Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005. / Includes bibliographical references (p. 203-208). / As we approach the end of traditional CMOS scaling, further improvements in integrated circuit performance and functionality will become limited by the inherently low carrier mobility and indirect bandgap of silicon. These performance shortcomings can be supplemented with high performance semiconductors such as Ge and GaAs, which have respectively improved carrier mobilities and a direct bandgap for efficient light emission. However, due to the economic superiority of Si-based microelectronics, it is unlikely that the CMOS industry will abandon Si entirely. Instead, it will be necessary to integrate materials such as Ge and GaAs with the Si platform by means of engineered substrates. In this thesis, thin Ge layers were transferred to Si by wafer bonding of compositionally graded structures. This approach combines the beneficial aspects of graded buffers with those of wafer bonding to provide a coplanar integration platform for lattice-mismatched semiconductors. The various innovations that were necessary to realize epitaxial layer transfer from virtual substrates stem from the fact that thin films of Ge are difficult to planarize. The large surface roughness of graded buffers requires smoothing of the surface prior to bonding. The poor surface passivation of GeO2 in aqueous chemo-mechanical planarization (CMP) slurries necessitates that Ge virtual substrates be planarized indirectly, using a deposited CMP layer. Furthermore, H-induced exfoliation is the only practical method of separating a thin Ge layer from the surface of a virtual substrate, leading to extensive surface damage of the transferred layer. / (cont.) This damage is traditionally removed using a CMP step for exfoliated Si layers. However for Ge transfer, a Sio.4Geo.6 etch-stop layer was incorporated for damage removal using a selective chemical etch. These techniques have enabled transferal of epitaxial Ge-on-insulator (GOI) structures to large diameter Si wafers. Tensilely strained layers have the ability to attract interstitially-dissolved hydrogen and accelerate the nucleation of platelets- both of which contribute to the layer exfoliation process. As a result, a strained Sio.4Geo.6 layer was used to enhance the exfoliation kinetics of Ge by providing a gettering site for ion-implanted hydrogen. During 250 C annealing of hydrogen-implanted Si0.4Ge.6/Ge gettering structures, preferentially-nucleated platelets are made to grow within the Sio.4Ge.6 layer with minimal loss of hydrogen to surface effusion. Subsequent annealing at a temperature exceeding >300 C yields significantly improved surface blistering kinetics over samples which do not contain a gettering layer. A platelet growth model was formulated accounting for both chemical and strain energy contributions to the free energy of platelet formation. Microstructure and strain relaxation data corroborate the free energy computations, revealing two kinetically- limited regimes of platelet growth within tensilely strained Sio.4Geo.6 layers. Low temperature annealing allows the platelets to grow in the strain-limited regime, resulting in a local platelet density of >1010 cm-2 and significantly improved exfoliation kinetics. Incorporation of strained layers has the potential of reducing the implantation dose necessary for layer transfer. Combined with virtual substrate bonding, gettering structures provide a promising solution for economical integration of high performance materials with silicon. / by Arthur Joseph Pitera. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 129-143). / Many biological structural materials exhibit "mechanical property amplification" through their intricate hierarchical composite designs. In the past several decades, significant progress has been achieved in elucidating the structure/mechanical property relationships of these materials. However, the design strategies of structural biomaterials with additional functional roles are still largely unexplored. This thesis, by selecting three unique mollusk shell model systems, explores the fundamental design strategies of multifunctional biomineralized materials with dual mechanical and optical functions: transparency, photonic coloration, and lens-mediated vision. The model systems are the bivalve Placuna placenta, the limpet Patella pellucida, and the chiton Acanthopleura granulata, respectively. By investigating the relationships between the mechanical and optical properties and the structural characteristics, this thesis uncovers novel design strategies used to integrate optical functions into mechanically-robust material systems. The high transmission property of the P. placenta shells (~99 wt% calcite), for example, is elucidated through experimental and theoretical analysis based on a light scattering model. This armor utilizes deformation twinning and additional mechanisms at the nanoscale to enhance the energy dissipation efficiency by almost an order of magnitude relative to abiotic calcite. 3D quantitative analysis of the damage zone resulting from high load indentations was performed via synchrotron X-ray micro-computed tomography, revealing the formation of a complex network of microcracks. A unique structural motif, screw dislocation-like connection centers, is identified to enable a high density of crack deflection and bridging. This thesis also leads to the discovery of a unique biomineralized photonic structure in the shell of the blue-rayed limpet P. pellucida. The photonic system consists of a calcite multilayer and underlying particles, which provide selective light reflection through constructive interference and contrast enhancement through light absorption, respectively. Lastly, this thesis presents a detailed investigation of the biomineralized lenses embedded in the armor plates of the chiton A. granulata. The image formation capability of these lenses is experimentally demonstrated for the first time. The optical performance of the eyes is studied via comprehensive ray-trace simulations that take into account the experimentally measured geometry and crystallography of the lens. Mechanical studies illustrate that trade-offs between protection and sensation are present in the plates. / by Ling Li. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1991. / Includes bibliographical references (leaves 138-142). / by Robert Paull Messner. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Material Science and Engineering, 1985. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibliography: leaves 208-229. / by Frank Remsen Field III. / Ph.D.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / Includes bibliographical references (leaves 50-53). / Applications, markets and a cost model are presented for III-V multi-junction solar cells built on compositionally graded SiGe buffer layers currently being developed by professors Steven Ringell of Ohio State University and Eugene Fitzgerald of MIT. Potential markets are similar to those currently occupied by high efficiency multi-junction space solar cells grown on a Germanium substrate. Initial cost analysis shows that at production volumes similar to those of the state of the art, cost could be reduced by a factor of' four. Significant market share may be gained in both the space and terrestrial PV markets due to improved performance associated with superior materials properties advantages as well as production cost reductions. / by Zachary Steele Judkins. / M.Eng.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Includes bibliographical references (leaf [15]). / This work presents an analysis of surface morphology changes in poly(acrylic acid)/polyacrylamide (PAA/PAAm) hydrogen-bonded multilayers. These changes were induced by immersion of the films in aqueous solutions of poly(allylamine hydrochloride), or PAH, at different levels of pH. Positive charges on PAH are attracted to negative charges on PAA, forming ionic bonds and locally decreasing the hydrophilicity of the multilayer. The degree of ionization for each polyelectrolyte, controlled by the pH of the treatment solution, determines the molecular conformations and the extent of electrostatic interactions. These factors, in turn, determine the resulting morphology of the film. Different surface morphologies appeared in four different pH regimes. Highly acidic solutions retained the film's original smooth surface, but wrinkled, honeycomb, or globular morphologies appeared as the pH increased. The three different surface morphologies correlate with the linear, pearl necklace, and globular conformations of PAH. / by Allison Kunz. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Includes bibliographical references (p. 125-129). / Optical fibers and semiconductor devices differ significantly in their properties and their processing approaches. The latter require an assembly of metal, insulator and semiconductor materials into complex geometries with small feature sizes (sub 100 nm), while maintaining high quality interfaces. Sophisticated logic and detection functions are realized through the integration of many such devices onto a chip. Conventional optical fibers have been restricted to insulating materials, much simpler cylindrically symmetric structures and larger feature sizes (order of a micrometer). Consequently a single fiber operates as a single optical transport device. Indeed the notion of integration has been foreign to optical fibers. Their processing however is much simpler compared to semiconductor devices as it utilizes a macroscopic perform to microscopic fiber fabrication approach. This process can efficiently yield highly uniform long and flexible fibers amenable to the formation of large area assemblies and woven fabrics. In this thesis, it is established that in-principle sophisticated semiconductor devices can be produced using simple preform-to-fiber thermal drawing techniques: In the first chapter a new materials processing paradigm is introduced where for the first time metals, insulators and semiconductors are thermally co-drawn in intimate contact and prescribed geometries. The second chapter focuses on unifunctional fiber devices and in particular the unique features of ID distributed photodetecting fibers. The concept of fiber device integration is then established by demonstrating optic, electronic and optoelectronic functionalities within a single fiber. In a third chapter a model for understanding the influence of geometric and structural changes on the performance of fiber photodetectors is derived. / (cont.) It is demonstrated in particular that similarly to the evolution of semiconductor devices, the reduction of the materials feature dimensions inside the fiber significantly impacts the fiber performance characteristics. This in turn enables an increase in device density integrated into a single fiber. Unprecedented angular and spectral resolutions are achieved using this approach as described in chapter four. Finally, the first field effect is observed in a thermally drawn semiconductor metal insulator fiber. This paves the way to further integration of ever more complex electronic functionalities inside fiber devices. / by Fabien Sorin. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 27-28). / In this study biomaterial scaffolds for regeneration of nucleus pulposus were developed by freeze drying slurries with different proportions of collagen II (CII), chondroitin-6-sulfate (CS), and hyaluronic acid (HA). The scaffolds were analyzed using biochemical assays to determine final composition. Chemically cross-linked scaffolds were analyzed to determine pore size and cross-link density. It was determined that every material type contained large enough pore size (275 gm) to seed nucleus pulposus cells and mesenchymal stem cells. The addition of CS to the scaffold increased pore size. It was also found that increasing levels of CS and HA resulted in lower cross-link density. These materials will be used next in In Vitro studies to determine their viability as regenerative tissue engineering constructs. / by Jacob A. Simson. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, February 2004. / Includes bibliographical references (p. 131-138). / The development of CMOS-compatible photodetectors capable of operating throughout the entire telecommunications wavelength spectrum will aid in the integration of photodetectors with Si microelectronics, thus offering a low cost platform for high performance photoreceivers. This thesis demonstrates the first CMOS process compatible high-responsivity Ge p-i-n diodes for 1.55 [mu]m wavelengths. The thermal expansion mismatch between Ge epilayers and Si substrates was used to engineer tensile strain upon cooling from the growth temperature. This 0.2% tensile strain results in a lowering of the direct transition energy in Ge by 30 meV and extends the responsivity curve to near 1.6[mu]m. Design rules are given for high speed and high responsivity, and the advantages of waveguide integration for simultaneous achievement of high speed and high responsivity are illustrated. It is shown that waveguide integration has advantages to vertical illumination when optical saturation is considered. Optical saturation will become important as photodetector sizes shrink to the order of a few tens of microns in diameter. High Ge content SiGe could have applications for a SiGe electro-optic modulator utilizing the Franz-Keldysh effect. High Ge content SiGe films have been grown on Si substrates. The Franz-Keldysh effect has been observed in our pure Ge films as an increase in responsivity with increasing reverse bias for wavelengths longer than the bandgap energy. . / by Douglas Dale Cannon. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / Includes bibliographical references (p. 118-125). / A high-pressure liquid infiltration process utilizing centrifugal force was designed and laboratory equipment developed. In this process, a mold containing reinforcing materials was located at the end of an elongated runner, which was filled with a molten metal. Rotation of the runner created centrifugal force driving infiltration. To obtain high pressures, the metal head was controlled to be long and constant throughout the process. Threshold pressures required for infiltration of several packed ceramic powders were determined using the laboratory equipment built. Achievable pressures were up to 150 atm for Sn-15 wt% Pb. The pressures allowed SiC, TiC, and A1203 powders ranging in sizes from 25 [mu]m to 300 [mu]m, packed to a high volume fraction, to be infiltrated by Sn-15 wt% Pb. Threshold pressure results obtained agree well with experimental results previously reported, and with calculated values. Observations of the resulting composite structures showed layering and porosity defects. Layering defects, but no porosity defects, were observed in the composite samples containing coarse powders. In contrast, the composites containing fine powders possess porosity defects, but not layering defects. The layering defect was attributed to the depacking mechanism of the powders during the cold pressing process. The porosity defect was attributed to insufficient applied pressures. A new packing process was proposed to avoid layering in coarse powders. Macrosegregation and microsegregation were limited in all samples. The interparticle spacings of these composites were smaller than the dendrite arm spacing would have been at equivalent cooling rates; thus, dendrite formation and microsegregation were effectively suppressed. / (cont.) Commercial viability of the process was assessed. Results show that the centrifugal infiltration process has several attributes, including a higher production rate and larger part size when compared with gas pressure infiltration and a wider variety of part geometry, part sizes, and materials systems capable of being produced when compared with squeeze casting. A feasibility study shows that an industrial-scale centrifuge would be able to fabricate aluminum metal matrix composites (MMCs) containing both coarse and fine reinforcements at a high volume fraction. The process should also be scalable to higher melting point MMCs. / by Jessada Wannasin. / Ph.D.