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Die fortschreitenden Entwicklungen in der Mikro- und Nanotechnologie erfordern eine solide Unterstützung durch Simulationen. Numerische Bauelementesimulationen waren und sind dabei unerlässliche Werkzeuge, die jedoch zunehmend an ihre Grenzen kommen. So basieren sie auf Parametern, die für beliebige Atomanordnungen nicht verfügbar sind, und scheitern für stark verkleinerte Strukturen infolge zunehmender Relevanz von Quanteneffekten. Diese Arbeit behandelt den Transport in Siliziumnanodrähten sowie durch NiSi2-Si-Grenzflächen. Dichtefunktionaltheorie wird dabei verwendet, um die stabile Atomanordnung und alle für den elektronischen Transport relevanten quantenmechanischen Effekte zu beschreiben. Bei der Untersuchung der Nanodrähte liegt das Hauptaugenmerk auf der radialen Abhängigkeit der elektronischen Struktur sowie deren Änderung bei Variation des Durchmessers. Dabei zeigt sich, dass der Kern der Nanodrähte für den Ladungstransport bestimmend ist. Weiterhin kann ein Durchmesser von ungefähr 5 nm identifiziert werden, oberhalb dessen die Zustandsdichte im Nanodraht große Ähnlichkeiten mit jener des Silizium-Volumenkristalls aufweist und der Draht somit zunehmend mit Näherungen für den perfekt periodischen Kristall beschrieben werden kann. Der Fokus bei der Untersuchung der NiSi2-Si-Grenzflächen liegt auf der Symmetrie von Elektron- und Lochströmen im Tunnelregime, welche für die Entwicklung von rekonfigurierbaren Feldeffekttransistoren besondere Relevanz hat. Verschiedene NiSi2-Si-Grenzflächen und Verzerrungszustände werden dabei systematisch untersucht. Je nach Grenzfläche ist die Symmetrie dabei sehr unterschiedlich und zeigt auch ein sehr unterschiedliches Verhalten bei externer Verzerrung. Weiterhin werden grundlegende physikalische Größen mit Bezug zu NiSi2-Si-Grenzflächen betrachtet. So wird beispielsweise die Stabilität anhand von Grenzflächen-Energien ermittelt. Am stabilsten sind {111}-Grenzflächen, was deren bevorzugtes Auftreten in Experimenten erklärt. Weitere wichtige Größen, deren Verzerrungsabhängigkeit untersucht wird, sind die Schottky-Barrierenhöhe, die effektive Masse der Ladungsträger sowie die Austrittsarbeiten von NiSi2- und Si-Oberflächen. Ein Beitrag zur Modellentwicklung numerischer Bauelementesimulationen wird durch einen Vergleich zwischen den Ergebnissen von Dichtefunktionaltheorie-basierten Transportrechnungen und denen eines vereinfachten Models basierend auf der Wentzel-Kramers-Brillouin-Näherung geliefert. Diese Näherung ist Teil vieler numerischer Bauelementesimulatoren und erlaubt die Berechnung des Tunnelstroms basierend auf grundlegenden physikalischen Größen. Der Vergleich ermöglicht eine Evaluierung des vereinfachten Models, welches anschließend genutzt wird, um den Einfluss der grundlegenden physikalischen Größen auf den Tunneltransport zu untersuchen.:Index of Abbreviations 1. Introduction 2. Silicon Based Devices and Silicon Nanowires 2.1. Introduction 2.2. The Reconfigurable Field-effect Transistor 2.2.1. Design and Functionality 2.2.2. Fabrication 2.3. Overview Over Silicon Nanowires 2.3.1. Geometric Structure 2.3.2. Fabrication Techniques 2.3.3. Electronic Properties 3. Simulation Tools 3.1. Introduction 3.2. Electronic Structure Calculations 3.2.1. Introduction and Basis Functions 3.2.2. Density Functional Theory 3.2.3. Description of Exchange and Correlation Effects 3.2.4. Practical Aspects of Density Functional Theory 3.3. Electron Transport 3.3.1. Introduction 3.3.2. Scattering Theory 3.3.3. Wentzel-Kramers-Brillouin Approximation for a Triangular Barrier 3.3.4. Non-equilibrium Green’s Function Formalism A. Radially Resolved Electronic Structure and Charge Carrier Transport in Silicon Nanowires A.1. Introduction A.2. Model System A.3. Results and Discussion A.4. Summary and Conclusions A.5. Appendix A: Computational Details A.6. Appendix B: Supplementary Material A.6.1. Comparison of the Band Gap Between Relaxed and Unrelaxed SiNWs A.6.2. Band Structures for Some of the Calculated SiNWs A.6.3. Radially Resolved Density of States for Some of the Calculated SiNWs B. Electron Transport Through NiSi2-Si Contacts and Their Role in Reconfigurable Field-effect Transistors B.1. Introduction B.2. Model for Reconfigurable Field-effect Transistors B.2.1. Atomistic Quantum Transport Model to Describe Transport Across the Contact Interface B.2.2. Simplified Compact Model to Calculate the Device Characteristics B.3. Results and Discussion B.3.1. Characteristics of a Reconfigurable Field-effect Transistor B.3.2. Variation of the Crystal Orientations and Influence of the Schottky Barrier B.3.3. Comparison to Fabricated Reconfigurable Field-effect Transistors B.4. Summary and Conclusions B.5. Appendix: Supplementary Material B.5.1. Band Structure and Density of States of the Contact Metal B.5.2. Relaxation Procedure B.5.3. Total Transmission Through Multiple Barriers C. Formation and Crystallographic Orientation of NiSi2-Si Interfaces C.1. Introduction C.2. Fabrication and characterization methods C.3. Model System and Simulation Details C.4. Results and discussion C.4.1. Atomic structure of the interface C.4.2. Discussion of ways to modify the interface orientation C.5. Summary C.6. Appendix: Supplementary Material D. NiSi2-Si Interfaces Under Strain: From Bulk and Interface Properties to Tunneling Transport D.1. Introduction D.2. Model System and Simulation Approach D.3. Computational Details D.3.1. Electronic Structure Calculations (Geometry Relaxations) D.3.2. Electronic Structure Calculations (Electronic Structure) D.3.3. Device Calculations D.4. Tunneling Transport From First-principles Calculations D.4.1. Evaluation of the Current D.4.2. Isotropic Strain D.4.3. Anisotropic Strain D.5. Transport Related Properties and Effective Modeling Schemes D.5.1. Schottky Barrier Height D.5.2. Simplified Transport Model D.5.3. Models for the Schottky Barrier Height D.6. Summary and Conclusions D.7. Appendix: Supplementary Material D.7.1. Schottky Barriers of the {110} Interface Under Anisotropic Strain D.7.2. Silicon Band Structure, Electric Field, and Number of Transmission Channels D.7.3. k∥-resolved Material Properties D.7.4. Evaluation of the Work Functions and Electron Affinities D.7.5. Verification of the Work Function Calculation 4. Discussion 5. Ongoing Work and Possible Extensions 6. Summary Bibliography List of Figures List of Tables Acknowledgements Selbstständigkeitserklärung Curriculum Vitae Scientific Contributions / The ongoing developments in micro- and nanotechnologies require a profound support from simulations. Numerical device simulations were and still are essential tools to support the device development. However, they gradually reach their limits as they rely on parameters, which are not always available, and neglect quantum effects for small structures. This work addresses the transport in silicon nanowires and through NiSi2-Si interfaces. By using density functional theory, the atomic structure is considered, and all electron transport related quantum effects are taken into account. Silicon nanowires are investigated with special attention to their radially resolved electronic structure and the corresponding modifications when the silicon diameter is reduced. The charge transport occurs mostly in the nanowire core. A diameter of around 5 nm can be identified, above which the nanowire core exhibits a similar density of states as bulk silicon. Thus, bulk approximations become increasingly valid above this diameter. NiSi2-Si interfaces are studied with focus on the symmetry between electron and hole currents in the tunneling regime. The symmetry is especially relevant for the development of reconfigurable field-effect transistors. Different NiSi2-Si interfaces and strain states are studied systematically. The symmetry is found to be different between the interfaces. Changes of the symmetry upon external strain are also very interface dependent. Furthermore, fundamental physical properties related to NiSi2-Si interfaces are evaluated. The stability of the different interfaces is compared in terms of interface energies. {111} interfaces are most stable, which explains their preferred occurrence in experiments. Other properties, whose strain dependence is studied, include the Schottky barrier height, the effective mass of the carriers, and work functions. A contribution to the development of numerical device simulators will be given by comparing the results from density functional theory based transport calculations and a model based on the Wentzel-Kramers-Brillouin approximation. This approximation, which is often employed in numerical device simulators, offers a relation between interface properties and the tunneling transport. The comparison allows an evaluation of the simplified model, which is then used to investigate the relation between the fundamental physical properties and the tunneling transport.:Index of Abbreviations 1. Introduction 2. Silicon Based Devices and Silicon Nanowires 2.1. Introduction 2.2. The Reconfigurable Field-effect Transistor 2.2.1. Design and Functionality 2.2.2. Fabrication 2.3. Overview Over Silicon Nanowires 2.3.1. Geometric Structure 2.3.2. Fabrication Techniques 2.3.3. Electronic Properties 3. Simulation Tools 3.1. Introduction 3.2. Electronic Structure Calculations 3.2.1. Introduction and Basis Functions 3.2.2. Density Functional Theory 3.2.3. Description of Exchange and Correlation Effects 3.2.4. Practical Aspects of Density Functional Theory 3.3. Electron Transport 3.3.1. Introduction 3.3.2. Scattering Theory 3.3.3. Wentzel-Kramers-Brillouin Approximation for a Triangular Barrier 3.3.4. Non-equilibrium Green’s Function Formalism A. Radially Resolved Electronic Structure and Charge Carrier Transport in Silicon Nanowires A.1. Introduction A.2. Model System A.3. Results and Discussion A.4. Summary and Conclusions A.5. Appendix A: Computational Details A.6. Appendix B: Supplementary Material A.6.1. Comparison of the Band Gap Between Relaxed and Unrelaxed SiNWs A.6.2. Band Structures for Some of the Calculated SiNWs A.6.3. Radially Resolved Density of States for Some of the Calculated SiNWs B. Electron Transport Through NiSi2-Si Contacts and Their Role in Reconfigurable Field-effect Transistors B.1. Introduction B.2. Model for Reconfigurable Field-effect Transistors B.2.1. Atomistic Quantum Transport Model to Describe Transport Across the Contact Interface B.2.2. Simplified Compact Model to Calculate the Device Characteristics B.3. Results and Discussion B.3.1. Characteristics of a Reconfigurable Field-effect Transistor B.3.2. Variation of the Crystal Orientations and Influence of the Schottky Barrier B.3.3. Comparison to Fabricated Reconfigurable Field-effect Transistors B.4. Summary and Conclusions B.5. Appendix: Supplementary Material B.5.1. Band Structure and Density of States of the Contact Metal B.5.2. Relaxation Procedure B.5.3. Total Transmission Through Multiple Barriers C. Formation and Crystallographic Orientation of NiSi2-Si Interfaces C.1. Introduction C.2. Fabrication and characterization methods C.3. Model System and Simulation Details C.4. Results and discussion C.4.1. Atomic structure of the interface C.4.2. Discussion of ways to modify the interface orientation C.5. Summary C.6. Appendix: Supplementary Material D. NiSi2-Si Interfaces Under Strain: From Bulk and Interface Properties to Tunneling Transport D.1. Introduction D.2. Model System and Simulation Approach D.3. Computational Details D.3.1. Electronic Structure Calculations (Geometry Relaxations) D.3.2. Electronic Structure Calculations (Electronic Structure) D.3.3. Device Calculations D.4. Tunneling Transport From First-principles Calculations D.4.1. Evaluation of the Current D.4.2. Isotropic Strain D.4.3. Anisotropic Strain D.5. Transport Related Properties and Effective Modeling Schemes D.5.1. Schottky Barrier Height D.5.2. Simplified Transport Model D.5.3. Models for the Schottky Barrier Height D.6. Summary and Conclusions D.7. Appendix: Supplementary Material D.7.1. Schottky Barriers of the {110} Interface Under Anisotropic Strain D.7.2. Silicon Band Structure, Electric Field, and Number of Transmission Channels D.7.3. k∥-resolved Material Properties D.7.4. Evaluation of the Work Functions and Electron Affinities D.7.5. Verification of the Work Function Calculation 4. Discussion 5. Ongoing Work and Possible Extensions 6. Summary Bibliography List of Figures List of Tables Acknowledgements Selbstständigkeitserklärung Curriculum Vitae Scientific Contributions