Made available in DSpace on 2015-03-26T13:35:11Z (GMT). No. of bitstreams: 1 texto completo.pdf: 2893346 bytes, checksum: 9ae656930afe35b3a72cddee0c2cc87c (MD5) Previous issue date: 2009-05-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The epitaxial growth technique by molecular beams (Molecular Beam Epitaxy – MBE) can be considered as one of the most important for obtaining thin fine films, heterostructures and nanostructures nowadays, allowing the production of high quality layers, and it also allows the in situ monitoring of process through several techniques of characterization. This work presents the project, the construction and the initial tests of a MBE system for the growing of compounds containing cadmium, tellurium, manganese and zinc. The work shows a bibliographic revision of the main types of epitaxy, some of the main techniques of growth, the basic principles of vacuum technology and the necessary tools to the construction of the system. The detailed project of the system and its main components represented. Finally, the functioning tests of the vacuum systems, the effusion cells, the system of controlling and automation and the results obtained with the first obtained samples represented. The total cost of the system in the current configuration is approximately R$150.000 which is about as less as one fourth of one commercial system with approximately the same characteristics. / A técnica de crescimento epitaxial por feixes moleculares (Molecular Beam Epitaxy – MBE) pode ser considerada como uma das mais importantes para a obtenção de filmes finos, heteroestruturas e nanoestruturas nos dias atuais, permitindo a obtenção de filmes de excelente qualidade, além de permitir o acompanhamento do crescimento in situ através de diversas técnicas de caracterização. Este trabalho aborda o projeto, a construção e os testes iniciais de um sistema de MBE para o crescimento de compostos contendo Cádmio, Telúrio, Manganês e Zinco. O trabalho apresenta uma revisão bibliográfica dos principais tipos de epitaxia, algumas das principais técnicas de crescimento, princípios básicos da tecnologia de vácuo e os instrumentos necessários à construção do sistema. É apresentado o projeto detalhado do sistema e seus principais componentes. Finalmente, descrevem-se os testes de funcionamento do sistema de vácuo, das células de efusão, o sistema de controle e automatização e os resultados obtidos com as primeiras amostras obtidas. O custo total do sistema na configuração atual é de aproximadamente R$ 150.000, cerca de 4 vezes menor que o de um sistema comercial com aproximadamente as mesmas características.

Made available in DSpace on 2015-03-26T13:35:12Z (GMT). No. of bitstreams: 1 texto completo.pdf: 4918198 bytes, checksum: d8295a3e9f68661e7fe4f267ae5aa01d (MD5) Previous issue date: 2010-04-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The presence of nuclear radiation detectors is extremily important in various industries such as medical, astronomy and of national security. There are many types of detectors. However, the detector constructed with CdTe and CdZnTe semiconductor films has become very popular due to some characteristics as convenience, density, energy resolution and for having the possibility of operating at room temperature. In this work, a review of nuclear radiation detectors is made, especially those built with semiconductor. Here are also presented structural, superficial and electric characterization methods to inform which type of sample is the most viable for such purpose. We also present in this work the results of the of CdTe films growth using HotWall Epitaxy technique (HWE) in temperatures from 150 C and 250 C over Si (111), simple glass and glass covered with tin oxide with fluorine . It is also presented the results ofcharacterization of CdTe films by x-ray diffraction and electrical characterization by curves I x V. / A presença de detectores de radiação nuclear é de extrema importância em várias indústrias como, por exemplo, a médica, a astronômica e de segurança nacional. Existem inúmeros tipos de detectores. Um deles, o detector construído com ligas semicondutoras de CdTe e CdZnTe, tem se tornado bastante popular devido às características peculiares como: praticidade, densidade, resolução energética e pela possibilidade de operarem a temperatura ambiente. Neste trabalho, faremos uma revisão de detectores de radiação nuclear, especialmente dos construídos com semicondutores. Apresentamos também métodos de caracterização estrutural, superficial e elétrica de amostras a fim de informar qual tipo de amostra é a mais viável para tal finalidade. Mostramos os resultados do crescimento de filmes espessos de CdTe, utilizando a técnica de Epitaxia de Paredes Quentes (HWE) nas temperaturas de 150 C e 250 C sobre Si (111), vidro simples e vidro coberto com óxido de estanho dopado com flúor. São também apresentados os resultados de caracterização dos filmes de CdTe por difração de raios-X e caracterização elétrica através de curvas I x V do filme.

Le poly(3-hexylthiophène) régiorégulier (rr-P3HT) est l’un des matériaux phare pour les applications en électronique plastique. Afin d’améliorer les performances des dispositifs, il est important de comprendre et de contrôler la structure et la morphologie de la couche active. Des films minces de rr-P3HT cristallins et orientés peuvent être obtenus par epitaxie directionnelle par cristallisation (DEC) de 1,3,5-trichlorobenzène (TCB). Dans ce travail de thèse, nous présentons les avantages de la méthode DEC pour (i) déterminer la structure cristalline du rr-P3HT et pour (ii) étudier les fibres « shish-kebab », une morphologie alternative du rr-P3HT. La première partie de ce manuscrit consiste en l’analyse structurale par diffraction électronique de la forme I du P3HT obtenue dans les films epitaxiés. Afin d’affiner le modèle structural, nous avons fait varier la conformation des chaînes latérales. Dans un deuxième temps, nous avons appliqué ce même protocole afin de résoudre la structure du second polymorphe du P3HT (forme II) grâce à l’obtention de monocristaux parself-seeding. La deuxième partie du manuscrit est consacrée à l’élaboration de fibres orientées dites en « shish-kebab » par épitaxie dans un mélange TCB/Pyridine. La cinétique de croissance ainsi que l’orientation des chaînes de P3HT sont étudiés afin d’optimiser la méthode de préparation des fibres. L’effet de la masse moléculaire sur leur formation a également été évalué. Finalement les propriétés de transport de charges au sein de ces fibres ont été étudiées grâce à l’élaboration de transistors à effet de champ. / Regioregular poly(3-hexylthiophene) (rr-P3HT) is one of the promising materials for plastic electronic applications. Understanding as well as controlling the structure and morphology of the active layer is essential to improve the device efficiency. Highly crystalline and highly oriented rr-P3HT thin films can be obtained via directional epitaxial crystallization (DEC) in 1,3,5-trichlorobenzene (TCB). In this dissertation, we present the benefits of DEC method to (i) determine the crystal structure of rr-P3HT and (ii) to study shish-kebab fibers, an alternative morphology of rr-P3HT. The first part of this dissertation is related to the structural analysis of rr-P3HT form-I by using electron diffraction analysis on the epitaxied film. Furthermore, the modification of side chain conformation is used to improve the structural model. Finally, the determination protocol is also applied to resolve the preliminary crystal structure of rr-P3HT form II prepared by self-seeding method. The second part of thisdissertation is related to highly oriented shish-kebab fibers of rr-P3HT prepared by epitaxial crystallization in a mixture of TCB and pyridine. The investigations of growth kinetics and orientation of P3HT chains in the fiber are performed in order to determine the optimization of preparation conditions and growth mechanism of the fibers. In addition, the effect of molecular weight on the formation of the fibers is investigated. Finally, the charge transport properties of the fibers are measured by using the field effect transistor configuration.

L'illumination par la face arrière amincie des imageurs CMOS est une des voies étudiées pour accroître le rapport signal à bruit et ainsi la sensibilité de ce capteur. Or cette configuration est adaptée à la détection des électrons dans la gamme d'énergie [[1 ; 12 keV]. L'électron incident crée, par multiplication, plusieurs centaines d'électrons secondaires, proche de la surface. Une couche de passivation par surdopage P++ de la face arrière est nécessaire afin de réduire le nombre de recombinaisons de surface des électrons. Par effet de champ électrique, la couche de passivation augmente le nombre de charges collectées, et ainsi le gain de collection du capteur. L'objectif de cette thèse est de développer des moyens de caractérisation pour déterminer in situ les performances sur le gain de collection de six procédés de passivation. Préalablement, le profil de dépôt d'énergie de l'électron incident est étudié au moyen d'une simulation Monte-Carlo puis d'un modèle analytique. Un modèle associé du gain de collection indique qu'à forte énergie, l'effet miroir de la passivation est déterminant tandis qu'à faible énergie, l'épaisseur de la passivation est un facteur clef. Une première expérience d'irradiation de diodes étendues P++=N permet de dégager l'influence du procédé de passivation sur les recombinaisons de surface. Grâce à une seconde caractérisation de type < événement unique >, directement sur capteur CMOS aminci, les passivations sont discriminées quant à leur effet miroir et l'étalement de la charge qu'elles induisent. Le recuit laser d'activation des dopants peut s'avérer une source d'inhomogénéités du gain sur la surface de la matrice / Backside illuminated thinned CMOS imaging system is a technology developed to increase the signal to noise ratio and the sensibility of such sensors. This configuration is adapted to the electrons detection from the energy range of [1 - 12 keV]. The impinging electron creates by multiplication several hundreds of secondary electrons close to the surface. A P++ highly-doped passivation layer of the rear face is required to reduce the secondary electron surface recombination rate. Thanks to the potential barrier induced by the P++ layer, the passivation layer increases the collected charges number and so the sensor collection gain. The goal of this study is to develop some experimental methods in order to determine the effect of six different passivation processes on the collection gain. Beforehand, the energy profile deposited by an incident electron is studied with the combination of Monte-Carlo simulations and some analytical calculations. The final collection gain model shows that the mirror effect from the passivation layer is a key factor at high energies whereas the passivation layer has to be as thin as possible at low energies. A first experimental setup which consists in irradiating P++=N large diodes allows to study the passivation process impacts on the surface recombinations. Thanks to a second setup based on a single event upset directly on thinned CMOS sensor, passivation techniques are discriminated in term of mirror effect and the implied spreading charges. The doping atoms activation laser annealing is turn out to be a multiplication gain inhomogeneity source impacting directly the matrix uniformity

Des films minces d’oxydes de cuivre (Cu2O, Cu4O3 et CuO) ont été déposés à température ambiante sur des substrats en verre et en silicium par pulvérisation magnétron réactive. Une attention particulière a été portée à l’influence des conditions de synthèse (débit d’oxygène et pression totale) sur la structure et l’orientation préférentielle des dépôts. La pression totale est le paramètre principal influençant la texture des films de Cu2O et de Cu4O3. En revanche l’orientation préférentielle des films de CuO est contrôlée par le débit d’oxygène. Pour des films de Cu2O et de Cu4O3, un phénomène de croissance épitaxique locale (CEL) a été mis en évidence. Il résulte de l’utilisation d’une première couche qui joue le rôle d’une couche de germination lors du processus de croissance. Ainsi, les films peuvent croître avec une texture donnée indépendamment de leurs conditions de synthèse. Cet effet de CEL a été mis à profit pour élaborer des films biphasés (Cu2O + Cu4O3) qui présentent une microstructure originale. L’augmentation de la transmittance optique et du gap optique de films de Cu2O a été rendue possible par des traitements thermiques dans l’air qui permettent de diminuer la densité de défauts dans les films. Finalement, les propriétés optiques et la structure électronique des oxydes de cuivre qui ont été calculées par la méthode GW sont en accord avec des résultats expérimentaux obtenus par absorption optique, photoémission et spectroscopie de perte d’énergie des électrons. / Copper oxide (Cu2O, Cu4O3 and CuO) thin films have been deposited on unmatched substrates by sputtering at room temperature. The influence of oxygen flow rate and total pressure on the film structure and preferred orientation has been studied. The total pressure is a relevant parameter to control the texture of Cu2O and Cu4O3 films, while the oxygen flow rate is effective to tune the preferred orientation of CuO films. Local epitaxial growth, where epitaxial relationship exists in columns of sputtered films, has been observed in Cu2O and Cu4O3 films by using a seed layer. The seed layer will govern the growth orientation of top layer via the local epitaxy, independently of the deposition conditions of top layer. Unusual microstructure that both phases have the vertically aligned columnar growth has been evidenced in biphase Cu2O and Cu4O3, which may relate to the local epitaxial growth of Cu2O. The lower resistivity than that in single phase films has been observed in this biphase film. Annealing in air can increase the transmittance of Cu2O films in visible region by the reduction of the impurity scattering, while the optical band gap is enlarged due to the partial removal of defect band tail. The optical properties and electronic structure of copper oxides calculated by GW approach with an empirical on-site potential for Cu d orbital, are in good accordance with experimental results from optical absorption, photoemission and electron energy loss spectroscopies

Weltweit wird ein großer Teil der Energie für die Kühlung unterschiedlichster Arten verwendet und der Bedarf steigt weiterhin an. Herkömmliche Kühlsysteme funktionieren mittels Kompression von Gasen mit sehr niedriger Verdampfungstemperatur. Diese Kältemittel sind entweder giftig, brennbar oder klimaschädlich. Deshalb zielen aktuelle Forschungsschwerpunkte auf alternative und nachhaltige Kühlsysteme. Eine vielversprechende Alternative ist der Einsatz von Festkörpern mit Phasenumwandlungen. Die durch verschiedene (magnetische, elektrische oder elastische) Felder induzierten Phasenübergänge ermöglichen die Nutzung kalorischer Effekte. Der magnetokalorische Effekt (MKE) beschreibt das physikalische Phänomen, bei dem ein sich veränderndes äußeres Magnetfeld unter adiabatischen Bedingungen zu einer Temperaturänderung in einem magnetischen Material führt. Für die Nutzung des MKE in Kühlsystemen stellen die Ni-Mn-X (X = Ga, In, Sb, Sn) Heusler-Legierungen eine geeignete Materialklasse dar. Sie besitzt mit ihrer gekoppelten magnetostrukturellen Umwandlung, bei der eine martensitische Phasenumwandlung auch die magnetischen Eigenschaften ändert, ein großes Potential für einen MKE. Beim Absenken der Temperatur unter die Umwandlungstemperatur kommt es zu einer diffusionslosen Strukturumwandlung von einer hohen zu einer niedrigeren Kristallsymmetrie. Dabei wird die Hochtemperaturphase als Austenit und die Niedrigtemperaturphase als Martensit bezeichnet. Werden einige Atomprozent Kobalt zu Ni-Mn-Ga hinzulegiert, ändern sich die magnetischen Eigenschaften der Phasen deutlich. So zeigt Ni-Mn-Ga-Co einen magnetostrukturellen Übergang zwischen der ferromagnetischen Austenitphase und der ferrimagnetischen Martensitphase und damit einen inversen MKE. Beim Anlegen eines äußeren magnetischen Feldes kommt es demnach zu einer Abkühlung des funktionalen Materials und damit zu positiven Werten der Entropieänderung. Für die Anwendung dieser Festkörper als Kühlelemente in Mikrosystemen ist die Entwicklung und Charakterisierung dünner Schichten nötig. Ihr hohes Oberflächen-zu-Volumen-Verhältnis ermöglicht einen schnellen Wärmeaustausch mit dem umgebenden Medium, wodurch hohe Zyklusfrequenzen erreichbar sind. Entsprechend können hohe spezifische Kühlleistungen erzielt werden. Epitaktische Ni-Mn-basierende Heusler-Legierungsschichten sind außerdem ein gutes Modellsystem für die Untersuchung des Einflusses von Ober- und Grenzflächen auf die Phasenumwandlung und die Materialeigenschaften und erlauben Untersuchungen zu den Ursachen der Hysterese, die bei einer martensitischen Phasenumwandlung auftritt. In dieser Arbeit werden epitaktisch gewachsene Ni-Mn-Ga-Co-Schichten, die eine gekoppelte strukturelle und magnetische Phasenumwandlung nahe Raumtemperatur besitzen, hergestellt und charakterisiert. Ausgehend von Vorarbeiten zu Ni-Mn-X-Schichten und vielversprechenden Zusammensetzungen, die von Massivmaterialproben bekannt sind, wird durch die Variation der Herstellungsparameter und der chemischen Zusammensetzung der Schichten, magnetostrukturelle Umwandlungen mit scharfen Umwandlungsbereichen und geringer thermischen Hysterese bei großer Magnetisierungsänderung erzielt. Anhand von zwei mittels Kombinatorik hergestellter Probenserien wird der Einfluss des Kobalt-Gehaltes auf strukturelle, magnetische und kalorische Eigenschaften untersucht und entspricht den Ergebnissen von Untersuchungen an Ni-Mn-Ga-Co-Massivmaterialien. Es wird gezeigt, wie sich die magnetischen und kalorischen Eigenschaften der Schichten nach der Ablösung vom Substrat ändern. Die Entropieänderung, die ein für die kalorischen Eigenschaften sehr wichtiger Parameter ist, wird indirekt mit Hilfe geeigneter Magnetisierungsmessungen bestimmt und zeigt vielversprechende Werte von bis zu 9,9 J/(kg K). Die Ergebnisse der verschiedenen Messwege durch den Magnetfeld-Temperatur-Phasenraum werden verglichen und die Unterschiede entsprechend des Nukleations- und Wachstumsmodells der martensitischen Umwandlung erläutert. Die Umwandlungszyklenzahl beeinflusst die Wiederholbarkeit der temperaturabhängigen Magnetisierungskurven und damit auf strukturelle und magnetische Eigenschaften der Schichten deutlich und reduziert die thermische Hysterese. Mittels unvollständiger Umwandlungszyklen kann die martensitische Umwandlung derart beeinflusst werden, dass sich die thermische Hysterese reduzieren lässt. Dadurch werden bestehende Nukleations- und Wachstumsmodelle der martensitischen Umwandlung bestätigt.

Perovskite based artificial superlattices has recently been extensively investigated due to the immense promise in various device applications. The major applications include non-volatile random access memories, microwave devices, phase shifters voltage tunable capacitor applications etc. In this thesis we have taken up the investigation of two different types of symmetric superlattices, viz. BaZrO3/BaTiO3 and SrTiO3/BaZrO3, with possible applicability to voltage tunable devices. Chapter 1 deals with the introduction to the perovskite based functional oxides. Their various applications and the specific requirements for voltage tunable device applications has also been discussed in detail. The basic properties of BaTiO3 and SrTiO3, which are well documented in the literature, have been reviewed. The fundamental physics of interfacial interactions that influence the properties of superlattices is also discussed using existing models. The reason behind the choice of constructing artificial superlattices of BaZrO3/BaTiO3 and SrTiO3/BaZrO3 and the motivation behind this thesis is outlined. Chapter 2 gives a brief description of the basic characterization techniques that has been employed for studying the thin films. These include pulsed laser deposition of oxide thin films, structural characterization using X-Ray Diffraction and Atomic Force Microscope and electrical characterization of thin film metal-insulator-metal structures. The basic principle behind the techniques has also been included in various sections of this chapter. Chapter 3 introduces the reader to basic properties of the less studied perovskite material BaZrO3, one of the parent components of Ba(Zr,Ti)O3 based ceramics for high frequency applications. BaZrO3 is the common material in both the types of superlattices studied in this thesis. Initially the growth of polycrystalline BaZrO3 on (111)Pt/TiO2/SiO2/Si has been elaborated in this chapter. After characterizing the crystalline quality of the films and optimizing the growth conditions, epitaxial BaZrO3 films has been grown on (001) SrTiO3 substates. Dielectric properties of epitaxial BaZrO3 film have been measured as a function of temperature and frequencies. The electric field tunability of BaZrO3 films has been calculated from capacitance-voltage data for comparison with superlattice structures. Chapter 4 deals with the basic considerations involving growth of artificial superlattices and multilayers using pulsed laser ablation technique. The fundamental differences between formation of multilayers and superlattices have also been discussed, and the basic considerations for optimizing growth parameters are analyzed in this chapter. X-ray θ-2θ and φ-scans have been performed to investigate crystal quality of superlattices. The growth rates calculated from the satellite reflections in X-ray θ-2θ scans indicate fair degree of control over the growth and φ-scans confirms epitaxial cube-on cube growth of both types of superlattices. Atomic Force microscopy has been used to hcaracterize the film quality and surface morphology of superlattice structures and it has been found that the films have a very smooth surface with rms roughness of the order of few nanometres. Chapter5 deals with the detailed electrical characterization of both types of superlattices structures. Dielectric response showed nearly temperature invariance for both types of superlattices. Polarization measurements show that the heterostructures are in paraelectric state. Even for paraelectric/ferroelectric BaZrO3/BaTiO3 superlattices, stress induced stabilization of the paraelectric state is exhibited in low period superlattices. Paraelectric/paraelectric-SrTiO3/BaZrO3 superlattices exhibited a tunability of ~20% at intermediate modulation periods and an extremely stable dissipation factor with respect to temperature which is very attractive for device application point of view. A maximum tunability of ~40% has been observed for lowest period BaZrO3/BaTiO3 superlattice. Relatively high Quality Factors has been observed for both type of superlattices and their dependence on the modulation periods has been analyzed. Dielectric relaxation data showed that Maxwell-Wanger type of behaviour is exhibited but the presence of a conductance component G had to be realized in the equivalent circuit representation, which originates from the observation of a square law dependence of the alternating current on the frequency. Finally DC electrical characteristics were investigated as a function of temperature to determine the type of conduction mechanism that is involoved. The data has been analyzed using existing theories of high field conduction in thin dielectric films and it has been found that at different temperature ranges, the conduction mechanism varied from bulk limited Poole-Frenkel to Space Charge limited conduction. The activation energy calculation indicate that the physical processes responsible for dielectric relaxation and dc conduction are identical.

<p>Shape memory alloys (SMAs) belong to an important class of active materials. Beyond shape memory, these alloys exhibit super-elasticity and pseudo-plasticity, all originating from a reversible phase transformation from a high-temperature austenitic phase to a low temperature martensitic phase. Their unique thermo-mechanical properties make these SMAs desirable for a wide range of applications in automobiles, robotics, aerospace, construction, and medicine. Only a fraction of the known metallic alloys exhibits martensitic transformations, and a relatively small subset exhibits shape memory. Given this limited pool of SMAs, tunability of this martensitic transformation and, hence, thermo-mechanical properties is a way to move forward for effectively designing the next-generation SMAs for specific applications. The modification in composition has always been at the heart of designing new SMAs for future applications. However, a relatively recent discovery of incorporating a second non-transforming phase in base martensitic materials to tune martensitic transformation to achieve unprecedented thermo-mechanical properties has shown great promise.</p><p><br></p><p>The objective of this work is to utilize the second phase to provide design guidelines for next-generation SMAs and to understand the detailed physics behind the experimentally observed unprecedented thermo-mechanical properties in SMAs as a result of the incorporation of coherent second phases. We first investigate Mg-Sc shape memory alloys that are attractive for a wide range of applications due to their low density. Unfortunately, the use of these alloys is hindered by a low martensitic transformation temperature (173 K). We observe from first-principles calculations that epitaxial strains arising from appropriate substrate or coherent second phase selection increase the martensitic transformation and operational temperature to room temperature. Next, we develop a novel approach to induce martensitic transformation in composite systems of two non-transforming materials. While we demonstrate this approach for the technologically relevant ultra-lightweight Mg/MgLi superlattices, however, our approach is general and will open a wide material space for the discovery and design of next-generation SMAs.</p><p><br></p><p>Finally, to bridge the gap between computationally studied single-crystalline materials and experimentally studied polycrystalline systems, we characterize the role of nanoscale precipitates on temperature- and stress-induced martensitic phase transformation in nanocrystalline Ni63Al37 SMAs using multi-million-atoms molecular dynamics simulations. Simulations provide the understanding of underlying atomistic mechanisms of experimentally observed unprecedented thermo-mechanical properties and the guidelines to design low-fatigue ultra-fine grain shape memory alloys. As a result of the exploration of novel thermomechanical properties in SMAs via coherent second phases, we also published a software package</p><p>to discover coherent precipitates within a base multi-component system by coupling highthroughput equilibrium thermodynamics calculations with strain-based lattice matching.</p>

Two important mechanisms in preparation of ultrathin films of magnetic oxides were systematically investigated in this work. First, influence of epitaxial strain on resulting magneto-optical properties of La2/3Sr1/3MnO3 (LSMO) ultrathin films was studied. The investigated films were grown by pulsed laser deposition on four different substrates, providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to the unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and off-diagonal elements of permittivity tensor. The off-diagonal elements confirmed presence of two previously reported electronic transitions in spectra of all films. Moreover, they revealed another electronic transition around 4.3 eV only in spectra of films grown under compressive strain. We proposed classification of this transition as crystal field paramagnetic Mn t2g → eg transition, which was further supported by ab initio calculations. A key role of strain in controlling electronic structure of ultrathin perovskite films was demonstrated. Dynamic application of...

Graphene nanoribbons (GNRs) are considered as major building blocks of future carbon-based electronics, in which the termination of the edges essentially defines the electronic properties. Theoretical predictions, such as tunable band gaps in armchair orientated GNRs, and the existence of topologically protected metallic states located at zigzag edges, make them a potential candidate for transistor applications as well as a new class of fully coherent devices. In this context, the fabrication of high-quality GNRs with precise edge geometries is of great interest. Atomistic details and the interaction with its support crucially influence and determine the charge propagation within such graphene nanostructures. Hence, the understanding of transport mechanisms on the nanoscale is indispensable in order to integrate GNRs in future nanoelectronics. This thesis presents a detailed study of the sublimation-assisted growth of different types of self-assembled GNRs on SiC crystals using scanning probe, electron microscopy, and electron diffraction experiments. First, natural SiC steps will be shown to trigger the formation of µm-long epitaxial monolayer GNRs (ML-GNRs), which laterally expand on the flat SiC(0001) surface. These ribbons can be transformed into bilayer GNRs (BL-GNRs) by annealing in air. During this process, oxygen-intercalation takes place, forming an oxide layer below the BL-GNRs. Charge transfer into the oxide layer results in strong p-type doping. Based on local multi-probe experiments, ML-GNRs and BL-GNRs revealed 1D diffusive transport characteristics inherent in the comparably high charge carrier densities in both types of ribbon. Moreover, temperature activated interlayer hopping was identified as an effective transport mechanism in BL-GNRs. Graphene nanoribbons grown on pre-processed SiC sidewalls exhibited superior crystalline and electronic quality on wafer-scales. Sidewalls aligned parallel to the [11-20] SiC direction are composed of a periodic array of mini-terraces hosting several approximately (3+-1) nm wide armchair terminated GNRs (ac-GNRs) at their step edges. By using a combined nanoprobe and conductive atomic force microscopy study, ac-GNRs revealed semi-conducting transport characteristics with band gaps of ~300 meV. Such debunching effects can be suppressed in sidewalls along the [1-100] SiC direction. Here, the graphene completely overgrows the sidewall resulting in ~40 nm wide freestanding zigzag GNRs (zz-GNRs). A robust ballistic edge channel was found to be the hallmark of zz-GNRs, which persists on µm-scales at room temperature suggesting the existence of a perfectly conducting channel. However, the roughness of the SiC and the mesa sidewalls limit the charge propagation in this edge mode due to strong short-range interactions. Moreover, ballistic transport was independently proven by utilizing non-invasive and invasive voltage probes. Tuning of the invasiveness was achieved using cleaning procedures of the tips, which lead to a subsequent decrease of contact resistance due to the removal of oxide from the tip surface. The measured resistance of the ballistic conductor was shown to be directly dependent on the invasiveness of the tips, pointing out the importance of the interplay between the probes and the GNR. Finally, spatially-resolved nanoprobe experiments with ultra-small probe spacings revealed several quantized conduction plateaus across zz-GNRs. These plateaus were attributed to edge and bulk transport channels, respectively. Based on tight-binding calculations, the occurrence of spatially-segregated ballistic channels was explained by transversal electric fields originating from asymmetric edge terminations on both sides of the GNR. These findings highlight that edge morphology is an essential parameter in order to understand electronic transport in GNRs. / Nanometerbreite Streifen aus Graphen, sogenannte Graphen-Nanoribbons (GNRs), gelten als wichtiges Bauelement in zukünftigen, kohlenstoffbasierten Elektroniken. Dabei sind die elektronischen Eigenschaften der GNRs wesentlich durch die Geometrie ihrer Kanten bestimmt. Basierend auf theoretischen Modellen, werden skalierbare Bandlücken in armchair-GNRs, sowie lokalisierte, metallische Kantenzustände in zigzag-GNRs vorhergesagt. Diese Eigenschaften könnten für Transistoranwendugen oder sogar für die Realisierung von Bauelementen, die auf kohärentem Ladungstransport basieren, genutzt werden. Dementsprechend ist die Herstellung hochwertiger GNRs mit präzisen Kantengeometrien sowie das Verständnis der zugrundeliegenden Transportmechanismen von großem Interesse. Die vorliegende Arbeit umfasst eine detaillierte Charakterisierung der strukturellen Eigenschaften verschiedener GNR-Typen, die mittels Sublimationsepitaxie auf SiC Kristallen hergestellt wurden. Es wird gezeigt, dass sich μm-lange Monolagen-GNRs (ML-GNRs) an natürlichen SiC Stufenkanten ausbilden, die durch Tempern an Luft zu Bilagen-GNRs (BL-GNRs) transformiert werden können. Während des Temperns findet die Interkalation von Sauerstoff statt, sodass sich unterhalb des BL-GNRs eine Oxidschicht bildet. Der Ladungstransfer in diese Oxidschicht führt zu einer starken p-Dotierung. Lokale Transportmessungen mittels eines 4-Spitzen STM/SEM zeigen, dass sowohl ML-GNRs als auch BL-GNRs 1D diffuse Leiter sind, deren Transporteigenschaften durch die hohen Ladungsträgerdichten dominiert werden. Darüber hinaus wird gezeigt, dass das thermisch aktivierte Tunneln zwischen Graphenlagen ein effektiver Transportmechanismus in BL-GNRs ist. Graphen-Nanoribbons, die durch präferenzielles Wachstum auf SiC-Seitenwänden hergestellt wurden, zeichnen sich durch herausragende strukturelle sowie elektronische Eigenschaften aus. Seitenwände parallel zur [11-20] Richtung wiesen hierbei eine periodische Struktur von Mini-Terrassen auf, an deren Stufen sich mehrere (3 ± 1) nm breite armchair-GNRs (ac-GNRs) ausbilden. Durch die Kombination von 4-Spitzen STM/SEM und Rasterkraftmikroskopie mit leitfähigen Spitzen wurde festgestellt, dass ac-GNRs halbleitende Eigenschaften aufweisen. Die Größe der ermittelten Bandlücken beträgt ∼ 300 meV. Das Zerfallen in Mini-Terrassen kann bei Seitenwänden entlang der [1-100] SiC Richtung unterdrückt werden. Hierbei wird die Seitenwand vollständig vom Graphen überwachsen, sodass sich ∼ 40 nm breite zigzag-GNRs (zz-GNRs) ausbilden. Diese zeichnen sich durch einen robusten, ballistischen (Kanten-) Transportkanal aus, der bei Raumtemperatur auf μm-Skalen nachweißbar ist. Lediglich Rauigkeiten des Substrats sowie der Seitenwände, die als starke Streuzentren dienen, limitieren die Ausbreitung der Ladungsträger in diesem Kantenzustand. Der ballistische Transport von Ladungsträgern in zz-GNRs wurde unabhängig, mit Hilfe von nicht-invasiven und invasiven Spannungskontakten (STM-Spitzen) nachgewiesen. Die Invasivität der Kontakte wurde durch spezielle Reinigungsverfahren der Spitzen verändert, die zu geringeren Kontaktwiderständen führten. Hierbei wird gezeigt, dass der gemessene Widerstand des ballistischen Leiters direkt von der Invasivität der Spitzen abhängt. Dies deutet darauf hin, dass die Interaktion zwischen Messspitze und GNR bezüglich der Transporteigenschaften von großer Bedeutung ist. Abschließend werden mittels ortsaufgelöster Transportmessungen mit ultrakleinen Spitzenabständen mehrere, quantisierte Leitungskanäle detektiert, die sich räumlich über die Breite der zz-GNRs verteilen. Diese Kanäle können jeweils Kanten- und Volumen-Zuständen zugeordnet werden. Gestützt durch tight-binding-Berechnungen werden die quantisierten Transportkanäle durch transversale elektrische Felder erklärt, die durch asymmetrische Bindungsverhältnisse der Kanten erzeugt werden. Diese Ergebnisse unterstreichen, dass die Kantenmorphologie ein wesentlicher Parameter ist, um den elektronischen Transport in GNRs zu verstehen.