GaAs nanowires (NW) have been grown on Si (111) substrates by the self-assisted vapor-liquid-solid (VLS) mechanism using molecular beam epitaxy (MBE). Substrates were prepared with nano-patterned oxide templates using electron beam lithography (EBL) in order to achieve position controlled NW growth. Early experiments uncovered several key issues with regards to the patterning process. Cross-sectional lamella prepared using the focused-ion beam (FIB) technique were used to study the NW-substrate interface using transmission electron microscopy (TEM). Undesirable growth outcomes were found to be caused in part by an unintended residual layer of oxide. Uniform NW dimensions were then obtained by improving the pattern transfer method. The effects of deposition parameters on the growth results were then explored in further experiments. The first systematic study of the axial and radial growth rates of vertical NWs in the positioned array was conducted. It was proposed that the observed expansion of the Ga droplet in Ga-rich growth conditions results in a slight inverse tapered morphology, promoting significant radial growth. While the growth rates were shown to be approximately constant in time, their measured values were found to increase with increasing pattern pitch and decrease with increasing hole diameter. A phenomenological model was then developed based on the principle of mass conservation. A fit to the experimental data was obtained by calculating the collection of growth material supplied by a secondary flux of both gallium and arsenic species desorbing from the oxide surface between the NWs, subsequently impinging on the liquid droplet and NW sidewalls. The reduction of this contribution due to shading of the incident and scattered flux by neighboring NWs in the array was able to account for the differences in final NW morphologies observed with increasing pattern pitch. This model demonstrates the significant impact of secondary adsorption in patterned self-assisted NW growth. / Thesis / Doctor of Science (PhD)

No description available.

The wide band-gap semiconductor silicon carbide has tremendous potential for use in high power, high temperature, and high frequency electronic devices. One of the more important design factors for these devices is the epitaxial layer. It is desirable that this thin film have uniform polytype, thickness, and impurity concentration, as well as be defect free. One method used for SiC to ensure epitaxial layers with homogenous polytype is to cut wafers from a boule that has been tilted towards a specific crystallographic face at a fixed angle (known as ?off cut?). The purpose of this thesis was to investigate the growth mechanisms of alternative boule tilting directions with 6H-SiC. Four alternative crystallographic tilting faces were chosen: <1230>, <1340>, <2130>, and <3140>. A lightly doped 1um-thick layer was grown on samples representing the four alternative off-cut directions and, as references, commercially available substrates off cut towards the traditional direction <1120>. The physical and electrical properties of the layers were characterized by means of optical microscopy, Fourier Transform Infrared Reflectance Spectroscopy, Atomic Force Microscopy, capacitance vs. voltage, and current vs. voltage. Three facts were observed: 1) the alternative off-cut directions affected the growth mechanisms and surface morphology, 2) the quality of the substrate affects the morphology of the epitaxy layer, and 3) the relative differences between the surface roughness attributed to the different off-cut directions affected the observed electrical characteristics of Schottky barrier diodes fabricated on the epi layers. The samples cut towards the <31-40> and <13-40> directions showed to the most promising alternative off-axis tilting direction.

No description available.

No description available.

Cette étude de la fonctionnalisation de graphène se base principalement sur la monocouche de graphène épitaxiée sur SiC. Les propriétés électroniques, structurales et les compositions chimiques du graphène fonctionnalisé sont étudiées. L'incorporation d'azote dans le graphène réalisée par les procédures à base de plasma montre un décalage de niveaux inoccupés du graphène vers EF , obtenue par les analyses spectroscopie de photoémission inverse en résolution angulaire. Ce dopage-n est attribué à la présence de graphitique-N. De plus, la configuration des espèces de N substitués dans le graphène peut être contrôlée efficacement par l'énergie, les espèces d'azote incidentes, et l'épaisseur du graphène de départ. L'hydrogénation de la couche tampon de graphène (BLG) à température variante sature les liaisons pendantes de Si de l'interface différemment, soit par la formation de nouvelles liaisons C-Si à température ambiente, soit par les hydrogènes intercalés. Le BLG devient fortement-isolant dans le premier cas, et devient une monocouche de graphène quasi-autoportante (QFSG) dans le second, permettant un nouveau concept de fabrication des dispositifs à base de graphène sur SiC. La réaction/couplage entre des molécules pi-conjugué et les graphène vierge ou fonctionnalisé est aussi étudiée. Les états inoccupés des molécules à base de perylene sont légèrement modiffiées sur le graphène dopé N à cause d'un renforcement de transfert de charge. Des réactions chimiques entre les molécules perylenes et le graphène sont observées aprés l'exposition aux électrons de basse énergie. En résumé, cette étude permettra une meilleure maîtrise des propriétés des matériaux 2D comme le graphène. / In order to promote 2D materials like graphene to their numerous applications, new methodsaltering their electronic and chemical properties have to be mastered. In this thesis, theprocesses of chemical doping and hydrogenation of monolayer graphene grown on SiC are investigated. Nitrogen atoms are successfully substituted in the graphene lattice using plasma-basedmethods. The bonding configurations of the incorporated N can be controlled via the nature and energy of exposing species and the thickness of the pristine graphene. An n-type doping, revealed by angle-resolved inverse photoemission spectroscopy (ARIPES), is found in most N-doped graphene and is assigned to the presence of graphitic-N. Hydrogenations of the buffer layer of graphene (BLG) on SiC at ambient or high temperatures saturate the remaining Si dangling bonds at BLG/SiC interface in two different ways, either by inducing additional C-Si bonds or by H intercalation. This results in 2D materials with distinct characters, an insulating, graphane-like H-BLG or a quasi-free-standing graphene, which may be used as a new concept for the engineering of graphene-based devices. The interactions between pi-conjugated molecules and the functionalized graphene are also investigated. The unoccupied states of molecules are altered by the presence of incorporated N, but the degradation of molecules due to low-energy electron exposure seems not enhanced by the doping nitrogen under the studied conditions. Nevertheless, the functionalization of graphene is demonstrated and its electronic and chemical properties are carefully studied, which should help to faster further applications employing functionalized graphene.

In dieser Arbeit werden die grundlegenden Wachstumsprozesse von Ga2O3 , mittels Transmissionselektronenmikroskopie analysiert. Dazu gehört die Untersuchung des heteroepitaktischen Wachstums von Galliumoxidschichten welche mittels Molekularstrahlepitaxie (molekular beam epitaxy MBE), der gepulsten Laser Abscheidung (pulsed laser deposition (PLD)) und der metallorganischen Gasphasenepitaxie (metalorganic vapor phase epitaxy (MOVPE)) auf (0001) orientierte Saphir Substraten abgeschieden wurden. Heteroepitaktisches Wachstum von Ga2O3 auf Saphir (0001) erfolgt bis zu einer Dicke von 3 Monolagen pseudomorph als α-Ga2O3 welches durch die Gitterfehlpassung zwischen Galliumoxid und dem Saphire Substrat induzierte Gitterverspannung stabilisiert wir. Weiterhin, im Fokus der Arbeit stehend, wird das homoepitaktische Wachstum von Galliumoxid auf (100) orientierten Galliumoxidsubstraten untersucht. Neben den Besonderheiten des Schichtwachstums, die sich aus den eingesetzten metallorganischen Präkursoren und Sauerstoffquellen ergeben, wird die Schichtstruktur in Abhängigkeit der typischen Wachstumsparameter (Wachstumstemperatur, Wachstumsrate, Kammerdruck und Fehlorientierung des Substrates) analysiert. Dabei wird gezeigt das homoepitaktischen Wachstum auf (100) orientiertem, β-Ga2O3, mittels MOVPE, die kristalline Perfektion der gewachsenen Schichten stark von den verwendeten Präkursoren (Trimethylgallium (TMGa) und Triethylgallium (TEGa) als metallorganische Ausgangsstoffe und H2 O oder purer Sauerstoff als Oxidant) und den chemischen Prozessen an der Oberfläche bestimmt wird. Des Weiteren wird die Entstehung von Zwillingslammelen in abhähngigkeit der Fehlorientierung untersucht. Durch die Einführung von vorbestimmten Fehlorientierungswinkeln der Substrate ist es möglich das Entstehen der Zwillingslamellen zu verhindern, und ein Stufenflusswachstum zu generieren. Durch die Anwendung eines Ratengleichungsansatzes ist es möglich die experimentell beobachteten Dichten an Zwillingslamellen zu erklären und einen Diffusionskoeffizienten zu bestimmen. (i) Heteroepitaktisches Wachstum von Ga2O3 auf Saphir (0001) erfolgt bis zu einer Dicke von 3 Monolagen pseudomorph als alpha-Ga2O3. Oberhalb dieser Schicht wächst relaxiertes ß-Ga2O3 in Form von 3 Rotationsdomänen auf. Die Stabilisation der dünnen alpha-Ga2O3 Schicht wird, durch die Gitterfehlpassung zwischen Galliumoxid und dem Saphire Substrat induzierte Gitterverspannung bewirkt. (ii) Beim homoepitaktischen Wachstum auf (100) orientiertem ß-Ga2O3 mittels MOVPE wird die kristalline Perfektion der gewachsenen Schichten stark von den verwendeten Präkursoren (Trimethylgallium (TMGa) und Triethylgallium (TEGa) als metallorganische Ausgangsstoffe und H2O oder purer Sauerstoff als Oxidant) und den chemischen Prozessen an der Oberfläche bestimmt. Während beim Wachstum mittels TMGa und O2 vorwiegend polykristalline Schichten entstehen, ergeben sich beim Wachstum mittels TMGa und H2O sowie TEGa und O2 geschlossenen epitaktische Schichten. Dieser signifikante Unterschiede lässt sich durch die unterschiedlichen Reaktionswege der Ausgangsstoffe sowie durch die katalytische Wirkung der (100) Flächen des ß-Ga2O3 erklären. (iii) Die Perfektion, mittels MOVPE gewachsener, homoepitaktischer Schichten, auf (100) orientierten Substraten, ist stark von der Fehlorientierung des Substrates bestimmt. Schichten die auf Substraten mit geringen Fehlorientierungen abgeschieden werden (< 2° bei Wachstumstemperaturen < 850°C) sind durch eine hohe Dichte an Zwillingslamellen gekennzeichnet. Die Entstehung der Zwillingslamellen ist ein Resultat eines Doppelpositionierungsprozesses der Atome auf der Oberfläche der Wachstumsebene. Durch die Einführung von vorbestimmten Fehlorientierungswinkeln der Substrate ist es möglich das Entstehen der Zwillingslamellen zu verhindern, und ein Stufenflusswachstum zu generieren. Durch die Anwendung eines Ratengleichungsansatzes, welcher die konkurrierenden Prozesse des Einbaus von Atomen in Oberflächenstufen sowie der Nukleation und des Wachstum von zweidimensionalen Inseln beschreibt, ist es möglich die experimentell beobachteten Dichten an Zwillingslamellen zu erklären und einen Diffusionskoeffizienten zu bestimmen, Dieser ist um zwei Größenordnungen geringer als bei klassischen Halbleitern, wie z. B. GaAs. In dieser Arbeit werden die grundlegenden Wachstumsprozesse von Ga2O3 , mittels Transmissionselektronenmikroskopie analysiert. Dazu gehört die Untersuchung des heteroepitaktischen Wachstums von Galliumoxidschichten welche mittels Molekularstrahlepitaxie (molekular beam epitaxy MBE), der gepulsten Laser Abscheidung (pulsed laser deposition (PLD)) und der metallorganischen Gasphasenepitaxie (metalorganic vapor phase epitaxy (MOVPE)) auf (0001) orientierte Saphir Substraten abgeschieden wurden. Heteroepitaktisches Wachstum von Ga2O3 auf Saphir (0001) erfolgt bis zu einer Dicke von 3 Monolagen pseudomorph als α-Ga2O3 welches durch die Gitterfehlpassung zwischen Galliumoxid und dem Saphire Substrat induzierte Gitterverspannung stabilisiert wir. Weiterhin, im Fokus der Arbeit stehend, wird das homoepitaktische Wachstum von Galliumoxid auf (100) orientierten Galliumoxidsubstraten untersucht. Neben den Besonderheiten des Schichtwachstums, die sich aus den eingesetzten metallorganischen Präkursoren und Sauerstoffquellen ergeben, wird die Schichtstruktur in Abhängigkeit der typischen Wachstumsparameter (Wachstumstemperatur, Wachstumsrate, Kammerdruck und Fehlorientierung des Substrates) analysiert. Dabei wird gezeigt das homoepitaktischen Wachstum auf (100) orientiertem, β-Ga2O3, mittels MOVPE, die kristalline Perfektion der gewachsenen Schichten stark von den verwendeten Präkursoren (Trimethylgallium (TMGa) und Triethylgallium (TEGa) als metallorganische Ausgangsstoffe und H2 O oder purer Sauerstoff als Oxidant) und den chemischen Prozessen an der Oberfläche bestimmt wird. Des Weiteren wird die Entstehung von Zwillingslammelen in abhähngigkeit der Fehlorientierung untersucht. Durch die Einführung von vorbestimmten Fehlorientierungswinkeln der Substrate ist es möglich das Entstehen der Zwillingslamellen zu verhindern, und ein Stufenflusswachstum zu generieren. Durch die Anwendung eines Ratengleichungsansatzes ist es möglich die experimentell beobachteten Dichten an Zwillingslamellen zu erklären und einen Diffusionskoeffizienten zu bestimmen. Des Weiteren wird das Wachstum im Mischsystem (InxGa1-x)2O3 untersucht und gezeigt das Indium als grenzflächenaktive Substanz wirken kann. / In this work we study the basic growth processes of epitaxial Ga2O3 films, by means of transmission electron microscopy. We investigate the heteroepitaxial growth of thin layers Ga2O3 on the (0001) plane of sapphire grown by molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and metal organic vapor phase epitaxy (MOVPE). Furthermore, we will focus on the homoepitaxial growth on the (100) plane by MOVPE. Beside the peculiarities of the layer growth dependence on different metal organic precursors and oxygen sources, we investigate the influence of typical growth parameters (i.e. growth temperature, growth-rate, chamber pressure and miscut angle of the substrate) on the layer morphology. Incase of heteroepitaxial growth of β-Ga2O3 on (0001) plane of sapphire, independent of the growth method, the formation of a 3 monolayer thick α-Ga2O3 layer is observed, which is stabilized through strain, as a result of the lattice mismatch between sapphire and α-Ga2O3. In case of homoepitaxial growth by MOVPE on (100) oriented β-Ga2O3, the crystalline quality of the grown layer strongly depends on the used precursor (tri-methyl-gallium (TMGa) or tri-ethyl-gallium (TEGa) as metal precursor and H2O or pure oxygen as oxidant) and the chemical processes on the surface, respectively. Further on is the crystalline perfection of homoepitaxial layers grown by MOVPE on (100) oriented β-Ga2O3 substrates strongly dependent on the miscut-angle of the substrates. Layer grown on substrate with a small miscut-angle (< 2°) show high amount of twin lamella. These twin lamella are a result of a possible double positioning mechanism of ad-atoms on the growth surface. By introducing appropriate miscut-angles of the substrate it is possible to suppress the formation of these twin lamellae, and enable step flow growth. By applying a rate equation approach, describing the competing processes of incorporation of ad-atoms at kink sites or nucleation and growth of two dimensional island, it is possible to quantitatively reproduce the experimentally observed twin lamella densities and to determine a surface diffusion coefficient of the ad-atoms. Furthermore, in case of the alloy system (InxGa1-x)2O3, it is shown that indium can act as an surfactant, by increasing the surface diffusion.

For decades, epitaxy is used in nanotechnologies and semiconductor fabrications. So far, it's the only affordable method of high quality crystal growth for many semiconductor materials. Heterostructures developed from these make it possible to solve the considerably more general problem of controlling the fundamental parameters inside the semiconductor crystals and devices. Moreover, as one newly arising study and application branch of epitaxy, selective area growth (SAG) is widely used to fabricate materials of different thicknesses and composition on different regions of a single wafer. All of these new and promising fields have caught the interests and attentions of all the researchers around the world. In this work, we will study the stress and strain analysis of epitaxy in nano-scale materials, in which we seek a methodology to bridge the gap between continuum mechanical models and incorporate surface excess energy effects, which can be obtained by molecular dynamical simulations. We will make a brief description of the elastic behavior of the bulk material, covering the concepts of stress, strain, elastic energy and especially, the elastic constants. After that, we explained in details about the definitions of surface/interface excess energy and their characteristic property tensors. For both elastic constants and surface excess energy, we will use molecular dynamic simulations to calculate them out, which is mainly about curve-fitting the parabola function between the total strain energy density and the strain. After this, we analyzed the stress and strain state in nanoisland during the selective area growth of epitaxy. When the nanoisland is relaxed, the lattice structure becomes equilibrated, which means the total strain energy of system need to be minimized. Compared to other researcher's work, our model is based on continuum mechanics but also adopts the outcome from MD simulations. By combining these microscopic informations and those macroscopic observable properties, such as bulk elastic constants, we can provide a novel way of analyzing the stress and strain profile in epitaxy. The most important idea behind this approach is that, whenever we can obtain the elastic constants and surface property tensors from MD simulations, we can follow the same methodology to analyse the stress and strain in any epitaxy process. This is the power of combining atomistic simulations and continuum method, which can take considerations of both the microscopic and macroscopic factors.

III-nitrides are a promising material system with unique material properties, which allows them to be utilized in a variety of semiconductor devices. III-nitrides grown by NH3-MOMBE are typically grown with high carbon levels (> 1021 cm-3) as a result of the incomplete surface pyrolysis of the metal-organic sources. Recent research has involved the compensating nature of carbon in III-nitrides to produce semi-insulating films, which can provide low-leakage buffer layers in transistor devices. The aim of this work is to investigate the possibility of forming a 2DEG, which utilizes the highly carbon-doped GaN layers grown by NH3-MOMBE to produce low-leakage buffer layers in the fabrication of HEMTs. These low leakage GaN buffers would provide increased HEMT performance, with better pinch-off, higher breakdown voltages and increased power densities. Additionally, methods of controlling and/or reducing the incorporation of carbon will be undertaken in an attempt to broaden the range of possible device applications for NH3-MOMBE. To realize these transistor devices, optimization and improved understanding of the growth conditions for both GaN and AlGaN will be explored with the ultimate goal of determining the feasibility of III-nitride transistors grown by NH3-MOMBE.

While modern epitaxial methods enable precise, monolayer (ML) control of the thin film deposition process, the complexity of certain device structures is ultimately limited by the capability and cost of the fabrication process. The objective of this work is to develop a pathway toward three-dimensional epitaxy (3DE) - the ability to intentionally and dynamically pattern regions of a film during the deposition process - in order to enable novel device concepts unbound by the traditional device fabrication paradigm. This work pioneers UV-assisted metal organic molecular beam epitaxy (MOMBE) as a particularly selective epitaxy technique to create a pathway toward 3DE of a crucial and topical material system - the III-Nitrides. A novel UV-assisted MOMBE system is developed enabling intense UV irradiation of films during growth. High quality, heavily (unintentionally) carbon-doped GaN is successfully grown by NH₃-based MOMBE and for the first time InGaN, AlGaN, and magnesium-doped GaN are demonstrated by NH₃-based MOMBE. Intense UV irradiation of films during NH₃-based MOMBE significantly enhances photo-desorption of species during the growth process, subsequently affecting the resultant InGaN alloy composition, carbon dopant concentration, or magnesium dopant concentration. A digital micromirror device is introduced to pattern incident UV radiation during InGaN growth, demonstrating that the effects of photoexcitation during MOMBE which have been proposed, discovered, and identified by this thesis indeed can be leveraged to deposit an InGaN film that is compositionally patterned within the growth plane. The results demonstrate that the new approach presented herein is possible for the 3DE of III-Nitrides if additional challenges in practical implementation can be overcome.