Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (leaves 66-68). / MRAM is a memory (RAM) technology that uses electron spin to store information. Often been called "the ideal memory", it can potentially combine the density of DRAM with the speed of SRAM and non-volatility of FLASH memory or hard disk, and all this while consuming a very low amount of power. However, it is the need for a fast and non-volatile computer memory that has been the key driver for evolution of this technology. At the moment, MRAM is in its final stages of development and much of the current research concentrates on issues like reducing the write current, increasing the density and making the process more reproducible. A lot of companies are pursuing research on this technology and are likely to introduce it into the market in the near future. However, it will be a while before MRAM can replace conventional memories. Nevertheless, since MRAM can resist high radiation, and can operate in extreme temperature conditions, it is likely that we will see the first MRAM in applications that need such properties. / by Rangarajan Vijayaraghavan. / M.Eng.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 163-171). / Optoelectronic devices based on III-V direct gap semiconductors enable efficient energy conversion for photovoltaic cells, light emission for LEDs, and on-chip communication via various microphotonic components. However, widespread adoption of III-V solar cells is limited by the expensive Germanium and III-V standard substrates required, while monolithic integration of III-V devices with Silicon CMOS circuitry is not yet well established. III-V solar cell cost reduction and direct Si/III-V integration can both be realized by depositing a thin layer (e.g. 1 [mu]m) of high quality Ge on relatively inexpensive Si substrates for which the decreased cost is due to Si's greater material abundance and larger possible wafer diameters. Efficient device performance will be retained if the Ge layer maintains a sufficiently low threading dislocation density (TDD) that does not adversely effect carrier lifetimes in epitaxially deposited III-V layers that inherit the Ge film's TDD. Assuming recombination at dislocations is carrier diffusion limited, an acceptable limit for most applications is below 10⁶ cm-² due to typical minority carrier diffusion lengths of ~ 10 [mu}m in III-V materials. However, direct deposition of Ge on Si will initially generate a TDD as high as 10¹² cm-² to plastically relax the 4.2% lattice mismatch between the two materials. State of the art approaches can reduce the TDD in large-area films to 10⁶ cm-² by including a 10+ m thick SiGe compositionally graded buffer, while TDD reduction in thinner films (e.g. 1 [mu]m) is limited to 10⁷ cm-² after cyclic annealing which enhances dislocation fusion and annihilation reactions. By introducing Ge film edges spaced approximately 10 [mu]m apart to serve as dislocation sinks during dislocation glide, the TDD has been reported to further decrease to 2:310⁶ cm-² in 1 m thick patterned Ge. However, these films are limited to areas too small for photovoltaic cells, and the sinks appear ineffective for thread reduction at the edges of faceted, selectively grown Ge. Thus, no solution has previously existed for a thin Ge-on-Si film grown over large areas that achieves a TDD of 10⁶ cm-² or below. This thesis first explores the limitations to dislocation reduction by sinks in selectively-grown Ge and provides structure and fabrication modifications to enable patterned Ge films with a TDD below 10⁶ cm-² throughout the patterned region. To use these films for large-area applications, overgrowth and coalescence of patterned Ge films are then evaluated in different pattern designs to determine the structures that optimize coalescence in terms of throughput as well as simultaneously avoid generation of additional defects as a result of coalescence. TDD reduction in patterned Ge films by glide to film edges requires uniform resolved shear stresses and minimum dislocation pinning during cyclic annealing. Because film facets allow for elastic relaxation of the applied thermal strain, the process of selective growth must be reversed: blanket Ge is to be grown instead to avoid faceting, followed by sidewall etching and filling before the cyclic anneal. Thermal expansion mismatch between Ge and the sidewall causes undesirable shear stress components while repulsive image forces are created if the sidewall surface's shear modulus is greater than that of Ge. Therefore, the ideal sidewall is primarily composed of Ge, separated from the primary Ge film by a thin SiO₂ layer. Monte Carlo simulations of dislocation glide were developed to estimate the limitations of glide due to the pinning effect of orthogonal dislocations. For small mesa widths w (or more generally, the spacing between adjacent dislocation sinks), TDD was found to scale with wa with a 4. The threshold of the small width regime and the value of a both increase for greater applied thermal stresses and thicker Ge films. Due to the high surface energy of the Ge/SiO₂ interface, lateral overgrowth and film coalescence do not readily occur. The rate was observed to strongly correlate with the Ge film perimeter concavity, delayed at convex mesa corners while relatively promoted at the ends of isolated SiO₂ lines surrounded by a concave Ge film perimeter. Ge mesa arrays were staggered to eliminate regions entirely dependent on overgrowth from mesa corners, decreasing the growth time until complete coalescence by at least 50% as compared to a regular gridded array. The faster overgrowth rates over isolated SiO² lines was observed to further increase for lines of reduced widths. Due to the facets that develop, orientation of SiO² lines relative to intersections of {111} planes with the substrate surface further affected overgrowth rates which maximized for slight offsets below 15°. Etch pit studies of coalesced, selectively-grown Ge films around SiO₂ sidewalls indicated a maximum TDD above the SiO² (6X10⁷ cm-² for staggered grids) while decreasing to 10⁷ cm-² further away in the film. As predicted by modeling, the dislocation pile-up near SiO₂ walls was due to inverted resolved shear stress and the reduced thickness at the Ge film edge. Significant improvement in TDD reduction is expected by these models if blanket Ge is instead grown, followed by etch and fill of sidewalls with additional Ge separated by a thin layer of SiO₂. While fabrication is more involved compared to the selective growth process, the structure will be successful at threading dislocation removal. With isolated line film edges of minimal width, oriented 5 from {111} surface intersection directions, the coalescence rate will be maximized. Coalescence-induced defects resulting from lattice misregistry over the SiO₂-coated Ge lines will be prevented as the Ge film is continuous at the line ends prior to overgrowth initiation. Assuming a pinning probability of 50%, a Ge film 1 [mu]m thick with a maximum distance between dislocation sinks < 6 [mu]m is expected to exhibit a TDD of 10⁵ cm-². At this density level, the performance of III-V devices will be unaffected, enabling both lower cost high efficiency III-V solar cells and LEDs as well as III-V/Si monolithic device integration. The multiple perspectives of analysis examined in this thesis are not limited to Ge-on-Si and can readily be applied to other high lattice-mismatched materials systems to obtain a low TDD surface in large areas while maintaining a buffer layer of minimal thickness. / by Brian Ross Albert. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 129-135). / The development of stable nanocrystalline binary alloys, which possess a large volume fraction of grain boundaries at elevated temperatures, is a promising route to high yield strength materials. Previous studies have focused on alloying by selecting solute elements that segregate at grain boundaries to stabilize the nanostructure. A selection criterion has been established for designing stable binary nanocrystalline materials. This thesis explores the extension of this concept to the design of multicomponent nanostructured systems. In contrast to the simplicity of a binary system where not many topological possibilities are accessible, multicomponent nanostructured systems are shown to occupy a vast space where the large majority of interesting configurations will be missed by a regular solution approximation. This thesis describes research to develop a conceptual basis for the thermodynamic properties of multicomponent nanocrystalline alloys, and to design interesting ternary configurations not accessible in binary systems. The conditions necessary to achieve the desired nanostructure configurations are developed in a model that takes solute interactions into consideration. Based on the model, we performed a systematic case study on one alloy system expected to exhibit nanocrystalline stability: Pt-Pd-Au. As a control, two binary systems (Pt-Au, Pt-Pd) were produced for comparison. While a uniform distribution of Pd is observed in binary Pt-Pd alloys at 400 °C, the results from scanning transmission electron microscopy (STEM) reveal that Pd segregation behavior was induced by the Au grain boundary segregation in the ternary system at 400 °C. Our work on induced co-segregation behavior of Pt-Pd-Au alloy is just a simple example of solute interaction in nanocrystalline alloys. Our approach more generally presents a new design framework to control the complex configurations possible in nanocrystalline materials by alloying element selection. / by Wenting Xing. / Ph. D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student submitted PDF version of thesis. / Includes bibliographical references (p. 49-50). / As environmental footprint considerations for companies gain greater importance, the need for quantitative impact assessment tools such as life cycle assessment (LCA) has become a higher priority. Currently, the cost and time burden associated with LCA has prevented it from becoming more prevalent. While several streamlining approaches have been suggested, questions regarding the effectiveness and efficiency of the streamlined results are still of concern. The streamlining method of probabilistic underspecification has shown initial success in its ability to reduce LCA efforts while simultaneously increasing certainty in the final impact assessment. Probabilistic underspecification streamlines LCA by prioritizing targets of more refined data collection and by implementing the use of underspecified surrogate data within LCI analysis. This thesis concentrates on further developing and improving the streamlining methodology of probabilistic underspecification through refinement of the materials classification systems for polymers and minerals and through additional case study analysis. The classification system allows for a better understanding of the relationship between the degree of materials specificity and the uncertainty in the resulting impact values. Additionally, the resulting polymer and mineral classifications were combined with existing materials classifications to conduct an alkaline battery case study in order to test the effectiveness of the streamlining method. The material classifications created through this research provide a logical and practical approach to underspecification while maintaining consistent and reasonable levels of uncertainty. Furthermore, the case study analysis showed that the streamlining methodology significantly lowered LCA burden by systematically reducing the number of product components requiring full specification. This research provides further evidence that probabilistic underspecification may provide a promising LCA streamlining method among a set of such strategies that can significantly reduce LCA efforts while maintaining the accuracy of the overall impact assessment. / by Abhishek Rampuria. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1987. / Bibliography: leaves 151-155. / by Walter Wilhelm Focke. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / Includes bibliographical references (p. 103-106). / This thesis presents fabrication process improvements for a RF MEMS resonator for the purpose of improving the quality factor (Q) and extending the frequency range. The process changes include the use of conformable contact lithography (CCL) and chlorine-based dry etching for improved fine-feature patterning and Moiré -based alignment techniques to allow for a non-self-aligned process. The resulting control over feature size and structure are expected to improve Q and enable higher frequency resonators. A CCL process utilizing moir6 alignment marks is described. An automated Moiré -based alignment system using Labview software is presented which demonstrates sub-100 nm alignment accuracy for a single alignment mark. A full-wafer alignment experiment is described that demonstrates average pattern placement errors of ... for the x- and y-directions respectively. The experimental limitations are analyzed and suggested improvements to the system are detailed. Chlorine dry etching experiments are conducted in order to produce a straight sidewall etch through the "stack" of resonator materials (chrome, aluminum nitride, and molybdenum). A combination of Cl₂/0₂, Cl₂/Ar, and CF₄0₂ plasmas at low pressure (2 mTorr), high microwave/source power (500W), and a moderate DC bias (-150V) demonstrates a straight sidewall angle (>80⁰ measured from horizontal) with no undercut for all layers of the stack. RF resonators fabricated with these process modifications are presented. An average overlay error of 55 nm (110 nm min-max) is recorded for 11 devices located closest to the line between the alignment marks in an aligned release of the resonators. The design modifications enabled by the new process are described and the prospect for higher-frequency devices and higher-Q device performance is discussed. / by Mark Sakai. / S.M.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (leaves 25-26). / Advances in nanochemistry will drive the development of technologies at the scale of 1 - 100 nm. Principles of biology are used for the self-assembly of structures and devices at this scale. The Ml 13 bacteriophage, a virus employed in phage-display libraries, serves as a scaffold for nanoscale structures. Phage are functionalized with inorganic materials, and controlled placement of phage at the nanoscale may lead to useful devices. Substrates patterned with dip-pen nanolithography (DPN) serve as templates for the deposition of phage. On gold substrates, 16-mercaptohexadecanoic acid (MHA) is deposited to form patterned lines. After surface passivation and activation chemistry, phage are deposited and adhere to the patterned substrate. Images from atomic force microscopy support that phage are covalently coupled to MHA lines and that cobalt precipitates on patterned phage. / by David S. Gray. / S.B.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 33-35). / The electronics manufacturing industry has been experiencing a fast-changing landscape with recent legislations targeting the supply chains for the 3TG minerals: tin, tantalum, tungsten, and gold mined from the Democratic Republic of Congo. These minerals exhibit unique properties that are crucial to their role in the manufacturing process and functionality of many electronic products such as computers, cell phones, and printers. This work focuses on using a bottom up model to quantify conflict mineral content within LaserJet printers and uses a market analysis to compare the conflict mineral composition between various IT products in order to obtain a measurement of impact the conflict minerals have in their respective IT product. On the global scale, the model estimates the market share of tin, tungsten, tantalum, and gold in printers to be 1.44%, 0.083%, 0.017%, and 16.5%, respectively. These results indicate a strong potential and improvement for the development of redefined materials selection processes for manufacturers of IT products in using alternative solutions or substitute materials. Current work in this field shows that it is imperative for future work to focus on decreasing the market share of these conflict minerals and shifting manufacturing focus to developing new conflict-free electronic products. / by Jason S. Lee. / S.B.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2001. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 41). / A method was devised for quenching undercooled samples of Fe-12Cr-16Ni. The samples were levitated in a magnetic field as a way of containerless processing. They were dropped onto a nucleation trigger and then into a quenching bath. This process was successful in producing quenched samples , but the layer of In-Ga wetting their surface showed problematic in the analysis of the samples. Data shows that double recalescence was observed in a few cases. SEM proved inconclusive and was not sensitive enough to detect the small chemical variations expected in the dendrites. Also, across the entirety of the sample there was no significant partitioning of Cr or Ni. / by Nicole Zacharia. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references. / The short-wave infrared region (SWIR; 1000-2000 nm) has excellent properties for in vivo imaging: low autofluorescence, reduced scattering, and little light absorption by blood and tissue. However, broad adoption of SWIR imaging in biomedical research is hampered by the availability of versatile and bright contrast materials. Quantum dots (QDs) are bright, compact SWIR emitters with narrow size distributions and emission spectra, qualities that make them ideal for labeling and multiplex SWIR imaging. Nevertheless, SWIR QDs have limited applications due to the shortcomings of established ligand systems. Established ligands result in QD probes with limited colloidal stability, large size and broad size distribution, or all three limitations. To address these limitations, we turned to polymeric ligands, beginning with the polymeric imidazole ligand (PIL) initially developed for visible-emitting CdSe/CdxZn₁₋xS QDs with L-type native ligands. We studied ligand exchange with PIL and InAs/CdSe/CdS SWIR QDs with native X-type ligands in a variety of conditions but only saw limited exchange. Our results combined with reports in the literature suggest that the mechanism of X-to-L ligand exchange is not amenable to polymeric ligands. These results led us to the concept of ligand-type matching: for straightforward exchange, QD native ligands should be the same type as the binding groups on the polymer. Thus, we synthesized InAs/CdSe/ZnS with L-type native ligands, which exchanged readily with PIL to produce probes with (<14 nm hydrodynamic diameter, Hd). We also synthesized a new ligand that is compatible with oleate-capped QDs: the polymeric acid ligand (PAL), which features carboxylic acids as the binding group and PEG₁₁ chains to solubilize the QD-ligand construct. We exchanged PAL with oleate-capped PbS and PbS/CdS QDs, resulting in compact probes ( <11 nm Hd) with narrow size distribution. The small size and narrow size distribution of these constructs are preserved for several months when stored in isotonic saline solution in air, addressing the size and stability limitations of existing ligand systems for SWIR QDs. Our constructs are bright in vivo and to demonstrate their suitability for imaging, we performed whole-body imaging as well as lymphatic imaging, including visualization of lymphatic flow. / by Daniel Mauricio Montana Fernandez. / Ph. D.