The Hall coefficient and conductivity of silicon films vacuum deposited on 0° and 60° sapphire at 850°C to 1050°C were measured from 100°K to 550°K. Films made from 0.094 Ω-cm p type and 1 Ω-cm n type silicon sources were prepared by electron bombardment heating of the source in a vacuum of 5 x 10⁻⁷ to 10⁻⁶ torr. The orientation and crystallinity of the films were investigated using electron diffraction. It was found that defects in the films introduced both donor and acceptor levels. The heavy compensation thus produced in films deposited at lower temperatures lead to a very low hole concentration. All films were p type at room temperature showing that the acceptor levels slightly dominated the donor levels. The films deposited on 0° sapphire indicated fewer defects than those deposited on 60° sapphire. At high temperature (> 950°C) doping of the silicon by aluminum atoms from the substrate was appreciable. The Hall mobility of the films made from the p type source material decreased with increasing temperature of deposition. This apparent anomaly is explained by the use of the polycrystalline film model suggested by Volger (1950). / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The etching of silicon by atomic and molecular chlorine and bromine was studied as a function of etchant pressure and reaction temperature. Various types of silicon were employed in the etching experiments including intrinsic and n-type polycrystalline silicon as well as the (100) face of intrinsic single crystal silicon. The pressures of Cl₂ and Br₂ varied from 0.1 to 30 Torr and the partial pressure of Cl and Br atoms was between 0.08 and 0.2 Torr. Temperatures of between 365 and 600°C were required for CI₂ and Br₂ etching, while lower temperatures of 25 to 470°C were sufficient for the more reactive Cl and Br atoms. The reaction between silicon and Br atoms was shown to be first order with respect to the partial pressure of atoms and a first order dependence was assumed for Cl atom etching. The rate constants were determined for the Cl and Br atom etching of intrinsic and n-type polycrystalline silicon, with a dopant concentration of 5x10¹⁸ atoms cm⁻³. The reactivity of Cl atoms with n-type silicon was approximately 90 times greater than with intrinsic silicon. This enhancement in reaction rate is primarily due to an increase in the preexponential factor in k₁, with the activation enthalpy for the process remaining unchanged at approximately 28 kJ mol⁻¹. For Br atom etching, the reaction rate for the n-type silicon was over 300 times greater than for intrinsic silicon and was characterized by activation enthalpies of 55 and 63 kJ mol⁻¹ respectively. The enhancement in reactivity can also be attributed principally to an increase in the preexponential factor. The preexponential factors for the rate constants are larger than those expected, based on the collision frequencies of Cl and Br atoms. This is interpreted as evidence for a preadsorption step in these reactions. The reactions of silicon with CI₂ and Br₂ were found to display a complex pressure dependence. The etch rates varied linearly with (etchant pressure)¹′² and the intercepts from a linear regression of the data were slightly negative. To account for the half order pressure dependencies observed in these etching reactions, a reversible dissociative adsorption mechanism is proposed whereby Br₂ (or CI₂) is dissociatively adsorbed, in a reversible reaction, onto the silicon surface yielding two atoms bound to the surface. This step is then followed by a first order reaction leading to the formation of a species which is either gaseous product or some precursor which forms that product in a subsequent non rate-determining step. From the slopes of etch rate versus (pressure)¹′² plots, composite half order rate constants were calculated and from the intercepts it was possible to evaluate the rate constant for dissociative adsorption of the halogen molecules. At high etchant pressures, where the reaction was half order with respect to Br₂ (or CI₂), a half order "composite" rate constant characterized the etching reaction. Values for the half order rate constant were determined for a number of wafers at various temperatures. From the temperature dependencies of these rate constants, activation enthalpies of 131±8 and 116±7 kJ mol⁻¹ were calculated for Br₂ and CI₂ etching of intrinsic polycrystalline silicon respectively. A value of 121±7 kJ mol⁻¹ was deterrnined for the Br₂ etching of silicon (100). Higher reaction rates were observed for the etching of n-type polycrystalline silicon, with greater enhancements observed for Br₂ relative to Cl₂ etching. The enhancements in etch rates were found to be principally due to a lower activation enthalpy for the half order rate constant. An activation enthalpy for the composite rate constant of 82±3 kJ mol⁻¹ was determined for Cl₂ etching of n-type silicon with a dopant atom concentration of 5x10¹⁸ atoms cm⁻³. Br₂ etching of the same wafer yielded an activation enthalpy of 86±3 kJ mol⁻¹. At low pressures, the reaction becomes first order and the temperature dependence of the corresponding first order rate constant yielded activation enthalpies of 109 and 83 kJ mol⁻¹ for intrinsic and n-type polycrystalline silicon. / Science, Faculty of / Chemistry, Department of / Graduate

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

The synthesis of 2:3-benzo-1,1-dimethyl-1-silacyclo-hex-2-ene has been reported. This compound was prepared by the simultaneous addition of 3-(o-bromophenyl)-propyl bromide and dichlorodimethylsilane to excess magnesium in tetra-hydrofuran. The six-membered ring organosilicon compound was readily brominated by N-bromosuccinimide to give the 4-derivative. Attempts were then made to synthesize the 4-cyano and the 4-carboxylate derivatives from this bromination product. Experimental data are also given for an attempted preparation of 2:3-benzo-4-(2-dimethylaminoethoxy)-1,1-dimethyl-1-silacyclohex-2-ene. / Pharmaceutical Sciences, Faculty of / Graduate

The introduction of boron into a silicon crystal lattice produces two sets of acceptor impurity states: (i) the "external" states (which include the acceptor ground state) which are associated with the P₃/₂ valence band maxima and are the usual impurity states lying in the forbidden gap, (ii) the "internal" states which are associated with the P₁/₂ valence band maximum and are degenerate with P₃/₂ continuum states. The energies of the transitions from the ground state to the so-called 2p', 3p' and 4p' internal acceptor states are such that the 2p', 3p' and 4p' internal absorption peaks are seen in the infrared. Three different experiments are performed to study the "internal" spectrum of boron-doped silicon. (i) The stress-perturbed behavior of the 2p' and 3p' absorption peaks is studied with the use of calibrated uniaxial stress (≤ 10⁹ dynes/cm² ) and polarized light. From these data it is deduced that the ground state has a ⌈8 symmetry while the 2p' and 3p' internal states each have a ⌈6 symmetry. In addition the deformation potential parameters of the ground state are calculated to be: b' = -0.66 ± 0.04 eV. and d' = -2.1 ± 0.2 eV. These calculated parameters for the ground state are used to obtain indirectly some information about the deformation potential parameters of the P₃/₂ valence band edge. (ii) For the temperature range 5°K ≤ T ≤ 60°K the temperature dependences of the breadths of the 2p' and 3p' peaks are measured. These temperature dependences are attributed to phonon broadening. The phonon broadening mechanism for the 2p' and 3p' peaks is shown to be primarily due to a lifetime effect caused by electron-phonon coupling of the 2p' and 3p' states to the P₃/₂ valence band states. (iii) For the impurity concentration range 2.6 x 10¹⁵ boron/cm³ ≤ N ≤ 4.5 x 10¹⁷ boron/cm³ the 2p' absorption peak is measured. With increasing concentration this peak is observed to broaden and become very asymmetrical. Explanations to the above data are presented. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The rapid growth in data communication technologies is at the heart of enriching the digital experiences for people around the world. Encoding high bandwidth data to the optical domain has drastically changed the bandwidth-distance trade-off imposed by electrical media. Silicon photonics, sharing the technological maturity of the semiconductor industry, is a platform poised to make optical interconnect components more robust, manufacturable, and ubiquitous. One of the most prominent device classes enabled by the silicon photonics platform is photonic switching, which describes the direct routing of optical signal carriers without the optical-electrical-optical conversions. While theoretical designs and prototypes of monolithic silicon photonic switch devices have been studied, realizing high-performance and feasible switch systems requires explorations of all design aspects from basic building blocks to control systems. This thesis provides a holistic collection of studies on silicon photonic switching in topics of novel switching element designs, multi-stage switch architectures, device calibration, topology scalability, smart routing strategies, and performance-aware control plane. First, component designs for assembling a silicon photonic switch device are presented. Structures that perform 2×2 optical switching functions are introduced. To realize switching granularities in both spatial and spectral domains, a resonator-assisted Mach-Zehnder interferometer design is demonstrated with high performance and design robustness. Next, multi-stage monolithic switching devices with microring resonator-based switching elements are investigated. An 8×8 switch device with dual-microring switching elements is presented with a well-balanced set of performance metrics in extinction ratio, crosstalk suppression, and optical bandwidth. Continued scaling in the switch port count requires both an economic increase in the number of switching elements integrated in a device and the preservation of signal quality through the switch fabric. A highly scalable switch architecture based on Clos network with microring switch-and-select sub-switches is presented as a solution to reach high switch radices while addressing key factors of insertion loss, crosstalk, and optical passband to ensure end-to-end switching performance. The thesis then explores calibration techniques to acquire and optimize system-wide control points for integrated silicon switch devices. Applicable to common rearrangeably non-blocking switch topologies, automated procedures are developed to calibrate entire switch devices without the need for built-in power monitors. Using Mach-Zehnder interferometer-based switching elements as a demonstration, calibration techniques for optimal control points are introduced to achieve balanced push-pull drive scheme and reduced crosstalk in switching operations. Furthermore, smart routing strategies are developed based on optical penalty estimations enabled by expedited lightpath characterization procedures. Leveraging configuration redundancies in the switch fabric, the routing strategies are capable of avoiding the worst penalty optical paths and effectively elevate the bottom-line performance of the switch device. Additional works are also presented on enhancing optical system control planes with machine learning techniques to accurately characterize complex systems and identify critical control parameters. Using flexgrid networks as a case study, light-weight machine learning workflows are tailored to devise control strategies for improving spectral power stability during wavelength assignment and defragmentation. This work affirms the efficacy of intelligent control planes to predict system dynamics and drive performance optimizations for optical interconnect systems.

The structural and electronic properties of graphene and graphene-related materials have been intensively investigated using the plane wave based periodic density func- tional theory (DFT). The Vienna ab initio simulation package (VASP) code employing the generalized gradient approximation (GGA) for the exchange correlation potential was used. In all calculations, the geometry optimization option was employed in allow- ing the structure to fully relax. Hydrogen adatoms were adsorbed on C, Si and SiC in the graphene structure in-volving (1x1),(2x2),(3x3) and (4x4) two dimensional unit cells. The density of states reveals that the adsorption of 50% hydrogen makes the system metallic but 100% coverage at the on top sites generates a band gap. Our results show that SiC in the graphene structure is a plausible structure with a wide band gap. For adsoption of lithium adatoms, we considered various configurations involving the (1x1), (2x1) and (2x2) two-dimensional unit cells, and we consider the isolated Li dimer on graphene. We consider more detailed configurations than have been studied before, and our results compare favourably with previously calculated results where such results exist. For 100% coverage, we have new results for Li on the on-top site, which suggests a staggered configuration for the lowest energy structure for which the Li adatoms are alternately pushed into and pulled out of the graphene layer. For 50% coverage, Li favours the hollow site. We discovered that a careful relaxation of the system also shows a staggered configuration, a result that has not been investigated before. / Dissertation (MSc)--University of Pretoria, 2011. / Physics / unrestricted

Silicon photonics accelerated the advent of complex integrated photonic systems where multiple devices and elements of the circuits synchronize to perform advanced functions such as beam formation for range detection, quantum computation, spectroscopy, and high-speed communication links. The key ingredient for silicon's growing dominance in integrated photonics is scalability: the ability to monolithically integrate large number of devices. There are emerging device designs and material platforms compatible with silicon photonics that offer performances superior to silicon alone, yet their lack of scalability often limits the demonstrations to device-level. Here we discuss two of such platforms, suspended air-cladded microresonators and graphene modulators. In this thesis, we demonstrate methods to scale these devices and enable more complex applications and higher performance than a single device can ever acheive. We present an effective method to thermally tune optical properties of suspended and air-cladded devices. We utilize released MEMs-like wire structures and integrated heaters and demonstrate efficient thermo-optic tuning of suspended microdisk resonators without affecting optical performance of the device. We further scale this method to a system of two evanescently coupled resonators and demonstrate on-demand control of their coupling dynamics. We present an approach to achieve large yield of high bandwidth graphene modulators to enable Tbits/s data transmission. Despite their high performance, graphene modulators have been demonstrated at single device-level primarily due to low yield, ultimately limiting their total data transmission capacity. We achieve large yield by minimizing performance variation of graphene modulators due to random inhomogeneous doping in graphene by optimizing device design and leveraging state-of-the-art electrochemical delamination graphene transfer. We present for the first time, to the best of our knowledge, a statistical analysis of graphene photonic devices. Finally, we present a graphene modulator that is versatile for photonic links at cryogenic temperature. We demonstrate the operation of high bandwidth graphene modulator at 4.9 K, a feat that is fundamentally challenging other electro-optic materials. We describe its performance enhancement at cryogenic temperature compared to ambient environment unlike modulators based on other electro-optic materials whose performance degrades at cryogenic temperature.

In this work, the development of a Thermoelectric Generator on Flexible Silicon Fabric is explored to extend silicon electronics for flexible platforms. Low cost, easily deployable plastic based flexible electronics are of great interest for smart textile, wearable electronics and many other exciting applications. However, low thermal budget processing and fundamentally limited electron mobility hinders its potential to be competitive with well established and highly developed silicon technology. The use of silicon in flexible electronics involve expensive and abrasive materials and processes. In this work, high performance flexible thermoelectric energy harvesters are demonstrated from low cost bulk silicon (100) wafers. The fabrication of the micro- harvesters was done using existing silicon processes on silicon (100) and then peeled them off from the original substrate leaving it for reuse. Peeled off silicon has 3.6% thickness of bulk silicon reducing the thermal loss significantly and generating nearly 30% more output power than unpeeled harvesters. The demonstrated generic batch processing shows a pragmatic way of peeling off a whole silicon circuitry after conventional fabrication on bulk silicon wafers for extremely deformable high performance integrated electronics. In summary, by using a novel, low cost process, this work has successfully integrated existing and highly developed fabrication techniques to introduce a flexible energy harvester for sustainable applications.

PhD - Science / The relationships between densification and microstructure, and between microstructure and mechanical and electrical properties of liquid phase sintered silicon carbide were studied in detail using hot pressing, gas pressure sintering and ultra–high pressure sintering techniques. Silicon carbide was sintered with 10 mass-% addition of the Y2O3-Al2O3 system, with various molar ratios. Hot pressing was carried out at 1925oC under 30 MPa, in argon, for half an hour. Materials were gas pressure sintered at 1925oC, under a final gas pressure of 80 bars (8MPa), in argon, for an hour. Ultra-high pressure sintering was done at ca. 1550oC, under 5.5 GPa pressure, for 15 minutes. The hot pressed and gas pressure sintered materials were subsequently heat treated at 1925oC and 1975oC. Most of the silicon carbide materials were sintered to a density around 99% of theoretical density. The heat treatment of the hot pressed materials resulted in an increase in density not changing the porosity. The densities of the heat treated hot pressed materials corresponded to the density of the gas pressure sintered materials. This resulted from the difference in composition of grain boundary phases – yttrium silicates in the hot pressed materials and yttrium aluminates in the gas pressure sintered and heat treated materials. The average silicon carbide grain size in the materials strongly depended on the densification method. In gas pressure sintered and heat treated materials the mean grain size was up to three times higher than that in the hot pressed materials. Grain growth appeared to be higher in the highest alumina-content materials. The heat treatment at 1975 °C resulted in more pronounced anisotropic grain growth. The ratio of the silicon carbide polytypes of sintered materials and materials heat treated materials at 1925oC, did not change significantly. In the materials heat treated at 1975oC Rietveld analysis revealed a decrease in SiC-6H polytype and an increase in amount of 4H and 15R polytypes, compared to the materials heat treated at 1925oC. This can be attributed to the increase in diffusion rates of aluminium into the SiC lattice at 1975oC. Segregation patterns were observed in the high yttria content materials, with Y2O3:Al2O3 molar ratios greater than or equal to two, after gas pressure sintering and heat treatments. This was suggested to be due to he poor wetting of the silicon carbide grains by the yttria-rich grain boundary phase. On heat treatment, the Vickers hardness of hot pressed materials was found to be increased from 20 to 26 GPa and elastic modulus from 318 to 338 GPa. In addition, the log of the electrical conductivity of liquid phase sintered silicon carbide (measured at 330oC) ranged from 10-8 to 10-3 with the changes in grain boundary phases observed after the heat treatments. The grain boundary phase composition also influenced the strength of the materials, The highest strength, 657 + 50 MPa, was measured for the hot pressed material containing the YAG phase. Indentation fracture toughness was mostly influenced by the SiC grain growth during heat treatments. The most significant increase in fracture toughness, the largest being from 3.7 MPa.m1/2 up to 5.6 MPa.m1/2, was observed in the higher alumina content materials after heat treatment at 1975oC. The increase in fracture toughness was attributed to the presence of a higher amount of platelet-like SiC grains within a broader grain size distribution. These elongated grains increased fracture toughness by increasing crack path deflection and crack bridging. The electrical properties were evaluated by Impedance Spectroscopy measurements between room temperature and 330oC. The LPS SiC materials can be classified into three groups with different electrical properties. This classification could be related to the grain boundary phases present in the materials. The materials with the lowest conductivity were all hot pressed materials, containing crystalline silicates and amorphous grain boundary phases. The materials with intermediate conductivity include gas pressure sintered materials and a hot pressed material, which contained crystalline aluminates (Y3Al5O12, YAlO3 and Y4Al2O9) in their grain boundaries. The materials with the highest conductivity only contained the aluminates, YAlO3 and Y4Al2O9. A pseudopercolation model of conduction was proposed, in which electrons move along a path which goes through the thinner intergranular layers, with largest nearest neighbour contact. The temperature dependence of the log of the conductivity of hot pressed and gas pressure sintered materials showed that the conduction mechanism in these liquid-phase sintered silicon carbide materials was variable range hopping conduction of electrons between defect sites. The non-Arrhenius behaviour, together with the observed wide range of peak frequencies, led to the conclusion that the effect of silicon carbide itself was not observed in the impedance spectra. The 1/T0.25 log conductivity dependence showed that the Cole-Cole arcs are due to insulating grain boundary phases rather than semiconducting SiC. In the Cole-Cole plots of the hot pressed and heat treated hot pressed materials only the effect of one phase could be observed. In the gas pressure sintered materials and the hot pressed material containing mainly YAG phase, the effects of two phases were seen in the frequency range measured. Ultra-high pressure liquid-phase sintered silicon carbide materials showed ultra-fine SiC grains, which were highly inter-grown. Segregated grain boundary “core-rim” structures, consisting of an inner core of nonequilibrium yttria and outer rim of equilibrium yttrium silicate were observed in materials containing 4 mass-% to 15 mass-% sintering additives. The hardness of ultra-high pressure sintered 10 mass-% materials increased with alumina-content, from 20 GPa – 22 GPa, and increased with decrease in sintering additive, up to 23 GPa (for the 4 mass-% material).