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Atomic-Scale Investigation of Epitaxial Graphene Grown on 6H-SiC(0001) Using Scanning Tunneling Microscopy and Spectroscopy Junghun Choi,† Hangil Lee,*,‡ and Sehun Kim*,† Molecular-LeVel Interface Research Center, Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea, and Department of Chemistry, Sookmyung Women’s UniVersity, Seoul 140-742, Republic of Korea ReceiVed: May 27, 2010; ReVised Manuscript ReceiVed: June 26, 2010
Graphene was epitaxially grown on a 6H-SiC(0001) substrate by thermal decomposition of SiC under ultrahigh vacuum conditions. Using scanning tunneling microscopy (STM), we monitored the evolution of the graphene growth as a function of the temperature. We found that the evaporation of Si occurred dominantly from the corner of the step rather than on the terrace. A carbon-rich (63 × 63)R30° layer, monolayer graphene, and bilayer graphene were identified by measuring the roughness, step height, and atomic structures. Defect structures such as nanotubes and scattering defects on the monolayer graphene are also discussed. Furthermore, we confirmed that the Dirac points (ED) of the monolayer and bilayer graphene were clearly resolved by scanning tunneling spectroscopy (STS). I. Introduction Since the discovery of graphene layers in 2004,1,2 the novel physical properties of these layers, such as ballistic transport, the quantum Hall effect, and minimum conductivity, have been intensively studied.3-5 These properties suggest that graphene may potentially be used in carbon-based electronics.6 In general, graphene consists of a flat monolayer of carbon atoms packed into a two-dimensional honeycomb lattice and is a building block for carbon materials, such as zero-dimensional fullerenes, one-dimensional nanotubes, and three-dimensional graphite.3 Graphene substrates can be fabricated by micromechanical cleavage, chemical vapor deposition (CVD) on metallic substrates, or thermal decomposition of SiC. Although the mobility of epitaxial graphene (EG) grown on SiC is at least 1 order of magnitude lower than that of the exfoliated graphene,7,8 EG on SiC substrates is versatile and suitable for applications due to its ease of processing.9 Most structural, growth, and electronic properties of EG originate from the Si-face graphene layer because orientations in the C-face graphene are disordered.8 In 6H-SiC, the stacking sequence is ABCACB... and the 6H-SiC cell is composed of six SiC bilayers. The lattice spacings of aSiC and cSiC are 3.0813 and 15.1198 Å, respectively. Prior to the formation of graphene, a sample is heated at 850 °C under a Si flux to remove oxides. After this treatment, under ultrahigh vacuum conditions, (3 × 3)R30° and (63 × 63)R30° phases are obtained as the temperature is increased from 900 to 1100 °C, and the graphene layer is grown during a short period of graphitization at 1200 °C.10,11 The thickness of the graphene can be controlled by the graphitization temperature and time. Over the last three decades, graphite substrates have been investigated by STM.12 The atomic structures of exfoliated graphene on SiO2 substrates were observed first,13 and the thickness-dependent structural and electronic properties of EG on SiC(001) were resolved at the atomic scale.14-29Initially, the * To whom correspondence should be addressed. E-mail: (S.K.) sehun-kim@ kaist.ac.kr; (H.L.)
[email protected]. † KAIST. ‡ Sookmyung Women’s University.
epitaxial growth of graphite was reported on 6H-SiC(0001) surfaces.14 Thickness-dependent atomic structures of graphene were identified by STM. Monolayer and multilayer graphene appeared, respectively, as hexagonal and triangular arrays of protrusions.15,19,20,25 STS measurements by several groups19,20,22 resolved the Dirac points of monolayer and multilayer graphene, which agreed with the ARPES observations.30,31 The growth mechanism was estimated by the structures or height differences between steps.18,19,21 However, the evolution of the graphene layer from a carbon-rich (63 × 63)R30° layer at the atomic level remained less understood. In this article, we present an STM and STS investigation of the atomic and electronic structures of EG grown on 6HSiC(0001) as a function of the annealing temperature to clarify the coverage-dependent structural and electronic changes in the EG layer. II. Experimental Methods STM (Omicron) measurements were performed in an ultrahigh vacuum chamber (base pressure ) 8.0 × 10-11 Torr) at room temperature with electrochemically etched tungsten (W) tips. The cleanness of the tip was characterized by performing STM and STS on monolayer and bilayer graphene. STS data were collected in grid mode during imaging with a sample bias of +0.1 V and a set point of 0.5 nA. During the STS measurement, feedback between the tip and the substrate was turned off. Normalized d ln I/d ln V spectra were obtained after approximately 2500 I/V curves were averaged. A nitrogen-doped (ND ≈ 9 × 1017 cm-3) Si-terminated 6HSiC(0001) substrate (Cree Research) was used to fabricate EG. The substrate was degassed, annealed at 900 °C under a Si flux (1 Å/min), and finally graphitized at temperatures up to 1200 °C (for 2 min) to produce a monolayer of graphene. The annealing temperature was monitored with an infrared pyrometer (emissivity of 0.9). The experimental details are described elsewhere.32,33 III. Results and Discussion Large-scale STM images of EG grown on a 6H-SiC(0001) substrate at several annealing temperatures were acquired, as
10.1021/jp1048716 2010 American Chemical Society Published on Web 07/15/2010
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Figure 1. Large-scale STM images of EG grown on 6H-SiC(0001) at (a) 1100 and (b) at 1200 °C. (63 × 63)R30° layer, monolayer graphene, and bilayer graphene are indicated by 6R3, M, and B, respectively. (Imaging conditions: Vs ) -2 V, It ) 0.1 nA, size ) 200 × 200 nm2.)
Figure 2. STM images of EG on 6H-SiC(0001). (a) Magnified image of Figure 1a. Monolayer graphene and a (63 × 63)R30° layer are indicated by M and 6R3, respectively; (63 × 63)R30° unit cell (yellow) and (6 × 6) quasi-cell (white) are indicated by diamonds. (b) Pristine monolayer and bilayer graphene layers (denoted as B). Nanotube defects are marked by an arrow. (c) Magnified STM image of Figure 1b. Circles represent trimer-like features. (d) Line profiles across the boundary of 6R3/M (a to a′) and M/B (b to b′). The corrugation of the 6R3, M, and B is clearly visible. (Imaging conditions: (a-c) Vs ) -2 V, It ) 0.1 nA; size ) (a and b) 40 × 40 nm2, (c) 20 × 20 nm2.)
shown in Figure 1. Figure 1a shows the filled-state STM image obtained after annealing the substrate at 1100 °C for 2 min. This image shows that the carbon-rich (63 × 63)R30° (marked as 6R3) reconstructed structure was dominant, although monolayer graphene (marked as M) began to emerge at low levels (∼5.5%), indicating that the annealing temperature was insufficient to form dominant monolayer graphene. Interestingly, the image clearly shows that the most prominent growth direction of the straight steps followed the hexagonal symmetry of the SiC substrate. We found that the growth of the monolayer graphene dominantly occurred from the corners of the step edges rather than from the terraces. The edge orientations of the monolayer graphene islands resembled those of the existing SiC steps.
To track the morphological variations as a function of annealing temperature, we further annealed the substrate at temperatures up to 1200 °C for 2 min, which is the formal annealing temperature reported in the literature. As shown in Figure 1b, we confirmed that the population of the fabricated monolayer graphene layer increased dramatically (∼65%) and the number of straight steps that followed the symmetry of the substrate decreased as the monolayer graphene coverage increased. Moreover, at this annealing temperature, bilayer graphene was observed to form. As a result, as the annealing temperature was controlled between 1150 and 1200 °C and the time between 1 and 6 min, the dominant monolayer graphene and other graphene layers could be fabricated to a variety of specifications.
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Figure 3. Atomic structure at the boundary of the monolayer (left, marked as M) and bilayer (right, marked as B) graphene. (Imaging conditions: Vs ) +0.1 V, It ) 0.5 nA, size ) 12 × 12 nm2.)
As mentioned above, our aim in this study was to determine the atomic-scale structure of the graphene layer as a function of annealing temperature and time. Hence, Figure 2 illustrates in greater detail the magnified image of the area enclosed by the white square in Figure 1, so that the exact structure of EG (monolayer and bilayer graphene) could be precisely described. Figure 2a shows a magnified view of Figure 1a, which was obtained after annealing at 1100 °C for 2 min. This image clearly shows a carbon-rich (63 × 63)R30° (marked as 6R3) surface together with monolayer graphene (marked as M). In the STM image, the (6 × 6) quasi-cell was often observed under normal tip conditions. The size of the unit cell was measured to be 1.86 nm, which was approximately six times the lattice constant (a ) 0.30813 nm) of 6H-SiC(0001).8 Line profiles showed that the height difference between the two 6R3 layers was 0.25 nm (a to a′ in Figure 2a), which corresponded to the SiC bilayer spacing. Moreover, we found that monolayer graphene showed a (6 × 6) honeycomb structure due to the underlying SiC substrate. From this image, one can clearly distinguish the monolayer graphene from the 6R3 layer by measuring the root-mean-square (rms) surface roughness. The measured average rms values, determined at a sample bias of -2 V, were 26 and 15 pm for the 6R3 surface and monolayer graphene, respectively, indicating the differences in height, in agreement with previous reports.19,20 Figure 2b confirms that (6 × 6) patterns were suppressed in the bilayer graphene, marked B. The roughness was measured to be 9 pm for bilayer graphene. The step height between the monolayer and bilayer was 0.33 nm (Figure 2d). Considering the contrast in the STM images (Figure 2a and 2b)
Choi et al. and the line profiles shown in Figure 2d, we speculated that the corrugation of the monolayer graphene was larger than that of the bilayer graphene but smaller than that of the 6R3 layer. Figure 2c shows a magnified region of Figure 1b, obtained after annealing at 1200 °C for 2 min, which includes 6R3, monolayer, and bilayer graphene. The morphologies of the three terraces were different from those shown in Figures 2a and 2b. However, usually, the tip conditions used to acquire the atomic-resolution images of graphene shown in Figures 2a and 2b were more suitable than those used to acquire the image shown in Figure 2c.35 In Figure 2c, the interface states of the graphene and SiC substrate were observed. Furthermore, trimer-like features (circles in Figure 2c) were observed. Atomic-scale STM images of monolayer and bilayer graphene are shown in Figure 3. Typically, the atomic structures of the graphene were resolved at sample bias voltages below 0.5 V in the filled- and empty-state images. This image shows that the morphologies of the monolayer (marked as M) and bilayer graphene (marked as B) are clearly discriminated because the six-membered ring and three-for-six structures are clearly resolved in the monolayer and bilayer graphene.21 Moreover, the step height between the two distinct layers was 0.08 nm, which corresponded to the difference between the interlayer spacing (0.34 nm) of bilayer graphene and the SiC bilayer (0.25 nm).21,22 The (6 × 6) superlattice formed by the two layers was not continuous. In other words, the monolayer graphene and the second layer of the bilayer were continuous over both the 6R3 layer and the first graphene layer of the bilayer graphene. However, the structural transition from a six-membered ring to a three-for-six structure did not correspond to the position of the step. Figure 4a shows monolayer graphene with partial (63 × 63)R30° layers. A magnified view of this image is displayed in Figure 4b. One can clearly observe that monolayer graphene continuously extended across four SiC bilayers, as confirmed by the line profile. A height difference of 1.0 nm, measured between the top and the bottom graphene layers, corresponded to four times the step height of the SiC bilayers (0.25 nm). As was observed in Figure 3, Figure 4 shows that the (6 × 6) superlattice between the two topmost layers was not matched. Note that monolayer graphene was grown over the straight SiC steps continuously. We conclude that the growth of graphene did not always smooth the step edges of SiC, although such a tendency was observed in the STM images in Figure 1. During fabrication of the EG, unexpected defect structures were sometimes observed, as shown in Figure 5.23,24 Figure 5a and 5b shows nanotube-shaped defects, which appear as rod-
Figure 4. Monolayer graphene over the SiC steps. (Imaging conditions: (a) Vs ) -2 V, It ) 0.1 nA; (b) Vs ) +0.2 V, It ) 0.5 nA; size ) (a) 60 × 60 and (b) 16 × 16 nm2.)
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Figure 5. Defect structures of monolayer graphene on 6H-SiC(0001): (a) nanotube defects, (b) magnified image, (c) line profiles of b, (d) scattering defects. (Imaging conditions: (a) Vs ) -2 V, It ) 0.1 nA, (b and d) Vs ) +0.2 V, It ) 1 nA; size ) (a) 40 × 40, (b and d) 8 × 8 nm2.)
shaped protrusions at high bias voltage. The presence of nanotube defect affects the electrical properties of graphene. However, gaseous species could be trapped inside the defect.36 Thus, the nanotube defect density of graphene is to be controlled. The atomic structures on top of one of these nanotubes are clearly resolved in the magnified image in Figure 5b. As shown in Figure 5c, the shape of the nanotube shown in Figure 5b was asymmetric along the long axis. The maximum height with respect to the graphene terrace was measured to be 0.3 nm. The length of the nanotube defect in these images was approximately 4 nm. The long axis was aligned with the underlying substrate structure. The formation of nanotube defects could have originated from subsurface SiC structures during graphitization or from intrinsic bumps at the interface. However, we were unable to locate asymmetric bump structures on the (63 × 63)R30° surfaces (Figures 1a, 2a, and 2c). It is unknown whether the nanotubes assumed a perfectly cylindrical shape or a half-cylinder shape because the subsurface structure was unresolved. Figure 5d shows a point-like defect that displayed 6-fold scattering. This defect may have indicated a carbon vacancy or a carbon atom that was substituted by a silicon atom because the atomic structure of the monolayer graphene was significantly altered in the vicinity of the defect (less than 10 nm).24 Finally, we performed STS measurements on pristine monolayer and bilayer graphene to investigate the electronic structures of monolayer and bilayer graphene. Figure 6 displays the normalized differential conductance spectra (d ln I/d ln V) of the pristine monolayer and bilayer EG. Generally, the normalized differential conductance is proportional to the local density of states (LDOS) of the sample, excluding the influence of tip-sample separation and applied bias voltages to the conductance.34 The overall shapes of the STS spectra, measured
Figure 6. Normalized differential conductance (d ln I/d ln V) STS spectra of pristine monolayer (black line) and bilayer (red line) graphene. The Dirac points (ED) are indicated by arrows.
on pristine monolayer (black line in Figure 6) and bilayer graphene (red line in Figure 6), are consistent with the previous STS spectra obtained at low temperatures.19,20 However, the issue of whether the graphene layer exhibits a band gap has been controversial.19,20 Our STS spectra show a finite density of states at the EF (Vs ) 0), indicating metallic behavior. Moreover, we clearly observed charge-neutral Dirac points (ED) at 401 meV (black arrow in Figure 6) and 314 meV (red arrow in Figure 6) below EF in the monolayer and bilayer graphene, respectively. The linear band structure near the Dirac point determines the unique property of graphene. Furthermore, the chemical and electrical doping of graphene could be identified by monitoring the changes in the electronic structure near the Dirac point. n-Type doping of EG originates from electron
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doping in the SiC substrate.30 As the thickness of the graphene layer decreases, the effect of doping is expected to decrease as well. IV. Conclusions We investigated the atomic and electronic structures of epitaxial graphene by STM and STS. Monolayer and bilayer graphene were clearly distinguishable by the measured roughness, step height, and atomic structures. Furthermore, we observed nanotube and scattering defects on monolayer graphene. The charge-neutral Dirac points (ED) were clearly resolved in both monolayer and bilayer graphene. Acknowledgment. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0001950). One of the authors (H.L.) was supported by National Research Foundation of Korea Grant funded by the Korean Government (KRF-2008314-C00169). References and Notes (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666–669. (2) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451–10453. (3) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183–191. (4) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Twodimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197–200. (5) Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Experimental Observation of the Quantum Hall Effect and Berry’s Phase in Graphene. Nature 2005, 438, 201–204. (6) Burghard, M.; Klauk, H.; Kern, K. Carbon-Based Field-Effect Transistors for Nanoelectronics. AdV. Mater. 2009, 21, 2586–2600. (7) Lin, Y.-M.; Jenkins, K. A.; Valdes-Garcia, A.; Small, J. P.; Farmer, D. B. Avouris Ph. Operation of Graphene Transistors at Gigahertz Frequencies. Nano Lett. 2009, 9, 422–426. (8) Hass, J.; de Heer, W. A.; Conrad, E. H. The Growth and Morphology of Epitaxial Multilayer Graphene. J. Phys.: Condens. Matter 2008, 20, 323202. (9) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Electronic Confinement and Coherence in Patterned Epitaxial Graphene. Science 2006, 312, 1191–1196. (10) Emtsev, K. V.; Speck, F.; Seyller, Th.; Ley, L.; Riley, J. D. Interaction, Growth, and Ordering of Epitaxial Graphene on SiC{0001} Surfaces: A Comparative Photoelectron Spectroscopy Study. Phys. ReV. B 2008, 77, 155303. (11) Kim, K.-j.; Lee, H.; Choi, J.; Lee, H.-K.; Kang, T.-H.; Kim, B.; Kim, S. Temperature Dependent Structural Changes of Graphene Layers on 6H-SiC(0001) Surfaces. J. Phys.: Condens. Matter 2008, 20, 225017. (12) Pong, W.-T.; Durkan, C. A Review and Outlook for an Anomaly of Scanning Tunnelling Ticroscopy (STM): Superlattices on Graphite. J. Phys. D: Appl. Phys. 2005, 38, R329–R355. (13) Stolyarova, E.; Rim, K. T.; Ryu, S.; Maultzsch, J.; Kim, P.; Brus, L. E.; Heinz, T. F.; Hybertsen, M. S.; Flynn, G. W. High-resolution Scanning Tunneling Microscopy Imaging of Mesoscopic Graphene Sheets on an Insulating Surface. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 9209–9212. (14) Charrier, A.; Coati, A.; Argunova, T.; Thibaudau, F.; Garreau, Y.; Pinchaux, R.; Forbeaux, I.; Debever, J.-M.; Sauvage-Simkin, M.; Themlin, J.-M. Solid-state Decomposition of Silicon Carbide for Growing Ultra-thin Heteroepitaxial Graphite Films. J. Appl. Phys. 2002, 92, 2479–2484.
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