Atomic-scale characterization of the interfacial phonon in graphene/SiC

Emi Minamitani, Ryuichi Arafune, Thomas Frederiksen, Tetsuya Suzuki, Syed Mohammad Fakruddin Shahed, Tomohiro Kobayashi, Norifumi Endo, Hirokazu Fukidome, Satoshi Watanabe, and Tadahiro Komeda

Phys. Rev. B 96, 155431 – Published 11 October 2017

Abstract

Epitaxial graphene on SiC that provides wafer-scale and high-quality graphene sheets on an insulating substrate is a promising material to realize graphene-based nanodevices. The presence of the insulating substrate changes the physical properties of free-standing graphene through the interfacial phonon, e.g., limiting the mobility. Despite such known impacts on the material properties, a complete and microscopic picture is missing. Here, we report on atomically resolved inelastic electron tunneling spectroscopy (IETS) with a scanning tunneling microscope for epitaxial graphene grown on $4 H$ -SiC(0001). Our data reveal a strong spatial dependence in the IETS spectrum, which cannot be explained by intrinsic graphene properties. We show that this variation in the IETS spectrum originates from a localized low-energy vibration of the interfacial Si atom with a dangling bond via ab initio electronic and phononic state calculations. This insight may help advancing graphene device performance through interfacial control.

Received 24 May 2017

DOI:https://doi.org/10.1103/PhysRevB.96.155431

Physics Subject Headings (PhySH)

Electron-phonon coupling Transport phenomena

Graphene Solid-solid interfaces

First-principles calculations Scanning tunneling microscopy Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Emi Minamitani^1,*, Ryuichi Arafune², Thomas Frederiksen^3,4, Tetsuya Suzuki⁵, Syed Mohammad Fakruddin Shahed⁵, Tomohiro Kobayashi⁵, Norifumi Endo⁶, Hirokazu Fukidome⁶, Satoshi Watanabe¹, and Tadahiro Komeda^5,†

¹Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
²International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 304-0044, Japan
³Donostia International Physics Center, Paseo Manuel de Lardizabal, 4, E-20018 Donostia-San Sebastián, Spain
⁴IKERBASQUE, Basque Foundation for Science, E-48013 Bilbao, Spain
⁵Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aobaku, Sendai, Miyagi 980-8577, Japan
⁶Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

^*eminamitani@cello.t.u-tokyo.ac.jp
^†komeda@tagen.tohoku-u.ac.jp

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Issue

Vol. 96, Iss. 15 — 15 October 2017

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Images

Figure 1
(a) STM topographic image of epitaxial graphene grown on a SiC(0001) surface. Area $14 \times 14 n m^{2}$ , and set point of $I = 0.8 nA$ and $V = - 200 mV$ . The brighter color represents the charge accumulation. (b) STS spectra obtained at the bright (B) and dark (D) areas of the topographic image. (c) Schematics of $d I / d V$ (STS) and $d^{2} I / d V^{2}$ (IETS) spectra for an excitation of a phonon with an excitation energy of hν. (d), (e) $d I / d V$ and $d^{2} I / d V^{2}$ spectra obtained at B and D areas. Black arrows in (d) and (e) indicate the energy at which the prominent inelastic phonon excitation occurs. (f) Spatial distribution of $d I / d V$ (left panel) and $d^{2} I / d V^{2}$ (right panel). The series of $d I / d V$ and $d^{2} I / d V^{2}$ is taken along the white line that connects the neighboring bright spots in the STM topographic image shown in the center panel. The horizontal axis corresponds to the sample bias and the vertical axis corresponds to the measured position (corresponding directly to the right panel) and the intensity of the $d^{2} I / d V^{2}$ spectrum is expressed according to a color table. For $d^{2} I / d V^{2}$ , the contrast is reversed in the positive and negative energy regions for a clear view.
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Figure 2
(a) STM simulation results at $- 100 mV$ sample bias voltage. (b) Partial charge distribution used for plotting (a). The black lines indicate the planes where the cross-sectional views shown in (c) and (d) are taken. (c), (d) Cross-sectional images of the partial charge distribution taken on the cut 1 and cut 2 plane in (b), respectively. The blue-gray surfaces are the tetrahedron of C centered by a Si atom. The charge distribution becomes dense at the Si atom with a dangling bond that is not surrounded by the tetrahedral plane.
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Figure 3
(a) Geometric structure of the $\sqrt{3}$ model of epitaxial graphene on $4 H$ -SiC (0001). Left: Side view of the structure; brown, C; blue, Si; light pink, H atoms. Right: Top view of the structure. The four different colors of the C atoms in the topmost graphene layer correspond to the different chemical environments; light green, C1 type atoms positioned on the top of the buffer C and interfacial Si atom; light blue, C2 type atoms also positioned on top of the buffer C atom, but with no interfacial Si atom beneath; orange, C3 type atoms where there is no buffer C atom or interfacial Si atom beneath; pink, C4 type atoms positioned on the top of the interfacial Si atom with dangling bond. (b)–(d) Phonon DOS of the $z$ -polarized mode projected on the interfacial Si atoms, the C atoms in the graphene layer, and the C atoms in the buffer layer, respectively.
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Figure 4
(a) Two structure models (D and B) used in the $d I / d V$ calculations. (b) $d I / d V$ simulations in the D and B models. Black lines are $d I / d V$ taking into account the extra broadening from the lock-in measurement technique $(V_{rms} = 5 meV)$ . Red lines are the intrinsic $d I / d V$ results (linewidth only due to a finite temperature of $T = 4.2 K$ ). The structural side views show the displacements of the atoms in the active phonon modes that contribute to the narrow (7.6 meV) and wide (20.4 meV) gaps, respectively.
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