| Summary: | 博士 === 國立清華大學 === 電子工程研究所 === 102 === Graphene, a single atomic layer of graphite comprising a planar two-dimensional hexagonal lattice of carbon atoms, is the building block for graphitic materials of all other dimensionalities. Based on the Mermin-Wagner Theorem, two-dimensional crystals were thermodynamically unstable and could not exist in ambient environment.The atomic monolayer materials have so far been known only as an integral part of larger three-dimensional structures. In 2004, A. K. Geim and K. S. Novoselov et al. provided a possible method to isolate two-dimensional single atomic carbon layer on SiO2 by using the mechanical cleavage of highly oriented pyrolytic graphite (HOPG).Since then, graphene has become an important research topic in the field of materials science and condensed-matter physics due to its unique properties. For instances, graphene has a host of characteristics that show great promise for the development of post silicon electronics,including a large room-temperature carrier mobility (20,000 cm2/V•s) and long-range ballistic transport.In addition to electrical properties, graphene is also an highly transparent material with an absorption of ∼ 2.3 % in visible light range Its thermal conductivity is measured to be ∼ 5,000 W mK-1 for a monolayer graphene at room temperature. The intrinsic mechanical properties of free-standing monolayer graphene are examined with the breaking strength of 42 N m-1 and the Young’s modulus of 1.0 TPa, indicating that it is one of the strongest materials ever measured.
This thesis focuses on the electrical and optical properties of graphene, including the synthesis, material characterizations and device applications. The first part presents the fabrication of graphene electronic devices, starting from the graphene synthesis by chemical vapor deposition (CVD), and the improvement of the device geometry in top-gate field-effect transistor (FET), addressing its emerging applications in flexible electronics. The second part presents the characterization of single-crystal graphene, which exhibits a high crystalline quality examined by transport analysis. We also study the fundamental issue of interlayer coupling between two rotational single-crystal bilayer graphene, which provides insights into the unique energy spectra of the two-dimensional carbon electron systems and may pave the way toward the opto-electronic applications.
Chapter 1 starts with the fundamentals of graphene, including the crystal structures as well as its energy band structure. The transport properties such as electric field-effect, minimum conductivity and scattering mechanism within graphene have been explained briefly. Chapter 2 presents an introduction of the Raman scattering, which is an important tool to examine the quality of graphene. We introduce the basic knowledge of Raman scattering and the phonon dispersion relation of graphene. Some extended studies such as electron-phonon coupling and doping effect have also been discussed. In Chapter 3, the characterization of polycrystalline graphene, such as Raman, transmission electron microscopy (TEM) and transport analysis have been presented. In order to improve the quality of graphene, we use the diluted concentration of hydrocarbon as precursor. This helps the self-limiting growth during the early stage of the growth on Cu. Single-crystal graphene can reduce the grain boundary scattering in micrometerscale electronics devices, which can enhance the device performance remarkably. In Chapter 4, a facile fabrication process for high-performance CVD graphene FETs with self-aligned drain/source contacts have been presented. In our process, an Al gate was directly defined on graphene by e-beam lithography, followed by air exposure or electrical annealing, forming a native oxide layer around the Al wire. We also characterize other device properties, such as charge neutrally point shifting, current saturation, and the complementary logic gate demonstration. Chapter 5 describes the fabrication of high-mobility low-voltage graphene FET array on a flexible plastic substrate using high-capacitance natural aluminum oxide as a gate dielectric in a self-aligned device configuration. The native aluminum oxide is resistant to mechanical bending and exhibits self-healing upon electrical breakdown. These results indicate that self-aligned graphene FETs can provide remarkably improved device performance and stability for a range of applications in flexible electronics. In Chapter 6, we present the study of the single-crystal bilayer graphene grown by ambient pressure CVD on polycrystalline Cu. Controlling the nucleation in early stage of growth allows the constituent layers to form single hexagonal crystals. New Raman active modes are shown to result from the twist, with the twist angle determined by the TEM analysis. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.
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