Graphene Epitaxy by Chemical Vapor Deposition on SiC - Nano

LETTER pubs.acs.org/NanoLett

Graphene Epitaxy by Chemical Vapor Deposition on SiC W. Strupinski,*,1 K. Grodecki,1,2 A. Wysmolek,2 R. Stepniewski,2 T. Szkopek,3 P. E. Gaskell,3 A. Gr€uneis,4,5 D. Haberer,4 R. Bozek,2 J. Krupka,6 and J. M. Baranowski1,2 1

Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland Faculty of Physics, University of Warsaw, Hoza 69, 00-681 Warsaw, Poland 3 Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, H3A-2A7, Canada 4 IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany 5 University of Vienna, Strudlhofgasse 4, 1090 Wien, Austria 6 Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland 2

ABSTRACT: We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades.1 CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ∼1800 cm2/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth. KEYWORDS: Graphene, epitaxy, CVD, sublimation, SiC, carbon deposition raphene, a single sp2-bonded carbon atomic sheet has great potential for microelectronics applications,1
3 including conventional components such as high-frequency analog devices,4 and devices in emerging fields such as spintronics,5 terahertz oscillators,6 and single-molecule gas sensors.7 However, a major factor hindering the development of technology for the large-scale production of graphene-based nanoelectronic devices is the lack of access to high-quality uniform graphene layers grown on large SiC or other semi-insulating substrates, which could meet the requirements dealing with extremely high carrier mobility. Concurrently, the chemical vapor deposition (CVD) epitaxial growth of graphene on metal substrates has lately received much attention8
16 following the works of May,17 Blakely et al.,18,19 and Oshima et al.20 It is worth noting that the work by Qingkai et al.21 preceded the work of the MIT group.8 Unfortunately, epitaxial growth on metals suffers from the disadvantage that electronic applications require graphene on an insulating substrate, and although wafer-scale transfer is possible, it is a difficult process.22 Thus, the growth on SiC is particularly attractive because graphene can be grown directly on semi-insulating SiC substrates of up to 150 mm diameter, which are anticipated to be commercially available. With the use of SiC wafers in microelectronics becoming increasingly popular (new

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SiC MOSFET transistors developed by Cree), its price is likely to gradually decline in coming years. During the past decade, several outstanding advancements in graphene growth technology have been achieved. Novoselov et al. demonstrated mechanical exfoliation of graphene from graphite, permitting the first observations of some of the highly attractive and novel physical properties of graphene.23,24 The growth of graphene by thermal desorption of Si from SiC (sublimated epitaxial graphene: S-EG) was developed by Berger et al.,2,25
27 following the early experiments of Van Bommel et al.1 Further improvements in the quality of S-EG were realized by sublimating Si at elevated temperatures in an Ar atmosphere rather than in vacuum.28 However, vacuum sublimation is still successfully applied with a Si vapor pressure control system.27 Graphene produced by sublimating Si from SiC heated to high temperatures (1200
2000 °C) is sensitive to the surface quality of the SiC substrates. Moreover, local scattering caused by charge buildup at the SiC substrate step edges can degrade electron transport properties29 and step edges give rise to the growth of additional one or two monolayers that deteriorate the graphene Received: February 1, 2011 Revised: March 13, 2011 Published: March 25, 2011 1786

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Nano Letters thickness uniformity.28,30 The formation of a SiC buffer epi-layer is connected to the central difficulty in the epitaxial growth of high-quality SiC on substrates without the usual intentional miscut of 4 or 8° from the c-axis. In this paper, we report the CVD growth of epitaxial graphene (CVD-EG) on SiC substrates using propane gas as the carbon precursor. The approach proposed here offers numerous benefits in comparison to S-EG, including the application of well-developed commercial epi-systems for SiC epitaxy. The most critical step is to protect the SiC substrate against Si sublimation at conditions of high temperature (T ≈ 1600 °C) and low Ar pressure (P). While protecting against Si sublimation, C deposition was enabled with one monolayer resolution by taking advantage of the high efficiency of kinetic processes at high T and low P. The proposed method permits the growth rate of graphene on C-face SiC(000-1) to be substantially lowered so as to enable the growth of 1 ML, which is extremely difficult to achieve in the case of S-EG. Additionally, multilayer graphene (MLG) can be grown on Si-face SiC(0001), which in comparison to a maximum S-EG thickness of 2
3 ML, creates opportunities for the study of graphene formed on the Si-face. Our proposed approach enables precise growth rate control by adjusting the mass transport of the carbon precursor in a similar way to the method used in MOCVD/CVD and permits the passivation of the SiC substrate by any substance prior to graphene growth. Moreover, one can tune the reactor conditions to grow both CVD-EG and S-EG in the same system. Various measurements of CVD-EG indicate that the proposed method enables the production of graphene with notably better quality than S-EG. Experimental Methods. Graphene layers were grown using a commercial horizontal CVD hot-wall reactor (Aixtron VP508), which is inductively heated with an rf generator.31 Epitaxial carbon films were deposited on the C- and Si-faces of both semi-insulating and conductive on-axis 4H
SiC substrates. To provide information at the atomic scale, all samples were characterized by scanning tunneling microscopy (STM). STM observations were conducted in ambient air at room temperature using a Veeco Multimode scanning probe microscope equipped with an STM head and a Nanoscope IIIa controller. The formation of graphitic structures was confirmed by Raman spectroscopy. Micro-Raman scattering experiments were performed at room temperature in a backscattering geometry using the 532 nm line of a continuous wave Nd:YAG laser. The laser spot size on the sample surface was about 2 μm in diameter. The excitation power of the laser was below 50 mW to avoid surface heating. The micro-Raman maps have been created using 530 points measured on a 2.3  2.2 mm2 area at the center of the sample. The signal was collected at 100 μm steps in both x- and ydirection. Histograms of the observed 2D line frequency from the sampled micro-Raman spectra were plotted against frequency ν with a (1 cm
1 bin size . The thickness of the graphene films was estimated by ellipsometry. A free space ellipsometer was built using a broadband Swarszchild objective as the laser focusing and imaging element with the details of the construction being reported elsewhere.32 The incident beam had an elongated 1/e spot (3 μm  5 μm) and the reflected beam was measured with (0.03° polarization sensitivity, corresponding to a sensitivity of (1 layer on SiC. The laser source is a temperature-stabilized λ = 635 nm, 2mW GaAs laser diode, which was chosen because the graphene optical response is nearly described by the universal optical conductance e2/4p at this wavelength. Angle-resolved photoemission spectroscopy (ARPES) measurements were done

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at the BESSY II synchrotron (Berlin, Germany) using the UE112-PGM-2 beamline with a Scienta R4000 analyzer. The samples were mounted on a three axis manipulator that was cooled down by liquid He yielding a sample temperature of ∼40 K and a total energy resolution of 30 meV. Prior to ARPES measurements, the samples were annealed at 800 °C for 20 min in a vacuum better than 10
10 mbar in order to clean the surface. The ARPES measurements were carried out with horizontal linearly polarized light and a photon energy of hν = 35 eV. A two-dimensional mapping of the photoemission intensity in the 2D Brillouin zone (BZ) of graphene was performed by varying the polar emission angle. This mapping yields the complete band structure information of the CVD-EG layers, and in particular it allows for the determination of the Fermi surface, charge carrier concentration and the Fermi velocity, providing valuable input for transport measurements and optical spectroscopies. The electron transport parameters of the graphene samples were measured with the van der Pauw method applying standard 10 mm  10 mm samples. Initially, a process for graphene growth by Si sublimation was developed.31,33 SiC substrates were first etched in H2/silane or H2/propane mixtures at 1600 °C. The reactor temperature T and Ar pressure P were optimized for controlling the Si sublimation rate. Graphene growth via Si sublimation can be completely prevented by increasing the Ar pressure P. The relation between the sublimation temperature and critical pressure was determined experimentally for the VP508 reactor used in this study.33 The Ar linear flow velocity can also be used to control the rate of Si sublimation. The epitaxial CVD of graphene relies critically on the creation of dynamic flow conditions in the reactor that simultaneously stop Si sublimation and enable the mass transport of propane to the SiC substrate. Tuning the value of the Reynolds number Re, which was realized experimentally, enables the formation of an Ar boundary layer (BL) that is thick enough to stop Si evaporation but thin enough to allow the diffusion of propane to the SiC surface. Laminar gas flow over the SiC surface consists of layers moving at different velocities due to the shear stress between adjacent gas layers. Re is a measure of the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two forces in a given gas flow. Re is dependent on the physical gas properties and on the reactor geometry according to Re ¼

pVd Vd ¼ μ v

ð1Þ

where V is the gas velocity, μ is the dynamic viscosity, v is the kinematic viscosity, p is the gas density, and d is the characteristic dimension of the reactor. The kinematic viscosity v = μ/p depends mostly on the gas type, pressure and temperature and increases with temperature unlike in the case of liquids. Although exact solution of the gas flow profile over the sample is nontrivial, it is known that increasing Re results in a thinner BL. With a fixed reactor geometry, we used the Ar flow, the Ar pressure, and temperature T as the three main parameters for tuning the Ar viscosity v thereby experimentally optimizing the BL thickness. Many SiC substrates were thermally annealed in Ar alone at the optimized conditions P = 20 mbar and T = 1600 °C. Raman and STM methods were used to verify the absence of a Sidepleted/C-enriched layer on the substrate surface for both Si and C polarities. With the conditions for the suppression of Si sublimation established, the dynamic flow conditions were 1787

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Figure 1. Atomically resolved STM image of a CVD-EG layer grown on a 4H
SiC(0001) substrate, taken over an area of 10  10 nm2.

further extended to enable propane diffusion through the encapsulating Ar BL. Propane that diffuses across the Ar BL to the SiC surface thermally decomposes, and the deposition of epitaxial graphene occurs on the SiC surface. CVD graphene layers were grown with this new technique. An additional advantage of this method is that the process can be easily directed toward two growth modes: one where Si sublimation is halted by a high number of slow-moving Ar atoms close to the substrate surface and propane diffusion through this Ar cap leads to CVD epitaxial graphene on SiC; and another in which Si sublimation dominates and is responsible for the sublimation of epitaxial graphene on SiC. The first growth mode enables classical epitaxy of carbon on SiC without parasitic carbon formation by Si sublimation and can be combined with carbon seed formation by a preceding Si sublimation step in the same growth run. Results and Discussion. CVD-EG layers were grown and subsequently characterized by ARPES, Raman, STM, ellipsometry, and van der Pauw methods. An atomically resolved STM image of a CVD-EG layer grown on a 4H-SiC(0001) substrate with a maximum miscut of 0.03° is shown in Figure 1. The CVDEG exhibits the well-known honeycomb characteristic observed on S-EG on 4H-SiC(0001). Long-range √ √ periodicity in charge density from the underlying 6 3  6 3 R30° reconstructed buffer layer was typically observed in CVD-EG over a 10  10 nm2 scan area. The calibration of the CVD growth rate and the estimation of the thickness uniformity were realized with ellipsometry. The number of graphene layers was extracted by fitting the measurements of ellipsometric angle Ψ to an infinitesimal thin film Fresnel theory.32 Reflection images and spot ellipsometry measurements of two samples are shown in Figure 2. A uniform layer thickness of 2 and 9 ML is observed on the Si-face of SiC(0001). It is known that in sublimation growth on SiC(0001), the upper limit of the graphene thickness is about 3 ML due to the blocking of Si diffusion by the graphene layer growth. Recent work has shown that the Si desorption rate decreases with an increasing number of grown graphene layers, which act as Si-diffusion barriers,34 and for Si terminated SiC surfaces the effective growth is limited to 2
3 ML.35 The measured graphene thickness of 9 ML indicates that the growth mode was indeed “real” epitaxial CVD of C rather than Si-sublimation. However, most CVD-EG layers grown on Si-face 4H
SiC were observed to have a thickness of 1
2 ML.

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Figure 2. Spot ellipsometry (bottom row) and false-color reflection images (top row, circles indicate a 4 μm diameter) of CVD-EG grown on 4H-SiC(0001) on-axis substrates with a mean CVD-EG layer thickness of (a) 1
2 ML and (b) 9 ML. Uniform graphene thickness is visible between the substrate terrace edges. Spot ellipsometry measurements were taken at distances ∼10
30 μm apart at the indicated incident angles θi.

ARPES is a powerful method to investigate the electronic structure as it gives access to the spectral function containing the information of the electron energy band dispersion. Thus ARPES was used to determine electronic properites of the CVD grown graphene on SiC. The ARPES measured bandstructure of CVDEG deposited on the 4H-SiC(0001) Si-face is illustrated in (Figure.3). In Figure 3a, we show the spectral function measured by ARPES of CVD-EG on SiC showing a single π-valence band and conduction band (VB and CB). The appearance of a single band indicates the absence of more than one layer in an AB stacking order that would otherwise appear as additional bands.36 The photoemission intensity distribution in the 2D BZ indicates that the CB(VB) has a maximum along the K-M (K-Γ) directions, consistent with previous measurements of graphene.37,38 Furthermore, the conduction band (CB) is partially occupied due to an electron transfer from the substrate. The cuts of Figure 3a,b are performed along the ky direction (refer to inset of Figure 3a for coordinate system). Individual energy dispersion curves (EDCs) are shown in Figure 3(b) with the cut through the K point indicated in red. The bottom of the conduction band is 245 meV below the Fermi level, and a gap of 345 meV is observed between VB and CB. The origin of the gap on SiC has been extensively debated for graphene grown by Si sublimation39,40 (electron-plasmon coupling versus substrate interaction). It is not clear whether these models can be applied to CVD-EG layers, which probably have a different structure on SiC as compared to graphene grown by sublimation, as indicated by a redshift in the 2D Raman line positions (discussed below). Since the doping level for S-EG and CVD-EG is comparable, doping is unlikely to be the origin of the redshift. We speculate that we have a slightly larger C
C bond length or a different superstructure for CVD-EG. Moreover, we have observed a reduced photoemission intensity in between VB and CB for the cut through K point (red line in Figure 3b), suggesting the possibility of symmetry breaking. The complete electronic structure was investigated with ARPES mapping of the band structure. In Figure 3c, we plot the equi-energy contours of the photoemission intensity for the Fermi level, 400 meV, 1 eV, and 1.5 eV binding energy. An analysis of the 1788

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Figure 3. ARPES spectrum of graphene formed by the CVD process on a 4H
SiC(0001) substrate (a) The ARPES intensity along a cut through K point in the ky direction (see inset for coordinate system). (b) The photoemission intensity profiles for fixed electron momentum as a function of binding energy (EDC). The red line denotes the EDC through the K point. (c) Equi-energy contours of the photoemission intensity around the K point for four different binding energies. (d) Graphene ML thickness measured by ellipsometry over a 100 μm  100 μm area.

equi-energy contours in the vicinity of EF yields a uniform Fermi velocity of 1.1  106 m/sec and integration of the carriers inside the Fermi surface yields an electron density of 3  1013/cm2. Structural analysis of CVD-EG layers was performed by means of Raman spectroscopy. The Raman spectra and histograms for CVD-EG and S-EG samples are shown in Figure 4a,b. We analyzed the 2D line in the Raman spectra, as this spectral region was least affected by subtraction of the SiC background, unlike

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the G line. In the case of epitaxial graphene grown on SiC (0001), a shift in 2D-band position is observed, which is attributed to strain as a consequence of the interaction between the SiC substrate and epitaxial graphene. Strain induces a change in the graphene lattice constant and thus in the Raman frequencies.41
45 The mean position of the CVD-EG 2D band is located at 2700 cm
1, which is blue shifted by 20 cm
1 in comparison to 2680 cm
1 observed for the 2D peak in micromechanically cleaved graphene (MCG).46 The histogram of S-EG (Figure 4b) clearly indicates a mean 2D peak position at 2743 cm
1, which is comparable with published data.45 The corresponding blue shift in comparison with MCG for this layer is 63 cm
1, which is attributed to the compressive character of the strain.47 The histogram of 2D peaks in Figure 4b indicates that the S-EG layer is under much higher compressive strain than CVD-EG. The position of 2D peak for CVD-EG varies between samples within the range 2685
2720 cm
1 depending on the growth conditions, in particular on thermal treatment procedure and SiC substrate surface preparation/passivation. However, in general, the CVD-EG 2D peak position is significantly red shifted in comparison to S-EG and there is a clear tendency of increasing 2D band frequency with graphene thickness. Raman maps for CVD-EG and S-EG are presented in (Figure.4d). Compressive strain at room temperature has been linked to a large difference in the coefficient of thermal expansion between graphene and SiC during cool down from growth temperature.47 The CVD-EG and S-EG layers have been grown at the same temperature 1600 °C. The thickness of both epitaxial graphene layers studied by Raman spectroscopy were 2
3 monolayers. Therefore, such a dramatic difference in the compressive strains between S-EG and CVD-EG layers proves that their occurrence cannot be explained by the same mechanism. We speculate that the strain is connected with different growth modes. In the case of S-EG growth, Si sublimation starts at atomic step edges and also at all kinds of defects present on the SiC surface, particularly at dislocations. Thus, the sublimated graphene layer will be pinned to SiC surface at many randomly distributed points. These will inevitably lead to a much larger strain during the cool down process and eventually to poorer quality. In the case of the CVD process, the nucleation sites of graphene layer seems to be connected with atomic steps on SiC surface enabling “step-flow” epitaxy. Thus, the CVD-EG layers may be less sensitive to surface defects. Subsequently grown layers will weakly interact with the first graphene layer grown, and strain can partly relax during cool down of the sample. This may explain superior quality of the CVD-EG layers. The electrical parameters of the graphene layers were measured by two techniques. The low-frequency conductance, carrier type, and carrier concentration was measured by the van der Pauw method at room temperature, using In contacts and 10  10 mm2 sample sizes. The electron density in several 1
2 ML graphene films grown in subsequent processes was typically 2
4  1012 cm
2, with a macroscopically averaged electron mobility inferred from Hall voltage in the range 1200
1800 cm2/(V s), demonstrating the high electronic quality of the CVD-EG layers on the wafer scale. This result confirms the observation of superior quality as revealed by ARPES and Raman spectroscopy. For higher carrier concentration (above 1013 cm
2) RT mobility decreased to the level of 900
1200 cm2/(V s). Contactless electrical measurements at microwave frequencies using the TE011 mode of a single postdielectric resonator (10-13.22 GHz) were also conducted with technical 1789

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Figure 4. (a) Raman spectra of the CVD-EG. (b) Histograms of the CVD-EG (left, red) and S-EG (right, blue), and the mean positions of 2D band are 2695 and 2743 cm
1, respectively. (Inset) spectra within 2D line range; lower (A, red) for CVD-EG, upper (B, blue) for S-EG. (c,d) Raman maps of CVD-EG and S-EG, respectively, measured over a 5.3 mm2 area with 530 points at steps of 100 μm. Graphene was grown on an on-axis 4H
SiC(0001) substrate. The background SiC spectrum has been subtracted.

details of the technique reported elsewhere.48 The 1
2 ML graphene films exhibited a large effective microwave conductivity of 6  106 S/m inferred from the measured surface resistance and graphene film thickness. The microwave conductance per square was in the range of (3.5
6.0)  10
3 S/square, which indicates the presence of a continuous (or nearly continuous) and highly conductive graphene layer on the SiC substrates. Conclusions. In summary, we successfully developed a technique for the CVD of graphene on SiC(0001) substrates. Our results reveal that the careful tuning of dynamic flow conditions in a CVD reactor can halt Si sublimation and at the same time enable the mass transport of propane to the SiC substrate, which is critical for the CVD epitaxy of graphene. Adjusting the reactor temperature and carrier gas flow modifies Re and enables the formation of a boundary layer. The BL thickness scaling argument gives qualitative guidance on the adjustment of reactor pressure and chamber temperature. The high quality and the uniform appearance of monolayer is evidenced by a combination of STM, ellipsometry, ARPES, and resonance Raman measurements that probe the surface morphology, and electronic and vibronic properties. In particular the redshift in Raman indicated that the CVD grown layers are under considerable less epitaxial strain than graphene grown on SiC by sublimation technique. The appearance of a single π-band in the ARPES corroborates the ellipsometry data that suggest only one layer over the whole sample. ARPES provides further valuable information on the doping level and Fermi velocity and suggests that CVD grown graphene has an electron energy gap that is important for transistor devices based on graphene. Measurements by the van der Pauw method reveal the high quality of CVD-EG layers with ∼1800 cm2/(V s) room temperature mobility. Electron concentration significantly depends on thermal treatment of the substrate prior to growth. CVD graphene growth on SiC

substrates is a stepping stone toward the development of graphene technology enabled by large-scale industrial production in commercial CVD equipment.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The work was partially supported by the Polish Ministry of Science and Higher Education, Projects No. 670/N- and No. 671/N- ESF-EPI/2010/0 within the EuroGRAPHENE program of the European Science Foundation, Grant 395/N-PICS-FR/ 2009/0 and by Projects POIG.01.01.02-00-015/09-00 and 0885/R/T02/2010/10. A.G. acknowledges an APART fellowship from the Austrian Academy of Sciences and Project DFG GR 3708/1-1 from the German Research Agency. We acknowledge Dr. Oleg Vilkov (BESSY II and TU-Dresden) for experimental assistance. W.S. conceived the study, developed the growth process of CVD- and sublimated-graphene, and wrote the manuscript with input from all other coauthors. Raman measurements were done by K.G. with help of A.W. ARPES studies were carried out by D.H. and A.G., R.B. performed STM measurements, P.E.G. and T.S. developed the analytical tool of ellipsometry, and J.K. designed and developed microwave frequency conductance measurements. J.B., R.S., A.G., and T.S. analyzed and interpreted all data, discussed the results and implications, and commented on the manuscript. ’ REFERENCES (1) van Bommel, A. J.; Crombeen, J. E.; van Tooren, A. Surf. Sci. 1975, 48, 463. 1790

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