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School of Physics

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https://hdl.handle.net/1853/70755

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College of Sciences

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Center for Relativistic Astrophysics

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Mourigal Lab

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https://finding-aids.library.gatech.edu/agents/corporate_entities/156

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Publication Search Results

Now showing 1 - 10 of 19

Scanning Probe Studies of Quasi-Two-Dimensional Organic Networks

(Georgia Institute of Technology, 2021-12-13) Enderson, Zachery A.

Organic based polymer materials have an abundance of potential applications due to the ability to tailor their molecular components and network topology. The experimental work featured in this thesis focuses on a small selection of quasi-two-dimensional materials within two subclasses of these polymers, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). Both are porous networks of organic molecules linked together by metal ions (MOFs) or covalent bonds (COFs). In layered 3D variants, this porosity grants the material an impressive internal surface area available for catalysis, gas storage, and molecular separation. In 2D, the organic molecular network produces a variety of electronic properties, with pi-bonding molecular orbitals predicted to create both Dirac bands and flat bands. This work presents studies of the atomic and electronic structures of these materials by ultrahigh vacuum (UHV) scanning probe microscopy (SPM).
Two-dimensional phases of silicon on silicon carbide

(Georgia Institute of Technology, 2020-07-14) Wu, Hsin-Ju

2D materials have been widely studied since single layer graphene, a 2D atomic honeycomb structure with unusual electronic properties, was obtained. Graphene grown on SiC by thermal decomposition is of interest because of its potential for fine control of the epitaxial growth directly on a crystalline semiconductor substrate. Control of this growth has been obtained by introducing Si vapor into the growth environment. Other work has shown that this approach is valuable for the production of unique nanostructures of graphene; in this work, we explore the possibility to grow 2D thin films of silicon on the SiC substrate. An understanding of growth dynamics and surface phases is of interest for the creation of electronic-grade graphene on SiC, but there is also the potential for electronically useful 2D phases of silicon. For instance, the Si 2D honeycomb structure, silicene, is considered a potential next-generation material for electronic devices. In this thesis, we describe experiments undertaken in order to understand growth dynamics and Si surface phases in the pre-graphene regime of quasi-equilibrium growth, determined by the temperature and Si vapor pressure. A novel LEED pattern (√43 x √43 R7.6 degree) for a complex 2D Si structure is found on both the Si-terminated and C-terminated faces of SiC. The hexagonal structure and epitaxial matching constraints are consistent with silicene, but ultimately we show that a different structure is a more likely explanation. Based on experimental data and established structures from other research groups, a model for the newly-discovered phase of Si on SiC(0001) is proposed. By comparing growth conditions and other experimental data collected, we conclude that the structure contains three tetramers similar to the Si-rich 3 x 3 structure and three bridge-atom formations, reducing the number of dangling bonds to just three per unit cell. Empirical methods underlying the development of this model are discussed. Similar approaches are potentially of use for other 2D phases of Si on SiC.
Studies of epitaxial graphene and silicon growth on silicon carbide under silane gas

(Georgia Institute of Technology, 2017-04-07) Hoang, Tien Manh

Two-dimensional (2D) materials have drawn much attention because of their superior and unique properties. Undoubtedly, the most well-known 2D material is graphene, an atomic-thick sheet of carbon in a honeycomb lattice. Up to date, many synthesized techniques were discovered; however, epitaxial graphene on silicon carbide (SiC) is still one of the most promising methods to produce high-quality graphene on semiconductor substrates. This thesis focuses on studying the epitaxial silicon/graphene growth on SiC under silane/argon gas mixtures using the confinement controlled sublimation technique. The morphology and layer coverage of the silicon/graphene thin films are characterized in-situ by LEED and Auger spectroscopy and ex-situ by AFM, SEM, and STM. Prior to the graphitization temperature, silicon deposits on SiC surface to grow thin film layers. On the Si-face, LEED images reveal several new reconstructions which have not reported elsewhere. At graphitization temperature, step bunching forms on vicinal silicon carbide with a power law relation between the average bunch size and the local angle. The formation and evolution of step bunching are compared with numerical solutions of the theory of Burton, Cabera, and Frank (BCF).
Nanostructured graphene on Si-terminated SiC and its electronic properties

(Georgia Institute of Technology, 2016-03-10) Li, Yuntao

Graphene nanostructures directly grown on SiC are appealing for their potential application to nano-scale electronic devices. In particular, epitaxial sidewall graphene nanoribbons have been a promising candidate in ballistic transport and band gap engineering. In this thesis, we study graphene nanoribbons by utilizing both nano-lithography and natural step bunching to control the step morphology of the SiC(0001) surface in order to guide the growth of graphene which initiates at step edges, and study their respective characteristics. With scanning tunneling microscopy and spectroscopy (STM/STS), we explore the local atomic and electronic structures of the graphene nanoribbons down to atomic scale. It is found that nanoribbon formation depends critically on nanofacet orientation, nanofacet density, and growth conditions. Under some conditions, nanoribbons grow predominantly on the nanofacet. Significant electronic density-of-states features, resolved by STS, are found to depend strongly on proximity to strained graphene near the step edge. Experimental results are compared to Molecular Dynamics simulations to better understand the origin of the discrete electronic states.
Growth and electronic properties of nanostructured epitaxial graphene on silicon carbide

(Georgia Institute of Technology, 2013-08-23) Torrance, David Britt

The two-dimensional phase of carbon known as graphene is actively being pursued as a primary material in future electronic devices. The goals of this thesis are to investigate the growth and electronic properties of epitaxial graphene on SiC, with a particular focus on nanostructured graphene. The first part of this thesis examines the kinetics of graphene growth on SiC(0001) and SiC(0001 ̅) by high-temperature sublimation of the substrate using a custom-built, ultra-high vacuum induction furnace. A first-principles kinetic theory of silicon sublimation and mass-transfer is developed to describe the functional dependence of the graphene growth rate on the furnace temperature and pressure. This theory can be used to calibrate other graphene growth furnaces which employ confinement controlled sublimation. The final chapter in this thesis involves a careful study of self-organized epitaxial graphene nanoribbons (GNRs) on SiC(0001). Scanning tunneling microscopy of the sidewall GNRs confirms that these self-organized nanostructures are susceptible to overgrowth onto nearby SiC terraces. Atomic-scale imaging of the overgrown sidewall GNRs detected local strained regions in the nanoribbon crystal lattice, with strain coefficients as high as 15%. Scanning tunneling spectroscopy (STS) of these strained regions demonstrate that the graphene electronic local density of states is strongly affected by distortions in the crystal lattice. Room temperature STS in regions with a large strain gradient found local energy gaps as high as 400 meV. Controllable, strain-induced quantum states in epitaxial graphene on SiC could be utilized in new electronic devices.
Local measurements of cyclotron states in graphene

(Georgia Institute of Technology, 2011-04-04) Kubista, Kevin Dean

Multilayer epitaxial graphene has been shown to contain "massless Dirac fermions" and is believed to provide a possible route to industrial-scale graphene electronics. We used scanning tunneling microscopy (STM) and spectroscopy (STS) in high magnetic fields to obtain local information on these fermions. A new STS technique was developed to directly measure graphene's energy-momentum relationship and resulted in the highest precision measurement of graphene's Dirac cone. STS spectra similar to ideal graphene were observed, but additional anomalies were also found. Extra peaks and an asymmetry between electron and hole states were shown to be caused by the work function difference between the Iridium STM tip and graphene. This tip effect was extracted using modeled potentials and performing a least square fit using degenerate perturbation theory on graphene's eigenstates solved in the symmetric gauge. Defects on graphene were then investigated and magnetic field effects were shown to be due to a mixture of potential effect from defects and the tip potential. New defect states were observed to localize around specific defects, and are believed to interact with the STM tip by Stark shifting in energy. This Stark shift gives a direct measurement of the capacitive coupling between the tip and graphene and agrees with the modeled results found when extracting the tip potential.
Atomic-scale spectroscopy and mapping of magnetic states in epitaxial graphene

(Georgia Institute of Technology, 2010-11-15) Miller, David Lee

Graphene grown epitaxially on silicon carbide provides a potential avenue toward industrial-scale graphene electronics. A predominant aspect of the multilayer graphene produced on the carbon-terminated (000 -1) face of SiC is the rotational stacking faults between graphene layers and their associated moire-pattern superlattice. We use scanning tunneling microscopy (STM) and spectroscopy (STS) in high magnetic fields to obtain detailed information about the "massless Dirac fermions" that carry charge in graphene. In agreement with prior investigations, we find that for small magnetic fields, the rotational stacking effectively decouples the electronic properties of the top graphene layer from those below. However, in maps of the wavefunction density at magnetic fields above 5 Tesla, we discover atomic-scale features that were not previously known or predicted. A phenomenological theory shows that this high-field symmetry-breaking is a consequence of small cyclotron-orbit wavefunctions, which are sensitive to the local layer stacking structures internal to the moire superlattice cell. The broken symmetry is sublattice degeneracy, predicated by atomic scale variations that derive from the sublattice polarization of graphene wavefunctions.
Microscopic and spectroscopic studies of growth and electronic structure of epitaxial graphene

(Georgia Institute of Technology, 2009-04-06) Sharma, Nikhil

It is generally believed that the Si technology is going to hit a road block soon. Amongst all the potential candidates, graphene shows the most promise as replacement material for the aging Si technology. This has caused a tremendous stir in the scientific community. This excitement stems from the fact that graphene exhibits unique electronic properties. Physically, it is a two-dimensional network of sp₂bonded carbon atoms. The unique symmetry of two equivalent sublattices gives rise to a linear energy dispersion for the charge carriers. As a consequence, the charge carriers behave like massless Dirac particles with a constant speed of c/300, where c is the speed of light. The sublattice symmetry gives rise to unique half-integer quantum hall effect, Klein's paradox, and weak antilocalization. In this research work, I was able to successfully study the growth and electronic structure of EG on SiC(0001), in ultra-high vacuum and low-vacuum furnace environment. I used STM to study the growth at an atomic scale and macroscopic scale. With STM imaging, I studied the distinct properties of commonly observed interface region (layer 0), first graphene layer, and the second graphene layer. I was able to clearly resolve graphene lattice in both layer 1 and 2. High resolution imaging of the defects showed a unique scattering pattern. Raman spectroscopy measurements were done to resolve the layer dependent signatures of EG. The characteristic Raman 2D peak was found to be suppressed in layer 1, and a single Lorentzian was seen in layer 2. Ni metal islands were grown on EG by e-beam deposition. STM/ STS measurements were done to study the changes in doping and the electronic structure of EG with distance from the metal islands.
Development of high-efficiency solar cells on thin silicon through design optimization and defect passivation

(Georgia Institute of Technology, 2009-03-24) Sheoran, Manav

The overall goal of this research is to improve fundamental understanding of the hydrogen passivation of defects in low-cost silicon and the fabrication of high-efficiency solar cells on thin crystalline silicon through low-cost technology development. A novel method was developed to estimate the flux of hydrogen, released from amorphous silicon nitride film, into the silicon. Rapid-firing-induced higher flux of hydrogen was found to be important for higher defect passivation. This was followed by the fabrication of solar cell efficiencies of ~ 17% on low-cost, planar cast multicrystalline silicon. Solar cell efficiencies and lifetime enhancement in the top, middle, and bottom regions of cast multicrystalline silicon ingots were explained on the basis of impurities and defects generally found in those regions. In an attempt to further reduce the cost, high-efficiency solar cells were fabricated on thin crystalline silicon wafers with full area aluminum-back surface field. In spite of loss in efficiency, wafer thinning reduced the module cost. Device modeling was performed to establish a roadmap towards high-efficiency thin cells and back surface recombination velocity and back surface reflectance were identified as critical parameters for high-efficiency thin cells. Screen-printed solar cells on float zone material, with efficiencies > 19% on 300 μm and > 18% on 140 μm were fabricated using a novel low-cost fabrication sequence that involved dielectric rear passivation along with local contacts and back surface field.
Structural characterization of epitaxial graphene on silicon carbide

(Georgia Institute of Technology, 2008-11-17) Hass, Joanna R.

Graphene, a single sheet of carbon atoms sp2-bonded in a honeycomb lattice, is a possible all-carbon successor to silicon electronics. Ballistic conduction at room temperature and a linear dispersion relation that causes carriers to behave as massless Dirac fermions are features that make graphene promising for high-speed, low-power devices. The critical advantage of epitaxial graphene (EG) grown on SiC is its compatibility with standard lithographic procedures. Surface X-ray diffraction (SXRD) and scanning tunneling microscopy (STM) results are presented on the domain structure, interface composition and stacking character of graphene grown on both polar faces of semi-insulating 4H-SiC. The data reveal intriguing differences between graphene grown on these two faces. Substrate roughening is more pronounced and graphene domain sizes are significantly smaller on the SiC (0001) Si-face. Specular X-ray reflectivity measurements show that both faces have a carbon rich, extended interface that is tightly bound to the first graphene layer, leading to a buffering effect that shields the first graphene layer from the bulk SiC, as predicted by ab initio calculations. In-plane X-ray crystal truncation rod analysis indicates that rotated graphene layers are interleaved in C-face graphene films and corresponding superstructures are observed in STM topographs. These rotational stacking faults in multilayer C-face graphene preserve the linear dispersion found in single layer graphene, making EG electronics possible even for a multilayer material.