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Study on the growth of an artificial graphene by chemical vapor deposition method and their local and non-local electronic transport properties

  • Jong-Hee, Lee (이종희, 대학원 물리학과 고체물리학전공)

초록/요약

Recently, the industry of the silicon-based Semiconductor device has reached the technical limits. Although this technology has grown continuously since Moore’s law was announced in 1965, it has congested more and more. Consequently, we try to find the suitable materials which can replace conventional silicon, and research on the reducing the size of devices. During the research, the discovery of graphene, as it is well known, was expected to develop the nano-scaled electronics. Graphene is an atomic planar sheet consisted of the hexagonal structure of carbon atoms, and it is the most promising material that expected for application to a variety of electronic devices due to their excellent electrical and mechanical properties. In this study, we discuss the synthesis of graphene on catalyst material using chemical vapor deposition and their structural, electrical properties with varying temperature and magnetic field. In the beginning stage of this thesis, we adopted the mechanical exfoliated method to extract graphene flake from HOPG and then transferred it to SiO2/Si substrate. By optical measurement, we confirmed that exfoliated graphene (so called, Ex-graphene) specimens were about few layers to graphite flake. To investigate the electrical properties of Ex-graphene, we fabricated two terminal devices using a typical semiconductor process. The transfer characteristics of current-gate voltage (Ids-Vg) were shown the linear curve and did not depend on applied gate voltage and this result indicated that our exfoliated graphene specimens are close to multilayer flake. In this way, we could obtain sufficient experiences for the fundamental properties of graphene before starting our main work. As the main work, we investigated the synthesis of graphene on catalyst material such as Nickel (Ni), Copper (Cu) by chemical vapor deposition (CVD). First of all, we conducted the synthesis of graphene using varying thickness of Ni film. When using a thick nickel film (>200nm), it could be obtained a continuous film type of graphene. In particular, in the case of using Ni thin film (<20nm), we observed the suspended type of graphene film which covered the segregated Ni nano-islands. This growth behavior is similar to that of lateral growth in carbon nanotubes, and it showed the possibilities of application to the electronic device without a further process such as transfer processing. However, Ni-CVD graphene films included all the films of 1-20 layers due to the high carbon solubility of Ni and as a result, it was difficult to examine the characteristic of monolayer CVD graphene. Therefore, to obtain a uniform and large scale graphene, we adopted Cu as a catalyst because it has the low carbon solubility. Cu-CVD graphene was grown uniformly large scale (more than 5mm at least), and we noticed that it is similar to monolayer graphene via Raman spectroscopy (I2D/IG: 2-4, ID/IG: 0.01-0.5, FWHM(G): about 14cm-1, FWHM(2D): about 32cm-1) and TEM measurement (HR images and SAED patterns). Lastly, we investigated the local and nonlocal electronic transport in CVD graphene. We etched to a type of nanoribbon and multi-terminal Hall Bar using synthesized graphene and then fabricated the device using e-beam process. The current response curve of CVD graphene FET showed the ambipolar behavior due to the linear energy band structure in graphene, and also the 1D characteristics observed in the narrow graphene channel due to quantum confinement effect which occurred by the limitation of carrier movement. In low temperature regime under applied high magnetic field, we observed half-integer Quantum Hall Effect (QHE), which is appeared in intrinsic graphene specimens. Meanwhile, to study the possibility as a spin medium, the non-local (so called, remote) voltage was measured in the device with voltage terminal far away from the terminals of the current path. Graphene has a weak spin-orbit coupling. Thus the Spin Hall Effect (SHE) is small, and the non-local signal is absent in none magnetic field. However, by increasing the amount of the defect (H2 atom), the non-local signal appeared more than 100Ω even at room temperature. To clarify the nature of the non-local behaviors, we compared the spin parameters estimated from defects for Raman of the synthesized graphene to those for non-local transport measurement of the graphene devices. In conclusion, we understood that the enhancement of the spin-orbit strength caused by both the lattice deformation due to hydrogenation and the remained Cu adatom acts as local spin-orbit scatters in our CVD graphene, and it was the reason why the spin Hall Effect appeared in effectively even at room temperature with low magnetic field.

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목차

ABSTRACT 1
CONTENTS 5
FIGURES 9
Chapter 1. Introduction 18
1. 1 Graphene 18
1. 2 Growth Mechanism of Graphene Films by Chemical Vapor Deposition (CVD) 23
1. 2. 1 Nickel (Ni) 24
1. 2. 2 Copper (Cu) 24
1. 3 Characterization of Graphene : Raman spectroscopy 26
Chapter 2. Preparation of Graphene Films by Mechanical Exfoliated Method from Natural Graphite Bulk 28
2. 1 Introduction 28
2. 2 Mechanical exfoliated graphene flake from Graphite (HOPG) 31
2. 3 Fabrication of Devices using the exfoliated graphene flakes and Their Electrical Characterization 36
2. 3. 1 Two-terminal Device fabrication with a back-gate electrode using Electron beam lithography 36
2. 3. 2 Measurement of electrical properties of two-terminal exfoliated graphene flake device 39
Chapter 3. Synthesis of Graphene by Chemical Vapor Deposition (CVD) 42
3. 1 Introduction 42
3. 2 Synthesis of graphene films on Nickel (Ni) film by CVD 43
3. 2. 1 Introduction 43
3. 2. 2 The sputtered Ni Film 43
3. 2. 3 The patterned Ni Stripe and Dot 52
3. 3 Synthesis of graphene films on Copper (Cu) by CVD 61
3. 3. 1 Introduction 61
3. 3. 2 Synthesis of graphene on Cu thin film by RT-CVD 61
3. 3. 3 Synthesis of graphene on Cu thick film by RT-CVD 67
3. 3. 4 Synthesis of graphene on Cu thick film by LP-CVD at high temperature 73
3. 3. 5 Synthesis of graphene on Cu foil by LP-CVD at high temperature 77
3. 4 Synthesis of graphene on the various types of catalyst by CVD 88
3. 4. 1 Introduction 88
3. 4. 2 The Process of graphene growth on the various forms of catalyst materials and transfer to the desired substrates 89
3. 4. 3 Synthesis of graphene on Cu wire by LP-CVD 90
3. 4. 4 Synthesis of graphene on TEM Grid by LP-CVD 94
3. 4. 4. 1 Ni-TEM Grid 95
3. 4. 4. 2 Cu-TEM Grid 97
Chapter 4. Electrical properties of CVD Graphene Films 102
4. 1 Introduction 102
4. 2 The Fabrication Process of the CVD graphene multi-terminal devices 103
4. 3 Graphene Nanoribbon Device : similar to 1D systems 109
4. 4 Graphene Multi-terminal Hall bar Device: QHE phenomena 118
4. 4. 1 Introduction : Quantum Hall Effect 118
4. 4. 2 Experimental Results and Discussions 121
4. 5 Graphene Multi-terminal Hall bar Device : Nonlocal transport and SHE phenomena 127
4. 5. 1 Introduction : Spintronics in Graphene 127
4. 5. 2 Experimental Results and Discussions 129
Chapter 5. Conclusions 143
Reference 146
Publication 164

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