Abstract
Semiconducting graphene plays an important part in graphene nanoelectronics because of the lack of an intrinsic bandgap in graphene1. In the past two decades, attempts to modify the bandgap either by quantum confinement or by chemical functionalization failed to produce viable semiconducting graphene. Here we demonstrate that semiconducting epigraphene (SEG) on single-crystal silicon carbide substrates has a band gap of 0.6 eV and room temperature mobilities exceeding 5,000 cm2 V−1 s−1, which is 10 times larger than that of silicon and 20 times larger than that of the other two-dimensional semiconductors. It is well known that when silicon evaporates from silicon carbide crystal surfaces, the carbon-rich surface crystallizes to produce graphene multilayers2. The first graphitic layer to form on the silicon-terminated face of SiC is an insulating epigraphene layer that is partially covalently bonded to the SiC surface3. Spectroscopic measurements of this buffer layer4 demonstrated semiconducting signatures4, but the mobilities of this layer were limited because of disorder5. Here we demonstrate a quasi-equilibrium annealing method that produces SEG (that is, a well-ordered buffer layer) on macroscopic atomically flat terraces. The SEG lattice is aligned with the SiC substrate. It is chemically, mechanically and thermally robust and can be patterned and seamlessly connected to semimetallic epigraphene using conventional semiconductor fabrication techniques. These essential properties make SEG suitable for nanoelectronics.
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The authors declare that all other data supporting the findings of this study are available in the Article and its Supplementary Information and are also available from the corresponding author on request.
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Acknowledgements
Most of the work reported here was performed at the Tianjin International Center for Nanoparticles and Nanosystems (TICNN), which is a research institute at the University of Tianjin campus established in 2015 by L. Ma and W. de Heer and designed by de Heer and constructed by Ma. TICNN has a comprehensive dedicated epigraphene laboratory designed to complement and coordinate with the Georgia Institute of Technology epigraphene project. de Heer was the TICNN director until 2020. At present, he serves as the scientific advisor for the epigraphene projects at the TICNN. We acknowledge the financial support from the Double First-Class Initiative of Tianjin University and from the Department of Education in China. We thank J. Li and M. Zhao for their unwavering support in establishing the TICNN centre. L.M. thanks B. von Issendorff of the University of Freiburg, Z. Jiang of the Georgia Institute of Technology and Y. Ma of the TICNN for their discussion and help. W.A.d.H. thanks GTRI for financial support, A. Juyal. for his work and C. Berger, Z. Jiang, P. First, J. Hankinson and A. Naeemi for their advice and support. We thank J. P. Turmaud for permission to use excerpts of text and figures in Supplementary Information section 3. W.A.d.H. thanks P.H. de Heer for her encouragement, patience and support.
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Contributions
J.Z. contributed to SEG growth, transport measurements, data processing and Supplementary Information section 4. P.J. contributed to STM measurements, data processing and Supplementary Information section 4. Y.L. was involved in materials growth and characterizations, transport measurement and data processing. R.L. contributed to device fabrication, transport measurement and data processing. K.Z. was involved in material characterization, device fabrication and data processing. H.T. contributed to material characterization and device fabrication. B.B. assisted in material growth and characterization and transport measurement. L.H. was involved in material characterization and device fabrication. K.Y. contributed to material characterization and device fabrication. X.X. contributed to material growth and characterization. R.M. edited the Supplementary Information section 4. L.M. initialized, conceptualized and supervised the project at TICNN and contributed to material growth, transport measurements, data analysis, Supplementary Information section 4 and editing of the paper. W.G. and N.D. helped with data and production of Fig. 1a, Fig. 2a,c, Fig. 3e, Extended Data Fig. 1a,b, Extended Data Figs. 2,5, Extended Data Figs. 3,6 and Supplementary Information section 1. W.A.d.H. directed the Georgia Institute of Technology projects and the TICNN transport measurements, analysis of Figs. 3 and 4 and Extended Data Fig. 8e and identified and analysed the stable SEG phase and wrote the paper.
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Extended data figures and tables
Extended Data Fig. 1 Face-to-face growth of SEG.
(a) Vertical furnace with improved temperature gradient control. (b) Overlapped images of the surface of a Si-face seed chip and mirror image of the corresponding C-face source chip (slightly shifted) showing identical complementary topological features, i.e., material removed from the C-face is deposited directly above it on the Si-face which demonstrates the close interaction between the two chips. (c) Approximate times to grow a buffer layer and to grow 100 nm SiC from the source chip on the seed chip where the latter is more than 10 C cooler than the former.
Extended Data Fig. 2 Si-face source to C-face seed growth.
(a) In this inverted geometry SEM images show SEG growth on the Si-face. (b) SEM of the C-face shows an irregular structure. (c) Contrast enhanced optical microscopy shows that the (0001) terraces on the Si-face are small but regular. (d) Contrast enhanced optical microscopy shows irregular step structure on the C-face which appears to be imposed by large steps (dark lines) the Si-face. Blue lines in (c) and (d) indicate the step directions of the unprocessed chips.
Extended Data Fig. 3 SEM image a single Si-face of a chip that is processed in a silicon saturated crucible showing the stability of SEG in a Si saturated environment.
(a) the surface is largely covered with narrow (0001) terraces covered with SEG (darker areas). The white areas are bare SiC. (b) zoom in of boxed area.
Extended Data Fig. 4 AFM measurement of an atomically flat SEG terrace between two approximately 100 nm high substrate steps 300 µm apart.
In a single line scan, spanning this distance, no SiC steps are detected, (these would be at least 250 pm high.) If there were substrate steps anywhere between the major steps, then this scan would have detected them. Topological 10 µm x 10 µm maps were made at three locations indicated, which did not detect any features larger than 50 pm, i.e., 5 times smaller than the minimal SiC substrate step heights, which verifies that SEG is atomically flat.
Extended Data Fig. 5 QFSG characterization.
(a) Low temperature STM of a 20 µm by 20 µm area of QFSG produced by hydrogen intercalation shows that it is defect free. (b) Raman map of a 25 µm × 25 µm area shows that it is completely covered with graphene with no bare SiC or buffer. The arrow labeled A points to a region with a I2D/IG = 3.73 (red scan) and the arrow labeled B points to a region with a I2D/IG = 1.75 (red scan). Variations of this magnitude are expected for graphene.
Extended Data Fig. 6 Transport measurements of a QFSG Hall bar.
(a) Resistivity versus temperature, (b) Charge density versus temperature (c) Mobility versus temperature. (d) Mean free path versus temperature. Note the absence of a significant temperature dependence compared with SEG (Fig. 3). Also note that at room temperature, the charge densities and the mobilities are comparable to those of SEG.
Extended Data Fig. 7 Example of a seamless SEG/QFSG junction.
The junction was produced by depositing an Al2O3 strip, 80 µm wide, and intercalating hydrogen at 700 C.
Extended Data Fig. 8 Characteristics of a SEG field effect transistor.
See also SI. Sec. 4. (a) Schematic of field effect transistor with SEG as channel. (b) Transfer characteristics (Ids-Vgs) at bias voltage of 0, 1 and 2 V. (c) Transfer curve of the device at Vds = 1 V and corresponding logarithmic plot. (d) Output characteristic curves of the device. The field effect mobility is µFET = 22 cm2 V−1 s−1. The large reduction compared with the intrinsic SEC properties is caused by scattering from the dielectric and large contact Schottky barriers. (e) Extrapolation of the linear rise of the output curves correspond well with the STS measured band gap (Fig. 2e).
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Zhao, J., Ji, P., Li, Y. et al. Ultrahigh-mobility semiconducting epitaxial graphene on silicon carbide. Nature 625, 60–65 (2024). https://doi.org/10.1038/s41586-023-06811-0
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DOI: https://doi.org/10.1038/s41586-023-06811-0
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