Theses and Dissertations from UMD
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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.
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Item Electronic Structure of SiC/SiO2 by Density Functional Theory(2012) Salemi, Shahrzad; Goldsman, Neil; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Silicon carbide (SiC) is a promising semiconductor material with desirable properties for many applications. SiC-based electronic devices and circuits are being developed for use in high-temperature, high-power, and high-radiation conditions under which conventional semiconductors cannot function. Additionally, it has the advantage of growing a native oxide, SiO2, by simple thermal oxidation. Despite all desirable properties, SiC-based devices still face major challenges. The main problem of SiC-based devices is the great density of imperfections at the SiC/SiO2 interface, which not only degrades the device performance but also causes reliability problems coming from the extreme operating conditions. The quality of the interface affects the channel mobility of MOSFETs, which is the most critical parameter of devices. In this work a hybrid functional density functional theory framework is employed to model the (0001)4H-SiC/SiO2 abrupt interface. Using this, defect energy levels in the bandgap have been calculated through the total and projected density of states. There is experimental evidence for improvement of the quality of the interface after passivation, However the atomic mechanisms of the improvement are not yet clear., Thus, the impact of various passivations on the potential defects has also been studied. Since the interface of SiC/SiO2 is not perfectly abrupt, several atomic configurations for (0001)4H-SiC/SiO2 transition layers have also been modeled, and their effect on the bandgap, and the near interface trap density has been studied. A DFT-based Monte Carlo carrier transport simulation technique is employed to compute the average velocities, phonon-limited and ionized-impurity-limited mobilities of the most probable transition layer structures. Finally, since low frequency noise calculation is a powerful tool to diagnose quality and reliability of semiconductor devices, a DFT-based method is presented to calculate the current spectral noise density of the (0001)4H-SiC/SiO2 transition layers.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 Molecular Structure and Surface Organization: A Study of Liquid/Vapor Interfaces Using Newly Developed Sum Frequency Methods(2004-08-19) Esenturk, Okan; Walker, Robert A; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Vibrational sum frequency spectroscopy (VSFS) is a surface specific nonlinear technique that provides vibrational spectra of molecules at interfaces. Studies presented in this thesis use VSFS to examine surface structure of common liquids at liquid/vapor interfaces. The goal of this work is to correlate molecular structure and molecules functionality with the structure adopted by molecules at liquid surfaces. The first part of this work describes the instrumental design and components of the newly developed VSFS. New methods are developed to overcome experimental difficulties associated with data collection for an entire spectral region of interest. The methods for data analysis and spectral post processing are also presented. Surface vibrational studies of linear alkanes ranging in length from 9 to 17 carbon atoms investigated dependence of surface order on chain length. The linear alkane liquid surfaces evince a surprising degree of conformational order and this surface structure becomes more disordered as chain length increases. Halogenated alkane studies showed that replacing one terminal methyl group of linear alkanes with CH2X (X=Cl or Br) for chain lengths 10, 14 and 16 leads to a mixed surface layer primarily occupied by CH2X. These results are attributed to dipole-dipole interactions between the CH2X and are indicative of a higher surface activity of the halogenated ends compared to the unsubstituted ends. Additional experiments have investigated the effect of stronger interactions on the surface structure of 1-, 3-, and 5-nonanols and their ketone analogs namely 1-, 3-, and 5-nonanones. Results showed an increasing order as the OH or =O groups advances toward the center of the molecule. A surprising result is a slightly lower surface order for nonanones compared to nonanols hints at the relative importance of simple dipolar interactions and highly directional hydrogen bonding in determining the surface structure. In addition, vibrational studies are carried out for 1- and 3-octanol at liquid/vapor interfaces. The results confirm observations from the nonanols by having a higher order for the 3-octanol. However, monolayer of 1-octanol on aqueous surface have a much higher surface order compared to 3-octanol. The increased order is attributed to the strong hydrogen bonding with water molecules.