A method was developed for finding charge and current distribution in nanoscale electronic devices such as MOS capacitors and resonant tunneling diodes. A system of differential equations, comprised of the Poisson and Schrödinger equations, was solved iteratively to find the electric field and charge distribution inside devices under simulation. The proposed solution method was based on the non-equilibrium Green’s function approach, but expands on that approach by using spatially varying quasi-Fermi levels to construct density operators. The proposed method was applied to several example device models. The simulation results are presented. Calculated charge distributions in FET transistors were found to have necessary features: for example, the results showed inversion layer formation. However, the calculated current-voltage curves differed significantly from published experimental results and other simulators. Other published methods for charge transport simulation are compared to the proposed method.

A method was developed for finding charge and current distribution in nanoscale electronic devices such as MOS capacitors and resonant tunneling diodes. A system of differential equations, comprised of the Poisson and Schrödinger equations, was solved iteratively to find the electric field and charge distribution inside devices under simulation. The proposed solution method was based on the non-equilibrium Green’s function approach, but expands on that approach by using spatially varying quasi-Fermi levels to construct density operators. The proposed method was applied to several example device models. The simulation results are presented. Calculated charge distributions in FET transistors were found to have necessary features: for example, the results showed inversion layer formation. However, the calculated current-voltage curves differed significantly from published experimental results and other simulators. Other published methods for charge transport simulation are compared to the proposed method.

The high surface to volume ratio which is a property of nanoscale devices means the interfacial effects from these devices on the mass transport of analyte can be significant. Quartz nanopipette effect on the mass transport behavior of inorganic monovalent salts such as potassium chloride is shown to differ from those of conical nanopore. Quartz nanopipettes demonstrate a more significant interfacial impact on the mass transport behavior of inorganic salts. This is evidenced by significant impacts on ionic transport even at high electrolyte concentration where nanopore interfacial effects do not significantly impact the ion transport. Nanopipettes have been use to precipitate salts such as lithium chloride in bulk concentrations three orders of magnitude below the saturation concentration. These novel interfacial interactions have opened new avenues for crystallization of more complex organic biomolecules using inorganic systems as model systems on which to base the approach for these more complex systems.

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Nesta tese investigamos a partir de cálculos de primeiros princípios, dispositivos e componentes de dispositivos baseados em grafeno. Abordamos os campos da nanoeletrônica e da spintrônica. Dentro da nanoeletrônica investigamos: (i) propriedades de transporte de um nanotransistor de bicamada de grafeno na presença de um gate duplo. Demonstramos que sobre a ação de um campo elétrico externo, mesmo utilizando um gate da ordem de 10 nm, à temperatura ambiente e 4.5K uma corrente nula nunca é exibida. Esses resultados são explicados por um regime de tunelamento; (ii) propriedades eletrônicas e de transporte de multicamadas de grafeno em função do número de camadas e tipo de empilhamento entre elas. Mostramos que a estrutura eletrônica do sistema depende fortemente desse novo grau de liberdade de empilhamento. Na presença de um campo elétrico externo aplicado perpendicular ao sistema, o empilhamento do tipo Bernal nunca exibe um gap de energia, ao contrário do empilhamento romboédrico que exige um gap ajustável através da intensidade do campo. Mostramos também que é possível diferenciar os tipos de empilhamentos através da resistência do sistema e variando-se a temperatura; (iii) dentro das componentes de um nanotransistor mais realista, estudamos as propriedades eletrônicas e estruturais de: (a) bicamadas de grafeno sobre um substrato de nitreto de boro hexagonal. Neste sistema o limite de voltagens que podem ser aplicadas depende fortemente do número de camadas de h-BN e da direção do campo, onde quanto menos camadas maior é a voltagem que pode ser aplicada; (b) heteroestruturas compostas de bicamadas de grafeno, nitreto de boro hexagonal e cobre. Demonstramos que para uma aplicação direta em um dispositivo a configuração com uma bicamada de grafeno depositada sobre um substrato de h-BN e este conjunto sobre a superfície de cobre é a mais favorável. Nessa configuração é possível tanto controlar o gap na bicamada como a dopagem do sistema, sem a abertura de canais de condução através do dielétrico (h-BN). Dentro do campo da spintrônica estudamos: (i) propriedades de transporte das nanofitas de grafeno (GNR) (3,0) pristinas e dopadas com boro e nitrogênio. Para as GNR pristinas mostramos com os eletrodos em um alinhamento de spin anti-paralelo o sistema apresenta um comportamento de filtro de spin, onde para tensões de bias positivos/negativos somente o canal up/down conduz. Para as GNR dopadas com boro e nitrogênio, mostramos que as correntes para os diferentes canais de spin são não degeneradas ao longo de todo o intervalo de tensões aplicadas, apresentando desse modo um comportamento de filtro de spin; (ii) finalmente estudamos as propriedades de transporte de uma junção túnel magnética, composta de GNR intercaladas por uma nanofita de nitreto de boro hexagonal. Mostramos que esse sistema pode ser utilizado tanto como filtros de spin como elementos para dispositivos de magnetoresistência gigante, onde para este último a sua eficiência é muito mais pronunciada. / In this thesis we investigated by first principle calculations, devices and components of devices based on graphene. We covered the fields of nanoelectronics and spintronics. On the field of nanoelectronics we investigated: (i) the transport properties of a dual gate bilayer graphene nanotransistor. We showed that under the action of an external electrical field, even with a gate length of 10 nm, at room temperature and 4.5K a zero current is never exhibited. These results could be explained by a tunneling regime; (ii) the electronic and transport properties of few layer graphene, as function of the number and type of stacking of the layers. We show that the electronic structure strong deppends of the stacking order. On the presence of a external electrical field applied to the system, the Bernal stacking never presents a gap, contrary to the rombohedrical one, that posses a tuneable energy gap. Also we showed that is possible to differentiate the types of stacking by the resistance of the system and varying the temperature;(iii) for the components of a more realistic nanodevice, we study the structural and electronic properties of: (a) bilayer graphene over a hexagonal boron nitride substrate. We show that the voltages that could be applied to the system strongly depends of the number 0 layers and the direction of the field, where with more layers, smaller is the field; (b) heterostructures composed with bilayer graphene, hexagonal boron nitride and cooper. We show that for a direct application on a device, the better configuration is with a bilayer graphene over the hexagonal boron nitride, and this set over a cooper. In this configuration is possible to control both the gap and the doping of the system, without the creation of conducting channels through the dielectric (h-BN). On the field of spintronics, we study: (i) the transport properties (3,0) graphene nanoribbons pristines and doped with nitrogen and boron. For the pristine GNR we show that for the electrodes in an anti-parallel alignment the system presents a spin filter behavior, where for positive/negative bias the transport is only by up/down channel. For the GNR doped with nitrogen and boron we show that the current is non-degenerated in all range of voltages applied, presenting a spin filter behavior; (ii) finally, we study the transport properties of a magnetic tunnel junction, consisting of a GNR intercalated with a hexagonal boron nitride nanoribbon. We show that such system could be used both as a spin filter as a device that uses the the giant magnetoresistance effect, where for the last the system if more efficient.

The development of resistance measurement techniques is very important for characterization of future nanoelectronics. Pulsed IV measurement techniques are very useful for accurate resistance measurements on nanoscale samples because of the efficient removal of e.g. EMF errors. In the project we have designed a pulsed IV-setup based on a state-of-the art current source (6221) and nanovoltmeter (2182A) from Keithley, and used the setup for resistance measurements on ferromagnetic samples. Two different samples were investigated using the pulsed IV system – ferromagnetic wires with a central nanoconstriction and amorphous microwires. We have tested the pulse delta system with different pulse widths, duty cycles and voltage levels. The results show a successful integration of the setup. From the measurement results we confirm that the pulse delta system provides accurate measurements with a low noise of about 0.02Ω. The resistance of the samples increases approximately quadratically with bias which is interpreted as a heating effect due to the very high current density of about 107A∙cm-2.

L'industrie du semiconducteur, dans son effort visant à réduire la taille des nanocomposants, éprouve le besoin de prédire les propriétés physiques des composants futures. Alors que la taille de tels composants se réduit, les modèles semi-classiques en vigueur perdent de leur validité, puisque des effets quantiques, qui sont d'ordinaire invisibles dans des dispositifs en silicium plus grands, prévalent dans des dispositifs plus petits ou à base de matériaux semiconducteurs III-V. Par conséquent, les outils de simulation et de modélisation devraient décrire adéquatement les options technologiques en faveur qui sont aujourd'hui étudiées. Par conséquent, des simulations quantiques sont nécessaires au développement de transistors à effet de champ modernes.Le but de cette thèse de doctorat est de développer les outils appropriés à ces simulations et les utiliser pour étudier certaines des options de conception les plus importantes dans la technologie du transistor.C'est pourquoi nous avons utilisé le formalisme des fonctions de Green hors équilibre pour simuler le transport des porteurs de charge and étudier les transistors à effet de champ.Les structures de bande des semiconducteurs ont été calculées dans le cadre du formalisme k.p, mais nous avons aussi développé une méthode par pseudopotentiel atomique effectif pour effectuer des simulations pleine bande avec une variété d'ingrédients comme une orientation cristalline arbitraire, de la rugosité de surface, une composition d'alliage arbitraire dans le canal du transistor, et ainsi de suite. Cette méthode par pseudopotentiel donne des résultats précis pour un large ensemble de configurations avec un effort de paramétrage inférieur au formalisme k.p.Nous avons utilisé ces outils de simulation pour évaluer les propriétés de transport de FinFETs à base de silicium et d'InAs, en nous concentrant sur l'adaptabilité de la tension d'alimentation de dispositifs à base de III-V comparés à leurs équivalents en silicium. En particulier, nous discutons de la faisabilité de l'obtention d'un fort courant on dans les dispositifs III-V.Ensuite, nous appliquons ce formalisme à des nanofils gate-all-around (GAA) tunnel-FETs (TFETs) à base de III-V. Les tunnel-FETs sont une architecture prometteuse pour les transistors futurs, qui rencontre des problématiques d'optimisation et de performance. Nous avons pour but de faire une évaluation de l'effet de boosters technologiques sur les performances des TFETs, en particulier l'utilisation de contraintes mécaniques, et d'une hétérojonction III-V. Nous avons montré que ces boosters permettent aux TFETs de surpasser en théorie la technologie MOSFET standard, mais que la contraint induit des effets indésirables.Pour concevoir des TFETs à haute performance sans l'utilisation de la contrainte, nous avons enfin introduit un choix de conception qui exploite une gradation de la fraction molaire d'un alliage ternaire, ou alternativement un puits quantique dans la source. Ces configurations augmentent de manière dramatique la densité d'états dans le TFET à la jonction source/canal et sont donc capable d'améliorer les performances électriques des TFETs par rapport aux MOSFET conventionnels. / The semiconductor industry, in its continued effort to scale down nanoscale components further, needs to predict the physical properties of future components. As the size of such devices shrinks down, the currently prevalent semi-classical models start to fall apart, as quantum effects that are usually invisible in larger silicon devices gain in relevance in smaller and/or III-V based semiconductor devices. Therefore, modeling and simulation tools should describe adequately the favorite technological options that are currently under investigation. Consequently, full quantum simulations are necessary to the development of modern field effect transistors.The purpose of this PhD thesis is to develop the tools suitable for those simulations and use them to look into some of the most relevant design options for transistor technology.Hence, we used the Non Equilibrium Green's Functions formalism to simulate charge carriers transport and investigate field effect transistors.The semiconductor band structures were calculated within a continuous kp formalism, but we also developed an atomistic effective pseudopotential method to perform full-band simulations with a variety of ingredients like arbitrary crystal orientation, surface roughness, arbitrary alloy composition in the transistor channel, and so on. This pseudopotential method provides accurate results for a wider array of configurations with a smaller parametrization effort than the k.p formalism.We used these simulation tools to evaluate the transport properties of silicon and InAs based FinFETs, focusing on the supply-voltage scalability of III-V based devices compared to silicon counterparts. In particular, the feasibility of obtaining large on-current values in III-V devices is discussed.Then, we applied that formalism to III-V based gate all-around (GAA) nanowire tunnel-FETs (TFETs). Tunnel-FETs are a promising architecture for future transistors, facing optimization and performance challenges. We aimed at benchmarking the effect of technological boosters on the performances of TFETs, namely the use of strain engineering and of III-V heterojunctions. We've shown that these boosters allow TFETs to theoretically outperform standard MOSFET technology, but that strain engineering induces undesirable drawbacks.In order to design high performance TFETs without the use of strain, we finally introduced novel design options by exploiting a molar fraction grading of a ternary alloy or alternatively a quantum well in the source region. These device configurations dramatically change the density of state of the TFET at the source/channel junction and are therefore able to improve the electrical performance of TFETs with respect to conventional MOSFETs.

Nesta tese investigamos a partir de cálculos de primeiros princípios, dispositivos e componentes de dispositivos baseados em grafeno. Abordamos os campos da nanoeletrônica e da spintrônica. Dentro da nanoeletrônica investigamos: (i) propriedades de transporte de um nanotransistor de bicamada de grafeno na presença de um gate duplo. Demonstramos que sobre a ação de um campo elétrico externo, mesmo utilizando um gate da ordem de 10 nm, à temperatura ambiente e 4.5K uma corrente nula nunca é exibida. Esses resultados são explicados por um regime de tunelamento; (ii) propriedades eletrônicas e de transporte de multicamadas de grafeno em função do número de camadas e tipo de empilhamento entre elas. Mostramos que a estrutura eletrônica do sistema depende fortemente desse novo grau de liberdade de empilhamento. Na presença de um campo elétrico externo aplicado perpendicular ao sistema, o empilhamento do tipo Bernal nunca exibe um gap de energia, ao contrário do empilhamento romboédrico que exige um gap ajustável através da intensidade do campo. Mostramos também que é possível diferenciar os tipos de empilhamentos através da resistência do sistema e variando-se a temperatura; (iii) dentro das componentes de um nanotransistor mais realista, estudamos as propriedades eletrônicas e estruturais de: (a) bicamadas de grafeno sobre um substrato de nitreto de boro hexagonal. Neste sistema o limite de voltagens que podem ser aplicadas depende fortemente do número de camadas de h-BN e da direção do campo, onde quanto menos camadas maior é a voltagem que pode ser aplicada; (b) heteroestruturas compostas de bicamadas de grafeno, nitreto de boro hexagonal e cobre. Demonstramos que para uma aplicação direta em um dispositivo a configuração com uma bicamada de grafeno depositada sobre um substrato de h-BN e este conjunto sobre a superfície de cobre é a mais favorável. Nessa configuração é possível tanto controlar o gap na bicamada como a dopagem do sistema, sem a abertura de canais de condução através do dielétrico (h-BN). Dentro do campo da spintrônica estudamos: (i) propriedades de transporte das nanofitas de grafeno (GNR) (3,0) pristinas e dopadas com boro e nitrogênio. Para as GNR pristinas mostramos com os eletrodos em um alinhamento de spin anti-paralelo o sistema apresenta um comportamento de filtro de spin, onde para tensões de bias positivos/negativos somente o canal up/down conduz. Para as GNR dopadas com boro e nitrogênio, mostramos que as correntes para os diferentes canais de spin são não degeneradas ao longo de todo o intervalo de tensões aplicadas, apresentando desse modo um comportamento de filtro de spin; (ii) finalmente estudamos as propriedades de transporte de uma junção túnel magnética, composta de GNR intercaladas por uma nanofita de nitreto de boro hexagonal. Mostramos que esse sistema pode ser utilizado tanto como filtros de spin como elementos para dispositivos de magnetoresistência gigante, onde para este último a sua eficiência é muito mais pronunciada. / In this thesis we investigated by first principle calculations, devices and components of devices based on graphene. We covered the fields of nanoelectronics and spintronics. On the field of nanoelectronics we investigated: (i) the transport properties of a dual gate bilayer graphene nanotransistor. We showed that under the action of an external electrical field, even with a gate length of 10 nm, at room temperature and 4.5K a zero current is never exhibited. These results could be explained by a tunneling regime; (ii) the electronic and transport properties of few layer graphene, as function of the number and type of stacking of the layers. We show that the electronic structure strong deppends of the stacking order. On the presence of a external electrical field applied to the system, the Bernal stacking never presents a gap, contrary to the rombohedrical one, that posses a tuneable energy gap. Also we showed that is possible to differentiate the types of stacking by the resistance of the system and varying the temperature;(iii) for the components of a more realistic nanodevice, we study the structural and electronic properties of: (a) bilayer graphene over a hexagonal boron nitride substrate. We show that the voltages that could be applied to the system strongly depends of the number 0 layers and the direction of the field, where with more layers, smaller is the field; (b) heterostructures composed with bilayer graphene, hexagonal boron nitride and cooper. We show that for a direct application on a device, the better configuration is with a bilayer graphene over the hexagonal boron nitride, and this set over a cooper. In this configuration is possible to control both the gap and the doping of the system, without the creation of conducting channels through the dielectric (h-BN). On the field of spintronics, we study: (i) the transport properties (3,0) graphene nanoribbons pristines and doped with nitrogen and boron. For the pristine GNR we show that for the electrodes in an anti-parallel alignment the system presents a spin filter behavior, where for positive/negative bias the transport is only by up/down channel. For the GNR doped with nitrogen and boron we show that the current is non-degenerated in all range of voltages applied, presenting a spin filter behavior; (ii) finally, we study the transport properties of a magnetic tunnel junction, consisting of a GNR intercalated with a hexagonal boron nitride nanoribbon. We show that such system could be used both as a spin filter as a device that uses the the giant magnetoresistance effect, where for the last the system if more efficient.

A eletrônica molecular vem sendo investigada intensivamente por mais de vinte anos. Nesse sentido, as pesquisas científicas estão sendo focadas na busca de estruturas que possam ser utilizadas na construção de dispositivos em escalas nanométricas, que possam substituir a tecnologia tradicional do silício. O objetivo principal deste trabalho foi explorar as propriedades físicas de sistemas a base de grafano, um dos mais promissores materiais para serem usados como nanodispositivos. Para isso, foi realizada uma investigação teórica, baseada em cálculos de primeiros princípios, das propriedades estruturais e eletrônicas do grafeno numa forma pura ou com defeitos intrínsecos e extrinsecos. O primeiro grupo de estruturas investigadas foi o grafeno e grafano como nanofolhas constituídas por elementos do grupo IV da tabela periódica (C, SiC, Si, Ge e Sn). Também foram analisadas as mudanças nas propriedades eletrônicas do grafano do grupo IV com a substituição dos átomos de hidrogênio por flúor. A segunda parte do trabalho explorou as propriedades de defeitos estruturais em grafeno, tais como a monovacância, divacância, trivacância e Stone-Wales, e também o grafeno com dopantes (boro e nitrogênio) em diversas configurações. Todos os cálculos foram feitos utilizando métodos ab initio com base na teoria do funcional densidade. Foram estudadas algumas possíveis aplicações para os grupos de estruturas de grafeno investigados, através da análise de algumas de suas propriedades, tais como as densidades de estados próximas ao nível de Fermi e as estruturas de bandas eletrônicas para cada sistema. / The molecular electronics has been investigated for more than twenty years. In this sense, the scientific research has been focused on the search for structures that could be used in nanoelectronic devices that could replace the traditional silicon technology. The major goal of this work is to explore the physical properties of systems based on graphene, one of the most promising materials to be used in nanoelectronics. For that, an ab initio investigation was carried on the structural and electronic properties of graphene in its pristine form and with intrinsic and extrinsic defects. The first investigation explored the properties of group IV nanosheets (of C, SiC, Si, Ge e Sn), and the modifications on their properties as result of hydrogenation or fluorination. The second part of this work explored the physical properties of structural intrinsic defects in graphene, such as monovacancy, divacancy, trivacancy, and Stone-Wales ones. The work also explored the properties of boron and nitrogen dopants. All the calculations were performed using the ab initio methodology, based on the density functional theory.

A eletrônica molecular vem sendo investigada intensivamente por mais de vinte anos. Nesse sentido, as pesquisas científicas estão sendo focadas na busca de estruturas que possam ser utilizadas na construção de dispositivos em escalas nanométricas, que possam substituir a tecnologia tradicional do silício. O objetivo principal deste trabalho foi explorar as propriedades físicas de sistemas a base de grafano, um dos mais promissores materiais para serem usados como nanodispositivos. Para isso, foi realizada uma investigação teórica, baseada em cálculos de primeiros princípios, das propriedades estruturais e eletrônicas do grafeno numa forma pura ou com defeitos intrínsecos e extrinsecos. O primeiro grupo de estruturas investigadas foi o grafeno e grafano como nanofolhas constituídas por elementos do grupo IV da tabela periódica (C, SiC, Si, Ge e Sn). Também foram analisadas as mudanças nas propriedades eletrônicas do grafano do grupo IV com a substituição dos átomos de hidrogênio por flúor. A segunda parte do trabalho explorou as propriedades de defeitos estruturais em grafeno, tais como a monovacância, divacância, trivacância e Stone-Wales, e também o grafeno com dopantes (boro e nitrogênio) em diversas configurações. Todos os cálculos foram feitos utilizando métodos ab initio com base na teoria do funcional densidade. Foram estudadas algumas possíveis aplicações para os grupos de estruturas de grafeno investigados, através da análise de algumas de suas propriedades, tais como as densidades de estados próximas ao nível de Fermi e as estruturas de bandas eletrônicas para cada sistema. / The molecular electronics has been investigated for more than twenty years. In this sense, the scientific research has been focused on the search for structures that could be used in nanoelectronic devices that could replace the traditional silicon technology. The major goal of this work is to explore the physical properties of systems based on graphene, one of the most promising materials to be used in nanoelectronics. For that, an ab initio investigation was carried on the structural and electronic properties of graphene in its pristine form and with intrinsic and extrinsic defects. The first investigation explored the properties of group IV nanosheets (of C, SiC, Si, Ge e Sn), and the modifications on their properties as result of hydrogenation or fluorination. The second part of this work explored the physical properties of structural intrinsic defects in graphene, such as monovacancy, divacancy, trivacancy, and Stone-Wales ones. The work also explored the properties of boron and nitrogen dopants. All the calculations were performed using the ab initio methodology, based on the density functional theory.