Graphitic Carbon Growth on MgO(100) by Molecular Beam Epitaxy

Article pubs.acs.org/JPCC

Graphitic Carbon Growth on MgO(100) by Molecular Beam Epitaxy S. K. Jerng,† J. H. Lee,† D. S. Yu,† Y. S. Kim,† Junga Ryou,† Suklyun Hong,† C. Kim,‡ S. Yoon,‡ and S. H. Chun*,† †

Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, Korea Department of Physics, Ewha Womans University, Seoul 151-747, Korea

‡

ABSTRACT: Direct graphene growth on insulating substrates is of great importance for graphene electronics. Limited accomplishments by using molecular beam epitaxy have been demonstrated on substrates of hexagonal symmetry. For comparison and further progress, we study the growth of graphitic carbon on cubic MgO substrates. Raman spectra clearly show D, G, and 2D peaks, confirming the formation of nanocrystalline graphite. The degree of graphitization is comparable to those of carbon layers grown on hexagonal substrates. X-ray photoelectron spectroscopy proves the dominance of carbon sp2 bonding, and transmission electron microscopy reveals nanometer-scale clusters directly. The flatness and the homogeneity of graphitic carbon on MgO(100) are also beneficial to potential applications of heterostructures containing graphitic carbon. First-principles calculations elucidate that the strong carbon−oxygen interaction limits the in-plane coherence length.

1. INTRODUCTION Recently graphene has attracted enormous attention due to its potential for fast and flexible electronics as well as its interesting physical properties.1 During the past years, graphene and its derivatives have demonstrated diverse applications, including flexible touch screen,2 high frequency field effect transistor,3,4 super capacitor,5 water purification,6 DNA translocation,7 thermal management,8,9 and analog communications and signal processing.10 These accomplishments and further developments are in parallel with the improvements of graphene synthesis. Since the first isolation of graphene by mechanical exfoliation (using a roll of adhesive tape) from highly oriented pyrolytic graphite (HOPG) flake,11 the preparation of highquality graphene has been of primary issue. Especially for electronic device applications, reproducible growth of large area graphene is imperative. Among the various approaches so far, thermal decomposition of 6H(4H)-SiC and chemical vapor deposition (CVD) on catalytic metals (e.g., Ni, Cu, Ir, and Ru) are probably the most successful ones.12−16 However, both methods have critical drawbacks: thermal decomposition requires expensive substrates and high annealing temperature over 1400 °C, and the CVD-grown graphene should be transferred to insulating substrates for useful device applications. Such drawbacks and other factors currently limit the carrier mobility below 10,000 cm/(V s). Therefore new routes to overcome these limitations are still needed, and molecular beam epitaxy (MBE) is one of the candidates. MBE is a wellknown physical growth method, widely used in the fabrication of semiconductor quantum structures. Direct growth of graphitic carbon films on Si(111), 6H(4H)-SiC(0001), sapphire, mica, SiO2, and glass by MBE were reported recently, showing the efforts and the potential of graphene growth by MBE.17−24 To achieve catalyst-free graphene growth by MBE, © 2012 American Chemical Society

it is necessary to investigate various substrates, carbon sources, or growth conditions, and thereby to provide further understanding of the carbon growth mechanism in the MBE process. Simulations at the atomic level from the first-principles are also required. So far the substrates having hexagonal symmetry were preferred in graphene-MBE, probably from the expectation of lattice-matched epitaxial growth of graphene. In the previous study on sapphire substrates of various cutting-directions, we have found that the symmetry of substrate affected little on the sample quality. Then, we may expand the list of candidates beyond those having hexagonal symmetry. We choose MgO(100) for several reasons. First, MgO(100) bears cubic symmetry with a lattice constant of 4.212 Å, and does not have a chance of a lattice-matching with graphene having hexagonal symmetry with a lattice constant of 2.455 Å. So the outcome will help identifying the MBE growth mechanism. Second, MgO(100) is very flat (average roughness, Ra, of 0.28 ± 0.02 nm) and its high melting point (∼2800 °C) is beneficial to high growth temperature around 1000 °C. Therefore, we can compare the growth on cubic MgO with previous studies, free from the issue of surface roughness or growth temperature. Finally, MgO can be used as a tunnel barrier especially for spintronic devices utilizing graphene. Wang et al. have reported the growth of smooth MgO films on graphene by MBE.25 The growth of graphene on MgO is an important step toward graphene electronics. A recent report of nanographene CVD synthesis on MgO also motivated this work.26 Received: November 13, 2011 Revised: March 6, 2012 Published: March 11, 2012 7380

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cutoff was set to 282.8 eV. The MgO(100) surface was modeled by a slab having a (2 × 2) and (4 × 4) surface supercell that consisted of six MgO layers with 12 Å thick vacuum region. The position of atoms in the two bottom layers were fixed, whereas the other layers were relaxed within residual forces smaller than 0.02 eV/Å. For the Brillouin-zone integration we used a 4 × 4 × 1 or 2 × 2 × 1 grid in the Monkhorst-Pack special k-point scheme depending on the size of surface unit cells.

Here, we report the growth of graphitic carbon on MgO(100) substrates by using solid-source carbon MBE. Raman spectra clearly show characteristic peaks of nanocrystalline graphite (NCG). Interestingly, the crystallinity of NCG on MgO(100) is similar to that on sapphire(0001), implying that the symmetry of substrate is not essential. X-ray photoelectron spectroscopy (XPS) near C 1s manifests the dominance of sp2 bonding, a promising sign for direct graphene synthesis on insulators. We also find a correlation of sp2 bonding fraction and the Raman 2D peak intensity. High-resolution crosssectional transmission electron microscopy (TEM) and atomic force microscopy (AFM) show that a continuous carbon film (∼2.6 nm-thick) of very flat surface morphology is formed by MBE. Furthermore, nanometer-scale clusters are directly observed by TEM. Ab initio calculations explain the short coherence length and the symmetry independence.

3. RESULTS AND DISCUSSION We have learned that the quality of MBE-grown NCG strongly depends on growth temperature (TG) and the optimum TG is near 1000 °C, from our previous study on NCG growth on sapphire.21 Therefore, we grew carbon films on MgO at TG = 900−1100 °C. Figure 1 shows the Raman spectra obtained

2. EXPERIMENTAL TECHNIQUES Samples were grown by a homemade MBE system dedicated to carbon growth. The base pressure was less than 2.0 × 10−10 Torr without liquid nitrogen (LN2) flowing in the shroud. Carbon was supplied from a commercial cell where an HOPG filament was heated to around 1600 °C for carbon sublimation (SUKO model from MBE-Komponenten GmbH). Electron beam irradiation from a tungsten filament behind the sample manipulator enabled the substrate temperature to reach beyond 1100 °C. The temperature reading was calibrated by the desorption temperature of the natural oxide on the Si substrate, confirmed by the reflection high energy electron diffraction (RHEED) pattern. During the growth, with LN2 flowing in the shroud, the pressure of the chamber was kept below 1.0 × 10−7 Torr. Single crystalline MgO(100) substrates, purchased from CrysTec GmbH, were used in the experiments. They were cleaned by acetone and isopropyl alcohol before the growth. After being introduced into the chamber, they were baked at 900 °C for 1 h to remove chemical residues, and the substrate temperature was changed to the target growth temperature. Raman scattering measurements were performed by using a McPherson model 207 monochromator with a 488 nm (2.54 eV) laser excitation source. The spectra recorded with a nitrogen-cooled charge-coupled device array detector. The laser spot size was around 1 μm and focused through a 100× objective. The incident laser had a power of 1.65 mW to avoid spectral change by laser-induced local heating damage.27−29 Additional Raman maps were collected by using a Renishaw inVia spectroscopy system to confirm the uniformity of graphitic carbon. XPS measurements were done by using a Kratos X-ray photoelectron spectrometer with a Mg Kα X-ray source. C 1s spectra were acquired at 150 W X-ray power with a pass energy of 20 eV and a resolution step of 0.1 eV. A commercial AFM from NanoFocus Inc. was used to investigate the surface and to quantify Ra of as-grown samples as well as substrates. TEM studies were conducted by Tecnai G2 F30 (operating at 300 kV). Electrical properties were measured by using a standard ac method after Hall bar patterning by photolithography. The first-principles calculations are based on the spin polarized density functional theory (DFT) using the Vienna ab initio simulation package (VASP).30 The generalized gradient approximation (GGA) formulated by Perdew, Burke and Ernzerhof (PBE) was employed for the exchangecorrelation functional. The ions were described by the projector augmented wave (PAW) potentials and the kinetic energy

Figure 1. Raman spectra of graphitic carbon films grown on MgO(100) by MBE at different temperatures. Characteristic peaks of sp2 bonds (D peak near 1350 cm−1, G peak near 1580 cm−1, and 2D peak near 2700 cm−1) are seen for all samples. The inset is the dependence of D peak to G peak and 2D peak to G peak intensity ratios on the growth temperatures.

from the samples grown at different temperatures. All of the samples show the characteristic peaks of graphite with disorder: the D peak (from intravalley scattering by defects) near 1350 cm−1, the G peak (graphite peak) near 1580 cm−1, the small shoulder near 1620 cm−1 as D′ peak (from intervalley scattering by defects), and the 2D peak (second order of D peak) near 2700 cm−1.31−33 According to the theoretical three-stage model of Ferrari and Robertson,34,35 these Raman spectra, especially the strong and sharp D peak, imply that the film mainly consists of graphite nanocrystals. The existence of clear 2D peak is also an important feature of NCG.32 Recent studies supported the validity of this model experimentally.36,37 As we have shown in Figure 1 (inset), the large ID/IG and the significant I2D/IG (where ID, IG, and I2D are the intensities of D, G, and 2D peak, respectively) confirm the formation of NCG. As we pointed out in the introduction, the comparison of carbon growth on hexagonal substrates and on cubic MgO(100) is informative to understand the growth mechanism and the effect of substrate. Among the available Raman spectra of NCG on hexagonal substrates (Figure 4 of ref 17, Figures 3, 7381

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peaks as shown in Figure 2b,c. Each peak corresponds to D, G, D′, and an unassigned peak, U, respectively (see Table 1 for the details). First the difference in the G peak position is noticeable. The shift of G peak in graphene is due to various reasons such as the coupling with the substrate, strain, or doping.24,38−40 In NCG, the G peak appears blue-shifted due to the development of the D′ peak.34 As shown in Table 1, after decomposition of G and D′ peaks, the G peak of NCG on sapphire is more blue-shifted. Calizo et al. reported a red-shift in graphene on A-plane sapphire (of rectangular symmetry) and interpreted it as a result of carbon-sapphire binding,40 while Tsukamoto et al. reported a blue-shift in case of graphene on Cplane sapphire (of hexagonal symmetry).39 It might be the case that the symmetry of the substrate also affects the degree of G peak shift. Second, there is a big difference in the U peak intensity. Although this peak has been known in the Raman spectrum of graphite, the origin is not clear yet.41 The location coincides with the peak related to O2 molecules,42 but the width is far different. Wurstbauer et al. also observed this peak in NCG on SiO2 and interpreted it as a Si phonon mode.24 In our experiments, the positions are almost the same in both NCG on MgO and sapphire, ruling out cation-related phonon modes. We speculate that the larger U peak intensity in NCG on sapphire is related to the smaller 2D peak intensity, and that the broad U peak is a characteristic of NCG, coming from disorder. A close look at the inset of Figure 1, one may notice that there is an increase of I2D/IG as TG increases beyond 900 °C, while ID/IG remains the same. To figure out the origin, we performed XPS measurements to identify the carbon bonding characteristics (Figure 3). C 1s spectra are fitted with several components, such as O−CO bonds (289 eV), CO bonds (288 eV), C−O bonds (287 eV), and C−C bonds or C−H bonds (284−285 eV).17,43−46 Especially 284.7 ± 0.2 and 285.6 ± 0.2 eV components are attributed to sp2 and sp3 bonds,45 namely sp2 hybridization of carbon atoms and sp3 hybridization of C−C or C−H bonds, respectively.46 XPS results show that there is a clear difference in the concentrations of sp2 and sp3 bonds depending on the growth temperatures. In the sample grown at TG = 900 °C, the fraction of sp3 bond is much larger than that of sp2 bond, while other samples consist of mainly sp2 bonds. Interestingly the growth of sp2 bond fraction coincides with the increase of I2D/IG (Figure 1 (inset)). This correlation is somewhat expected, but has not been verified in NCG films so far. It is not certain, however, why the growth of sp2 fraction increases only the 2D peak intensity, leaving the intensity of D peak unchanged. This calls for a refined model for D and 2D peaks in nanocrystalline graphite.32 The fact that even the best sample contains more than 10% of sp3 bonds may explain the small cluster size and the large D peak. A modification of growth method or an appropriate postannealing method is

5, and 8 of ref 19, and Figure 2 of ref 21), we compare our results with those on C-plane sapphire, where the D and G peaks are well-separated and the 2D peak is the most pronounced.21 Figure 2a shows that the crystallinity inferred

Figure 2. Comparison of Raman spectra of graphitic carbon films grown on MgO(100) (this work) and sapphire(0001)21 at 1000 °C (a). Raman spectra between D and G band were fitted by four Lorentzian peaks (b and c). The peaks were attributed to D (blue), G (green), D′ (pupple), and U (red). The details are described in the text.

from ID/IG is almost the same, regardless of the symmetry of substrates. The average cluster size calculated by using the eq (10) in ref 32 is about 2 nm for both cases. This confirms that the growth is not simply governed by the symmetry. Rather, as we will show later from the simulation results, the strong bonding between oxygen in the substrate surface and carbon from the source induces angle disorders and reduces the cluster size. In order to clarify the possible substrate effect, we fitted the Raman spectra between D and G band by four Lorentzian Table 1. Fitting Results of the Raman Spectra in Figure 2a substrate MgO(100)

sapphire(0001)

a

ν FWHM I/IG ν FWHM I/IG

D

U

G

D′

2D

1354.7 52.0 1.89 1353.2 61.1 1.76

1467.8 107.3 0.06 1470.3 214.5 0.21

1591.6 49.7 1 1597.2 65.1 1

1621.5 30.9 0.41 1624.8 33.8 0.29

2701.3 87.0 0.43 2698.4 85.2 0.28

Lorentzian functions are used to fit the data. Peak positions (ν) and FWHM are units of cm−1. 7382

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(Figure 1). Further TEM and polarized Raman studies will be conducted to clarify this observation. The surface morphology of MBE-grown graphitic carbon is also promising for the integration with other materials. The AFM images show that the surfaces of as-grown samples are very flat with the average roughness, Ra, about 0.30 nm, only a slight increase from that of MgO(100) substrate (Figure 5a,b).

Figure 3. XPS results for samples grown at (a) 900 °C, (b) 950 °C, (c) 1000 °C, and (d) 1100 °C. C 1s spectra are fitted with four components, and the relative fractions of sp2 and sp3 bonds are shown: O−CO (dotted light-gray line), C−O (dotted gray line), sp3 C−C (dark-yellow line), and sp2 C−C (red line). Note the dominance of sp2 bond for all samples except one grown at 900 °C.

required to reduce sp3 bond and to improve the film quality close to that of perfect graphene. Despite the disorder in the MBE-grown NCGs, structural analyses imply that they can be improved and even useful for some applications. Scanning electron microscopy images (not shown) and AFM images do not show any sign of island growth despite the differences in the symmetry and the lattice constants between graphene and MgO(100). Neither clusters nor mosaic patterns are found. A high-resolution cross-sectional TEM study confirms that a continuous layer of carbon was formed by MBE (Figure 4a). A magnified view clearly shows

Figure 5. AFM images (1 μm × 1 μm) of (a) MgO(100) substrate and (b) graphitic carbon grown at 1000 °C. The mean roughness parameters, Ra, from 1 μm × 1 μm scan are (a) 0.28 nm and (b) 0.30 nm, respectively. Cross-sectional profiles along lines in panels a and b are shown in panels c and d, respectively, indicating little change of surface morphology. The homogeneity is confirmed by a 10 μm × 10 μm Raman map of the intensity ratio of D peak to G peak (e). Figure 4. High-resolution cross-sectional TEM images of the sample grown at 1000 °C. Pt was deposited prior to measurement to prevent the surface damage during the FIB process. A continuous layer of carbon is seen in panel a. A magnified view with 3 nm scale bar shows in panel b: the single crystalline MgO(100) (bottom: vertical patterns), the nanocrystalline graphite (middle: partially diagonal patterns), and the amorphous Pt (top: dark region).

Cross-sectional profiles of MgO (Figure 5c) and NCG on MgO (Figure 5d) also support this. In order to check the homogeneity, we performed the Raman mapping experiment. As shown in Figure 5e, ID/IG varies little (±6.1% in a 10 μm × 10 μm scan) and we can say that the NCG is fairly homogeneous. Although the crystallinity of graphitic carbon should be improved much, the flatness and the homogeneity are good signs for multilayer growth. We have previously shown that the transport property of NCG can be manipulated by a gate voltage, as in graphene.21 Graphitic carbon or NCG may find an application in the near future, and the flatness and the homogeneity are important properties for the fabrication of heterostructures. In this regards, the transport properties of NCGs on MgO(100) are of high interest. Unfortunately, we failed in

the crystallinity of carbon with a lateral coherence over a few nanometers. As far as we know, this is the first direct observation of clusters in MBE-grown NCGs. We also find that the growth direction is not perpendicular to the substrate plane at least for some clusters. Since the Raman spectra were taken without using polarizers, this diagonal growth is not contradictory to the observation of characteristic peaks of NCG 7383

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the fabrication of top-gated structure from the samples due to NCG peeling-off during the process. Instead we provide the carrier density and Hall mobility taken from Hall bars (channel width = 100 μm) at room temperature. As shown in Figure 6,

Figure 7. (a) Top and (b) side views of the optimized structure of 20 carbon atoms adsorbed on the MgO(100) surface (yellow = C, orange = O, red = Mg). The carbon atoms form a distorted honeycomb-like structure attached to an oxygen atom.

4. CONCLUSIONS We have shown that nanocrystalline graphite thin films can be grown on MgO(100) by MBE using solid source carbon. The Raman spectra show D, G, and 2D bands clearly. The increase of 2D peak intensity is correlated with the increase of sp2 bonding fraction probed by XPS. TEM and AFM studies show that a continuous carbon film of very flat surface morphology is formed by MBE. The homogeneity is confirmed by the Raman mapping. Interestingly the degree of graphitization is similar to that of MBE-grown graphitic carbon on hexagonal sapphire substrate. Ab initio calculations explain why the growth is independent of the substrate symmetry. The spectroscopic, structural, and electrical properties of NCG provided here will be useful in the development of direct graphene growth by MBE.

Figure 6. Growth temperature dependence of the carrier density and the Hall mobility at room temperature. A weak correlation between the mobility and the sp2 fraction is found.

both the carrier density and the mobility increase at first and then decrease as the growth temperature increases. The mobility is in the range of few tens of cm2/(V s), an order of magnitude larger than that of NCG on sapphire.21 We note that there is a weak correlation between the mobility and the sp2 fraction probed by XPS (Figure 3). This implies that the electrical properties can be improved by the increase of the sp2 fraction. Simulation of the growth mechanism from the first-principles would be an important guide for a successful growth via MBE. To study the initial growth of graphene on the MgO(100), we investigate the adsorption behavior of carbon atoms on the surface. The most stable site of a single carbon atom adsorbed on the MgO(100) surface is atop site on an oxygen atom. The binding energy between carbon and oxygen atoms is 2.73 eV, which is smaller than the C−O binding energy (3.48 eV) of sapphire. For the adsorption of two carbon atoms, carbon atoms form a dimer on the surface. Noticeably, one carbon atom always binds an oxygen atom on the MgO(100) surface, of which behavior has been already observed in the previous study.21 We increase the number of carbon atoms up to 20 on the MgO(100) surface to study the possible formation of graphene-like structure. Such structure can be a nucleation seed for graphene itself. Six carbon atoms form the carbon linear chain on the surface, where one carbon atom always binds to an oxygen atom on the surface and the oxygen atom protrudes outward from the top-layer plane of the MgO substrate. In the optimized structure of 20 carbon atoms adsorbed on the surface (see Figure 7), the carbon atoms form a distorted honeycomblike structure on the surface and some carbon atoms of the entire carbon structure make bonds with oxygen atoms of MgO(100) due to strong binding between carbon and oxygen atoms. Similarly to the case of carbon atoms adsorbed on sapphire,21 binding between carbon and oxygen atoms is expected to occur somewhat randomly, so it may lead to segregation of graphene-like structures in a limited area rather than a perfect graphene formation. Thus, ab initio calculations may (partly) explain the strong Raman D peak and the symmetry independence.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +82 2 3408 4316. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Priority Research Centers Program (2011-0018395), the Converging Research Center Program (2011K000620), the Basic Science Research Program (2011-0026292, KRF-2008-3130C00279), and the Center for Topological Matter in POSTECH (2011-0030786) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST).

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dx.doi.org/10.1021/jp210910u | J. Phys. Chem. C 2012, 116, 7380−7385