Browsing by Subject "Silicon"
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Item Advanced semi-classical Monte Carlo modeling of Si, Ge, InGaAs, and MoS₂ n-channel FETs for novel CMOS(2020-02-03) Bhatti, Aqyan Ahmed; Banerjee, Sanjay; Register, Leonard F.Scaling-down of silicon (Si) based complementary-metal-oxide-semiconductor (CMOS) technologies are approaching material limits. For high-performance applications, high thermal velocity channel materials, such as indium-gallium-arsenide (InGaAs) and germanium (Ge), are viable alternatives to Si to extend the limits of CMOS downscaling. The unique mechanical and electrical properties of two-dimensional atomic crystals, such as single-layer molybdenum disulfide (MoS₂), combined with soft, flexible, and curvilinear substrates, enable new device functionalities and concepts in the field of low-power flexible electronics not achievable with Si channels. While the intrinsic electron mobility of MoS₂ is rather low, strain engineering may provide a pathway for improving electron transport. Silicon, InGaAs, Ge, and MoS₂ n-channel MOSFETs were explored via first-principles computational tools including density functional theory and particle-based ensemble semi-classical Monte Carlo methods to better understand and enable the rational design of end-of-the-roadmap CMOS and potential beyond-CMOS technologies. The impact of contact geometry and transmissivity and gate length scaling on quasi-ballistic nanoscale Si, Ge, and InGaAs n-channel FinFETs was studied. FinFETs with end, saddle/slot, and raised source and drain contacts and the same saddle/slot contact geometry with different gate lengths, according to the projections of industry roadmaps, were simulated. Simulated Si FinFETs exhibited relatively limited degradation in performance due to non-ideal contact transmissivities, more limited sensitivity to contact geometry with non-ideal contact transmissivities, some contact-related advantage for Si 〈110〉 channel devices, and limited sensitivity to gate length scaling. Simulated InGaAs FinFETs were highly sensitive to modeled contact geometry, specific contact resistivity, the band structure model, and gate length scaling. Simulated Ge FinFETs showed substantial degradation due to non-ideal contact transmissivities, sensitivity to gate length scaling, and a large orientation-related advantage for Ge 〈110〉 channel devices. The impact of tensile strain on the intrinsic performance limits of monolayer MoS₂ n-channel MOSFETs was studied. 200 and 15 nm gate length MoS₂ MOSFETs with end contacts subject to different types and amounts of strain were simulated. Simulated MoS₂ MOSFETs displayed improved performance with strain due to lower effective mass and larger inter-valley separation, which is largely reduced due to non-ideal contact transmissivities.Item Alignment effects of hydrogen reflection by Si(100)(2015-08) Stevens, Robert Gardiner; Sitz, Greg Orman; Fink, Manfred; Downer, Michael; Keto, John; Henkelman, GraemePrevious scattering experiments have shown a dependence of reflectivity on the alignment of the angular momentum vector of molecular hydrogen incident on a surface of Pd(111), Isakson 2001. In these past experiments orthohydrogen was preferentially aligned relative to the surface by preparing the J=3 state. This J=3 state has multiple values for its magnetic quantum number and therefore there is a distribution in J=3 alignment. Within I will discuss the design of a new laser that can efficiently pump parahydrogen from the J=0 state, which has no distribution in magnetic quantum numbers and therefore the resulting J=2 state can be aligned with much better precision. Evidence suggests (Isakson, 2001) that the perpendicular alignment of the angular momentum vector with respect to the surface (helicopter-type motion) was less reflective than the parallel alignment (cartwheeler-type motion) for orthohydrogen when interacting with Pd(111). Within a study of the preservation of these alignments, both helicopter and cartwheeler, for aligned J=3 initial states will be attempted as they reflect off of the unreactive Si(100) surface. This study will strongly influence future studies, ones off of reactive surfaces such as Pd(111), and dictate if loss of alignment can explain the perceived decreased reflectivity.Item Assembly of colloidal nanocrystals into phospholipid structures and photothermal materials(2012-08) Rasch, Michael; Korgel, Brian Allan, 1969-There has been growing interest in developing colloidal metal and semiconductor nanocrystals as biomedical imaging contrast agents and therapeutics, since light excitation can cause the nanocrystals to fluoresce or heat up. Recent advances in synthetic chemistry produced fluorescent 2-4 nm diameter silicon and 1-2 nm diaemeter CuInSSe nanocrystals, as well as 16 nm diameter copper selenide (Cu₂₋[subscript x]Se) nanocrystals exhibiting strong absorbance of near infrared light suitable for biomedical applications. However, the syntheses yield nanocrystals that are stabilized by an adsorbed layer of hydrocarbons, making the nanocrystals hydrophobic and non-dispersible in aqueous solution. Encapsulating these nanocrystals in amphiphilic polymer micelles enables the nanocrystals to disperse in water. Subsequently, the Si nanocrystals were injected into tissue to demonstrate fluorescence imaging, the photothermal transduction efficiency of copper selenide nanocrystals was characterized in water, and the copper selenide nanocrystals were used enhance the photothermal destruction of cancer cells in vitro. The polymer-encapsulated copper selenide nanocrystals were found to have higher photothermal transduction efficiency than 140 nm diameter Au nanoshells, which have been widely investigated for photothermal therapy. Combining the optical properties of metal and semiconductor nanocrystals with the drug-carrying capability of lipid vesicles has received attention lately since it may create a nanomaterial capable of performing simultaneous drug delivery, optical contrast enhancement, and photo-induced therapy. Hydrophobic, dodecanethiol-coated Au nanocrystals were dispersed in water with phosphatidylcholine lipids and characterized using cryo transmission electron microscopy. 1.8 nm diameter Au nanocrystals completely load the bilayer of unsaturated lipid vesicles when the vesicles contain residual chloroform, and without chloroform the nanocrystals do not incorporate into the vesicle bilayer. 1.8 nm Au nanocrystals dispersed in water with saturated lipids to form lipid-coated nanocrystal agglomerates, which sometimes adhered to vesicles, and the shape of the agglomerates varied from linear nanocrystal chains, to flat sheets, to spherical clusters as the lipid fatty acid length was increased from 12 to 18 carbons. Including squalene formed lipid-stabilized emulsion droplets which were fully loaded with the Au nanocrystals. Results with 4.1 nm Au and 2-3 nm diameter Si nanocrystals were similar, but these nanocrystals could not completely load the bilayers of unsaturated lipids.Item Atomic-scale modeling and experimental studies for dopants and defects in Si and SiGe nano-scale CMOS devices(2010-05) Kim, Yonghyun; Banerjee, Sanjay; Kirichenko, Taras A.; Lee, Jack Chung-Yeung; Register, Leonard F.; Tutuc, Emanuel; Henkelman, GraemeContinued scaling of CMOS devices with Si and SixGe1-x down to 22 nm design node or beyond will require the formation of ever shallower and more abrupt junctions with higher doping levels in order to manage the short channel effects. With the increasing importance of surface proximity and stress effects, the lateral diffusion in gate-extension overlap region strongly influences both threshold voltage roll-off degradation and DIBL increase by requiring an optimized abruptness and diffusion for better device performance. Therefore, the detailed understanding of defect-dopant interactions in the disordered and/or strained systems is essential to develop a predictive kinetic model for the evolution of dopant concentration and electrical activation profiles. Our density functional theory calculations provide the guidance for experimental designs to realize ultra-shallow junction formation required for future generations of nano-scale CMOS devices. Few systematic studies in epitaxially-grown SixGe1-x channel CMOS have been reported. The physical mechanisms of boron diffusion in strained SixGe1-x/Si heterojunction layers with different SixGe1-x layer thicknesses and Ge content (>50%) are addressed, especially with high temperature annealing. In addition, the effects of the fluorine incorporated during BF2 implant on boron diffusion are investigated to provide more insight into short channel device design. In this study, we investigate how short channel margins are affected by Ge mole fraction and SixGe1-x layer thickness in a compressively strained SixGe1-x/Si heterojunction PMOS with high temperature annealing. Series resistance characterization in S/D extension region and gate oxide interface trap characterization for Si, SixGe1-x, and Ge nMOSFETs are done. TCAD device simulation is also performed to evaluate which distributions of interface traps will significantly affect the electrical characteristics such as flatband voltage (VFB) shift and threshold voltage (Vth) shift based on capacitance-voltage (CV) and current-voltage (IV) curves. n+/p and p+/n diode structures are studied in order to decouple the electrical characteristics from the gated-diode (GD) MOSFETs. With the extraction of S/D series resistance from various channel lengths, possible reasons for performance degradation in SixGe1-x and Ge nMOSFETs, based on simulations, are proposed.Item Chemical modification of nanocolumnar semiconductor electrodes for enhanced performance as lithium and sodium-ion battery anode materials(2014-08) Abel, Paul Robert; Mullins, C. B.Chemical EngineeringItem Colloidal nanocrystals with near-infrared optical properties : synthesis, characterization, and applications(2011-12) Panthani, Matthew George; Korgel, Brian Allan, 1969-; Dodabalapur, Ananth; Chelikowsky, James; Mullins, C. Buddie; Manthiram, ArumugamColloidal nanocrystals with optical properties in the near-infrared (NIR) are of interest for many applications such as photovoltaic (PV) energy conversion, bioimaging, and therapeutics. For PVs and other electronic devices, challenges in using colloidal nanomaterials often deal with the surfaces. Because of the high surface-to-volume ratio of small nanocrystals, surfaces and interfaces play an enhanced role in the properties of nanocrystal films and devices. Organic ligand-capped CuInSe2 (CIS) and Cu(InXGa1-X)Se2 (CIGS) nanocrystals were synthesized and used as the absorber layer in prototype solar cells. By fabricating devices from spray-coated CuInSe nanocrystals under ambient conditions, solar-to-electric power conversion efficiencies as high as 3.1% were achieved. Many treatments of the nanocrystal films were explored. Although some treatments increased the conductivity of the nanocrystal films, the best devices were from untreated CIS films. By modifying the reaction chemistry, quantum-confined CuInSeXS2-X (CISS) nanocrystals were produced. The potential of the CISS nanocrystals for targeted bioimaging was demonstrated via oral delivery to mice and imaging of nanocrystal fluorescence. The size-dependent photoluminescence of Si nanocrystals was measured. Si nanocrystals supported on graphene were characterized by conventional transmission electron microscopy and spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM). Enhanced imaging contrast and resolution was achieved by using Cs-corrected STEM with a graphene support. In addition, clear imaging of defects and the organic-inorganic interface was enabled by utilizing this technique.Item A comprehensive study of 3D nano structures characteristics and novel devices(2008-12) Zaman, Rownak Jyoti; Banerjee, SanjaySilicon based 3D fin structure is believed to be the potential future of current semiconductor technology. However, there are significant challenges still exist in realizing a manufacturable fin based process. In this work, we have studied the effects of hydrogen anneal on the structural and electrical characteristics of silicon fin based devices: tri-gate, finFET to name a few. H₂ anneal is shown to play a major role in structural integrity and manufacturability of 3D fin structure which is the most critical feature for these types of devices. Both the temperature and the pressure of H₂ anneal can result in significant alteration of fin height and shape as well as electrical characteristics. Optimum H₂ anneal is required in order to improve carrier mobility and device reliability as shown in this work. A new hard-mask based process was developed to retain H₂ anneal related benefit while eliminating detrimental effects such as reduction of device drive current due to fin height reduction. We have also demonstrated a novel 1T-1C pseudo Static Random Access Memory (1T-1C pseudo SRAM) memory cell using low cost conventional tri-gate process by utilizing selective H₂ anneal and other clever process techniques. TCAD-based simulation was also provided to show its competitive advantage over other types of static and dynamic memories in 45nm and beyond technologies. A high gain bipolar based on silicon fin process flow was proposed for the first time that can be used in BiCMOS technology suitable for low cost mixed signal and RF products. TCAD-based simulation results proved the concept with gain as high 100 for a NPN device using single additional mask. Overall, this work has shown that several novel process techniques and selective use of optimum H₂ anneal can lead to various high performance and low cost devices and memory cells those are much better than the devices using current conventional 3D fin based process techniques.Item Controlled synthesis and characterization of silicon nanocrystals(2004) Pell, Lindsay Erin; Korgel, Brian AllanIn response to the demand for shrinking feature sizes and faster electronics, many resources have been dedicated to the research of nanotechnology. At present, silicon is undoubtedly the building block and key to the microelectronics world. In its bulk form, silicon is an inefficient emitter in the infrared, but as its dimensions shrink to the nanoscale, silicon exhibits unique optical and electrical properties such as size tunable photoluminescence. The more successful methods of the synthesis of silicon nanocrystals include laser ablation and silane pyrolysis; however these methods offer little in the way of particle stabilization which would prevent oxidation and allow for manipulation through dispersion in organic solvents. A novel supercritical fluid synthesis is investigated with respect to various silicon precursors such as diphenylsilane, silicon tetrachloride and trisilane. The electrochemical and luminescent properties of silicon nanocrystals, synthesized via the thermal decomposition of diphenylsilane, were studied. Differential pulse voltammetry of silicon nanocrystals in DMF and acetonitrile exhibit quantized double layer charging as previously reported for Au and CdS nanocrystals. Additionally, electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redoxactive coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak red shifted from the photoluminescence maximum. Single nanocrystal photoluminescence was investigated via Argon laser excitation and confocal microscopy. The single nanocrystals demonstrate stochastic single-step “blinking” behavior and size-dependent PL spectra with line widths approximately only three times greater than those measured for CdSe nanocrystals at room temperature. Investigation of trisilane as a viable silicon precursor in a supercritical fluid synthesis led to the formation of well formed, sub-micron, amorphous silicon colloids in high yield. Manipulation of temperature, pressure and precursor concentration allowed for the synthesis of amorphous silicon particles 60-400 nm in diameter. Polydisperse samples exhibited two dimensional, longviii range orientational order in the absence of translational order which has been compared to the Reverse Brazil Nut Effect. Additionally, metal induced crystallization was observed in amorphous silicon particles annealed in a vacuum evaporator.Item Design, fabrication, and analysis of enhanced mobility silicon germanium transistors(2001-08) Kim, Taehoon; Banerjee, SanjaySilicon-germanium is a very compatible material with silicon. It can improve the performance of the current silicon-based semiconductor devices. A temperature measurement system based on infrared light absorption by the silicon wafer was constructed for a Rapid Thermal Processing Chemical Vapor Deposition system. The details of the temperature measurement system are described here. This system can provide very sensitive temperature measurement for the important 650 – 850 °C range. A relaxed silicon-germanium structure with very smooth surface was grown successfully using this temperature measurement system. A new way to improve the growth of the structure was found. It was also found what the optimum temperature condition for the growth of the structure should be. MOSFETs based on silicon-germanium were fabricated and measured. PMOSFET with a buried channel of silicon-germanium-carbon was fabricated vii and measured to quantify its characteristics. A new method to calculate hole mobility of a buried channel of silicon-germanium-carbon has been proposed. This method requires the low temperature measurement of the device and computer simulation of the device. When this method was used for our PMOSFET, the result successfully revealed hole mobility characteristics of silicon-germanium-carbon. This study also could quantify these characteristics using the well-known Lombardi mobility model for silicon. This device study demonstrated enhanced hole mobility for a certain range of a germanium in silicon-germanium-carbon.Item Design, fabrication, and testing of a MEMS z-axis Directional Piezoelectric Microphone(2012-05) Kirk, Karen Denise; Hall, Neal A.; Neikirk, Dean P.Directional microphones, which suppress noise coming from unwanted directions while preserving sound signals arriving from a desired direction, are essential to hearing aid technology. The device presented in this paper abandons the principles of standard pressure sensor microphones, dual port microphones, and multi-chip array systems and instead employs a new method of operation. The proposed device uses a lightweight silicon micromachined structure that becomes “entrained” in the oscillatory motion of air vibrations, and thus maintains the vector component of the air velocity. The mechanical structures are made as compliant as possible so that the motion of the diaphragm directly replicates the motion of the sound wave as it travels through air. The microphone discussed in this paper achieves the bi-directionality seen in a ribbon microphone but is built using standard semiconductor fabrication techniques and utilizes piezoelectric readout of a circular diaphragm suspended on compliant silicon springs. Finite element analysis and lumped element modeling have been performed to aid in structural design and device verification. The proposed microphone was successfully fabricated in a cleanroom facility at The University of Texas at Austin. Testing procedures verified that the resonant frequency of the microphone, as expected, was much lower than in traditional microphones. This report discusses the theory, modeling, fabrication and testing of the microphone.Item Developing and implementing a Raman NSOM for the characterization of semiconductor materials(2010-05) Furst-Pikus, Greyhm Matthew; Campion, Alan; Barbara, Paul; Mullins, C. B.; Stevenson, Keith J.; Vanden Bout, David A.We have designed and constructed a novel Raman near-field scanning optical microscope (NSOM) and evaluated its performance characteristics with the goal of characterizing the strain in nanoscopic silicon structures. The Raman NSOM was built around a commercial Raman microscope to which a custom built stage was added to provide precise control over the tip position above the sample (z) using shear-force microscopy feedback as well as sample scanning in the x-y plane. The motion control axes were calibrated to better than 1 nm in z and approximately 20 nm in x and y. The NSOM provides both topographical images and Raman mapping with a lateral spectral resolution of 150-300 nm. The experiments described herein were enabled by gold-coated chemically etched NSOM tips with aperture diameters ranging between 60 and 150 nm. The sensitivity of the instrument was demonstrated by the high signal-to-noise ratios observed for Raman scattering by diamond and silicon in reflection mode. Spatial resolution and spectral sensitivity were demonstrated by obtaining well-resolved tip-sample separation curves that provide an accurate estimate of tip aperture size during an experiment.Item Epioptics of stepped silicon surfaces(2011-05) Ehlert, Robert; Downer, Michael Coffin; Demkov, Alexander A.; Ekerdt, John G.; Fink, Manfred; Li, XiaoqinSpectroscopic second-harmonic generation (SHG) and reflectance-anisotropy spectroscopy (RAS) are used to probe molecular adsorption on clean reconstructed single-domain stepped Si(001) in ultra-high vacuum (UHV). We implement a simplified bond hyperpolarizability model (SBHM) as a common microscopic analysis for SHG and RAS. Three different scenarios are studied: (i) The dissociative adsorption of molecular hydrogen on dangling bonds of D[subscript B] step-edges. (ii) Structural changes to rebonded r-D[subscript B] steps induced by exposure to atomic hydrogen. (iii) The adsorption of cyclopentene on Si(001)(2x1) terrace dimers in a [2+2] cycloaddition pathway. Using the SBHM we develop a new optical fingerprinting method to isolate, identify and monitor individual types of bonds (e.g. dimers, rebonds, dangling bonds, backbonds) and their chemical activity on a single-domain stepped Si(001) surface using nonresonant, but rotationally-anisotropic, second-harmonic generation (RA-SHG). The methods presented here will be applicable to many material systems and allow to track, in-situ and in real-time, the chemical action of adsorbates on surfaces.Item Epitaxial germanium via Ge:C and its use in non-classical semiconductor devices(2015-12) Mantey, Jason Christopher; Banerjee, Sanjay; Lee, Jack C; Register, Leonard F; Akinwande, Deji; Ferreira, Paulo JThe microelectronics industry has been using Silicon (Si) as the primary material for complementary metal-oxide-semiconductor (CMOS) chip fabrication for more than six decades. Throughout this time, these CMOS devices have gotten exponentially smaller, faster, and cheaper. While new materials and fabrication processes have been slowly added over the years, the CMOS device of today is largely the same as it was decades ago. However, field-effect transistors (FETs) have now scaled so far that Si is approaching physical limits. Thus, new channel materials and new fundamental device structures are being investigated to replace traditional CMOS. Germanium is one of the prime candidates to replace Si in the FET channel, with its increased electron and hole mobilities compared to Si. Perhaps more importantly, it is compatible with the existing Si manufacturing techniques by epitaxially growing thin layers of Ge crystal on the starting Si wafer. Because these two crystals do not share a lattice constant, there will inevitably be crystal defects in the thin Ge layer that can be catastrophic for device functionality. Several approaches have been introduced to reduce defects, but most of them are wastefully thick (>1 um) or require complex manufacturing methods. In this work, we utilize an extremely thin (~10 nm) buffer layer of carbon-doped Ge (Ge:C) to grow Ge and SiGe layers for FET and virtual substrate applications with improved crystalline quality and reduced surface roughnesses. These thin Ge layers not only offer new pathways for MOSFETs, but can also be used in non-classical structures. Semiconductor nanowires (NWs) and tunnel-FETs (TFETs) are two of the most promising device architectures, and both can be used with Ge. This dissertation presents a simulated Si/Ge heterostructure interface TFET that can be fabricated on a virtual substrate made with the Ge:C buffer layer. Detailed analysis on device operation is given. Also in this work is the fabrication process for individually addressable Ge NW-FETs. The NWs offer excellent electrostatic gate control through reduced dimensions and offer another potential pathway for Ge in a post-CMOS world.Item Fingerprinting of Si surface bonds using non-resonant optical second-harmonic generation(2018-08-21) Loumakos, Loucas K.; Downer, Michael Coffin; Demkov, Alexander A; Sitz, Greg O; Ekerdt, John G; Fink, ManfredModern electronic device structures require monitoring and control of surface structure at the atomic level during epitaxial growth. We demonstrate an optical fingerprinting technique that isolates, identifies and monitors individual types of bonds (e.g. step-edge rebonds, terrace dimers) and their chemical activity on a single-domain, vicinal Si(001) surface in ultra-high vacuum. The method uses optical second-harmonic generation (SHG) at a single wavelength, but at multiple incidence angles and polarizations (MAP) hence we call it SHG-MAP. SHG-MAP identifies bonds via the unique dependence of their SHG response on azimuthal sample rotation. Using a simplified bond hyper-polarizability model (SBHM), we developed an automated two-step algorithm for identifying all opportunities for isolating a certain bond type geometrically without multi-parameter fitting: firstly, the full parameter space is used to create a 4-D model of the expected macroscopic SHG radiation and secondly a search is preformed to isolate unique bond group contributions. We demonstrate SHG-MAP by monitoring adsorption of atomic hydrogen and chemical etching of rebonded r-D [subscript B] steps on clean vicinal Si(001) in ultra high vacuum.Item First principles study of point-like defects and impurities in silicon, carbon, and oxide materials(2012-05) Kweon, Kyoung Eun, 1981-; Hwang, Gyeong S.; Banerjee, SanjaySince materials properties are determined by the interactions between the constituent atoms, an accurate description of the inter-atomic interactions is crucial to characterize and control material properties. Particularly, a quantitative understanding of the formation and nature of defects and impurities becomes increasingly important in the era of nanotechnology, as the imperfections largely influence many properties of nanoscale materials. Indeed, due to its technological importance and scientific interest, there have been significant efforts to better understand their behavior in semiconductors and oxides, and their interfaces, yet many fundamental aspects are still ambiguous due largely to the difficulty of direct characterization. Hence, our study has focused on developing a better understanding of atomic-scale defects and impurities using first principles quantum mechanical calculations. In addition, based on the improved understanding, we have attempted to address some engineering problems encountered in the current technology. The first part of this thesis focuses on mechanisms underlying the transient enhanced diffusion of arsenic (As) during post-implantation annealing by examining the interaction of As with vacancies in silicon. In the second part, we address some fundamental features related to plasma-assisted nitridation of silicon dioxide; this study shows that oxygen vacancy related defects play an important role in (experimentally observed) peculiar nitridation at the Si/SiO2 interface during post O2 annealing. In the third part, we examine the interaction between vacancies and dopants in sp2–bonded carbon such as graphene and nanotube, specifically the formation and dynamics of boron-vacancy complexes and their influence on the electrical properties of host materials. In the fourth part, we study the interfacial interaction between amorphous silica (a-SiO2) and graphene in the presence of surface defects in a-SiO2; this study shows possible modifications in the electronic structure of graphene upon the surface defect assisted chemical binding onto the a-SiO2 surface. In the last part, we examine the structural and electronic properties of bismuth vanadate (BiVO4) which is a promising photocatalyst for water splitting to produce hydrogen; this study successfully explains the underlying mechanism of the interesting photocatalytic performance of BiVO4 that has been experimentally found to strongly depend on structural phase and doping.Item First principles study of silicon-based nanomaterials for lithium ion battery anodes(2014-05) Chou, Chia-Yun Ph. D.; Hwang, Gyeong S.; Mullins, Charles B; Manthiram, Arunmugam; Ekerdt, John G; Stevenson, KeithSilicon (Si)-based materials have recently emerged as a promising candidate for anodes in lithium-ion batteries because they exhibit much higher energy-storage capacities than the conventional graphite anode. However, the practical use of Si is hampered by its poor cycleability; during lithiation, Si forms alloys with Li and undergoes significant structural and volume changes, which can cause severe cracking/pulverization and consequent capacity fading arising from the loss of electrical contacts. To overcome these drawbacks, many innovative approaches have been explored with encouraging results; however, many fundamental aspects of the lithiation behavior remain ambiguous. Hence, the focus of this work is to develop a better understanding of the lithiation process at the atomistic scale using quantum mechanical calculations. In addition, based on the improved understanding, we attempt to address the fundamental mechanisms behind the successful approaches to enhance the anode performance. To lay a foundation for the investigation of alloy-type anodes, in Chapter 3, we first examine how lithiation occurs in Si and the formation of crystalline and amorphous LixSi alloys (0 ≤ x ≤ 4); followed by assessing the lithiation-induced changes in the energetics, atomic structure, electronic and mechanical properties, and Li diffusivity. The same approach is then extended to analyze the lithiation behavior of germanium (Ge) and tin (Sn) for developing a generalized understanding on the Group IV alloy-type anodes. Along this comparative study, we notice a few distinguishing features pertain only to Si (or Ge), such as the facile Li diffusion in Ge and facet-dependent lithiation in Si, which are discussed in Chapter 4. Beyond the fundamental research, we also look into factors that may contribute to the improved anode performance, including (i) finetuning of the oxidation effects in Si-rich oxides, [alpha] -SiO [subscript 1/3] (Chapter 5), (ii) maximizing the surface effects through nano-engineered structures (Chapters 6 & 7), and finally (iii) the role of interface in Si-graphene (carbon) composites (Chapter 8).Item First-principles and kinetic Monte Carlo simulation of dopant diffusion in strained Si and other materials(2006) Lin, Li, 1973-; Banerjee, SanjayThe TCAD tools of today are based on the atomic mechanisms underlying particular processes. This allows the simulators to be more predictive and to be utilized for a wider range of device architectures. The models in the simulators are also becoming increasingly sophisticated as new physics and processes are incorporated. Current diffusion simulators include models for ion implantation, dopant and defect diffusion, point defect/dopant /extended defect interactions, diffusion transients, stress, electric fields, charged defects and more. The study for this Ph.D. focuses on dopant diffusion in strained Si for metal oxide semiconductor field effect transistor (MOSFET) by using first-principles and Kinetic Monte Carlo simulation. Tensile-strained silicon is a promising candidate for the channel of MOSFET due to the high mobility of the carriers. We have performed density functional theory based first-principles calculations to investigate the effect of biaxial strain on boron diffusion in Si by a more direct approach than previous researchers. The results suggest that tensile biaxial strain lowers the activation enthalpy, and furthermore causes an anisotropy of boron diffusion. On the contrary, compressive biaxial strain increases the activation enthalpy. We study strain effect in different charge systems. Basically the trend of decreasing diffusion barrier persists. The analysis of charged system is much more complicated than neutral system due to the well-known reason that strain can change the band structure of Si. A lot of experiments show that fluorine can suppress boron transient enhanced diffusion (TED) if it is co-implanted with boron, but the reason of this effect is not very clear yet. We propose our own theory about the effect of fluorine on boron diffusion based on the fluorine-vacancy complex theory. Fluorine-vacancy complex is found to be a trap for mobile boron atoms as well as Si interstitials by ab initio calculations. The suppression of boron diffusion is due to not only the lower Si interstitial concentration, but also the direct interaction between fluorine-vacancy complex and boron. Kinetic Monte Carlo simulations are performed to confirm this effect and agree with the experimental results. Furthermore, the same theory also explains why boron can enhance the diffusion of fluorine.Item First-principles atomistic modeling for property prediction in silicon-based materials(2010-12) Bondi, Robert James; Hwang, Gyeong S.; Mullins, C. B.; Ekerdt, John G.; Chelikowsky, James R.; Banerjee, Sanjay K.The power of parallel supercomputing resources has progressed to the point where first-principles calculations involving systems up to 10³ atoms are feasible, allowing ab initio exploration of increasingly complex systems such as amorphous networks, nanostructures, and large defect clusters. Expansion of our fundamental understanding of modified Si-based materials is paramount, as these materials will likely flourish in the foreseeable cost-driven future in diverse micro- and nanotechnologies. Here, density-functional theory calculations within the generalized gradient approximation are applied to refine configurations of Si-based materials generated from Metropolis Monte Carlo simulations and study their resultant structural properties. Particular emphasis is given to the contributions of strain and disorder on the mechanical, optical, and electronic properties of modified Si-based materials in which aspects of compositional variation, phase, strain scheme, morphology, native defect incorporation, and quantum confinement are considered. The simulation strategies discussed are easily extendable to other semiconductor systems.Item Functional oxide heterostructures on semiconductors(2013-08) Seo, Hosung; Demkov, Alexander A.Complex oxides exhibiting a wide variety of novel functional properties such as ferromagnetism and ferroelectricity have been extensively studied during the past decades. Recent advances in the field of oxide heteroepitaxy have made it possible to create and control hybrid oxide heterostructures with abrupt epitaxial interfaces. The oxide heteroepitaxy with the capability of controlling interface composition, strain, length scales, etc. has opened the totally new and exciting scientific avenue and has offered potential device applications to be explored. Epitaxial integration of functional oxides on semiconductor such as Si (001) and Ge(001) is of great interest, as it potentially leads to further technological development of these interesting oxide systems. In this dissertation, using density functional theory we explore physics and chemistry of novel oxide heterostructures and issues related to the integration of functional oxides on semiconductors. Oxide materials that are studied in this dissertation include polar LaAlO₃, high-k dielectric SrTiO₃, photocatalytic anatase TiO₂ and CoO, and strongly correlated magnetic oxide LaCoO₃.Item Germanium and silicon nanowires for use in water purification(2022-05-11) Sullivan, William (M.S. in chemical engineering); Korgel, Brain Allan, 1969-Germanium and silicon nanowires present an exciting opportunity for broadening the scope of membrane fouling mitigation research. Germanium nanowires provide a highly effective model system for investigating how to incorporate silicon nanowires into polymeric membranes, while providing relative ease in synthesis and workability compared to silicon nanowires. Silicon nanowires present an exciting area of investigation for fouling mitigation for two main reasons: they can be surface passivated to achieve desired chemical properties and they are photoactive. This work explores how to effectively incorporate germanium nanowires into polymeric membranes as a model to be used for silicon nanowires. Then the integration of silicon nanowires is further explored to determine the most effective methods of silicon nanowire incorporation into polymeric membranes. Successful integration of silicon nanowires into polymeric membrane systems is demonstrated, providing the groundwork for further exploration of the use of nanowires in water purification, specifically for fouling mitigation.
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