Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
10 results
Search Results
Item Exciton Photophysics at Fluorescent Quantum Defects(2018) Kim, Mijin; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluorescent quantum defect is an emerging synthetic structure that can be covalently attached to a semiconducting single-walled carbon nanotube. Incorporation of fluorescent quantum defect breaks the symmetry of carbon nanotubes at a defect center, creating new optically allowed, low-lying states in the electronic structure of carbon nanotube. Exciting electronic and optical properties arise from the defects, including the generation of new photoluminescence features, which can be used for applications, such as chemical sensing, bioimaging, and quantum light source. As excitons dominate the optical properties of carbon nanotubes, understanding the exciton photophysics in a defect-tailored carbon nanotube is essential to efficiently harness the emission properties of fluorescent quantum defects. In this dissertation, I aim to understand the exciton photophysics in fluorescent quantum defects in order to explain the origins and behavior of novel phenomena arising from them. First, the structure-property relationships of fluorescent quantum defects are discussed; these guide the systematic tuning of defect-induced emission and the binding energy of defect-trapped excitons. Then, the discussion moves to the exciton dynamics at fluorescent quantum defects. Particularly, I describe how the chemical nature of defects or the density of defects influences the thermal detrapping energy of excitons. The exciton-electron interaction at a fluorescent defect is also discussed. Our results suggest that a fluorescent quantum defect colocalizes an exciton and an electron as a tri-charge carrier and the brightening at the defect can be chemically tuned. Finally, I introduce super-resolved, hyperspectral photoluminescence spectroscopy, enabling both direct probing of a single fluorescent defect and the quantitative evaluation of the brightening of dark excitons.Item Electrical Properties of a Tube-in-a-Tube Semiconductor(2016) Ng, Allen Lee; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tube-in-a-tube (Tube^2) nanostructures were synthesized through the outer-wall selective covalent functionalization of double-walled carbon nanotubes (DWCNTs) at high functional densities. Upon functionalization, the properties of individual walls within the structure decouple resulting in an electrically insulating functional outer tube while the inner tube retains exceptional CNT properties. The exceptional electrical properties of Tube^2 semiconductor structures were demonstrated for applications that include molecular and biological sensors and patterning of CNTbased structures with electronic type specificity. Tube^2 thin film transistor (TFT) sensors exhibited simultaneous ultrahigh sensitivity and selectivity towards chemical and biological targets. Carboxylic acid terminated Tube^2 sensors displayed an NH3 sensitivity of 60 nM, which is comparable with small molecule aqueous solution detection using state-of-the-art TFT sensors while simultaneously attaining 6,000 times higher chemical selectivity towards a variety of amine containing analyte molecules over carboxylic acids. Similarly, 23-base ii oligonucleotide terminated Tube^2 sensors demonstrated concomitant sensitivity down to 5 nM towards their complementary sequence without amplification techniques and single mismatch selectivity without the use of a gate electrode. Unique sensor architectures can be designed with the requirement of a gate electrode, such as the creation of millimeter-scale point sensors. The optical features and unique structural features of Tube^2 thin films were also exploited to address the challenge of patterning CNT nanostructures with electronic type specificity. Patterned dot arrays and conductive pathways were created on an initially insulating Tube^2 thin film by tuning the resonance of the direct-writing laser with the electronic type of the inner tube (i.e., metallic or semiconducting). The successful patterning of Tube^2 thin films was unambiguously confirmed with in situ Raman spectral imaging and electrical characterization. Furthermore, a hybrid 2-D carbon nanostructure comprised of a functionalized graphene that covers a semiconducting (6,5) SWCNT network (fG/sSWCNT) was developed. The hybrid fG/sSWCNT nanostructure exhibits similar structural and electrical properties as a semiconducting Tube^2 thin film, but possesses a transconductance that is an order of magnitude larger than Tube^2 and ON/OFF ratios as high as 5400 without the useful of further processing steps such as electrical breakdown.Item PLASMA INTERACTIONS WITH MASKING MATERIALS FOR NANOFABRICATION(2011) Weilnboeck, Florian; Oehrlein, Gottlieb; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma-based transfer of patterns into other materials is a key process for production of nano-scale devices used in micro-electronic technology. With the continuously decreasing feature-size of integrated circuits, manufacturing tolerances are becoming increasingly smaller and complex interactions of plasmas and patterned mask materials require an atomistic understanding to meet future processing tolerances. In this work, we investigated how plasma-material interactions in typical low-k pattern transfer processes depend on individual plasma components and properties of polymeric and metallic masks. First, we studied modifications of 193nm and 248nm photoresist (PR) by plasma ultraviolet/vacuum ultraviolet (UV) radiation, quantifying contributions of plasma radiation to the overall material modifications for direct interaction with plasma. Energetic ions (~125 eV) led to rapid (~3-5 s) formation of a graphitic ion-crust (~1.8 nm) and introduced together with simultaneous UV modifications of the material bulk significantly higher roughness for 193nm PR (~6 nm) than for 248nm PR (~1 nm). During ion-crust formation, 193nm PR softened by chain-scissioning and pendant group detachment in a depth of ~60 nm by UV radiation, while 248nm PR was radiation stable showing surface-close cross-linking (~4 nm). Pretreating 193nm PR with a radiationdominated He plasma and introducing UV modifications before ion-crust formation in the subsequent plasma etch reduces synergistic roughness formation as explained by wrinkling theory. Second, we studied interactions of fluorocarbon (FC) plasmas with Ti and TiN and compared these with organosilicate glass (OSG). Metal hardmasks are expected to provide improved etching selectivity (ES) and low-k damage compared to PR during pattern transfer. Erosion stages and dependencies of etch rates (ER) on FC layer thickness and energy deposition by ions were identified. ES were low (~4-8) in the diffusion-limited regime (thick FC layers) where OSG experienced strong reduction in ER, but high (up to 15) in the chemical sputtering regime (thin FC layers) at low ion energies where removal of Ti etch products was limited. TiN exhibited higher ER and lower ES than Ti due to increased surface reactivity after rapid removal of N. Overall, findings give directions for rational design of masking materials and plasma discharges for future nanofabrication pattern transfer processes.Item MULTI-SCALE MODELING AND COMPUTATIONS(2009) Zhang, Linbao; Liu, Jian-Guo; Mathematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the rarefied gas dynamics, the classic kinetic models are more accurate and complicated, while the fluid models are much simpler but fail in some cases. In this thesis, we propose a new local up-scaling model to couple Euler equations with the kinetic model when the previous up-scaling model in [19] does not apply, e.g. when the Boltzmann equation is solved by the particle method, like DSMC. By means of the first order Chapman-Enskog expansion we propose a new NSLU model to couple the Navier-Stokes equations with the kinetic models. We also propose the zero-moment projection based on the macro-micro decomposition ([34]) to correct the non-fluid part in the up-scaling models. Numerical tests of these local up-scaling models have been done in various multi-scale problems, including the Jin-Xin relaxation model for the traveling shock, 1D1D BGK model for the dynamics of a small perturbation of an equilibrium, 1D3D BGK model for the stationary shock and the simulation of a planar Couette flow by direct simulation of Monte Carlo (DSMC) for the Boltzmann equation. The implicit-explicit scheme for the relaxation models is applied, which is shown to preserve the positiveness of the distribution function, the conservation laws and entropy inequality. Numerical results show that the zero-projection is necessary to ensure the stability and accuracy for the up-scaling models, especially when non-kinetic schemes are applied in the moment equations. NSLU model must be applied to replace the up-scaling model in [19] if the macroscopic approximation is the viscous fluid. The similar scaling exists in the relaxation-time model for the semiconductor device when electric field is low. The DrDiLU model based on drift-diffusion model for the diode is proposed which is similar to NSLU model for the rarefied gas. Numerical experiments show it is stable and accurate compared with the results from the relaxation-time model.Item INTERFACE EFFECTS ON NANOELECTRONICS(2009) Conrad, Brad Richard; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanoelectronics consist of devices with active electronic components on the nanometer length scale. At such dimensions most, if not all, atoms or molecules composing the active device region must be on or near a surface. Also, materials effectively confined to two dimensions, or when subject to abrupt boundary conditions, generally do not behave the same as materials inside three dimensional, continuous structures. This thesis is a quantitative determination of how surfaces and interfaces in organic nanoelectronic devices affect properties such as charge transport, electronic structure, and material fluctuations. Si/SiO2 is a model gate/gate dielectric for organic thin film transistors, therefore proper characterization and measurement of the effects of the SiO2/organic interface on device structures is extremely important. I fabricated pentacene thin film transistors on Si/SiO2 and varied the conduction channel thickness from effectively bulk (~40nm) to 2 continuous conducting layers to examine the effect of substrate on noise generation. The electronic spectral noise was measured and the generator of the noise was determined to be due to the random spatial dependence of grain boundaries, independent of proximity to the gate oxide. This result led me to investigate the mechanisms of pentacene grain formation, including the role of small quantities of impurities, on silicon dioxide substrates. Through a series of nucleation, growth and morphology studies, I determined that impurities assist in nucleation on SiO2, decreasing the stable nucleus size by a third and increasing the overall number of grains. The pentacene growth and morphology studies prompted further exploration of pentacene crystal growth on SiO2. I developed a method of making atomically clean ultra-thin oxide films, with surface chemistry and growth properties similar to the standard thick oxides. These ultra-thin oxides were measured to be as smooth as cleaned silicon and then used as substrates for scanning tunneling microscopy of pentacene films. The increased spatial resolution of this technique allowed for the first molecular resolution characterization of the standing-up pentacene crystal structure near the gate dielectric, with molecules oriented perpendicular to the SiO2 surface. Further studies probed how growth of C60 films on SiO2 and pentacene surfaces affected C60 morphology and electronic structure to better understand solar cell heterojunctions.Item Spectroscopic & Structural Investigation of the Thermal Evolution of Undoped and Phosphorus Doped ZnO and Implications for Unipolar and Bipolar Device Fabrication(2006-11-28) Pugel, Diane; Venkatesan, Thirumalai; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The main objective of this dissertation was to explore the structural, electrical, and optical properties of undoped and extrinsically doped thin film and single crystal ZnO under various growth and processing thermal conditions in the context of understanding intrinsic defect formation and extrinsic dopant incorporation. Undoped (000-1) ZnO thin films were grown by on-axis RF sputter deposition at a range of temperatures and in oxygen-rich and oxygen-deficient atmospheres. For comparison, ZnO single crystals were thermally processed under similar conditions. Samples were examined for temperature-dependent effects on surface and bulk properties for temperature-dependent changes in structure, semiconducting band gap, and Schottky barrier height in order to isolate temperature regions that may support conditions that minimize defect production. Phosphorus-doped (000-1) ZnO thin films were grown and doped ZnO crystals were prepared under the same conditions described above. Phosphorus was selected as a potential p-type dopant due to reduced concerns for outdiffusion of the dopant from the host crystal. Films were grown via sputter deposition. Crystals were prepared via planar (vapor) doping. By investigating undoped ZnO, this work expands current understanding of the fabrication of ZnO-based unipolar devices, such as Schottky diodes. To this end, the structure (surface and bulk), composition, optical, and electrical properties of ZnO single crystals were investigated as a function of annealing temperature and atmosphere. Near-surface diffusion of Zn atoms was found to influence the Schottky barrier height. Annealing conditions that minimize donor defect states, as detected by photoluminescence, were found. By investigating extrinsically doped ZnO, this work sheds light on the feasibility of bipolar device fabrication using ZnO. For film growth, we found a narrow window of deposition temperature and pressure that optimizes crystallinity and transmission in the ultraviolet spectrum for the preparation of p-type doped material. For single crystals, we found optimal conditions for p-type doping ZnO using phosphorus vapor. Results from Hall measurements of these doped single crystals allowed for a revision of the limits defined by previously existing experimental results in the "failure to dope" rule for ZnO.Item Quantum Transport in Nanoscale Semiconductor Devices(2006-08-02) Jones, Gregory Millington; Yang, Chia-Hung; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Because of technological advancement, transistor dimensions are approaching the length scale of the electron Fermi wavelength, on the order of only nanometers. In this regime, quantum mechanical phenomena will dominate electron transport. Using InAs single quantum wells, we have fabricated Y-shaped electron waveguides whose lengths are smaller than the elastic mean free path. Electron transport in these waveguides is ballistic, a quantum mechanical phenomenon. Coupled to the electron waveguide are two gates used to coherently steer the electron wave. We demonstrate for the first time that gating modifies the electron's wave function, by changing its geometrical resonance in the waveguide. Evidence of this alteration is the observation of anti-correlated, oscillatory transconductances. Our data provides direct evidence of wavefunction steering in a transistor structure and has applications in high-speed, low-power electronics. Quantum computing, if realized, will have a significant impact in computer security. The development of quantum computers has been hindered by challenges in producing the basic building block, the qubit. Qubit approaches using semiconductors promise upscalability and can take the form of a single electron transistor. We have designed, fabricated, and characterized single electron transistors in InAs, and separately in silicon, for the application of quantum computing. With the InAs single electron transistor, we have demonstrated one-electron quantum dots using a single-top-gate transistor configuration on a composite quantum well. Electrical transport data indicates a 15meV charging energy and a 20meV orbital energy spacing, which implies a quantum dot of 20nm in diameter. InAs is attractive due to its large electron Landé g-factor. With the silicon-based single electron transistor, we have demonstrated a structure that is similar to conventional silicon-based metal-oxide-semiconductor field effect transistors. The substrate is undoped and becomes insulating at low temperatures. There are two layers of gates that when properly biased define the single electron transistor potential profile. The measured stability chart at 4.2K indicates a charging energy of 18meV. Our silicon-based single electron transistor is promising, because spin coherence times in silicon are orders of magnitude longer than those in GaAs.Item DEVELOPMENT OF AN OBJECT-ORIENTED FRAMEWORK FOR MODULAR CHEMICAL PROCESS SIMULATION WITH SEMICONDUCTOR MANUFACTURING APPLICATIONS(2006-04-27) Chen, Jing; Adomaitis, Raymond A.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chemical Vapor Deposition (CVD) processes constitute an important unit operation for micro electronic device fabrication in the semiconductor industry. Simulators of the deposition process are powerful tools for understanding the transport and reaction conditions inside the deposition chamber and can be used to optimize and control the deposition process. This thesis discusses the development of a set of object-oriented modular simulation tools for solving lumped and spatially distributed models generated from chemical process design and simulation problems. The application of object-oriented design and modular approach greatly reduces the software development cycle time associated with designing process systems and improves the overall efficiency of the simulation process. The framework facilitates an evolutionary approach to simulator development, starting with a simple process description and building model complexity and testing modeling hypothesis in a step-by-step manner. Modularized components can be easily assembled to form a modeling system for a desired process. The framework also brings a fresh approach to many traditional scientific computing procedures to make a greater range of computational tools available for solving engineering problems. Two examples of tungsten chemical vapor deposition simulation are presented to illustrate the capability of the tools developed to facilitate an evolutionary simulation approach. The first example demonstrates how the framework is applied for solving systems assembled from separate modules by simulating a tungsten CVD deposition process occurring in a single wafer LPCVD system both at steady-state and dynamically over a true processing cycle. The second example considers the development of a multi-segment simulator describing the gas concentration profiles in the newly designed Programmable CVD reactor system. The simulation model is validated by deposition experiments conducted in the three-segment prototype. To facilitate the CVD system design, experimental data archiving, and distributed simulation, a three-tier Java and XML-based integrated information technology system has also been developed.Item semiconducting carbon nanotube transistors: electron and spin transport properties(2006-04-25) Chen, Yung-Fu; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWNTs) have attracted great interest both scientifically and technologically due to their long mean free paths and high carrier velocities at room temperature, and possibly very long spin-scattering lengths. This thesis will describe experiments to probe the charge-and spin-transport properties of long, clean individual SWNTs prepared by chemical vapor deposition and contacted by metal electrodes. A SWNT field-effect transistor (SWNT-FET) has been shown to be sensitive to single electrons in charge traps. A single charge trap near a SWNT-FET is explored here using both electronic and scanned-probe techniques, and a simple model is developed to determine the capacitances of the trap to the SWNT and gate electrode. SWNTs are contacted with ferromagnetic electrodes in order to explore the transport of spin-polarized current through the SWNT. In some cases spin-dependent transport was observed, verifying long spin scattering lengths in SWNT. However, in many cases no spin-dependent effects were observed; these results will be discussed in the context of the present state of results in the literature. Semiconducting SWNTs (s-SWNTs) with Schottky-barrier contacts are measured at high bias. Nearly symmetric ambipolar transport is observed, with electron and hole currents significantly exceeding 25 µA, the reported current limit in m-SWNTs. Four simple models for the field-dependent velocity (ballistic, current saturation, velocity saturation, and constant mobility) are studied in the unipolar regime; the high-bias behavior is best explained by a velocity saturation model with a saturation velocity of 2 x 10^7 cm/s. A simple Boltzmann equation model for charge transport in s-SWNTs is developed with two adjustable parameters, the elastic and inelastic scattering lengths. The model predicts velocity saturation rather than current saturation in s-SWNTs, in agreement with experiment. Contact effects in s-SWNT-FET are explored by electrically heating the devices. These experiments resolve the origin of nanotube p-type behavior in air by showing that the observed p-type behavior upon air exposure cannot be explained by change in contact work function, but is instead due to doping of the nanotube. Modest doping of the SWNT narrows the Schottky Barriers and provides a high-conductance Ohmic tunnel contact from electrode to SWNT.Item Ab initio Lattice Dynamics and Infrared Dielectric Response(2004-11-24) lawler, hadley Mark; Shirley, Eric L.; Drew, H. Dennis; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Methods for theoretically evaluating lattice dynamics, anharmonic effects and related optical properties from first principles are designed and implemented. Applications of density-functional theory and the pseudopotential approximation are adapted, via the Born-Oppenheimer approximation, the Hellmann-Feynman force theorem, and wave-commensurate supercells, to a direct calculation of the Born-von Karman force constants. With a symmetry analysis and interpolation of Born-von Karman force constants, the complete phonon spectra are obtained for the cubic systems Ar, Si, Ge, and diamond, and for the stacked hexagonal system, graphite. The phonon spectra for the polar materials GaAs and GaP, in which the degeneracy between longitudinal and transverse optical modes is lifted, are also calculated. The splitting is a consequence of the macroscopic field associated with long-range Coulombic interactions and longitudinal displacements. Diagramatically-derived expressions for the finite lifetime of the Raman mode arising from phonon-phonon interactions are calculated for Si, Ge, and diamond from first principles, and agree with experiment to within uncertainty. The infrared absorption spectra of GaAs and GaP are calculated from first principles through the phonon anharmonic self-energy (phonon-phonon interaction) and the Born effective charges (photon-phonon interaction). Several aspects of the spectra are in detailed agreement with the experimental spectra, including the strong temperature dependence of the far-infrared absorption due to the onset of difference processes; the linewidth and asymmetric lineshape of the reststrahlen; the spectral structure of the absorption by two-phonon modes, and overall oscillator strengths. The theory allows for the identification of narrow spectral transmission bands with an ionic mass mismatch in the case of GaP. Analytic and complete calculations are performed for the ion-ion displacement correlation function in solid Ar, and agree well. The correlations are evaluated for arbitrary lattice vector and Cartesian displacement directions, and their pressure dependence leads to the conjecture that anharmonic effects are less prominent at higher pressures.