| Abstract |
This thesis concerns experimental research with the scope of creating
scientific understanding and knowhow for the epitaxial growth of heterostructures-nanostructures
of III-nitride semiconductors on new substrates and/or substrate
orientations. Specifically, the epitaxial growth of a-plane GaN on substrates of r-plane
sapphire was investigated extensively and became the main part of this work and an
initial exploration of the growth of c-plane GaN on diamond with orientation {100} or
{110} or {111}, using molecular beam epitaxy (MBE) with a nitrogen RF plasma
source [RF-MBE or plasma assisted MBE (PA-MBE)].
The properties of the GaN heterostructures were investigated by a wide variety
of experimental techniques, with emphasis on the study of surface morphologies and
structural properties by Atomic Force Microscopy (AFM) and X-ray diffraction
(XRD), respectively. The optoelectronic properties of the materials were evaluated by
photoluminescence spectroscopy (PL).
R-plane sapphire (Al2O3) substrates were employed for the growth of nonpolar
a-plane GaN heterostructures. Initially, the nitridation of r-plane sapphire was
studied, followed by the study of the nucleation of a-plane GaN or AlN layers on
sapphire and, finally, of the epitaxial growth of a-plane GaN thin films.
It was found that the nitridation procces causes the formation of periodic steps
on the surface of sapphire, attributed to an unintentional miscut angle of the substrate
from the r-plane. The duration of nitridation is crucial. The surface roughness of the
substrate, the spacing between steps and hence the height of some of them, increased
with increasing duration of the nitridation process. Temperature is another factor that
affects nitridation. Increasing the temperature increases the amount of nitridation and
the surface roughness of the substrate. The optimal nitridation is indicated by
observations of Reflection High-Energy Electron Diffraction (RHEED), using as
criterion the appearance of a surface reconstruction, which disappears after a critical
time, followed by increasing surface roughness. It has been estimated that effective rplane
sapphire nitridation is achieved at the substrate temperature of 400 °C, which
results to rapid onset of the surface reconstruction and low surface roughness.
Then, the nucleation of a-plane GaN or AlN layers on the r-plane sapphire
surface at various temperatures was studied. At high temperatures, three-dimensional
(3D) islands nucleate between the steps of the substrate and they do not fully coalesce
into a compact film. The growth of compact a-plane GaN thin films, regardless of
their thickness, requires the existence of a compact thin nucleation layer, which is
obtained at low temperatures below 500 °C. According to these results, a two-step
growth process was adopted for the growth of a-plane GaN thin films, where initially
a layer of 20 nm GaN or AlN is grown at low temperature. The growth of an 1,24 μm
GaN film directly at 800 °C, under nitrogen-rich conditions, resulted to a very smooth
surface but extremely high crystal mosaicity.
The research effort was continued with the study of the PA-MBE growth of aplane
GaN films on optimized nucleation layers. The investigation focused on
determining the effects of substrate temperature and III/V flux ratio greater than 1
(Ga-rich) on the mechanisms of epitaxial growth and the properties of a-plane GaN
thin films. Growth at high temperatures (750-800 °C), with constant III/V flux ratio of
1.8, was found to lead to formation of compact films consisting of elongated (along caxis)
3D-islands of reduced epitaxial thickness, because of high re-evaporation of Ga
atoms from the surface. These islands exhibited very smooth surfaces at their central
area. At lower temperatures and lower III/V ratio (1,5-1,6 for ~720 °C), the lateral
growth of GaN was more homogeneous, despite the prevalence of 3D growth mode.
In these conditions, the a-plane GaN films exhibited minimum crystal mosaicity and
maximum intensity with minimum linewidth of the emitted PL. Moreover, an
isotropic behavior of the crystal mosaicity along the in-plane c- and m-axes was
observed. The results were attributed to isotropic diffusion of the Ga atoms on the
surface of a-plane GaN, when the surface is covered by a Ga adlayer, in agreement
with theoretical calculations.
Finally, the epitaxial growth of GaN on diamond substrates with orientation
{100} or {110} or {111} was studied. In all cases, thin films of monocrystalline cplane
GaN were grown and only on the diamond {100} substrate two alternative
epitaxial arrangements of the GaN crystal coexisted, corresponding to a 30° rotation
around the c-axis. The GaN films were of N-polarity and contained narrow Ga-face
inversion domains (IDs). The crystalline quality of the samples was better on the
diamond {111} substrates. The GaN layers are under tensile elastic stress/strain due to
the different thermal expansion coefficients of GaN and diamond, but there were no
micro-cracks in 1.4 μm thick GaN films on diamond (111).
In conclusion, this thesis led to the physical understanding and optimization of
the heteroepitaxy of a-plane GaN semiconductor on r-plane Al2O3 by PA-MBE. GaN
heterostructures along this non-polar direction can improve the performance of light
emitting diodes (LEDs) and laser diodes (LD), due to the elimination of electric fields
inside the quantum wells’ active region, causing a spatial separation of electrons and
holes. In the case of diamond substrates, the growth of monocrystalline c-plane GaN,
regardless of the substrates’ orientation, is promising for the development of IIInitride
heterostructures suitable for applications in power devices on polycrystalline
substrates. These diamond substrates are available in large sizes and offer high
thermal conductivity to maximize the power performance of the III-nitride devices.
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Επίταξη με μοριακές δέσμες του ημιαγωγού GaN πάνω σε υποστρώματα αδάμαντος και σαπφείρου επιπέδου-r
