Graphene

Properties, Preparation, Characterisation and Devices

Viera Skakalova, Alan B. Kaiser

• Publisher: Woodhead Publishing
• Year: 2014
• Topic: Technology & Engineering
• eBook ISBN:
  9780857099334
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Graphene: Properties, Preparation, Characterisation and Devices reviews the preparation and properties of this exciting material. Graphene is a single-atom-thick sheet of carbon with properties, such as the ability to conduct light and electrons, which could make it potentially suitable for a variety of devices and applications, including electronics, sensors, and photonics. Chapters in part one explore the preparation of , including epitaxial growth of graphene on silicon carbide, chemical vapor deposition (CVD) growth of graphene films, chemically derived graphene, and graphene produced by electrochemical exfoliation. Part two focuses on the characterization of graphene using techniques including transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and Raman spectroscopy. These chapters also discuss photoemission of low dimensional carbon systems. Finally, chapters in part three discuss electronic transport properties of graphene and graphene devices. This part highlights electronic transport in bilayer graphene, single charge transport, and the effect of adsorbents on electronic transport in graphene. It also explores graphene spintronics and nano-electro-mechanics (NEMS). Graphene is a comprehensive resource for academics, materials scientists, and electrical engineers working in the microelectronics and optoelectronics industries. - Explores the graphene preparation techniques, including epitaxial growth on silicon carbide, chemical vapor deposition (CVD), chemical derivation, and electrochemical exfoliation - Focuses on the characterization of graphene using transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and Raman spectroscopy - A comprehensive resource for academics, materials scientists, and electrical engineers

Part I

Preparation of graphene

Outline

1 Epitaxial growth of graphene on silicon carbide (SiC)

2 Chemical vapor deposition (CVD) growth of graphene films

3 Chemically derived graphene

4 Graphene produced by electrochemical exfoliation

1

Epitaxial growth of graphene on silicon carbide (SiC)

H. Huang, National University of Singapore, Singapore

S. Chen, Nanyang Technological University, Singapore

A.T.S. Wee and W. Chen, National University of Singapore, Singapore

Abstract:

This chapter provides an overview of the epitaxial growth of graphene films on various silicon carbide (SiC) substrates, their growth mechanism, and atomic scale characterization. The chapter focuses on the growth of epitaxial graphene (EG) via the thermal decomposition of single-crystal SiC in ultrahigh vacuum (UHV) and under ambient pressure. There is also a discussion of the thermal decomposition of polycrystalline SiC thin films and the intercalation methods used to produce EG.

Key words

thermal decomposition; epitaxial graphene; silicon carbide

1.1 Introduction

The realization of technologically feasible graphene-based electronic, optoelectronic, chemical- and bio-sensing devices greatly relies on the development of large-scale production of high-quality graphene thin films. In the last few years, intensive research efforts have been devoted to methods for production of single-layer or few-layer graphene films, including the micromechanical exfoliation from bulk graphite using sticky tape,^1,2 chemical exfoliation from bulk graphite powders,³ chemical or physical reduction from graphene oxides,^4–7 chemical vapour deposition of hydrocarbons on transition metal substrates^8–17 such as Cu, Ni, Ru, Ir and Pt, thermal decomposition of solid carbon sources on metals, semiconductors or insulators substrates, and thermal decomposition of commercial silicon carbide (SiC) substrates in vacuum or under atmospheric pressure conditions.^18,19 Epitaxial graphene (EG) films thermally grown on SiC can be patterned using CMOS-compatible nanolithography methods, making it compatible with current semiconductor technology and hence a promising growth process for future graphene-based devices.^20,21 In particular, high-performance devices, such as field-effect transistors,²² photodetectors,²³ and chemical sensors²⁴ have been demonstrated using EG on SiC. The aim of this chapter is to provide an overview of the epitaxial growth of graphene films on various SiC substrates, their growth mechanism, and atomic-scale characterization. The chapter focuses on the growth of EG via the thermal decomposition of single-crystal SiC in ultrahigh vacuum (UHV) and under ambient pressure, followed by a discussion of the thermal decomposition of polycrystalline SiC thin films and intercalation methods to produce EG.

1.2 Ultrahigh vacuum (UHV) thermal decomposition of single-crystal SiC

The formation of crystalline graphite layers on SiC via thermal heating at high temperature in UHV was first observed by van Bommel et al. in 1975.²⁵ However, the crystalline graphite layers later known as EG received little attention initially. About the time when the first isolation of free-standing graphene by mechanical exfoliation was reported, Berger and coworkers demonstrated that EG on SiC has nearly identical properties to those of free-standing graphene and is compatible with the CMOS-compatible lithography process for device fabrication.¹⁸ Single-crystal graphene with a controlled number of atomic layers can be epitaxially grown on SiC, depending only on the annealing temperature and time. The growth of EG depends on SiC surface polarity (i.e. silicon or carbon face) but shows little variation for different SiC polytypes (such as 3C, 4H and 6H). To understand this growth behaviour, we provide a brief description of the SiC structure.

SiC contains carbon and silicon in 1:1 stoichiometry. Each Si (or C) atom is covalently bonded to four nearest-neighboring C (or Si) atoms in a tetrahedral coordination (sp³ configuration). These tetrahedral Si–C bonds are arranged in a hexagonal bilayer with carbon and silicon in alternating positions. The Si–C bilayers can be stacked in various stacking and orientation sequences along the direction perpendicular to the bilayer plane, leading to more than 200 polytypes in the SiC bulk structure. There are two main configurations for these polytypes: one has cubic symmetry, i.e. face-centred cubic (fcc); the other has hexagonal symmetry, i.e. hexagonal close-packed (hcp). Among those polytypes, 3C-SiC, 4H-SiC and 6H-SiC are the most important, where the number 3 (4 or 6) indicates the number of bilayers per unit cell and C (H) denotes the cubic (hexagonal) symmetry. Thus, the bilayer plane in 3C-SiC is the (111) plane, and it is the (0001) plane in 4H-SiC and 6H-SiC. Figure 1.1 displays the stacking sequence for 3C-SiC (ABCABC. …), 4H-SiC (ABACABAC. …) and 6H-SiC (ABCACBABCACB. …) along the cross sectional plane perpendicular to the bilayer; this corresponds to the (110) plane in 3C-SiC and

plane in 4H-SiC and 6H-SiC. The unique physical and electrical properties of each polytype are attributed to the different stacking sequences. The termination of Si–C bilayer (i.e. Si on top or C on top) influences the silicon sublimation and carbon segregation processes, and thus results in distinct differences in graphene formation. We first focus on the Si-terminated or Si-face 6H-SiC(0001) to elucidate the properties of graphene on SiC and then discuss graphene formation on C-terminated or C-face SiC.

1.1 Stacking sequence for three typical polytypes of SiC: (a) 3C-SiC(111), (b) 4H- and (c) 6H-SiC(0001).

The growth of EG can be achieved via thermal decomposition of bulk SiC. At high temperatures, Si atoms start to evaporate from the surface. C atoms segregate on the surface to form C-rich surface layers, ranging from the interfacial graphene (IG) layer, to single-layer EG, bilayer EG and few-layer EG. Because this growth process involves a series of surface reconstructions, we use the growth of EG on 6H-SiC(0001) as an example to describe the evolution of these surface reconstructions as a function of substrate annealing temperature, as characterized by in situ low-energy electron diffraction (LEED, upper panel in Fig. 1.2) and scanning tunneling microscopy (STM, lower panel in Fig. 1.2).²⁶ After annealing the bare 6H-SiC(0001) at around 850 °C under a Si flux in UHV, a Si-rich 3 × 3 superstructure appears, comprising a twisted Si adlayer and Si tetramers on bulk SiC substrate (Fig. 1.2(a) and (e)).

1.2 The LEED patterns (upper row) and corresponding STM images (lower row) of annealing-induced 6H-SiC(0001) surface reconstructions: (a) and (e) 3 × 3; (b) and (f)

(c) and (g)

and (d) and (h) single-layer EG. (Reprinted from reference 26, with permission from IOP Publishing Limited, copyright 2007.)

Thus, the coverage of the surface Si layer is

ML, where ML is monolayer.^27,28 Further annealing of the substrate at 950 °C in the absence of a Si flux causes more Si atoms to evaporate, resulting in a less Si-rich reconstruction (Fig. 1.2(b) and (f)) with Si adatoms at the tetrahedral or T₄ positions of a Si-terminated bulk crystal (Si coverage is

ML). This is the

reconstruction.^29,30 Heating the substrate to 1100 °C leads to the evaporation of Si atoms from bulk SiC, accompanied by the accumulation of surface carbon atoms that form a honeycomb superstructure with a periodicity of around 1.8 nm, as shown in Fig. 1.2(c) and (g). This is the well-known

reconstruction, which has been also referred to as the ‘graphene buffer layer’ or IG.^31,32 For consistency, we refer to this phase as ‘IG’ henceforth. Annealing the 6H-SiC sample at 1200 to 1400 °C leads to the formation of single-crystal EG layers with thickness ranging from a single layer to a few layers atop IG. The LEED and STM images of a single layer EG on SiC are shown in Fig. 1.2(d) and (h), respectively.

The formation of EG on SiC can be clearly evidenced by C 1s x-ray photoemission spectroscopy (XPS). The synchrotron-based high-resolution C 1s XPS spectra of SiC as a function of annealing temperature are shown in Fig. 1.3. To enhance the surface sensitivity, a photon energy of 350 eV and an emission angle of 40° were chosen. On the Si-rich

reconstructed surface, only the bulk SiC related peak at 282.9 eV below Fermi energy (E_F) appears in the C 1s spectrum. On a surface with partial IG coverage, an IG-related component appears at 285.1 eV. The C 1s spectrum from the full-coverage IG surface is dominated by the peak at 285.1 eV, accompanied by a shoulder at 283.9 eV. The graphene-related C 1s peak at 284.4 eV in the spectrum is recorded from the sample annealed at higher than 1100 °C, indicating graphitization of the surface. The C 1s spectrum of full coverage EG surface is dominated by this peak.

1.3 The evolution of synchrotron-based high-resolution C 1s spectra of various annealing-induced SiC surface reconstructions. (Reprinted from referen...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Electronic and Optical Materials
• Preface
• Part I: Preparation of graphene
• Part II: Characterisation of graphene
• Part III: Electronic transport properties of graphene and graphene devices
• Index