Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1987. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographies. / by G. William Tasker. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1987. / Bibliography: leaves 114-116. / by Yongun Lee. / M.S.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / 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 142-149). / III-V semiconducting nanostructures present a promising platform for the realization of advanced optoelectronic devices due to their superior intrinsic materials properties including direct band gap energies that span the visible light spectrum and high carrier mobilities. Additionally, the inherently high surface-to-volume ratio of nanostructures allows for the efficient relaxation of stress enabling the realization of defect free heterostructures between highly mismatched materials. As a result, nanostructures are being investigated as a route towards the direct integration of III-V materials on silicon substrates and as platforms for the fabrication of novel heterostructures not achievable in a thin film geometry. Due to their small size, however, many of the methods used to calculate stress and strain in 2D bulk systems are no longer valid as free surface effects allow for relaxation creating more complicated stress and strain fields. These inhomogeneous strain fields could have significant impacts on both device fabrication and operation. Therefore, it will be vital to develop techniques that can accurately predict and measure the stress and strain in individual nanostructures. In this thesis, we demonstrate how the combination of advanced transmission electron microscopy (TEM) and continuum modeling techniques can provide a quantitative understanding of the complex strain fields in nanostructures with high spatial resolutions. Using techniques such as convergent beam electron diffraction, nanobeam electron diffraction, and geometric phase analysis we quantify and map the strain fields in top-down fabricated InAlN/GaN high electron mobility transistor structures and GaAs/GaAsP core-shell nanowires grown by a particle-mediated vapor-liquid-solid mechanism. By comparing our experimental results to strain fields calculated by finite element analysis, we show that these techniques can provide quantitative strain information with spatial resolutions on the order of 1 nm. Our results highlight the importance of nanoscale characterization of strain in nanostructures and point to future opportunities for strain engineering to precisely tune the behavior and operation of these highly relevant structures. / by Eric James Jones. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015. / 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. / Dynamically controlling magnetism at the nanoscale is the foundation for realizing high-performance, low-power solid-state spintronic devices. The manipulation of nonuniform magnetization textures such as domain walls and skyrmions provide both a means to control magnetism in devices, and to gain new fundamental insights into spin-charge and spin-orbital interactions in magnetic materials. In this thesis, we examine novel mechanisms for the evolution and control of nonuniform magnetization textures. We first show that magnonic spin currents due to spin wave propagation can couple to and drive magnetic domain walls in nanowires mainly with experiments, supported by micromagnetic simulations. This work highlights a route towards integrating domain walls and spin waves for enhanced functionality in spintronics applications. We then focus on pure spin currents generated at the interface between a metallic ferromagnet and a heavy metal due to the spin Hall effect induced by strong spin orbit interaction. We demonstrate that the spin Hall effect can efficiently amplify or attenuate spin waves in an adjacent ferromagnet, and the efficiency of spin current generation can also be dramatically enhanced by optimizing the ferromagnet/heavy-metal interfaces. Moreover, we describe the microscopic mechanisms by which the spin Hall effect leads to magnetization switching, in the presence of chiral exchange interactions due to interfacial Dzyaloshinskii-Moriya interaction. This work shows the essential role that Dzyaloshinskii-Moriya interaction plays in magnetization switching. Finally, by harnessing this effect in carefully engineered materials, we show for the first time that the Dzyaloshinskii-Moriya interaction can stabilize topologically-protected skyrmions, whose statics and dynamics we have imaged for the first time in transition-metal ferromagnets at room temperature without any static external bias field. This finding provides not only experimental evidence of recent predictions but also opens the door to room-temperature skyrmion spintronics in robust thin-film heterostructures / by Seonghoon Woo. / Ph. D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 33-34). / Quantum dots (QDs) are nanometer-sized crystallites of inorganic semiconductors with tunable optoelectronic properties, which has led to a variety of real-world applications beginning in the 1980s, ranging including electronic displays, solar cells, and quantum computers [(Lee, SID), (Tang, Nature Mater.), (Puri, Phys Rev. B)]. However, most high-quality QD materials explored to date have been limited for large-scale application due to toxicity concerns or difficult-to-scale preparative methods. This thesis explores the synthesis and properties of colloidal nanocrystals composed of the non-toxic semiconductor copper indium sulfide (CulnS₂). We report an improved core nanoparticle synthesis with unique compositional control, a rationally-designed precursor for the synthesis of high-quality CulnS₂/ZnS nanocomposites, and describe the dependence on the photophysical properties of CulnS₂/ZnS on core CulnS₂ elemental composition. / by Trevor Walton Romero. / S.B. / S.B. Massachusetts Institute of Technology, Department of Materials Science and Engineering

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 30). / Lithium-ion batteries are widespread in use due to their thermal stability and high energy density. The most common design uses an organic electrolyte and lithium-cobalt electrode. While safe under typical operating conditions, the use of an organic electrolyte subjects the battery user to certain risks; in particular, Li-ion liquid batteries are explosive when exposed to air and subject to thermal runoff, making them highly sensitive to any physical damage. The use of cobalt also poses a moral concern, as the mining and sourcing of cobalt is geographically restricted and most commonly sourced from countries that have a history of foreign exploitation and child labor. An all solid state battery is suggested as a possible alternative battery that reduces operation risks and maintains similar performance characteristics. Lithium-lanthanum-zirconium oxide is presented as a suitable electrolyte replacement. Coupled with cobalt-free electrodes, this battery design would provide a safer, more responsible battery. / by Erick Ivan Hernandez Alvarez. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. / Includes bibliographical references (p. 164-171). / The dramatic advances in Si/SiO2-based microelectronic processing witnessed over the past several decades can largely be attributed to relatively material-independent device scaling. However, with physical and economic limitations to the continued scaling of such devices appearing on the horizon, it is likely that near-term advances will come from the integration of novel and previously underrepresented materials. One of the most promising ways to enhance performance comes from the integration of judiciously chosen lattice-mismatched materials with Si. However, the integration of such structures typically poses significant technical challenges. The work contained in this thesis seeks to address several of these important issues, primarily through the use of relaxed, graded SiGe buffers on Si (i.e. Vx[Si1-xGex]/Si). Several new phenomena in relaxed graded SiGe buffers are developed in this thesis. A rise in threading dislocation density was observed in high-Ge content relaxed graded SiGe layers grown at relatively high temperatures, which was attributed to dislocation nucleation. This observation is contrary to conventional graded buffer theory in which high growth temperatures are expected to result in reduced threading dislocation densities (TDDs). / (cont.) Additionally, a coupling effect between the effective strain and the growth rate was observed, as evidenced by increased TDD values at reduced growth rates. This observation is attributed to reduced growth rates allowing more time for the surface to evolve (i.e. roughen) during growth, thereby trapping mobile dislocations and necessitating the nucleation of additional dislocations to continue relaxing the structure. Also detailed in this thesis is the creation of two novel CMOS-compatible platforms for high-power applications: strained-silicon on silicon (SSOS) and strained-silicon on silicon-germanium on silicon (SGOS). SSOS substrate has an epitaxially-defined, tensilely strained silicon (-Si) layer directly on bulk silicon wafer without an intermediate SiGe or oxide layer. SSOS is a homochemical heterojunction, i.e. a heterojunction defined by strain state only and not by an accompanying compositional change, and therefore in principle SSOS may ease metal-oxide-semiconductor (MOS) -Si fabrication as SiGe is absent from the structure. SGOS has an epitaxially-defined SiGe layer between the strained silicon channel and the Si substrate, which is likely necessary to prevent excessive off-state leakage in MOS devices due to overlap of the source-drain contacts and the interfacial misfit array. / (cont.) The thesis concludes with a study of utilizing buried -Si layers for improving the fabrication of SSOI substrate via the hydrogen induced layer exfoliation process. Previous work involving tensile -Si.4Geo.6 layers in relaxed Ge/Vx[SiixGex/Si demonstrated that significant hydrogen gettering via the formation of strain-relieving platelets occurred within the tensile -Sio.4Ge.6 layers, leading to an overall increase in layer transfer efficiency for GOI substrate fabrication. Buried tensile -Si layers in relaxed SilGex for SSOI fabrication, however, demonstrate markedly different hydrogen gettering behavior that is dependent on a combination of both the degree of tensile strain as well the amount of damage present in the adjacent Si.xGex alloy. It was determined that a tensile strain level of approximately 1.6% in Si (corresponding to a Sio.6Ge.4-based donor structure) was needed to create sufficient engineered damage to overcome the implantation damage in the adjacent Sio.6Ge.4 layers and result in enhanced layer exfoliation. Lastly, an advanced Sio.6Geo.4-based structure which incorporated -Si layers as transfer, hydrogen gettering, and etchstop layers was demonstrated. Such a structure may prove useful for the reuse of significant portions of the original SSOI donor structure, thereby potentially speeding commercial adoption of the SSOI platform. / by David Michael Isaacson. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references. / The optical logic unit cell is the photonic analog to transistor-transistor logic in electronic devices. Active devices such as InP-based semiconductor optical amplifiers (SOA) emitting at 1550 nm are vertically integrated with passive waveguides using the asymmetric twin waveguide technique and the SOAs are placed in a Mach-Zehnder interferometer (MZI) configuration. By sending in high-intensity pulses, the gain characteristics, phase-shifting, and refractive indices of the SOA can be altered, creating constructive or deconstructive interference at the MZI output. Boolean logic and wavelength conversion can be achieved using this technique, building blocks for optical switching and signal regeneration. The fabrication of these devices is complex and the fabrication of two generations of devices is described in this thesis, including optimization of the mask design, photolithography, etching, and backside processing techniques. Testing and characterization of the active and passive components is also reported, confirming gain and emission at 1550 nm for the SOAs, as well as verifying evanescent coupling between the active and passive waveguides. In addition to the vertical integration of photonic waveguides, Esaki tunnel junctions are investigated for vertical electronic integration. Quantum dot formation and growth via molecular beam epitaxy is investigated for emission at the technologically important wavelength of 1310 nm. The effect of indium incorporation on tunnel junctions is investigated. The tunnel junctions are used to epitaxially link multiple quantum dot active regions in series and lasers are designed, fabricated, and tested. / by Ryan Daniel Williams. / Ph.D.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1988. / Includes bibliographical references. / by Robert I. Park. / B.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1996. / Includes bibliographical references (leaves 59-61). / by Lynn Louise Chalfoun. / M.S.