Magister Scientiae - MSc / The growth of amorphous hydrogenated silicon (a-Si:H) thin films deposided by hot wire chemical vapor deposition (HWCVD) has been studied. The films have been characterised for optical and structural properties by means of UV/VIS,FITR,ERDA, XRD.XTEM and Raman spectroscopy. Low subtrate heater temperatures in the range form 130 to 200 degrees celcius were used in this thesis because it is believed to allow for the deposition of device quality a-Si:H which can be used for electronic photovoltaic devices. Furthermore, low temperatures allows the deposition of a-Si:H on any subtrate and thus offers the possibility of making large area devices on flexible organic substances. We showed that the optical and structural properties of grown a-Si:H films depended critically upon whether the films were produced with silane gas or silane diluted with hydrogen gas. We also showed that it is possible to to deposit crystalline materials at low temperature under high hydrogen dilution ratio of silane gas. / South Africa

Epitaxial oxide thin films are used in many technologically important device applications. This work deals with the deposition and characterization of epitaxial WO3 and SrBi2Ta2O9 (SBT) thin films on single crystal oxide substrates. WO3 thin films were chosen as a subject of study because of recent findings of superconductivity at surfaces and twin boundaries in the bulk form of this oxide. Highly epitaxial thin films would be desirable in order to be able to create a device within a film without patterning it, by locally creating superconducting regions (e.g. twins) within an otherwise defect free film by reducing or doping the film with Na. Films were deposited by reactive magnetron sputtering at various temperatures on single crystal SrTiO3 (100) and R-sapphire substrates. X-ray diffraction studies showed that the optimised films were highly (001) oriented, quality of epitaxy improving with decreasing deposition temperature. AFM studies revealed columnar growth of these films. Films were heat treated with Na vapour in order to reduce or dope them with Na. Low temperature measurements of the reduced films did not show existence of any superconductivity. SBT is a ferroelectric oxide and its thin films are attractive candidates for non-volatile ferroelectric random access memory (FRAM) applications. High structural anisotropy leads to a high degree of anisotropy in its ferroelectric properties which makes it essential to study epitaxial SBT films of different orientations. In this study, SBT films of different orientations were deposited on different single crystal substrates by pulsed laser ablation. Highly epitaxial c-axis oriented and smooth SBT films were deposited on SrTiO3 (100) substrates. AFM studies revealed the growth of these films by 3-D Stranski-Krastanov mode. However, these films did not exhibit any ferroelectric activity. Highly epitaxial (116)-oriented films were deposited on SrTiO3 (110) substrates. These films were also very smooth with root mean square (RMS) roughness of 15-20 Å. Films deposited on TiO2 (110) were partially a-/b-axis oriented and showed the formation of c-axis oriented SBT and many impurities. Completely a-/b-axis oriented SBT films were deposited on LaSrAlO4 (110) substrates. Films deposited at non-optimal growth temperatures showed the formation of many impurities. Attempts were also made towards depositing Sr2RuO4 films on LaSrAlO4 (110) substrates, which can act as a bottom electrode for ferroelectric SBT films.

This thesis explores different physical phenomena observed in, or involving thin surface films. Thin surface layers are ubiquitous. Found in nature and used in almost every aspect of daily life, thin surface films are invaluable. While the applications and roles may be varied, to be used effectively, the physical properties of these films and the factors influencing their stability need to be well understood. Surfaces can have a strong effect on the stability of thin films. In thin films of polymer blends, wetting layers rich in one component often form at the film interface prior to phase separation. Here the formation of these wetting layers are seen to result in destabilisation of the film, even when the blend is far from phase coexistence. A spinodal like instability with a characteristic wavelength is shown to form. A theoretical model is developed, which describes the observed behaviour in terms of coupled height and composition fluctuations in the wetting layer. Spin coating is a common technique for the formation of thin polymer films. Films formed in this way however, are often seen to exhibit anomalous properties, which strongly differ from that of the bulk behaviour of the material. Here the rheological properties and stored stresses in spin cast films are explored, with focus on the role that the casting solvent plays in the properties of the film. The results suggest that the observed deviation comes from a lowered density of chain entanglements. The effective viscosity and residual stresses in the as-spun film are seen to strongly depend on the casting solvent properties and the solvent-polymer interactions. The use of organometallic polymers as precursors for the formation of magnetic ceramics is investigated. Emphasis is placed on doping the polymers with metallic compounds prior to pyrolysis, allowing for the formation of technologically interesting metallic alloys, without the need for new polymers to be synthesised. The formation of iron-palladium alloys is demonstrated using this method. These are highly desirable due to their potential use in hard-disk drive technologies. Thin films can be used to influence the optical signature of a material and are widely used in nature to produce vibrant, pure, iridescent colours. Here the optical properties of the tropical plant Selaginella willdenowii are explored. The bright blue colouration is seen to arise from a multilayer lamella structure on the upper surface of the leaves. Light is important to plants, who use it both as an energy source and an environmental signal. Blue iridescence occurs in a wide range of plant species, suggesting that it has some adaptive benefit. These are considered and discussed.

An automated ellipsometer was used to study three topics associated with the anodic oxide films of tantalum and niobium. The electro-optic effect was measured on tantalum and niobium oxides and was found to be quadratic. The change in refractive index upon application of a field occurred in two phases: an instantaneous change followed by a slower change. The effects on Nb₂O₅ were greater than on Ta₂O₅. The effect of ultra violet light on the two oxides was found to cause a change in the refractive index before appreciable photo-induced growth occurred. The results indicated that it was unlikely that the u.v.-induced change in refractive index occurred uniformly throughout the film. The effects of annealing and temperature are discussed in relation to the constant field current transient. The change in the refractive index during the transient was monitored with the ellipsometer. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A flash evaporation system is constructed in order to deposit thin films of Gallium Antimonide. The system includes a substrate heater-holder, a film thickness monitoring device and a powdered evaporant feeder. Thin films of GaSb, 2 to 4 microns thick are deposited on glass, sapphire and silicon substrates maintained at temperatures up to 365°C. The deposited films are observed to have many structural defects. The films exhibit semiconductor-like properties. Optical studies of the fundamental absorption edge at liquid-nitrogen temperature show that the films deposited on heated sapphire substrates have a polycrystalline structure and a band gap of 0.78 eV. The nature of the films is dependent on the type and temperature of the substrate. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The present thesis is concerned mainly with space charge and high field effects on the electrical properties of thin amorphous films. A theory of space charge contribution to the polarization current in thin dielectric films is proposed. The transient current on short-circuiting a thin dielectric film is believed to consist of two components, one due to the dielectric polarization and the other due to trapped space charge. The space charge contribution is investigated using a model for a film containing distributed traps. Computed results seem to be consistent with experimental results on Ta/Ta₂O₅/Au diodes, so that space charge effects are more important at low preapplied fields. The applicability of step response techniques to determine low frequency dielectric losses is discussed and the effect of space charge on the dielectric losses is analysed. The theory of thermoluminescence and thermally stimulated currents is extended to the case of traps with distributed binding energies to investigate the possibility of distinguishing between distributed and discrete trap levels. It seems possible to distinguish experimentally between distributed and discrete traps by using different doses of optical radiation to obtain initially different amounts of trapped charges, and by varying the frequency of optical excitation over a suitable frequency range to allow only certain energy levels to be occupied by excited electrons. High field electronic conduction through very thin films sandwiched between two metal electrodes is analysed. In view of the fast tunneling time of electrons through very thin films, MIM structures can be used for microwave detection. It is shown that the maximum responsivity-bandwidth product of such detectors is obtained when they are biased at a voltage equal to the anode work function (in volts), and that the presence of invariant positive space charge increases the magnitude of this maximum. In considering high field switching in thin films of semiconducting glasses, it is suggested that Joule heating, which could account for the delay times observed experimentally, serves only to initiate an electronic switching mechanism. A model for current-controlled negative resistance due to space charge formation is proposed and its dc characteristics are computed. Carrier injection from the electrodes is taken to occur either by Schottky thermionic emission or a Fowler-Nordheim tunneling mechanism. The injected carriers develop space charge regions near the electrodes by impact ionization. The position dependent generation-recombination rate is discussed. The small ac signal equivalent circuit of the model is given. The formation of current filaments is analysed. Memory devices are discussed in terms of filament formation and phase change mechanisms due to excessive heating. Filamentary breakdown has been observed in anodic films grown on Ta, Al, Nb and Ti. A detailed experimental study of film growth and the effects of growth conditions, film thickness, counterelectrodes and temperature on breakdown strength has been carried out. A possible mode of breakdown, in which breakdown can result from thermal effects following a non-destructive electron avalanche, is proposed and its limitations are pointed out. It is concluded that breakdown in thin anodic films would occur due to disruption of the chemical bonds as the applied field approaches the formation field. The product of the molecular dissociation and the presence of energetic electrons could start an accumulative process which might end with the formation of a highly conducting channel. The injected electrons, field distortion and thermal runaway could assist in the channel development. Once the channel is developed, the sample's stored energy starts to dissipate through the channel. The voltage collapse has been found experimentally to occur in a time of less than 200 nanoseconds. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The Y₁Ba₂Cu₃0x superconducting thin films were made by both sequential electron-beam evaporation and co-evaporation of Cu, BaF₂ and Y, followed with high temperature post-deposition annealing. The possibility of making Y₁Ba₂Cu₃0x superconducting film in situ with BaF₂ as the Ba source was investigated by studying the conditions of in situ conversion of BaF₂ to BaO. The BaO concentration in the film as function of the substrate temperature was estimated with an equilibrium thermodynamic model and found to agree with the experimental results. A brief experimental and theoretical exploration of BaCl₂ also showed that BaCl₂ does not have any advantage over BaF2 as the Ba source for in situ growth of Y-Ba-Cu-0 films from the growth temperature point of view. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The classical theory of rectilinear vortex motion has been generalized to include vortices in thin fluids of varying depth on curved surfaces. The equations of motion are examined to lowest order in a perturbation expansion in which the depth of fluid is considered small in comparison with the principal radii of curvature of the surface. Existence of a generalized vortex streamfunction is proved and used to generate conservation laws. A number of simple vortex systems are described. In particular, criteria for the stability of rings of vortices on surfaces of revolution are found. In contradistinction to the result of von Karman, double rings (vortex streets) in both staggered and symmetric configurations may be stable. The effects of finite core size are examined. Departures from radial symmetry in core vorticity distributions are shown to introduce small wobbles in the vortex motion. The case of an elliptical core is treated in detail. Applications of the theory to atmospheric cyclones and superfluid vortices are discussed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The first study investigated the deposition and characterization of the CoO and Co3O4 phases of cobalt oxide. It was determined that both phases can be easily distinguishable by XPS, LEED and EELS and grown by only altering the oxygen partial pressure during MBE deposition. This fundamental knowledge gives a foundation for further experiments involving graphene growth on cobalt oxides. The second study focused on the layer-by-layer growth of graphene on another metal oxide, MgO. Past research gives promise of favorable interfacial interactions at the graphene/MgO interface though the exact growth mechanism is unknown. Layer by layer growth by MBE resulted in the characterization of a complex graphene oxide/buckled graphene/ graphene heterostructure confirmed by XPS, AES, LEED and EELS and supported by DFT calculations performed by the project's collaborators at the California Institute of Technology. This detailed look into graphene growth give valuable information that can be allied to graphene growth on similar oxide surfaces. The last project deviates from graphene-based studies and instead focused on interfacial interactions between two metal oxides, chrome oxide and titanium oxide. A corundum phase TiO2-x film was grown on Al2O3 via MBE and characterized using XPS, AES, LEED, and EELS. Data taken gives evidence of the presence of a two-dimensional election gas at the TiO2-x surface because of oxygen vacancies present after deposition. Deposition of chrome in UHV results in the formation of an oxidized chrome overlayer by abstraction of oxygen from the TiO2-x underlayer increasing the number of vacancies present. MOKE measurements taken by the project's collaborators at the University of Nebraska-Lincoln indicate that there is an interfacial exchange bias at the interface of the two oxides, a favorable property for magnetic device applications.

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