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In this dissertation, Zinc oxide (ZnO) nanorods grown by a two-step chemical bath deposition method on Si substrate is characterized. Research was conducted on ZnO nanorods for the understanding of their optical properties at room temperature (RT), with the emphasis on the visible luminescence. To this end, controlled thermal treatments of as-grown ZnO nanorods were conducted under different conditions, such as annealing time and environment, at atmospheric pressure. Results related to the following studies are reported: an investigation of the structure of ZnO nanorods, an analysis of the chemical composition of the surface, an investigation of the surface stoichiometry of the rods, and a study of defect-related photoluminescence of ZnO nanorods upon thermal treatment in different ambients.To achieve this, the samples were investigated by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES) and room temperature (RT) photoluminescence Spectroscopy (PL). As-grown ZnO nanorods exhibit a hexagonal shape and have the wurtzite structure; they have, respectively, an average length and diameter of ~900 nm and ~50 nm, and most of the rods are perpendicular to the substrate. The main extrinsic species found in as-grown nanostructures are C, H, F, S, and Cl. ToF-SIMS and XPS confirmed the presence of H related-defects, and the oxygen 1 S XPS peak at 531.5 eV is therefore assigned to oxygen bound to H-related defects. Based on stoichiometry studies, it is found that the near surface regions of as-grown ZnO nanorods (2 to 10 monolayers) are rich in Zn. The RT luminescence of as-grown ZnO nanorods exhibits a near band edge emission centered at ~379.5 nm and deep level emission extending from ~450 nm to ~850 nm. When these nanorods are thermally treated at high temperatures (>850 oC), it is found that even though their crystalline quality is preserved, their morphology is significantly affected, regardless of annealing ambient. Furthermore, in the near surface regions of annealed ZnO nanorods it is found that the Zn/O stoichiometric ratios deviate from unity. Specifically, oxygen vacancies form within the first 100 nm from the sample surface. Further from the surface, the material is deficient in Zn. It is deduced from XPS and AES that the ambient affects the activation rate of intrinsic defects. Furthermore, the only extrinsic defects that are affected by thermal treatment are found to be H-related defects. At high annealing temperatures (300 oC to ~700 oC), H-related defects are removed, and this removal process is found to affect significantly the RT luminescence properties of ZnO nanorods. Specifically, hydrogen passivates vacancy-related defects, depending on the thermal treatment. PL spectroscopy is used to follow this passivation effect as a function of annealing temperature, which causes an initial quenching followed by an enhancement of the green and the red luminescence, regardless of the ambient. Finally, the green luminescence that arises following annealing above ~800 oC is assigned to Zn vacancy-related defects, while the red luminescence that dominates the visible band of ZnO nanorods upon annealing between 400 oC and 600 oC is suggested to be due to oxygen vacancy-related defects.

MgxZn1-xO has emerged as a material of great technological importance. Having a direct energy band gap that is tunable throughout much of the ultraviolet (UV) region of the spectrum from the near-UV (~370 nm) to the deep-UV (~176 nm), this compound is of interest for a variety of optoelectronic devices operating in this part of the electromagnetic spectrum. MgxZn1-xO offers advantages over the more mature compound semiconductor AlGaN which stem mainly from the unusually high exciton binding energy (60 meV in ZnO). In this study the growth of ZnO and MgxZn1-xO thin films using metal organic chemical vapour deposition (MOCVD) is systematically investigated. The films are mainly grown on c-Al2O3 and Si (100) and characterized using various techniques, such as photoluminescence (PL), x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and auger electron spectroscopy (AES). The optical and the structural properties are essentially inspected in order to improve their quality. In this thesis the optimisation of ZnO grown using oxygen gas as a new oxidant in our reactor is investigated. The growth temperature and VI/II ratio are varied in order to find optimum parameters giving high quality layers. The effects of Si (100), Si (111), c- and r-sapphire, glass, GaAs and ZnO substrates on the optical, structural and morphological properties of ZnO thin films grown with tert-butanol (TBOH) is examined. Similar morphologies are observed for all substrates, with the films comprising hexagonal columns having cone shaped ends. The photoluminescence spectra are similar, but the various transitions have different relative intensities. It is clear that the different substrates influence neither the orientation of the films, nor the surface morphology, significantly. The photoluminescence hints at larger stacking fault densities in films grown on silicon and glass, however, as evidenced by stronger basal plane stacking fault-related luminescence at ~3.319 eV in the relevant low temperature photoluminescence spectra. The morphology changes with Mg incorporation, from hexagonal columnar structures to cubic faceted columns. From PL, the full with at half maximum is found to gradually increase with Mg content due to alloy broadening. The deep level emission (DLE) is observed to shift with Mg content. By changing the Mg content, the band gap of MgxZn1-xO film is tuned by ~450 meV, which provides an excellent opportunity for band gap engineering for optoelectronic applications. The c-lattice constant of ZnO (5.205 Å) decreases by only 0.6% when the Mg content reaches x=0.39. The introduction of Mg into ZnO is shown to increase the relative PL intensity of stacking fault-related transitions (at 3.314 eV for ZnO). This becomes the dominant near band edge emission. Using TEM a thin Mg rich layer is observed at the interface between the film and the Si or Al2O3. Temperature dependent PL measurements on layers with low Mg concentration (x=0.05 and 0.1) show that the main bound exciton peak exhibits an “s-shaped” temperature dependence, characteristic of localization in a disordered alloy. The origin of the PL line broadening of MgxZn1-xO (x≤0.04) is also analyzed with respect to alloy broadening, taking into account a random cation distribution and alloy clustering. The influence of various MOCVD growth parameters such as growth temperature and VI/II ratio is studied. Varying the temperature from 280 ˚C to 580 ˚C reveals strong morphological changes and optical degradation of the films. Low (<280 ˚C) and high (>580 ˚C) growth temperatures reduce the Mg incorporation. High VI/II ratios also decrease the Mg incorporation, as evidenced by the red-shift of the donor bound exciton (D°X) line. This is ascribed to a stronger premature reaction between (MeCp)2Mg and the oxidant or a preferential heterogeneous interaction between the Mg and oxygen species on the growth front. For both oxidizing agents (O2 and TBOH), the growth at 420 ˚C and a VI-II ratio of 60 on c-Al2O3 gave optimal quality layers in terms of their optical and structural quality. A comparison of films grown using TBOH and O2 gas as oxidizing agent shows no major difference in terms of Mg incorporation. The effect of annealing, the inclusion of a buffer layer and the influence of growth rate on the properties MgxZn1-xO thin films are also reported.

This work focuses on the impact of defects, intrinsic or artificially introduced, on the functional properties of thin, epitaxial oxide films. In the first part, the origin of the ferromagnetic properties of Mn-doped and undoped zinc oxide is studied. The deposition conditions are found to have a significant impact on the structural, transport and magnetic properties of the thin films. Combining x-ray magnetic circular dichroism and magnetometry experiments, it is established that the transition metal dopants (i.e. Mn) have no influence on the ferromagnetic nature of the zinc oxide, but that localised magnetic moments on intrinsic defects are in fact responsible for the ferromagnetic behaviour. A relation between strain (related to defect concentration) and magnetisation is established. In the second part of this dissertation, artificially introduced defects are employed in order to discover the fundamental conduction mechanism behind the two-dimensionally conductive LaAlO3/SrTiO3 interface. All experiments, from varying deposition temperature, to oxygen pressure, to laser fluence or to the insertion of (doped) perovskite layers, point towards a structurally governed conduction mechanism, although the exact details are still unclear. Distinct transitions in the resistance versus temperature curves are observed at different values than the bulk phase transformation temperature. These transitions form the boundaries of different conduction modes, with tendencies towards non-Fermi-liquid behaviour observed in certain two-dimensionally conducting samples in limited temperature regimes. By optimising the (defect) structure at the interface, i.e. by introducing a single unit cell of (La0.5,Sr0.5)TiO3 or SnTiO3, it is shown that the sheet carrier density can be dramatically enhanced, up to an order of magnitude higher than unmodified LaAlO3/SrTiO3 interfaces with a value of 1e14 cm−2 at 200 K. Finally, attempts at functionalising the conductive heterointerface by doping and inserting (anti)ferromagnetic layers are made.

The MOS device is the basic switching element in modern integrated circuits, and its reliability is vital to the successful operation of electronic equipment. Exposure to ionising radiation seriously affects MOS devices because of charge trapping and the formation of defects at the silicon-silicon dioxide interface. After an introductory chapter on MOS devices and radiation effects, experiments are described which give information about the nature of the interface defects and how they interact with each other. A particular device current Irec is measured whose magnitude depends on the recombination of charge carriers at the defects. The device is so minute, and the interface so thin, that the paramagnetic defects are too few in number to be detected and identified by conventional electron spin resonance methods. However, the static and microwave magnetic fields corresponding to spin resonance affect the recombination of carriers on the defects, and this causes a detectable change in Irec. This phenomenon is called Spin-Dependent Recombination (SOR), and a survey of SOR studies in semiconductors is given in Chapter 2 . The experimental results confirm a model which suggests that SOR occurs between adjacent trapped pairs. The results of the experiments are compared with ESR data on similar (but much larger) MOS structures. Spin-Dependent Generation of carriers is also investigated. The recombination is also found to be strongly dependent on a static magnetic field of zero to 5 milliTesla, even with no microwaves. Results of experiments on these "non-resonant" spin-dependent effects are presented with a model, relating them to the resonance experiments, which involves the recombination of singlet and triplet electron-hole pairs in a magnetic field. Electrical charge injection can affect MOS devices in similar ways to ionising radiation, and this is discussed in Chapter 6. Experimental results are presented which show that there are spin-dependent effects associated with defects produced by electrical charge injection. There are two Appendices, on slow radiation-induced instabilities in MOS structures, and on the size of the recombination current Irec�

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A study has been made of the dissolution of naturally occurring α-iron oxide hydrate in acidified aqueous solutions of sulphur dioxide at 110°C. The dissolution was found to be independent of acidity at low concentrations of sulphur dioxide and inversely dependent on acidity at higher concentrations of sulphur dioxide. Both homogeneous and heterogeneous control of the reaction was observed. The addition of cupric ion to the system catalysed the rate. Dissolution is thought to occur via hydration of the oxide surface and subsequent reaction of undissociated sulphurous acid at the surface to form a ferric-sulphite complex. The rate determining step is considered to be the desorption of the complex from the surface. A limited study of the direct dissolution of iron oxide hydrate in sulphuric and perchloric acids at temperatures between 120 and 150°C is included. Under these conditions, the hydrated oxide surface is thought to undergo anion exchange during dissolution. Work carried out on the preparation and identification of the isomeric α- and γ-iron oxide hydrates is reported. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The Electronic transition ²Σ-X²Π of N₂0⁺ has been observed at high resolution and a more thorough analysis of the spectrum has been undertaken than was previously available in the literature. Twenty-four bands of the spectrum have been fitted into a vibrational energy level scheme. Of these, eighteen were found to involve only the vibrations Ʋ₁ and Ʋ₃. The remainder of the bands were ascribed to the vibration Ʋ₂, and were found to be explainable in terms of Renner effects coupling the transverse vibration and the orbital angular momentum of the ²π Electronic state. Rotational analyses of varying degrees of thoroughness have been made of seven bands. Various molecular constants have been deduced, including α₁, α₃, p ( the Λ - doubling constant), and q ( the 1-doubling constant of the upper state vibrational level 010). / Science, Faculty of / Physics and Astronomy, Department of / Graduate

[No abstract available] / Science, Faculty of / Chemistry, Department of / Graduate