Physics
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Item EXPLORATION OF NOVEL METHODS FOR THE FABRICATION AND CHARACTERIZATION OF ORGANIC FIELD-EFFECT TRANSISTORS AND EXAMINATION OF FACTORS INFLUENCING OFET PERFORMANCE(2009) Southard, Adrian Edward; Fuhrer, Michael S.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis explores novel methods for fabricating organic field effect transistors (OFETs) and characterizing OFET devices. Transfer printing is a promising process for fabricating organic thin-film devices. In this work, a transfer-printing process is developed for the polymer organic semiconductor P3HT. Pre-patterned P3HT is printed onto different dielectrics such as PMMA, polystyrene and polycarbonate. The P3HT layer is spun on a smooth silicon interface made hydrophobic by treatment with octyltrichlorosilane, which functions as a release layer. This method has distinct advantages over standard OFET fabrication methods in that 1) the active layer can be pre-patterned, 2) the solvent for the P3HT need not be compatible with the target substrate, and 3) the electrical contact formed mimics the properties of top contacts but with the spatial resolution of bottom contacts. Transparent, conducting films of carbon nanotubes (CNTs) are prepared by airbrushing, and characterized optically and electronically. OFETs with CNT films as source and drain electrodes are fabricated using various patterning techniques, and the organic/CNT contact resistance is characterized. CNT films make transparent, flexible electrodes with contact resistance comparable to that found for Au bottom-contacted P3HT transistors and comparable to CNT-film bottom-contacted pentacene transistors with CNTs deposited by other less flexible methods. A transparent OFET is demonstrated using transfer printing for the assembly of an organic semiconductor (pentacene), CNT film source, drain, and gate electrodes, and polymer gate dielectric and substrate. The dependence of the conductance and mobility in pentacene OFETs on temperature, gate voltage, and source-drain electric field is studied. The data are analyzed by extending a multiple trapping and release model to account for lowering of the energy required to excite carriers into the valence band (Poole-Frenkel effect). The temperature-dependent conductivity shows activated behavior, and the activation energy is lowered roughly linearly with the square-root of electric field, as expected for the Poole-Frenkel effect. The gate voltage dependence of the activation energy is used to extract the trap density of states, in good agreement with other measurements in the literature.Item STEPS ON VICINAL SURFACES: DENSITY-FUNCTIONAL THEORY CALCULATIONS AND TRANSCENDING MINIMAL STATISTICAL-MECHANICAL MODELS(2009) Sathiyanarayanan, Rajesh; Einstein, Theodore L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Using both density-functional theory calculations and Monte Carlo simulations, we compute various key parameters that are used to model steps on vicinal surfaces. In the first part, we discuss the importance of multi-site interactions (trios and quartos) in the lattice-gas characterization of adatom interactions. Using density-functional theory calculations, we show that multi-site interactions with substantial contributions from direct interactions are sensitive to adatom relaxations. Such sensitivity to adatom relaxations complicates the lattice-gas approach to modeling overlayer systems. Our results show that a careful consideration of relaxation effects is required to make connections with experiments. In the second part, we use both density-functional theory calculations and kinetic Monte Carlo simulations to identify the impurity atom responsible for growth instabilities on Cu vicinals. In addition to that, we also show that a small quantity of codeposited impurities significantly alters the growth behavior. Our results indicate that growth morphologies could be controlled through the codeposition of an appropriate impurity. Hence, impurities could play a crucial role in nanostructuring of surfaces. Step configurations have fruitfully been related to the worldlines of spinless fermions in one dimension. However, in addition to the realistic no-crossing condition, the fermion picture imposes a more restrictive non-touching condition. in the third part of this thesis, we use Metropolis Monte Carlo method to study the effects of loosening this non-touching condition on the resulting TWDs. Our results show that allowing step touching leads to an effective attraction in the step-step interaction strength measurements. We show that this effective attraction can be incorporated into the fermion picture as a finite-size effect.Item DEVELOPMENT OF ION-MOBILITY AND MASS SPECTROMETRY FOR PROBING THE REACTIVITY OF NANOPARTICLES AND NANOCOMPOSITES(2009) Zhou, Lei; Zachariah, Michael; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerosols of diameter smaller than 100 nm, usually are referred as nanoparticles or ultrafines, have received considerable interests lately as a source of building blocks to novel materials. However, our capabilities for charactering these materials are greatly limited by lack of appropriate diagnostic tools. The objective of this work is to develop new aerosol-based techniques for the characterization of nanoparticles and nanocomposites. The scope of this dissertation can be categorized in two ways. First, to provide knowledge of just how reactive a material is, we develop particle ion-mobility spectrometry and Single Particle Mass Spectrometry methods to probe the intrinsic size-dependent reactivity of individual metal particles. And second, the development of a new Time-of-Flight mass spectrometer (TOFMS) combined with a temperature jump (T-Jump) technique to study particle-particle reaction, and probe the reactivity of nanocomposite materials under combustion-like condition.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 Nanogap Junctions and Carbon Nanotube Networks for Chemical Sensing and Molecular Electronics(2006-11-20) Esen, Gokhan; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis work may be divided into two parts. The first part (chapters 2-7) focuses on the fabrication of a particular test structure, the electromigration (EM) formed metal nanogap junction, for studying the conduction through single molecules and for hydrogen sensing. The second part (chapters 8 and 9) focuses on carbon nanotube networks as electronic devices for chemical sensing. Chapters 2-4 discuss the formation of nanogap junctions in thin gold lines fabricated via feedback controlled electromigration. Using a feedback algorithm and experimenting on thin gold lines of different cross sections, I show that the feedback controls nanogap formation via controlling the temperature of the junction. Chapters 5 and 6 discuss the background and my experimental efforts towards fabricating superconducting electrodes for single molecule electronics research. Chapter 7 discusses the application of the techniques of chapters 2-4 to form palladium nanogaps via electromigration. I show that such devices can be used as hydrogen sensors, but suffer from slow response times (on the order of minutes). The results are discussed in the context of the in-plane stress buildup between the palladium metallization and the SiO2 substrate. The use of nanotube networks as chemical sensors is discussed in the second part of the thesis (chapters 8 and 9). I show measurements of the resistance and frequency-dependent (50 Hz - 20 KHz) gate capacitance of carbon nanotube thin film transistors (CNT-TFTs) as a function of DC gate bias in ultra-high vacuum as well as low-pressure gaseous environments of water, acetone, and argon. The results are analyzed by modeling the CNT-TFT as an RC transmission line. I show that changes in the measured capacitance as a function of gate bias and analyte pressure are consistent with changes in the capacitive part of the transmission line impedance due to changes in the CNT film resistivity alone, and that the electrostatic gate capacitance of the CNT film does not depend on gate voltage or chemical analyte adsorption to within the resolution of my measurements. However, the resistance of the CNT-TFT is enormously sensitive to small partial pressure (< 10-6 Torr) of analytes, and the gate voltage dependence of the resistance changes upon analyte adsorption show analyte-dependent signatures.Item Carbon Nanotube Devices: Growth, Imaging, and Electronic Properties(2006-01-11) Brintlinger, Todd Harold; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation focuses on growth, fabrication, and electronic characterization of carbon nanotube (CNT) devices. A technique for imaging CNTs on insulating substrates with the scanning electron microscope (SEM) will be described. This technique relies on differential charging of the CNT relative to the surrounding insulator. In addition, it is not only quicker than using scanning probe microscopy (SPM), but is also useful for identifying conducting pathways within an assortment of CNTs and metallic contacts. CNT field effect transitors (FETs) fabricated on strontium titanate gate dielectric show transconductances normalized by channel width of 8900 S/m, greatly exceeding that in Si FETs. Intriguingly, the transconductance cannot be explained within the conventional FET or Schottky-barrier models. To explain this, it is proposed that there is Schottky-barrier lowering due to high electric fields at the nanotube/contact interface. Exploring novel CNT-FET lithography, I demonstrate focused electron beam induced deposition (FEBID) of pure gold for CNT device electrodes. In examination of the CNT/electrode interface, equivalence between FEBID leads and leads deposited using conventional electron beam lithography is found with the majority device resistance in the CNT. Lastly, CNTs are suspended across wide trenches (>100 microns). These trenches are formed without lithography or etching and have metallic leads on either side of the trench for electrical transport measurements. Using a mechanical probe as a mobile gate, electrical transport can be performed on these suspended CNT devices, which show minimal hysteresis consistent with the absence of charge trapping.Item Analysis and Control of Microstructure in Binary Alloys(2004-12-20) Lee, Kyuyong; Losert, Wolfgang; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)When metallic alloys solidify, various microstructures form inside the alloys. Most solidified alloys have a polycrystalline structure, which is an assembly of crystalline grains with boundaries between any two grains. Each grain is a single crystal with a unique crystalline orientation. Many physical properties of polycrystalline alloys are determined by the arrangement of these grains and grain boundaries. During solidification of a single crystal, microstructures with even smaller microscopic lengthscales form, such as dendritic and eutectic structures. The physical properties of single crystal alloys are largely influenced by the lengthscales of these structures. Therefore, the understanding and control of microstructure formation in solidification is important in order to achieve desired properties. Microstructures form while the system is not in equilibrium. What microstructures form is not based on minimization of free energy of the system, but depends on the dynamics of the solidification process, which is the focus of our study. We used an alloy model system, Succinonitrile-Coumarin152, to experimentally investigate dynamic selection and control of grain boundary structures and dendritic structures in binary alloys. We found that in a temperature gradient the grain boundaries drift toward the high temperature region in addition to the migration due to grain coarsening. We show how we can control grain boundary orientations by generating local temperature gradient through UV or laser heatings. We show that perturbations also permit accurate control of the microstructure within a single crystal during the directional solidification process. Dendritic patterns can be controlled either by guiding the initial formation of the pattern or by triggering subcritical transitions between stable microstructures. We also investigated the role of surface tension anisotropy on the stability of cellular/dendritic arrays using three crystals of different growth orientations with respect to the surface tension anisotropy. We found that the surface tension anisotropy affects the spacing between dendrites and stability via the surface tension perpendicular to the growth direction.Item Electron Transport Simulations and Band Structure Calculations of New Materials for Electronics: Silicon Carbide and Carbon Nanotubes.(2003-12-03) Pennington, Gary Wayne; Goldsman, Neil; PhysicsSilicon carbide (SiC) and carbon nanotubes (CNTs) are two materials which have promising potential in electronics. Due to its large bandgap and large thermal conductivity, SiC is targeted as a potential material for use in high-power high-temperature electronics. Carbon nanotubes are at the forefront of current research in nanoelectronics, and field-effect nanotube transistors have already been developed in research laboratories. The small dimensions of these materials suggests their possible use in densely packed CNT-integrated circuits. Carbon nanotubes also appear to have very large electron mobilities, and may have applications in high-speed electronic devices. In this work the properties of the electronic structure and electron transport in silicon carbide and in semiconducting zig-zag carbon nanotubes are studied. For SiC, a new method to calculate the bulk band structure is developed. The conduction band minimum is found to lie at the $L$ and $M$ points in the Brillouin zones of 4H and 6H-SiC respectively. The quasi-2D band structure of hexagonal SiC is also determined for a number of lattice orientations. Electron transport in SiC is investigated in the bulk and at the SiC/oxide interface. The dependence of transport on the lattice temperature, applied field, and crystal orientation is studied. A methodology for semiclassical transport of electrons in semiconducting carbon nanotubes is also developed. Monte Carlo simulations predict large low-field mobilities (4000-13000 cm*cm/Vs) agreeing with experiments. The simulations also predict high electron drift velocities (500 km/s) and negative differential resistance.Item IR Hall Angle Measurements On Single Crystal Bi-2212 (BSCCO)(2003-12-08) Jenkins, Gregory S.; Drew, Howard D; PhysicsThe far-infrared complex ac- Hall angle was studied in thin optimally doped single crystal Bi-2212 as a continuous function of temperature in both the normal and superconducting states. The temperature was varied from 25 to 300 K at a discrete set of frequencies in the range of 25 to 175 cm^-1. Much of this dissertation focuses on the design and construction of an instrument that is capable of measuring the FIR ac-Hall angle in BSCCO as a continuous function of temperature with high sensitivity. The heterodyne system is capable of measuring the real and imaginary part of the Hall angle to an accuracy of ~ 0.1 mrad over a temperature range of 20 to 320 K, and a frequency range of 20 to 240 cm^-1. The normal state properties are measured from Tc up to room temperature at four discrete frequencies, demonstrating clear non-Drude frequency dependence. Furthermore, a recently proposed square- Lorentzian model does not reasonably describe the present data, although previous relatively noisy FIR Hall angle measurements which the model appeared to describe are consistent with the present measurements. Specifically, the real part of the inverse Hall angle obeys a temperature power law, T^a, where a = 1.65 +/- 0.1 which is consistent with the dc- value of 1.75 +/- 0.05. The values show a decrease with increasing frequency, clearly displaying non-Drude behavior. For the three frequencies below 90 cm^-1, the Hall frequency is a constant in temperature and frequency to within 20% from Tc up to room temperature. The Hall frequency is 0.38 +/- 0.03 cm^-1/T which corresponds to an effective mass of 2.5 +/- 0.21 m_e in reasonable agreement with the values found in FIR optical measurements (3.0 +/- 0.4), ARPES dispersion results along the (pi,pi) nodal direction (2.9 m_e), and near-IR ac- Hall measurements (2.8 m_e) where m_e is the bare electron mass. The salient features of the superconducting state are qualitatively consistent with a simple model of the conductivity which contains a quasi-particle cyclotron resonance, a zero frequency (London) superfluid resonance, and a finite frequency chiral oscillator at ~ 35 cm^-1. The high frequency (175 cm^-1) data is consistent with one collective mode cyclotron resonance. A frequency dependent feature in the imaginary part of the Hall angle is observed 10 K above Tc, suggestive of precursive superconducting state behavior.