Physics
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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 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.