Graphene nanoribbons on vicinal SiC surfaces by molecular beam epitaxy

We present a method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consists of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [1$\overline{1}$00], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high-density GNRs, enable us to observe the electronic structure at $K$ points by angle-resolved photoemission spectroscopy, showing a clear band-gap opening of at least 0.14 eV.

Figure 1

An AFM image and a structural model of SiC nanosurface. (a) An AFM 3D view of the vicinal SiC surface after hydrogen etching, showing periodic array of terraces and facet structures. (b) A schematic drawing of the SiC surface in (a). Each pair of array consists of the (0001) terrace and (1$\overline{1}$0$n$) facet and is ordered with the periodic distance of $\sim$20 nm. The width of each (0001) terrace is $\sim$10 nm.

Figure 2

In situ RHEED analysis during MBE growth and a LEED image after growth. (a) A RHEED image after 20 min growth, where both $R3$ and $6R3$ structures are visible. (b) A LEED image after 33 min growth, indicating clear $6R3$ satellite spots. (c) RHEED intensity profiles as a function of growth time. The intensities of a specular spot, an $R3$ and a $6R3$ streak, as indicated in (a), are monitored. Note that the initial $R3$ intensity is increased after 5 min growth and the $R3$ and $6R3$ intensities show a complementary relationship.

Figure 3

AFM analyses of the hydrogen-intercalated samples. AFM height (upper) and phase images (lower) of the samples after 20 min growth [(a) and (c)] and 33 min growth [(b) and (d)]. Panel (e) represents the cross-section at the line A-B indicated in (a) and (c). $\sim$0.25 nm higher regions shaded in (e) show darker contrast in the phase image, i.e., graphene on the hydrogen-terminated SiC surface.

Figure 4

A Raman analyses of GNRs. (a) Raman spectra of the sample after 33 min growth before (blue) and after (red) hydrogen intercalation. (b) Polarization angle dependence of the $D$-band intensity. $D$-band intensity at each polarization angle $\theta$, defined in the insert, is fitted as a function of cos${}^4$$\theta$ shown by a dotted curve.

Figure 5

Intensity map and EDCs of the ARPES spectra around the $K$ point. Intensity map (a) and EDCs (b) of the ARPES spectra around the $K$ point of the samples after 33 min growth. The spectra are taken along the $\Gamma$-$K$-$M$ line. The red dots in (a) indicate the positions of the EDC peaks. The intensity map clearly indicates a linear dispersion of the valence band of the single-layer graphene. The EDCs show the folding of the valence band at the $K$ point below $E_F$.