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Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also in a digital version from UMI/Dissertation Abstracts International.

The systematic study of the formation of β-SiC formed by low energy carbon ion (C-)implantation into Si followed by high temperature annealing is presented. The research is performed to explore the optimal annealing conditions. The formation of crystalline β-SiC is clearly observed in the sample annealed at 1100 °C for a period of 1 hr. Quantitative analysis is performed in the formation of β-SiC by the process of implantation of different carbon ion fluences of 1×1017, 2×1017, 5×1017, and 8×1017 atoms /cm2 at an ion energy of 65 keV into Si. It is observed that the average size of β-SiC crystals decreased and the amount of β-SiC crystals increased with the increase in the implanted fluences when the samples were annealed at 1100°C for 1 hr. However, it is observed that the amount of β-SiC linearly increased with the implanted fluences up to 5×1017 atoms /cm2. Above this fluence the amount of β-SiC appears to saturate. The stability of graphitic C-C bonds at 1100°C limits the growth of SiC precipitates in the sample implanted at a fluence of 8×1017 atoms /cm2 which results in the saturation behavior of SiC formation in the present study. Secondly, the carbon cluster formation process in silica and the characterization of formed clusters is presented. Silicon dioxide layers ~500 nm thick are thermally grown on a Si (100) wafer. The SiO2 layers are then implanted with 70 keV carbon ions at a fluence of 5×1017 atoms/cm2. The implanted samples are annealed 1100 °C for different time periods of 10 min., 30 min., 60 min., 90 min., and 120 min., in the mixture of argon and hydrogen gas (96 % Ar + 4% hydrogen). Photoluminescence spectroscopy reveals UV to visible emission from the samples. A detail mechanism of the photoluminescence and its possible origin is discussed by correlating the structural and optical properties of the samples. Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, spectroscopy, photoluminescence spectroscopy, and transmission electron microscopy are used to characterize the samples.

Silicon-on-insulator formed by high dose oxygen ion implantation and subsequent epitaxially grown silicon layers were studied and compared with silicon on sapphire materials. Czochralski grown, (100) silicon wafers were implanted with molecular oxygen ions, 0+2, to a total dose of 2.12 x 10^18 0+/cm^2 at an energy of 150 keV/atom.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Ph.D., Faculty of Engineering and the Built Environment, University of the Witwatersand, 2012 / This thesis focuses on the boron suboxide B6O, a boron-rich super-hard ceramic material. With hardness values previously reported between 24 GPa and 45 GPa, B6O is one of the hardest known materials. Although first reports on boron suboxides date back as far as 1909 (Weintraub E., Transactions of the American Electrochemical Society, 16 (1909) 165), it is the B6O-based composites that have attracted considerable interest in recent years due to their enormous technological potential, especially as an alternative to polycrystalline diamond and polycrystalline cubic boron nitride for wear and abrasive applications. Investigations into the properties of B6O itself appear to have been neglected in favour of the improvement of densification and fracture toughness of the composites. The B6O samples used for the work reported in this work was hot-pressed was prepared under an argon environment at 1800 C and 50 MPa for 20 minutes followed metallographical preparation. The density of the hot-pressed compacts measured 2.44 g/cm3. The starting B6O powder material was supplied from the Fraunhofer Institute for Ceramic Technologies and Systems, Dresden. This thesis primarily seeks to generate and report as much practical data for polycrystalline B6O materials prepared by uniaxial hot-pressing as possible from a variety of characterization techniques. Firstly, the Raman spectra of hot-pressed B6O, which was until now poorly understood, was investigated using using a 514.5 nm green Ar+ laser excitation source. Secondly, the fundamental nature of some mechanical properties of hot-pressed B6O were investigated by means of Vickers and Berkovich indentation techniques. New approaches for data analysis, especially the investigations of the nanomechanical properties of hot-pressed B6O by Berkovich nanoindentation, were also suggested. Thirdly, the intrinsic hardness of hot-pressed B6O was investigated by means of a comprehensive inter-model comparison study. Fourthly, a combined experimental and simulation approach for determining mechanical properties of hot-pressed B6O by nanoindentation was also carried out, based on the outcomes of the study, the deformation response of the material under dynamic indentation was investigated at di erent stages using a custom developed finite element model. Finally, based on the preliminary ab initio density functional calculations of the structural properties of B6O conducted by Lowther showing that the presence of a high electronegativity interstitial in the B6O structure could enhance the strength of the bonding in B6O, fluorine ion implantation into B6O were conducted. The e ects to the structural and the nanomechanical properties of radiation damage induced by ion implantation was investigated. The possible formation of novel nanostructures in the ion-irradiated B6O matrix near-surface was also investigated. Results obtained from this study provides a vast amount of practical data for hot-pressed B6O materials as well as a number of novel analysis approaches for the extraction useful properties from the measured raw data. Firstly, using an automated background subtraction method, observable first- and second-order Raman spectra of B6O were obtained. A comparative analysis with previously reported spectra of other -rhombohedral boron-rich ceramic materials demonstrate a good agreement. Results also confirm the existence of highly resolved Raman modes measured at ambient conditions using a green Ar+ excitation source which is contrary to the conventional understanding. Secondly, results from the micro-indentation investigations indicate the measured microhardness exhibits indentation load dependence. A model inter-comparison study of indentation size e ects in the microhardness measurements of hot-pressed B6O is comprehensively discussed. Thirdly, the intrinsic hardness value of 30 GPa was deduced. Fourthly, a quantitative analysis approach was developed to simulate multi-cycling loading load-displacement curves from a single measured load-displacement nanoindentation curve. Based on the results, the nature of the indentation size e ect in the nanoindentation hardness as well as the intrinsic nanomechanical properties of hot-pressed B6O were established. Fifthly, a combined experimental and finite element method simulation approach for determining mechanical properties of hot-pressed B6O by nanoindentation was developed. Based on the outcomes of the combined experimental and simulation studies, the deformation response of the material under dynamic indentation was also investigated at di erent stages using a custom developed finite element model. Finally, results from the structural characterization of the ion implanted B6O material demonstrates the formation of novel nanostructures by means of the ion bombardment of B6O. In addition, the study presented here also seeks to investigate the e ects of the fluorine ion implantation on the near-surface nanomechanical properties of hot-pressed B6O. The principal conclusions that the study provide are both comprehensive practical data for B6O materials prepared by uniaxial hot-pressing. A number of properties, including the Raman spectra, the intrinsic hardness, and the radiation resistance and the e ects of radiation damage are reported in the thesis.

PhD, Faculty of Science, University of the Witwatersrand

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy. / High purity natural type Il a diamond specimens were used in this study. Conducting layers in the surfaces of these diamonds were generated using low-ion dose multiple implantation-annealing steps. The implantation energies and the ion-doses were spread evenly to intermix the point-defects, thereby increasing the probability of interstitialvacancy recombinations and promoting dopant-interstitial-vacancy combination resulting in activated dopant sites in the implanted layers. The process used to prepare our samples is known as cold-implantation-rapid-annealing (CIRA). Carbon-ion and boron-ion implantation was used to prepare the diamond specimens, and de-conductivity measurements in the temperature range of 1.5-300 K were made following each CIRA sequence. An electrical conductivity crossover from the Mott variable range hopping (VRH) to the Efros-Shklovskii VRH conduction was observed when the temperature of insulating samples was lowered. The conductivity crossover temperature Tcross decreases with increasing concentration of the boron-ion dose in the implanted layers, indicating the narrowing of the Coulomb gap in the single-particle density of states near the Fermi energy. (Abbreviation abstract) / Andrew Chakane 2019

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, June, 2013 / Diamond with its outstanding and unique physical properties offers the opportunity to be used as semiconductor material in future device technologies. Promising ap- plications are, among others, high speed and high-power electronic devices working under extreme conditions, such as high temperature and harsh chemical environments. With respect to electronic applications, a controlled doping of the material is neces- sary which is preferably done by ion implantation. The ion implantation technique allows incorporation of foreign atoms at de¯ned depths and with controlled spatial distribution which is not achievable with other methods. However, the ion implanta- tion process is always connected with the formation of defects which compensate and trap charge carriers thus degrading the electrical behaviour. It is therefore essential to understand the nature of defects produced under various implantation conditions. In this respect, this study involves the investigation of the nature of the radiation damage produced during the multi-implantation of carbon ions in synthetic high- pressure, high-temperature (HPHT) type Ib diamond spread over a range of energies from 50 to 150 keV and °uences, using the cold-implantation-rapid-annealing (CIRA) routine. Single energy implantation of carbon ions in synthetic HPHT (type Ib), at room temperature, was also performed. Both ion milling and FIB (Focused Ion Beam) milling were used to prepare thin specimen for transmission electron micro- scope (TEM) analysis. The unimplanted, implanted and annealed samples were characterized using trans- mission electron microscopy based techniques and Raman spectroscopy. ii iii In unimplanted type Ia natural diamond, a high density of platelets, exhibiting the typical contrast of both edge-on and inclined platelets on f100g planes was found. As-implanted HPHT type Ib diamond, implanted with single energy of 150 keV car- bon ions and °uence of 7£1015 ions cm¡2 revealed an amorphous diamond layer of about 80 nm in thickness while, for low °uence implantations, the damaged diamond retained its crystallinity after annealing at 1600 K. In addition, damaged diamond transformed into disordered carbon comprising regions with bent (002) graphitic fringes and regions of amorphous carbon when high °uence, i.e., one above the amor- phization/graphitisation threshold were used followed by rapid thermal annealing at 1600 K. Furthermore, the interface between the implanted and annealed layer and the diamond substrate at the end of the range, showed diamond crystallites, inter- spersed between regions of amorphous carbon and partially graphitized carbon. This indicates that solid phase epitaxial recrystallization regrowth in diamond does not occur.