SUPPLEMENTARY INFORMATION Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide Supporting online material Konstantin V. Emtsev 1, Aaron Bostwick 2, Karsten Horn 3, Johannes Jobst 4, Gary L. Kellogg 5, Lothar Ley 1, Jessica L. McChesney 2, Taisuke Ohta 5, Sergey A. Reshanov 4, Jonas Röhrl 1, Eli Rotenberg 2, Andreas K. Schmid 6, Daniel Waldmann 4, Heiko B. Weber 4, Thomas Seyller 1,* 1 Lehrstuhl für Technische Physik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 3 Department of Molecular Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany 4 Lehrstuhl für Angewandte Physik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany 5 Sandia National Laboratories, Albuquerque, NM, USA 6 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA, USA * Corresponding author: thomas.seyller@physik.uni-erlangen.de The initial stages of graphene formation on the SiC(0001) surface were monitored by XPS. C1s spectra are shown in Fig. S1(a). Comparison of the spectra before (green curve) and after annealing in Ar atmosphere at 1450 C (yellow curve) reveals that no surface carbon enrichment takes place upon annealing in argon atmosphere at this temperature. The surface remains stoichiometric with respect to Si:C ratio but strong step bunching is already observed as evident from fig. S1(b). Annealing at 1550 C in Ar (blue curve) leads to the onset of Si sublimation and subsequent C enrichment. As in the case of vacuum annealing, graphitization begins first with the formation of the 6 3 reconstruction buffer layer at T~1550 C. In UHV this stage is already reached at 1150 C (dotted curve). The buffer layer is structurally similar to graphene with the exception that -orbitals are covalently bound to the SiC substrate 1-3. In the C1s spectra the buffer layer gives rise to characteristic broad asymmetric peak 1 with a maximum at 285.5 ev clearly identified in the spectrum obtained after annealing at 1150 C in UHV (dotted curve). In the spectrum of the Ar annealed sample (blue curve) an additional small peak at 284.8 ev is due to the onset of graphene nucleation (~0.2 ML). Increasing the temperature to 1650 C in Ar (red curve) results in the formation of a graphene film with a thickness of 1.2 ML which is visible by the peak at 284.8 ev. No difference in the C1s spectra is observed for graphene grown in vacuum (dashed curve) or argon environment (red curve). The signal due to the buffer layer residing at the interface between SiC and graphene is still distinctly visible as reported previously 1 for UHV growth. Submonolayer graphene samples were additionally investigated by AFM and LEEM (Fig. S2). A submonolayer coverage was achieved by shortening the annealing time from 15 min nature materials www.nature.com/naturematerials 1 1
supplementary information as described in the text to 3 min at temperature of 1650 C. Despite that very short annealing time the significant step bunching is observed already at this stage. For this sample the average terrace width increases by a factor of five as compared to the initial surface (see inset in Fig. S2(a)). Correspondingly, the average step height increases from 1.5 nm to 7.5 nm. The spatial distribution of graphene is shown in a LEEM micrograph in Fig. S2(b). The medium grey, light grey and dark grey regions correspond to the buffer layer, monolayer graphene and bilayer graphene, respectively. Graphene islands appear in the form of narrow stripes extending along the substrate steps. They are separated by wider regions covered with the buffer layer only. Apparently, the separation between adjacent graphene stripes correlates with the width of macroterraces. Interestingly, all graphene stripes have the same width of about 500 nm. In the AFM image a shallow depression about 4Å deep (marked by arrows) and of the same width is observed at the step edge of every terrace. The step height is consistent with the density arguments where decomposition of 3 layers of SiC (7.5 Å) is required to form a layer of graphene (3.3 Å). Hence, the nucleation of the first graphene layer takes place at the edges of the macrosteps of the substrate. Traces of bilayer graphene regions (~100 nm wide stripes) located at the very edge of the steps are also detected in LEEM. The formation of bilayer graphene is slower than that of a monolayer graphene. We explain this by the fact that the out-diffusion of Si from the interface is hampered by already existing graphene layers. In order to evaluate the insulating properties of the SiC substrate after processing all Hall bars had an additional test contact as shown in fig. S3. The leakage currents between these test contacts and the graphene Hall bars were determined for all structures and it was found to be at least six orders of magnitude smaller than the source-drain currents. In agreement with that we observed a strong asymmetry in the resistance on submonolayer samples such as that shown above. The resistance along the terraces was generally six orders of magnitude smaller than that measured perpendicular to the steps. Thus we conclude that our high temperature process does not significantly alter the conductivity of the substrate. The Raman spectra in the frequency region of the D- and G-line of graphene on SiC are overlayed by strong multi phonon signals from the SiC substrate (see fig. S4). Therefore, a suitable reference spectrum must be subtracted from the raw data 4. The procedure is illustrated in fig. S4. Figure S5 shows the temperature dependence of the electron mobility of Ar grown graphene measured in van der Pauw geometry. The linear (T) dependence is unexpected. Scattering at acoustic phonons of graphene would result in T -4 behavior at low temperatures 5. A theoretical treatment of the effect of static impurities in graphene predicts 1/T dependence of the mobility 6. Candidates for such impurities are certainly dangling bonds below the graphene layer. Also adsorbates might play a certain role. The linear dependence of the 2 nature MATERIALS www.nature.com/naturematerials
supplementary information scattering rates rather fits to the case of electron-electron interaction in a 2D electron gas 7. More work is required to understand the temperature dependence of the mobility in epitaxial graphene. References 1 2 3 4 5 6 7 Emtsev, K. V. et al., Interaction, growth, and ordering of epitaxial graphene on SiC{0001} surfaces: A comparative photoelectron spectroscopy study. Phys. Rev. B 77, 155303 (2008). Mattausch, A. and Pankratov, O., Ab Initio Study of Graphene on SiC. Physical Review Letters 99, 076802 (2007). Varchon, F. et al., Electronic Structure of Epitaxial Graphene Layers on SiC: Effect of the Substrate. Physical Review Letters 99, 126805 (2007). Röhrl, J. et al., Raman spectra of epitaxial graphene on SiC(0001). Appl. Phys. Lett. 92, 201918 (2008). Hwang, E. H. and Sarma, S. Das, Acoustic phonon scattering limited carrier mobility in two-dimensional extrinsic graphene. Phys. Rev. B 77, 115449 (2008). Cheianov, V. V. and Fal'ko, V. I., Friedel Oscillations, Impurity Scattering, and Temperature Dependence of Resistivity in Graphene. Phys. Rev. Lett. 97, 226801 (2006). Zala, G., Narozhny, B. N., and Aleiner, I. L., Interaction corrections at intermediate temperatures: Longitudinal conductivity and kinetic equation. Phys. Rev. B 64, 214204 (2001). nature materials www.nature.com/naturematerials 3
supplementary information a buffer layer b 2.0 m Fig. S1: (a) C1s spectra of the SiC(0001) surface taken during initial stages of graphene formation in argon atmosphere at different temperatures. The processing temperature and graphene coverage are indicated. The annealing time was 15 min for all samples. The spectra obtained on the SiC samples prepared in ultra high vacuum (UHV) are shown for comparison (dashed and dotted lines). The ratio of SiC bulk component to surface related components appears different to that shown in Fig 2(b) of the main text due to different excitation energy used (Here Al K, 1486.74 ev, main text 700 ev). (b) AFM image of 6H- SiC(0001) after annealing in Ar at 1450 C indicating strong step bunching prior to graphene growth. 4 nature MATERIALS www.nature.com/naturematerials
a supplementary information b 0ML 1ML 2ML Fig S2: (a) AFM and (b) LEEM images of submonolayer graphene on the 6H-SiC(0001) surface. Inset in (a) shows the morphology of the initial SiC sample obtained after hydrogen etching at the same lateral scale. The contrast in LEEM is due to buffer layer (0 ML), monolayer graphene (1 ML), and bilayer graphene (2 ML), respectively. Fig. S3: Example of a Hall bar structure (50 m 4 m) imaged by scanning electron microscopy. The graphene layer is seen in balck. All Hall bars measured in the present work had an additional test contact (bottom left, highlighted) used for measuring the resistance of the SiC substrate. nature materials www.nature.com/naturematerials 5
supplementary information Intensity (arb. units) D G SiC UHV grown Ar grown normalized raw data 0 difference 1200 1300 1400 1500 1600 1700 1800 1900 2000 Raman shift (cm -1 ) Fig. S4: Raman spectra of epitaxial graphene grown in UHV (blue) and Ar (red) and of a 6H- SiC substrate (black). The substrate leads to strong signals due to multi-phonon bands. The lower spectra labeled with difference are obtained after subtracting the SiC reference spectrum from the raw data. The difference spectra are offset for clarity. mobility (cm 2 /Vs) 2000 Van der Pauw Hall bar 1800 1600 1400 1200 1000 0 50 100 150 200 250 temperature (K) Fig. S5: Temperature dependence of electron mobility in Ar-grown monolayer epitaxial graphene. The mobility values were derived from Hall measurements in van der Pauw and Hall bar geometry. The experimental data display a linear T dependence. 6 nature MATERIALS www.nature.com/naturematerials