| Summary: | 博士 === 國立清華大學 === 電子工程研究所 === 100 === Two-dimensional electron gas is an interesting system in semiconducting materials, in which electrons move freely in two dimensions, but the third degree-of-freedom is confined. The electron gas is artificially embedded in three-dimensional (3D) world such as the inversion layer in a field-effect transistor or the quantum well in semiconductor heterojunctions, but it is very difficult to realize ideal 2D structure by lithographical top-down method. In 2010 the Nobel Prize in physics was awarded to A. Geim and K. S. Novoselov for their pioneer work in isolating single-atomic-layer of graphite sheet on silicon dioxide. Graphene is the thinnest natural 2D material consisting of hexagonal carbon network in only one atom thick. Now a days, graphene can be synthesized by chemical vapor deposition on transition metal surface in wafer scale and even up to the 30 inches. To realize the applications to graphene-based electronic circuits and optoelectronics devices, first we have to precisely control the electronic conduction for n-type or p-type doping, and the most important issue is how to keep graphene surface clean to utilize the natural two-dimensional electron gas system sufficiently.
This thesis outlines the process of making graphene electronic devices, starting from the fundamental study of the graphene transfer from metal, electrical transport, molecular doping, and theoretical electronic structure calculation. Particular emphasis is the ultra clean graphene transfer technique in combination with Raman spectroscopy and transmission electron microscopy (TEM). This technique becomes the step tone to improve the quality of graphene electronic and optoelectronic device.
In Chapter 1, we start from the introduction of basic carbon allotropes in different dimensions and then focus to graphene, the 2D carbon material. We present an introductory overview to graphene structures and its energy band structure as well as the electrical properties and also giving an introduction to the fundamental optical properties.
Chapter 2 gives an introduction of the Raman spectroscopy which is an important tool to identify the quality of graphene. To understand Raman spectroscopy, we first describe the point group theory and the relation between vibration modes and the phonon dispersion. Base on these knowledge, we can understand the origin of each Raman feature of graphene.
At the end, we introduce the fingerprint of staked graphene layers.
Chapter 3 discusses the fundamental electrical properties of graphene including field-effect behavior and the doping induced Raman G peak shift. In the following experiment, we take advantage of the large surface area and ambipolar conduction properties to perform p-type and n-type graphene field-effect transistor by using molecular doping and plasma treatment, respectively.
Since we can utilize molecule adsorption or chemical functionalization to doped graphene, the surface cleanness of graphene turns into the key issue to determine the electrical or optical properties of graphene. In Chapter 4, we point out the contamination problems on graphene surface due to the strong dipole interaction with the lithographical resist, i.e. Poly(methyl methacrylate), PMMA. We build up a suspension technique to combine Raman spectroscopy and TEM to quantitatively estimate the thickness of residual PMMA and the figure out how does the residues affect to the graphene electronic properties.
In Chapter 5, we further demonstrate a new method to transfer atomic clean graphene from the CVD growing material Cu. The surface cleanness is confirmed by TEM and we also establish a standard reference to judge the cleanness by Raman spectroscopy. We also build-up a new technique to transfer CVD graphene without using polymer. This technique push the surface cleanness from 100 $\times$ 100 nm$^{2}$ to an order larger area.
This quality of such ultra-clean transferred graphene is also proved by quantum Hall effect measurement.
In Chapter 6, single-crystal bilayer graphemes with distinct stacking orders and interlayer rotation angles are synthesized by chemical vapor deposition at ambient pressure. We use Raman spectroscopy to characterize the electronic structures of different rotation angles, where the rotation angles are identified precisely by selected area electron diffraction pattern in TEM. Fundamental knowledges of twisted bilayer graphene system are built up here and the electronic structures are further calculated by density functional theory. We found two new Raman excited mode on the twist bilayer graphene and also G peak enhancement phenomena at the critical twisting angle where the overlapped electronic structure rehybridization resulted in van Hove singularities and high local joint density of states. Based on these studies, we can further estimate the twisting angle in a more convenient method with high accuracy. Furthermore, the angle-dependent electronic structures, optical properties, Fermi velocity, and electrical transport can be further investigated on the CVD single-crystal bilayer graphene.
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