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Nielsen, Jon F.Energetically and Kinetically Driven Step Formation and Evolution on Silicon Surfaces
Doctor of Philosophy, The Ohio State University, 2001, Physics

Energetically and kinetically driven step formation and evolution on Si(001) and Si(111) surfaces has been investigated experimentally using scanning tunneling microscopy (STM), atomic force microscopy (AFM), optical microscopy, and low-energy electron microscopy (LEEM). Four systems are investigated:

(1) Detailed STM measurements of boron-doped Si(001) surfaces is presented, along with large-scale AFM and LEEM observations of the well-known boron-induced ‘striped’ phase at elevated temperatures. Boron is shown to induce a variety of related atomic-scale structures, some of which tend to decorate surface step-edges. This, in turn, could provide an explanation for the observed boron-induced reduction in step formation energy. However, the observed boron-accumulation at step-edges does not appear to vary systematically with annealing temperature, leaving the well-known temperature dependence of the striped phase unresolved. Real-time LEEM observations of striped step formation on Si(001) during diborane (B2H6) exposure at elevated temperatures are used to demonstrate the controlled formation of large (>5 mm) surface regions with highly uniform striped step structures.

(2) Large-scale step rearrangements have been investigated on Si(001) and Si(111) surfaces heated to sublimation temperatures (>900 °C) using a direct current. These surfaces undergo dramatic morphological changes, which are believed to arise from a directional drift of diffusing surface atoms in the presence of an applied electric field. Such ‘electromigration’ phenomena include step ‘bunching’ and step ‘wandering’, as well as a predicted step ‘bending’ instability. Using AFM and optical microscopy, we argue that the direction of surface atom electromigration on Si(001) can be parallel, anti-parallel, or even sideways to the applied electric field, depending on the direction of the applied field with the high-symmetry <110> crystal directions. In addition, the first experimental evidence for the predicted step bending instability is presented.

(3) Sublimation pit formation is studied on Si(001) surfaces heated to ~1000 °C. Real-time LEEM and microscopic modeling of step dynamics is used to show that – for a given net sublimation rate – adding a small Si flux during heating increases the stability of atomically flat surface terraces against sublimation pit formation. This makes it practical to produce much larger step-free terraces than have been reported previously.

(4) The anisotropy of surface diffusion on Si(001) has been analyzed from the formation of ‘denuded’ zones during Si(001) two-dimensional homo-epitaxial growth. Comparison with a simple model for surface atom diffusion shows that diffusion is at least 10 times faster along the surface ‘dimer’ rows than across them. Furthermore, contrary to previous reports, Si(001) surface diffusion is shown to be anisotropic at temperatures up to at least 800 °C. The apparent contradiction between our results and those reported by previous authors is resolved by observing denuded zone formation for several different growth conditions.

Committee:

Jon Pelz (Advisor)

Subjects:

Physics, Condensed Matter

Keywords:

silicon; Si(001); Si(100); Si(111); surface morphology; surface diffusion; electromigration; step bunching; diffusion anisotropy; sublimation; Si(001) boron-induced striped step structure; large terrace formation; scanning tunneling microscopy; STM

Cai, WeiBallistic Electron Emission Microscopy and Internal Photoemission Study on Metal Bi-layer/Oxide/Si, High-k Oxide/Si, and “End-on” Metal Contacts to Vertical Si Nanowires
Doctor of Philosophy, The Ohio State University, 2010, Physics

In this thesis, three separate experiments involving electronic materials are described. Nanometer-spatial resolution ballistic electron emission microscopy (BEEM) is used to study the inhomogeneity of metal-bialyer/SiO2 interface, and small size and restricted geometry effect of “end-on” metal contacts to vertical Si NWs embedded in spin-on glass. BEEM and internal photoemission (Int-PE) are also combined for the first time to study conduction and valence band offsets between Si and promising high-k (high dielectric constant) oxides: Sc2O3 and Lu2O3.

In the first experiment, a comparison of the dependence on gate voltage of the average energy barrier measured by BEEM of a metal bi-layer Pt/Al/SiO2/Si sample and a Pt/SiO2/Si sample suggests that the metal/oxide interface of the Pt/Al/SiO2/Si sample is laterally inhomogeneous at nm length scales. However, BEEM images of the bi-layer sample do not show significantly larger lateral variations than observed on a (uniform) Pt/SiO2/Si sample, indicating that any inhomogeneous “patches” of lower energy barrier height have size smaller than the lateral resolution of BEEM, estimated for these samples to be ~10 nm. Finite element electrostatic simulations of an assumed inhomogeneous interface with nm size patches of different effective work function can fit the experimental data of the bi-layer sample much better than an assumed homogenous interface, indicating that bi-layer film is in fact laterally inhomogeneous at the nm scale.

Int-PE measurements on 20 nm-thick epitaxial Sc2O3 and Lu2O3 film on Si (111) show the existence of a lower “tail state” conduction band (CB) extending ~1 eV below the upper CB (similar to that reported for amorphous Sc2O3 and Lu2O3 films), indicating that these states are not simply due to disorder in amorphous films. BEEM measurements on epitaxial Sc2O3/Si also show that this lower CB supports elastic hot-electron transport even against an applied electric field, indicating transport via extended rather than localized states.

BEEM measurements on “end-on” Au contacts to vertical Si NWs show strong suppression of hot-electron injection at higher injected current in the NWs (produced either by increasing the tip voltage or the tunneling current) comparing to a regular Au/Si junction, suggesting that this current suppression is due to a steady-state charge build-up in the NW that increase as more current is injected into the NW. The BEEM current suppression of most NWs was also found to increase strongly at lower temperature, indicating more charge build-up at lower temperature. Time-dependent current suppression due to changing steady-state charge build-up in the NWs was observed when the tunnel current was abruptly increased and decreased, directly supporting a model in which the BEEM current suppression is due to (temperature-dependent) steady-state charge build up. Those electrons might be trapped at the NW/SiO2 interface close to the metal/Si NW contact, consistent with finite element simulations. Our BEEM measurements also show that the local Schottky barrier height (SBH) at the edge of two separate NWs is ~20 meV lower than at the NWs center. Finite element simulation suggests that the lower SBH at the contact edge might be due to the NW/oxide interface states charge.

Committee:

Jonathan P. Pelz, PhD (Advisor); David G. Stroud, PhD (Committee Member); Thomas J. Humanic, PhD (Committee Member); Fengyuan Yang, PhD (Committee Member)

Subjects:

Physics

Keywords:

ballistic electron emission microscopy; BEEM; internal photoemission; metal-bialyer/SiO2 interface; high-k oxide/Si; metal/oxide/semiconductor; end-on metal contact; vertical Si NWs; current suppression; Schottky barrier; interface state charge

CHAKRAVARTY, SRINIVAS L.N.DEVELOPMENT OF SCRATCH RESISTANT PECVD SILICA-LIKE FILMS
MS, University of Cincinnati, 2000, Engineering : Materials Science
Plasma-enhanced chemical vapor deposition (PECVD) was used to deposit silica-like films onto various substrates to improve their scratch resistance. These films were deposited using a dual-frequency (microwave/low frequency) plasma reactor, which allowed the concentration of the active species in the plasma and the energy of bombardment of the species on the substrate to be controlled separately. External plasma parameters, including microwave (MW) power, low frequency (LF) power, and deposition time were varied to observe their effects on the thickness and hydroxyl content of the silica-like films. These parameters were optimized on ferroplate substrates to obtain thick silica-like films with low hydroxyl content to enhance their scratch resistance. However, it was found that when these silica-like films were deposited for deposition times longer than 2500 seconds, some undesirable powder particles were formed. Incorporation of powder particles into the film was detrimental to the scratch resistance properties. This problem was overcome by depositing films at low pressures (10 Pa). In order to optimize the external plasma parameters for enhancement of scratch resistance on ferroplate substrates at these low pressures, Taguchi's design of experiments was used. It was shown that silica-like films at a thickness of 4 (m or more could be deposited at the rate of 300 Å/min using these optimized parameters. These films exhibited excellent scratch resistance on ferroplate substrates. These films were also found to enhance the scratch resistance of polycarbonate substrates. Optical microscopy of the silica-like films directly deposited on FKM substrates revealed the presence of cracks in the films due to the compressive stresses in the film. Crack-free films were obtained on FKM substrates by depositing 2-layered films comprising a siloxane film as the first layer and silica-like film on top of the siloxane film. It was shown that these 2-layered films exhibited significantly better scratch resistance on FKM substrates than the directly deposited silica-like films. Tape peel tests were used to qualitatively evaluate the adhesion of the films to the FKM substrates. It was shown that the adhesion of the directly deposited silica-like films and multi-layered films to the FKM substrates was very good.

Committee:

F. James Boerio (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

stresses in films; powder formation; 90 degree peel test; optical microscopy; Si-O-Si band assignments

Pan, YueTopological Origin of the Urbach Tail
Doctor of Philosophy (PhD), Ohio University, 2009, Physics and Astronomy (Arts and Sciences)
Urbach tail, the universal exponential band tail observed in impure crystals and disordered materials, was first identified in 1953. Despite theories presented to explain the Urbach tail, the topological origin of this nearly universal phenomenon still remains unknown. The topological nature of the band tail states with the Urbach tail energies is important for the understanding of several properties of amorphous material including, for instance, electronic transport. The main target of my study is to determine the topological origin of the Urbach tail and develop a basic understanding of the problem. My study is carried out via the investigation of networks of high quality amorphous silicon for topological details and via ab intio calculations for electronic properties. In this dissertation, the topological filaments, and the electronic filaments, reflecting topology-electron correlation, are defined and observed. The band tail states are directly related to explicit topological elements (correlations) and the topological origin of Urbach tail is identified in at least one important material. Alongside the study of the Urbach tail, a semi-quantitative scattering theory is presented to explain basic electronic quantities of amorphous solid. Electron transport in solids and Boron doping a-Si:H are also studied: results are concisely presented.

Committee:

David A. Drabold, Dr. (Advisor); David Ingram, Dr. (Committee Member); Gang Chen, Dr. (Committee Member); Ralph Whaley, Dr. (Committee Member)

Subjects:

Physics

Keywords:

Urbach; a-Si; topological filament

Jin, NiuSi-based quantum functional tunneling devices and their applications to logic and other future circuit topologies
Doctor of Philosophy, The Ohio State University, 2004, Electrical Engineering
The aim of this Ph.D. project is to develop high performance Si-based tunneling structures for integrated circuitry applications. In this work, three kinds of Si-based tunneling structures, namely, Si-based resonant interband tunneling diodes (RITD) for mixed signal integrated circuit and low power digital applications, Si-based backward diode for millimeterwave detection applications, and fabrication of Ge quantum dots for single electron transistor (SET) and quantum-dot cellular automata (QCA) applications, were studied. Both the RITD structure and post-growth annealing process were optimized to raise the peak-to-valley current ratio. Studies were performed to both maximize and minimize the peak current density for two different types of circuit applications. A low power one-transistor tunnel diode SRAM was demonstrated using the low current density RITD developed. Vertically integrated RITDs showing two successive NDR regions under the forward biasing condition are presented. Subsequently, tri-state logic was demonstrated. A comprehensive model which combines both the small and large signal models of RITDs was developed. The microwave performance of RITDs is also discussed. Several simple RITD circuits are simulated using this model. Radiation experiment on the Si-based RITD provides experimental evidence for resonant tunneling as opposed to Esaki-like tunneling. RITDs were grown on SiGe substrate to engineer the band offsets by strain modification. Si-based backward diodes for zero-biased millimeter wave detection are studied. The high sensitivity and SiGe HBT compatibility of the Si-based backward diodes make them very attractive for zero-bias millimeter-wave detector applications. Fabrication of Ge quantum dot by oxidizing Si and SiGe nano-pillars for QCA applications was presented. Overall, the work presented here was to extend Si technology, as articulated on the International Technology Roadmap for semiconductors, and can serve as a guide for the future exploitation of Si-based RITDs.

Committee:

Paul Berger (Advisor)

Keywords:

RITD; diodes; tunnel diodes; PVCR; TUNNELING; Si; tunnel

Li, ChangrongStudies on Silicon Carbide: Heteroepitaxy on Silicon and Titanium Alloy Ohmic Contacts
Doctor of Philosophy, Case Western Reserve University, 2009, Materials Science and Engineering

The first half of this dissertation describes the design and construction of a new low pressure MOCVD reactor. Various problems and concerns with leakage, fitting and safety were solved to make the reactor more reliable and safer.

For SiC growth on Si substrates, the so-called two-step growth procedure was adopted and a single precursor hexamethyldisilane was used. XRD results showed that the films on Si(100) were polycrystalline, while epilayers on Si(111) are single crystalline. A few GaN films were also epitaxially grown on Si(111) using a buffer layer of SiC. A possible explanation for the advantages of the two-step over the one-step growth procedure is proposed involving the interfacial energy and total energy change during nucleation.

The second half of the dissertation deals with a microstructural investigation of multi-layer Ti alloy contacts to SiC. For this purpose, numerous sitespecific TEM thin foils were prepared for (Au/)TaSi2/Ti ohmic contacts with different annealing histories using the Focus Ion Beam liftout technique. Subsequently, the thin foils were studied by various imaging and analytical techniques.

On either sides of the Ti layer in the TaSi2/Ti/SiC multi-layer structure, two interfaces could be clearly observed. While no C or Si were detected in the Ti contact layer of the as-deposited sample, the annealed samples exhibited strong Si and C signals throughout the Ti layer. This is indicative of the formation of new phases such as silicide(s) and/or carbide(s) of titanium. Moreover, the TaSi2 layer had acted as a diffusion barrier and prevented the diffusion of Ti to the surface and in-diffusion of oxygen.

In samples with four more layers (i.e. Au/Ti/TaSi2/Ti in addition to TaSi2/Ti/SiC, making a seven-layer contact), which were annealed from 1 to 50 hours, a thick interface had formed between the upper TaSi2 layer and the top Au layer. In addition, on the surface of the contacts, cross-linked Au micro-wires had formed under which many irregular dark patches were present with high concentrations of C, O and Si.

Using the microcharacterization information, we have proposed a failure mode and mechanism for the Ti alloy contacts.

Committee:

Pirouz Pirouz (Advisor); Kathleen Kash (Committee Member); Frank Ernst (Committee Member); Peter Lagerlof (Committee Member); Robert Okojie (Committee Member)

Keywords:

SiC; TaSi2; Si; AuL; Ti; HAADF

Li, JunSilicon Phthalocyanines for Photodynamic Therapy Studies
Doctor of Philosophy, Case Western Reserve University, 2008, Chemistry
Four sulfonated silicon phthalocyanines SiPc(1-SO3H)[OSi(n-C6H13)3]2, 2, Pc 177; SiPc(2-SO3H)[OSi(n-C6H13)3]2, 4, Pc 178; SiPc(1-SO3H)[OSi(CH3)2(CH2)3N(CH3)2]2, 3, Pc 179; and SiPc(2-SO3H)[OSi(CH3)2(CH2)3N(CH3)2]2, 5, Pc 180, were prepared and characterized. The first two were made for an abandoned external study and the second two for an internal photodynamic therapy (PDT) study. In further work, acid SiPc[OSi(CH3)2(CH2)10C(O)OH]2, 28, Pc 61 was prepared and was conjugated with trimer H(C9H10N2O2)3OCH3, 15, a trimer first synthesized by Hamilton. This trimer has the potential to bind to Bcl-2 and Bcl-xL proteins (proteins which are overexpressed in cancer cells). The two conjugates obtained, SiPc[OSi(CH3)2(CH2)10C(O)(C9H10N2O2)3OCH3]2, 30, Pc 212; and HOSiPcOSi(CH3)2(CH2)10C(O)(C9H10N2O2)3OCH3, 31, Pc 213, have the potential to complex with the Bcl-2 and Bcl-xL through their aminotrimer units and with photodynamic action to destroy these proteins and their associated cancer cells. Also, two alkyl silicon phthalocyanines, HS(CH2)3SiPcOSi(CH3)2(CH2)3N(CH3)2, 52, Pc 227; and CH3S(CH2)3SiPcOSi(CH3)2(CH2)3N(CH3)2, 58, Pc 223, were prepared as “masked Pc 4s”. These two alkyl phthalocyanines have the potential to bind to Au nanoparticles through their SH and SCH3 functional groups respectively. xviii The phthalocyanine-Au nanoparticle conjugates appear to have the potential to target tumor tissues, and when irradiated with red light in the presence of H2O and O2 to yield Pc 4. Pc 4 has the ability to destroy the tumor tissues when used in PDT.

Committee:

Malcolm Kenney (Advisor)

Subjects:

Chemistry, Inorganic

Keywords:

CH2; Si; CH3; SiPc; OSi; Pc; PHTHALOCYANINES

Liu, YiningDesign of an Optical Response System for Characterization of Hyperoped Silicon Photodetectors
Master of Science (M.S.), University of Dayton, 2016, Electro-Optics
Silicon photonics requires the realization of CMOS-compatible infrared detectors for large scale integration. One possible solution for infrared detection in Si is through the use of hyperdoping, where supersaturated solutions of impurities in Si are produced in order to create intermediate bands in between the valence and conduction bands in Si. Sub-band gap photoconductivity was recently demonstrated in a prototype photodetector fabricated from Si hyperdoped with Au, with infrared response at wavelengths as long as 2 µm. The thesis focuses on the design and fabrication of an optical and electrical measurement system for the characterization of prototype photodiode detectors fabricated from Au-hyperdoped material in order to further its development. Measurement of detector response at long wavelengths requires design of an optical system that can deliver long wavelength light to the device and an electrical system that can measure the (possibly small) photocurrent induced in the detector. An optical system was designed and implemented using a purpose-built tungsten halogen lamp and monochromator as the illumination source, with a custom infrared lensed fiber to deliver the light. In order to characterize on-chip devices, a micro-probe station was employed for electrical connectivity. The system measures electrical response by modulating the incident light with an optical chopper and measuring the change in voltage across a series resistor using a lock-in amplifier as the wavelength is changed. The intensity output of the optical system was measured using calibrated photodiodes covering the wavelength range 800-2600 nm. The system showed measureable light over the entire wavelength range. To couple the max amount of IR to the detector, a large NA and core size silica fiber and a small filament, high power light source are considering to substitute into the system. And the ultimately illumination power can be increased by 25 times.This demonstrates that this system can be used to characterize detectors fabricated from Au-hyperdoped Si.

Committee:

Jay Mathews (Advisor); Andrew Sarangan (Committee Member); Imad Agha (Committee Member)

Subjects:

Electrical Engineering; Materials Science

Keywords:

Hyperdoped Si; optical system; IR material

Nguyen, QuynhGiao N.High Temperature Volatility and Oxidation Measurements of Titanium and Silicon Containing Ceramic Materials
Doctor of Philosophy in Clinical-Bioanalytical Chemistry, Cleveland State University, 2008, College of Science
Titanium (Ti) and silicon (Si) containing materials are of high interest to the aerospace industry due to its high temperature capability, strength, and light weight. A continuous exterior oxide layer is desirable to reduce the oxidation rate of these two materials. At high temperatures, water vapor plays a key role in the volatility of materials including oxide surfaces. This study first evaluated several hot-pressed Ti and Si-containing compositions at high temperatures as a function of oxidation resistance. This study also evaluated cold pressed titanium dioxide (TiO2) powder pellets at a temperature range of 1400°C - 1200°C in water containing environments to determine the volatile hydoxyl species using the transpiration method. The water content ranged from 0-76 mole % and the oxygen content range was 0-100 mole % during the 20-250 hour exposure times. Results indicate that oxygen is not a key contributor at these temperatures and a volatile Ti-O-H species has been identified.

Committee:

Lily M. Ng, PhD (Committee Chair); James L. Smialek, PhD (Advisor); Kang N. Lee, PhD (Committee Member); John F. Turner II, PhD (Committee Member); Mary V. Zeller, PhD (Committee Member)

Subjects:

Aerospace Materials; Chemistry; Materials Science

Keywords:

aerospace; ceramic; combustion environment; high temperature; hydroxyl species; oxidation; materials; Si; silicon; titanium dioxide; TiO2; Ti-O-H; transpiration method; transpiration technique; volatility; water vapor

Mirous, Brian KSYNTHESIS AND PRESUMPTIVE CROSSLINKING OF STIMULI-RESPONSIVE DIBLOCK POLYMER BRUSHES
Doctor of Philosophy, University of Akron, 2006, Polymer Science
The objective of this research was to create diblock polymer brushes containing a functional polymer as the bottom block and a polystyrene (PS) top block to obtain a “switchable surface” that displays a rearrangement into pinned micelles upon treatment with block selective solvents. The goal was also to crosslink these brushes to stabilize any changes in morphology and surface composition after rearrangement. Effectiveness of the presumed crosslinking was determined by further exposure of the brushes to solvent or thermal treatment. Brushes of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) were prepared by an in situ surface-initiated polymerization from flat silicon substrates via atom transfer radical polymerization (ATRP). Reaction conditions were further optimized so that a sample could be removed from the polymerization solution, analyzed, and then subjected to a subsequent polymerization to prepare an additional PDMAEMA block of similar thickness, suggesting efficient re-initiation of chains. Styrene was polymerized from an optimized Si/SiO2//PDMAEMA brush to give a Si/SiO2//PDMAEMA-b-PS brush. This brush exhibited switching properties, as measured by water contact angles, when exposed to solvent or thermal treatment. No pinned micelles were observed; presumably the low glass transition temperature (19 °C) of the PDMAEMA prevented rough surface features at ambient temperature. Crosslinking was investigated by exposing the switched brush to an á,ù-dihaloalkane. The presumptive crosslinking did not always make surface composition permanent, as evidenced by water contact angle changes upon further solvent or thermal treatment. Similar experiments were done to prepare poly(2-hydroxyethyl methacrylate) (HEMA) and poly(glycidyl methacrylate) (PGMA) brushes. The brushes were extended with styrene to give Si/SiO2//PHEMA-b-PS and Si/SiO2//PGMA-b-PS brushes that again exhibited reversible switching properties. A pinned micelle morphology was not observed, and the surface roughness of the brushes in the extended and switched states was similar to that of Si/SiO2//PHEMA and Si/SiO2//PGMA homopolymer brushes. Crosslinking was investigated by exposing the switched Si/SiO2//PHEMA-b-PS and Si/SiO2//PGMA-b-PS brushes to a difunctional acyl chloride and multifunctional primary amines, respectively. In all cases the presumed crosslinking was relatively ineffective in preventing the brushes from further responding to solvent or thermal treatment, as measured by water contact angles.

Committee:

William Brittain (Advisor)

Subjects:

Chemistry, Polymer

Keywords:

brush; BRUSHES; PDMAEMA; ATRP; Si/SiO2; polymerization; PHEMA

Ratcliff, ChristopherGrowth and Characterization of III-Phosphide Materials and Solar Cells for III-V/Si Photovoltaic Applications
Doctor of Philosophy, The Ohio State University, 2014, Electrical and Computer Engineering
Utilizing major advances in III-V/Si epitaxy is a promising approach to realize high-efficiency, cost efficient terrestrial solar power. The successful demonstration of planar GaP epitaxial layers free of nucleation-related defects grown on Si by both MBE and MOCVD establishes extremely fertile ground for the development of high quality III V devices on Si substrates. Solar cell devices in particular stand to benefit, and not only by the merging of the best performing photovoltaic materials with the cost, availability, and scalability advantages of Si substrates, but also because of the advanced state of Si PV itself. With this motivation in mind, several aspects of a III-V/Si multijunction solar cell are investigated in this dissertation. Such a device requires many advanced and novel components. The use of GaP as the bridge from Si to the wide range of III-V materials renews interest in a material whose activity in research efforts had faded significantly since the advent of modern growth techniques and GaAs-based heterostructures in the 1980s. GaP-based research is also motivated by the fact that it represents the far lesser studied binary component of both GaxIn1-xP and GaAsyP1-y, two alloys essential to the wider scope of high efficiency PV research. Efforts to advance the ability to grow GaP by MBE and MOCVD were undertaken. Post-growth surface morphologies of homoepitaxial GaP films grown by MBE and MOCVD have been studied. Smooth, stepped surface morphologies of MBE-grown layers, measured by atomic force microscopy, were found for a wide range of substrate temperatures and P2:Ga flux ratios. A MOCVD-based growth study performed under similar conditions to MBE-grown samples show a nearly identical smooth, step-flow surface morphology, presenting a convergence of growth conditions for the two different methods. Electrical characterization of these films was then carried out by Hall effect measurements and DLTS to determine transport properties and to test for the presence of defect levels. Si-doped GaP showed mobilities that compare well to previous studies concerning n-type GaP grown by vapor- and liquid-phase growth methods. An activation energy greater than 80 meV, confirming the relatively deep position of the Si donor atom in these GaP films, was extracted from fits of temperature dependent Hall effect measurements, agreeing with similar measurements carried out several decades ago on vapor-phase grown samples. DLTS measurements revealed seven distinct majority-carrier deep levels for the films grown by MBE and current experiments to compare MBE and MOCVD films are discussed. The other major material of interest in this work is GaxIn1-xP. A significant effort is currently being made throughout the PV area to develop high bandgap top junction subcells which are needed for a variety of technologies to continue to push the boundaries of high-efficiency solar cells. Ga-rich compositions of GaxIn1-xP are prime candidates that can achieve direct bandgaps of up to about 2.2 eV. Prototype Ga0.57In0.43P solar cells have been grown by MBE and MOCVD on GaAsyP1-y compositionally graded buffers in order to optimize the cell structures, compare performance between the two growth methods, and to investigate the effects of threading dislocation density on solar cell performance metrics and material properties. Major improvements were made in the subcells by both growth techniques. The MOCVD-grown subcells, at this juncture, have shown superior performance likely due to the greater degree of freedom in growth parameters, most notably the ability to grow at higher temperature to increase crystalline quality; a conclusion supported by the fact that post-growth annealing of the MBE-grown subcells resulted in better material properties as measured by photoluminescence as well as lower levels of depletion recombination current in the solar cell junction itself and better photovoltaic performance: larger VOC and FF. In addition to standard photovoltaic characterization methods, DLTS and DLOS were used to determine the presence of bandgap states in the MBE-grown solar cell devices and to observe their sensitivity to threading dislocation density. Three total majority carrier levels were found within the Ga0.57In0.43P bandgap, one by DLTS (EV + 0.70 eV) and two by DLOS (EV + 0.90 eV, EV + 1.82 eV). The results obtained during the course of this research present significant strides in the understanding of several challenging components essential to the development of a III-V/Si multijunction device. Experience gained for the growth and material properties of GaP epitaxial layers and for the development of high bandgap GaxIn1-xP solar cells are of interest to a wide range of semiconductor technologies. The comparison of the materials and devices between two of the most important and common modern semiconductor growth methods in both the research and commercial worlds expands the impact of the work.

Committee:

Steven Ringel, Dr. (Advisor)

Subjects:

Electrical Engineering

Keywords:

Semiconductor, III-V, Si, InGaP, GaP, Gallium Phosphide, MBE, MOCVD, Epitaxy, Metamorphic, Solar, Photovoltaics

Ravichandran, KarthikNano-scale process and device simulation
Master of Science, The Ohio State University, 2005, Materials Science and Engineering

With the tremendous increase in computational power over the last two decades, computer simulation has become an indispensable tool in device engineering. In this work, atomistic simulation based on ab-initio density functional theory (DFT) was used in modeling Si-based Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and Carbon Nanotube Field Effect Transistors (CNTFET).

In the case of MOSFETs, the dopant dose loss due to segregation and pileup at the Si/SiO2 interface have a strong influence in device performance, especially in the deep submicron regime. The previously suggested segregation model for arsenic at Si/SiO2 interfaces based on a combined trapping/pairing model [J. Dabrowski, H.-J. Mussig, V. Zavodinsky, R. Baierle, and M. J. Caldas, Phys. Rev. B 65, 245305 (2002)] requires high binding energies for the interface vacancies, which our results of ~0.2 eV cannot confirm. As an alternative explanation, we present ab initio results that show that As and hydrogen bond with an energy gain of 1.5-3 eV with their minimum-energy position at the interface, which creates additional trapping sites for As segregation. The inclusion of hydrogen into the modeling might thus be able to explain the perceived differences between the previous model and experiments.

With the continuous downscaling of MOSFETs, the physical limits for the current Si based CMOS technology will be reached, in terms of device dimensions and materials properties, within the next few years. The future transistors will be based on molecular electronics, like carbon nanotubes, utilizing their ballistic electron transport property. In CNTFETs, the nanotube/metal contact forms a crucial region which exerts considerable influence on the device characteristics, similar to the role of pn-junctions in traditional MOSFETs. Whereas in MOSFETs the junction formation is reasonably well understood and can be virtually engineered by process simulation within technology computer aided design (TCAD), such a capability does not really exist to date for molecular devices. Here, we suggest such molecular process modeling and study the evolution of the atomic structure of the contact between Ti-metal and a CNT by ab-initio temperature-accelerated dynamics and the respective change in the electron transport properties. Our results indicate that the CNT disintegrates on top of Ti to form a flat graphene-like structure causing a decrease in the current through the device by one order of magnitude in comparison to the optimum structure, in agreement with experiments. Thus, we show that the use of “molecular process simulation” to follow the evolution of the appropriate contact structure for the interface is a key factor in modeling electron transport through molecular devices.

Committee:

Wolfgang Windl (Advisor)

Keywords:

CNT; Si/SiO2; Nanotube; dopant; electron

Kovarik, LiborMicrostructural study and modeling of metastable phases and their effect on strenghthening in Al-Mg-Cu-Si alloying system
Doctor of Philosophy, The Ohio State University, 2006, Materials Science and Engineering
In this work, microstructure evolution of an Al-3Mg-0.4Cu-0.12Si (wt%) alloy was studied during artificial aging at 180°C and 200°C. The primary investigation methods used were conventional and high resolution transmission electron microscopy (TEM and HRTEM), Z-contrast scanning TEM, and atom probe tomography (APT). Two new metastable phases are found to successively form during 180°C aging, designated as GPB and GPBII zones. The first forming GPB zones are rod-like regions aligned along <100>Al directions. As supported by ab initio calculation, the GPB zones are L10 ordered, with alternating Mg enriched and Si, Cu, Al enriched layers. In the later stages of aging, GPBII are detected as nanometer size-scale, homogeneously dispersed, coherent particles. The crystal structure of the GPBII can be explained by a hexagonal lattice, with parameters a=0.7, c=0.4nm and space group P-62m. The stoichiometry of the proposed GPBII zone is Al3Mg3Si2Cu1, which in terms of the solute elements agrees with the elemental analysis. Ab initio calculations show very favorable enthalpy of formation of –14.4 kJ/mol. At higher temperature aging and longer aging times, such as 16 hours at 200°C, the microstructure is found to consist of two different crystallographic types of GPBII zones, which can be differentiated based on their location in the matrix. The isolated GPBII zones are comparable to those from lower temperature aging; however, the GPBII zones associated with S-phase particles have a different crystal structure, which seem to be crystallographically related to the structure of lath-like particles in some 6xxx series alloys. At the later stages of aging, the structure also consists of isolated S-phase particles, which have a rod-like morphology and modified orientation relationship (S''-phase). The origin of the modified orientation relationship for S''-phase was studied. It is found that its establishment is related to coherency at the (021)S//(014)Al interface, which is shown to coexist with the commonly present (001)S//(021)Al. Apart from the microstructural studies, attention was paid to the mechanical properties of alloys of similar composition from various aging treatments. The observed strengthening from the early stages of aging was modeled based on a contribution from GPB zones considering three different mechanisms.

Committee:

Michael Mills (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

Al-Mg-Cu-(Si) alloys; Age hardening; GPB and GPBII zones; S''-phase; HRTEM

Dahal, Lila RSpectroscopic Ellipsometry Studies of Thin Film a-Si:H Solar Cell Fabrication by Multichamber Deposition in the n-i-p Substrate Configuration
Doctor of Philosophy, University of Toledo, 2013, College of Arts and Sciences
Real time spectroscopic ellipsometry (RTSE), and ex-situ mapping spectroscopic ellipsometry (SE) are powerful characterization techniques capable of performance optimization and scale-up evaluation of thin film solar cells used in various photovoltaics technologies. These non-invasive optical probes employ multichannel spectral detection for high speed and provide high precision parameters that describe (i) thin film structure, such as layer thicknesses, and (ii) thin film optical properties, such as oscillator variables in analytical expressions for the complex dielectric function. These parameters are critical for evaluating the electronic performance of materials in thin film solar cells and also can be used as inputs for simulating their multilayer optical performance. In this Thesis, the component layers of thin film hydrogenated silicon (Si:H) solar cells in the n-i-p or substrate configuration on rigid and flexible substrate materials have been studied by RTSE and ex-situ mapping SE. Depositions were performed by magnetron sputtering for the metal and transparent conducting oxide contacts and by plasma enhanced chemical vapor deposition (PECVD) for the semiconductor doped contacts and intrinsic absorber layers. The motivations are first to optimize the thin film Si:H solar cell in n-i-p substrate configuration for single-junction small-area dot cells and ultimately to scale-up the optimized process to larger areas with minimum loss in device performance. Deposition phase diagrams for both i- and p-layers on 2" x 2" rigid borosilicate glass substrate were developed as functions of the hydrogen-to-silane flow ratio in PECVD. These phase diagrams were correlated with the performance parameters of the corresponding solar cells, fabricated in the Cr/Ag/ZnO/n/i/p/ITO structure. In both cases, optimization was achieved when the layers were deposited in the protocrystalline phase. Identical solar cell structures were fabricated on 6" x 6" borosilicate glass with 256 cells followed by ex-situ mapping SE on each cell to achieve better statistics for solar cell optimization by correlating local structural parameters with solar cell parameters. Solar cells of similar structure were also fabricated on flexible polymer substrates in the roll-to-roll configuration. In this configuration as well, RTSE was demonstrated as an effective process monitoring and control tool for thin film photovoltaics.

Committee:

Robert Collins , Dr. (Committee Chair); Nikolas Podraza, Dr. (Committee Member); Song Cheng, Dr. (Committee Member); Sanjay Khare, Dr. (Committee Member); Andre Ferlauto, Dr. (Committee Member)

Subjects:

Physics

Keywords:

real time spectroscopic ellipsometry; photovoltaics; a-Si:H solar cell; deposition phase diagram; spatial phase diagram; roll-to-roll deposition; back-reflector; phasmon resonance; surface roughness evolution; effective medium theory

Gopalakrishna, KeshavaFrequency Characterization of Si, SiC, and GaN MOSFETs Using Buck Converter In CCM as an Application
Master of Science in Engineering (MSEgr), Wright State University, 2013, Electrical Engineering
Present day applications using power electronic converters are focusing towards improving the speed, efficiency, and robustness. This led to the implementation of new devices in such converters where speed and efficiency are of concern. As silicon (Si) based power devices are approaching their operational performance limits with respect to speed, it is essential to analyze the properties of new devices, which are capable of replacing silicon based devices. Wide band-gap (WBG) semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN) are such materials, whose material properties show promising advantages for power electronic applications. This thesis focuses on the comparison of Si, SiC, and GaN based power devices. A detailed comparison in terms of the material performance based on their figures-of-merit will be discussed. In this thesis, a performance evaluation of Si, SiC, and GaN based power devices used as a high-side switch in a buck DC-DC converter will be performed. A buck converter having specifications: output voltage of 12 V and output power of 120 W. Initially, a design example for switching frequency of 100 kHz will be discussed. Further, an evaluation of the same for increase in switching frequencies will be performed. Finally, analyses of the power loss and efficiency of these devices will be made along with its validation using PSpice, SABER and MATLAB simulation software. It will be shown that the theoretical performance analyses are in accordance with the obtained simulated results. Finally, it will be shown that GaN based power devices have improved operational capabilities at high frequencies than those of Si and SiC.

Committee:

Marian K. Kazimierczuk, Ph.D. (Advisor); Henry Chen, Ph.D. (Committee Member); Saiyu Ren, Ph.D. (Committee Member)

Subjects:

Aerospace Materials; Electrical Engineering; Engineering; Materials Science; Technology

Keywords:

power MOSFETs, Gan, SiC, Si, high frequency buck converter

Kulisek, Jonathan AndrewThe Effects of Nuclear Radiation on Schottky Power Diodes and Power MOSFETs
Doctor of Philosophy, The Ohio State University, 2010, Nuclear Engineering

NASA is exploring the potential use of nuclear reactors as power sources for future space missions. These missions will require electrical components, consisting of power circuits and semiconductor devices, to be placed in close vicinity to the reactor, in the midst of a high neutron and gamma-ray radiation field. Therefore, the primary goal of this research is to examine the effects of a mixed neutron and gamma-ray radiation field on the static and dynamic electrical performance of power Schottky diodes and power MOSFETs in order to support future design efforts of radiation-hard power semiconductors and circuits.

In order to accomplish this goal, commercial Si and 4H-SiC Schottky barrier power diodes were irradiated in the mixed neutron and gamma-ray radiation field of The Ohio State University research reactor (OSURR). The forward I-V characteristics were measured before and immediately after each successive radiation dose and the carrier-removal rates were compared, on the basis of NIEL, to a previous study, for which the same diode models were irradiated with a 203 MeV proton beam. In addition, a number of SiC Schottky barrier diodes were also irradiated in the OSURR and subsequently functionally tested in half-wave rectifier circuits, for which the voltage and current waveforms in the circuit were recorded. The results from the functional testing of these half-wave rectifier circuits were analyzed using results from I-V characterization, PSpice simulations, and an analytical formulation.

In addition, boost and buck converters containing commercial power MOSFETs and Schottky diodes that were irradiated to various doses in a mixed neutron and gamma-ray radiation field, were tested. In addition to overall circuit performance in terms of output voltage and efficiency, the individual voltage and current waveforms of the MOSFET and diode in each circuit were examined. Radiation-induced changes in the switching characteristics of the MOSFETs were observed. Furthermore, changes in overall circuit performance and increased power dissipation in the MOSFETs during the over-voltage turn-off transient and on-state conduction portions of the switching cycle were observed. In addition to I-V characterization of the MOSFETs and diodes, PSpice simulations were performed in order to aid in the analysis and interpretation of the experimental results.

Committee:

Thomas Blue, PhD (Advisor); Tunc Aldemir, PhD (Committee Member); Don Miller, PhD (Committee Member)

Subjects:

Electrical Engineering; Nuclear Physics

Keywords:

power MOSFET; SiC; Si; Schottky diode; NIEL; displacement damage; TID; DC-to-DC switching converters; radiation effects

Andre, Carrie L.III-V semiconductors on SiGe substrates for multi-junction photovoltaics
Doctor of Philosophy, The Ohio State University, 2004, Electrical Engineering

The epitaxial integration of III-V semiconductors with Si is of interest for photovoltaics since Si substrates offer a lighter, stronger, and cost effective platform for device production. By using compositionally step-graded SiGe layers to 100% Ge, the 4% lattice-mismatch between Si and GaAs and In 0.49 Ga 0.51 P is accommodated; this method has produced record low threading dislocation densities (TDD) of 1x10 6 cm -2 in fully relaxed the Ge/SiGe/Si (SiGe) substrates. In this dissertation, this method of III-V/Si integration is used for the development of GaAs and In 0.49 Ga 0.51 P single junction (SJ) solar cells and In 0.49 Ga 0.51 P /GaAs dual junction (DJ) solar cells, integrated on a Si platform. As such, we report that the minority carrier electron lifetime in p-type GaAs grown on Si is lower than that of holes in n-type GaAs at a given TDD and is a consequence of the higher mobility of electrons. This lower lifetime produced higher reverse saturation currents and lower open-circuit voltages for n+/p compared to p+/n configuration GaAs cells grown on SiGe with the same TDD. The higher performance of the p+/n GaAs/Si cell, by virtue of its higher open-circuit voltage, has demonstrated a record terrestrial efficiency of 18.1% and has been produced in areas up to 4 cm 2 with no degradation in cell performance. In 0.49 Ga 0.51 P SJ cells were integrated on Si substrates and an increase in depletion region recombination component of the reverse saturation current with TDD was also measured. A p+/n polarity preference for In 0.49 Ga 0.51 P on Si was demonstrated, although, the lower mobility of both carriers in In 0.49 Ga 0.51 P compared to GaAs, suggests a greater TDD tolerance. Based on these SJ results, the first realization of an In 0.49 Ga 0.51 P/GaAs DJ cell on Si with an output voltage greater than 2 V was demonstrated. A comparison with an identical DJ cell on GaAs found that the DJ cell on Si retained 91% of open-circuit voltage and 99% of short-circuit current; moreover, the p+/n In 0.49 Ga 0.51 P/GaAs DJ cell grown on Si demonstrated an open-circuit voltage that was consistent with the value predicted by the metamorphic dual junction device model developed in this thesis.

Committee:

Steven Ringel (Advisor)

Subjects:

Physics, Condensed Matter

Keywords:

Si; SiGe; GaAs; InGaP; GaInP; solar cells; photovoltaics; dual junction; heteroepitaxy; lattice-mismatch; threading dislocation; minority carrier lifetime; reverse saturation current; open-circuit voltage

Munbodh, KineshmaEvanescent Microwave Characterization of carbon Nanotube Films Grown on Silicon Carbide Substrate
Master of Science (MS), Wright State University, 2007, Physics
The electromagnetic characterization of carbon nanotube films (CNT) grown by the surface decomposition of silicon carbide (SiC) has been performed. The CNT films formed on the carbon and silicon terminated face of the SiC substrate were uncapped by an annealing process at a temperature of 4000 C with dwelling time up to 60 minutes in oxygen or carbon dioxide atmosphere. X-Y scans of the quality factor were used to deduce the local conductive properties of the films measured by evanescent microwave microscopy. Real and imaginary permittivity values, as determined by these electromagnetic measurements, provided valuable information for future field emission testing on these films. A theoretical model, adapted from the literature, was used to find the real and imaginary component of the permittivity of the CNT films. The results showed improvement in the surface conductivity of the samples after the annealing treatment.

Committee:

Gregory Kozlowski (Advisor)

Subjects:

Physics, General

Keywords:

CNT; CNT films; Si-face; NANOTUBE; CARBON NANOTUBE; CARBON; Fig

Brenner, Mark R.GaP/Si Heteroepitaxy (Suppression of Nucleation Related Defects)
Master of Science, The Ohio State University, 2009, Electrical and Computer Engineering

Integration of III-V’s with Si has been under investigation for many years, but the physical material mismatch issues between the two have impaired all previous attempts at defect-free hetero-integration. The lattice constant mismatch and thermal expansion coefficient differences along with the heterovalent (i.e. polar/nonpolar) nature of the IIIV/Si interface are the sources of numerous harmful defects and material issues. Any credible solution to the hetero-integration of III-V/Si must include the control of these material concerns. GaP/Si is the most promising pathway toward accomplishing this goal, using the nearly lattice matched GaP to decouple the lattice mismatch obstacle from the other material issues.

This thesis focuses on the development of a process for the heteroepitaxial growth of GaP on Si(001) via MBE, demonstrating the suppression of nucleation-related defects, including antiphase domains (APD), stacking faults (SF) and microtwins (MT) arising from the polar/nonpolar interface. To accomplish this task, an extensive silicon substrate preparation study was conducted to create a suitable silicon surface for GaP nucleation at the initial stages of heteroepitaxy. Through these experiments, a clean DASR silicon surface was produced, creating an optimal surface template for GaP heteroepitaxial growth. GaP was then grown through migration enhanced epitaxy (MEE) to initiate planar GaP wetting layer at the polar/nonpolar interface, with GaP MBE subsequently grown on the GaP-MEE layer. The resultant GaP/Si heterointegrated materials system was characterized using AFM, TEM, XRD and SIMS finding for the first time, GaP/Si heteroepitaxially integrated using a process that effectively and simultaneously eliminated APDs, SFs and MTs. In addition, bulk GaP crystallinity was found to be exceptional and autodoping at the GaP/Si interface is sufficiently suppressed.

Committee:

Ringel Ringel, PhD (Advisor); Siddharth Rajan, PhD (Committee Member)

Subjects:

Electrical Engineering; Materials Science

Keywords:

GaP/Si; MBE; migration enhanced epitaxy; MEE; APDs; stacking faults

Parker, William DavidSpeeding Up and Quantifying Approximation Error in Continuum Quantum Monte Carlo Solid-State Calculations
Doctor of Philosophy, The Ohio State University, 2010, Physics

Quantum theory has successfully explained the mechanics of much of the microscopic world. However, Schrödinger's equations are difficult to solve for many-particle systems. Mean-field theories such as Hartree-Fock and density functional theory account for much of the total energy of electronic systems but fail on the crucial correlation energy that predicts solid cohesion and material properties.

Monte Carlo methods solve differential and integral equations with error independent of the number of dimensions in the problem. Variational Monte Carlo (VMC) applies the variational principle to optimize the wave function used in the Monte Carlo integration of Schrödinger's time-independent equation. Diffusion Monte Carlo (DMC) represents the wave function by electron configurations diffusing stochastically in imaginary time to the ground state.

Approximations in VMC and DMC make the problem tractable but introduce error in parameter-controlled and uncontrolled ways. The many-electron wave function consists of single-particle orbitals. The orbitals are combined in a functional form to account for electron exchange and correlation. Plane waves are a convenient basis for the orbitals. However, plane-wave orbitals grow in evaluation cost with basis-set completeness and system size. To speed up the calculation, polynomials approximate the plane-wave sum. Four polynomial methods tested are: Lagrange interpolation, pp-spline interpolation, B-spline interpolation and B-spline approximation. The polynomials all increase speed by an order of the number of particles. B-spline approximation most consistently maintains accuracy in the seven systems tested. However, polynomials increase the memory needed by a factor of two to eight. B-spline approximation with a separate approximation for the Laplacian of the orbitals increases the memory by a factor of four over plane waves.

Polynomial-based orbitals enable larger calculations and careful examination of error introduced by approximations in VMC and DMC. In silicon bulk and interstitial defects, tens of variational parameters in the wave function converge the VMC energy. A basis set cutoff ≅1000 eV converges the VMC energy to within 10 meV. Controlling the population of electron configurations representing the DMC wave function does not bias the energy above 24 configurations. An imaginary time step for the configurations of 10-2 hartree-1 introduces no error above the 10 meV level. Finite-size correction methods on the 16-atom cell size with difference up to 2 eV error and 1 eV discrepancy between 16- and 64-atom cells indicate finite-size error is still significant. Pseudopotentials constructed with and without scalar relativistic correction agree in DMC energy differences at the 100 meV level, and mean-field calculations with and without pseudopotentials suggest a correction of 50-100 meV. Using the VMC wave function to evaluate the nonlocal portion of the pseudopotential introduces an error on the 1 meV level. DMC energies using orbitals produced with varying mean-field approximations produce a 1 eV range in the defect formation energies while applying a backflow transformation to the electron coordinates reduces Monte Carlo fluctuations. The backflow-transformed average also permits an extrapolation to zero fluctuation. The extrapolated value estimates the formation energy unbiased by the starting wave function to be 4.5-5 eV.

Committee:

John W. Wilkins (Advisor); Nandini Trivedi (Committee Member); Jay A. Gupta (Committee Member); Michael G. Poirier (Committee Member); Dhabaleswar Panda (Committee Member)

Subjects:

Materials Science; Physics

Keywords:

quantum Monte Carlo; polynomial approximation; approximation error; Si interstitial defects

O'Dell, Benjamin DanielHenry Jekyll, Sherlock Holmes, and Dorian Gray: Narrative Politics and the Representation of Character in Late-Victorian Gothic Romance
Master of Arts, Miami University, 2008, English
This thesis explores the function of iconic literary characters in late-Victorian gothic romance as expressed through the contemporary debates they embody as narrative types. Chapter 1 examines the paradoxical position of the Victorian gentleman's public identity through a reading of Robert Louis Stevenson's Strange Case of Dr. Jekyll and Mr. Hyde. Chapter 2 analyzes the relationship between Sherlock Holmes's position on the social periphery and the tale of imperial corruption he exposes in Arthur Conan Doyle's The Sign of Four. Chapter 3 discusses the dandy's ambiguous moral state as a product of economic and cultural changes among wealthier residents in London's West End that were connected to debates about their group's role in relation to charity, consumerism, and culture. These findings suggest characters that are often read as personifying complex literary aspirations may also be approached productively as vessels that are capable of addressing difficult issues on innocuous terms.

Committee:

Susan Morgan, PhD (Committee Chair); Mary Jean Corbett, PhD (Committee Member); Madelyn Detloff, PhD (Committee Member)

Subjects:

English literature

Keywords:

Robert Louis Stevenson; Strange Case of Dr. Jekyll and Mr. Hyde; Arthur Conan Doyle; The Sign of Four; Oscar Wilde; The Picture of Dorian Gray; character; fin de si&232;cle; late Victorian gothic romance; city; London; performance; class

Guzman-Verri, Gian GiacomoElectronic Properties of Silicon-based Nanostructures
Master of Science (MS), Wright State University, 2006, Physics
We have developed a new unifying tight-binding theory that can account for the electronic properties of recently proposed Si-based nanostructures, namely, Si graphene-like sheets and Si nanotubes. We considered the sp2s* and sp3 models up to first- and second-nearest neighbors, respectively. Our results show that the corresponding Si nanotubes follow the so-called Hamada's rule. Comparison to a recent ab initio calculation is made.

Committee:

Lok Lew Yan Voon (Advisor)

Subjects:

Physics, Condensed Matter

Keywords:

Si; nanotubes; graphene; tight-binding; sp2s*; sp3

Gladysz, Gary M.Remote microwave-enhanced chemical vapor deposition of silicon-nitrogen (Si xN y) thin films
Master of Science (MS), Ohio University, 1991, Chemical Engineering (Engineering)
Remote microwave-enhanced chemical vapor deposition of silicon-nitrogen (Si xN y) thin films

Committee:

Daniel Gulino (Advisor)

Subjects:

Engineering, Chemical

Keywords:

Remote Microwave-Enhanced Chemical; Vapor Deposition; Silicon-nitrogen (Si xN y) Thin Films

Pakiru, SwapnaEffect of Step Change in Growth Speed During Directional Solidification on Array Morphology of Al-7 wt% Si Alloy
Master of Science in Chemical Engineering, Cleveland State University, 2011, Fenn College of Engineering

Dendritic single crystals of Al-7wt%Si alloy have been directionally solidified at a thermal gradient of 40 K cm-1 over growth speeds, ranging from 5 to 85 to 31µm s-1, using aluminum single crystal seed aligned along [100] crystallographic orientation. Mushy zone morphology parameters, such as, primary dendrite nearest neighbor spacing, primary dendrite trunk diameter, side (secondary dendrite) branch length and side branch orientation, have been characterized in to investigate the transients introduced by step increase and step decrease in growth speed.

An increase in the growth speed shows a decrease in both nearest neighbor spacing and trunk diameter. A comparison between the two parameters suggests that the trunk-diameter may be a better metric to quantify differences due to small change in processing parameters, such as, growth in low-gravity in the absence of convection. The trend of decreasing trunk diameter with increasing growth speed is similar to the primary dendrite tip radii trend (predicted by theoretical models). Statistical analysis of the transverse microstructures after the step decrease in growth speed (from 85 to 31µm s-1) showed that primary dendrites which survive the transient are the ones with larger neighbor spacing. However, the predictive ability of nearest-neighbor distance (NNS-1), mean of four nearest neighbor distances (NNS-4) or the mean of six nearest neighbor distances (NNS-6) in terms of which dendrites are likely to dissolve-off appears to be similar. Average side-branch length of the surviving primary dendrites increases and those of the disappearing primary dendrites decreases after the growth speed decrease. Primary dendrites whose side-arms are not orthogonal are more likely to dissolve-off than those which are aligned closer to [100]. Porosity formation during directional solidification can lead to spurious grain formation which will seriously degrade the high temperature creep properties of directionally solidified components.

Committee:

Surendra N Tewari, PhD (Committee Chair); Orhan Talu, PhD (Committee Member); Dhananjai B Shah, PhD (Committee Member)

Subjects:

Chemical Engineering; Materials Science; Metallurgy

Keywords:

Directional Solidification; Step change; Growth Speed; Morphology; Nearest Neighbor Spacing; Trunk diameter; Side branch length; Side branch orientation; Al- 7 wt% Si alloy; thermal gradient; step increase; step decrease; steady state; transition.

Guo, MingSYNTHESIS AND CHARACTERIZATION OF SILICON PHTHALOCYANINES FOR PHOTODYNAMIC THERAPY
Doctor of Philosophy, Case Western Reserve University, 2008, Chemistry

A series of Pc 4 related compounds consisting of OH-substituted derivatives, Pc 4 salts and Pc 4 OH-substituted derivative salts has been prepared. The salts have been synthesized by straightforward acid-base reactions. Solubility studies of the Pc 4, the OH-substituted derivatives, the Pc 4 salts and the OH-substituted derivative salts in representative organic solvents and in aqueous dispersant-containing solutions have been carried out. Different dosage forms of Pc 4 including a liquid salt, aqueous solutions, a mineral oil solution, gels, a cream, and water-swellable films have been formulated.

In addition, membrane penetration studies of the formulation of Pc 4 used in a completed Phase I clinical trial done at University Hospitals of Cleveland have been conducted. Polyurethane, cellulose, polyvinylidene fluoride, silicone, pig keratome, and human roof blister membranes have been used in the study.

In unrelated work, the known polymers, chrysotile-derived trimethylsiloxy scroll polymer, and chrysotile-derived vinyldimethylsiloxy scroll polymer have been made. Also, the new polymers chrysotile-derived trimethylsiloxy-3-cyanopropylmethylmethoxysiloxy scroll polymer and chrysotile-derived vinyldimethylsiloxy-3-cyanopropylmethylmethoxysiloxy scroll polymer, have been made by a novel pendent group-exchange reaction. Finally, partially platinum-filled scrolls have been prepared.

Committee:

Malcolm Kenney (Advisor); Thomas Gray (Committee Chair); John Protasiewicz (Committee Member); Clemens Burda (Committee Member); Nancy Oleinick (Committee Member)

Subjects:

Chemistry

Keywords:

Pc; Si; CH2; CH3; 3NH; HOSiPcOSi; CH2Cl2

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