Carbon encapsulated metal nanoparticles are an increasingly important class of materials due to the wide range of electronic, magnetic and mechanical properties they display. However, traditional deposition techniques are often complex or lead to a poor quality film. Ionised magnetron sputter deposition is a promising development to traditional magnetron sputtering which combines film deposition with ion bombardment. By adding an RF powered, inductively coupled plasma positioned between the deposition targets and the substrate, the ionisation fraction of the depositing flux is greatly increased. This additional ion flux can then be controlled through the use of an electrical substrate biasing. This controls the energy flux to the surface and therefore the resulting microstructure. Carbon-nickel thin films were grown by ionised magnetron sputter deposition. The films themselves were characterised using a wide variety of techniques to measure not only their structure but their properties. Additionally, the inductively coupled plasma itself was characterised using a Langmuir probe. It was determined that upon application of a negative substrate biasing, the ion flux to the growing film remained constant, however the energy of the species increased. This resulted in a columnar structure of nickel carbide which coarsened as the bias (and therefore the energy of the ions) was increased. Conversely the application of a positive bias gives a large flux of low energy bombardment. This led to the formation of metallic nickel nanoparticles (? 30 nm diameter) which were surrounded by several layers of ordered graphitic shells forming a so-called "nano-onion" structure. The transition between these phases is a result of an increase in adatom mobility when there is a high flux, low energy ion bombardment which allows the nickel and carbon to phase separate. Upon separating, the nickel templates graphite growth due to their similar bond lengths leading to the formation of the graphitic cages. The transition between these two structures is measured through X-ray diffraction which shows a transition from the hexagonal carbide phase to the cubic nickel phase. This is accompanied by an increase in ordering of the carbon as the bias is increased as measured by Raman spectroscopy. Additionally, it is observed that there is an increase in carbon ordering when a negative bias is applied, due to the additional energy from ion bombardment leading to graphite formation. Magnetic measurements showed a transition from a non-magnetic state when the structures were largely carbide, to a magnetic state when metallic. However at room temperature the structures display superparamagnetic behaviour due to the small size of the particles. Measurements of electronic conductivity showed a negative temperature coeffcient of resistivity for all samples demonstrating no metallic conduction path was present. A large drop in resistivity as the temperature increases was assigned to thermally activated conduction. At low temperatures the conductivity is dependent on tunnelling across small regions of amorphous carbon, while at higher temperatures it is possible to excite the electrons into a conduction band allowing them to conduct more easily.

A study of the structural and electrical properties of low-temperature-sublimed silicon films indicates that they are characterized by a high density of grain boundaries, hence crystal defects. A trapping model has been proposed to explain the experimentally observed temperature-dependencies of resistivity and carrier concentration of these films. The result shows that the defect density at the grain boundaries is of the order of 10¹² cmˉ², and that it is independent of the doping concentrations in the films. It has been shown that the thin-film metal-insulator-semi-conductor (MIS) structure can be reduced to a transmission line problem by expressing the equivalent capacitance of the structure as a series combination of the depletion capacitance and the insulator capacitance. The variations of both the capacitance and channel conductance of the MIS structure have been utilized to make notch filters in which the notch frequency can be varied over 200% by an external biasing voltage. In view of the need for maintaining a constant null depth in the semiconductor notch filter under various biasing potentials, a new notch network has been proposed in which the optimal notch condition could be maintained simply by designing the ratios of the lengths and widths of the MIS structure to the appropriate values. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A study has been made of the properties of thin yttrium oxide dielectric films prepared by the electron beam evaporation of high purity Y₂O₃ powder. Films deposited on freshly cleaved NaCl crystals and on polished n-type silicon were examined in the electron microscope. The specimens were found to be polycrystalline, with a crystal size of the order of 100 Å. The structure was found to be essentially the same as found for bulk Y₂O₃. D.C. conduction measurements were made on films of various thicknesses. The characteristics were found to be bulk-limited, with the conductivity decreasing at lower pressures. An activation energy of 0.6 eV was found. The conduction mechanism was believed to be Poole-Frenkel emission of electrons from donor centers into the Y₂O₃ conduction band. The donor centers were believed to be interstitial yttrium atoms rather than oxygen vacancies because of the pressure dependence observed in conductivity. Step response measurements were made, and the results explained on the basis of a loss peak with a most probable relaxation time of 200 seconds. The relaxation of oxygen atoms dissolved in the anion defective Y₂O₃ lattice was assumed to be the mechanism. The results of step response and A.C. bridge loss measurements indicated that different relaxation mechanisms are dominant in different frequency ranges. Internal photoemission measurements were made on Al-Y₂O₃-Al sandwiches. The energy barrier between the electrodes was found to be trapezoidal, with barrier heights of 3.14 and 3.72 eV. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The work contained in this thesis is concerned mainly with conduction mechanisms and polarization processes in thin amorphous insulating films. A model has been proposed and its d.c. conduction characteristics computed. The numerical results show the possibility of obtaining either space charge, Schottky, or Poole-Frenkel characteristics depending on the model parameters. The transient electronic discharge current has been analysed and the results show that this electronic current is approximately independent of the preapplied voltage in contrast to the ionic discharge current which is linearly dependent on preapplied voltage. This result, together with the experimental results obtained on Ta/Ta₂O₅/Au diodes, suggests that the calculations of low frequency dielectric losses using step response measurements are complicated by space charge effects only when the preapplied field is relatively low (≤lMV/cm for Ta₂0₅ films). Ta/Ta₂O₅/Au diodes were prepared by solution anodization or plasma anodization. All prepared diodes exhibited a rectification behaviour. Over the frequency range 100 Hz -100 kHz capacitance and loss tangent were found to decrease slightly with increasing frequency while the equivalent series resistance was found to be approximately proportional to ω⁻¹‧º⁵. All prepared diodes, with gold counter electrodes less than 1000 Å thick, were found to withstand, under a slowly applied field, field strengths approaching the formation field value. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A microcomputer software package was written to calculate the tranmission and reflection characteristics of multilayer thin films on a substrate. The program was written to be as "user-friendly", versatile and modular as possible. To test the program, a transparent oxide-type heat-mirror film capped with an antireflection coating was studied for greenhouse applications. Si0₂ and ZnO were considered as representative antireflection and heat-mirror materials respectively. The results of calculations of heat transfer coefficient and transmittance of the glazing construction polyethylene/air gap/Si0₂/ZnO/polyethylene are presented. The resulting structure is shown to give a visible light transmission spectrum which closely matches the plant sensitivity curve for plant growth and has higher thermal insulation compared to uncoated polyethylene. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The magnetic behavior of cubic B20 crystal structure FeSi thin film has been previously probed at macroscopic level using a magnetometer. The results revealed ferromagnetic state with significant hysteresis. This is contrary to the bulk with the same cubic B20 crystal structure that is paramagnetic. The origin of ferromagnetism in thin films in contrast to paramagnetism in the bulk is unclear and unexplained. Electron spin resonance technique (ESR) was used as a tool to characterize the magnetic behavior of FeSi thin films at microscopic level. With ESR technique, B20 crystalline FeSi show microwave power absorption centred at zero field (HDC = 0) termed low-field microwave absorption (LFA) in addition to usual ferromagnetic resonance (FMR) typical of magnetic materials. LFA was observed as a distinct signal in these films. This signal has been observed in several other materials other than FeSi thin films. However in FeSi thin films it was for the first time that LFA signal was observed. The LFA is closely connected to the magnetization process that occurs at low applied field. LFA is a new technique that has recently been used to detect the magnetic transition in materials, sensitive detection of magnetic order and more importantly to distinguish between different dynamics of microwave absorption centres. The LFA measurements were made at 9.4 GHz (X-band) on pulse laser deposited (PLD) polycrystalline B20 cubic structure FeSi thin film grown on Si (111) substrate. PLD is regarded as a powerful tool for thin film growth. The LFA properties of the films were investigated as a function of DC field, temperature, microwave power and orientation of DC field with respect to the film surface. The LFA signal is very strong when the DC field is parallel to the film surface and diminishes at higher angles. This is attributed to induced anisotropy field (IAF) and surface anisotropy field (SAF) contributing to total anisotropy field (TAF). The LFA signal strength increases as the microwave power is increased, such increase is due to impedance and thus showing that LFA and magnetoimpedance (MI) has common origin. The LFA signal disappears around 340 K which can be attributed to the disappearance of long range order giving us a positive signature of surviving magnetic state well above room temperature in these films. We believe that domain structure evolution in low-fields, which in turn modifies the low field permeability as well as the anisotropy, could be the origin of LFA observed in these films. MI and LFA can be understood as the absorption of electromagnetic radiation by spin systems that are modified by domain configuration and strongly depend on anisotropy field. The observation of LFA opens the possibility of FeSi films to be used as potential candidates of low magnetic field sensors in the microwave and radio frequency regions. Copyright / Dissertation (MSc)--University of Pretoria, 2012. / Physics / unrestricted

No description available.

Spintronic-based magnetic random-access memory (MRAM) implementing the tunnel magnetoresistance (TMR) effect has various advantages over conventional semiconductor base memory devices, such as non-volatility and potentially high density and scalability. Traditional MRAM design implemented in-plane magnetic switching for the read/write operation which is now recognized to suffer from poor scalability below 60 nm. With the discovery of the spin-transfer torque (STT) effect, where the spin-polarized current is used to switch the ferromagnet, the MRAM design simplified considerably as it eliminated one of the two current-carrying wires that are used to generate the magnetic field required for switching. The thermal stability is further enhanced by using magnetic materials with perpendicular magnetic anisotropy (PMA). In current devices, perpendicular anisotropy is developed at the free magnetic layer (CoFeB) interface with the tunnel barrier (MgO). It is called interfacial-perpendicular anisotropy. However, it has been shown that this design has scaling issues below 20 nm. Materials with volume (bulk) perpendicular magnetic anisotropy should show better scaling without compromising on thermal stability.This dissertation work is focused on growth and physical property investigations of thin films of novel magnetic and electronic materials which are promising for MRAM devices. Leveraging on prior identified materials (both theory and bulk materials experiment) with tetragonal and hexagonal symmetry that support PMA, we have successfully implemented several manganese-based hexagonal Heusler-like Mn3-xFexSn (X=0,1,2) alloys predicted to be high PMA materials. While Mn3Sn thin films are reported in the literature, we are not aware of any thin film reports elsewhere on Fe2MnSn and Mn2FeSn thin films discussed here. All these materials are stabilized in the hexagonal structure which inherently supports perpendicular anisotropy. Specifically, we found that Mn3Sn has low saturation magnetization and high Tc but low magnetic anisotropy. Mn2FeSn has a moderate magnetic moment but low Tc (272 K). Fe2MnSn is the most favorable material among our investigations, with high magnetic anisotropy and high Curie temperature of 548 K, but with a higher than desired magnetization value. The magnetic anisotropy value of Fe2MnSn is estimated to be 0.56 MJ/m3. Such value is in the desirable range for MRAM devices. Our thermal stability calculations indicate that STT-MRAM with Fe2MnSn free layer can scale below 20 nm lateral size for 3nm free layer thickness. While the scaling behavior remains to be investigated experimentally, my work has demonstrated that research into new materials is always an exciting prospect particularly if combined with a theory-driven design approach.

Surface tension driven flow is studied in films of viscous polymer liquid by monitoring the spreading of droplets or the capillary levelling of films with excess surface area. The research presented in this thesis is focused on three major themes where molecular details are important to flow: molecular anisotropy, interfacial friction, and the initial state of the film. The effect of molecular anisotropy is studied by examining the dynamics of partially wetting diblock copolymer droplets using optical microscopy. The shape of the droplets is measured as they evolve towards equilibrium. In this system, it is found that energy is dissipated at the base of the droplets. This is consistent with a reduced interfacial friction at the liquid-substrate interface. Flow dynamics are also found to depend on the symmetry of the initial film thickness profile. Thickness perturbations with different degrees of symmetry were created in an initially flat film using focused laser spike annealing. The films were allowed to flow under the driving force of surface tension, and using atomic force microscopy, the film thickness profile was measured as a function of time. We find the depth of the perturbations decreases as a power law in time, with a power law exponent that depends on the symmetry of the thickness perturbation. The role of interfacial effects are explored by studying the flow in a film with zero interfacial friction: a freely-suspended film. Flow is measured in films with no interfacial friction using a technique which creates a film with a sharp step in the initial thickness profile. The excess surface area at the edge of the step drives flow, and information about the dynamics of the fluid is obtained by measuring the width of the step over time with atomic force microscopy. We observe flow that is consistent with plug flow: where the velocity of the fluid in the plane of the film is constant along the direction perpendicular to the film. Finally, freely-suspended films provide a model system to study the nucleation and growth of pores in a membrane. By purposefully creating pores of different initial size, the critical radius for nucleation is measured as a function of the membrane thickness. The experimental results agree with a simple model in which the free energy cost at the perimeter of a pore is determined by the excess surface area due to the curved interface of the pore edge. / Thesis / Doctor of Philosophy (PhD)

Thin film heaters, corrosion-resistance electrode, thin film inductors / Stretchable electronics are soft and light weight. Compared with conventional wafer-based electronics, which are rigid and planar, stretchable electronics can conform to curved surfaces and movable parts. The unique properties of stretchable electronics enable their integration with the human body, and open the door for ever more compelling applications, such as advanced surgical tools, wearable monitoring electronics, implantable prosthesis, and many others. However, the development of stretchable electronics is still at an early stage since their mechanical robustness and electrical performance are still far from satisfying. In this work, I have developed a method to fabricate thin film stretchable devices by solvent-assisted transfer of wrinkled thin films from rigid polystyrene (PS) substrates to elastomeric polydimethylsiloxane (PDMS) substrates. Using this approach, structured thin films containing multiple materials and hybrid structures could be lifted off simultaneously, facilitating the fabrication of stretchable thin film devices. With this approach, I have built corrosion-resistant stretchable electrodes, stretchable thin film heaters, and stretchable thin film inductors. These applications demonstrate the simplicity and effectiveness of this stretchable electronics fabrication strategy. Finally, I made the first step towards fabricating dye-sensitized solar cells (DSSCs) with room temperature processes, including the preparation of mesoporous TiO2 layers through mechanical compression and the integration of an interdigitated electrode that was fabricated solely by bench-top patterning, alignment, and sputtering deposition. These steps lay the foundation for the future development of stretchable DSSC. I anticipate that the fabricated stretchable thin films electronic components will contribute to the advancement of wearable and implantable electronics. / Thesis / Doctor of Philosophy (PhD) / Electronics that can be deformed and conform to the irregular surfaces are attractive because they can be better integrated with the human body. For example, they could improve disease diagnostics and therapeutic treatments by providing wearable continuous monitoring devices and more advanced surgical tools. In this work, I created wrinkled thin films that could be affixed onto an elastic substrate and stretched. The principle of operation of these wrinkled devices mimics the way that the wrinkled skin on our knuckles and elbows allows us to bend our fingers and elbows. This approach makes wrinkled thin films stretchable and could lead to robust electronic devices. I have showcased this approach building a corrosion-resistant stretchable electrode, thin films heaters that can closely conform to joints, and a spiral-shaped inductor that could be used to wirelessly transfer data or power wearable devices. I believe that this work will contribute to the development of electronics that can be worn or implanted in the human body.