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School of Earth and Atmospheric Sciences

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School of Mathematics

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School of Physics

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Fabrication of arrays of ballistic epitaxial graphene nanoribbons

(Georgia Institute of Technology, 2018-04-05) Deniz, Dogukan

Epitaxial graphene nanoribbons have recently demonstrated exceptional one dimensional ballistic transport where charge carriers can travel without scattering up to 16 m at room temperature (STM probes in UHV). These transport properties are not yet fully understood, and they can only be exploited if nanoribbons can be produced at large scale with good properties in ambient conditions. Following up on these results, in this thesis I will summarize my work on arrays of epitaxial graphene nanoribbons grown on the sidewalls of trenches and steps of silicon carbide. I will discuss nanoribbon growth, nanofabrication, factors that affect nanoribbon transport and how mean free paths larger than one micron are consistently found at room temperature for thousands of nanoribbons.
Spin dependent current injection into epitaxial graphene nanoribbons

(Georgia Institute of Technology, 2015-05-15) Hankinson, John H.

Over the past decade there has been a great deal of interest in graphene, a 2-dimensional allotrope of carbon with exceptional mechanical and electrical properties. Its outstanding mobility, minimal size, and mechanical stability make it an appealing material for use in next generation electronic devices. Epitaxial graphene growth on silicon carbide is a reliable, scalable method for the production of high quality graphene films. Recent work has shown that the SiC can be patterned prior to graphitization, in order to selectively grow graphene nanostructures. Graphene nanoribbons grown using this technique do not suffer from the rough edges caused by lithographic patterning, and recent measurements have revealed extraordinary transport properties. In this thesis the magnetic properties of these nanoribbons are investigated through spin polarized current injection. The sensitivity of these nanoribbons to spin polarized current is interesting from a fundamental physics standpoint, and may find applications in future spintronic devices.
Pre-growth structures for high quality epitaxial graphene nanoelectronics grown on silicon carbide

(Georgia Institute of Technology, 2014-11-11) Palmer, James Matthew

For graphene to be a viable platform for nanoscale devices, high quality growth and structures are necessary. This means structuring the SiC surface to prevent graphene from having to be patterned using standard microelectronic processes. Presented in this thesis are new processes aimed at improving the graphene as well as devices based on high quality graphene nanoribbons. Amorphous carbon (aC) corrals deposited prior to graphene growth are demonstrated to control SiC step-flow. SiC steps are shown to be aligned by the presence of the corrals and can increase SiC terrace widths. aC contacts deposited and crystallized during graphene growth are shown as a way to contact graphene without metal lift-off. Observation of the Quantum Hall Effect demonstrates the high quality of the graphene grown alongside the nanocrystalline graphite contacts. Continuing the ballistic transport measurements on sidewall graphene nanoribbons, the invasive probe effect is observed using an atomic force microscope (AFM) based technique that spatially maps the invasive probe effect. Cleaning experiments demonstrate the role of scattering due to resist residues and environmental adsorbates on graphene nanoribbons. Finally, switches based on junctions formed in the graphene nanoribbons are shown as a route toward graphene based devices.
Mono-layer C-face epitaxial graphene for high frequency electronics

(Georgia Institute of Technology, 2014-06-04) Guo, Zelei

As the thinnest material ever with high carrier mobility and saturation velocity, graphene is considered as a candidate for future high speed electronics. After pioneering research on graphene-based electronics at Georgia Tech, epitaxial graphene on SiC, along with other synthesized graphene, has been extensively investigated for possible applications in high frequency analog circuits. With a combined effort from academic and industrial research institutions, the best cut-off frequency of graphene radio-frequency (RF) transistors is already comparable to the best result of III-V material-based devices. However, the power gain performance of graphene transistors remained low, and the absence of a band gap inhibits the possibility of graphene in digital electronics. Aiming at solving these problems, this thesis will demonstrate the effort toward better high frequency power gain performance based on mono-layer epitaxial graphene on C-face SiC. Besides, a graphene/Si integration scheme will be proposed that utilizes the high speed potential of graphene electronics and logic functionality and maturity of Si-CMOS platform at the same time.
Production and properties of epitaxial graphene on the carbon terminated face of hexagonal silicon carbide

(Georgia Institute of Technology, 2013-08-13) Hu, Yike

Graphene is widely considered to be a promising candidate for a new generation of electronics, but there are many outstanding fundamental issues that need to be addressed before this promise can be realized. This thesis focuses on the production and properties of graphene grown epitaxially on the carbon terminated face (C-face) of hexagonal silicon carbide leading to the construction of a novel graphene transistor structure. C-face epitaxial graphene multilayers are unique due to their rotational stacking that causes the individual layers to be electronically decoupled from each other. Well-formed C-face epitaxial graphene single layers have exceptionally high mobilities (exceeding 10,000 cm ²/Vs), which are significantly greater than those of Si-face graphene monolayers. This thesis investigates the growth and properties of C-face single layer graphene. A field effect transistor based on single layer graphene was fabricated and characterized for the first time. Aluminum oxide or boron nitride was used for the gate dielectric. Additionally, an all graphene/SiC Schottky barrier transistor on the C-face of SiC composed of 2DEG in SiC/Si₂O ₃ interface and multilayer graphene contacts was demonstrated. A multiple growth scheme was adopted to achieve this unique structure.
Structured epitaxial graphene for electronics

(Georgia Institute of Technology, 2012-06-28) Ruan, Ming

After the pioneering investigations into graphene-based electronics at Georgia Tech, great strides have been made developing epitaxial graphene on silicon carbide (EG) as a new electronic material. EG has not only demonstrated its potential for large scale applications, it also has become an important material for fundamental two-dimensional electron gas physics. Graphene is generally considered to be a strong candidate to succeed silicon as an electronic material. However, to date, it actually has not yet demonstrated capabilities that exceed standard semiconducting materials. One disadvantage of conventionally fabricated graphene devices is that nanoscopically patterned graphene tends to have disordered edges that severely reduce mobilities thereby obviating its advantage over other materials. The other disadvantage is that pristine graphene does not contain a band gap, which is critical for standard field effect transistor to operate. This thesis will show that graphene grown on structured silicon carbide surfaces overcomes the edge roughness and promises to provide an inroad into nanoscale patterning of graphene. High-quality ribbons and rings can be made using this technique.
Electric dipole moments, cluster metallicity, and the magnetism of rare earth clusters

(Georgia Institute of Technology, 2010-07-06) Bowlan, John

One of the fundamental properties of bulk metals is the cancellation of electric fields. The free charges inside of a metal will move until they find an arrangement where the internal electric field is zero. This implies that the electric dipole moment of a metal particle should be exactly zero, because an electric dipole moment requires a net separation of charge and thus a nonzero internal electric field. This thesis is an experimental study to see if this property continues to hold for tiny sub- nanometer metal particles called clusters (2 - 200 atom, R < 1 nm). We have measured the electric dipole moments of metal clusters made from 15 pure elements using a molecular beam electric deflection technique. We find that the observed dipole moments vary a great deal across the periodic table. Alkali metals have zero dipole moments, while transition metals and lanthanides all have dipole moments which are highly size dependent. In most cases, the measured dipole moments are independent of temperature (T = 20 - 50 K), and when there is a strong temperature dependence this suggests that there is a new state of matter present. Our interpretation of these results are that those clusters which have a non- zero dipole moment are non-metallic, in the sense that their electrons must be localized and prevented from moving to screen the internal field associated with a permanent dipole moment. This interpretation gives insight to several related phenomena and applications. We briefly discuss an example cluster system RhN where the measured electric dipole moments appear to be correlated with a the N2O reactivity. Finally, we discuss a series of magnetic deflection experiments on lanthanide clusters (Pr, Ho, Tb, and Tm). The magnetic response of these clusters is very complex and highly sensitive to size and temperature. We find that PrN (which is non-magnetic in the bulk) becomes magnetic in clusters and TmN clusters have magnetic moments lower than the atomic value as well as the bulk saturation value implying that the magnetic order in the cluster involves non-collinear or antiferromagnetic order. HoN and TbN show very similar size dependent trends suggesting that these clusters have similar structures.
Epitaxial graphene on silicon carbide: low-vacuum growth, characterization, and device fabrication

(Georgia Institute of Technology, 2010-06-04) Sprinkle, Michael W.

In the past several years, epitaxial graphene on silicon carbide has been transformed from an academic curiosity of social scientists to a leading candidate material to replace silicon in post-CMOS electronics. This has come with rapid development of growth technologies, improved understanding of epitaxial graphene on the polar faces of silicon carbide, and new device fabrication techniques. The contributions of this thesis include refinement and improved understanding of graphene growth on the silicon- and carbon-faces in the context of managed local silicon partial pressure, high-throughput epitaxial graphene thickness measurement and uniformity characterization by ellipsometry, observations of nearly ideal graphene band structures on rotationally stacked carbon-face multilayer epitaxial graphene, presentation of initial experiments on localized in situ chemical modification of epitaxial graphene for an alternate path to semiconducting behavior, and novel device fabrication methods to exploit the crystal structure of the silicon carbide substrate. The latter is a particularly exciting foray into three dimensional patterning of the substrate that may eliminate the critical problem of edge roughness in graphene nanoribbons.
Electric deflection measurements of sodium clusters in a molecular beam

(Georgia Institute of Technology, 2009-11-10) Liang, Anthony

Rotationally averaged polarizabilities and intrinsic electric dipole moments of sodium clusters are measured and reported. The experimental method is a molecular beam deflection. Our precision is the highest (<5%) and the range of the cluster sizes is the broadest to date (Na₁₀ ∼ Na₃₀₀). Compared to the earlier measurements, our data covers all sizes with no gaps up to the largest cluster. The fine structure in the polarizability curve is previously unobserved. We have carefully ruled out several possible explanations. And we find an earlier existing theory could explain the facts but will lead to magic numbers which were not seen in some previous experiments. A detailed theory is needed to understand the behaviors we see. Intrinsic electric dipole moments (EDM) of sodium clusters are probed to answer the intriguing question: Do metal clusters develop electric dipole moments like molecules? Some theories have predicted the existence of EDM in ground state sodium clusters and gave their magnitudes. We put upper bounds on the EDM of sodium clusters and find that they are orders of magnitude smaller than the predictions. This provokes an interesting question: how can one define metallicity in metal clusters? Our measurements are performed at cryogenic temperature 20 Kelvin. At this temperature the clusters are believed to be in their vibronic ground states.
Epitaxial graphene films on SiC: growth, characterization, and devices

(Georgia Institute of Technology, 2008-05-13) Li, Xuebin

Graphene is a single sheet of graphite. While bulk graphite is semimetal, graphene is a zero bandgap semiconductor. Band structure calculations show graphene has a linear energy dispersion relation in the low energy region close to the Dirac points where the conduction band and the valence band touch. Carriers in graphene are described as massless Dirac fermions in contrast to massive carriers in normal metals and semiconductors that obey a parabolic energy dispersion relation. The uniqueness of graphene band structure indicates its peculiar electronic transport properties. In this thesis work, single- and multi-layer graphene films epitaxially grow on either the Si face or the C face of SiC substrates in a homemade induction vacuum chamber by thermal decomposition of SiC at high temperatures. The surface morphology and crystal structure of epitaxial graphene are studied with surface analysis tools. The transport properties of epitaxial graphene are studied by magnetotransport experiments. An epitaxial graphene film turns out to be a multilayered graphene because carriers in epitaxial graphene act as those in single layer graphene. Top gated and side gated epitaxial graphene field effect transistors (FETs) have also been successfully fabricated. These systematic studies unambiguously demonstrate the high quality of epitaxial graphene and the great potential of epitaxial graphene for electronic applications