STM 1

Atomic and electronic structure of Si dangling bonds in quasi-free-standing monolayer graphene

Quasi-free-standing monolayer graphene (QFMLG), obtained by intercalating hydrogen at the interface of buffer layer and SiC(0001), is efficiently decoupled from the substrate and a promising material for wafer-scale graphene-based nanoelectronics. However, the mobility of QFMLG is limited to a value lower than epitaxial monolayer graphene (~3000 cm2/Vs), and the carrier scattering has not been fully understood. Recently it has been reported that the mobility of QFMLG depends on the substrate temperature during the hydrogen intercalation process, and the highest mobility is obtained at 700-800°C. These measurements suggested that the carrier scattering is mainly caused by charged impurities at 600 and 800°C, and by defects at 950°C. We have used scanning tunneling microscopy (STM) to study the surface structure of QFMLG formed at several hydrogen intercalation temperatures, and investigated the relationship with transport measurements.

Fig1Fig 1: STM images on QFMLG. H atoms were intercalated at a temperature of (a-c) 600°C, (d-f) 800°C, and (g-i and l) 1000°C. Black arrows in (a) and (b) indicate a bright and a small dark spot, respectively. Blue arrow in (e) indicates a SiC(0001) <11-20> direction. Scan size, bias voltage, and tunneling current are (a) 200 nm, -0.6 V, 0.5 nA, (b) 50 nm, 0.5 V, 0.5 nA, (c) 8 nm, -0.6 V, 0.4 nA, (d) 200 nm, 0.6 V, 0.5 nA, (e) 50 nm, 0.6 V, 0.4 nA (f) 8 nm, 0.5 V, 0.1 nA, (g) 200nm, 0.6 V, 0.5 nA, (h) 50 nm, 0.8 V, 0.4 nA, (i) 8 nm, -0.1 V, 0.6 nA, (l) 8 nm, 0.8V, 0.4 nA. Inset in (l) shows a two-dimensional Fourier transform of (l). White and blue arrows indicate a graphene 1×1 and √3×√3 spot, respectively. (j) Line profile along red line in (b). (k) Line profile along red line in (h).

Our STM observations reveal that the QFMLG formed at 600°C and 800°C shows defects (dark spots) with a diameter of 1.5 nm, depth of 20-80 pm, and density of 1×1013 cm-2, while samples formed at 1000°C show dark spots with diameter 4-10 nm, depth 250 pm, and density 6×1010 cm-2 (see Fig. 1). The dark spots at 600°C and 800°C partially align with a periodicity of 1.8 nm, corresponding to the quasi-(6×6) reconstruction of the buffer layer. This implies that hydrogen intercalation in our samples is not complete at 600°C and 800°C, and the remaining patches of buffer layer are observed as dark spots. Since the depth of the dark spot defects at 1000°C corresponds to the height of a SiC(0001) bilayer, they are identified as holes in the SiC substrate, probably due to etching by hydrogen or redistribution of surface atoms at high temperature. This is consistent with transport measurements and suggests that the incomplete hydrogen intercalation at 600 and 800°C results in Si dangling bonds at the interface of QFMLG which act as charged scattering centers and severely affect carrier mobility.

Fig 2: (a) and (b) STS curves recorded for the A and B features and for a flat area. The energy range is (a) –0.2 to +0.5 V and (b) –1.7 to +1.7 V. The measurements were performed at T = 5 K, with V(stab) = 0.5 (a) and 1.7 V (b), and I(stab) = 0.01 nA for both (a) and (b). (c) Calculated DOS for vacancies with one, three, and four missing hydrogen atoms and for free graphene.

Next, we investigated the atomic and electronic structure of Si dangling bonds in QFMLG using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and density functional theory (DFT) calculations. Two types of defects with different contrast were observed on a flat graphene terrace by STM and AFM which we label A and B; in particular, their STM contrast varied with the bias voltage. Moreover, these defects showed characteristic STS peaks at different energies, 1.1 and 1.4 eV, see Fig. 2. The comparison of the experimental data with the DFT calculations indicates that the defects with STS peak energies of 1.1 and 1.4 eV consist of clusters of three and four Si dangling bonds, respectively.

Publications:

  1. Yuya Murata, Torge Mashoff, Makoto Takamura, Shinichi Tanabe, Hiroki Hibino, Fabio Beltram, and Stefan Heun: Correlation between morphology and transport properties of quasi-free-standing monolayer graphene, arXiv:1409.0457 [cond-mat.mes-hall].
  2. Y. Murata, T. Mashoff, M. Takamura, S. Tanabe, H. Hibino, F. Beltram, and S. Heun: Correlation between morphology and transport properties of quasi-free-standing monolayer graphene, Appl. Phys. Lett. 105 (2014) 221604.
  3. Yuya Murata, Tommaso Cavallucci, Valentina Tozzini, Niko Pavliček, Leo Gross, Gerhard Meyer, Makoto Takamura, Hiroki Hibino, Fabio Beltram, Stefan Heun: Atomic and Electronic Structure of Si Dangling Bonds in Quasi-Free-Standing Monolayer Graphene, arXiv:1706.01422 [cond-mat.mtrl-sci].
  4. Y. Murata, T. Cavallucci, V. Tozzini, N. Pavlicek, L. Gross, G. Meyer, M. Takamura, H. Hibino, F. Beltram, and S. Heun: Atomic and electronic structure of Si dangling bonds in quasi-free standing monolayer graphene, Nano Research 11 (2018) 864 – 873.
  5. Tommaso Cavallucci, Yuya Murata, Makoto Takamura, Hiroki Hibino, Stefan Heun, Valentina Tozzini: Revealing the nature of defects in quasi free standing monolayer graphene on SiC(0001) by means of Density Functional Theory, arXiv:1710.03993 [cond-mat.mes-hall].
  6. Tommaso Cavallucci, Yuya Murata, Makoto Takamura, Hiroki Hibino, Stefan Heun, Valentina Tozzini: Unraveling localized states in quasi free standing monolayer graphene by means of Density Functional Theory, Carbon 130 (2018) 466 – 474.
  7. Valentina Tozzini and Stefano Veronesi: Graphene engineering: new opportunities for controlled functionalization and energy storage, CNR Nano Activity Report 2020 [Page 66].
  8. Hydrogen storage in functionalized graphene, NEST Scientific Report 2014 – 2020.

Presented at:

  1. Yuya Murata, Torge Mashoff, Makoto Takamura, Shinichi Tanabe, Hiroki Hibino, Fabio Beltram, and Stefan Heun: Correlation between morphology and transport properties of quasi-free-standing monolayer graphene, ECOSS 30, Antalya, Turkey, 31 August – 5 September 2014 (oral). [Abstract] [Talk]
  2. Yuya Murata, Torge Mashoff, Makoto Takamura, Shinichi Tanabe, Hiroki Hibino, Fabio Beltram, and Stefan Heun: Correlation between morphology and transport properties of quasi-free-standing monolayer graphene, NTT Basic Research Laboratories, Atsugi, Japan (Dr. H. Hibino), 31 October 2014. [Abstract] [Talk]
  3. Yuya Murata, Torge Mashoff, Makoto Takamura, Shinichi Tanabe, Hiroki Hibino, Fabio Beltram, and Stefan Heun: Correlation between morphology and transport properties of quasi-free-standing monolayer graphene, The 7th International Symposium on Surface Science ISSS-7, Matsue, Japan, 2 – 6 November 2014 (oral). [Abstract] [Talk]
  4. Y. Murata, T. Mashoff, M. Takamura, S. Tanabe, H. Hibino, F. Beltram, and S. Heun: Correlation between Morphology and Transport Properties of Quasi-Free-Standing Monolayer Graphene, International Workshop on SPA-LEED 2015, Hannover, Germany, 28 – 29 May 2015 (oral). [Abstract] [Talk]
  5. Y. Murata, T. Mashoff, M. Takamura, S. Tanabe, H. Hibino, F. Beltram, and S. Heun: Correlation between Morphology and Transport Properties of Quasi-Free-Standing Monolayer Graphene, EP2DS-21/MSS-17, Sendai, Japan, 26 – 31 July 2015 (poster). [Abstract] [Poster]
  6. S. Heun: Hydrogenated Graphene, Tohoku University, Sendai, Japan (Prof. M. Suemitsu), 31 July 2015. [Abstract] [Talk]
  7. S. Heun: Hydrogenated graphene, FisMat 2015, Palermo, Italy, 28 September – 2 October 2015 (invited). [Abstract] [Talk]
  8. Yuya Murata, Torge Mashoff, Niko Pavliček, Gerhard Meyer, Makoto Takamura, Shinichi Tanabe, Hiroki Hibino, Fabio Beltram, and Stefan Heun: Correlation between morphology and transport properties of quasi-free-standing monolayer graphene (QFMLG), 2016 MRS Spring Meeting, Phoenix, Arizona, USA, March 28-April 1, 2016 (oral). [Abstract] [Talk]
  9. Yuya Murata: Hydrogen interaction with graphene, NEST Colloquium, Pisa, Italy, 16 June 2016 (oral). [Abstract] [Talk]
  10. S. Heun: Hydrogenated Graphene, Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia (Prof. V. Khrapai), 05 July 2016 (invited seminar). [Abstract] [Talk]
  11. Y. Murata, T. Mashoff, N. Pavlicek, G. Meyer, T. Cavallucci, V. Tozzini, M. Takamura, H. Hibino, F. Beltram, and S. Heun: Hydrogen vacancies in quasi-free-standing monolayer graphene, ECOSS-32, Grenoble, France, 28 August – 02 September 2016 (oral). [Abstract] [Talk]
  12. T. Cavallucci, Y. Murata, S. Heun, and V. Tozzini: H coverage defects in quasi free standing graphene, Graphene 2017, Barcelona, Spain, 28 – 31 March 2017 (poster). [Abstract] [Poster]
  13. Yuya Murata, Tommaso Cavallucci, Valentina Tozzini, Niko Pavliček, Leo Gross, Gerhard Meyer, Makoto Takamura, Hiroki Hibino, Fabio Beltram, and Stefan Heun: Atomic and electronic structure of Si dangling bonds in quasi-free-standing monolayer graphene, University of Seoul, S. Korea (Prof. Jeil Jung), 18 September 2017 (invited). [Abstract] [Talk]
  14. T. Cavallucci, Y. Murata, S. Heun, and V. Tozzini: H coverage defects in quasi-free-standing monolayer graphene on SiC, Fismat 2017, Trieste, Italy, 1 – 5 October 2017 (oral). [Abstract] [Talk]
  15. V. Tozzini, T Cavallucci, S Heun, L Bellucci: Substrate induced defects in graphene as opportunities for advanced applications, NanoMeeting 2018, Pisa, Italy, 29 – 30 October 2018 (poster). [Abstract] [Poster]