SOUZA, Aldilene Saraiva. Electronic Transport in Molecular Systems. 2012. 107 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2012. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-06-08T19:17:13Z No. of bitstreams: 1 2012_tese_assouza.pdf: 11520100 bytes, checksum: 04bcbbf301130ab097bf5c01340034df (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-06-08T19:52:21Z (GMT) No. of bitstreams: 1 2012_tese_assouza.pdf: 11520100 bytes, checksum: 04bcbbf301130ab097bf5c01340034df (MD5) / Made available in DSpace on 2015-06-08T19:52:21Z (GMT). No. of bitstreams: 1 2012_tese_assouza.pdf: 11520100 bytes, checksum: 04bcbbf301130ab097bf5c01340034df (MD5) Previous issue date: 2012 / Nesta tese apresentamos o estudo teórico de transporte eletrônico de dispositivos moleculares em dois problemas distintos. No primeiro, comparamos medidas via microscopia de tunelamento (STM) com cálculos de primeiros princípios onde a tensão aplicada em uma mono camada de moléculas auto-montadas, denominadas: 5-(4-piridina)-1,3,4-oxadiazol-2-tiol (HPYT) e 5-(4-fenil)-1,3,4-oxadiazol-2-tiol (HPOT) mostram a distribuição local de carga. Essas moléculas são depositadas sobre um substrato de ouro tipo (1 1 1). A formação destas camadas moleculares foi confirmada por medidas de STM. Cálculos baseados na teoria do funcional da densidade (DFT) foram realizados para obter a conformação mais estável da interação molécula/substrato. Verificamos uma grande semelhança entre os resultados teóricos e as medidas de imagem de STM. A partir desta comparação, sugerimos que o átomo de enxofre na molécula HPYT e HPOT está ligado à superfície de ouro por uma ligação direta à um único átomo de ouro. Para descrever a corrente de tunelamento ao longo da mono camada molecular sobre a superfície de Au (1 1 1) foi proposto um modelo quântico baseado na técnica de equação mestra. Nós investigamos também, propriedades de transporte de spin em uma cadeia de poliacetileno (como ponte) acoplada à uma nano fita de carbono tipo zigue-zague (ZGNRs) funcionando como eletrodos. Os cálculos de transporte foram efetuados usando técnica de funções de Green fora do equilíbrio (NEGF), combinada com a teoria do funcional da densidade (DFT). Trabalhos anteriores demonstraram que as ZGNRs exibem um ordenamento antiferromagnético (AF) e meia-metalicidade nos estados provenientes da borda, que podem ser destruídos com aplicação de um forte campo elétrico externo. Neste trabalho, nós demonstramos que a ligação entre a ponte molecular e átomos não-equivalentes de carbono (A/B) na sub rede de grafeno ZGNRs pode ocorrer de duas formas produzindo um sistema metálico ou semicondutor fortemente dependente do acoplamento local. Ao considerar o anel de carbono onde a cadeia está ligada, uma ligação se assemelha a uma ligação para no benzeno, enquanto a outra ligação é semelhante a uma ligação meta. Estas geometrias geram transmissão eletrônica distinta, que pode ser controlada sob um campo elétrico transversal.

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Modelling of phosphorus donor-based silicon (Si:P) qubit devices and mesoscopic single-electron devices is presented in this thesis. This theoretical analysis is motivated by the use of Si:P devices for scalable quantum computing. Modelling of Si:P single-electron devices (SEDs) using readily available simulation tools is presented. The mesoscopic properties of single and double island devices with source-drain leads is investigated through ion implantation simulation (using Crystal-TRIM), 3D capacitance extraction (FastCap) and single-electron circuit simulation (SIMON). Results from modelling two generations of single and double island Si:P devices are given, which are shown to accurately capture their charging behaviour. The trends extracted are used to forecast limits to the reduction in size of this Si:P architecture. Theoretical analysis of P2+:Si charge qubits is then presented. Calculations show large ranges for the SET measurement signal, Δq, and geometric ratio factor, α, are possible given the 'top-down' fabrication procedure. The charge qubit energy levels are calculated using the atomistic simulator NEMO 3-D coupled to TCAD calculations of the electrostatic potential distribution, further demonstrating the precise control required over the position of the donors. Theory has also been developed to simulate the microwave spectroscopy of P2+:Si charge qubits in a decohering environment using Floquet theory. This theory uses TCAD finite-volume modelling to incorporate realistic fields from actual device gate geometries. The theory is applied to a specific P2+:Si charge qubit device design to study the effects of fabrication variations on the measurement signal. The signal is shown to be a sensitive function of donor position. Design and analysis of two different spin qubit architectures concludes this thesis. The first uses a high-barrier Schottky contact, SET and an implanted P donor to create a double-well suitable for implementation as a qubit. The second architecture is a MOS device that combines an electron reservoir and SET into a single structure, formed from a locally depleted accumulation layer. The design parameters of both architectures are explored through capacitance modelling, TCAD simulation, tunnel barrier transmission and NEMO 3-D calculations. The results presented strengthen the viability of each architecture, and show a large Δq (> 0.1e) can be expected.

Modelling of phosphorus donor-based silicon (Si:P) qubit devices and mesoscopic single-electron devices is presented in this thesis. This theoretical analysis is motivated by the use of Si:P devices for scalable quantum computing. Modelling of Si:P single-electron devices (SEDs) using readily available simulation tools is presented. The mesoscopic properties of single and double island devices with source-drain leads is investigated through ion implantation simulation (using Crystal-TRIM), 3D capacitance extraction (FastCap) and single-electron circuit simulation (SIMON). Results from modelling two generations of single and double island Si:P devices are given, which are shown to accurately capture their charging behaviour. The trends extracted are used to forecast limits to the reduction in size of this Si:P architecture. Theoretical analysis of P2+:Si charge qubits is then presented. Calculations show large ranges for the SET measurement signal, Δq, and geometric ratio factor, α, are possible given the 'top-down' fabrication procedure. The charge qubit energy levels are calculated using the atomistic simulator NEMO 3-D coupled to TCAD calculations of the electrostatic potential distribution, further demonstrating the precise control required over the position of the donors. Theory has also been developed to simulate the microwave spectroscopy of P2+:Si charge qubits in a decohering environment using Floquet theory. This theory uses TCAD finite-volume modelling to incorporate realistic fields from actual device gate geometries. The theory is applied to a specific P2+:Si charge qubit device design to study the effects of fabrication variations on the measurement signal. The signal is shown to be a sensitive function of donor position. Design and analysis of two different spin qubit architectures concludes this thesis. The first uses a high-barrier Schottky contact, SET and an implanted P donor to create a double-well suitable for implementation as a qubit. The second architecture is a MOS device that combines an electron reservoir and SET into a single structure, formed from a locally depleted accumulation layer. The design parameters of both architectures are explored through capacitance modelling, TCAD simulation, tunnel barrier transmission and NEMO 3-D calculations. The results presented strengthen the viability of each architecture, and show a large Δq (> 0.1e) can be expected.

CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / Nesta tese apresentamos o estudo teÃrico de transporte eletrÃnico de dispositivos moleculares em dois problemas distintos. No primeiro, comparamos medidas via microscopia de tunelamento (STM) com cÃlculos de primeiros princÃpios onde a tensÃo aplicada em uma mono camada de molÃculas auto-montadas, denominadas: 5-(4-piridina)-1,3,4-oxadiazol-2-tiol (HPYT) e 5-(4-fenil)-1,3,4-oxadiazol-2-tiol (HPOT) mostram a distribuiÃÃo local de carga. Essas molÃculas sÃo depositadas sobre um substrato de ouro tipo (1 1 1). A formaÃÃo destas camadas moleculares foi confirmada por medidas de STM. CÃlculos baseados na teoria do funcional da densidade (DFT) foram realizados para obter a conformaÃÃo mais estÃvel da interaÃÃo molÃcula/substrato. Verificamos uma grande semelhanÃa entre os resultados teÃricos e as medidas de imagem de STM. A partir desta comparaÃÃo, sugerimos que o Ãtomo de enxofre na molÃcula HPYT e HPOT està ligado à superfÃcie de ouro por uma ligaÃÃo direta à um Ãnico Ãtomo de ouro. Para descrever a corrente de tunelamento ao longo da mono camada molecular sobre a superfÃcie de Au (1 1 1) foi proposto um modelo quÃntico baseado na tÃcnica de equaÃÃo mestra. NÃs investigamos tambÃm, propriedades de transporte de spin em uma cadeia de poliacetileno (como ponte) acoplada à uma nano fita de carbono tipo zigue-zague (ZGNRs) funcionando como eletrodos. Os cÃlculos de transporte foram efetuados usando tÃcnica de funÃÃes de Green fora do equilÃbrio (NEGF), combinada com a teoria do funcional da densidade (DFT). Trabalhos anteriores demonstraram que as ZGNRs exibem um ordenamento antiferromagnÃtico (AF) e meia-metalicidade nos estados provenientes da borda, que podem ser destruÃdos com aplicaÃÃo de um forte campo elÃtrico externo. Neste trabalho, nÃs demonstramos que a ligaÃÃo entre a ponte molecular e Ãtomos nÃo-equivalentes de carbono (A/B) na sub rede de grafeno ZGNRs pode ocorrer de duas formas produzindo um sistema metÃlico ou semicondutor fortemente dependente do acoplamento local. Ao considerar o anel de carbono onde a cadeia està ligada, uma ligaÃÃo se assemelha a uma ligaÃÃo para no benzeno, enquanto a outra ligaÃÃo à semelhante a uma ligaÃÃo meta. Estas geometrias geram transmissÃo eletrÃnica distinta, que pode ser controlada sob um campo elÃtrico transversal.

<p>The miniaturization of a MOSFET is the constant driving force in semiconductor technology over the decades. This scaling enables the realization of the ever complex and functional integration on a single chip where over tens of billions of transistors densely packed. Silicon (Si) is always the golden performer until recent years when the shrinking of a transistor becomes more and more difficult, due to phenomena such as short channel effect and mobility degradation, which is a challenge especially for atomic level scaling. The dawning of low dimensional materials, such as graphene, transition metal dichalcogenides (TMDs), black phosphorus (BP), with their natural atomically thin two-dimension (2D) layered structure and other novel properties, might serve as an alternative solution for ultimate scaling. However, the understanding of the electronic transport in these Van der Waals materials is still lacking. </p><p> In this research, the exploration of this material was first initiated on the vertical heterojunctions where two materials’ interfaces meet. Many previous literatures claimed this hetero-interface creates a P/N junction that results in a diode-like rectification. Yet, by careful analysis and comparing with our “real” vertical structures where the lateral components were eliminated, it is proved this rectification is a direct result from the contact region. The Schottky barrier on the drain side together with the gate effect is the true culprit.</p><p> Realizing how the Schottky barrier could be dominating in these 2D FETs, the second study is the Schottky barrier effect on the contact resistances and furthermore the mobility of the device. Because of the existence of the Schottky barrier between the channel and contact, the contact resistance is not negligible, unlike the ohmic contact for conventional Si MOSFETs. By comparing the intrinsic and extrinsic mobilities of TMD materials, It is found that the contact resistance’s response to the back gate, namely, the rate of how it changes with the back gate has a huge factor in determining whether the extrinsic field-effect mobility underestimates or overestimates its intrinsic mobility. This opens a new insight on the understanding of the transport mechanism under contacts for different TMDs.</p> With the understanding of the Schottky barrier FETs, lastly, the flexibility of these 2D materials is utilized to create high performance three-dimensionally stacked multi-channel FETs, from the inspiration of the Si gate-all-around nanosheet structure. A first-ever 3D integrated high performance MoS₂ device with two channels on top of each other was designed and fabricated, where the current is doubled with an extra layer of channel. The potential of these novel material to be implemented on the future generations of high-performance devices is demonstrated, shedding light on the prospect for extending the Moore’s Law with proper assistance from new materials.

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Recently, the proposal to use voltage amplification from ferroelectric negative capacitance (NC) to reduce the power dissipation in nanoelectronic devices has attracted significant attention. Homogeneous Landau theory predicts, that by connecting a ferroelectric in series with a dielectric capacitor, a hysteresis-free NC state can be stabilized in the ferroelectric below a critical film thickness. However, there is a strong discrepancy between experimental results and the current theory. Here, we present a comprehensive revision of the theory of NC stabilization with respect to scaling of material and device dimensions based on multi-domain Ginzburg–Landau theory. It is shown that the use of a metal layer in between the ferroelectric and the dielectric will inherently destabilize NC due to domain formation. However, even without this metal layer, domain formation can reduce the critical ferroelectric thickness considerably, limiting not only the range of NC stabilization, but also the maximum amplification attainable. To overcome these obstacles, the downscaling of lateral device dimensions is proposed as a way to prevent domain formation and to enhance the voltage amplification due to NC. These insights will be crucial for future NC device design and scaling towards nanoscale dimensions.

Jahresbericht des Instituts für Halbleiter- und Mikrosystemtechnik der Technischen Universität Dresden

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