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Enhanced field emission (EFE) presents the main impediment to higher acceleration gradients in superconducting niobium (Nb) radiofrequency cavities for particle accelerators. The strength, number and sources of EFE sites strongly depend on surface preparation and handling. The main objective of this thesis project is to systematically investigate the sources of EFE from Nb, to evaluate the best available surface preparation techniques with respect to resulting field emission, and to establish an optimized process to minimize or eliminate EFE. To achieve these goals, a scanning field emission microscope (SFEM) was designed and built as an extension to an existing commercial scanning electron microscope (SEM). In the SFEM chamber of ultra high vacuum, a sample is moved laterally in a raster pattern under a high voltage anode tip for EFE detection and localization. The sample is then transferred under vacuum to the SEM chamber equipped with an energy-dispersive x-ray spectrometer for individual emitting site characterization. Compared to other systems built for similar purposes, this apparatus has low cost and maintenance, high operational flexibility, considerably bigger scan area, as well as reliable performance. EFE sources from planar Nb have been studied after various surface preparation, including chemical etching and electropolishing, combined with ultrasonic or high-pressure water rinse. Emitters have been identified, analyzed and the preparation process has been examined and improved based on EFE results. As a result, field-emission-free or near field-emission-free surfaces at ~140 MV/m have been consistently achieved with the above techniques. Characterization on the remaining emitters leads to the conclusion that no evidence of intrinsic emitters, i.e., no fundamental electric field limit induced by EFE, has been observed up to ~140 MV/m. Chemically etched and electropolished Nb are compared and no significant difference is observed up to ~140 MV/m. To address concerns on the effect of natural air drying process on EFE, a comparative study was conducted on Nb and the results showed insignificant difference under the experimental conditions. Nb thin films deposited on Cu present a possible alternative to bulk Nb in superconducting cavities. The EFE performance of a preliminary energetically deposited Nb thin film sample are presented. / Ph. D.

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The aim of this work is to produce a manufacturing strategy for high current (>10 mA) field emission (FE) devices for military (microwave generation) and civilian (particle accelerator electron beam) applications using carbon nanotubes (CNT) as base material. With a particular focus on the relationship of the laser time pulse duration used for cutting CNTs and how this affects the field emission performance. Material selection for this work was narrowed to CNT as they possess unique characteristics such as: high aspect ratio; high thermal conductivity; high chemical stability and high current carrying capacities up to a theoretical limit of 1,200 MA∙cm-1 making them an ideal material for FE. The CNT material studied in this work is produced in two distinct forms, fibres (∅~7-10 μm) and films (h~30 μm), using a floating catalyst chemical vapour deposition process which produces high quantities of CNT material with mixed mechanical and electrical properties. The material is difficult to handle because of its dimensions and is susceptible to environmental changes i.e. electrostatic forces. In order to reduce the variability in electrical properties, a laser purification process was developed. The process consists of locally irradiating an infra-red (IR) laser several microseconds directly at the material. A percentage is vaporised (mainly non-conductive or defective material) and the remaining CNT material shows very high crystallinity with an increase of up to ten times (G/D ratio > 100) compared to the original material and electron mean free path is increased by an order of magnitude. The production strategy is based on directly coating the CNT material with copper using an electroplating process. This allowed for CNT fibre and film to be easily handled and improved the overall electrical contact. Emitter geometry was customised by a laser cutting process to achieve increased enhancement factor geometries, in this case, triangles with 29 tips whilst reducing FE variability. FE performance was quantified by testing the devices in a continuous DC mode with a sweep up to 1,000 V until the material suffered catastrophic failure. The gap distance between the tip of the triangles and the anode was varied to increase the electric field until failure. FE results using the production strategy improved more than 400% compared to untreated material. Applications for these devices are intended to be in the creation of high energy electron beam lines and generation of high powered directed microwaves.

No description available.

Vacuum microelectronic (VME) devices are of interest for the development of flat-screen displays and microwave devices. In many cases, their operation depends on the field emission of electrons from micron-sized cathodes (semiconductor or metal), into a vacuum. Major challenges to be met before these devices can be fully exploited include obtaining - low operating voltages, high maximum emission currents, uniform emission characteristics, and long-term emission stability. The research in this thesis concerns the production of silicon field emitters and the improvement of their emission properties by the process of anodisation. Anodisation was carried out for short times, in order to form a very thin layer of porous silicon (PS) at the surface of both p and p⁺-type silicon emitters. The aim in doing this was to form a high density of asperities over the surface of the emitters. It was the intention that these asperities, rather than the "macroscopic" apex of the emitter, would control emission. This was the first work of its kind to be carried out. Transmission electron microscopy was used to characterise the morphology of p and p⁺-type silicon emitters before and after anodisation. Both the structure and arrangement of the surface fibrils, the thickness of the PS layers at the apex and nature of PS cross-sections were studied. The morphology was correlated to subsequent field emission measurements. Field emission characteristics, before and after anodisation, were obtained using a scanning electron microscope adapted for field emission measurements, and a field emission microscope. Extensive measurements showed that, following anodisation, there was substantial improvement in emission behaviour. After anodisation, the following was found to be true: i) The starting voltage was reduced by up to 50% (with p<sup>+</sup -type PS emitters exhibiting a greater reduction in starting voltage than p-type PS emitters). ii) Number of emitting tips per array was increased. iii) Higher maximum currents (up to 3 times higher) were obtained before tips underwent destruction. iv) The resistive effect of the PS layer at the apex was important in determining the maximum current obtained from a tip. In addition, both field emission and field ion microscopy were used to identify the emission source following anodisation. It was shown that individual fibrils on the emission surface caused an increase in field enhancement over a flat plane, leading to emission at lower voltage. Overall, porous silicon appears to be a very promising material for field emission displays.

The electron emission characteristics of aluminum, molybdenum and carbon nanotubes were studied. The experiments were setup to study the emission behavior as a function of temperature and exposure to oxygen. Changes in the surface work function as a result of thermal annealing were monitored with low energy ultra-violet photoelectron spectroscopy for flat samples while field emission energy distributions were used on tip samples. The change in the field emission from fabricated single tips exposed to oxygen while in operation was measured using simultaneous Fowler-Nordheim plots and electron energy distributions. From the results a mechanism for the degradation in the emission was concluded. Thermal experiments on molybdenum and aluminum showed that these two materials can be reduced at elevated temperatures, while carbon nanotubes on the other hand show effects of oxidation. To purely reduce molybdenum a temperature in excess of 750 ºC is required. This temperature exceeds that allowed by current display device technology. Aluminum on the other hand shows reduction at a much lower temperature of at least 125 ºC; however, its extreme reactivity towards oxygen containing species produces re-oxidation. It is believed that this reduction is due to the outward diffusion of aluminum atoms through the oxide. Carbon nanotubes on the other hand show signs of oxidation as they are heated above 700 ºC. In this case the elevated temperatures cause the opening of the end caps allowing the uptake of water. Oxygen exposure experiments indicate that degradation in field emission is two-fold and is ultimately dependent on the emission current at which the tip is operated. At low emission currents the degradation is exclusively due to oxidation. At high emission currents ion bombardment results in the degradation of the emitter. In between the two extremes, molybdenum tips are capable of stable emission.

Fung Yun Ming. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaves 134-140). / Abstracts in English and Chinese. / Abstract --- p.I / Acknowledgement --- p.III / Contents --- p.IV / List of Figure captions --- p.VIII / List of Table captions --- p.XIII / Chapter Chapter 1 --- Introduction --- p.1 / Chapter Chapter 2 --- Theory and Applications / Chapter 2.1 --- Principle of field emission / Chapter 2.1.1 --- The Fowler-Nordheim Theory --- p.3 / Chapter 2.1.2 --- Field emission from metals --- p.6 / Chapter 2.1.3 --- Field emission from semiconductors --- p.8 / Chapter 2.1.3.1 --- Advantages and limitations of silicon --- p.9 / Chapter 2.1.4 --- Application of the Fowler-Nordheim theory --- p.10 / Chapter 2.1.5 --- Factors influencing field emission efficiency --- p.11 / Chapter 2.2 --- Applications --- p.11 / Chapter 2.2.1 --- Operation of a Field Emission Displays --- p.11 / Chapter 2.2.2 --- Basic structure of a Field Emission Displays --- p.13 / Chapter 2.2.3 --- Parameters relevant to applications --- p.15 / Chapter 2.3 --- The fabrication processes --- p.17 / Chapter 2.3.1 --- The anisotropic wet etching method --- p.18 / Chapter 2.3.2 --- The isotropic wet etching method --- p.19 / Chapter 2.3.3 --- Field emission from coating materials --- p.20 / Chapter 2.3.3.1 --- Coating enhancement --- p.20 / Chapter 2.3.3.2 --- Diamond and diamond-like films --- p.21 / Chapter 2.3.3.3 --- Metallic coatings --- p.22 / Chapter 2.3.3.4 --- Porous silicon coatings --- p.22 / Chapter 2.3.3.5 --- Silicon carbide coatings --- p.22 / Chapter 2.3.4 --- Fabrication of field emitters with gate --- p.23 / Chapter Chapter 3 --- Sample Preparation and Characterization Methods / Chapter 3.1 --- Sample preparation --- p.25 / Chapter 3.2 --- The fabrication process / Chapter 3.2.1 --- Isotropic etching of silicon / Chapter 3.2.1.1 --- The anodization process --- p.25 / Chapter 3.2.1.2 --- Porous silicon formation --- p.26 / Chapter 3.2.2 --- Anistropic etching of silicon --- p.27 / Chapter 3.2.3 --- The sputtering system --- p.28 / Chapter 3.2.4 --- The MEVVA Ion Source Implanter --- p.30 / Chapter 3.3 --- Characterization Methods / Chapter 3.3.1 --- Atomic Force Microscopy (AFM) --- p.32 / Chapter 3.3.2 --- Scanning Electron Microscopy (SEM) --- p.34 / Chapter 3.3.3 --- Field emission measurement / Chapter 3.3.3.1 --- Vacuum requirements --- p.35 / Chapter 3.3.3.2 --- Testing system / Chapter 3.3.3.3 --- Fluctuation of field emission --- p.38 / Chapter Chapter 4 --- Fabrication of Silicon Tips and their field emission charateristics / Chapter 4.1 --- The anodization etching process / Chapter 4.1.1 --- Introduction --- p.40 / Chapter 4.1.2 --- Experimental details --- p.42 / Chapter 4.1.3 --- Results and Discussions / Chapter 4.1.3.1 --- N type (100) sample --- p.45 / Chapter 4.1.3.2 --- Ntype(lll) sample --- p.60 / Chapter 4.1.3.3 --- Fluctuations of the emission current --- p.64 / Chapter 4.1.3.4 --- The effect of Concentration of HF solution on First Step Anodization --- p.68 / Chapter 4.1.3.5 --- The effect of the Concentration of HF solution on Second Step Anodization --- p.70 / Chapter 4.1.3.6 --- Gated silicon field emitter --- p.70 / Chapter 4.1.4 --- Conclusions --- p.73 / Chapter 4.2 --- Anisotropic texturing process / Chapter 4.2.1 --- Introduction --- p.74 / Chapter 4.2.2 --- Experimental details --- p.76 / Chapter 4.2.3 --- Results and Discussions --- p.78 / Chapter 4.2.4 --- Conclusion --- p.92 / Chapter 4.3 --- Formation of Porous Silicon Layer on silicon / Chapter 4.3.1 --- Introduction --- p.93 / Chapter 4.3.2 --- Experimental details --- p.94 / Chapter 4.3.3 --- Results and Discussions --- p.95 / Chapter 4.3.4 --- Conclusion --- p.100 / Chapter 4.4 --- Chapter Summary --- p.101 / Chapter Chapter 5 --- Improvement in the field emission characteristics of the silicon tips upon coating with low work function materials / Chapter 5.1 --- Amorphous carbon coating / Chapter 5.1.1 --- Introduction --- p.102 / Chapter 5.1.2 --- Experimental details --- p.103 / Chapter 5.1.3 --- Results and Discussions --- p.104 / Chapter 5.1.4 --- Conclusion --- p.118 / Chapter 5.2 --- Silicon carbide coated Silicon emitter by MEWA / Chapter 5.2.1 --- Introduction --- p.119 / Chapter 5.2.2 --- Experimental details --- p.120 / Chapter 5.2.3 --- Results and Discussions --- p.121 / Chapter 5.2.4 --- Conclusion --- p.125 / Chapter 5.3 --- Chapter Summary --- p.126 / Chapter Chapter 6 --- Conclusions --- p.127 / Reference --- p.134 / List of publications --- p.140

The objective of this research is to build a prototype of a carbon nanotube (CNT)-based micro X-ray tube array, which can be used in a real-time cone-beam computed tomography (CT) scanner for cancer research. The X-ray tube array consists of an electron source, control grids, focusing electrodes, and an anode plate. All the experiments have been executed in an ultra high vacuum environment at a pressure of 10⁻⁷ Torr. A thin film consisting of multi-wall carbon nanotubes (MWNTs) was used as the electron source. A diode configuration was employed to test the field emission performance of the CNT thin film. The current density achieved was 1mA/cm² at 10V/µm. After the initial burn-in process, a relatively steady emission current was obtained for duration of 170 hours. The control grid was made of 25% opening space stainless steels mesh. Meshes with different wire diameters were tested in a triode structure, and some differences were observed. Multi-anode field emission tests and multi-tube electric field simulations were executed. Experiments and simulations have revealed crosstalk between pixels during field emission. Based on the above experiments and simulations, a signal pixel prototype has been fabricated and is being tested. Moreover, some potential optimizations that will be used in the second prototype are also discussed

The objective of this research is to build a prototype of a carbon nanotube (CNT)-based micro X-ray tube array, which can be used in a real-time cone-beam computed tomography (CT) scanner for cancer research. The X-ray tube array consists of an electron source, control grids, focusing electrodes, and an anode plate. All the experiments have been executed in an ultra high vacuum environment at a pressure of 10⁻⁷ Torr. A thin film consisting of multi-wall carbon nanotubes (MWNTs) was used as the electron source. A diode configuration was employed to test the field emission performance of the CNT thin film. The current density achieved was 1mA/cm² at 10V/µm. After the initial burn-in process, a relatively steady emission current was obtained for duration of 170 hours. The control grid was made of 25% opening space stainless steels mesh. Meshes with different wire diameters were tested in a triode structure, and some differences were observed. Multi-anode field emission tests and multi-tube electric field simulations were executed. Experiments and simulations have revealed crosstalk between pixels during field emission. Based on the above experiments and simulations, a signal pixel prototype has been fabricated and is being tested. Moreover, some potential optimizations that will be used in the second prototype are also discussed