Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010. / Includes bibliographical references. / Difficulties with traditional methods of synthesis and film formation for conducting polymers, many of which are insoluble, motivate the development of CVD methods. Indeed, conjugated polymers with rigid linear backbones typically crystallize readily and overcoming the resultant heat of crystallization makes them difficult to dissolve. Poly(3,4-ethylenedioxythiophene) (PEDOT) thin films were obtained through oxidative chemical vapor deposition (oCVD) by using a new oxidant- bromine. The use of bromine eliminates any post processing rinsing step required with other oxidants like iron chloride and hence makes the process completely dry. Accelerated aging experiments show longer retention of electrical conductivity for the PEDOT films obtained using bromine as the oxidant. Conductivities as high as 380 S/cm were obtained for PEDOT films deposited using bromine as the oxidant at 80 'C, which is significantly higher than that for PEDOT films deposited using iron chloride as the oxidant at the same temperature. Cross-sectional SEM of the PEDOT films deposited using bromine on silicon trench wafers demonstrates high conformal deposition of the films. All the results show the possibility of depositing highly conducting, conformal PEDOT films on any substrate including silicon, glass, paper, plastic. One of the many applications of conducting polymer is as hole-transport layer in light emitting diode. To be competitive in the LED market, improvements in hybrid-LED quantum efficiencies as well as demonstrations of long-lived HLED structures are necessary. In this work, we consider improvement in the stability of the HLED. The device fabricated can be configured as ITO/ Poly (EDOT-co-TAA)/CdSe (ZnS)/ Au. All the materials used in the device synthesis are stable in ambient conditions and all the synthesis steps on ITO substrate are done either in air or in very moderate pressure conditions. This significantly reduces the cost of the device fabrication by obviating the need of packaging layers and ultrahigh vacuum tools. The operating voltage as low as 4.3 V have been obtained for red-LEDs. We believe that with optimization of various layers in the device, further improvements can be made. For green LEDs we obtained the characteristic IV curve of a diode, but we still need to work on getting a functioning green LED. / by Hitesh Chelawat. / S.M.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 41-42). / Infrared sensors are used in a variety of applications, from gas and moisture analysers, to human body detection to spectrophotometers. Available IR technology falls on two ends of the spectrum: state-of-art photon detectors are high-quality but expensive and cumbersome due to the need for cryogenic cooling, while thermal detectors are inexpensive but not very sensitive. The goal of this project is to develop materials for uncooled IR sensors with improved performance. Lead selenide (PbSe) detectors are direct narrow band gap materials that have shown promise for relatively inexpensive IR sensing with modest cooling requirements. Adapting the vapour-liquid-solid (VLS) growth mechanism traditionally used for growing nanowires to growing PbSe thin films circumvents the very slow adsorption of a gas phase into a solid surface by introducing a catalytic liquid alloy phase, while simultaneously retaining the stoichiometric control, simplicity, and economy of vapor phase growth. We have set the stage for further experimentation by demonstrating that we can attain a single phase PbSe thin film via VLS growth on an epitaxially matched substrate. We have explored the effects of VLS growth vs. vapor growth on crystal quality as well as the factors that influence diffusion and nucleation rates, such as film thickness, growth temperature, and the presence of a capping layer. / by Alexandra Andreevna Sourakov. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004. / Includes bibliographical references (v.2, leaves 267-272). / Thin films are defined by an extremely high ratio of the in-plane dimensions to the thickness, resulting in very high surface-to-volume ratios. For this reason, the surface characteristics of a film play an important role in the properties of the film as a whole. This research focused on the connections between the properties and the surface state of metallic thin films. High resolution in-situ stress measurements were made during the ultra-high vacuum evaporative deposition of polycrystalline Cu films and homoepitaxial (111)-oriented Cu and Ag films. The measurements were enabled through the development of two high resolution in-situ stress monitoring devices that minimized sample placement and vibrational error through compact and monolithic ultra-high vacuum compatible designs. These devices allowed in-situ stress measurements with unprecedented sensitivities while the device electronics enabled the study of systems in real-time with high data acquisition rates. Measurements were made during growth and interruptions of growth as the films formed and thickened. In the earliest stages of a growth cycle, when only a small fraction of a monolayer is deposited, the elastic component of the adatom-surface interaction dominates the stress evolution. The early stage observations are analyzed through a first- order expansion of the thermodynamic surface stress which allows for direct extraction and measurement of the force-dipole associated with the adatom-surface interaction. / (cont.) Values of the force-dipole magnitude determined from experiments compared well with values obtained from embedded atom method molecular dynamics calculations. On a longer timescale, evolution of the atomic-scale surface roughness was tracked using reflection high energy electron diffraction (RHEED), and a strong correlation with the measured stress changes was found. Significant portions of both the stress and surface roughness changed reversibly upon interrupting and restarting the growth. These results demonstrate that reversible stress changes observed during interruptions of the growth of polycrystalline and epitaxial films are due to a far-from-equilibrium atomic scale surface defect concentration present during growth. In-situ stress measurements were also made during heteroepitaxial growth in several metallic systems: Ag/Au(1 11), Cu/Au(1 11), Pt/Au(l 11), Al/Au(l 11), Ag/Cu(1 11), Ag/Pt(111), and Ag/Al(111). Each system has different lattice misfits and different relative electronic structures, leading to a large range of stress and structure evolution phenomenology. In all cases the large stress changes observed at the very beginning of growth were influenced by both misfit strain and chemical differences, with the latter often dominating. Stress evolution during continued film thickening was also always more complex than the behavior expected for epitaxial misfit and dislocation-mediated relaxation of epitaxial misfit. As was the case for Volmer-Weber growth of polycrystalline films and for homoepitaxial growth, transient defect concentrations lead to large reversible stresses in these systems during growth interruptions ... / by Cody A. Friesen. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1994. / Includes bibliographical references. / Yuying Tang. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 34-35). / Co/Ni multilayers display perpendicular magnetic anisotropy and have applications in magnetic devices that could lead to a large increase in the density of magnetic storage. Co/Ni 10-(2 Å Co/ 8Å Ni) and 10-(2 Å Co/ 4 Å Ni) multilayers were deposited with ion beam sputtering on either ion beam sputtered copper or direct current magnetron sputtered gold buffer layers of various thicknesses. The effect of the the roughness and the degree of (1 1 1) texture of the buffer layers and the multilayers on the perpendicular magnetic anisotropy of the deposited multilayers was examined. In addition the effect of the deposition method used to fabricate the samples, ion beam sputtering, was analyzed. The magnetic behavior of the multilayers was examined with alternating gradient magnetometry and vibrating sample magnetometery, the structure of the buffer layers and the multilayers was characterized with X-ray diffraction, and the roughness of the surface of the multilayers was characterized with atomic force microscopy. None of the deposited films showed perpendicular magnetic anisotropy and instead showed parallel magnetic anisotropy which was found to have occurred for every sample due to either a low degree of (1 1 1) texture in the buffer layer and the Co/Ni multilayer, a too high degree of roughness in the buffer layer and the Co/Ni multilayer or a combination of these two factors. In addition it was hypothesized that as the samples were deposited with sputtering, diffusion and alloying at the multilayer interfaces may have contributed to the multilayers having parallel magnetic anisotropy instead of perpendicular magnetic anisotropy. / by Boris Rasin. / S.B.

Thesis (S.M.)--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. / In title on title page, "1̳", "-x̳" and "x̳" in "Bi1̳-x̳Sbx̳" appear as subscript script. Cataloged from student submitted PDF version of thesis. / Includes bibliographical references (p. 56-61). / The study on the electronic band structures of Bi1-xSbx thin films is a very interesting topic. Recall that in bulk Bi1-xSbx, the electronic band structure can be varied as a function of temperature T, pressure P and stoichiometry. The electronic band structure does not change with T significantly in the cryogenic temperature range under the atmospherical presure. The conduction band edge and the valence band edge are very close to each other at the three L points within the first Brillouin zone such that they are strongly coupled, and the energy band at the L points is non-parabolic dispersive. At certain conditions, the conduction band edge and the valence band edge will touch each other at the three L points, and the dispersion relation at the L points will become linear, which leads to the formation of three-dimensional Dirac points. By synthesizing Bi1-xSbx thin films, we have two more parameters to control the band structure, namely film thickness and growth orientation. We have developed the iterative-two-dimensional-two-band model to study the two- dimensional L-point non-parabolically dispersive electronic band structure of the Bi1-xSbx thin films system. The Lax model based on the k - p model describes the the L-point non- parabolic dispersion relations very well consistent with experimental results for bulk bis- muth. Because the band gap is narrow, the number of bands that are needed in the per- turbation is small. A satisfactory representation over a limited region of k-space has been archived in terms of the two coupled bands, which means that the Hamiltonian could be approximately diagonalized, and which gives a very simple form for the Lax model. In the thin films system, the anylysis is more different due to the non-parabolic quantum confinement effect. The L-point band gap is increased in a thin film compared to the L-point band gap in a bulk system. As the film thickness decreases, the L-point band gap increases. The L-point band gap and the L-point inverse-effective-mass tensor are coupled together and are different from the values for the bulk materials. Thus, iterative procedures are employed for getting the accurate values of the L-point band gap and its corresponding inverse-effective-mass tensor. The iterative-two-dimensional-two-band model can be gen- eralized to study other two-dimensional narrow-gap systems, for example lead telluride thin films and silicon-germanium alloys thin films. The model can also be modified to study one-dimensional narrow-gap systems such as Bi1-xSbx nanowires. The electronic band structure of Bi1-xSbx thin films for different growth orientations are studied. The results shows that by growing the Bi1-xSbx thin film normal to a low symmetry crystalline direction other than the trigonal axis, the three-fold symmetry of the three L points in the bulk Bi1-xSbx can be broken. Specifically, by growing the Bi1-xSbx thin film along the bisectrix axis, anisotropic single-Dirac-cone can be constructed at the L point associated with this bisectrix axis. In similar ways, by choosing proper antimony compositions, growth orientations and film thicknesses, a large variety of Dirac-cone materials can be constructed based on the Bi1-xSbx thin films system, including single-Dirac-cone materials with different aisotropies, bi-Dirac-cone materials, tri-Dirac-cone materials, quasi-Dirac-cone materials and semi- Dirac-cone materials. / by Shuang Tang. / S.M.

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.