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

The oxidation of dopamine in aqueous solutions deposits thin conformal films on a wide variety of material surfaces. These films consist of a material known as polydopamine (PDA), and they exhibit chemical and structural similarities to melanin pigments and adhesive proteins secreted by mussels. The facile synthesis and versatile adhesion of PDA enable the functional modification of numerous material surfaces for applications in biomedical devices, energy storage, and water purification. This thesis details fundamental investigations into the deposition, oxidation, and adhesive mechanisms of PDA films. Depositing PDA films on substrates with different controlled chemistries revealed the importance of solution pH and initial deposition rates on the morphology of the films. The deposition of PDA molecules with increasing pH depends on two competing factors: increased generation rate of PDA molecules versus increased solubility due to catechol ionization. The areal density and coverage of three-dimensional PDA islands is influenced by the surface charge and hydrophobicity of the substrate in aqueous solutions. Spectroscopic and electrochemical characterization of PDA films revealed that redox-inactive metal cations can accelerate the oxidation of PDA. The generation of radicals of 5,6-dihydroxyindole were monitored in situ via ultraviolet-visible spectroscopy as a function of cation concentration and pH. The extent of oxidation was quantified by cyclic voltammetry. The resulting oxidation modifies the metal sorption properties of PDA by generating more carboxylic acid groups and enhancing the iron chelation of the films. The adhesive stability of PDA films was characterized by delamination kinetics of films on SiO2 and indium tin oxide (ITO). PDA film adhesion is a substrate, salt, and oxidation-dependent phenomenon. Long-term adhesive stability of PDA films can be promoted by use of higher dopamine concentrations during synthesis, incorporation of multivalent cations, and avoiding alkaline conditions and strongly oxidizing electrical bias. Elastic moduli of PDA films were quantified by compressive thin film wrinkling, and the measured value of 2.0 ± 0.9 GPA agrees with simulations of PDA based on an oligomeric aggregate model. This thesis helps develop a framework for understanding the synthesis, composition, microstructure, and stability of PDA films.

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.

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.

Bismuth layer-structured ferroelectrics have been recognised as promising film materials for ferroelectric random access memory application due to their excellent fatigue resistance and other electrical properties. This work deals with the deposition and characterisation of epitaxial and polycrystalline W-doped SrBi2Ta2O9 (SBT) and lanthanide-doped bismuth titanate (BiT) films. SBT and W-doped SBT films were fabricated by pulsed laser deposition (PLD) on platinised silicon substrates. The effects of fabrication temperature and W-doping level on film properties were studied. The crystallinity of SBTW films improved with increasing fabrication temperatures, resulting in enhanced ferroelectric properties and dielectric properties above the fabrication temperature of 750 °C. Dense ceramic samples of Nd- and Sm-doped BiT (BNdT and BSmT) were successfully fabricated for PLD targets by solid state processing. Highly epitaxially (001)-, (118)-, and(104)-oriented Nd-doped bismuth titanate (BNdT) films were grown by PLD on (001)-, (011)-,and (111)-oriented SrTiO3 (STO) single crystal substrates, respectively. A three-dimensional orientation relationship between films and substrates was derived as: BNdT(001)//STO(001),BNdT[ 110 ]//STO[100]. Films showed strong dependence of structural and ferroelectric properties on the crystal orientation. PLD-grown BSmT films on platinised silicon substrates were studied as a function of fabrication temperature, effects of Pt bottom layer orientation, Sm doping level, and LaNiO3 buffer layer. An alkoxide-salt chemical solution deposition (CSD) method was adopted to prepare the precursors for BSmT (BNdT) film fabrication. Precursors of Bi-Sm(Nd)-Ti which were stable for at least eight months in air ambient were successfully developed. In-situ FT-IR studies suggest that acetic acid serves as chelating agent to improve the homogeneity of the precursor solution by generating a dense and homogeneous Ti-O-Ti polymeric network. The electrical properties of the films fabricated in this study (dielectric and ferroelectric properties, leakage current characteristics and electrical fatigue properties), are comparable or superior to these previously reported for similar films developed by other techniques or with other doping elements. Low temperature electrical properties of BSmT films suggest that the films are very promising for extremely low temperature nonvolatile memory applications. The results of BNdT films annealed at different oxygen partial pressure (O2, air, N2) showed that oxygen ambience affected structural properties of the films by enhancing the growth of perovskite phase (phase formation), increasing grain size (grain growth), and assisting the growth of (117)-oriented grains (crystallographic orientations). Piezoresponse force microscopy (PFM) was adopted to characterise BSmT films. Domain structures were clearly observed in a PLD-grown BSmT film, which were closely related to the grain structures. Domain manipulation was carried out in a CSD-derived BSmT film, showing that the film can be nearly uniformly polarised, which can be used in nanoscale device fabrication. Clear hysteresis loops were measured by PFM, which was an important proof of ferroelectricity. Large spatial variations of piezoelectric hysteresis loops of a CSD-derived BSmT film were observed across the film surface. Effective electrostriction coefficient (Qeff) of a PLD-grown BSmT film was measured, showing that BSmT films had better piezoelectric properties (higher Qeff, higher dzz) than SBT films, un-doped BiT ceramics and films. It suggests that BSmT films are promising piezoelectric materials for MEMS use.

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.

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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