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@PHDTHESIS{Mauder:64546,
author = {Mauder, Christof},
othercontributors = {Heuken, Michael},
title = {{P}hysics, {MOVPE} growth, and investigation of m-plane
{G}a{N} films and {I}n{G}a{N}/{G}a{N} quantum wells on
gamma-{L}i{A}l{O} 2 substrates},
address = {Aachen},
publisher = {Publikationsserver der RWTH Aachen University},
reportid = {RWTH-CONV-125845},
pages = {132 S. : Ill., graph. Darst.},
year = {2011},
note = {Prüfungsjahr: 2011. - Publikationsjahr: 2012; Aachen,
Techn. Hochsch., Diss., 2011},
abstract = {The growth of InGaN/GaN quantum well structures along a
nonpolar orientation avoids the negative effects of the
so-called "Quantum Confined Stark Effect" and is therefore
considered as promising approach to improve wavelength
stability and efficiency of future optoelectronic devices.
This work describes physical principles and experimental
results on metal-organic vapor phase epitaxy and
characterization of GaN layers and InGaN/GaN quantum well
structures, which grow along the nonpolar (1-100) m-plane on
(100) lithium aluminum oxide (LiAlO2) substrates. The
limited thermal and chemical stability of the LiAlO2
substrate can be improved by a nitridation step, which
causes the formation of a thin (1-100) AlN layer on the
surface of the LiAlO2. This enables the phase-pure
deposition of high-quality and smooth (1-100) GaN layers.
The low lattice mismatch of (1-100) GaN to (100) LiAlO2
allows for a coherent growth of thin films, which show
strong in-plane compressive strain. Due to the absence of a
suitable slip plane, this strain relaxes only partly for
layer thicknesses up to 1.7 µm. Low densities of line and
planar defects compared to other heteroepitaxially deposited
nonpolar GaN layers were assessed by X-ray diffraction
(XRD), transmission electron microscopy (TEM) and electron
channelling contrast imaging microscopy (ECCI). The surface
of the GaN layers is dominated by macroscopic hillocks,
which are elongated along the c-axis direction and result in
an average root mean square (RMS) roughness of ~ 20 nm in a
50 • 50 µm2 scan area. Spiral growth around line defects
is seen as most likely cause for this effect. In a
microscopic scale, one can detect a stripe pattern, which is
formed by 2 - 3 nm high steps aligned parallel to the
c-axis. An anisotropic growth mode is assumed responsible
for this appearance. Between these steps, much smoother
areas with typical RMS roughness of 0.2 nm (for a 0.5 •
0.5 µm2 scan) is found, which is also an indication for
high quality on this small scale. As a consequence of the
anisotropic growth mechanism, the line widths of XRD
omega-scans taken with the incident direction perpendicular
to the c-axis are strongly broadened compared to the
perpendicular direction. The larger extension of coherent
crystal regions along the c-axis is also reflected in the
electron mobility, which is on average by $13\%$ larger for
carriers moving in this direction and takes values of up to
130 cm2/Vs. (1-100) GaN layers on LiAlO2 are always n-type
conductive with a background doping in the range of 1 •
1019 cm-3. The introduction of large amounts of magnesium
allows for an overcompensation to achieve p-type
conductivity. The reason for the strong background doping is
the incorporation of oxygen, which may evaporate from the
heated substrate and effectively re-incorporate on the
growing film since the (1-100) GaN surface exhibits a strong
affinity to oxygen at the relatively low growth
temperatures. The typical physical oxygen concentration of 1
• 1019 cm-3 is in agreement with the measured electron
density. Lithium can also escape from the substrate and act
as a crystal impurity, but the measured concentrations range
only in the order of 1 • 1016 cm-3. (1-100) InGaN/GaN
multi-quantum well structures (MQW) with different indium
contents of 5 - $30\%$ were successfully deposited and
characterized. A lower indium incorporation efficiency
compared to equally prepared MQW with (0001) orientation is
in accordance to literature. All MQW exhibit smooth surfaces
and abrupt interfaces. A few triangular-shaped pits with
typical diameter of 100 nm are found on the surface, which
arise from defects in the underlying GaN. The MQW are also
deposited on the tilted facets of these pits, which is
accompanied by a local change in MQW thickness and indium
content. Photoluminescence spectra of InGaN/GaN MQW with
indium fractions below $16\%$ show strong, blue emission
with excellent wavelength stability at increased excitation
levels. For higher indium contents, the peaks become broader
and weaker and exhibit a slight wavelength shift at higher
intensities. Indium accumulation near defects or surface
pits is seen as most likely origin. Higher indium contents
on a nm scale are also blamed for the lower degree of
polarization of emission compared to literature reports on
nonpolar MQW. Indium clusters change the spatial
distribution of holes within the valence subbands and
therefore affect the recombination properties. LED based on
(1-100) InGaN/GaN MQW were successfully fabricated. Although
the light output is still significantly lower compared to
devices based on layers deposited along the (0001)
orientation, the strong blue emission at a forward voltage
of only 4.1 V appears already quite promising. Main
challenges for further improvement are the optimization of
the upper p-type contact layer and the MQW layer stack.},
keywords = {Galliumnitrid (SWD) / MOCVD-Verfahren (SWD) / Quantenwell
(SWD) / Photolumineszenz (SWD) / Röntgendiffraktometrie
(SWD) / Polarisation (SWD)},
cin = {612020},
ddc = {530},
cid = {$I:(DE-82)612020_20140620$},
shelfmark = {81.15.Kk * 81.05.Ea * 78.55.Cr * 81.65.-b * 68.55.-a *
61.05.cp},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-opus-39241},
url = {https://publications.rwth-aachen.de/record/64546},
}
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