Plenarvorträge - DPG-Tagungen

Halbleiterphysik Mittwoch
HL 27.10 Mi 17:30 H15
Zeitaufgelöste Photolumineszenz an GaInN/GaN-Strukturen:
Laserqualität contra Quantenausbeute — •C. Netzel, F. Hitzel,
S. Lahmann, U. Rossow und A. Hangleiter — Institut für Technische
Physik, Technische Universität Braunschweig, D-38106 Braunschweig
GaN-basierte Halbleiterstrukturen, ob LEDs oder Laserdioden, mit
Emissionswellenlängen von grün bis ins UV, werden seit einigen Jahren
hergestellt. Trotz der Entwicklungserfolge konnten grundlegende Fragen,
die strahlende und nichtstrahlende Rekombination in dem defektreichen
Materialsystem betreffend, bisher nicht eindeutig geklärt werden.
In dieser Arbeit soll durch Untersuchungen von GaInN/GaN-
Heterostrukturen, die mit MOVPE auf Saphir/SiC gewachsen wurden,
ein tieferer Einblick in die Rekombinationsabläufe gewonnen werden. Es
wurden auf hohe optische Verstärkung optimierte Laserstrukturen mit
zeitaufgelöster Photolumineszenz untersucht und mit auf hohe Quantenausbeute
optimierten MQWs verglichen. Sämtliche Probenreihen zeigen
in der Photolumineszenz neben der erwarteten Linienposition mindestens
eine weitere hochenergetische Emission. Die Ausprägungen der Emissionslinien
sind abhängig vom jeweiligen Optimierungsgrad. Vergleichsmessungen
mittels SNOM (Scanning Nearfield Optical Microscope) deuten
auf eine räumliche Trennung der Emissionen hin. Zeitaufgelöste Messungen
zeigen eine unterschiedliche Zerfallskinetik der Emissionen auf.
Eine Erhöhung der Quantenausbeute scheint mit dem verstärkten Auftreten
der hochenergetischen Emission einherzugehen. Die Laserqualität
wird dabei jedoch verschlechtert.
HL 27.11 Mi 17:45 H15
MOVPE growth of GaN/AlGaN quantum well structures and
its impact on the optical properties — •D. Fuhrmann 1 , M.
Greve 1 , N. Riedel 1 , U. Rossow 1 , G. Ade 2 , P. Hinze 2 , J. Bläsing 3 ,
A. Krost 3 , and A. Hangleiter 1 — 1 TU Braunschweig, Inst. f. Techn.
Phys., 38106 Braunschweig; d.fuhrmann@tu-bs.de — 2 Physikalisch Technische
Bundesanstalt, 38116 Braunschweig — 3 Otto-von-Guericke-Univ.,
Inst. f. Exp. Phys., 39016 Magdeburg
GaN/AlxGa1−xN (x≥20%) quantum well (QW) structures were grown
by MOVPE on sapphire and SiC substrates in order to realize efficient UV
light emitters. It will be shown how growth parameters (substrate, III-
V-ratio, Si-doping, pressure) affect the optical properties, determined by
photoluminescence (PL) spectroscopy. TEM and XRD measurements reveal
smooth surfaces for the upper and lower GaN/AlxGa1−xN-interface.
AFM pictures of GaN QW-structures on sapphire show microholes, which
can be correlated to defects inside the AlGaN buffer layer. In addition,
a PL emission around 3.65 eV arising from these defects is observed,
whereas for structures on SiC both AFM and PL do not show these features.
Varying QW-thickness for QWs on SiC or sapphire yields a nearly
linear shift for the PL peak position with different slopes. It is found
that for short growth times a nucleation phase takes place for QWs on
sapphire which is not seen in the case of SiC substrates. In order to increase
the potential barrier, AlN/GaN MQW structures have been grown
for comparison. Differences between AlxGa1−xN barriers and AlN barriers
arising from a different morphology and structural quality as well as
different electric fields inside the quantum well will be discussed.
HL 27.12 Mi 18:00 H15
Temperature dependence of the built-in electric field strength
in AlGaN/GaN Heterostructures grown on Si(111) — •A.T.
Winzer 1 , R. Goldhahn 1 , G. Gobsch 1 , A. Dadgar 2 , A. Krtschil 2 ,
H. Witte 2 , and A. Krost 2 — 1 Institute of Physics, TU Ilmenau, 98684
Ilmenau — 2 Institute of Experimental Physics, Otto-von-Guericke University
Magdeburg, 39016 Magdeburg
Previous studies of the barrier electric field strength F of AlGaN/GaN
heterostructures grown on sapphire revealed a strong temperature dependence.
This was attributed to the change of piezoelectric polarization
due to the large mismatch of thermal expansion coefficients between the
nitride layers and the sapphire.
Here, we report for the first time on similar studies for AlGaN/GaN
heterostructures on Si(111)-substrate. The samples were grown by
MOCVD with different Al-contents in the top AlGaN layer, utilizing a
thin low temperature AlN interlayer for strain reduction.
Hall and C(V) measurements as well as spatially resolved surface potential
measurements were carried out in order to determine the density
of the 2DEG. In the AlGaN layers, F was determined by analyzing the
Franz-Keldysh-Oscillations in photoreflectance spectroscopy. Their energetic
splittings indicate that F is nearly independent from the temper-
ature, which can be attributed to a constant piezoelectric polarization
and thus to a constant strain state.
This interpretation is corroborated by temperature dependent reflectance
measurements. The results are compared with self-consistent
conduction band calculations.
HL 27.13 Mi 18:15 H15
Silicon doping of heteroepitaxial AlN films grown by MBE —
•Martin Hermann, Florian Furtmayr, Martin Stutzmann, and
Martin Eickhoff — Walter Schottky Institut, Am Coulombwall 3,
85748 Garching
Recently, Si-doped AlN has attracted increasing interest due to its
possible application in wide bandgap semiconductor devices, such as n-
AlN/p-diamond heterojunction diodes. So far, these devices suffer from
a large serial resistance because of the high ionization energy of Si in
AlN. We have investigated Si doping of heteroepitaxial AlN films grown
by plasma assisted molecular beam epitaxy.
The influence of the Si concentration on the structural properties has
been investigated by high resolution X-ray diffraction measurements. The
surface morphology of the AlN:Si samples has been examined by atomic
force microscopy. The dependence of the Si donor activation energy on
the doping concentration was analyzed by temperature dependent conductivity
measurements. Optical absorption measurements were used to
detect the formation of impurity induced defects, especially the by Si
donor, and to examine their influence on the optical properties of AlN:Si.
HL 27.14 Mi 18:30 H15
Verspannungsgradienten in dicken GaN-Schichten auf Silizium
Substrat — •U. Haboeck 1 , A. Hoffmann 1 , C. Thomsen 1 , T. Riemann
2 , F. Bertram 2 , J. Christen 2 , A. Dadgar 2 und A. Krost 2
— 1 Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr.
36, 10623 Berlin, Germany — 2 Institut für Experimentelle
Physik, Otto-von-Guericke-Universität Magdeburg, Universitätsplatz 2,
39106 Magdeburg, Germany
Die Gitterfehlanpassung sowie die extrem unterschiedlichen thermischen
Ausdehnungskoeffizienten erschweren das Wachstum von Galliumnitrid
(GaN) auf Siliziumsubstrat (Si (111)). Selbst dünne Schichten sind
in der Regel deutlich tensil verspannt und können reissen. Mittels Mikro-
Raman-Spektroskopie und Kathodolumineszenz Mikroskopie haben wir
mehrere dicke GaN/Si-Schichten untersucht. Unsere Experimente belegen
den günstigen Einfluss von eingefügten Aluminiumnitridzwischenschichten
und Siliziumnitridmasken auf die strukturellen und optischen
Eigenschaften von nominell undotierten und Si-dotierten Proben. Ortsaufgelöste
Messungen über die Probenkanten ermöglichen einen Einblick
in die Entwicklung der Verspannung vom Substrat zur Oberfläche hin.
Obwohl die Relaxation in den ca. 6 Mikrometer dicken Schichten sprunghaft
erfolgt, waren die Oberflächen homogen und frei von Rissen.
HL 27.15 Mi 18:45 H15
Electro-optic Effects in Nitride Semiconductors: Investigation
and Application for Characterisation — •S. Shokhovets 1 , G.
Gobsch 1 , O. Ambacher 2 , M. Hermann 3 , and M. Eickhoff 3 —
1 Institute of Physics, TU Ilmenau, PF 100565, 98684 Ilmenau, Germany
— 2 Center for Micro- and Nanotechnologies, TU Ilmenau, PF 100565,
98684 Ilmenau — 3 Walter Schottky Institute, TU Munich, Am Coulombwall
3, 85748 Garching
We report on the results of electroreflectance (ER) and impedancevoltage
(ZV) studies of GaN and AlGaN diodes with a semitransparent
Pt Schottky gate. The data analysis was carried out using the multi-layer
formalism which allows us to consider the electric field inhomogeneity.
The field-dependent contribution of excitons and excitonic continuum
to the dielectric function (a) and the linear electro-optic effect (b) were
identified to be the main mechanisms of the electro-optical response. The
first effect leads to the so-called rotation ER spectra in a region of the
excitonic absorption and reduces to the Franz-Keldysh-like oscillations
above the band gap. The second one is responsible for the electro-optical
response below the band gap. Further, applications for characterisation
(exciton energies and broadening parameters for the zero-field limit, band
bending, concentration of ionised impurities and electric field strength in
the depletion region as a function of the temperature) are presented and
compared with the ZV measurements.