Am
Coulombwall 3, 85748 Garching
Field effect transistors based on AlGaN/GaN heterostructures are of
great interest for chemical and biochemical sensor applications. The electrical
properties of the polarization induced 2DEG, which is formed at the
AlGaN/GaN heterointerface are highly sensitive to changes in the surface
potential. Additionally, the device surface is chemically inert in aqueous
solutions and non-toxic to living cells. We have studied the response
of nonmetallized gate regions of AlGaN/GaN transistors and transistor
arrays to changes in the concentration of specific ions in the ambient electrolyte.
The potential drop at the interface between the gate surface and
the electrolyte solution was measured and used as a sensor signal. A linear
sensor response of approximately 55 mV/pH was found for variations
in the H + -concentration between 10 -1 and 10 -13 mol/l, which is close to
the theoretical Nernst limit, determined by the native oxide layer present
on the surface. Sensitivity towards Na + , K + , Ca 2+ and Cl – -ions and long
term stability of the transistor devices in aqueous solutions were also investigated.
As a first biophysical application of these sensors we discuss
the detection and analysis of ionic signals of cells cultivated directly on
the device surface.
HL 16.8 Di 12:00 H13
Optimized Specific Contact Resistance of p-GaN — •Jens Dennemarck
1 , Tim Böttcher 1 , Detlef Hommel 1 und Anna Piotrowska
2 — 1 Institute of Solid State Physics, University of Bremen, D-
28359 Bremen — 2 Institute of Electron Technology, Al. Lotnikow 32/46,
02-668 Warsaw, Poland
Nowadays GaN-based high performance devices are available, which
needs high current densities during operation. For example, a GaN-based
laser diode needs a current density in the range of some 1 kA/cm 2 . To
operate devices reliable with such high driving conditions, ohmic contacts
with quite low specific contact resistances are necessary.
In the case of p-type GaN a metal with a large work function is necessary.
Then the difference between vacuum and Fermi level is large,
that holes can get into the metal without a barrier.
Two approaches were performed to reduce the specific contact resistance:
first a highly doped cap layer with a thickness of 15nm was added.
This should increase the number of acceptor levels with the effect of a
decreased barrier height. The second approach was a InGaN subcontact
layer about 20 nm. In respect of the induced strain the thickness of the
barrier decreased. To measure the specific contact resistance, the transfer
length method (TLM) was used. The problems of this measurement are
current spreading and current crowding. The cTLM method (circular
TLM) was considered to be the better method.
A number of metallization with different work functions were applied.
The best results were achieved by Pd/Au contacts with a specific contact
resistance of 1-2*10-5 A/cm 2 .
HL 16.9 Di 12:15 H13
Heat Dissipation in GaN LEDs and LDs - Simulation and
Experiment — •Stephan Figge 1 , Tim Böttcher 1 , Sven
Einfeldt 1 , Detlef Hommel 1 , Christoph Zellweger 2 , Mark
Ilegems 2 , Patrick Waltereit 3 , and James Speck 3 — 1 Institut
für Festkörperphysik, Universität Bremen — 2 Institut de photonique et
d’electronique quantiques, Ecole Polytechnique Federale de Lausanne
— 3 The Interdisciplinary Center for Wide Band-Gap Semiconductors,
University of California, Santa Barbara
The heating due to joule heat plays an important role in the device
characteristics of high power GaN light emitting diodes (LEDs) and laser
diodes (LDs). In this talk we will show two and three dimensional finite
elements calculations on the heat dissipation in LEDs and LDs and will
compare them with experiential results taken from devices grown on sapphire
substrates. The data of the simulations are in excellent agreement
with the experiential data such as the wavelength shift and the dependence
of output power taken from devices in DC and pulsed operation.
The talk will discuss the influence of the duty cycle and the pulse parameters
on the heating within the active region. In addition the dependence
of the thermal resistance on the device geometry (layer thicknesses, die
size, substrate material) will be shown.
HL 16.10 Di 12:30 H13
In,Ga,Al,N-Wellenleiter- und Laserstrukturen im blauen Spektralbereich
auf Si-Substraten — •Andre Strittmatter, Lars
Reissmann und Dieter Bimberg — Technische Universität Berlin,
Institut für Festköerperphysik, Sekr. PN 5-2, Hardenbergstr. 36, 10623
Berlin
Mittels Transfermatrizen und Lösung der Modengleichung wurden aus
epitaktischer Sicht realisierbare In,Ga,Al,N-Wellenleiterstrukturen auf
Si-Substraten für den Wellenlängenbereich 400-460 nm untersucht. Es
zeigt sich, dass bei geeigneter Dimensionierung einer GaN-Pufferschicht
und der AlGaN-Mantelschichten auch Fundamentalmoden innerhalb des
Wellenleiterkerns geführt werden. Dies ist zunächst überraschend, da der
Realteil des Brechungsindexes des Si-Substrats im interessierenden Spektralbereich
deutlich über den Brechungsindizes der Nitrid-Schichten liegt.
Die Modellierungen ergeben jedoch, daß gerade die absorbierenden Eigenschaften
des Si zu der gewünschten Wellenführung beitragen. Des weiteren
sind auch die bisher beim Wachstum rissfreier Nitridschichten unverzichtbaren
AlN-Zwischenschichten für die Führung der optischen Mode
vorteilhaft, da sie zu einer Reduktion der Eindringtiefe der optischen Mode
in die Mantelschichten führen. Ausgehend von diesen Erkenntnissen
präsentieren wir den Aufbau von Laserdioden auf Si-Substraten bei einer
Wellenlänge von 460 nm, die bei einer Gesamtdicke von 1.4-1.6 mum mit
nur 2-3 AlN-Zwischenschichten im n-dotierten Bereich der Diodenstruktur
realisiert wurden.