Thesis (MSc)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: In this investigation the formation of thin film manganites and their electrical characteristics is studied. In order to see the effect of oxidation states on magneto-resistivity, 80% of Mn is replaced by Fe. Pulsed laser deposition (3 J/cm2), carried out in oxygen partial pressures ranging from 0.01 mbar to 1.00 mbar was used to fabricate the thin films from two target compositions, namely La2CaMn2.94Feo.0609 and La2CaMno.6Fe2.409. Films were deposited on Si< 100 >, MgO< 100 >, SrTi03< 100 > and LaAl03< 100 > single crystal substrates. Samples were characterized by RBS, AFM, SEM, and XRD. Electrical measurements were also carried out. One of the main characterization techniques in this investigation is Rutherford Backscattering Spectrometry (RBS). It has been shown that RBS is a very powerful characterization technique when used in conjunction with the RUMP simulation program. The effect of various parameters can be determined beforehand by RUMP simulation of the thin film structures to be investigated. Simulation shows that RBS is an excellent characterization tool for determining film thickness and stoichiometry. The role of oxygen uptake in La2CaMn3_xFexOg was investigated as the oxidation states of elements in manganite materials have a large effect on their magnetoresistive properties. The height of the La signal can be used as a measure of the oxygen content. RBS spectra of films deposited on single crystal silicon substrates at different ambient pressures show that the fit between simulated and measured RBS spectra improves with higher oxygen pressures, thereby indicating better quality manganite material. The RBS spectra also show that the films have good stoichiometry. Atomic force microscopy was used to determine the roughness of the thin films. The annealed film (average roughness 4.5 nm) shows a surface smoother than the non-annealed film (average roughness 5.3 nm). SEM measurements show that in the case of samples having a high Fe content, the crystallite size varies between about 0.04 11m and 0.10 11m, while for samples with high manganese content, the crystallinity varies between 0.03 jJ,m and 0.06jLm. Manganites were analyzed using Bragg-Brentano (28) X-ray diffraction. Measurements show that manganite films cannot be grown epitaxially on Si< 100 > and MgO< 100 > single crystals due to a large lattice mismatch. In the case of SrTi03 and LaAl03 several reflections and sharp peaks from the film can be seen, indicating reasonable epitaxial growth. SEM measurements of the samples however show polycrystallinity. Complete epitaxy has thus not occurred, but many grains have an epitaxial orientation. Resistance versus temperature (the room temperature to about 100 K) in zero magnetic field was measured for a La2CaMno.06Fe2.409 thin film and maximum resistance corresponding to about 108 K was found. At higher temperatures the resistance decreases as temperature increases. The manganite thin film therefore shows semiconductor behaviour. Resistance measurements carried out at different magnetic fields (0 - 1 T) show a small positive magnetoresistance of 0.83 %. Usually the magnetoresistance phenomenon is measured at higher magnetic fields and this could be the reason for our low value as well as the fact that the iron content could be too high. / AFRIKAANSE OPSOMMING: In hierdie ondersoek is die formasie en karakterisering van dunlagie manganiete ondersoek. Om die effek van oksidasie-toestand op magnetoresistiwiteit te bepaal, is 80% van die Mn verplaas deur Fe. Pulseerde laser deposissie(3 J/cm2), is uitgevoer by 'n parsiële suurstof druk tussen 0.10 en 1.00 mbar deur gebruik te maak van La2Ca Mn2.94Feo.o609 en La2CaMno.6Fe2.409 teiken skywe. Dunlagies was gedeponeer op Si<IOO>, MgO<IOO>, SrTi03<100> en LaAl03<100> enkelkristal substrate. Die dunlagies is daarna ge-karakteriseer met behulp van Rutherford terugverstrooing (RBS), atoom krag mikroskopie(AFM), skandeer elektronmikroskopie (SEM) en xstraal diffraksie(XRD). Elektriese metings is ook uitgevoer. Een van die hoof tegnieke wat gebruik is in hierdie ondersoek is Rutherford terugverstrooing (RBS) van 2 Mev alfa-deeltjies. In hierdie navorsing is aangetoon dat RBS saam met spektra simulasie(RUMP), 'n besondere kragtige metode is om die stoichiometrie en dikte van manganiet lagies te bepaal. Die rol van die opname van suurstof in die dunlagies was ondersoek, aangesien die oksidasie toestand van manganiet lagies 'n groot effek het op hulle magnetoresistiwiteit. Die hoogte van die La sein is gebruik as 'n maatstaf van suurstof inhoud. RBS spektra van dunlagies gevorm op enkelkristal silikon substrate by verskillende parsiële suurstof drukke wys dat die passing tussen gemete en gesimuleerde spektra verbeter by hoër suurstof drukke, wat beter kwaliteit manganiet materiaal aandui. Die RBS spektra het ook aangetoon dat die stoichiometrie van die lagies uitstekend is. Atoom krag mikroskopie(AFM) is gebruik om die grofheid van die oppervlaktes van die dunlagies te bepaal. Lagies wat by 750 grade celsius uitgegloei is ( gemiddelde gladheid van 4.5 nm) was gladder as films wat nie na ablasie uitgegloei is nie (gemiddelde gladheid van 5.3 nm). SEM metings toon ook dat dunlagies met 'n hoë Fe inhoud 'n kristalliet deursnit het van 0.04 tot 0.10 mikrometer en die met 'n hoë mangaan inhoud 'n poli-kristalliniteit het van tussen 0.03 en 0.06 mikrometer het. Bragg-Brentano(twee-theta) X-straal diffraksie meting wys dat manganiet films nie epitaksieël op Si<IOO> en MgO<IOO> enkelkristal substrate gevorm kan word nie, weens 'n groot verskil in die kristal-rooster parameters. SEM metings van die monsters wys polikristalliniteit. Algehele epitaksie het dus nie plaasgevind nie, maar verskeie kristalliete het 'n epitaksiële orientasie. Weerstand metings is gemaak by temperature so laag as 100 Kelvin vir La2CaMno.o6Fe2.409dunlagies en 'n maksimum weerstand is by 108 Kelvin gevind. By hoër temperature het die weerstand afgeneem soos die temperatuur toeneem, wat halfgeleier gedrag aandui. Weerstand metings by verskillende magneetvelde (0 tot I Tesla) wys 'n klein magnetoresistiwiteits effek van 0.83%. Gewoonlik word magnetoresistiwiteit gemeet by hoë magneet velde (ongeveer 6 Tesla). Dit, sowel as die hoë Fe samestelling van die monsters kan die rede wees vir die lae magnetoresistiwiteit wat waargeneem word.

published_or_final_version / Electrical Engineering / Master / Master of Science in Engineering

Geotechnical characterization of crystalline rock is often dependent upon the influence of the rock's fracture system. To test ensemble fracture behavior in situ, a series of cross-hole and single-hole electrical conductivity measurements were made within saturated Oracle granite. The tests were conducted with a point source and a point reference electrode and employed electrode separations ranging from 8 inches to over 100 feet. A volume of rock approximately 50 x 50 x 150 feet was tested (as bounded by the vertical test borings). Analysis of the data in terms of an equivalent homogeneous material showed that the effective electrical conductivity increased with electrode separation. The cross-hole data indicate that the rock can be treated as a non-homogeneous, isotropic material. Further, the spatial variation of measured conductivities along a line can be fit to a fractal model (fractional Brownian motion), implying that the scale-dependence is a result of a fractal process supported by the fracture system. Scale-dependence exists at the upper scale limit of the measurements, hence a classical representative elemental volume was not attained. Two directions were taken to understand the scale-dependence. The rock mass is treated in terms of a disordered material, a continuum with spatially varying conductivity. First, a percolation-based model of a disordered material was examined that relates the conductivity pathways within the rock to the backbone of a critical percolation cluster. Using the field data, a fractal dimension of 2.40 was derived for the dimensionality of the subvolume within the rock that supports current flow. The second approach considers an analytic solution for a non-homogeneous, isotropic material known as the alpha center model (Stefanescu, 1950). This model, an analytic solution for a continuously varying conductivity in three dimensions, is a non-linear transform to Laplace's equation. It is employed over a regular grid of support points as an alternative to spatially discretized (piece-wise continuous) numerical methods. The model is shown to be capable of approximating the scale-dependent behavior of the field tests. Scaling arises as a natural consequence of the disordered electrical structure caused by the fracture system.

The key challenge for a nanomaterial based sensor is how to synthesize in bulk quantity and fabricate an actual device with insightful understanding of operational mechanisms during performance. I report here effective, controllable methods that exploit the concepts of the "green approach" to synthesize two different one-dimensional nanostructures, including tin oxide nanowires and carbon nanotubes. The syntheses are followed by product characterization and sensing device fabrications as well as sensor performance understanding at the molecular level. Sensor-analyte response and recovery kinetics are also presented. The first part of the thesis describes bulk-scale synthesis and characterization of tin oxide nanowires by the molten salt synthetic method and the nanowire doping with antimony (n-types) and lithium. The work builds on the success of using n-doped SnO2 nanoparticles to selectively detect chlorine gas at room temperature. Replacing n-doped nanoparticles with n-doped nanowires reduces the number of inter-particle electron hops between sensing electrodes. The nanowire based sensors show unprecedented 5 ppb detectability of corrosive Cl2 gas concentration in air. At the higher range, 10 ppm of Cl2 gas leads to a 250 fold increase in the device resistance. During sensor recovery, FT-IR studies show that dichlorine monoxide (Cl2O) and chlorine dioxide (ClO2) are the desorbing species. Long term stability of devices is affected by lattice oxygen vacancies replaced by chlorine atoms. Bulk-scale synthesis of multiwall carbon nanotube (MWCNTs) was achieved by a novel inexpensive synthetic method. The green chemistry method uses the non-toxic and easy to handle solid carbon source naphthalene. The synthesis is carried out by simply heating naphthalene and organometallic precursors as catalysts in a sealed glass tube. Synthesis at 610º C leads to MWCNTs of 50 nm diameter and lengths exceeding well over microns. MWCNT doping is attempted with nitrogen (n-type) and boron (p-type) precursors. Palladium nanoparticles decorated on as-synthesized MWCNTs are employed for specific detection of explosive hydrogen gas with concentrations far below the explosive concentration limits. During performance, the sensor exhibits abnormal response behaviors at hydrogen gas concentrations higher than 1%. A model of charge carrier inversion, brought about by reduction of MWCNT by hydrogen molecules dissociated by Pd nanoparticles is proposed.

This thesis investigation concerns the optical and nonlinear electrical properties of n-InSb. Two specific areas have been studied. First is the magneto-optical study of magneto-donors, and second is the nonlinear dynamic study of nonlinear and chaotic oscillations in InSb. The magneto-optical study of InSb provides a physical picture of the magneto-donor levels, which has an important impact on the physical model of nonlinear and chaotic oscillations. Thus, the subjects discussed in this thesis connect the discipline of semiconductor physics with the field of nonlinear dynamics.

M.Sc. / Techniques for the fabrication of polycrystalline silicon solar cells have advanced in recent years with efficiencies exceeding 17%. The major advantage of polycrystalline silicon is its low cost relative to single-crystalline silicon. The disadvantage is the significantly smaller minoritycarrier bulk diffusion length and inhomogeneous nature of the material. These two drawbacks are due to the presence of grain boundaries as well as high concentrations of dislocations and other physical and chemical defects. In this study the experimental conditions were determined to fabricate solar cells on polycrystalline silicon substrates. The controlled diffusion of phosphorous into silicon and subsequent evaluation of the doped layers (by spreading resistance profiling and chemical staining) were important aspects of this study. From these results the diffusion parameters (i.e. temperature and reaction times) could be optimized in order to improve the solar cell output parameters. Additional material improvement (increase in surface- and bulk minority carrier lifetimes) was demonstrated by the hydrogen passivation of electrically active defects in polycrystalline silicon. However. measurements on hydrogenated silicon samples also indicated that excess passivation can result in surface damage and subsequent reduction in the minority carrier lifetimes. Preliminary solar cells were fabricated on polycrystalline silicon with efficiencies ranging between 0.5 and 6% (total area = 16 cm2).

O titanato de estrôncio (SrTiO3) possui estrutura cristalina do tipo perovsquita. Materiais com este tipo de estrutura são utilizados para diversas aplicações, tais como, sensores, atuadores, em células a combustível de óxido sólido, entre outros. Devido as suas interessantes propriedades físicas, o SrTiO3 vem sendo intensamente estudado, em especial com a introdução de dopantes. Portanto, neste trabalho foi investigada a influência de diferentes teores de Ca (1; 2,5 e 5% mol) e Pr (0,025; 0,050; 0,075 e 1% mol) na microestrutura e propriedades elétricas e dielétricas do SrTiO3, assim como o material sem aditivos (puro). Os resultados mostram que após a sinterização do SrTiO3 puro, a microestrutura consiste de grãos poligonais com tamanho médio micrométrico, além de texturas lisas e rugosas. A condutividade elétrica das amostras sintetizadas sinterizadas a 1450 e 1500ºC é máxima para 2 horas de patamar. Apenas as amostras de SrTiO3 contendo 1% em mol de Ca apresentam fase única. O tamanho médio de grãos das amostras contendo 1% em mol de Ca é 10,65 &plusmn; 0,28 &micro;m e para teores acima deste valor ocorre crescimento significativo dos grãos. As medidas de condutividade elétrica mostraram que as amostras contendo a adição de 1% em mol de Ca possuem maior condutividade dos grãos em relação ao material puro. Para as amostras contendo teores de até 0,075% mol de Pr, pode-se observar alguns grãos lisos e outros rugosos e não há variação considerável do tamanho médio de grãos. As amostras contendo menor teor de Pr (0,025% mol) apresentam maior condutividade dos grãos e contornos de grãos. As amostras de SrTiO3 sintetizado sinterizadas a 1450ºC/10 h apresentaram permissividade elétrica colossal em temperatura ambiente em altas frequências. / Strontium titanate (SrTiO3) exhibits cubic perovskite type crystalline structure at room temperature. Polycrystalline ceramics with this structure are potential candidates for a number of applications including sensors, actuators and in solid oxide fuel cells. Several properties of SrTiO3 are strongly dependent on addition of both donors and acceptors additives. Then, there is a growing interest for studying its properties as a function of type and concentration of additives. In this study, the effects of Ca (1, 2.5 and 5 mol%) and Pr (0.025 to 1 mol%) additions on microstructure and electric and dielectric properties of SrTiO3 were investigated. The microstructure of pure SrTiO3 consists of polygonal grains with average grain size in the micrometer range, and the electric conductivity is maximized after sintering for 2 h at 1450 and 1500ºC. Specimens containing 1 mol% Ca are single phase and the average grain size is 10.65 &plusmn; 0.28 &micro;m, but for higher additive contents grain growth is observed. The electric conductivity of SrTiO3 with 1 mol% Ca is higher than that of the pure ceramic. Specimens containing Pr do not show significant grain growth, and the higher conductivity of grains and grain boundaries was achieved with 0.025 mol% Pr. Pure SrTiO3 sintered at 1450ºC for 10 h shows colossal dielectric permittivity (> 1.000) at room temperature, in contrast to specimens prepared with commercial powder (dielectric permittivity = 300), at high frequencies.

The research into materials for use as cathode materials for solid oxide fuel cells (SOFC) is ongoing, with many different avenues being investigated. Copper based delafossites were studied for cathode side applications in SOFCs, as a novel and comparatively cheap material. The aim was to identify suitable materials with appropriate electrical conductivity, thermal, chemical and mechanical stability in air. Furthermore, understanding the behaviour of the delafossites during the thermal oxidation to spinel and copper oxide would be beneficial to further development of the materials. The structure and properties of the copper based delafossites CuFeO₂, CuAlO₂ and CuCrO₂ were studied, alongside several doped compositions for each parent composition. The electronic conductivity of the CuFeO₂ family was improved by doping fluorine into the structure, with 1 atomic % doping producing ~3.8 S cm⁻¹ at 800 °C. However, as reported in literature the structure is vulnerable to oxidation at higher temperatures. In contrast, CuAlO₂ was stable over the SOFC temperature range, and therefore had appropriate thermal expansion coefficients (TEC) of ~11 x 10⁻⁶ K⁻¹, but relatively low electronic conductivity. CuCrO₂ compositions had good overall TECs, but aliovalent doping of Mg²⁺ improved the conductivity to ~17.1 S cm⁻¹ at 800°C for 2.5 atomic % doped CuCrO₂. Neutron diffraction was utilised to study members of the solid solution CuFe₁₋ₓCrₓO₂ (x = 0, 0.25 and 0.5) during in-situ oxidation at high temperature. Points of positive scattering density were identified within the CuFeO₂ structure, which were attributed to the location of the intercalated oxygen ions before the transformation proceeded. Additionally, the cation distribution between the tetrahedral and octahedral sites within the developing spinel were characterised for x = 0, and partially for the x = 0.25 and 0.5 compositions using complimentary XRD patterns. Finally, magnesium doped CuCrO₂ delafossites were used in several different preliminary symmetrical cells for study using electrochemical impedance spectroscopy (EIS). Pure delafossite inks gave relatively large area specific resistance (ASR) values, 1.29 - 2.69 Ω cm² at 800 °C. It was attempted to improve upon these values through infiltration of CeO₂ and through change in microstructure using composite type inks, without much success. Inks using CuCr₀.₈Fe₀.₂O₂ were also tested as both a single phase electrode and as a composite type electrode. The pure delafossite electrode still had a large ASR value, (~33.4 Ω cm² at 800 °C) while composite electrodes obtained much more respectable ASR values ~0.75 Ω cm² at 800 °C.

Finally, we also propose and demonstrate an integrated Mach-Zehnder optical diplexer (IMZOD) for possible use in an integrated silicon optical amplifier. The diplexer is based on two rnultimode interferometers (MMIs) and a Mach-Zehnder interferometer (MZI), and has potential use in an integrated silicon waveguide optical amplifier, to combine or separate the pump signal (1440nm) and probe signal (1556nm) for monolithic implementation of a silicon Raman amplifier. / Helium ion implantation can not only reduce the free-carrier loss, but can also enhance the detection responsivity of below-bandgap wavelengths (1440 1590 nm). We propose and demonstrate an in-line channel power monitor (ICPM) based on helium ion implanted silicon waveguides. The implanted waveguide can detect light at 1440 1590 nm which are normally not detectable by silicon. We study the enhanced photoresponse of helium ion implanted waveguide samples which were annealed at different temperatures and for different durations. / Recently there has been much interest in silicon optical amplifiers and lasers relying on stimulated Raman scattering (SRS), which, despite the much shorter waveguide lengths possible in silicon compared with silica optical fiber, can still provide large optical gain because of the large Raman coefficient of silicon and small mode field areas. However, two-photon absorption (TPA) generated free-carrier absorption (FCA) loss can exceed the Raman gain. In this thesis, experiments and theoretical model will he discussed and analyzed, showing that helium ion implantation can successfully reduce the optical losses due to free-carriers and allow net gain to be attained by continuous-wave (CW)-pumped SRS without requiring external bias to remove the photo-generated free carriers. The theoretical study of dynamics of free carrier lifetime of the silicon waveguides will be described. The effective nonlinear length of the silicon waveguides is defined and studied. The theoretical and experimental studies of the enhanced spectral broaden induced by self-phase-modulation (SPM) are carried out in helium on implanted silicon waveguides. / Silicon-on-insulator (SOI) wafers are an attractive platform for the fabrication of planar lightwave circuits (PLCs) because they offer the potential for low-cost fabrication using mature complementary metal--organic--semiconductor (CMOS) compatible processes developed in the microelectronics industry. At the wavelengths of interest for telecommunications, SOI waveguides can have low optical losses (0.1dB/cm). Besides, the strong optical confinement offered by the high index contrast between silicon (Si) (n=3.45) and silicon dioxide (SiO2) (n=1.45) makes it possible to scale photonic devices to sub-micron level. In addition, the high optical intensity arising from the strong optical confinement inside the waveguide makes it possible to observe nonlinear optical effects, such as Raman and Kerr effects, in chip-scale devices. / We then make use of the ICPM to perform a system application, called optical-burst-and-transient-equalizer (OBTE). The OBTE may provide a compact and low-cost solution to compensate gain-transient, gain-spectrum-tilt and to equalize the upstream packet amplitude in erbium doped fiber amplifier (EDFA) amplified hybrid dense-wavelength-division-multiplexed (DWDM) and time-division-multiplexed (TDM) passive-optical-networks (PONs). The OBTE may be monolithically integrated on SOI platform and is potentially low cost and compact. The OBTE can compensate complicated gain slope shape, which may be generated in cascaded EDFAs or deliberate channel add/drop, based on individual channel equalization. 15-dB receiver sensitivity improvement at 10 Gbit/s bit-error-rate (BER) measurements of 10-9 was achieved by the compensation. / Liu, Yang. / "August 2007." / Adviser: Hon Ki Tsang. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1212. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract in English and Chinese. / School code: 1307.

Cobalt doped anatase TiO2 films show room temperature ferromagnetism. Doping was provided by implantation using a MEVVA ion source. The enhancement of ferromagnetic properties was obtained by post-implantation annealing. The microstructure, magnetic properties and the dependence on the annealing conditions have being studied using various characterization techniques. Interestingly, the output referring to the saturation magnetization per Co atom with a value as high as 3.16 muB/Co atom, exceeds considerably that of the bulk cobalt which suggests that contribution to the overall magnetic behavior is not only a function of the concentration of inherently magnetic elements, but there must exist also sources of magnetisms. One of these sources are oxygen vacancies as discussed within this work. It is also interesting that instead of the more commonly observed hcp structure, the Co nanoclusters are found in fcc structure probably being stabilized by the TiO2 matrix. / In this work, we study the properties of cobalt-implanted titanium dioxide, a room temperature dilute ferromagnetic semiconductor discovered in 2001. The ferromagnetic interaction mechanism is however controversial. By using metal vapor vacuum arc (MEVVA) ion source, different doses of cobalt ions were implanted into anatase structures of titanium dioxide (TiO2) thin films. The TiO2 films which were sputtered on SiO2 (100nm)/Si (110) substrates and rutile structure of TiO2. The cobalt implanted TiO2 thin films were prepared with different atomic fraction and then thermally treated at different temperature after ion implantation. The structural properties of the anatase titanium dioxide were also studied as a comparison to rutile titanium dioxide. / Rutherford backscattering spectrometry (RBS) was performed to determine the composition of cobalt. The crystal structure of the thin films and rutile single crystal was mainly anatase as detected in XRD spectra. X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) were also used in sample analysis. Vibrating sample magnetometer (VSM) was employed to study the magnetic properties of the cobalt implanted films. Ferromagnetic behaviors of these films were observed at room temperature. / Semiconductor spintronics is a promising new field of study in the ongoing quest to make electronic devices faster, cheaper, and more efficient. While current spintronics utilize the spin property of electrons to achieve greater functionally, the integration of spintronics into conventional semiconductor electronics will lead to advances optoelectronics, quantum computing, and other emerging fields of technology. This integration relies on effective generation; injection, transport, and detection of spin polarized electron current. To these end, mastering synthesis of room temperature ferromagnetic semiconductors is inevitable. / Luk, Wing Yan. / Adviser: H. P. Ho. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3730. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.