School of Physics - Theses

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    TEM and structural investigations of synthesized and modified carbon materials
    Lai, Pooi-Fun ( 1999-08)
    Due to the extreme properties of diamond, such as extreme hardness, high thermal conductivity, high electrical breakdown strength, high electron and hole mobilities and large band gap, it is of interest to study this material in detail. Before advantage can be taken of diamond’s properties for high-temperature, high-power electronic applications successful doping/ion implantation of diamond must be achieved. This requires an understanding of the types of defects produced during ion irradiation. In the present work, type IIa diamond has been irradiated with various doses of 320keV Xe ions at room temperature. Analytical techniques used are electron spin resonance spectroscopy, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. Previous models have suggested that upon ion impact, amorphous and/or graphitized clusters are formed in diamond, which will overlap at a critical dose to form a semi-continuous graphitized layer. (For complete abstract open document)
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    The study of defect and trapping levels in CVD polycrystalline diamond with applications to ultraviolet dosimetry
    TRAJKOV, ELIZABETH ( 2004-05)
    The unique properties of diamond make it an excellent material for electronic and optical applications. It is particularly attractive for ultraviolet radiation dosimetry due to its intrinsic properties, which include biological tissue equivalence and visible blindness. Importantly, the advent of synthetic diamond, especially Chemical Vapour Deposition (CVD) diamond, has made it more economically viable for such applications. A thorough understanding of the electronic properties of diamond is needed before these applications can be fully explored. Consequently, this thesis investigates charge carrier trapping states in CVD polycrystalline diamond for the optimization of ultraviolet radiation dosimetry. The technique of Thermally Stimulated Conductivity (TSC) is used to probe electrically active defects and is also applied for dosimetric read-out. A range of as-grown CVD polycrystalline diamond films are studied to determine attributes that favour dosimetric-related TSC. In doing so, we establish that dosimetric TSC in these films originate from defects at the grain boundaries with a correlation to high crystalline quality. With this finding in hand, we then investigate the possibility of optimising diamond for dosimetry by controllably introducing extrinsic dosimetric defects using ion implantation. However, it is shown that these defects are not suitable for dosimetry and have a detrimental effect on the indigenous TSC signal. This study verifies the importance of crystalline quality on the indigenous dosimetric properties of CVD polycrystalline diamond. The possibility of doping CVD diamond during growth is also investigated as a means for intentionally introducing extrinsic dosimetric defects. Sulphur is selected as the dopant based on the theoretical energy levels formed by this defect, and because the prospect of S doping in diamond remains an actively debated issue in the literature. We report for the first time defect levels extracted from TSC analysis of S-doped CVD diamond and find consistency with theoretical predications. In addition, the dominant TSC trap level in S-doped diamond shows promise for radiation dosimetry with certain properties exceeding many current radiation dosimeters. The experimental results in this thesis lead to a deeper understanding of defect and trapping mechanisms in CVD polycrystalline diamond and establish attributes that favour TSC and related dosimetric properties in such films. This knowledge is fundamental to the realisation of diamond for ultraviolet dosimetry.