<p>The goal of the project work has been to study the symmetry of the phonons in 4H and 6H-SiC for different measuring geometries by using two experimental techniques, Raman and infrared absorption (IR) spectroscopy, and a theoretical model. The Raman spectra were measured in different scattering configurations in order to obtain experimental data for detailed investigation of the phonon symmetries.</p><p>The gross features of the spectra obtained in different geometries can be explained using general group-theoretical arguments. Using a lattice-dynamics model, we have also calculated the angular dependence of the phonon energies near the centre of the Brillouin zone, as well as the phonon displacements in some high-symmetry directions. The theoretical results are used to interpret the Raman lines in different configurations, and it was possible to estimate that if ionicity of the bonding of 12% is taken in the theoretical model for 4H-SiC, the splitting of the polar TO mode and the shift of the polar LO mode observed in our spectra are well reproduced theoretically. It was also observed that these polar modes have to be classified as longitudinal and transversal with respect to the direction of phonon wave vector, while the rest of the modes remain longitudinal or transversal with respect to the c-axis of the crystal. The Raman lines in the case of 4H SiC have been tentatively labelled with the irreducible representations of the point group of the crystal (C6v).</p>

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Silicon carbide bipolar junction transistors (BJTs) are attractive power switching devices because of the unique material properties of SiC with high breakdown electric field, high thermal conductivity and high saturated drift velocity of electrons. The SiC BJT has potential for very low specific on-resistances and this together with high temperature operation makes it very suitable for applications with high power densities. For SiC BJTs the common emitter current gain (β), the specific on-resistance (RSP_ON), and the breakdown voltage are important to optimize for competition with silicon based power devices. In this thesis, power SiC BJTs with high current gain β ≈ 60 , low on-resistance RSP_ON ≈ 5 mΩcm2, and high breakdown voltage BVCEO ≈ 1200 V have been demonstrated. The 1200 V SiC BJT that has been demonstrated has about 80 % lower on-state power losses compared to a typical 1200 V Si IGBT chip. A continuous epitaxial growth of the base-emitter layers has been used to reduce interface defects and thus improve the current gain. A significant influence of surface recombination on the current gain was identified by comparing the experiments with device simulations. In order to reduce the surface recombination, different passivation layers were investigated in SiC BJTs, and thermal oxidation in N2O ambient was identified as an efficient passivation method to increase the current gain. To obtain a low contact resistance, especially to the p-type base contact, is one critical issue to fabricate SiC power BJTs with low on-resistance. Low temperature anneal (~ 800 oC) of a p-type Ni/Ti/Al contact on 4H-SiC has been demonstrated. The contact resistivity on the ion implanted base region of the BJT was 1.3 × 10-4 Ωcm2 after annealing. The Ni/Ti/Al p-type ohmic contact was adapted to 4H-SiC BJTs fabrication indicating that the base contact plays a role for achieving a low on-resistance of SiC BJTs. To achieve a high breakdown voltage, optimized junction termination is important in a power device. A guard ring assisted Junction Termination Extension (JTE) structure was used to improve the breakdown voltage of the SiC BJTs. The highest breakdown voltage of the fabricated SiC BJTs was obtained for devices with guard ring assisted JTE using the base contact implant step for a simultaneous formation of guard rings. As a new approach to fabricate SiC BJTs, epitaxial regrowth of an extrinsic base layer was demonstrated. SiC BJTs without any ion implantation were successfully demonstrated using epitaxial regrowth of a highly doped p-type region and an etched JTE using the epitaxial base. A maximum current gain of 42 was measured for a 1.8 mm × 1.8 mm BJT with a stable and reproducible open base breakdown voltage of 1800 V. / QC 20100819

The design, modeling, fabrication, and validation of an optical magnetic response in dielectric-based metamaterials are studied. These metamaterials consist of either periodic or random arrays of dielectric particle inclusions, which may be spheres, coated spheres, or completely randomly shaped. It is demonstrated that because of the simple particle shapes and dielectric materials, these metamaterials are quite easy and feasible to implement in a bulk, three-dimensional sample, and the response is isotropic. This in is contrast to other predominant designs of optical metamaterials, which are planar and anisotropic arrays of complicated metallic fishnet or split-ring resonator structures, which require stringent tolerances and sophisticated assembly. It is shown that SiC is one of many materials from which such infrared magnetic metamaterials can be constructed. A simple SiC powder is used to verify these claims. The milled micropowder of crystalline SiC is comprised of particles of random shapes and sizes. A model of the electromagnetic response of such powders is developed, whereby the induced magnetic dipole response is modeled by equivalently-sized spheres of SiC, whereas the electric dipole response is modeled by a continuous distribution of ellipsoidal particles. Infrared spectroscopic measurements and numerical calculations are performed, verifying both the magnetic and electric response of the powder. A alternate approach is also described, where uniform SiC microspheres are fabricated using more sophisticated nanochemical techniques. In the final portion of the dissertation, the mutual near-field coupling between ideal magnetic dipoles induced in dielectric spheres is studied. This is implemented for microwave frequencies using large permittivity ceramic spheres. An approximate coupled dipole model of the multiple scattering among the spheres is developed, and a transition matrix method is implemented to calculate the exact scattering by the clusters. Experimental measurements are performed, confirming the two models. The results for pairs, chains, and rings of spheres indicates that the magnetic dipole modes hybridize in analogy to atomic bonding. A notable result is that certain hybridized magnetic dipole modes may have a net electric dipole moment. The similarity to atomic and molecular bonding should prove useful in conceptualizing and designing more sophisticated metamaterials.

The design, modeling, fabrication, and validation of an optical magnetic response in dielectric-based metamaterials are studied. These metamaterials consist of either periodic or random arrays of dielectric particle inclusions, which may be spheres, coated spheres, or completely randomly shaped. It is demonstrated that because of the simple particle shapes and dielectric materials, these metamaterials are quite easy and feasible to implement in a bulk, three-dimensional sample, and the response is isotropic. This in is contrast to other predominant designs of optical metamaterials, which are planar and anisotropic arrays of complicated metallic fishnet or split-ring resonator structures, which require stringent tolerances and sophisticated assembly. It is shown that SiC is one of many materials from which such infrared magnetic metamaterials can be constructed. A simple SiC powder is used to verify these claims. The milled micropowder of crystalline SiC is comprised of particles of random shapes and sizes. A model of the electromagnetic response of such powders is developed, whereby the induced magnetic dipole response is modeled by equivalently-sized spheres of SiC, whereas the electric dipole response is modeled by a continuous distribution of ellipsoidal particles. Infrared spectroscopic measurements and numerical calculations are performed, verifying both the magnetic and electric response of the powder. A alternate approach is also described, where uniform SiC microspheres are fabricated using more sophisticated nanochemical techniques. In the final portion of the dissertation, the mutual near-field coupling between ideal magnetic dipoles induced in dielectric spheres is studied. This is implemented for microwave frequencies using large permittivity ceramic spheres. An approximate coupled dipole model of the multiple scattering among the spheres is developed, and a transition matrix method is implemented to calculate the exact scattering by the clusters. Experimental measurements are performed, confirming the two models. The results for pairs, chains, and rings of spheres indicates that the magnetic dipole modes hybridize in analogy to atomic bonding. A notable result is that certain hybridized magnetic dipole modes may have a net electric dipole moment. The similarity to atomic and molecular bonding should prove useful in conceptualizing and designing more sophisticated metamaterials.

The goal of the project work has been to study the symmetry of the phonons in 4H and 6H-SiC for different measuring geometries by using two experimental techniques, Raman and infrared absorption (IR) spectroscopy, and a theoretical model. The Raman spectra were measured in different scattering configurations in order to obtain experimental data for detailed investigation of the phonon symmetries. The gross features of the spectra obtained in different geometries can be explained using general group-theoretical arguments. Using a lattice-dynamics model, we have also calculated the angular dependence of the phonon energies near the centre of the Brillouin zone, as well as the phonon displacements in some high-symmetry directions. The theoretical results are used to interpret the Raman lines in different configurations, and it was possible to estimate that if ionicity of the bonding of 12% is taken in the theoretical model for 4H-SiC, the splitting of the polar TO mode and the shift of the polar LO mode observed in our spectra are well reproduced theoretically. It was also observed that these polar modes have to be classified as longitudinal and transversal with respect to the direction of phonon wave vector, while the rest of the modes remain longitudinal or transversal with respect to the c-axis of the crystal. The Raman lines in the case of 4H SiC have been tentatively labelled with the irreducible representations of the point group of the crystal (C6v).

This work resulted in the development of multiple distinct and novel methods of cheaply producing large numbers of biologically derived, complex, 3-dimensional microstructures in a multitude of possible compositions. The biologically derived structures employed in this work were diatoms, a type of single celled algae, which grow complex silica shells in species-specific shapes. Due to the wide diversity of naturally occurring diatom shapes (on the order of 105), and the flexibility in tailoring chemical compositions using the methods developed here, real potential exists for cheaply mass-producing industrially relevant quantities of controlled shape and size 3-d particles for the first time. The central theme of this research is the use of diatoms as a transient scaffold onto which a coating is applied. After curing the coating, and in some cases firing the coating to form ceramic, the diatom can be selectively etched away leaving a free standing replica of the original structure with the salient features of the pre-form intact, but now composed of a completely different material. Using this concept, specific methods were developed to suit various precursors. Dip coating techniques were used to create epoxy diatoms, and silicon carbide diatoms. The Sol-Gel method was used to synthesize zirconia diatoms in both the tetragonal and monoclinic phases. A multi step method was developed in which previously synthesized epoxy diatoms were used as a template for deposition of a silicon carbide precursor and then heat treated to produce a silicon carbide/carbon multi-component ceramic. A hydrothermal reaction was also developed to convert Titania diatoms to barium titanate by reaction with barium hydroxide. Finally, the device potential of diatom-derived structures was conclusively demonstrated by constructing a gas sensor from a single Titania diatom. Under suitable conditions, the sensor was found to have the fastest response and recovery time of any sensor of this type reported in the literature. Furthermore, this work has laid the groundwork for the synthesis of many other tailored compositions of diatoms, and provided several compositions for device creation.

The impedance response of silicon carbide whisker-alumina composites is investigated utilizing novel stereological techniques along with a microstructural simulation. The stereological techniques developed allow for a measurement of the trivariate length, radius and orientation distribution of whiskers in the composite from measurements made on two-dimensional sectioning planes. The measured distributions are then utilized in a Monte Carlo simulation that predicts connectivity in the composite for a given volume fraction. It is assumed in the simulation that connectivity factors dominate the electrical response, not interfacial phenomena. The results of the simulation are compared with impedance spectra taken from real samples, and conclusions are drawn regarding the nature of the impedance response.

Si-based anodes have recently received considerable attention for use in Li-ion batteries, due to their extremely high specific capacity - an order of magnitude beyond that offered by conventional graphite anode materials. However, during the lithiation process, Si-based anodes undergo extreme increases in volume, potentially by more than 300 %. The stresses produced within the electrode by these volume changes can damage the electrode binder, the active Si particles and the solid electrolyte interphase (SEI), causing the electrode to rapidly fail and lose capacity. These problems can be overcome by producing new anode materials incorporating both Si and C, which may offer a favorable combination of the best properties of both materials, and which can be designed with internal porosity, thereby buffering the high strains produced during battery charge and discharge with minimal overall volume changes. However, in order to develop useful anode materials, we must gain a thorough understanding of the structural, microstructural and chemical changes occurring within the electrode during the lithiation and delithiation process, and we must develop new processes for synthesizing composite anode particles which can survive the extreme strains produced during lithium intercalation of Si and exhibit no volume changes in spite of the volume changes in Si. In this work we have developed several novel synthesis processes for producing internally porous Si-C nanocomposite anode materials for Li-ion batteries. These nanocomposites possess excellent specific capacity, Coulombic efficiency, cycle lifetime, and rate capability. We have also investigated the influence of a range of different parameters on the electrochemical performance of these materials, including pore size and shape, carbon and silicon film thickness and microstructure, and binder chemistry.

Ceramic-matrix composites of alumina and silicon carbide whiskers have recently found novel commercial application as electromagnetic absorbers. However, a detailed understanding of how materials issues influence the composite electrical response, which underpins this application, has been absent until now. In this project, such composites were electrically measured over a wide range of conditions and modeled in terms of various aspects of the microstructure in order to understand how they work. For this purpose, three types of composites were made by different methods from the same set of ceramic powder blends loaded with different volume fractions of whiskers. In doing so, the interfaces between whiskers, the preferred orientations of whiskers, and the structure of electrically-connected whisker clusters were varied; the whisker aspect-ratio distributions were the same for all methods. At the electrode interfaces, Schottky barriers at the junctions of the electrically-percolating wide-bandgap semiconductor whiskers on the surface were responsible for a significant portion of the total measured impedance. The associated electrical response was studied on the microscopic and macroscopic level, and the gap between these different scales was bridged. Also, a modeling approach was developed for the non-linear behavior of the composite which results from these barriers. In regards to the whiskers within the composite bulk, the effects of various factors on the wide-band frequency dependence of the dielectric response and dc conductivity were explained and contextualized for the electromagnetic absorber application. Such factors include whisker preferred orientation, electrical percolation and cluster structure, the interfaces between electrically-connected SiC whiskers, and porosity. A quantitative correlation between the anisotropy of the microstructure and that of the conductivity was found, and was understood in terms of the interfacial SiC-Al2O3-SiC conduction mechanism. This behavior was shown to differ from the behavior commonly observed for other disordered mixtures of relatively conductive particles dispersed inside insulating polymer hosts. A description of this new mechanism was developed based on an observed correlation between the temperature dependencies of the static and radio-frequency electrical responses. Also, the aforementioned non-linear response model was expanded upon to describe conduction through and across electrically-percolated clusters. The model demonstrates how loading and interface behavior influence the topology and the strength of the non-linear response of the clusters.