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While investigations of semiconductor nanowires (NWs) has a long history, a significant progress is yet to be made in silicon carbide (SiC) NW technologies before they are ready to be utilized in electronic applications. In this dissertation work, SiC NW polytype control, NW axis orientation with respect to the growth substrate and other issues of potential technological importance are investigated. A new method for growing SiC NWs by vapor-liquid-solid mechanism was developed. The method is based on an in-situ vapor phase delivery of a metal catalyst to the growth surface during chemical vapor deposition. This approach is an alternative to the existing seeded catalyst method based on ex-situ catalyst deposition on the target substrate. The new SiC NW growth method provided an improved control of the NW density. It was established that the NW density is influenced by the distance from the catalyst source to the substrate and is affected by both the gas flow rate and the catalyst diffusion in the gas phase. An important convenience of the new method is that it yields NW growth on the horizontal substrate surfaces as well as on titled and vertical sidewalls of 4H-SiC mesas. This feature facilitates investigation of the NW growth trends on SiC substrate surfaces having different crystallographic orientations simultaneously, which is very promising for future NW device applications. It was established that only certain orientations of the NW axes were allowed when growing on a SiC substrate. The allowed orientations of NWs of a particular polytype were determined by the crystallographic orientation of the substrate. This substrate-dependent (i.e., epitaxial) growth resulted in growth of 3C-SiC NWs in total six allowed crystallographic orientations with respect to the 4H-SiC substrate. This NW axis alignment offers an opportunity to achieve a limited number of NW axis directions depending on the surface orientation of the substrate. The ease of controlling the NW density enabled by the vapor-phase catalyst delivery approach developed in this work, combined with the newly obtained knowledge about how to grow unidirectional (wellaligned) NW arrays, offer new opportunities for developing novel SiC NW electronic and photonic devices.

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The extraordinary properties of the single wall carbon nanotubes (SWCNTs) have stimulated an enormous amount of research towards the realization of SWCNT-based products for different applications ranging form nanocomposites to nanoelectronics. Their high charge mobility, exceedingly good current-carrying capacities and ability to be either semiconducting or metallic render them ideal building blocks for nanoelectronics. For nanoelectronic applications, either individual or parallel aligned SWCNTs are advantageous. Moreover, closely packed arrays of parallel SWCNTs are required in order to sustain the relatively large currents found in high frequency devices. Two key areas still require further development before the realization of large-scale nanoelectronics. They are the reproducible control of the nanotubes spatial position/orientation and chiral management. In terms of nanotube orientation, different techniques have been demonstrated for the fabrication of horizontally aligned SWCNTs with either post synthesis routes (e.g. dielectrophoresis and Langmuir-Blodgett approach) or direct growth (e.g. chemical vapor deposition (CVD)). The low temperature of the production process, allowing the formation of aligned nanotubes on pretty much any substrate, is the main advantage of the post synthesis routes, while the poor levels of reproducibility and spatial control, and the limited quality of the aligned tubes due to the inherently required process steps are limitations. The simplicity, up-scalability, along with the reproducible growth of clean high quality SWCNTs with well-controlled spatial, orientation and length, make CVD the most promising for producing dense horizontally well-aligned SWCNTs. These CVD techniques suffer some drawbacks, namely, that because they are synthesized using catalyst particles (metals or non-metals) the catalyst material can contaminate the tubes and affect their intrinsic properties. Thus, the catalyst-free synthesis of aligned SWNT is very desirable. This thesis comprises detailed and systematic experimental investigations in to the fabrication of horizontally aligned SWCNTs using both post growth (Dielectrophoresis) and direct growth (CVD) methods. Both catalyst-assisted and catalyst-free SWCNTs are synthesized by CVD. While metallic nanoparticles nucleate and grow SWCNTs, opened and activated fullerene structures are used for all carbon catalyst-free growth of single wall and double wall carbon nanotubes. The systematic studies allow for a detailed understanding of the growth mechanisms of catalyst and catalyst-free grown SWCNTs to be elucidated. The data significantly advances our understanding of horizontally aligned carbon nanotubes by both post synthesis alignment as well as directly as-synthesized routes. Indeed, the knowledge enables such tubes to be grown in high yield and with a high degree of special control. It is shown, for the first time, how one can grow horizontally aligned carbon nanotubes in crossbar configurations in a single step and with bespoke crossing angles. In addition, the transport properties of the aligned tubes at room temperature are also investigated through the fabrication of devices based on these tubes.:Introduction ……………………………………………………………….…………… 11 1 Carbon nanotubes basics ……………………………………………………. 15 1.1 sp2 hybridization …………………………………………………….……… 16 1.2 Graphene basics ………………………………………………………...… 16 1.3 Single wall carbon nanotubes Basics …………………………… 18 1.4 Synthesis of single wall carbon nanotubes ………………… 24 1.4.1 Arc discharge ………………………………………………… 24 1.4.2 Laser ablation ……………………………………………… 24 1.4.3 Chemical vapor deposition …………………………… 25 1.4.4 The as-produced carbon nanotubes …………… 25 1.5 Potential applications of single wall carbon nanotubes 26 1.6 Challenges face single wall carbon nanotubes ………… 27 2 Horizontally aligned single wall carbon nanotubes: a review of fabrication and characterization ………………………………………………… 29 2.1 Introduction …………………………………………...………………………………………… 29 2.2 The requisites of horizontally aligned single wall carbon nanotubes 31 2.3 Characterization of Horizontally aligned single wall carbon nanotubes 32 2.3.1 Electron microscopy …………………………………………………………… 32 2.3.2 Scanning probe microscopy ……………………………………...…………… 34 2.3.3 Spectroscopy ……………………………………………………………………… 35 2.4 Fabrication of horizontally aligned single wall carbon nanotubes ……… 36 2.4.1 Dielectrophoresis (Growth-then-place) …………………….…………… 36 2.4.2 Chemical vapor deposition (Growth-in-place) ………...…………… 40 2.5 Transistor performance from horizontally aligned single wall carbon nanotubes ……… 67 2.5.1 Field effect transistor ……………….…………...………………………….…… 67 2.5.2 Thin film transistor …………………………….…...…………………….……… 68 3 Dielelectrophoretic deposition of single wall carbon nanotubes 69 3.1 Deposition of single wall carbon nanotubes in between metallic electrodes ………………… 69 3.1.1 Dispersion of single wall carbon nanotubes ………………………… 69 3.1.2 Dielectrophoretic alignment of single wall carbon nanotubes 70 3.2 CNTFET topographical characterization …………..………………………..……… 70 3.3 Dielectrophoresis advantages and drawbacks ………………………….....……… 72 4 Growth of catalyst-assisted horizontally aligned single wall carbon nanotubes …..………..... 75 4.1 Experimental procedure ….………………………………………………………...……… 76 4.1.1 ST-cut quartz substrates preparation ……………………….....……… 76 4.1.2 Catalyst solutions preparation ……………………………........……… 76 4.1.3 Growth of single wall carbon nanotubes ……………………………… 77 4.1.4 Single wall carbon nanotubes transfer into silicon substrates 78 4.2 Substrate thermal treatment ………………………………………………..........……… 79 4.3 Formed catalyst nanoparticles ………………………………………………...……… 82 4.4 As-grown single wall carbon nanotubes ………………...……………..…………… 84 4.5 Transferred single wall carbon nanotubes ………………...………….……...…… 91 4.6 Chapter summary ………………………………………………...…………………………… 92 5 Growth of catalyst-free horizontally aligned single wall carbon nanotubes … 93 5.1 Experimental procedure ………………………………………………………………….… 94 5.1.1 Different fullerene-based structure ……………………...……………… 94 5.1.2 Pre-treatment of fullerene structures …………………………...…….. 95 5.1.3 Growth of catalyst-free single wall carbon nanotubes ………… 96 5.2 Different fullerene structures nucleate the growth of single wall carbon nanotubes …… 97 5.3 C60 nucleated aligned single wall carbon nanotubes .……………...………… 98 5.3.1 Orientation of the as-grown nanotubes …………………………..… 98 5.3.2 Yield of the grown nanotubes ……………………………………………… 99 5.3.3 Activated sp2 caps ……………………………………………………...……….… 103 5.3.4 Type of the grown nanotubes …………………………………...………… 106 5.3.5 Growth mechanism of carbon nanotubes nucleated from fullerene … 109 6 Electrical characterization of the aligned single wall carbon nanotubes ……… 113 6.1 Device fabrication …………………………………………………………………..…………… 114 6.1.1 FET fabrication over the dielectrophoretic deposited carbon nanotubes … 114 6.1.2 Fabrication of the CVD grown nanotubes based device …………114 6.2 Electrical characterization of dielectrophoretic deposited single wall carbon nanotubes 115 6.2.1 I-V characteristics of the dielectrophoretic deposited nanotubes 115 6.2.2 Defect detection ………………………………………………………………..…… 117 6.3 Electrical characterization of the CVD grown nanotubes ……………………… 120 6.3.1 IV-Characteristics of the metal-assisted single wall carbon nanotubes ……… 120 6.3.2 Electrical behaviour of the catalyst-free single wall carbon nanotubes …………122 7 Conclusions and outlook ……………..……………………..………………………… 125 Appendix ……..……………………………………..………………………….……………. 129 Bibliography …...…………………………………..………………………….……………. 133 List of figures ….…………………………………..………………………….……………. 143 Glossary …………..…………………………………..………………………….……………. 147 Publications ………………………………………..………………………….……………. 149 Curriculum vitae ……………………………………..………………..…………………. 153 Acknowledgment ……..…………………………………..…..…………………………. 155 Declaration …………………………………………………..…..…………………………. 157 / Die außergewöhnlichen Eigenschaften von einwandigen Kohlenstoffnanoröhren (engl. single wall carbon nanotubes, SWCNTs) haben bemerkenswerte Forschungsaktivitäten zur Verwirklichung von auf SWCNTs basierenden Anwendungen für verschiedene Bereiche, die von Nanokompositen bis hin zur Nanoelektronik reichen, stimuliert. Ihre hohe Ladungsträgermobilität und die außerordentlichen hohen Ladungsdichten, die in SWCNTs erreicht werden können sowie ihre Eigenschaft, entweder halbleitend oder metallisch zu sein, machen sie zu idealen Konstituenten von nanoelektronischen Schaltkreisen. Für Anwendungen in der Nanoelektronik sind entweder einzelne oder parallel angeordnete SWCNTs vorteilhaft. Darüber hinaus sind dicht gepackte Anordnungen von SWCNTs erforderlich, um die relativ hohen Ströme in Hochfrequenzbauelementen zu transportieren. Für eine erfolgreiche Realisierung von großskaligen nanoelektronischen Bauteilen, die auf SWCNTs basieren, sind noch zwei enorm wichtige Kernprobleme zu lösen, die weitere Forschungsanstrengungen erfordern: die reproduzierbare und verlässliche Kontrolle der räumlichen Positionierung und Orientierung der Nanoröhren sowie die Kontrolle der Chiralität der einzelnen SWCNTs. Hinsichtlich der Orientierung der Nanoröhren kann die horizontal parallele Ausrichtung von SWCNTs mit verschiedenen Techniken erreicht werden. Diese setzen entweder nach dem eigentlichen Wachstum der Röhren ein (Post-Synthese-Methoden wie z.B. Dielektrophorese oder Langmuir-Blodgett-Techniken) oder erreichen direkt während des Wachstums (z.B. durch Chemical-Vapor-Deposition-Methoden (CVD)) die parallele Anordnung. Durch die niedrigen Prozesstemperaturen, die während des Herstellungsprozesses erforderlich sind, erlauben die nach der eigentlichen Synthese stattfindenden Ausrichtungsmethoden die parallele Anordnung von Nanoröhren auf nahezu jedem Substrat, jedoch stellen die geringe Reproduzierbarkeit dieser Prozesse, die schwierige Kontrollierbarkeit der räumlichen Anordnung und die limitierte Qualität der ausgerichteten Röhren aufgrund der erforderlichen Prozessschritte natürliche Beschränkungen dieser Techniken dar. Die einfache Durchführung und ihre Skalierbarkeit, zusammen mit dem reproduzierbaren Wachstum qualitativ sehr hochwertiger SWCNTs mit hoher Kontrolle von räumlicher Anordnung, Orientierung und Länge machen die CVD-Methode zur erfolgversprechendsten Technik für die Herstellung von dichtgepackten hochparallelen horizontalen Anordnungen von SWCNTs. Diese CVD-Ansätze weisen jedoch auch einige Nachteile auf, die in den bei der Synthese verwendeten Katalysatorpartikeln (metallisch oder nicht-metallisch) begründet liegen, da das Katalysatormaterial die Röhren kontaminieren und dadurch ihre intrinsischen Eigenschaften beeinflussen kann. Daher ist eine katalysatorfreie Synthesemethode für ausgerichtete SWCNTs ein höchst erstrebenswertes Ziel. Die vorliegende Arbeit beschreibt detaillierte und systematische experimentelle Untersuchungen zur Herstellung von horizontalen, parallel ausgerichteten Anordnungen von SWCNTs unter Verwendung von Methoden, die sowohl nach dem eigentlichen Wachstum der Nanoröhren (Dielektrophorese) als auch während des Wachstums ansetzen (CVD). Bei den CVD-Methoden werden sowohl solche, die auf der Verwendung von Katalysatoren basieren, als auch katalysatorfreie Techniken verwendet. Während metallische Nanopartikel den Ausgangspunkt für das Wachstum von SWCNTs darstellen, werden geöffnete und aktivierte Fullerenstrukturen verwendet, um das katalysatorfreie Wachstum von reinen ein- oder mehrwandigen Nanoröhren zu erreichen. Die systematischen Untersuchungen ermöglichen ein tiefgehendes Verständnis der Wachstumsmechanismen von SWCNTs, die unter Verwendung von Katalysatoren oder katalysatorfrei erzeugt synthetisiert wurden. Die erzielten Ergebnisse erhöhen in einem hohen Maß das Verständnis der Herstellung von horizontal parallel angeordneten Nanoröhren, die durch Post-Synthese-Methoden oder direkt während des Wachstumsprozesses ausgerichtet wurden. Die erzielten Einsichten erlauben die Herstellung solcher Strukturen mit hoher Ausbeute und mit einem hohen Maß an räumlicher Kontrolle der Anordnung. Zum ersten Male kann ein Verfahren präsentiert werden, mit dem horizontal parallel angeordnete Nanoröhren in gekreuzten Strukturen mit wohldefinierten Kreuzungswinkeln hergestellt werden können. Zusätzlich werden die Transporteigenschaften von parallel ausgerichteten Nanoröhren bei Raumtemperatur, durch die Herstellung von auf den dargestellten Strukturen basierenden Bauelementen, untersucht.:Introduction ……………………………………………………………….…………… 11 1 Carbon nanotubes basics ……………………………………………………. 15 1.1 sp2 hybridization …………………………………………………….……… 16 1.2 Graphene basics ………………………………………………………...… 16 1.3 Single wall carbon nanotubes Basics …………………………… 18 1.4 Synthesis of single wall carbon nanotubes ………………… 24 1.4.1 Arc discharge ………………………………………………… 24 1.4.2 Laser ablation ……………………………………………… 24 1.4.3 Chemical vapor deposition …………………………… 25 1.4.4 The as-produced carbon nanotubes …………… 25 1.5 Potential applications of single wall carbon nanotubes 26 1.6 Challenges face single wall carbon nanotubes ………… 27 2 Horizontally aligned single wall carbon nanotubes: a review of fabrication and characterization ………………………………………………… 29 2.1 Introduction …………………………………………...………………………………………… 29 2.2 The requisites of horizontally aligned single wall carbon nanotubes 31 2.3 Characterization of Horizontally aligned single wall carbon nanotubes 32 2.3.1 Electron microscopy …………………………………………………………… 32 2.3.2 Scanning probe microscopy ……………………………………...…………… 34 2.3.3 Spectroscopy ……………………………………………………………………… 35 2.4 Fabrication of horizontally aligned single wall carbon nanotubes ……… 36 2.4.1 Dielectrophoresis (Growth-then-place) …………………….…………… 36 2.4.2 Chemical vapor deposition (Growth-in-place) ………...…………… 40 2.5 Transistor performance from horizontally aligned single wall carbon nanotubes ……… 67 2.5.1 Field effect transistor ……………….…………...………………………….…… 67 2.5.2 Thin film transistor …………………………….…...…………………….……… 68 3 Dielelectrophoretic deposition of single wall carbon nanotubes 69 3.1 Deposition of single wall carbon nanotubes in between metallic electrodes ………………… 69 3.1.1 Dispersion of single wall carbon nanotubes ………………………… 69 3.1.2 Dielectrophoretic alignment of single wall carbon nanotubes 70 3.2 CNTFET topographical characterization …………..………………………..……… 70 3.3 Dielectrophoresis advantages and drawbacks ………………………….....……… 72 4 Growth of catalyst-assisted horizontally aligned single wall carbon nanotubes …..………..... 75 4.1 Experimental procedure ….………………………………………………………...……… 76 4.1.1 ST-cut quartz substrates preparation ……………………….....……… 76 4.1.2 Catalyst solutions preparation ……………………………........……… 76 4.1.3 Growth of single wall carbon nanotubes ……………………………… 77 4.1.4 Single wall carbon nanotubes transfer into silicon substrates 78 4.2 Substrate thermal treatment ………………………………………………..........……… 79 4.3 Formed catalyst nanoparticles ………………………………………………...……… 82 4.4 As-grown single wall carbon nanotubes ………………...……………..…………… 84 4.5 Transferred single wall carbon nanotubes ………………...………….……...…… 91 4.6 Chapter summary ………………………………………………...…………………………… 92 5 Growth of catalyst-free horizontally aligned single wall carbon nanotubes … 93 5.1 Experimental procedure ………………………………………………………………….… 94 5.1.1 Different fullerene-based structure ……………………...……………… 94 5.1.2 Pre-treatment of fullerene structures …………………………...…….. 95 5.1.3 Growth of catalyst-free single wall carbon nanotubes ………… 96 5.2 Different fullerene structures nucleate the growth of single wall carbon nanotubes …… 97 5.3 C60 nucleated aligned single wall carbon nanotubes .……………...………… 98 5.3.1 Orientation of the as-grown nanotubes …………………………..… 98 5.3.2 Yield of the grown nanotubes ……………………………………………… 99 5.3.3 Activated sp2 caps ……………………………………………………...……….… 103 5.3.4 Type of the grown nanotubes …………………………………...………… 106 5.3.5 Growth mechanism of carbon nanotubes nucleated from fullerene … 109 6 Electrical characterization of the aligned single wall carbon nanotubes ……… 113 6.1 Device fabrication …………………………………………………………………..…………… 114 6.1.1 FET fabrication over the dielectrophoretic deposited carbon nanotubes … 114 6.1.2 Fabrication of the CVD grown nanotubes based device …………114 6.2 Electrical characterization of dielectrophoretic deposited single wall carbon nanotubes 115 6.2.1 I-V characteristics of the dielectrophoretic deposited nanotubes 115 6.2.2 Defect detection ………………………………………………………………..…… 117 6.3 Electrical characterization of the CVD grown nanotubes ……………………… 120 6.3.1 IV-Characteristics of the metal-assisted single wall carbon nanotubes ……… 120 6.3.2 Electrical behaviour of the catalyst-free single wall carbon nanotubes …………122 7 Conclusions and outlook ……………..……………………..………………………… 125 Appendix ……..……………………………………..………………………….……………. 129 Bibliography …...…………………………………..………………………….……………. 133 List of figures ….…………………………………..………………………….……………. 143 Glossary …………..…………………………………..………………………….……………. 147 Publications ………………………………………..………………………….……………. 149 Curriculum vitae ……………………………………..………………..…………………. 153 Acknowledgment ……..…………………………………..…..…………………………. 155 Declaration …………………………………………………..…..…………………………. 157

Functional oxides are versatile materials that can simultaneously enable efficiency gains and cost reductions in crystalline silicon (c-Si) solar cells. In this work, the deposition of functional oxide materials using atmospheric pressure chemical vapor deposition (APCVD) and the integration of these materials into c-Si solar cells are explored. Specifically, thin oxide films and multi-layer film stacks are utilized for the following purposes: (1) to minimize front surface reflectance without increasing parasitic absorption within the anti-reflection coating(s); (2) to maximize internal back reflectance of rear passivated cells, thereby increasing optical absorption of weakly absorbed long wavelength photons (? > 900 nm); (3) to minimize recombination losses by providing excellent surface passivation; and (4) to improve doping processes during cell manufacturing (e.g., emitter and surface field formation) by functioning as highly controllable dopant sources compatible with in-line diffusion processes. The oxide materials deposited by APCVD include amorphous and polycrystalline titanium oxide, aluminum oxide, boron-doped aluminum oxide, silicon oxide, phosphosilicate glass, and borosilicate glass. The microstructure, optical properties, and electronic properties of these films are characterized for different deposition conditions. Additionally, the impact of these materials on the performance of different types of c-Si solar cells is presented using both simulated and experimental current-voltage curves.

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