Depuis sa découverte, le graphène a attiré beaucoup d’intérêt et ses propriétés remarquables font de lui un matériau très étudié par la communauté scientifique. Ce travail de thèse porte non pas sur ces propriétés intrinsèques, mais sur les possibilités de dopage et de fonctionnalisation du graphène pour d’éventuelles applications futures. Le choix du graphène épitaxié sur SiC comme matériau de base nous a permis d’avoir des échantillons adaptés aux études spectroscopiques (XPS, ARPES, NEXAFS) effectuées au synchrotron SOLEIL. Ces études sont indispensables pour la caractérisation macroscopique du graphène dopé et fonctionnalisé. La croissance epitaxiale permet à la fois le dopage in-situ et ex-situ. Dans un premier temps nous avons étudié l’influence de l’azote, élément voisin du carbone. Nous avons opté pour une technique de dopage in-situ, ce qui nous a permis d’avoir du graphène dopé dans un seul et même processus de fabrication. De plus nous avons pu déterminer les conditions de croissance pour obtenir une couche de nitrure de silicium (Si3N4) entre le graphène et le substrat. D’autre part nous avons utilisé l’oxygène pour fonctionnaliser le graphène. En exposant le graphène vierge à l’oxygène atomique et moléculaire, on a pu étudier l’évolution des états vide du graphène en présence d’oxygène. Les bords des grains de graphène sont particulièrement adaptés pour la fonctionnalisation à cause de leur activité chimique. Nous avons ainsi synthétisé du graphène avec des grains de petites dimension (~100 nm) pour avoir une forte densité de bords dans l’échantillon. De cette manière nous avons pu détecter, par absorption des rayons X, la signature de ces états de bord. / Since its discovery, graphene has attracted tremendous interest and its remarkable properties make it a material intensively studied by the scientific community. This thesis is not directly concerned with its intrinsic properties, but the possibilities of doping and functionalization of graphene for future possible applications and devices. The choice of epitaxial graphene on SiC as basic material allowed us to have samples well adapted for spectroscopic studies (XPS, ARPES and NEXAFS) carried out on a synchrotron facility (SOLEIL). These studies are essential for the macroscopic characterization of doped graphene and its functionalization. Epitaxial growth provides us the possibility to dope graphene both in-situ and ex-situ. We first opted for an in-situ doping technique studying the influence of nitrogen as a chemical dopant on the growth process. This allowed us to fabricate doped graphene in a one-step process. By tuning the parameters for epitaxial growth the creation of a silicon nitride layer was also observed. We also used atomic and molecular oxygen for the functionalization of graphene. By exposing pristine graphene to oxygen in an ex-situ process, we were able to study the evolution of empty states of graphene and the consequences on the electronic structure. The edges of graphene crystallites are particularly adapted for functionalization because of their chemical activity. The epitaxial growth on a 3C-SiC substrate allowed us to synthesize graphene with a reduced lateral size (~100 nm) and to have a higher density of edges in our sample. In this way we were able to detect the signature of these edge states using non-local spectroscopic methods.

Graphene is widely considered to be a promising candidate for a new generation of electronics, but there are many outstanding fundamental issues that need to be addressed before this promise can be realized. This thesis focuses on the production and properties of graphene grown epitaxially on the carbon terminated face (C-face) of hexagonal silicon carbide leading to the construction of a novel graphene transistor structure. C-face epitaxial graphene multilayers are unique due to their rotational stacking that causes the individual layers to be electronically decoupled from each other. Well-formed C-face epitaxial graphene single layers have exceptionally high mobilities (exceeding 10,000 cm ²/Vs), which are significantly greater than those of Si-face graphene monolayers. This thesis investigates the growth and properties of C-face single layer graphene. A field effect transistor based on single layer graphene was fabricated and characterized for the first time. Aluminum oxide or boron nitride was used for the gate dielectric. Additionally, an all graphene/SiC Schottky barrier transistor on the C-face of SiC composed of 2DEG in SiC/Si₂O ₃ interface and multilayer graphene contacts was demonstrated. A multiple growth scheme was adopted to achieve this unique structure.

As the thinnest material ever with high carrier mobility and saturation velocity, graphene is considered as a candidate for future high speed electronics. After pioneering research on graphene-based electronics at Georgia Tech, epitaxial graphene on SiC, along with other synthesized graphene, has been extensively investigated for possible applications in high frequency analog circuits. With a combined effort from academic and industrial research institutions, the best cut-off frequency of graphene radio-frequency (RF) transistors is already comparable to the best result of III-V material-based devices. However, the power gain performance of graphene transistors remained low, and the absence of a band gap inhibits the possibility of graphene in digital electronics. Aiming at solving these problems, this thesis will demonstrate the effort toward better high frequency power gain performance based on mono-layer epitaxial graphene on C-face SiC. Besides, a graphene/Si integration scheme will be proposed that utilizes the high speed potential of graphene electronics and logic functionality and maturity of Si-CMOS platform at the same time.

Graphene is a single sheet of graphite. While bulk graphite is semimetal, graphene is a zero bandgap semiconductor. Band structure calculations show graphene has a linear energy dispersion relation in the low energy region close to the Dirac points where the conduction band and the valence band touch. Carriers in graphene are described as massless Dirac fermions in contrast to massive carriers in normal metals and semiconductors that obey a parabolic energy dispersion relation. The uniqueness of graphene band structure indicates its peculiar electronic transport properties. In this thesis work, single- and multi-layer graphene films epitaxially grow on either the Si face or the C face of SiC substrates in a homemade induction vacuum chamber by thermal decomposition of SiC at high temperatures. The surface morphology and crystal structure of epitaxial graphene are studied with surface analysis tools. The transport properties of epitaxial graphene are studied by magnetotransport experiments. An epitaxial graphene film turns out to be a multilayered graphene because carriers in epitaxial graphene act as those in single layer graphene. Top gated and side gated epitaxial graphene field effect transistors (FETs) have also been successfully fabricated. These systematic studies unambiguously demonstrate the high quality of epitaxial graphene and the great potential of epitaxial graphene for electronic applications

Graphene, a honeycomb lattice of sp2-bonded carbon atoms, has received considerable attention in the scientific community due to its unique electronic properties. Distinct symmetries of the graphene wave functions lead to unusual quantum properties, such as a unique half-integer quantum Hall effect. As an added consequence of these symmetries, back-scattering in graphene is strongly prohibited leading to long coherence lengths of carriers. These charge carriers at low energy exhibit linear energy-momentum dispersion, much like neutrinos. Thus, carriers in graphene can be described as massless Dirac fermions. Graphene grown epitaxially on semiconducting substrates offers the possibility of large-scale production and deterministic patterning of graphene for nanoelectronics. In this work, epitaxial graphene is created on SiC(0001) by annealing in vacuum. Sequential scanning tunneling microscopy (STM) and spectroscopy (STS) are performed in ultrahigh vacuum at a temperature of 4.2 K and 300 K. These atomic-scale studies address the growth, interfacial properties, stacking order, and quasiparticle coherence in epitaxial graphene. STM topographic images show the atomic structure of successive graphene layers on the SiC substrate, as well as the character of defects and adatoms within and below the graphene plane. STS differential conductance (dI/dV) maps provide spatially and energy resolved snapshots of the local density of states. Such maps clearly show that scattering from atomic defects in graphene gives rise to energy-dependent standing wave patterns. We derive the carrier energy dispersion of epitaxial graphene from these data sets by quantifying the dominant wave vectors of the standing waves for each tunneling bias.

Nous présentons dans ce travail la caractérisation, essentiellement en transport, de couches de graphène épitaxiés élaborées par sublimation contrôlée de carbure de silicium (SiC). Des mesures de transport électroniques sont effectuées à basse température (T~1,6 K) et à fort champ magnétique. Dans une première partie, Il est indispensable de se focaliser sur la méthode spécifique (‘graphite cap') utilisée pour la fabrication de tous les échantillons étudiés dans ce travail au CNM, Barcelone. La méthode de ‘graphite cap' permet d'obtenir des couches de graphène en formes de rubans suffisamment isolés entre eux pour la fabrication de dispositifs électroniques. La croissance de graphène donne des résultats très différents suivant les conditions de croissance et les spécificités du substrat de carbure de silicium employé : les échantillons obtenus sur face carbone, et les échantillons sur face silicium.Sur face carbone, deux polytypes de SiC ont été utilisés pour l'élaboration de graphène : (i) sur le polytype ‘6H-SiC (on axis)', des rubans de graphène de l'ordre de 600 µm de longueur et de 6 µm de largeur sont obtenus. La largeur de graphène reste faible car le graphène suit la formation des marches sur le SiC résultant de la reconstruction de la surface pendant la croissance (‘step bunching'). Des monocouches ont été identifiées par spectroscopie Raman. Les résultats de transport sur ces monocouches montrent que la concentration de porteurs, de type trous, varie entre 5x1012cm-2 et 5x1013cm-2. L'effet Hall quantique n'est pas observé à cause du dopage élevé. Mais des oscillations de Shubnikov de Haas ont été bien résolues et étudiées pour extraire leurs phases. La phase des oscillations est égale à zéro, ce qui est une signature de la présence d'une monocouche de graphène.(ii) sur le polytype ‘4H-SiC (8° off axis)', les rubans obtenus sont plus larges et peuvent atteindre une longueur de 600 µm et une largeur de 50 µm. L'utilisation d'un substrat SiC avec une désorientation intentionnelle lors du clivage de la surface initiale permet la coalescence des rubans de graphène. Les résultats de transport sur les monocouches montrent que les porteurs sont toujours de type trous, mais beaucoup moins dopé sur plusieurs monocouches (de l'ordre 8x1011cm-2). L'effet Hall quantique est reporté sur un échantillon dont la mobilité atteint 11 000 cm²/V.s. Une étude à bas champ magnétique est encore réalisée et donnent des informations intéressantes sur l'(anti)localisation faible. Tous les phénomènes quantiques observés sont des signatures sur les propriétés intrinsèques des monocouches de graphène. Pour mieux appréhender le graphène épitaxié, il est important de faire varier la concentration de porteurs. Pour cela, une autre approche est proposée. Nous avons fabriqué une face arrière d'un échantillon semi-isolant par implantation d'ions d'azotes dans le SiC avant la croissance de graphène. Les résultats de transport obtenus sur les monocouches de graphène ont montré l'efficacité de cette grille pour contrôler le type de porteurs. L'effet Hall quantique a été observé pour les deux types de porteurs avec des plateaux de Hall remarquables en largeur (23 T).Sur la face Si, des multicouches de graphène couvrent uniformément toute la surface du substrat. Les multicouches de graphène sont plus épaisses sur les bords de marches que sur les terrasses, identifiées par spectroscopie Raman. Les porteurs sont maintenant de type électrons grâce à la couche de tampon qui existe sur la face Si. Les résultats de transport en champ magnétique et à basse température détectent l'existence d'une anisotropie électrique dues principalement aux marches du substrat SiC. / In this work, we present the characterization, mainly in transport, of epitaxial graphene layers produced by controlled sublimation of silicon carbide substrate (SiC). Electronic transport measurements are performed at low temperature (T ~ 1.6 K) and high magnetic field. In the first part, we explain the specific method ('graphite cap') used for growth of the samples studied in this work at CNM, Barcelona. The method of 'graphite cap' provides graphene ribbons homogeneous and isolated for the fabrication of electronic devices.Graphene on SiC gives very different results depending on the conditions of growth (temperature, pressure…) and the face of SiC substrate used: carbon face (C-face) or silicon face (Si-face).On the carbon face, two SiC polytypes have been used for the graphene growth:(i) On axis 6H-SiC: graphene ribbons are obtained on the whole surface. The length of ribbon approaches 600 µm and the width do not exceed 6 µm. The graphene follows the formation of steps on the SiC resulting from surface reconstruction during growth (‘step bunching'), which affects the graphene width. Monolayers were identified by Raman spectroscopy. For all measured samples, we found that the graphene is p-typed doped with a Hall concentration between 5x1012 and 5x1013cm-2. The quantum Hall effect is not observed because of the high doping level. But the Shubnikov de Haas oscillations (SdH) have been well resolved and studied. The phase of the oscillations is equal to zero, which is a signature from the presence of graphene monolayer.(ii) 8° off axis 4H-SiC: graphene ribbons obtained are larger and can reach a length of 600 µm and a width of 50 µm. The use of a SiC substrate with intentional disorientation upon cleavage of the initial surface allows the coalescence of the graphene ribbons. For all measured devices on this sample, we found that the graphene is p-typed doped (as determined from the sign of the Hall effect) with a Hall concentration between 8x1011 and 1013 cm-2. Mobilities varied between 1000 and 11000 cm²/Vs from device to device at 4K. Magnetoresistance revealed both Shubnikov-de Haas (SdH) oscillations, and interference phenomena (weak localization and antilocalization). For some low doped devices, Quantum Hall effect was observed. All quantum phenomena observed are signatures on the intrinsic properties of graphene monolayers.The main drawback of the epitaxial growth technique is the difficulty to control of the carrier density. Here, we investigate a bottom gate of a graphene device, epitaxially grown on the C-face of SiC substrate. The gate was realized by Nitrogen atoms implantation in the SiC crystal. The transport measurements have shown the effectiveness of the gate to control the type of carriers. The quantum Hall effect was observed for both types of carriers with remarkable Hall plateaus width (23 T).On the silicon face, we discuss results obtained from few layer graphene (FLG) grown epitaxially on the (0001) surface of a 6H-SiC substrate. Carriers are now like electrons through the buffer layer that exists on the Si face. The resulting FLG uniformly covers the substrate on which large step bunched terraces are also visible. The FLG is thicker at the step edges, as evidenced by micro-Raman analysis. Indeed, a noticeable anisotropy of the resistance has been detected by magnetotransport measurements at low temperature and high magnetic field. We will argue that this anisotropy originates from different mobilities, in the terraces and at the step edges.

In dieser Arbeit wird das direkte Wachstum von Graphen auf isolierenden Substraten untersucht. Die hergestellten Schichten werden mittels verschiedener Methoden untersucht, unter anderem Rasterkraftmikroskopie, Ramanspektroskopie und Synchrotron-Röontgendiffraktometrie. Zwei verschiedene Synthetisierungsmethoden kommen hierbei zur Anwendung. Zuerst wird das Wachstum von epitaktischem Graphen mittels thermischer Zersetzung von hexagonalen Siliciumcarbid-Oberflächen vorgestellt. Ein Fokus der Untersuchungen liegt hierbei auf den Stufen, welche auf der Substratoberfläche vorhanden sind. Wir zeigen, dass die initiale Oberflächenkonfiguration keinen unmittelbaren Einfluss auf den Wachstumsprozess und die entstehenden Graphenschichten besitzt. Die Stufen beeinflussen jedoch die elektrischen Transporteigenschaften im Quanten-Hall-Regime. Dieses Phänomen wird genauer untersucht und durch ein schematisches Modell erklärt. Die Struktur der epitaktischen Graphenschichten wird analysiert, inklusive präzieser Messungen der Gitterkonstanten. Anschließend werden Untersuchungen über das Wachstum von EG auf Kohlenstoff-terminierten SiC-Oberflächen vorgestellt und diskutiert. Als zweite Herstellungsmethode wird Molekularstrahlepitaxie verwendet. Wir demonstrieren Wachstum von Graphen auf zwei verschiedenen Substraten. Die Abhängigkeit der Morphologie und der strukturellen Qualität der Proben von den Wachstumsbedingungen wird untersucht. Wir zeigen, dass die Graphenschichten aus nanokristallinen Domänen bestehen, deren laterale Abmessungen 30 nm überschreiten. Die strukturelle Qualität der Graphenschichten nimmt mit zunehmender Substrattemperatur zu. Schließlich wird gezeigt, dass die Graphendomänen eine epitaktische Beziehung zu ihrem jeweiligen Substrat besitzen und dass eine beobachtete Reduzierung der Gitterparameter durch die Existenz von Punktdefekten zu erklären ist. / This thesis presents an investigation of graphene growth directly on insulating substrates. The graphene films are characterized using different techniques, including atomic force microscopy, Raman spectroscopy, and grazing-incidence X-ray diffraction. These allowed insight into the morphological, structural, and electrical properties of the graphene layers. Two different preparation methods were employed. The growth of epitaxial graphene on SiC(0001) by surface Si depletion is presented first. An important parameter in this type of growth is the surface steps present on the SiC substrate. We show that the initial SiC surface step configuration has little influence on the growth process, and the resulting graphene layers. The surface steps do impact the magneto-transport properties of graphene on SiC, which is investigated closely and can be explained by a schematic model. The structure of the epitaxial graphene layers is also analyzed, including precise measurements of the lattice constants. Additionally, the growth of graphene on the C-face of SiC is investigated. Graphene films were also synthesized directly on insulating substrates using molecular beam epitaxy. With the accurate deposition rates and sub-monolayer thickness control, MBE allows for fundamental studies of the growth process. We demonstrate graphene growth on two different substrates. The dependence of the morphology and structural quality of the graphene samples on the growth parameters is evaluated and discussed. We find that graphene films grown by MBE consist of nanocrystalline graphene domains with lateral dimensions exceeding 30 nm. The structural quality of the graphene layers improves with increasing substrate temperature during growth. Finally, we show that the nanocrystalline domains of the graphene films possess an epitaxial relation to either substrate, and attribute an observed contraction of the graphene lattice constant to the presence of point-defects within the film.

Als Graphen bezeichnet man eine einzelne freistehende Lage des Schichtkristalls Graphit. Im Gegensatz zur mechanischen Isolation von Graphit bietet die Züchtung auf Siliziumkarbid eine Methode zur großflächigen Herstellung von Graphen. Aufgrund der besonderen physikalischen Eigenschaften werden für Graphen viele verschieden Einsatzmöglichkeiten in diversen Bereichen prognostiziert. Mit seiner hohen Ladungsträgerbeweglichkeit ist Graphen besonders als Kanalmaterial für Feldeffekttransistoren (FET) interessant. Allerdings muss hierfür unter anderem ein geeignetes FET-Isolatormaterial gefunden werden. In dieser Arbeit wird eine detaillierte, theoretische Beschreibung der Graphen-FETs vorgestellt, die es erlaubt die steuerspannungsabhängige Hall-Konstante zu berechnen. Mit der dadurch möglichen Analyse können wichtige Kenngrößen, wie z. B. die Grenzflächenzustandsdichte des Materialsystems bestimmt werden. Außerdem wurden zwei Methoden zur Isolatorabscheidung auf Graphen untersucht. Siliziumnitrid, welches mittels plasmaangeregter Gasphasenabscheidung aufgetragen wurde, zeichnet sich durch seine n-dotierende Eigenschaft aus. Damit ist es vor allem für quasi-freistehendes Graphen auf Siliziumkarbid interessant. Bei der zweiten Methode handelt es sich um einen atomaren Schichtabscheidungsprozess, der ohne eine Saatschicht auskommt. An beiden Graphen- Isolator-Kombinationen wurde die neue Charakterisierung mittels der Hall-Datenanalyse angewandt.

The two-dimensional phase of carbon known as graphene is actively being pursued as a primary material in future electronic devices. The goals of this thesis are to investigate the growth and electronic properties of epitaxial graphene on SiC, with a particular focus on nanostructured graphene. The first part of this thesis examines the kinetics of graphene growth on SiC(0001) and SiC(0001 ̅) by high-temperature sublimation of the substrate using a custom-built, ultra-high vacuum induction furnace. A first-principles kinetic theory of silicon sublimation and mass-transfer is developed to describe the functional dependence of the graphene growth rate on the furnace temperature and pressure. This theory can be used to calibrate other graphene growth furnaces which employ confinement controlled sublimation. The final chapter in this thesis involves a careful study of self-organized epitaxial graphene nanoribbons (GNRs) on SiC(0001). Scanning tunneling microscopy of the sidewall GNRs confirms that these self-organized nanostructures are susceptible to overgrowth onto nearby SiC terraces. Atomic-scale imaging of the overgrown sidewall GNRs detected local strained regions in the nanoribbon crystal lattice, with strain coefficients as high as 15%. Scanning tunneling spectroscopy (STS) of these strained regions demonstrate that the graphene electronic local density of states is strongly affected by distortions in the crystal lattice. Room temperature STS in regions with a large strain gradient found local energy gaps as high as 400 meV. Controllable, strain-induced quantum states in epitaxial graphene on SiC could be utilized in new electronic devices. / Per request of the author and the advisor, and with the approval of the graduate office, the Acknowledgements page was replaced with an errata.

In this thesis work several manufacturing methods for graphene is discussed followed by an indepth study of graphene grown by a high temperature sublimation method (sublimation of siliconcarbide). The graphene surfaces studied have been grown by Graphensic AB, both graphenegrown on the Si-face and the C-face of the silicon carbide were studied. Six graphene samplesgrown 4H-SiC substrates were examined for homogeneity and surface morphology as well assome surface roughness parameters using Atomic Force Microscopy (AFM). The graphene wasstudied to get a better understanding of the surfaces and the growth mechanisms to improvemanufacturing parameters while also being informative for graphene sample customers. Anadditional graphene sample grown on 6H-SiC epitaxial layer was also studied to get a betterunderstanding of the sublimation mechanism. If graphene could be manufactured in a cheaprepeatable way the applications are endless and a new era of technology could emerge muchlike the silicon era that began several decades ago. In this thesis work the results are presentedas topography images as well as tables and histograms in the results section. The growth onthe Si-face is found to be well ordered when compared to the C-face which shows signs of alargely complex growth. The graphene on the Si-face lies on top of silicon carbide steps like acarpet with a buer layer interface against the silicon carbide. On the C-face this buer layeris not present but the graphene is deformed by buckling which is suspected to originate fromdierences in thermal properties between the graphene and the C-face. The in uence of AFMsettings for characterization of graphene while using intermittent mode have been evaluated andrecommendations are given. Finally a method for evaluating the homogeneity of the graphenelm is proposed but is in need of further verication.