Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1991. / Includes bibliographical references (p. 353-358). / by Waldemar Furdanowicz. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2003. / Includes bibliographical references (p. 144-152). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Reactively sputtered ZnO thin film gas sensors were fabricated onto Si wafers. The atmosphere dependent electrical response of the ZnO micro arrays was examined. The effects of processing conditions on the properties and sensor performance of ZnO films were investigated. Using AFM, SEM, XRD and WDS, the 02/Ar ratios during sputtering and Al dopant were found to control the property of ZnO films. Subsequent annealing at 700 C improved the sensor response of the films considerably although it had only minor effects on the microstructure. DC resistance, I-V curves and AC impedance were utilized to investigate the gas response of ZnO sensors. ZnO films prepared with high O2/Ar ratios showed better sensitivity to various gases, a feature believed to be related to their lower carrier density. Al doped ZnO showed measurable sensitivity even with lower resistance attributable to their porous microstructure. AC impedance identified two major components of the total resistance including Schottky barriers at the Pt-ZnO interfaces and a DC bias induced constriction resistance within the ZnO films. Time dependent drift in resistance of ZnO films has been observed. Without applied bias, the ZnO films showed a fast and a slow resistance change response when exposed to gases with varying oxygen partial pressure with both response components dependent on operating temperature. Even at the relatively low operating temperatures of these thin film sensors, bulk diffusion cannot be discounted. The oxygen partial pressure dependence of the sensor resistance and its corresponding activation energy were related to defect process controlling the reduction/oxidation behavior of the ZnO. / (cont.) In this study, time dependent DC bias effects on resistance drift were first discovered and characterized. The DC bias creates particularly high electric fields in these micro devices given that the spacing of the interdigited electrodes falls in the range of microns. The high electric field is believed to initiate ion migration and/or modulate grain boundary barrier heights, inducing resistance drift with time. Such DC bias resistance induced drift is expected to contribute to the instability of thin film micro array sensors designed for practical applications. Suggestions for stabilizing sensor response are provided. / by Yongki Min. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (p. 29). / Introduction: New materials are being developing each year that could revolutionize the world. However, while development of novel materials in the lab brings us one step closer to next latest-and-greatest innovation, the following and perhaps similarly difficult step requires bringing these materials to the world market. Indeed, "although U.S. firms have invested the majority of materials introduced over the past half-century, they have failed to commercialize many of these innovations" (Eagar, 98). For a material introduction to be successful, it will be useful to understand the trends involved within the market for such an introduction and for continuing survival. / by Yao-Chung King. / S.B.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1995. / Vita. / Includes bibliographical references (leaves 171-179). / by Filippos Patsiogiannis. / Sc.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000. / Vita. / Includes bibliographical references (leaves 236-240). / A fundamental study of the Direct Current Electric Arc Furnace (DC-EAF) for steel-making has been carried out through the development of a rigorous mathematical model. The mathematical representation involves the simultaneous solution of Maxwell's equations for the electromagnetic fields, and the turbulent fluid flow and heat transfer equations. In solving the arc and bath regions it was assumed ( and justified) that the arc-bath interactions are dominated by the behavior of the arc. In contrast to previous modeling investigations, this work relaxes some critical assumptions and provides a more realistic and comprehensive representation of the system. This work also examines and compares the relative merits of alternative electromagnetic and turbulence formulations, and addresses the role of induced currents and compressibility effects in the representation of the arc. Furthermore, due allowance was made to represent and analyze the effect of gas injection, the presence of a slag layer in the bath and changes in anode configuration at the bottom of the reactor. Because of a lack of experimental information on actual or pilot plant DC-EAF systems, different aspects of the model were validated using several sources of experimental data reported in the literature for related systems. These included measurements on welding arcs, laboratory scale high-intensity carbon arcs, electromagnetically driven metallic systems, and ladle metallurgy physical models. It was found that, in general, the agreement between measurements and predictions was good. A detailed analysis was carried out to examine the effect of process parameters (e.g., arc current, arc length, bath dimensions, anode arrangements, etc) on the behavior of the furnace (e.g., heat transfer to the bath, heating efficiency, mixing times in the bath, etc). Predictions from the arc model show that all the arc characteristics are strongly coupled and that the arc physics is governed by the expansion of the arc. From a parametric study it was found that when the arc region (defined by the 10,000 K isotherm) is plotted in dimensionless form, a universal shape for the arc can be defined, regardless of the values of arc current or arc length. This universality was restricted to the range of conditions analyzed in this thesis, to arcs struck between graphite cathodes in air, and does not include the jet impingement region on the bath surface. This common arc expansion behavior suggested the universal nature of other arc characteristics. Universal maps of temperature, magnetic: flux density, and axial velocity are also reported in terms of simple analytical expressions. The practical effects of the two main process parameters of the arc region,. i.e. the arc current and the arc length, were analyzed. It was found that increasing the arc length significantly increases the arc resistance and, consequently, the arc power, although this behavior reached asymptotic values at larger arc lengths. Increasing the arc current, however, does not affect the arc voltage. Thus, it is found that increasing the arc power increases the amount of energy transferred into the bath, but the heat transfer efficiency decreases. Therefore, the shorter the arc the more efficient is the heat transfer to the bath. It is also recognized that heat transfer from the arc to the bath is controlled by convection, although radiation can become an important mechanism, especially for large arc lengths. Results of the bath model indicate that, in the absence of inert gas stirring and with no slag present in the system, electromagnetic body forces dominate and are responsible for the fluid flow patterns in the system. The effects of the arc determine the distributions of temperature and other mixing characteristics in the bath. The bath model was used to evaluate the effect of the main process parameters and design variables on mixing, refractory wear, temperature stratification, and heat transfer efficiency. An increase in the arc length is detrimental to mixing but increases the rate of heating in the melt as a result of the increased arc power. Increasing arc current improves mixing and the heat transferred to the bath, but is likely to be detrimental to the life of the bottom refractory. The results also suggest that high furnace aspect ratios (taller and thinner arc furnaces) are highly recommended because an increase in the aspect ratio increases mixing, prevents refractory wear, and promotes arc heating efficiency. The arc configuration in the furnace can be changed to control fluid flow patterns in the bath to meet specific needs, such as better mixing, or to prevent refractory wear. The presence of a top layer of slag reduces mixing and increases overall liquid temperatures. Injection of gases through the bottom in eccentric operations generates complex flow patterns that improve mixing in regions away from the symmetry axis. It is the author's belief that this model is a useful tool for process analysis in the DC-EAF. It has the capability to address many issues of current and future concern and represents one component of a fundamental approach to the optimization of DC-EAF operations. / by Marco Aurelio Ramírez. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1990. / Includes bibliographical references (leaf 143). / by Eva Jiran. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Includes bibliographical references (leaves 157-164). / by Brent Victor Chambers. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002. / Includes bibliographical references (p. 115-119). / We use what is known about transition metal (TM) defect thermodynamic driving forces and kinetic responses to make predictive simulation of gettering during solar cell fabrication possible. We have developed a simulator to explore the impact of various device and process parameters on gettering effectiveness. The relevant heat treatments are ramps up in temperature, isothermal annealing, and cools from high temperature down to room temperature. We explore a range of surface conditions, density and size of heterogeneous nucleation sites in the bulk, and the degree of contamination in order to create a framework in which to examine these mechanisms acting in concert. Such simulations enable process optimization for gettering. For solar cell processing, segregation to an Al back contact layer is routine. We have estimated the segregation coefficient between a p-type Si wafer and a molten Al layer by the Calphad method and use these results to estimate the thermodynamic driving force for redistribution of Fe into the Al layer. We simulate gettering treatments of supersaturated levels of Fe contamination in Si samples with FeSi2 and Al contacts and compare these results with data at various temperatures. The gettering data for FeSi2 contacts follow a simple exponential decay and can be simulated with appropriate choice of internal gettering time constant. We recognize that radiative heating dominates the temperature ramp for samples in evacuated quartz ampoules and use reasonable parameters to include this effect in our simulations. Fitting parameters for [Fe] data taken from heat treatments at 755Ê»C on samples with FeSi2 and Al contacts successfully predict the gettering data of Al coated samples treated at 810Ê»C. / (cont.) Discrepencies in the data for Al coated samples treated at 6950C and data for Al coated samples treated at 755Ê»C after long times have exposed a new mechanism dominating internal gettering processes. We propose the existence of a silicide precipitate growth retardation mechanism as a result of supersaturation of the Si vacancy (V). Accumulation of V reduces the ability of precipitates to relax strain free-energy ([Delta]g strain) by further V emission. We performed Cu gettering experiments on p/p+ epitaxial wafers. Photoluminescence measurements revealed significant Cu removal from the epitaxial region compared to similarly doped uniformly doped float zone (FZ) Si wafers. Step etching revealed haze, indicating the presence of silicide precipitates below the epitaxial layer in the heavily doped substrates. Uncontaminated heat treated epitaxial wafers did not demonstrate the presence of haze after step etching. This finding demonstrates that redistribution of Cu from the lightly doped expitaxial layer to the heavily doped substrate as predicted by the dopant enhanced solubility model has occurred. The commonly used T3/2 model for effective density of conduction and valence band states (Nc and Nv respectively) is not accurate for Si, even in the device operation regime, and the available experimentally determined relations of Green do not extend past 500K. We have constructed a DOS model using ab initio calculations and temperature appropriate Fermi-Dirac ... / by Aimée Louise Smith. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000. / Includes bibliographical references (leaves 245-251). / The stringent performance and reliability demands that will accompany the development of next-generation circuits and new metallization technologies will require new and more accurate means of assessing interconnect reliability. Reliability assessments based on conventional methodologies are flawed in a number of very important ways, including the disregard of the effects of complex interconnect geometries on reliability. New models, simulations and experimental methodologies are required for the development of tools for circuit-level and process-sensitive reliability assessments. Most modeling and experimental characterization of interconnect reliability has focused on simple straight lines terminating at pads or vias. However, laid-out integrated circuits usually have many interconnects with junctions and wide-to-narrow transitions. In carrying out circuit-level reliability assessments it is important to be able to assess the reliability of these more complex shapes, generally referred to as "trees". An interconnect tree consists of continuously connected high-conductivity metal within one layer of metallization. Trees terminate at diffusion barriers at vias and contacts, and, in the general case, can have more than one terminating branch when the tree includes junctions. We have extended the understanding of "immortality" demonstrated and analyzed for straight stud-to-stud lines, to trees of arbitrary complexity. We verified the concept of immortality in interconnect trees through experiments on simple tree structures. This leads to a hierarchical approach for identifying immortal trees for specific circuit layouts and models for operation. We suggest a computationally efficient and flexible strategy for assessment of the reliability of entire integrated circuits. The proposed hierarchical reliability analysis can provide reliability assessments during the design and layout process (Reliability Computer Aided Design, RCAD). Design rules are suggested based on calculations of the electromigration-induced development of inhomogeneous steadystate mechanical stress states. Failure of interconnects by void nucleation in single-layermetallization, as well as failure by void growth in the presence of refractory metal shunt layers are taken into account. The proposed methodology identifies a large fraction of interconnect trees in a typical design as immune to electromigration-induced failure. To complete a circuit-level-reliability analysis, it is also necessary to estimate the lifetimes of the mortal trees. We have developed simulation tools that allow modeling of stress evolution and failure in arbitrarily complex trees. We have demonstrated the validity of these models and simulations through comparisons with experiments on simple trees, such as "L"- and "T"-shaped trees with different current configurations. Because analyses made using simulations are computationally intensive, simulations should be used for analysis of the least reliable trees. The reliability of the majority of the mortal trees can be assessed using a conservative default model based on nodal reliability analyses for the assessment of electromigration-limited reliability of interconnect trees. The lifetimes of the nodes are calculated by estimating the times for void nucleation, void growth to failure, and formation of extrusions. The differences between straight stud-to-stud lines and interconnect trees are studied by investigating the effects of passive and active reservoirs on electromigration. Models and simulations were validated through comparisons with experiments on simple tree structures, such as lines broken into two limbs with different currents in each limb. Models, simulations and experimental results on the reliability of interconnect trees are shown to yield mutually consistent results. Taken together, the results from this research have provided the basis for the development of the first RCAD tool capable of accurate circuit-level, processing sensitive and layout-specific reliability analyses. / by Stefan P. Hau-Riege. / Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1992. / Includes bibliographical references (leaves 246-262). / by Hai Pham Longworth. / Sc.D.