AVS 59th Annual International Symposium and Exhibition
    Graphene and Related Materials Focus Topic Thursday Sessions
       Session GR+AS+NS+SS-ThM

Paper GR+AS+NS+SS-ThM1
Atomic and Electronic Structures of Graphene Nanoribbon made by MBE on Vicinal SiC Substrate

Thursday, November 1, 2012, 8:00 am, Room 13

Session: Graphene Nanostructures
Presenter: F. Komori, University of Tokyo, Japan
Authors: F. Komori, University of Tokyo, Japan
K. Nakatsuji, University of Tokyo, Japan
T. Yoshimura, University of Tokyo, Japan
T. Kajiwara, Kyushu University, Japan
K. Takagi, Kyushu University, Japan
S. Tanaka, Kyushu University, Japan
Correspondent: Click to Email
Electronic states of graphene nanoribbon attract much interest because its intrinsic metallic band is modified to have a gap or a one-dimensional edge state at the Dirac energy ED. Actually, microfabricated graphene [1] showed an energy gap at ED, and the gap size increases with decreasing the width. Fabrication of well-controlled graphene nanoribbons on macroscopic area of a semiconductor substrate is, however, still one of the challenging issues in graphene research. Here, we report characterizations of graphene nanoribbon made by carbon molecular beam epitaxy (MBE) and a hydrogen treatment on a vicinal SiC(0001) substrate. Use of MBE is essential because graphene is made over the step edges of the SiC substrate in the case of graphene formation by widely-used thermal decomposition.

In the experiment, a 6√3 x 6√3 structure was first made by MBE on the anisotropic terrace of the Si-terminated surface of a nitrogen-doped 6H-SiC(0001) substrate vicinal to the [1-100] direction. The tilting angle of the substrate was 4 degree, and a well-ordered step-and-terrace structure was made after cleaning the substrate by annealing in hydrogen as confirmed by atomic force microscopy. We optimized the substrate temperature and the carbon deposition rate to make a homogeneous 6√3 x 6√3 structure on the terraces without thermal decomposition of the substrate. The surface structure was in situ monitored by reflection high energy electron diffraction, and the width of the 6√3 x 6√3 area on the terrace was adjusted by monitoring the 6√3 x 6√3 spots. After stopping the growth, the sample was exposed to hydrogen molecules at 600 °C to transform the surface 6√3 x 6√3 layer to single-layer graphene by inserting hydrogen atoms at the interface. [2]

Graphene honeycomb lattice without the 6√3 x 6√3 structure was confirmed by low energy electron diffraction and scanning tunneling microscopy (STM). Few point defects are seen at the graphene on the terrace in the STM images of atomic resolution. The width of graphene nanoribbon on the substrate terrace is 10-15 nm, depending on the growth condition. The electronic states of the graphene nanoribbon were studied using angle-resolved photoemission spectroscopy (ARPES) at 130 K as in the previous report. [3] The top of the π band of the graphene nanoribbon was 0.05 ~ 0.25 eV below the Fermi energy. No signal from the π* band was detected by ARPES above the top of the π band, indicating the gap formation at ED.

References

1. M. Y. Han et al., Phys. Rev. Lett. 98, 206805 (2007).

2. C. Riedl et al., Phys. Rev. Lett. 103, 246804 (2009).

3. K. Nakatsuji et al., Phys. Rev. B82 045428 (2010).