Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 232-243).

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 206-210). Available also in a digital version from Dissertation Abstracts.

Inside your microelectronic devices there are up to a billion transistors working in flawless operation. Silicon has been the workhorse semiconductor used for the transistor; however, there must be a transition to materials other than silicon, such as germanium, with future device sizes. In addition, new dielectric oxide materials are needed. My research has examined a type of crystalline oxide, known as a perovskite, which is selected for its ability to bond chemically to Si and Ge, and eliminate the electrical defects that affect performance. Many perovskite oxides are lattice-matched to the Si (001) and Ge (001) surface spacing, enabling heteroepitaxy. To date, the majority of research on crystalline oxides integrated with semiconductors has been based on strontium titanate, SrTiO3, epitaxially grown on Si (001) by molecular beam epitaxy. Alternative low-temperature growth methods, such as atomic layer deposition (ALD), offer both practical and economic benefits for the integration of crystalline oxides on semiconductors. My initial research informed the broader community that four unit cells (~1.5 nm) of SrTiO3 are required to enable heteroepitaxy on Si. The research has also shown that heteroepitaxial layers can be monolithically integrated with Si (001) without the formation of a SiOx interlayer between the Si (001) surface and the SrTiO3 layer because ALD is performed at lower temperatures than are typical for MBE. Thus, a combined MBE-ALD growth technique creates possible advantages in device designs that require the crystalline oxide to be in contact with the Si (001) surface. In recent work, I have demonstrated a method for integrating crystalline oxides directly on Ge by ALD. Germanium is being explored as an alternative channel material due to its higher hole and electron mobilities than Si, potentially enabling device operation at higher speed. This all-chemical growth process is expected to be scalable, is inherently less costly from a manufacturing cost of ownership, and is based on current manufacturing tool infrastructure. The impact of my research will be in continued scaling of device dimensions with novel materials that will provide faster speed and lower power consumption for microelectronic devices. / text

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This dissertation is divided into two parts. In the first, the general paradigm of scanned probe microscopy is outlined with a focus on atomic force microscopy and a few of its variations. Magnetic force microscopy is covered in detail as it forms the basis of the second part of this dissertation. The core elements and extra features of the instrument are described with attention paid to the upgrades made by the author. In the second part of this dissertation, background information on perovskite oxides and the inverse spinel system, magnetite, is given. Magnetic force microscopy studies were done on three thin film systems and are detailed. In the first study, ferromagnetic manganite films were subjected to discontinuous changes in strain due to structural transitions in their barium titanate substrates. The resulting effect on the magnetic domains was observed. In the second study, the ferromagnetism of a tensile-strained LaCoO₃ film was studied across temperatures from 4.3 K to 90 K and applied fields up to [mu]₀H=1.1 T. Finally, the properties of antiphase domains in magnetite films of varying film strain due to transition metal buffer layers was probed by imaging the magnetic domains which are pinned to the antiphase boundaries. / text

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Our understanding of the nature of Earth’s D” region was changed significantly by a recent finding by Murakami et al. (2004), who revealed a phase transition from perovskite to post-perovskite structure in MgSiO3 at about 125 GPa and 2500 K, corresponding to conditions of the lowermost mantle. A perovskite to post-perovskite phase transition accounts for many unusual features of the D” region, including its notable seismic anisotropy, and also accounts for the unusual topology of the D” discontinuity. However, the experimentally synthesised post-perovskite phase of MgSiO3 is not quenchable to ambient conditions, which means that many of its physical properties remain difficult to determine. On the other hand, there are several post-perovskite oxides, CaIrO3, CaPtO3, CaRhO3 and CaRuO3, which can be quenched to ambient conditions, maintaining their structure. High pressure synthesis of CaIr1-xPtxO3 solid solutions (x = 0, 0.3, 0.5, 0.7) and CaIr0.5Rh0.5O3 was conducted at the University of Edinburgh and Geodynamics Research Center, Ehime University, and structures and physical properties of these novel post-perovskite materials determined. Substantial [100] grain growth was observed in all solid solutions leading to pronounced texture even in powdered materials. Temperature-independent paramagnetism above 150 K and small magnetic entropy observed in heat capacity measurements suggest that CaIrO3 is an intrinsically weak itinerant ferromagnetic metal, while electrical resistivity measurements show that it is a narrow bandgap semiconductor, possibly due to grain boundary effects. CaIrO3 undergoes a magnetic transition at 108K and possesses a saturated magnetic moment of 0.04 μB. Doping with Pt or Rh induces Curie-Weiss paramagnetism and suppresses the magnetic transition. The anisotropic structure and morphology of CaIrO3 combined with the Ir4+ spin-orbit coupling results in a large magnetic anisotropy constant of 1.77 x 106 Jm-3, comparable to values for permanent magnet materials. A new high-pressure phase of CaIr0.5Pt0.5O3 was synthesised at 60GPa, 1900K using a laser-heated DAC (diamond anvil cell) at GRC, Ehime University. Its Raman spectra resemble those of perovskite phases of CaIrO3 and CaMnO3, implying that CaIr0.5Pt0.5O3 undergoes a post-perovskite to perovskite phase transition with increasing pressure. I estimate an increase in thermodynamic Grüneisen parameter γth across the post-perovskite to perovskite transition of 34 %, with similar magnitude to (Mg,Fe)SiO3 and MgGeO3, suggesting that CaIr0.5Pt0.5O3 is a promising analogue for experimentally simulating the competitive stability between perovskite and post-perovskite phase of magnesium silicates in Earth’s lowermost mantle. Such estimation is reliable since the estimated and directly calculated thermodynamic Grüneisen parameter γth from heat capacity show consistent values. The marked effect that Pt has on stabilising the post-perovskite structure in CaIr1-xPtxO3 solid solutions explains why the post-perovskite to perovskite phase transition has not been observed for CaPtO3 in contrast to other quenchable post-perovskite oxides: CaIrO3, CaRhO3 and CaRuO3.Work presented here demonstrates that CaIrO3 solid solutions can be used to provide new insight into factors stabilising post-perovskite structures in Earth’s lowermost mantle.

This work focused on the study of three independent projects involving perovskite oxide materials and their applications as solid oxide fuel cell (SOFC) anodes and electrochemical oxygen sensors. The underlying theme is the versatility and tune-ability of the perovskite structure. Reactivity and conductivity (ionic as well as electronic) are modified to optimize performance in a specific application. The effect of Ce doping on the structure and the conductivity of BaFeO3 perovskite materials is investigated and the resulting materials are applied as oxygen sensors. The new perovskite family, Ba1-xCexFeO3-δ (x=0, 0.01, 0.03, and 0.05), was prepared via a sol-gel method. Powder XRD indicates a hexagonal structure for BaFeO3 with a change to a cubic perovskite upon Cerium doping at the A site. The solubility limit of Ce at the A site was experimentally determined to be between 5-7 mol %. Bulk, electronic and ionic conductivities of BaFeO3-δ and Ba0.95Ce0.05FeO3-δ were measured in air at temperatures up to 1000˚C. Cerium doping increases the conductivity throughout the entire temperature range with a more pronounced effect at higher temperatures. At 800˚C the conductivity of Ba0.95Ce.05FeO3-δ reaches 3.3 S/cm. Pellets of Ba0.95Ce.05FeO3-δ were tested as gas sensors at 500 and 700˚C and show a linear, reproducible response to O2. Promising perovskite anodes have been tested in high sulfur fuel feeds. A series of perovskite solid oxide fuel cell (SOFC) anode materials: Sm0.95Ce0.05FeO3-δ, Sm0.95Ce0.05Fe0.97Ni0.03O3-δ and Sm0.95Ce0.05Fe0.97Co0.03O3-δ have been tested for sulfur tolerance at 500°C. The introduction of the extreme 5% H2S enhances the performance of these anodes, verified by EIS and CA experiments. Post mortem analyses indicate that the performance XII enhancement arises from the partial sulfidation of the anode, leading to the formation of FeS2, Sm3S4 and S on the perovskite surface. Testing in lower concentrations of sulfur, more common in sour fuels, 0.5% H2S, also enhances the performance of these materials. The SCF-Co anode shows promising stability and an increase in exchange current density, io, from 13.72 to 127.02 mA/cm2 when switching from H2 to 0.5% H2S/99.5% H2 fuel composition. Recovery tests performed on the SCF-Co anode conclude that the open cell voltage (OCV) and power density of these cells recover within 4 hours of H2S removal. We conclude that the formation of metal sulfide species is only partially reversible, yielding an anode material with an overall lower Rct upon switching back to pure H2. Combining their performance in sulfur containing fuels with their previously reported coke tolerance makes these perovskites especially attractive as low temperature SOFC anodes in sour fuels. A new perovskite family Ba1-xYxMoO3 (x=0-0.05) has been investigated in regards to electrical conductivity and performance as IT-SOFC anode materials for the oxidation of H2. Refinement of p-XRD spectra as well as SEM imaging conclude that the solubility limit of Y doping at the A site is 5 mol%, beyond which Y2O3 segregation occurs. The undoped BaMoO3 sample has a colossal room temperature conductivity of 2500 S/cm in dry H2. All materials maintain metallic conductivity in the temperature range of 25-1000°C with resistance increasing with Y doping. The Ba1-xYxMoO3 (x=0, 0.05) materials exhibit good performance as SOFC anode materials between 500-800°C, with Rct values at 500°C in dry H2 of 3.15 and 6.33 ohm*cm2 respectively. The catalytic performance of these perovskite anodes is directly related to electronic conductivity, as concluded from composite anode performance.

An oxygen barometer to estimate fO2 during the crystallization of kimberlites is developed using the Fe content of perovskite (Pv), a common groundmass phase in these rocks. With increasing fO2, more Fe exists in the kimberlitic liquid as Fe3+, and thus partitions into Pv, which accepts only Fe3+ into its crystal structure. Experiments to study the partitioning of Fe3+ between Pv and kimberlite liquid were conducted on simple and complex anhydrous kimberlite bulk compositions at 100 kPa over a range temperatures (1130 - 1300°C) and of fO2's from NN0+4 to NNO-5 (NNO, nickel-nickel oxide buffer) and at Nb and REE levels of 0 to 1.5 wt% and 1500 ppm respectively. For Nb-free experiments, the Fe2O3 content of Pv increases with fO2 according to the relation (at 2σ): Fe2O3 Pv (wt%) = 0.25 (0.04) ?NNO + 1.83(0.06) For experiments doped with Nb, two universal equations for the relationship between the Fe and Nb in Pv, and fO2 were defined. Based on a slope intercept fitting method (SIM) we obtain: Log Fe (0.04)=0.058(0.004)* ?NNO + 0.26(0.02)*Log Nb - 0.91(0.03) Based on a multiple linear regression method (MLR): Fe (0.031)= 0.404(0)* NNO + 0.50(0.021)*Nb + 0.030(0.001) with uncertainties at 2o, and Nb and Fe as cation units per 3 oxygens in both equations. Over the range of conditions of our experiments, these relationships show no temperature (T) dependence and are not affected by the bulk Fe content of the kimberlite starting material. The Fe2O3 content of Pv from natural kimberlites compiled from the literature corresponds to fO2 conditions of NNO-5 to NNO+1. Data on zoned Pv from a single kimberlite, the Phoenix pipe, suggests that cores record lower fO2 than rims, NNO-1 in = cores compared to NNO+1 in rims. Within the Somerset Island cluster, Pv from six pipes display an average relative f02 of NNO-4 to NNO+I . Within individual kimberlite pipes, however, the fO2 range recorded by Pv narrows to NNO-2.6 +/- 0.6 within the Nikos 1 pipe, and NNO +1- 2 within the Zulu pipe. Within the Lac de Gras cluster, Pv from five pipes display an average relative fO2 of NNO-2.5 to NNO+6. However, within a single kimberlite, the Grizzly pipe, the fO2 range recorded by Pv narrows to NNO+/- 1. The range in fO2 recorded by Pv may result from the diversity and complexity of processes that ensue during the emplacement of kimberlite magma (crystallization, assimilation and degassing), a detailed record of which is revealed by a comprehensive study of perovskite parageneses in these complex rocks.