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|Title:||Diamond and silicon carbide surface modification for hostile environment applications|
|Abstract:||Wide bandgap semiconductors represent exciting new areas of research where they hold considerable advantages over traditional semiconductors, in particular the area of power electronics. Specifically, silicon carbide (SiC) and diamond have received much attention over the years for their high breakdown field, thermal conductivity and resistance to radiation. Materials that can maintain operation with considerable device longevity in extreme environments that enhance efficiency and performance of a given system are highly sought after and attract considerable levels of research. Additionally, demands for more performance out of each device requires a competitive price point, utilising scalable fabrication methods resulting in transistors of smaller dimensions at each generation. The scaling down of these electronics has necessitated atomic level control over thin films of high uniformity and quality. Atomic layer deposition makes possible the formation of monolayer and even sub-monolayer surface coverages. If realised, the potential to radically modify a surface through a thin layer of this type is substantial. Experimentally, creating these layers and subsequently measuring and visualising their electronic properties and structure remains challenging. Constructing theoretical models of such surfaces provides atomic level insight, acting as a validatory platform for experimentally observed surface characteristics and an introductory viability test for new ideas. Density functional theory supplies the means to accurately represent the structure and energy of a given system, where theory and experiment are shown to be in excellent agreement in a number of key areas. In this thesis, it is shown that both SiC and diamond can be modified by chemical termination. Key data include the electronic properties and binding energies. It has been found that SiC modification through the addition of basic elements like hydrogen, chlorine and fluorine leads to positive EAs. Intriguingly lithium termination leads to an EA of almost zero. Lithium termination is unexpectedly non-metallic, revealed only through analysis of the electronic band structure. Additionally, it is necessary to obtain information on the energetic favourability of the position of single atoms to single monolayers atop the surface. Crystallogens (Si and Ge) on diamond are studied as a function of fractional monolayer coverage. Indeed, periodicity and geometry play a crucial role in uncovering the lowest energy structure. A 67% coverage is shown to represent the equilibrium coverage of the diamond surface terminated by Si and Ge, creating negative EA surfaces of −1 eV. Links can be drawn between experimentally observed resonant states of the same coverage with band structure data, supporting our conclusions. Electron affinity forms a key feature of the project, where its value factors in numerous surface focused applications. In the area of emission, a low or negative electron affinity puts the vacuum level just above ii or even below the conduction band. Tailoring surfaces to emit more efficiently reduces power consumption and increases electron beam current density, where >1 A/cm2 applications are at the cutting edge of research areas including particle physics and energy harvesting. This project explores different surfaces of diamond and silicon carbide under a range of novel terminations, focusing mainly on structural, thermodynamic and electronic properties.|
|Appears in Collections:||School of Mathematics, Statistics and Physics|
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|BeattieJ2020.pdf||Thesis||108.21 MB||Adobe PDF||View/Open|
|dspacelicence.pdf||Licence||43.82 kB||Adobe PDF||View/Open|
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