| 1. |
- Huang, Jing-Jia, 1990-
(författare)
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Surface-Controlled Chemical Vapor Deposition of Silicon Carbide
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polycrystalline cubic silicon carbide, 3C-SiC, has long been investigated in the field of hard coating materials. The typical synthesis method for 3C-SiC coatings is thermal chemical vapor deposition (CVD) using either multicomponent precursors, e.g. methyltrichlorosilane, or a combination of single component precursors, e.g. silane and propane. In this thesis, the fabrication of polycrystalline SiC coatings has been explored from the new aspects on the basis of thermal CVD utilizing silicon tetrachloride (SiCl4) and various hydrocarbons, i.e. toluene (C7H8), methane (CH4) and ethylene (C2H4) as the precursors. The goal of this thesis is to control the surface chemistry in the SiCl4-based SiC CVD and has been accomplished by the following three different approaches: In the first approach to control the surface chemistry of SiC CVD, the difference in the adsorption energy of aromatic and aliphatic hydrocarbons on different SiC crystal planes was utilized. Under identical deposition conditions, a highly <111>-oriented 3C-SiC coating was deposited using C7H8 as the carbon precursor, whereas using CH4 resulted in a randomly oriented 3C-SiC. The results from quantum chemical calculation showed that the active film forming carbon species, i.e. C6H6 in the C7H8 process and CH3 in both C7H8 and CH4 processes, behaved differently when they adsorbed on the 3C-SiC (111) and (110) planes. CH3 is strongly chemisorbed on both planes, while C6H6 is chemisorbed on the (111) plane, but only physiosorbed on the other. The significant difference in the adsorption energy of CH3 and C6H6 on the (111) and (110) planes therefore explains the resulting highly <111>-oriented 3C-SiC from the C7H8 process. Furthermore, the ability to deposit 3C-SiC coatings with alternating highly <111>- and randomly oriented layers by merely switching the carbon precursor between C7H8 and CH4 or C2H4 in a single CVD deposition has further proven that the effect of aromatic hydrocarbons on the preferred growth orientation of 3C-SiC was controlled primarily by the surface chemistry. The second approach to the surface-controlled SiC CVD was based on the reduction of surface reaction probability (β) for conformal film growth via low-temperature, low-pressure CVD, which was originally proposed by Abelson and Girolami. Their strategies in reducing β, including lowering the temperature and increasing the precursor partial pressure, were successfully adapted to the SiC CVD growth using SiCl4 and C2H4 as the precursors in this thesis, where an elevated temperature and a moderate pressure were used. Moreover, the addition of Cl species as a growth inhibitor to the process further reduced the β, leading to a superconformal SiC growth. The third approach employed in this thesis for the SiC growth was pulsed CVD. Instead of a continuous and simultaneous SiCl4 and C2H4 flow, the precursors were pulsed alternately into the chamber with each precursor pulse being separated by a H2 purge. In this precursor delivery mode, the gas phase reactions between SiCl4 and C2H4 were avoided and hence the SiC growth was mostly controlled by the surface chemistry. Altering the pulse durations of the precursors led to a variation of growth per cycle (GPC), which was explained by a two-step mechanism. During the SiCl4 pulse, a thin layer of Si is deposited, which is carburized by carbon species produced during the C2H4 pulse. Additionally, the separation of precursor pulses should lead to a large increase in the surface coverage of Cl species, further enhancing the inhibition effect and resulting in a superconformal SiC growth. By using this approach, superconformal SiC coatings were achieved at temperatures where conventional CVD only yielded nonconformal SiC coatings. The observed decline in coating conformality with an elongated purge implied that more surface Cl species were replaced by H during the H2 purge and consequently the inhibition effect was diminished.
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| 2. |
- Giri, Suvendu, 1988-
(författare)
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New horizons in string theory : bubble babble in search of darkness
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It was discovered nearly two decades ago that we live in an accelerating universe that is dominated by dark energy. Understanding the origin of such an energy has turned out to be a very difficult open question in physics, and calls on the need for a fundamental theory like string theory. However, despite decades-long effort, string theory has proven incredibly resilient to a satisfactory construction of dark energy within its framework.In the first part of this thesis and the included papers, we examine this problem and propose two possible solutions. The first is a construction within the framework of M-theory, the eleven dimensional cousin of string theory. Using only well-understood geometric ingredients and higher-derivative corrections to eleven dimensional supergravity, we present a new class of four dimensional vacua that contain dark energy. In the process, we also construct a new class of non-supersymmetric Minkowski vacua that were previously not known. Our second idea is a novel proposal that our universe could be embedded on the surface of an enormous spherical bubble that is expanding in a five dimensional anti de Sitter spacetime. The bubble is made of branes in string theory and its expansion is driven by the difference in the cosmological constants across it. We argued that such a construction arises naturally in string theory, and showed how four dimensional gravity arises in such a universe. We further showed that four dimensional matter and radiation arise from quantities that are innately five dimensional.Another challenging problem in physics concerns the nature of black holes – the presence of an event horizon in particular. This poses a paradox between well understood physical principles, and requires a fundamental theory for its resolution. Towards this goal, we constructed a novel class of horizonless objects that mimics black holes, and proposed these objects as an alternative end point of gravitational collapse. Subsequently, we constructed slowly rotating versions of these "black shells" and proposed an observational signature that could distinguish them from black holes in cosmological experiments. This is discussed in the second part of the thesis and in the included papers.
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| 3. |
- Panizo Pérez, Daniel
(författare)
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Blowing Bubbles from String Theory
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the discovery of the accelerated expansion of the universe at the end of the nineties, physicists have tried to describe this cosmological behaviour from theories that are complete in the ultraviolet regime, i.e. theories that are well defined at arbitrarily high energies. The most fundamental property that any proposed model must reproduce is that of a de Sitter space in the lower-dimensional description. In other words, an accelerated expanding four-dimensional cosmology driven by an unknown energy dubbed as dark energy.String theory seems to be the most promising candidate for a theory of quantum gravity, as it naturally accommodates the graviton, the quantum propagator of gravity and fulfills all the requirements to be an ultraviolet-complete theory. However, despite all the efforts of recent decades, no lower-dimensional effective field theory description with the desired positive cosmological constant has yet been obtained. This suspicious conspiracy has motivated the physics community to take a different approach; to investigate what criteria lower-dimensional effective field theories of gauge fields coupled to gravity should satisfy to be derived from string theory rather than aiming to derive four-dimensional effective field theories directly from string theory. One of the most striking conclusions that can be drawn from these criteria is that four-dimensional de Sitter cosmology may belong to the swampland, the set of effective field theories that cannot be obtained from string theory.In the first part of this monograph we will review both classical and quantum approaches to four-dimensional cosmology and then introduce the main aspects and issues faced by the aforementioned string cosmology constructions. The second part of this thesis presents an alternative and novel realisation of a four-dimensional expanding cosmology from string theory. This low-dimensional universe rides a three-dimensional brane that mediates the decay between two different five-dimensional and non-supersymmetric anti-de Sitter vacua. This proposal, known as the Dark Bubble model, turns into advantages some of the liabilities found within the swampland programme, granting an innovative way of realising dark energy from string theory. In this work, we will discuss the top-down construction of the Dark Bubble model from ten-dimensional supergravity, the higher-dimensional origin of four-dimensional dark energy and the new intrinsic hierarchy of energy scales that emerges from this stringy construction. Furthermore, we will also show how four-dimensional matter, radiation and both gravitational and electromagnetic waves will arise naturally from features of the higher-dimensional spacetime.
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