This thesis describes the conductivity fluctuations or noise measurements in graphenebased field effect transistors. The main motivation was to study the effect of disorder on the electronic transport in graphene. In chapter 4, we report the noise measurements in graphene field effect (GraFET) transistors with varying layer numbers. We found that the density dependence of noise behaves oppositely for single and multilayer graphene. An analytical model has been proposed to understand the microscopic mechanism of noise in GraFETs, which reveals that noise is intimately connected to the band structure of graphene. Our results outline a simple portable method to separate the single layer devices from multi layered ones. Chapter 5 discusses the noise measurements in two systems with a bandgap: biased bilayer graphene and graphene nanoribbon. We show that noise is sensitive to the presence of a bandgap and becomes minimum when the bandgap is zero. At low temperature, mesoscopic graphene devices exhibit universal conductance fluctuations (UCF) arising due to quantum interference effect. In chapter 6, we have studied UCF in single layer graphene and show that it can be sensitive to the presence of various physical symmetries. We report that time reversal symmetry exists in graphene at low temperature and, for the first time, we observed enhanced UCF at lower carrier density where the scattering is dominated by the long-range Coulomb scattering. Chapter 7 presents the transport and noise measurements in single layer graphene in the quantum Hall regime. At ultra-low temperature several broken symmetry states appear in the lowest Landau level, which originate possibly due to strong electron-electron interactions. Our preliminary noise measurements in the quantum Hall regime reveal that the noise is sensitive to the bulk to edge transport and can be a powerful tool to investigate these new quantum states.

L'objectif de cette thèse dans le domaine de la nanoélectronique est de contribuer à l'analyse des phénomènes de transport électronique quantique dans les nanostructures. Nous développons ainsi spécifiquement la théorie de raccordement des champs de phase (PFMT). Cette approche algébrique décrit les propriétés électroniques du système par les liaisons fortes, mais repose fondamentalement sur la technique de raccordement de phase des états électroniques des électrodes avec ceux sur les nanojonctions moléculaires. En comparant certains de nos résultats avec ceux des méthodes de principes premiers, nous avons montré la justesse et fonctionnalité de notre approche. Une alternative pratique et générale aux nombreuses techniques basées sur la fonction de Green, elle est appliquée dans ce travail de thèse pour modéliser le transport électronique à travers de nanojonctions sous forme de fils mono et diatomiques, constitués d'éléments de Na, Cu, Co, C, Si, Ga et As, mono et multivalents. / The aim of this thesis in the nanoelectronics domain is to present a contribution to the analysis of the quantum electronic transport phenomena in nanostructures. For this purpose, we specifically develop the phase field matching theory (PFMT). Within this algebraic approach the electronic properties of the system are described by the tight-binding formalism, whereas the analysis of the transport properties based on the phase matching of the electronic states of the leads to the states of the molecular nanojunctions. By comparing some of our results with those of the first principles methods, we have shown the correctness and fonctionality of our approach. Moreover, our method can be considered as a practical and general alternative to the Green’s function-based techniques, and is applied in this work to model the electronic transport across mono and diatomic nanojunctions, consisting of mono and multivalent Na, Cu, Co, C, Si, Ga and As elements.

Ferromagnétisme et supraconductivité conventionnelle sont deux phases antagonistes dont la compétition peut facilement être étudiée dans les structures hybrides S/F. En particulier, une supraconductivité triplet odd-frequency peut être induite dans une couche ferromagnétique en régime diffusif.Dans une première partie, on étudie les courants qui circulent à l'équilibre dans de telles jonctions hybrides S/F. On prédit l'existence d'un courant triplet longue portée dans une bicouche ferromagnétique d'aimantation non homogène. La relation courant phase a la particularité d'être superharmonique. Ceci peut être interprété comme un effet Josephson entre un supraconducteur conventionelle et un supraconducteur triplet artificiellement induit à l'extrémité de la bicouche ferromagnétique. La compétition entre supraconductivité singulet et triplet peut aussi être observée dans le courant critique de certaines jonctions hybrides : le courant circulant entre deux reservoirs triplets au travers d'une couche supraconductrice conventionnelle peut présenter un maximum à température finie.Dans une seconde partie, on explore la combinaison de la supraconductivité et du ferromagnétisme avec en perspective la génération de courants de spin pour la spintronique. La resonance ferromagnetique (RFM) est un mécanisme de pompage de spin qui permet de générer des courants de spin sans appliquer de tension. Grâce à des effets d'interface, les signatures d'un courant de spin induit par RFM ont déjà été mesurées à température ambiante au bord d'un métal normal attaché à une couche ferromagnétique sous RFM. On prédit que l'effet survit à basse température quand le métal normal devient supraconducteur. / While ferromagnetism and conventional superconductivity appear as antagonist phases of nature, the proximity effect in hybrid S/F structures offers a unique opportunity to study their interplay. In particular, spin-triplet odd-frequency superconducting correlations may be induced in a diffusive ferromagnet.In a first part, we study the equilibrium current that may flow in hybrid S/F Josephson junctions. We exhibit signatures of odd-frequency triplet correlations. In particular, we predict the existence of a long range triplet current through a non-collinear bilayer ferromagnet with a peculiar superharmonic current phase relation. This can be viewed as the Josephson effect between a conventional superconductor and an effective triplet superconductor generated at the end of the bilayer ferromagnet. Then, we study the competition between triplet and singlet superconductivity in the temperature dependence of the critical current. Namely, the critical current flowing between two effective triplet reservoirs through a conventional superconducting layer may display a maximum at finite temperature.

Ce manuscrit porte sur la conception d’un transistor à effet de champ destiné à l’observation de la photoluminescence de l’exciton et des complexes excitoniques chargés du ZnO sous l’influence d’un champ électrique. Pour cela, des simulations ont permis de définir un cahier des charges de la structure du transistor afin de bloquer la conductivité dans le canal de ZnO et d’appliquer un champ électrique intense. La seconde partie concerne le choix du matériau de grille et de l’électrode transparente de surface pour l’observation de la photoluminescence dans le canal. L’oxyde de gallium (-Ga2O3) a été choisi car il présente un grand gap, des propriétés d’isolant et de semi-conducteur avec dopage. Cependant les films de Ga2O3 dopés avec Ti, Sn, Zn et Mg élaborés par MOCVD n’ont pas révélé de conductivité. Les films d’alliages (Ga,Sn)2O3 n’ont pas non plus montré de conductivité et leur structure est étudiée intensivement. Des traitements plasma radiofréquence sous flux d’argon, d’oxygène ou d’hydrogène ont permis de montrer que l’implantation de l’hydrogène donne lieu à un niveau donneur avec une énergie d’activation de 7 meV. La conductivité est toutefois modulée par le dopage en Sn et les traitements s’accompagnent d’un changement de la sous-stœchiométrie en oxygène qui diminue la transparence à cause de la formation de niveau profond de lacune d’oxygène. La structure finale de la grille transparente dans l’ultraviolet pour l’observation de la photoluminescence du ZnO peut donc être élaborée par une grille diélectrique de -Ga2O3 puis une électrode conductrice transparente de (Ga,Sn)2O3 traitée superficiellement par un plasma d’hydrogène. / This manuscript covers the design of a field transistor for the observation of photoluminescence of the exciton and the charged excitonic complex of ZnO under the influence of an electric field. For this, simulations have helped to define the specifications of the transistor structure to block the conductivity in the ZnO channel and applying a strong electric field. The second part concerns the choice of gate material and the surface transparent electrode for the observation of photoluminescence in the channel. The gallium oxide (-Ga2O3) was chosen because it has a large gap, insulating properties and semiconductor properties with doping. However, Ga2O3 films doped with Ti, Sn, Zn and Mg MOCVD did not show conductivity. Films of alloys (Ga,Sn)2O3 have not shown either conductivity and their structure is studied intensively. Radio frequency plasma treatment under a flux of argon, oxygen or hydrogen have shown that implantation of hydrogen gives rise to a donor level with 7 meV activation energy. However, the conductivity is modulated by doping Sn and treatments are accompanied by a change of sub-stoichiometry in oxygen, which reduces the transparency due to the formation of deep level of oxygen vacancy. The final structure of the transparent gate in the ultraviolet for the observation of photoluminescence of ZnO can be prepared by a dielectric gate -Ga2O3 and a transparent conductive electrode of (Ga,Sn)2O3 surface treated by a plasma of hydrogen.

Uma das metas para a expansão do conhecimento em eletrônica molecular pode ser encontrada no projeto relacionado com a criação de circuitos em nanoescala com base em características de corrente-tensão não lineares, composto por moléculas ligadas a eletrodos metálicos sob a ação de um campo elétrico externo. O desenvolvimento de dispositivos eletrônicos moleculares que utilizam sistemas exibindo recursos semelhantes como materiais semicondutores intrínsecos é uma das metas a serem atingidas por uma extensa pesquisa em nanotecnologia. Assim, este trabalho tem como objetivo a expansão do conhecimento não só do transporte eletrônico, mas também as características físicas que justificam o regime de ressonância para o transporte eletrônico, como a condutância e Espectroscopia de Voltagem de Transição. Foi investigado teoricamente o transporte de carga eletrônica em um sistema molecular composto por estruturas de carbyne levando em conta as variações nos tipos de ligações −≡− (ligações simples e triplas para cada carbono). Os cálculos e aproximações ab initio são realizados para investigar a distribuição de estados de elétrons através da molécula na presença de um campo elétrico externo. Este novo dispositivo nanoeletrônico suscitará vantagem para o projeto de grandes circuitos orgânicos/metálicos híbridos 1D com um aumento do fluxo eletrônico que é importante para as necessidades da nanotecnologia. / One of the goals for the expansion of knowledge in molecular electronics may be found in the Project related to the creation of nanoscale circuits based on nonlinear current–voltage characteristics composed by molecules connected to metallic electrodes under the action of an external electric field. The development of molecular electronic devices using systems exhibiting similar feature as intrinsic semiconductor materials is one of the goals to be achieved by an extensive research in nanotechnology. Thus, this work aims at expanding of knowledge not only of the electronic transport, but also the physical features that justify the resonant regime for electronic transport, such as conductance and Transition Voltage Spectroscopy. We theoretically investigate the electronic charge transport in a molecular system composed by carbyne structures taking into account the variations in the bonds −≡− type (simple and triple bonds for each carbon). Ab initio calculations approximations are performed to investigate the distribution of electron states over the molecule in the presence of an external electric field. This new nanoelectronic device raise up advantage for the design of large 1D hybrid organic/metallic circuits with an increased electronic flow that is importante for the needs of nanotechnology.

This thesis studies electron spin phenomena in lateral semi-conductor quantum dots/anti-dots and electron conductance in graphene nanoribbons by numerical modelling. In paper I we have investigated spin-dependent transport through open quantum dots, i.e., dots strongly coupled to their leads, within the Hubbard model. Results in this model were found consistent with experimental data and suggest that spin-degeneracy is lifted inside the dot – even at zero magnetic field. Similar systems were also studied with electron-electron effects incorporated via Density Functional Theory (DFT) in the Local Spin Density Approximation (LSDA) in paper II and III. In paper II we found a significant spin-polarisation in the dot at low electron densities. As the electron density increases the spin polarisation in the dot gradually diminishes. These findings are consistent with available experimental observations. Notably, the polarisation is qualitatively different from the one found in the Hubbard model. Paper III investigates spin polarisation in a quantum wire with a realistic external potential due to split gates and a random distribution of charged donors. At low electron densities we recover spin polarisation and a metalinsulator transition when electrons are localised to electron lakes due to ragged potential profile from the donors. In paper IV we propose a spin-filter device based on resonant backscattering of edge states against a quantum anti-dot embedded in a quantum wire. A magnetic field is applied and the spin up/spin down states are separated through Zeeman splitting. Their respective resonant states may be tuned so that the device can be used to filter either spin in a controlled way. Paper V analyses the details of low energy electron transport through a magnetic barrier in a quantum wire. At sufficiently large magnetisation of the barrier the conductance is pinched off completely. Furthermore, if the barrier is sharp we find a resonant reflection close to the pinch off point. This feature is due to interference between a propagating edge state and quasibond state inside the magnetic barrier. Paper VI adapts an efficient numerical method for computing the surface Green’s function in photonic crystals to graphene nanoribbons (GNR). The method is used to investigate magnetic barriers in GNR. In contrast to quantum wires, magnetic barriers in GNRs cannot pinch-off the lowest propagating state. The method is further applied to study edge dislocation defects for realistically sized GNRs in paper VII. In this study we conclude that even modest edge dislocations are sufficient to explain both the energy gap in narrow GNRs, and the lack of dependance on the edge structure for electronic properties in the GNRs.

The objective of this work was to study the noise in semiconductors and relate the transport mechanisms in the devices with the noise from the devices. The main part of the work was to set up a system for the measurement of noise in semiconductor devices. To establish the sensitivity of the system, it was calibrated at different temperatures. Some of the results from GaAs pn-junction showed some anomaly from that available in the literature. But certain points are yet to be clarified. This requires certain developments in the measurement system. In the case of QWIPS structures, studies on some samples with varying number of wells are required and in order to study the GR noise spectra and other activated processes, we need to study the temperature dependence of the noise and a larger bias variation for studying the low frequency current noise.

This thesis deals with inelastic effects in electronic currents. We developed a time-dependent technique and show that this approach gives rich insight into electron-phonon coupling during transport. We compare our results with a time-independent technique and analyse the validity of our model. Finally, the results of a quantum chemistry calculation are presented in the framework of scanning tunneling miscroscopy (STM). We study the chemisorption of a tetrathiafulvalene molecule on a gold surface by performing the calculation of the charge transfer, the induced dipole, and the STM images using the density functional theory.

Organic and inorganic nanostructured materials, nano- and mesoscale objects and devices, and their integration into existing microelectronic technologies have been at the center of recent fundamental and applied research in nanotechnology. One of the critical needs is to develop an enhanced predictive capability of structure-property correlations and enable robust high performance systems by design. My thesis work was concerned with the theoretical and experimental studies of electronic and mechanical properties of chemically functionalized nanowires. I will describe a theoretical approach for investigating structure-property correlations in atomic-sized metallic wires based on the Density Functional Theory (DFT) for structure calculations and the Non-equilibrium Green's Function (NEGF) technique for electronic transport properties simulations. This synergistic approach is shown to yield the atomic structure of the smallest niobium nanowires. Furthermore, the method was applied to simulate electronic properties of chemically functionalized graphene nanoribbons. Further, I will demonstrate an experimental technique for simultaneous measurements of force and conductance in atomic-size objects based on quartz tuning fork piezoelectric sensors. A peculiar scaling effect, relevant for a broad range of test and measurement applications, namely the squeeze film effect, was observed during the development of the sensors. Using theoretical analysis based on finite element simulations of the hydrodynamic behavior of the sensors in a broad range of ambient conditions, I explain the observed phenomenon.

SOUSA, Duarte José Pereira de. Transporte eletrônico em anéis quânticos de grafeno. 2015. 83 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2015. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2016-01-05T19:55:02Z No. of bitstreams: 1 2015_dis_djpsousa.pdf: 9267324 bytes, checksum: 1dab83c7a9473498a415cb6cb9e5bf4b (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2016-01-05T19:55:40Z (GMT) No. of bitstreams: 1 2015_dis_djpsousa.pdf: 9267324 bytes, checksum: 1dab83c7a9473498a415cb6cb9e5bf4b (MD5) / Made available in DSpace on 2016-01-05T19:55:40Z (GMT). No. of bitstreams: 1 2015_dis_djpsousa.pdf: 9267324 bytes, checksum: 1dab83c7a9473498a415cb6cb9e5bf4b (MD5) Previous issue date: 2015 / In this work, we propose a current switch device that exploits the phase acquired by a charge carrier as it tunnels through a potential barrier in graphene in the ballistic regime without the need of the presence of a gap in the spectrum. The system acts as an interferometer based on an armchair graphene quantum ring, where the phase difference between interfering electronic wave functions for each path can be controlled by tuning the height of a potential barrier in the ring arms. By varying the parameters of the potential barriers the interference can become completely destructive. We demonstrate how this interference effect can be used for developing a simple graphene-based logic gate. / Neste trabalho, é proposto um dispositivo de controle de corrente que explora a fase adquirida por um portador de carga quando este tunela através de uma barreira de potencial no grafeno no regime balístico sem a necessidade da presença de um gap no espectro de energias. O sistema atua como um interferômetro baseado em um anel quântico de grafeno com bordas armchair, onde a diferença de fase entre as funções de onda para elétrons que tomam diferentes caminhos pode ser controlada através da intensidade das barreiras de potencial nos braços do anel. Variando os parâmetros das barreiras a interferência pode tornar-se completamente destrutiva. É demonstrado como esse efeito de interferência pode ser utilizado para o desenvolvimento de portas lógicas simples baseadas em grafeno.