Kouhei Morita

Anisotropic transport in graphene on SiC substrate with periodic nanofacets

Anisotropic transport in graphene field-effect transistors fabricated on a vicinal SiC substrate with a self-organized periodic nanofacet structure is investigated. Graphene thermally grown on a vicinal substrate contains two following regions: atomically flat terraces and nanofacets (atomically stepped slopes). The graphene film at a nanofacet is continuously connected between two neighboring terrace films. Anisotropic transport properties are clearly observed, indicating a difference in the graphene properties of the two regions. The observed anisotropic properties are discussed in terms of the effects of nanofacet structures on conductivity and electron mobility.

Few-layer epitaxial graphene grown on vicinal 6H–SiC studied by deep ultraviolet Raman spectroscopy

Few layer epitaxial graphenes (1.8–3.0 layers) grown on vicinal 6H–SiC (0001) were characterized by deep ultraviolet Raman spectroscopy. Shallow penetration depth of the probe laser enabled us to observe G-peak of graphene without subtraction of the SiC substrate signal from observed spectra. The G-peak was greatly shifted to higher frequency compared to that of graphite due to in-plane compressive stress deriving from the substrate. The frequency shift decreased with the number of graphene layers because of stress relaxation from layer to layer. Our experiment suggests that the stress is completely relaxed within five to six graphene layers.

Role of atomic terraces and steps in the electron transport properties of epitaxial graphene grown on SiC

Thermal decomposition of vicinal SiC substrates with self-organized periodic nanofacets is a promising method to produce large graphene sheets toward the commercial exploitation of graphenes superior electronic properties. The epitaxial graphene films grown on vicinal SiC comprise two distinct regions of terrace and step; and typically exhibit anisotropic electron transport behavior, although limited areas in the graphene film showed ballistic transport. To evaluate the role of terraces and steps in electron transport properties, we compared graphene samples with terrace and step regions grown on 4H-SiC(0001). Arrays of field effect transistors were fabricated on comparable graphene samples with their channels parallel or perpendicular to the nanofacets to identify the source of measured reduced mobility. Minimum conductivity and electron mobility increased with the larger proportional terrace region area; therefore, the terrace region has superior transport properties to step regions. The measured elect...

Formation of graphene nanostructures on vicinal SiC surfaces

After the successful fabrication of graphene by the mechanical exfoliation method by Novoselov et al. [1], there have been many significant studies using graphene flakes. For future applications and further simplification of the experiments, it is essential to scale up the size and obtain high quality films. The surface decomposition of silicon carbide (SiC) in vacuum [2, 3] or inert gas environment [4] is one of the promising approaches for growth of epitaxial graphene. In the future, it will be possible to fabricate single layer graphene and a few layers of graphene (FLG) over the entire surface of the SiC substrate. Emtsev et al. demonstrated the fabrication of monolayer graphene with sufficiently large domain sizes on SiC substrates by high temperature annealing in an Ar atmosphere and nearly atmospheric pressure [4]. Graphene nanostructures such as nanoribbons (GNRs) and periodic ripples are of interest because of their electronic structures modified by quantization and potential fluctuations. Especially, GNRs gather interests due to the presence of edges, which are theoretically predicted to possess unique electronic characteristics [5]. However, it has been difficult to fabricate GNRs having very narrow widths below ~20nm, even with the use of the state-of-the-art lithographic techniques. Several ideas to realize GNRs are proposed: unzipping of carbon nanotubes (CNT) [6] and chemical synthesis [7]. In this study, we demonstrate a new approach, i.e. epitaxial growth of GNRs, by noticing ‘SiC nanosurfaces’ as a template and molecular beam epitaxy (MBE) [8]. We found after H2-gas etching the vicinal SiC surfaces exhibited self-ordered nanofacet structures consisting of pairs of a (0001) plane and a (1-10 n) nanofacet with a characteristic distance of 10/20 nm [9, 10], as shown in Figs. 1. Such unique vicinal SiC surfaces (SiC nanosurface) would play a significant role in obtaining self-ordered graphene nanostructures.

Deep UV Raman Spectroscopy of Epitaxial Graphenes on Vicinal 6H-SiC Substrates

We report microscopic Raman scattering studies of epitaxial graphene grown on SiC substrates using a deep-ultraviolet (UV) laser excitation at 266 nm to elucidate the interaction between the graphene layer and the substrate. The samples were grown on the Si-face of vicinal 6H-SiC (0001) substrates by sublimation of Si from SiC. The G band of the epitaxial graphene layer was clearly observed without any data manipulation. Increasing the number of graphene layers, the peak frequency of the G-band decreases linearly, while the peak width and the intensity increase. The G-band frequency of the graphene layers on SiC is higher than those of exfoliated graphene, which has been ascribed to compression from the substrate.