The focus of this thesis is the study of the field electron emission (FEE) of diamond and related films synthesized by plasma enhanced chemical vapor deposition. The diamond and related films with different morphologies and compositions were prepared in a microwave plasma-enhanced chemical vapor deposition (CVD) reactor and a hot filament CVD reactor. Various analytical techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy were employed to characterize the surface morphology and chemical composition.<p>The influence of surface morphology on the field electron emission property of diamond films was studied. The emission current of well-oriented microcrystalline diamond films is relatively small compared to that of randomly oriented microcrystalline diamond films. Meanwhile, the nanocrystalline diamond film has demonstrated a larger emission current than microcrystalline diamond films. The nanocone structure significantly improves the electron emission current of diamond films due to its strong field enhancement effect.<p>The sp2 phase concentration also has significant influence on the field electron emission property of diamond films. For the diamond films synthesized by gas mixture of hydrogen and methane, their field electron emission properties were enhanced with the increase of methane concentration. The field electron emission enhancement was attributed to the increase of sp2 phase concentration, which increases the electrical conductivity of diamond films. For the diamond films synthesized through graphite etching, the growth rate and nucleation density of diamond films increase significantly with decreasing hydrogen flow rate. The field electron emission properties of the diamond films were also enhanced with the decrease of hydrogen flow rate. The field electron emission enhancement can be also attributed to the increase of the sp2 phase concentration. <p>In addition, the deviation of the experimental Fowler-Nordheim (F-N) plot from a straight line was observed for graphitic nanocone films. The deviation can be mainly attributed to the nonuniform field enhancement factor of the graphitic nanocones. In low macroscopic electric field regions, electrons are emitted mainly from nanocone or nanocones with the largest field enhancement factor, which corresponds to the smallest slope magnitude. With the increase of electric field, nanocones with small field enhancement factors also contribute to the emission current, which results in a reduced average field enhancement factor and therefore a large slope magnitude.

The focus of this thesis is the study of the field electron emission (FEE) of diamond and related films synthesized by plasma enhanced chemical vapor deposition. The diamond and related films with different morphologies and compositions were prepared in a microwave plasma-enhanced chemical vapor deposition (CVD) reactor and a hot filament CVD reactor. Various analytical techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy were employed to characterize the surface morphology and chemical composition.<p>The influence of surface morphology on the field electron emission property of diamond films was studied. The emission current of well-oriented microcrystalline diamond films is relatively small compared to that of randomly oriented microcrystalline diamond films. Meanwhile, the nanocrystalline diamond film has demonstrated a larger emission current than microcrystalline diamond films. The nanocone structure significantly improves the electron emission current of diamond films due to its strong field enhancement effect.<p>The sp2 phase concentration also has significant influence on the field electron emission property of diamond films. For the diamond films synthesized by gas mixture of hydrogen and methane, their field electron emission properties were enhanced with the increase of methane concentration. The field electron emission enhancement was attributed to the increase of sp2 phase concentration, which increases the electrical conductivity of diamond films. For the diamond films synthesized through graphite etching, the growth rate and nucleation density of diamond films increase significantly with decreasing hydrogen flow rate. The field electron emission properties of the diamond films were also enhanced with the decrease of hydrogen flow rate. The field electron emission enhancement can be also attributed to the increase of the sp2 phase concentration. <p>In addition, the deviation of the experimental Fowler-Nordheim (F-N) plot from a straight line was observed for graphitic nanocone films. The deviation can be mainly attributed to the nonuniform field enhancement factor of the graphitic nanocones. In low macroscopic electric field regions, electrons are emitted mainly from nanocone or nanocones with the largest field enhancement factor, which corresponds to the smallest slope magnitude. With the increase of electric field, nanocones with small field enhancement factors also contribute to the emission current, which results in a reduced average field enhancement factor and therefore a large slope magnitude.

published_or_final_version / Physics / Master / Master of Philosophy

No description available.

<p>As electronic device dimensions decrease to micro and nanoscale, Paschen’s law (PL)—the standard theory used to predict breakdown voltage (<i>V_b</i>) governed by Townsend avalanche (TA)—fails due to ion-enhanced field emission (FE). Analytic models to predict <i>V_b</i>at these scales are necessary to elucidate the underlying physics driving breakdown and electron emission in these regimes. Starting from a previously-derived breakdown criterion coupling TA and FE, this dissertation derives a universal (true for any gas) breakdown equation. Further simplifying this equation using a matched asymptotic analysis, dependent on the product of the ionization coefficient and the gap distance, yields an analytic theory for dimensionless <i>V_b</i>. This analytic model unifies the coupled FE/TA regime to a universal PL derived by applying scaling parameters to the standard PL. This model enables parametric analyses to assess the effects of different parameters (such as pressure, gap distance, and field enhancement factor) on breakdown and quantify the relative contribution of FE and TA to identify the transition to the universal PL. This dissertation applies this general theory to experimental cases of different gap width, gap pressure and electrode surface roughness before exploring unification across electron emission regimes, validation with molecular dynamics simulations, and extensions to alternating current (AC).</p> <p> </p> <p>One application of this theory to experimental data used data from a collaborator at Xi’an Jiaotong University, who used an electrical-optical measurement system to measure the breakdown voltage and determine breakdown morphology as a function of gap width. An empirical fit showed that the breakdown voltage varied linearly with gap distance at smaller gaps as in vacuum breakdown. This dissertation demonstrates that applying the matched asymptotic theory in the appropriate limits recovers this scaling with the slope as a function of field emission properties. </p> <p> </p> <p>Pressure also plays a critical role in gas breakdown behavior. This dissertation derives a new analytic equation that predicts breakdown voltage <i>V_b</i> within 4% of the exact numerical results of the exact theory and new experimental results at subatmospheric pressure for gap distances from 1-25 . At atmospheric pressure, <i>V_b</i> transitions to PL near the product of pressure and gap distance, <i>pd</i>, corresponding to the Paschen minimum; at lower pressures, the transition to PL occurs to the left of the minimum. We further show that the work function plays a major role in determining whether <i>V_b</i> transitions from the coupled FE/TA equation back to the traditional PL to the right or the left of the Paschen minimum as pressure increases, while field enhancement and the secondary emission coefficient play smaller roles. These results indicate that appropriate combinations of these parameters cause <i>V_b</i> to transition to PL to the left of the Paschen minimum, which would yield an extended plateau similar to some microscale gas breakdown experimental observations. </p> <p> </p> <p>Finally, the importance of electrode surface structure on microscale gas breakdown remains poorly understand. This dissertation provides the next step at assessing this by applying the asymptotic theory to microscale gas breakdown measurements for a pin-to-plate electrode setup in air at atmospheric pressure with different cathode surface roughness. Multiple discharges created circular craters on the flat cathode up to 40 μm deep with more pronounced craters created at smaller gap sizes and greater cathode surface roughness. The theory showed that breakdown voltage and ionization coefficient for subsequent breakdown events followed our earlier breakdown theory when we replaced the gap distance <i>d</i> with an effective gap distance <i>d_eff</i> defined as the sum of cathode placement distance and crater depth. Moreover, the theory indicated that <i>d_eff</i> could become sufficient large to exceed the Meek criterion for streamer formation, motivating future studies to assess whether the cathode damage could drive changes in the breakdown mechanism could for a single electrode separation distance or the Meek criterion requires modification at microscale. </p> <p> </p> <p>We next unified field emission with other electron emission mechanisms, including Mott-Gurney (MG), Child-Langmuir (CL), and quantum space-charge-limited current (QSCL) to develop a common framework for characterizing electron emission from nanoscale to the classical PL. This approach reproduced the conditions for transitions across multiple mechanisms, such as QSCL to CL, CL to FE, CL to MG to FE, and microscale gas breakdown to PL using a common nondimensional framework. Furthermore, we demonstrated the conditions for more complicated nexuses where multiple asymptotic solutions matched, such as matching QSCL, CSCL, MG, and FE to gas breakdown. A unified model for radiofrequency and microwave gas breakdown will be compared to experimental results from Purdue University to elucidate breakdown mechanism. </p> <p>The results from this dissertation will have applications in microscale gas breakdown for applications including microelectromechanical system design, combustion, environmental mitigation, carbon nanotube emission for directed energy systems, and characterizing breakdown in accelerators and fusion devices.</p>

Theoretical formulations of secondary electron emission over the past 20 years have exceeded the confirming ability of available measurements. An instrument has been developed and tested for the purpose of obtaining simultaneous angle- and energy-resolved (AER) secondary and backscattered electron measurements for energetic electrons incident on conducting surfaces. The instrument is found to be in good working order and the data quality found to be excellent for nearly all angles and energies investigated. A representative set of AER measurements has been acquired for 1500 e V electrons normally incident on polycrystalline gold. The data have been used to construct angle-resolved (AR) spectra and energy-resolved (ER) angular distributions, which have been examined both as surface plots and cross sections. Analysis of the measurements strongly suggests that secondary electrons comprise the bulk of emitted electrons at energies much greater than the traditionally accepted maximum secondary electron energy of 50 eV. Additional evidence suggests the ability to investigate dominant secondary and backscattered electron production mechanisms in several energy domains.

Secondary electron emission (SEE) and incident-particle backscattering are important processes accompanying the impact of energetic electrons and ions on surfaces. The phenomena play a key role in the buildup of electrical charge on spacecraft surfaces, and are therefore of particular interest to scientists attempting to model spacecraft charging. In response to a demonstrated need for data, techniques for determining total secondary electron (SE) and backscatter (BS) yields (del) and (neu), and associated scattering-angle-resolved,scattering-energy-resolved, and simultaneous angle-energy-resolved yields have been developed. Further, an apparatus capable of making the necessary measurements for experimental determination of these quantities---for conducting materials in an ultra-high vacuum environment-has been designed, constructed, and partially tested. The apparatus is found to be in working order, though in need of fine-tuning, and the measurement technique successful. Investigations using a 1-3 Kev beam of monoenergetic electrons normally incident on bulk AI have been undertaken with the new apparatus. Electron-stimulated desorption of surface contaminants has been observed, as has been beam-induced carbon deposition, and an empirical model describing the resulting dynamic evolution of (del)is presented. Totalb and 11 values obtained in the present investigation are found to be in qualitative agreement with the results of previously reported investigations, though quantitative disagreement of b-values is substantial. Specifically, evidence is presented suggesting that previously reported SE yields for clean AI under electron bombardment (in the 1-3 Kev energy range) are in error by as much as 30 %.

In a wide variety of applications such as the Hall thruster and the tokamak, understanding the plasma-material interactions which take place at the wall is important for improving performance and preventing failure due to material degradation. In the region near a surface, the plasma sheath forms and regulates the electron and ion fluxes into the material. Emission from the material has the potential to change sheath structure drastically, and must be modeled rigorously to produce accurate predictions of the fluxes into the wall. Continuum kinetic codes offer significant advantages for the modeling of sheath physics, but the complexity of emission physics makes it difficult to implement accurately. This difficulty results in major simplifications which often neglect important energy-dependent physics. A focus of the work is on proper simulation of the sheath. The implementation of source and collision terms is discussed, alongside a brief study of the Weibel instability in the sheath demonstrating the necessity of proper collision implementation to avoid missing relevant physics. A novel implementation of semi-empirical models for electron-impact secondary electron emission into the boundary conditions of a continuum kinetic code is presented here. The features of both high and low energy regimes of emission are represented self-consistently, and the underlying algorithms are flexible and can be easily extended to other emission mechanisms, such as ion-impact secondary electron emission. The models are applied to simulations of oxidized and clean lithium for fusion-relevant plasma regimes. Oxidized lithium has a high emission coefficent and the sheath transitions into space-charge limited and inverse modes for different parameters. The breakdown of the classical sheath results in an increase of energy fluxes to the surface, with potential ramification for applications. / Doctor of Philosophy / Great advances are being made in a variety of promising applications of plasma physics, such as the development of spacecraft thrusters and fusion devices. Many of these devices constrain the flow of plasma within a material channel, leading degradation of the wall due to particle impact to be a serious concern for durability and lifespan. The plasma sheath is a region next to these material surfaces where ions are accelerated towards the wall, while electrons are repelled. As particles from the sheath impact the material, they cause the emission of secondary particles back into the sheath. This can drastically change the expected fluxes into the material and consequently the degradation expected to occur. Continuum kinetic simulations are a valuable tool for predicting and modeling the evolution of the sheath, but they are limited in their ability to rigorously do material emission physics by their inability to directly represent particle interactions with the surface. As such, past treatments of material emission in continuum kinetics tend to sacrifice valuable energy-dependent physics for simplified models.par To facilitate better understanding of the effects of emission on the sheath and the ramifications it might have for applications, the work here seeks to develop a framework for capturing the entire range of energy-dependent emission physics within a continuum kinetic framework. The implementation relies on semi-empirical models of beam emission data, focusing on simplicity and flexibility while still capturing the separate emission mechanisms which dominate in different energy regimes. The model is applied to simulations of lithium, an important material for fusion applications. Oxidized lithium has significantly enhanced emission properties over clean lithium, and is found to undergo a shift to non-monotonic sheath modes. The results show that the fundamental changes in the sheath structure due to the increased emission lead to greater energy fluxes into the surface. In this work, only secondary electron emission from the impact of electrons on a surface is examined. However, the underlying algorithms are easily extended to other energy-dependent energy mechanisms, such as ion-impact secondary electron emission.

No description available.

No description available.