Epitaxial growth of graphene on silicon carbide (SiC)

Epitaxial growth of graphene on silicon carbide (SiC)

1 Epitaxial growth of graphene on silicon carbide (SiC) H. H UA N G, National University of Singapore, Singapore, S. C H E N, Nanyang Technological Un...

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1 Epitaxial growth of graphene on silicon carbide (SiC) H. H UA N G, National University of Singapore, Singapore, S. C H E N, Nanyang Technological University, Singapore and A. T. S. W E E and W. C H E N, National University of Singapore, Singapore DOI: 10.1533/9780857099334.1.3 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 substrates8–17 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, highperformance 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 3 © 2014 Woodhead Publishing Limited

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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 (1120) 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

Epitaxial growth of graphene on silicon carbide (SiC)

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C atom Si atom

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

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. 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)). 4 Thus, the coverage of the surface Si layer is ML, where ML is 9 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 1 positions of a Si-terminated bulk crystal (Si coverage is ML ). This is the 3 3 × 3R30° 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

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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) 3 × 3R 30°; (c) and (g) 6 3 × 6 3R 30°; and (d) and (h) single-layer EG. (Reprinted from reference 26, with permission from IOP Publishing Limited, copyright 2007.)

periodicity of around 1.8 nm, as shown in Fig. 1.2(c) and (g). This is the well-known 6 3 × 6 3R30° 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 3 × 3R30° reconstructed surface, only the bulk SiC related peak at 282.9 eV below Fermi energy (EF) 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.

Epitaxial growth of graphene on silicon carbide (SiC)

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284.4 285.1

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1.3 The evolution of synchrotron-based high-resolution C 1s spectra of various annealing-induced SiC surface reconstructions. (Reprinted from reference 31, with permission from Elsevier B.V., copyright 2005.)

The EG formed on SiC can be characterized by Raman spectroscopy. In Fig. 1.4, typical Raman spectra of single-layer and bilayer EG on 6H-SiC(0001), single-layer mechanically cleaved graphene (MCG), bulk graphite, and bare 6H-SiC(0001) substrate are shown.33 On the single-layer or bilayer EG, the bulk SiC related peaks appear at ∼1520 and ∼1713 cm−1, as on the bare SiC substrate. Three other peaks related to EG are observed: the defect-induced D band at ∼1368 cm−1, in-plane vibrational G band at ∼1597 cm−1 and the two-phonon 2D band at ∼2725 cm−1. The insertion clearly shows that the 2D band of bilayer EG is wider than that of single-layer EG (95 compared with 60 cm−1) and occurs at higher frequency (2736 compared with 2715 cm−1), in line with the trend observed on MCG. The interfacial stress caused by the large lattice mismatch between SiC (a = 3.07 Å) and graphene (a = 2.46 Å) leads to a significant blue shift of the G (1597 cm−1) and 2D (2715 cm−1) bands of single-layer EG relative to that of the singlelayer MCG (G band at 1580 cm−1 and 2D band at 2673 cm−1).

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1.4 Typical Raman spectra of single-layer and bilayer EG on SiC, compared with bulk graphite and single-layer MCG. (Reprinted from reference 33, with permission from American Physical Society, copyright 2008.)

The IG layer is carbon rich and is believed to have a similar atomic structure to graphene. However, the IG layer does not have the same electronic properties as graphene. For comparison, the entire valence band structures of IG and single-layer EG are shown in the angular-resolved photoemission spectroscopy (ARPES) images in Fig. 1.5(a) and (b), respectively.34 Both show a strongly dispersing σ-band between ∼23 and ∼8 eV below EF, indicating that IG has a similar carbon–carbon distance to that of graphene. The features in the region from about 12.5 to 2.5 eV are dominated by the emission from the bulk valence bands of SiC. Figure 1.5(b) clearly shows graphene’s linearly dispersed π-band at K point around EF. Instead, broad structures are observed on IG as shown in Fig. 1.5(a). Moreover, except for surface-related hardly dispersing features at 1.8 and 0.6 eV below EF, there are no occupied states near EF, suggesting partial coupling between the carbon pz orbitals in IG and the dangling bonds of SiC(0001). The dispersion of π-band at the K point around EF of single-layer, bilayer and multilayer EG on SiC varies considerably, giving rise to differing transport properties. Figure 1.6(a) to (d) displays the ARPES results and the corresponding tight-binding simulation of the π-band in the vicinity of the Dirac point (ED) of single-layer to quadlayer EG on SiC, respectively.35 The position of ED below EF is attributed to charge transfer from the SiC substrate to graphene via IG. The EF position relative to ED point of EG can be adjusted by intercalation, as discussed in 1.5. The bands of

Epitaxial growth of graphene on silicon carbide (SiC) (6√3 × 6√3) R 30° Γ'

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1.5 ARPES images of (a) IG and (b) single-layer EG on 6H-SiC(0001) (hν = 50 eV). (Reprinted from reference 34, with permission from American Physical Society, copyright 2008.)

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1.6 (a)–(d) The π- and π*-bands near EF for 1–4 EG layers on 6H-SiC(0001), respectively (hν = 94 eV, T = ∼30 K), where k|| = 1.703 Å−1 corresponds to the K point, the corner of the hexagonal Brillouin zone. The dashed lines are from a calculated tight binding band structure. The number of π-bands increases with the number of layers owing to interlayer splitting. (Reprinted from reference 35, with permission from American Physical Society, copyright 2007.)

single-layer EG shown in Fig. 1.6(a) are strongly renormalized, i.e. they deviate from the expected linear dispersion, owing to many-body interactions. The band of bilayer EG in Fig. 1.6(b) shows a band gap of 0.15 eV, owing to charge transfer from the substrate. This leads to a graded charge carrier concentration and thus the two layers have different onsite Coulomb potential.36 A complex band structure is observed in Fig. 1.6(c) and (d) for trilayer and quadlayer EG, respectively.

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Graphene

EG on SiC grown in UHV is usually inhomogeneous. Various graphene layers coexist on the same sample. This is because the growth temperature is sufficient for silicon desorption but is not sufficient for homogeneous growth of graphene films. As shown in the low-energy electron microscopy (LEEM) image in Fig. 1.7(a), EG grown on SiC(0001) in UHV is a mixture of single-layer to quadlayer. The regions with different contrast correspond to a different number of EG layers, marked (1)–(4). The electron reflectivity versus kinetic energy of the incident electron beam

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1.7 The nonuniformity of EG on SiC(0001). (a) LEEM image from an area with various numbers of graphene layers. The field of view (FOV) is 20 μm and the electron energy is 4.2 eV. Insets are μ-LEED patterns collected at E = 53.3 eV from the four labelled areas. (b) Electron reflectivity spectra extracted from the four representative areas labelled (1)–(4) and corresponding to graphene of 1–4 ML thick, respectively. (c) Raman spectral map of 2D peak position and (d) the corresponding AFM image from an area without macro-defects. ((a) and (b) are reprinted from reference 37, with permission from IOP Publishing Limited, copyright 2010; (c) and (d) are reprinted from reference 38, with permission from American Chemical Society, copyright 2009.)

Epitaxial growth of graphene on silicon carbide (SiC)

11

allows a direct determination of the number of graphene layers, as shown in Fig. 1.7(b).37,38 The number of local minima (dips) in the reflectivity curve represents the number of graphene layers and it is clear that the areas correspond to 1–4 layers of graphene, respectively. Alternatively, the number of graphene layers can also be determined by μ-LEED as shown in Fig. 1.7(a). Owing to the short penetration depth of low-energy electrons, when the number of graphene layers increases, the six buffer-layer diffraction spots around the graphene spot (the centre/ middle spot) gradually fade away. These six spots are barely visible from areas with three layers and are not possible to detect when the number of layers is larger than three. The inhomogeneity of the prepared graphene samples is also manifested in inhomogeneous strain with the film. For single-layer EG on SiC, whose thickness is independently verified by photoemission spectroscopy, the strain can vary over a distance shorter than 300 nm; such strain may also be uniform over roughly 1 μm. The Raman spectral map of the 2D peak position shown in Fig. 1.7(c) is seen to be correlated with the physical topography of the graphene film as revealed by AFM in Fig. 1.7(d), suggesting that changes in the physical topography may lead to corresponding changes in the strain of the graphene film. The EG on SiC undergoes a bottom-up growth mode, as confirmed by STM observations. STM/STS is a powerful method to study the local structural and electronic properties of EG grown on SiC(0001). Figure 1.8(a) and (b) are the large-scale and corresponding zoomed-in images, showing the coexistence of single-layer and bilayer EG.39 The superimposed line profile shows the interlayer height difference is only 0.07 ± 0.01 nm. Figure 1.8(b) shows the atomically-resolved STM image zoomed-in from the black square in Fig. 1.8(a). The structure of graphene can be better resolved at low tunnelling bias conditions, as shown in Fig. 1.8(d), where the top graphene layer is physically continuous from the single-layer region to the bilayer region. The inserted images in Fig. 1.8(b) reveals a honeycomb structure in the single-layer region, and a triangular lattice in the bilayer region, as highlighted by hexagons. The bilayer or thicker EG involves the Bernal or AB stacking of bulk graphite, which breaks the symmetry of the graphene hexagonal lattice resulting in two inequivalent carbon atoms per unit cell. As such, STM reveals a three-fold symmetry pattern for bilayer EG. The honeycomb in the inset at the upper-right corner highlights two inequivalent carbon atoms in one unit cell. This transition from a honeycomb lattice to a three-fold symmetry pattern is used to differentiate single-layer and bi- or multilayer EG on SiC. The local electronic properties of 1–4 layer graphene is revealed by STS data, as shown in Fig. 1.8(c).40 Consistent with the ARPES results shown in Fig. 1.6, the ED shifts towards EF as the thickness increases.

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Graphene (a)

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1.8 STM images showing the coexistence of single-layer and bilayer graphene: (a) 100 × 100 nm2, VT = 1.78 V; (b) 20 × 20 nm2, VT = 0.5 V; and the physical continuum at the domain boundaries (d) 8 × 8 nm2, VT = −0.1 V. The insets in (b) display the corresponding atomically resolved STM images of single-layer and bilayer EG, respectively; the hexagon in the lower left part highlights the hexagonal lattice of the monolayer EG, and the hexagon in the upper right part represents the two inequivalent triangular sublattices of the bilayer EG. (c) STS results taken from 1–4 layer EG, respectively. ((a), (b) and (d) are reprinted from reference 39, with permission from American Chemical Society, Copyright 2008; (c) is reprinted from reference 40, with permission from American Physical Society, copyright 2008.)

The surface density of C atoms in a graphene layer (3.82 × 1015 cm−2) is triple that in a SiC bilayer (1.22 × 1015 cm−2). This suggests that three consecutive SiC bilayers are consumed to form a graphene layer. Figure 1.9 proposes a possible mechanism. The line profile in Fig. 1.9(b) taken along line AD in the STM image in Fig. 1.9(a) reveals a 0.07 nm high step and a 0.25 nm high step. The latter is attributed to a SiC bilayer step. The former is much smaller than the SiC bilayer height and the interlayer spacing in bulk graphite (0.335 nm), but is in good agreement with the difference

Epitaxial growth of graphene on silicon carbide (SiC)

13

between them. Therefore, the model in Fig. 1.9(c) is proposed to illustrate the atomic structures of neighbouring single-layer and bilayer EG. A single SiC bilayer thermally decomposes underneath the IG of single layer EG, accompanied by the sublimation of Si species from the interface and the release of carbon species to form a new IG layer. This leads to the transformation of the original IG layer to a new first EG layer atop the newly formed IG, thereby resulting in a transition from single-layer to bilayer EG. From this bottom-up growth model, the top EG layers of the neighbouring bilayer and single-layer EG remain continuous as they originate from the same EG layer. This explains the observed physical continuum at the boundary between single-layer and bilayer EG. The lowering of the EG layer owing to the decomposition of the underlying SiC bilayer is compensated by the formation of a second EG layer with interlayer spacing of 0.34 nm, consistent with the measured height difference between the monolayer and the bilayer EG (i.e., 0.07 ± 0.01 nm).41 Graphene also grows on SiC (0001) but in quite a different way from the growth mechanism on SiC(0001). The graphitization process is usually much faster on the C face than the Si face. UHV-grown EG shows many rotational domains and tends to form 3D structures, whereas furnace-grown

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1.9 (a) Large-scale STM image (150 × 100 nm2, VT = 1.5 V) of epitaxial single-layer and bilayer graphene on 6H-SiC(0001); (b) line profile along the line AD in (a). (c) Proposed model. (Reprinted from reference 39, with permission from American Chemical Society, copyright 2008.)

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1.10 Multilayer EG on SiC(000 1) with rotational stacking faults of (√13 × √13)R46.1° superlattice: (a) tentative model, (b) large-scale and (c) zoomed-in STM images. (d) ARPES image of an 11-layer EG film measured at a temperature of 6K, showing three linear Dirac cones. (e) Momentum distribution curve at BE = EF − 0.675 eV showing all three cones; the heavy solid line is a fit to the sum of six Lorentzians (thin solid lines). ((a)–(c) Reprinted from reference 42, with permission from American Physical Society, Copyright 2008. (d) and (e) reprinted from reference 43, with permission from American Physical Society, copyright 2009.)

C-face multilayer graphene films exhibit good planarity with unique layerstacking and behave effectively as n independent graphene monolayers (cf. 1.3). LEED patterns from furnace-grown 4H-SiC (0001) with ∼10 graphene layers reveal that such graphene can grow in multiple forms: layers rotated 30° (R30) or ±2.2° (R2±) from the SiC bulk [1010] direction. All three rotated phases are interleaved in the film, leading to a high density of stacking fault boundaries between the R30 and R2± layers. Such stacking faults possess a 13 × 13R46.1° unit cell schematically shown in Fig. 1.10(a) and can be directly observed in STM as shown in Fig. 1.10(b) and (c).42 These stacking faults decouple adjacent graphene sheets so that their band structure is nearly identical to isolated graphene, as revealed by ARPES of the topmost (neutral) layers in the multilayer epitaxial graphere (MEG) stack in Fig. 1.10(d) and (e).43 The Dirac cones of the multilayer EG remain unperturbed and distinct from one another. The k⊥ displacement of the cone sections in Fig. 1.10(d) and (e) is the result of the rotation angle between layers.

Epitaxial growth of graphene on silicon carbide (SiC)

1.3

15

Thermal decomposition of single-crystal SiC under ambient pressure conditions

To improve the homogeneity of EG, the growth condition should shift towards thermodynamic equilibrium, i.e. increasing the growth temperature without increasing the silicon sublimation rate. At higher temperature, C atoms at the surface have greater kinetic energy and transform into the graphene layer with fewer defects and grain boundaries. One viable method first used by Emtsev et al.19 is to introduce inert gas, specifically argon, to the growth environment. Compared with UHV, where silicon atoms are free to escape from the surface, the presence of argon molecules give a finite possibility to bounce the silicon atoms back and therefore reduce the silicon sublimation rate. When argon pressure is close to ambient pressure, graphene films with large domains and few defects can be readily obtained. The ambient pressure growth of graphene on SiC is performed in specially designed air-tight furnaces. These furnaces are either quartz tubes or vacuum chambers. At ambient pressure, the heat can be more effectively dissipated via convection and thus proper water cooling to the furnace wall is required. A graphite holder is used to withstand high annealing temperature. One furnace made by water-cooled quartz tube via inductive heating is shown in Fig. 1.11.44 Methods to minimize the temperature gradient need to be considered when wafer size SiC samples are used. The SiC samples are slowly heated and cooled at a rate of 2–3 °C s−1. The annealing temperature, 1500–2000 °C, is maintained for 15 min. Argon flow at 0.9–1 bar is introduced into the furnace during the growth on Si-face SiC. On C-face, the annealing temperature is kept at 1500 °C and an argon pressure up to 9 bar is used, to enhance the suppression of fast silicon sublimation at this surface.45 In Fig. 1.12, EG prepared by UHV and ambient annealing are shown. Figure 1.12(a) shows the initial surface of SiC(0001) after hydrogen etching. Large terrace widths around 300–700 nm and step heights about 1.5 nm are observed. On the UHV-prepared sample (Fig. 1.12(b) and (c), the surface morphology is substantially changed and LEEM suggests that the graphene islands are about a few hundred nanometers in size. In contrast, graphene grown under ambient pressure has a greatly improved morphology. The atomic fluorescence microscopy (AFM) image (Fig. 1.12(e)) shows increased terrace sizes from hundreds of nanometers to a few micrometers and average step heights of 8–15 nm. LEEM reveals a continuous graphene layer over all terraces with bi- or trilayer graphene strips at the terrace edges. Except for the domain size, the defects in graphene film are significantly reduced in the ambient prepared sample. Figure 1.12(d) shows the Raman spectra of samples prepared in UHV and in Ar. For vacuumgrown sample, the G-, D- and 2D-peaks at 1596, 1356 and 2706 cm−1 are observed, respectively. For the sample grown in Ar atmosphere, the G-peak

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1.11 A furnace made by quartz tube for graphene growth in Ar atmosphere by inductive heating and water cooling. (Reprinted from reference 44, with permission from National Academy of Sciences, copyright 2011.)

Epitaxial growth of graphene on silicon carbide (SiC) (a)

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Raman shift (cm–1)

1.12 Characterization of graphene growth in vacuum and in Ar atmosphere. (a) Initial surface after H-etching imaged by AFM. The step height is 15Å. (b) AFM image of graphene on 6H-SiC(0001) with a nominal thickness of 1 ML formed by annealing in UHV at a temperature of about 1280 °C. (c) LEEM image of an UHV-grown graphene film on SiC(0001) with a nominal thickness of 1.2 ML. The image contrast is caused by the locally different layer thickness. Light, medium and dark grey correspond to a local thickness of 0, 1 and 2 ML, respectively. (d) Raman spectra of Ar-grown (lower trace) and UHV-grown (upper trace) epitaxial graphene on 6H-SiC(0001). The spectra of the D- and G-line shown here are corrected for the emission of the substrate by subtraction of a reference spectrum. (e) AFM image of graphene on 6H-SiC(0001) with a nominal thickness of 1.2 ML formed by annealing in Ar (p = 900 mbar, T = 1650 °C). (f) LEEM image of a sample equivalent to that of (d) revealing macro-terraces covered with graphene up to 50 μm long and at least 1 μm wide. (Reprinted from reference 19, with permission from Nature Publishing Group, copyright 2009.)

and 2D-peak are observed at 1592 and 2717 cm−1, respectively. However, the D-peak is not present and the full-width at half-maximum of the 2D peak is 37 cm−1, narrower than that of the UHV-grown sample (54 cm−1). The Raman spectra suggest the Ar-grown graphene layer has fewer defects than the vacuum-grown sample. The transport performance of graphene prepared in Ar can be evaluated by carrier mobility measurements. In Table 1.1, the mobility of Ar-grown and vacuum-grown samples is measured by Hall

18

Graphene

Table 1.1 Hall mobilities (cm2 V−1 s−1) for Hall bars and van der Pauw structures on UHV- and Ar-grown graphene measured at T = 300K and 27K19 Method

Structure

300K

27K

Ar

Hall Van Hall Van

900 930 470 550

1850 2000 – 710

UHV

bar der Pauw bar der Pauw

effect in Hall bar and van der Pauw geometry. The mobility of Ar-grown graphene is two to three times higher than vacuum-grown graphene layers. The maximum mobility value obtained is 2000 cm2 V−1 s−1 at 27K. After the discovery of Ar-assisted graphene decomposition on SiC, further improved growth methods based on controlling the net flux of silicon atoms leaving the surface have been suggested. This could be done by directly controlling the Si vapour pressure using disilane gas46 or confining the sample in a graphite enclosure with a pin hole.44 The latter method known as the confinement controlled sublimation (CCS) method can provide additional suppression to the silicon sublimation rate in the order of 10−3.

1.4

Thermal decomposition of single-crystal SiC thin films and polycrystalline SiC substrates

Graphene devices using epitaxial graphene on SiC have been widely constructed in the laboratory. However, a single-crystal SiC wafer is expensive and has a diameter not more than 100 mm. The high cost and small size of the SiC substrate impede the large-scale manufacture of graphene devices in a cost-effective manner. The growth of graphene films only requires decomposition of a few SiC atomic layers, and epitaxial SiC thin films and polycrystalline SiC substrates could provide more costeffective alternatives for the growth of EG. Cubic SiC (3C-SiC) thin films can be readily grown on top of many low-index surfaces of Si wafers. These low-index surfaces include Si(111), Si(100) and Si(110). Thin 3C-SiC films with thicknesses from 100 nm to a few micrometres can be grown on Si substrates using chemical vapour deposition with binary (SiH4/C3H8) or single sources (SiCH6). Graphene on 3C-SiC thin films are grown in vacuo at 1200 to 1300 °C, similar to the EG growth on single-crystal SiC in UHV. Ar-mediated graphene growth on these thin films is not possible as the annealing temperature can exceed the melting point of the silicon substrate (1414 °C). The characterization of graphene grown in UHV on SiC thin films by Raman spectroscopy confirms graphene formation. In Fig. 1.13, the

Epitaxial growth of graphene on silicon carbide (SiC) (a)

(b)

2D

SiC 2D* (i)

(ii)

(iii) (iv) 1500

2500 3000 2000 Raman shift (cm–1)

3500

Intensity (arbitrary units)

Intensity (arbitrary units)

G

D

19

G SiC

D

1 nm

1500

2000 2500 Raman shift (cm–1)

2D

3000

1.13 Raman spectra of graphene on 3C-SiC thin film and polycrystalline SiC substrate. (a) Graphene spectra prepared by various methods: (i) exfoliated SLG on SiO2/Si(100), (ii) exfoliated 5LG on SiO2/Si(100), (iii) EG on 3C-SiC(110)/Si(110) and (iv) EG on 6H-SiC(0001) bulk crystal. Spectrum (D) was obtained after subtracting the 6H-SiC reference spectrum from the raw data. In the subtracted spectra, the subtraction is not complete and therefore the component arising from bulk SiC cannot be removed. (b) Raman spectra taken from two points on the sample showing graphene-related D, G, and 2D bands. The inset shows a STM image of graphene lattice on polycrystalline SiC substrate. ((a) Reprinted from reference 47, with permission from IOP Publishing Limited, copyright 2010. (b) Reprinted from reference 48, with permission from American Physical Society, copyright 2011.)

characteristic graphene peaks, e.g. G-peak and 2D-peak and additional defect related peaks (D-peak and D+G peak) are observed. The strong intensity of D-peak and distinguishable D+G peak of graphene on SiC(111) and SiC(110) suggests the incorporation of significant amount of defects within graphene.47 Polycrystalline 3C-SiC can also be used in graphene growth, and produces a similar quality to the graphene on single-crystal SiC from Raman observations.48 Polycrystalline SiC is commercially available at much lower prices and larger sizes than single-crystal SiC. Polycrystalline SiC can be decomposed in UHV or Ar atmosphere, though the latter method has not been reported. The Raman spectra of a graphene layer prepared in UHV are shown in Fig. 1.13(b). Both graphene-related peaks (G-peak, 2D-peak and D-peak) and SiC-related peaks at ∼1524 and ∼1716 cm−1 are shown. No apparent D+G peak is observed in the spectra, suggesting that the quality of graphene is similar to EG on single-crystal substrate.48 A STM image of graphene layer on polycrystalline substrate is shown in Fig. 1.13(b) inset. The graphene lattice with honeycomb structure clearly reveals the singlelayer graphene growth.

20

Graphene

1.5

Epitaxial graphene formed by intercalation

Graphene sheets interact weakly with their substrate by van der Waals interaction. Thus, insertion of molecules or atoms into the graphene– substrate interface or intercalation is energetically favourable and is observed both on SiC and metal substrates. For instance, EG on Ni(111) can be intercalated by other metal atoms, such as Fe, Au or Al, as confirmed by XPS, ARPES and STM studies.49,50 Intercalated atoms act as a buffer layer and can remove the strong graphene–substrate interaction and restore the linear dispersion of graphene at its K point. Intercalation can affect the chemical reactivity of graphene. For instance, Pt intercalated graphene on Ru(0001) shows lower reactivity to oxidation in O2.51 On SiC substrates, intercalation is accompanied by additional chemical interactions. The atoms intercalated between IG and SiC substrate can react with Si dangling bonds at the interface and release IG to form an additional graphene layer. The reactive intercalation occurs with or without the top graphene layer. It provides an opportunity to reversibly change the IG layer into graphene and eliminate the n-type doping effect of IG on to EG. Many intercalated atoms are found to react with silicon atoms, including hydrogen, oxygen, fluorine, gold, iron, lithium, germanium and even silicon itself.52–59 However, exceptions have also been found. Cs and Rb do not intercalate into epitaxial graphene probably because of their large atomic radius.60 Intercalation is usually completed in two steps. First atoms are deposited (a few atomic layers) on the graphene surface. Second, the surface is annealed to cause the adsorbed atoms to diffuse and intercalate through defects or grain boundaries and to react with interfacial silicon atoms. The annealing temperature varies with the element used. For Ge, the annealing temperature is as high as 920 °C55 whereas for Li, intercalation occurs at room temperature although annealing at 330 °C helps Li atoms distribute uniformly.57 Table 1.2 summarizes the temperature needed to intercalate various atoms. For gas molecules, intercalation can be done using atomic sources (hydrogen),52 high-pressure annealing (oxygen, 1 bar 250 °C)53 or by molecular decomposition (C60F48, fluorine).54 The transformation from IG into single-layer EG on SiC by hydrogen intercalation has been confirmed by ARPES as shown in Fig. 1.14(a)–(e).52

Table 1.2 Annealing temperature for intercalation of solid atoms at graphene surface Element

Au59

Fe

Li57

Ge55

Si56

Temperature

727 °C

600 °C



720–920 °C

800 °C

Epitaxial growth of graphene on silicon carbide (SiC) (a) Clean ZL (b)

ZL+H (c)

700 °C (d)

800 °C (e)

EF=0 (f) Clean ML (g)

ML+H (h)

700 °C (i)

900 °C

E-EF (eV)

E-EF (eV)

EF=0

21 900 °C

–0.5

–0.5

(j)

1000 °C

ED

–0.15 0 0.15 –0.15 0 0.15 –0.15 0 0.15 –0.15 0 0.15 –0.15 0 0.15 k (Å–1) k (Å–1) k (Å–1) k (Å–1) k (Å–1)

1.14 Dispersion of the π bands measured with ARPES perpendicular to the Γ direction of the graphene Brillouin zone for: (a) an as-grown graphene zero layer (ZL) on SiC(0001); (b) after hydrogen treatment and (c)–(e) subsequent annealing steps; (f) for an as-grown monolayer (ML), (g) after hydrogen treatment and (h)–(j) subsequent annealing steps. (Reprinted from reference 52, with permission from American Physical Society, copyright 2009.)

The linear dispersion at K point only appears after hydrogen intercalation and disappears above 900 °C, i.e., hydrogen is desorbed at this temperature. Likewise, hydrogen intercalation can transform single-layer EG into bilayer EG on SiC as shown in Fig. 1.14(f)–(j). Notably, after intercalation, EF coincides with the Dirac point indicating that the intrinsic n-type doping of graphene is diminished. The intercalation of other elements may introduce additional n-type doping (Li), p-type doping (F) or ambipolar doping depending on annealing temperature (Ge).

1.6

Conclusion

This chapter reviews the growth and the atomic-scale characterization of EG grown on single-crystal SiC wafers, polycrystalline SiC and SiC thin films via thermal decomposition under UHV or ambient-pressure conditions. High-quality graphene films with controllable layer numbers and relatively large domain sizes exceeding 1 μm can be obtained on single-crystal SiC substrates. Moreover, EG grown on SiC is compatible with current Si-based microfabrication lithography processes, making EG a promising candidate for the large-scale production of graphene-based electronic devices. Because

22

Graphene

of the atomic flat, chemically inert and structurally simple surface, EG on SiC can also be used as an excellent platform for the growth of the highquality topological insulator Bi2Se3 or superconductor FeSe, or graphenebased heterojunctions with organic overlayers. However, a few technical problems associated with EG growth on SiC need to be properly addressed to further extend their practical applications towards the realization of commercialized graphene-based devices, including: (1) the high cost of SiC substrates, in particular single-crystal SiC substrates; the search for low-cost alternatives that allow the growth of high-quality EG films; (2) the current technology requires the growth of EG at high temperatures usually exceeding 1200 °C, and hence a significant energy input; the realization of a low-temperature process for the growth of high-quality EG on SiC is essential for the further development of EG-based devices; (3) the electronic properties of EG are severely affected by the coupling with the underlying SiC; this degrades the EG-based device performance relative to micromechanical exfoliated graphene. The development of practically feasible methods to electronically decouple EG with the underlying SiC is needed for the ease of integration of EG-based device fabrication process; (4) the EG on SiC has more defects with smaller domain size than the exfoliated graphene; therefore optimization of EG growth technology is necessary to achieve macroscopic domain sizes.

1.7

Acknowledgements

The authors acknowledge financial support from NRF-CRP grants R-144000-295-281 ‘Novel 2D materials with tailored properties – beyond graphene’, and R-143-000-360-281 ‘Graphene related devices and materials’, MOE grant R143-000-542-112 and NUS YIA grant R143-000-452-101.

1.8

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