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This thesis presents a computational study of water and ion dynamics in modified hydrophobic silicon nitride (Si3N4) nanopores, aimed at enhancing protein sequenc- ing technologies. By employing molecular dynamics (MD) simulations, the research investigates the wetting-dewetting behavior within nanopores as an indirect measure of amino acid residue hydrophobicity, focusing on how post-translational modifications (PTMs) of lysine, particularly the acetylation of lysine residues, influence nanopore hydrophobicity and ionic conductance. The study reveals that nanopore radius and hydrophobicity significantly affect water and ion permeation, with smaller nanopores oscillating between open and closed states, while larger ones remain open.  Using umbrella sampling and the Weighted Histogram Analysis Method (WHAM), the potential of mean force (PMF) for potassium (K+), chloride (Cl−), and water within the nanopores was determined, showing distinct PMF profiles based on lysine and acetyl- lysine presence. The modulation of ionic currents as a tool for protein sequencing was explored, demonstrating that different amino acid residues affect ionic currents by par- tially blocking the pore and altering local permeability, thereby enabling differentiation based on size, shape, charge, and hydrophobicity.  The findings suggest that silicon nitride pore hydrophobicity can be tailored for nanopore sequencing, correlating changes in ionic currents with amino acid residue translocation. This research enhances the understanding of interactions within nanopore environments, potentially leading to more precise nanopore-based sequencing methods.

In the rapidly advancing field of quantum technologies, integrated quantum photonics merges quantum mechanics with photonics, promising breakthroughs in communication, sensing, computing, and security. This doctoral thesis investigates the generation of correlated photons via spontaneous four-wave mixing (sFWM) on silicon-based platforms. Through a comparative analysis of various intramodal and intermodal sources, the research focuses on two main areas: applications in sensing within the 2 μm region and the development of sources and other integrated structures in the visible-near infrared region for quantum algorithms, such as variational quantum eigensolver and boson sampler. For sensing, the study enhances quantum ghost spectroscopy to enable efficient gas detection using non-degenerate intermodal silicon sFWM. In the context of quantum simulation, silicon-nitride-based integrated photonic structures were realized to generate and manipulate quantum light within a photonic integrated circuit. Additionally, a proof-of-concept implementation of a two-qubit SWAP test in silicon nitride material showcased significant potential in quantum machine learning.

With the fast development of information technology (IT) industry, the demand and market volume for off-line power supplies keeps increasing, especially those for desktop, flat-panel TV, telecommunication, computer server and datacenter. An off-line power supply normally consists of electromagnetic interference (EMI) filter, power factor correction (PFC) circuit and isolated DC/DC converter. Isolated DC/DC converter occupies more than half of the volume in an off-line power supply and takes the most control responsibilities, so isolated DC/DC converter is the key aspect to improve the overall performance and reduce the total cost for off-line power supply. On the other hand, of all the power supplies for industrial applications, those for the data center servers are the most performance driven, energy and cost conscious due to the large electricity consumption. The total power consumption of today's data centers is becoming noticeable. Moreover, with the increase in cloud computing and big data, energy use of data centers is expected to continue rapidly increasing in the near future. It is very challenging to design isolated DC/DC converters for datacenters since they are required to provide low-voltage high-current output and fast transient response. The LLC resonant converters have been widely used as the DC-DC converter in off-line power supplies and datacenters due to its high efficiency and hold-up capability. Using LLC converters can minimize switching losses and reduce electromagnetic interference. Almost all the high-end offline power supplies employs LLC converters as the DC/DC converter. But there are three major challenges in LLC converters. Firstly, the control characteristics of the LLC resonant converters are very complex due to the dynamics of the resonant tank. This dissertation proposes to implement a special LLC control method, state-trajectory control, with a low-cost microcontroller (MCU). And further efforts have been made to integrate all the state-trajectory control function into one MCU for high-frequency LLC converters, including start-up and short-circuit protection, fast transient response, light load efficiency improvement and SR driving. Secondly, the transformer in power supplies for IT industry is very bulky and it is very challenging to design. By pushing switching frequency up to MHz with gallium nitride (GaN) devices, the magnetics can be integrated into printed circuit board (PCB) windings. This dissertation proposes a novel matrix transformer structure and its design methodology. On the other hand, shielding technique can be employed to suppress the CM noise for PCB winding transformer. This dissertation proposes a novel shielding technique, which not only suppresses CM noise, but also improves the efficiency. The proposed transformer design and shielding technique is applied to an 800W 400V/12V LLC converter design. Thirdly, the LLC converters have sinusoidal current shape due to the nature of resonance, which has larger root mean square (RMS) of current, as well as larger conduction loss, compared to pulse width modulation (PWM) converter. This dissertation employs three-phase interleaved LLC converters to reduce the circulating energy by inter-connecting the three phases in certain way, and proposed a novel magnetic structure to integrated three inductors and three transformers into one magnetic core. By pushing switching frequency up to 1MHz, all the magnetics can be implemented with 4-layer PCB winding. Additional 2-layer shielding can be integrated to reduce CM noise. The proposed magnetic structure is applied to a 3kW 400V/12V LLC converter. This dissertation solves the challenges in analysis, digital control, magnetic design and EMI in high-frequency DC/DC converters in off-line power supplies. With the academic contribution in this dissertation, GaN devices can be successfully applied to high-frequency DC/DC converters with MHz switching frequency to achieve high efficiency, high power density, simplified but high-performance digital control and automatic manufacturing. The cost will be reduced and the performance will be improved significantly. / Ph. D. / With the fast development of information technology (IT) industry, the demand and market volume for off-line power supplies keeps increasing, especially those for desktop, flat-panel TV, telecommunication, computer server and datacenter. The total power consumption of today’s data centers is becoming noticeable. Moreover, with the increase in cloud computing and big data, energy use of data centers is expected to continue rapidly increasing in the near future. The efficiency of off-line power supplies is very critical for the whole human society in order to reduce the total electricity consumption. And the cost is also a key driving force for the development of novel technology in off-line power supplies due to the large market volume. An off-line power supply normally consists of electromagnetic interference (EMI) filter, power factor correction (PFC) circuit and isolated DC/DC converter. Isolated DC/DC converter occupies more than half of the volume in an off-line power supply and takes the most control responsibilities, so isolated DC/DC converter is the key aspect to improve the overall performance and reduce the total cost for off-line power supply. Among all the DC/DC converter topologies, the LLC resonant converters have been most widely used as the DC/DC converter due to its high efficiency and hold-up capability. But there are three major challenges in LLC converters. Firstly, the control characteristics are very complex due to the dynamics of the resonant tank. To achieve good control performance, very complex and expensive digital controller has to be employed. Secondly, the magnetic components are very bulky, and it is expensive to manufacture them. Thirdly, there is circulating energy in LLC converters due to the nature of resonance, which increases the total loss. To solve these challenges, this dissertation proposes to implement a special control method, state-trajectory control, with a low-cost microcontroller (MCU). All the control functions can be integrated into one simple, low-cost MCU to replace the previous complex and expensive controller. By pushing switching frequency up to MHz with next generation power devices, this dissertation proposes a novel magnetics structure that can be integrated into printed circuit board (PCB) windings to achieve low-cost and automatic manufacturing. Furthermore, this dissertation employs three-phase interleaved LLC converters topology to reduce the circulating energy, and proposed a novel magnetic structure to integrated three inductors and three transformers into one magnetic core with simple 4-layer PCB winding. All the proposed technologies have been verified on hardware prototypes, and significant improvements over industrial state-of-art designs have been demonstrated. To sum up, this dissertation solves the challenges in analysis, digital control, magnetic design and EMI in DC/DC converters for off-line power supplies. With the academic contribution in this dissertation, the cost can be reduced due to the simplified control and automatic manufactured magnetics, and the efficiency can be improved with proper utilization of next generation power devices. This dissertation will improve future DC/DC converter for IT industrial in the three most important aspects of efficiency, power density and cost.

With a higher power rating and broader application, Gallium nitride (GaN) is a promising next-generation power switch. The current four GaN HEMTs in paralleled phase leg that can block 400 V and conduct 200 A current is very beneficial, thus making the protection method on a GaN phase leg an urgent topic. This thesis starts with an overview of shortcircuit robustness among silicon (Si), silicon carbide (SiC) and GaN devices. An approximately safe operation area (SOA) for a GaN power switch will also be determined. The various common shortcircuit protection methods are mentioned. Additionally, current research on a GaN semiconductor is summarized. Among all of the protection methods, desaturation detection is selected and analyzed through simulation and then implemented in a parallel enhancement-mode high-electron-mobility transistor (E-HEMT) GaN phase leg. With this desaturation detection feature, the GaN E-HEMT can be turned off as quickly as 200 ns, and in the worst case, 500 ns, during a shortcircuit test. The phase leg survived a series of shortcircuit tests with shortcircuit protection. For the proposed protection scheme, the best-case reaction time (200 ns) is similar to others in the literature, while the shortcircuit peak current and peak energy are higher. The worst-case performance of this design is limited by both the gate driver and the device shortcircuit robustness. Due to the fast switching speed of the GaN HEMT, the false turn-on phenomenon caused by the Miller effect can be a problem. A shoot through may occur with one switch false turn on. The Miller clamp is added to the phase leg to improve its reliability. After the hardware was implemented, the Miller clamp was tested and verified through a double pulse test (DPT). Compared to the phase leg without the Miller clamp, the gate is better protected from gate voltage overshoot and undershoot. The switching loss is reduced by 20 percent by using a new gate driver IC with higher current driving capability. The degradation effect of GaN power switches in different shortcircuit pulses was also studied. The device passes through the shortcircuit tests, but any degradation effect that may change its parameters and influence its normal operation characteristic need to be addressed. Several GaN devices were selected and characterized after several shortcircuit tests to observe any degradation effect caused by the shortcircuit. The degradation test results reveal a "recovery effect" of the GaN HEMT used in this project. The parameter variations on threshold voltage and on-resistance recover to the original state, several hours after the shortcircuit test. The test results match with the conclusion drawn in degradation test conducts by other research groups that the parameter variation during shortcircuit test is negligible. Also, repetitively fast shortcircuit tests on the GaN HEMT show that the shortcircuit protection limit for this device under 400 V bus should be limited to 300 ns. / Master of Science / A phase leg consists of two power switches: a top switch and a bottom switch. As a result of a wrong gate signal or the Miller effect, shoot through problems may occur that lead to a shortcircuit current running through the channel. The excessive heat brought by the shortcircuit current will kill the device if not turned off in time. The failure of the phase leg may also have a hazardous impact on the rest of the system. To improve the overall system stability, a shortcircuit protection feature can be added on the gate-drive level. The shortcircuit protection turns off the device when it runs into shortcircuit mode, and before device failure. In this thesis, desaturation detection is selected to implement on a paralleled Gallium nitride (GaN) phase leg based on the device characteristic and configuration. Desaturation detection takes the device under test (DUT) as a current sensing component. By sensing the voltage across the DUT, the desaturation detection decides whether the DUT is operating under shortcircuit. If it is, a signal is sent to the gate driver to turn off the DUT when high voltage is sensed. A series of shortcircuit tests were conducted to verify the function of shortcircuit protection. A Miller clamp is also implemented and tested on the same phase leg to prevent a false turn on problem and to protect the gate. Both the Miller clamp and desaturation v detection features are tested on the same phase leg. The GaN devices survive the shortcircuit tests, with shortcircuit protection times between 200 ns to 500 ns. The design is successfully validated. Along with the implemented protection features, device degradation and shortcircuit robustness tests are also included in this work. The test results show that 300 ns shortcircuit time under 400 V bus is a safe turn off goal for this device.

The possibility of using methylamines instead of ammonia as a nitrogen precursor for the CVD of nitrides is studied using quantum chemical computations of reaction energies: reaction electronic energy (Δ𝑟𝐸𝑒𝑙𝑒𝑐) reaction enthalpy (Δ𝑟𝐻) and reaction free energy (Δ𝑟𝐺). The reaction energies were calculated for three types of reactions: Uni- and bimolecular decomposition to more reactive nitrogen species, adduct forming with trimethylgallium (TMG) and trimethylaluminum (TMA) followed by a release of methane or ethane and surface adsorption to gallium nitride for both the unreacted ammonia or methylamines or the decomposition products. The calculations for the reaction entropy and free energy were made at both STP and CVD conditions (300°C-1300°C and 50 mbar). The ab inito Gaussian 4 (G4) theory were used for the calculations of the decomposition and adduct reactions while the surface adsorptions were calculated using the Density Functional Theory method B3LYP. From the reactions energies it can be concluded that the decomposition was facilitated by the increasing number of methyl groups on the nitrogen. The adducts with mono- and dimethylamine were more favorable than ammonia and trimethylamine. 𝑁𝐻2 was found to be most readily to adsorb to 𝐺𝑎𝑁 while the undecomposed ammonia and methylamines was not willingly to adsorb.

Cette thèse a pour objectif d’apporter des informations quantitatives sur la mobilité de l’hélium dans des céramiques nucléaires avancées comme TiC, TiN et ZrC, soumises à des traitements thermiques ou bien en présence de dommages d’irradiation. L’approche expérimentale développée au cours de ce travail est basée sur l’implantation ionique d’ions d’hélium-3 de 3 MeV en profondeur dans les trois matériaux précédemment cités et sur la mesure du profil de concentration en profondeur de l’isotope 3He au moyen d’une réaction nucléaire spécifique induite par des deutérons, 3He(d, p0)4He. La microscopie électronique à transmission et la spectrométrie Raman sont couplées à l’analyse par réaction nucléaire.Parmi les principaux résultats obtenus :- aucun relâchement d’hélium n’est observé à température ambiante pour les trois composés. Les valeurs d’énergie d’activation associée au relâchement d’hélium après un recuit thermique dans l’intervalle 1100 – 1600°C sont comprises entre 0,77 et 1,2 eV et semblent étroitement liées à la microstructure initiale du composé (stoéchiométrie et taille de grains). La capacité de rétention de l’hélium-3 dans des carbures ou nitrures de métaux de transition soumis à des traitements thermiques en conditions contrôlées croît dans l’ordre ZrC < TiC < TiN.- la formation de blisters n’et observée qu’à la surface de ZrC.- les profils d’implantation d’hélium présentent deux composantes pour les trois matériaux, l’une située au voisinage de la fin de parcours des ions et la seconde plus proche de la surface. Cette dernière résulte probablement du piégeage d’atomes d’hélium par les lacunes natives présentes.- les valeurs obtenues pour le coefficient apparent de diffusion de l’hélium varient dans l’intervalle 3,58E-19 – 5,296E-18 m^2s^-1 pour TiN et 4,20E-18 – 2,59E-17 m^2s^-1 pour TiC.Les valeurs correspondantes obtenues pour l’énergie d’activation sont respectivement de 2,50 eV pour TiC et de 1,05 eV pour TiN. Le mécanisme impliqué repose sur une dissociation des amas atomes d’hélium – lacunes au voisinage de la fin de parcours des ions. Plus en surface, la diffusion est plutôt du type substitutionnel.- l’observation au MET de sections transverses de TiN préparées par la technique FIB révèlent la présence de bulles d’hélium dès recuit à 1100°C et montrent la croissance des bulles avec la température. L’énergie d’activation de croissance des bulles a été estimée à 0,38 eV. A partir de 1400°C, cette croissance résulte vraisemblablement de l’absorption de lacunes par les amas.- la pression interne des bulles a été calculée à l’aide du modèle de Trinkaus, et nous avons montré qu’à partir de 1500°C, cette pression tendait à s’approcher de la valeur du module de cisaillement de TiN (240 GPa) et qu’elle atteignait la pression d’équilibre de 2 GPa à 1600°C.- à 1100°C, il semble que la densité des bulles présentes dans TiN varie linéairement avec la fluence d’implantation. A 1500°C, la taille des bulles est d’autant plus grande que la fluence est faible.- pour ZrC, l’effet de la fluence sur la mobilité de l’hélium est comparable à celui observé pour TiN. A la plus basse fluence, le relâchement d’hélium est très faible. Il croît avec la température de recuit et avec la fluence d’implantation.- la pré-Irradiation des trois composés par des auto-Ions avant implantation d’hélium provoque une augmentation de la dureté au moins =jusqu’à une dose de 27 dpa. Une très faible augmentation du paramètre de maille est alors détectée (≤ 0.5%).- dans le cas de ces matériaux non amorphisables sous irradiation aux ions, le recuit par perte d’énergie électronique ou bien le pré-Endommagement balistique ne jouent a priori aucun rôle sur la mobilité de l’hélium, étudiée sous l’angle d’une activation thermique. / While the current second and third generation nuclear plant designs provides an economically, technically, and publicly acceptable electricity supply in many markets, further advances in nuclear energy system design can broaden the opportunities for the use of nuclear energy. The fourth generation of nuclear reactors is under development. These new reactors are designed with the following objective in mind: sustainability, safety and reliability, economics, proliferation resistance. Out of six Generation IV systems namely, Gas-Cooled Fast Reactor (GFR), Lead-Cooled fast reactor (LFR), Molten Salt Reactor (MSR), Sodium-Cooled Fast Reactor (SFR), Supercritical-Water-Cooled Reactor (SCWR), Very-High-Temperature Reactor (VHTR), this work is dedicated to identify specific fuel type that is compatible with gas-Cooled fast reactor (GFR) in-Core service conditions and could be extended to diagnose potential cladding material for SFR. The French strategy is mainly oriented towards the development of sodium-Cooled fast reactors (SFR) and very slightly focused on GFR. This dissertation is focused on the study of transition metal ceramics which are candidates for fuel coatings in GFR and have been considered as potential cladding materials for SFR. The specific fuel type in GFR should consists of spherical fuel particle made up of UC or UN, surrounded by a ceramic coating which provides structural integrity and containment of fission products. The most promising candidates for ceramic coatings are ZrN, ZrC, TiN, TiC & SiC due to a combination of neutronic performance, thermal properties, chemical behavior, crystal structure, and physical properties. It is obvious that these ceramics would be exposed to energetic fission products from fuel such as heavy ions and neutrons. These high-Energy neutron will knock the atoms in the surrounding materials and can induce (n, α) reactions, thus producing high concentration of helium atoms during and after reactor operation. The helium atoms produced are energetic and can easily penetrate into the surrounding material. Helium atoms are considered to be highly insoluble in previously studied structural nuclear materials. The accumulation of helium into solid matrix, can lead to the formation of bubbles, cavity, swelling, embrittlement etc. Helium can strongly induce grain boundary cavitation that can produce formation of inter-Granular channels, which may serve as pathways for release of radioactive elements to the environment or lead to grain-Boundary weakening and de-Cohesion. Particularly in ceramics, large quantities of helium can also lead to dimensional changes and cracks due to over-Pressurized helium bubbles. Therefore, study of helium behavior in advanced nuclear ceramics under high operating temperatures and extreme radiation conditions predicted for GFRs is viewed as crucial. In this thesis, ion-Implantation technique and material characterization techniques are used to study diffusion of helium in transition metal ceramics under thermal and extreme irradiation environments. Our main aim during this thesis is: 1) To calculate diffusion and migration energies of helium under different experimental conditions by applying theoretical models on experimental data.2) To investigate the role of microstructure such as grain boundaries, native vacancies and porosity on helium accumulation and its evolution after helium accumulation.3) To know the role of helium introduction conditions on helium diffusion. 4) To establish and validate an approach to calculate pressure built by helium gas inside the bubbles and to verify if the pressure approaches mechanical stability limit.

Im Rahmen dieser Arbeit wurde der pseudo-halogenide Gasphasenepitaxie (PHVPE)-Prozess für die GaN-Kristallzüchtung entwickelt. Dieser Prozess basiert auf dem Zyanid als Transportmittel für Ga. Das HCN wurde aus der Reaktion von heißem NH3 entweder mit Graphit oder einem gasförmigen Kohlenstoffträger gewonnen. Als Quelle für reaktiven Stickstoff diente NH3. Im ersten Ansatz wurde ein Reaktor aus Graphit genutzt. In diesem Fall wurden Wachstumsraten von 60 um=h erreicht. Außerdem zeigte der Kristall eine geringe Perfektion mit hoher V-Grubendichte. Im zweiten Ansatz bestand der Reaktor aus mit pyrolytischem Graphit beschichteten Teilen. Diese Änderung des Konzeptes half die Kristallqualität zu verbessern, reduzierte aber gleichzeitig die Wachstumsrate drastisch, weil das Ga-transportmittel nicht mehr ausreichend zur Verfügung stand. Der neu konstruierte, graphitfreie Aufbau stellt den dritten Zugang zur PHVPE dar. In diesem Fall entsteht HCN während eines Degussa-Prozesses am Pt-Katalysator im Züchtungsreaktor. Zur Untersuchung der Reaktionswege wurde ein FTIR-basiertes insitu Abgasmesssystem entwickelt. GaN-Kristalle wurden auf Saphir und Ga2O3 Substraten, AlN/Al2O3 und GaN/Al2O3 Templates gezüchtet. Eine Selbstseparation wurde für dicke GaN-Schichten auf Ga2O3 erreicht. Die Proben wurden mit verschiedenen Methoden charakterisiert, z.B. mit der Röntgenbeugungs-Spektroskopie (XRD) und Elektronenrückstreubeugung (EBSD) für die Kristallperfektion und kristallographische Orientierung, der Transmissionselektronenmikroskopie (TEM) zur Untersuchung von Versetzungen und der Grenzfläche zwischen GaN und dem Ga2O3, der Rasterelektronenmikroskopie (REM) für die Oberflächenmorphologie und Schichtdicke, der energiedispersiven Röntgenspektroskopie (EDX) für die Kristallzusammensetzung, sowie der ex-situ und in-situ Abgasanalyse mit der Fourier-Transform-Infrarotspektroskopie (FTIR) zum Studium der Reaktionswege. / Within the frame of this work the pseudo halide vapor phase epitaxy process (PHVPE) was developed for GaN crystals growth. The process is based on cyanide as a transport agent for Ga. The source of HCN was the reaction of hot NH3 with either graphite or gaseous carbon precursor. Source of reactive nitrogen was NH3. In the first approach the reactor made of graphite was used. In this case growth rate of 60 um/h was achieved. Additionally, the crystals exhibit poor quality with high V-pit density. The second approach was to provide the reactor with pyrolytical boron nitride covered parts. Changing the concept helped to improve the crystals'' quality but simultaneously reduced drastically the growth rate, due to the lack of sufficient supply of Ga transport agent. Newly designed graphite free setup is used in the third approach for PHVPE. In this case, HCN forms during Degussa process on Pt catalyst, inside the growth reactor. For investigation of the reaction paths, an in-situ exhaust gas measurement system based on FTIR was developed. GaN crystals were grown on sapphire and Ga2O3 substrates, AlN/Al2O3 and GaN/Al2O3 templates. Self separation was achieved for thick GaN crystals grown on Ga2O3. The samples were characterized by various methods i.e. x-ray diffraction spectroscopy (XRD) and electron back scattering diffraction EBSD for crystal quality and crystallographic orientation, transmission electron microscopy (TEM) for investigating dislocations and interface between GaN and Ga2O3, scanning electron microscopy (SEM) for surface morphology and layer thickness, energy dispersive x-ray spectroscopy (EDX) for crystals compositions, ex-situ and in-situ exhaust gas analysis by Fourier transform infrared spectroscopy (FTIR) for investigation of the reaction paths.

In dieser Arbeit untersuchen wir die grundlegenden Wachstums- und Relaxationsprozesse, die es erlauben den Verzerrungszustand von GaN (0001) beim Wachstum auf Silizium (111) Substraten einzustellen und die resultierende Dichte an Durchstoßversetzungen zu reduzieren. Zu deren Analyse werden GaN (0001) Schichten, die mittels metallorganischer Gasphasenepitaxy abgeschieden worden sind, hauptsächlich mit transmissionselekronenmikroskopischen Methoden untersucht. Die wesentlichen Erkenntnisse der Arbeit sind: (i) Der Aufbau einer kompressiven Verzerrung von GaN (0001) Filmen mittels AlGaN Zwischenschichten beruht auf einer Asymmetrie der plastischen Relaxation an den beiden Grenzflächen der AlGaN Zwischenschicht. Fehlpassungsversetzungen bilden sich zwar an beiden Grenzflächen aus, jedoch ist der mittlere Abstand zwischen Versetzungslinien an der unteren Grenzfläche kleiner, als an der oberen. (ii) Plastische Relaxation von verzerrten (0001) Wurtzit Schichten erfolgt im Wesentlichen durch Bildung von a-Typ Fehlpassungsversetzungen im 1/3 |{0001} Gleitsystem. Diese bilden sich aber nur dann, wenn die verzerrten Schichten eine 3-D Morphologie aufweisen. Eine quantitative Modellierung dieses Prozesses zeigt, dass die kritische Schichtdicke für das Einsetzen der plastischen Relaxation wesentlich vom Wachstumsmodus bestimmt wird. (iii) Eine Silizium Delta-Dotierung der GaN (0001) Oberfläche führt zum Wachstum einer kohärenten Sub-Monolage SiGaN3, die eine periodisch Anordnung von Silizium- und Galliumatomen, sowie Galliumvakanzen aufweist. Da das Wachstum von GaN direkt auf der SiGaN3-Monolage unterdrückt ist, tritt ein Übergang zu 3-D Inselwachstum auf, das zunächst ausschließlich in Löchern der SiGaN3-Monolage anfängt. Eine hohe Konzentration von Silizium auf der GaN (0001) Oberfläche wirkt also als Anti-Surfactant beim epitaktischen Wachstum von GaN. Rechnungen mittels der Dichtefunktionaltheorie liefern Erklärungen für das beobachtete Wachstumsverhalten. / In this work we study the basic growth and relaxation processes that are used for strain and dislocation engineering in the growth of GaN (0001) films on silicon (111) substrates. To analyse these processes, samples, grown by metalorganic vapour phase epitaxy were investigate by means of transmission electron microscopy. Our investigations have revealed the following main results: (i) Strain engineering and build-up of compressive strain in GaN (0001) films by means of AlGaN interlayer is based on an asymmetry in plastic relaxation between the two interfaces of the AlGaN interlayer. Although misfit dislocation networks form at both interfaces of the interlayer, the average spacing of dislocation lines at the lower interface is smaller than that at the upper one. (ii) Plastic relaxation of strained (0001) wurtzite films is caused mainly by formation of a-type misfit dislocations in the 1/3 |{0001} slip-system. These a-type misfit dislocations form once the strained films undergo a transition to a 3-D surface morphology, e.g. by island growth or cracking. Quantitative modelling of this process reveals that the critical thickness for nucleation of a-type misfit dislocations depends next to the lattice mismatch mainly on the growth mode of the film. (iii) Silicon delta-doping of the GaN (0001) surface leads to the growth of a coherent sub-monolayer of SiGaN3 that shows a periodic arrangement of silicon and gallium atoms and gallium vacancies. Since growth of thick GaN layers directly on top of the SiGaN3-monolayer is inhibited a transition towards 3-D island growth occurs, whereby GaN islands exclusively nucleate at openings in the SiGaN3-monolayer. A high concentration of silicon on the GaN (0001) surface thus acts as an anti-surfactant in the epitaxial growth of GaN. Our density functional theory calculations provide an explanation for both the self-limited growth of the SiGaN3-monolayer, as well as for the blocking of GaN growth on top of the SiGaN3-monolayer.

Previous research in InGaN/GaN light emitting diodes (LEDs) employing semi-classical drift-diffusion models has used reduced polarization constants without much physical explanantion. This paper investigates possible physical explanations for this effective polarization reduction in InGaN LEDs through the use of the simulation software SiLENSe. One major problem of current LED simulations is the assumption of perfectly discrete transitions between the quantum well (QW) and blocking layers when experiments have shown this to not be the case. The In concentration profile within InGaN multiple quantum well (MQW) devices shows much smoother and delayed transitions indicative of indium diffusion and drift during common atomic deposition techniques (e.g. molecular beam epitaxy, chemical vapor deposition). In this case the InGaN square QW approximation may not be valid in modeling the devices' true electronic behavior. A simulation of a 3QW InGaN/GaN LED heterostructure with an AlGaN electron blocking layer is discussed in this paper. Polarization coefficients were reduced to 70% and 40% empirical values to simulate polarization shielding effects. QW shapes of square (3 nm), trapezoidal, and triangular profiles were used to simulate realistic QW shapes. The J-V characteristic and electron-hole wavefunctions of each device were monitored. Polarization reduction decreased the onset voltage from 4.0 V to 3.0 V while QW size reduction decreased the onset voltage from 4.0 V to 3.5 V. The increased current density in both cases can be attributed to increased wavefunction overlap in the QWs.