Browsing by Subject "silicon"

Now showing 1 - 3 of 3
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    Analysis of Droplets and Bubbles in Crystal Growth Processes: Migration and Engulfment
    (2024) Pawar, Swanand
    The crystal growth process for certain materials faces the problem of foreign particles, drops or bubbles getting lodged into the solid as it is grown. There can be significant risks associated with such impurities. In materials such as sapphire and silicon being grown from the melt, bubbles could enter the melt phase and make their way into the crystal by the process of engulfment. This is very commonly observed in sapphire crystals. There is also a chance for growth instabilities during certain solution growth processes, which could result in pockets of high solute concentration in certain places. This has been seen in scintillator crystals such as cadmium zinc telluride and even Arctic ice. This work is divided into two parts. In the first part, the engulfment of bubbles into a crystal is studied using detailed finite element modelling. The effect of various process parameters is studied and an effort is made to understand process conditions under which such engulfment can be avoided. In the second part, a simplified one-dimensional transport model is developed for the temperature gradient zone melting technique, in which a thermal gradient is applied across a solid to move any liquid drops stuck inside it to one end of the sample. Analytical expressions for drop size, speed and position as a function of time are derived for a drop undergoing such thermal migration. The derivation is also used to gain insight into process physics and some important time scales.
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    An in-situ analytical scanning and transmission electron microscopy investigation of structure-property relationships in electronic materials
    (2014-08) Wagner, Andrew
    As electronic and mechanical devices are scaled downward in size and upward in complexity, macroscopic principles no longer apply. Synthesis of three-dimensionally confined structures exhibit quantum confinement effects allowing, for example, silicon nanoparticles to luminesce. The reduction in size of classically brittle materials reveals a ductile-to-brittle transition. Such a transition, attributed to a reduction in defects, increases elasticity. In the case of silicon, elastic deformation can improve electronic carrier mobility by over 50%, a vital attribute of modern integrated circuits. The scalability of such principles and the changing atomistic processes which contribute to them presents a vitally important field of research. Beginning with the direct observation of dislocations and lattice planes in the 1950s, the transmission electron microscope has been a powerful tool in materials science. More recently, as nanoscale technologies have proliferated modern life, their unique ability to spatially resolve nano- and atomic-scale structures has become a critical component of materials research and characterization. Signals produced by an incident beam of high-energy electrons enables researchers to both image and chemically analyze materials at the atomic scale. Coherently and elastically-scattered electrons can be collected to produce atomic-scale images of a crystalline sample. New specimen stages have enabled routine investigation of samples heated up to 1000 °C and cooled to liquid nitrogen temperatures. MEMS-based transducers allow for sub-nm scale mechanical testing and ultrathin membranes allow study of liquids and gases. Investigation of a myriad of previously "unseeable" processes can now be observed within the TEM, and sometimes something new is found within the old. High-temperature annealing of pure a Si:H films leads to crystallization of the film. Such films provide higher carrier mobility compared to amorphous films, offering improved photovoltaic performance. The annealing process, however, requires exceptionally high temperature (> 600 °C) and time (tens of hours), limiting throughput and costing energy. In an effort to fabricate polycrystalline solar cells at lower cost, large (~30 nm) silicon nanocrystals were incorporated into hydrogenated amorphous silicon (a Si:H) thin films. When annealed, the embedded nanocrystals were expected to act as heterogeneous nucleation sites and crystallize the surrounding amorphous matrix. When observed in the TEM, an additional and unexpected event was observed. At the boundary between the nanocrystal and amorphous matrix, nanocavities were observed to form. Continued annealing resulted in movement of the cavities away from the nanocrystal while leaving behind a crystalline tail. The origins and fundamental mechanisms of this phenomenon were examined by in-situ heating TEM and ex-situ crystallographic TEM techniques. We demonstrate a mechanism of solid-phase crystallization (SPC) enabled by nanoscale cavities formed at the interface between an hydrogenated amorphous silicon film and embedded 30 nm to 40 nm Si nanocrystals. The nanocavities, 10 nm to 25 nm across, have the unique property of an internal surface that is part amorphous and part crystalline, enabling capillarity-driven diffusion from the amorphous to the crystalline domain. The nanocavities propagate rapidly through the amorphous phase, up to five times faster than the SPC growth rate, while "pulling behind" a crystalline tail. It is shown that twin boundaries exposed on the crystalline surface accelerate crystal growth and influence the direction of nanocavity propagation. HASH(0x7febe3ca40b8) The mechanical properties and mechanisms of plasticity in these same silicon nanocubes have also been investigated. The strain-dependent mechanical properties and the underlying mechanisms governing the elastic-plastic response are explored in detail. Elastic strains approaching 7% and flow stresses of 11 GPa were observed, significantly higher than that observed in other nanoscale volumes of Si. In-situ imaging revealed the formation of 5 nm dislocation embryos at 7% strain, giving way at 20% strain to continuous nucleation of leading partial dislocations with {111}-habit at the embryo surface.
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    Peatland Hydrological Dynamics in Northern Minnesota
    (2015-09) Roush, Benjamin
    I investigated peatland water table elevation responses to large precipitation events and long precipitation-free periods for a fen, poor fen, and bog, and pore water chemistry trends in a fen boundary zone, in northern Minnesota. Water table change compared to both precipitation and dry periods was slower in the fen than the poor fen or bog, a response attributed to connections between the fen and the regional groundwater aquifer. Water table change compared to larger dry periods remained consistent over a 51-year period and among peatlands. Calcium-silicon ratios in fen pore water were collected along transects perpendicular to the fen boundary. Larger calcium-silicon ratios at edge of the fen were interpreted as originating from a regional aquifer source, with minimal influence from vegetative calcium uptake and upland subsurface runoff. The extent of the fen-upland boundary zone was demarcated where calcium-silicon ratios matched average fen and stream outlet calcium-silicon ratios.