Stacking of adjacent graphene layers grown on C-face SiC

Graphene was grown on the C-face of nominally on-axis SiC substrates using high-temperature sublimation with Ar as the buffer inert gas. The results of studies of the morphology, thickness, and electronic structure of these samples using low-energy electron microscopy (LEEM), x-ray photoelectron emission microscopy, photoelectron spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), and low-energy electron diffraction (LEED) are presented. The graphene thickness is determined to vary from 1 or 2 to 6 or 7 monolayers (MLs), depending on the specific growth conditions utilized. The formation of fairly large grains (i.e., crystallographic domains) of graphene exhibiting sharp 1×1 spots in micro-LEED is revealed. Adjacent grains are found to show different azimuthal orientations. Macro-LEED patterns recorded mimic previously published, strongly modulated, diffraction ring LEED patterns, indicating contribution from several grains of different azimuthal orientations. We collected selected area constant initial energy photoelectron angular distribution patterns that show the same results. When utilizing a small aperture size, one Dirac cone centered on each of the six K-points in the Brillouin zone is clearly resolved. When using a larger aperture, several Dirac cones from differently oriented grains are detected. Our findings thus clearly show the existence of distinct graphene grains with different azimuthal orientations; they do not show adjacent graphene layers are rotationally disordered, as previously reported for C-face graphene. The graphene grain size is shown to be different on the different samples. In some cases, a probing area of 400 nm is needed to detect the grains. On one sample, a probing area of 5 μm can be used to collect a 1×1 LEED pattern from a multilayer graphene grain. ARPES is used to determine the position of the Dirac point relative to the Fermi level on two samples that LEEM shows have dominant coverage of 2 and 3 MLs of graphene, respectively. The Dirac point is found to be located within 75 meV of the Fermi level on both samples, which indicates that the electron carrier concentration induced in the second and third graphene layers on the C-face is less than ∼4×10${}^{11}$ cm${}^{-2}$. Formation of patches of silicate is revealed on some samples, but the graphene formed on such nonhomogenous surfaces can contain fairly large ordered multilayer graphene grains.

Figure 1

(a) LEEM image recorded at 0.4 eV and a field of view of 20 μm. (b) Electron reflectivity curves extracted from areas A–D. (c) A LEED pattern collected from the 3-ML graphene area at 44 eV using a probing area of 5 μm is shown in the far left panel labeled 1. The other four panels show LEED patterns collected at the same energy but using a probing area of 400 nm at four equidistant positions, from position 1 to position 7 in (a). This sample was grown at 1850 °C for 15 min in an Ar pressure of 500 mbar.

Figure 2

(a) Normal emission C 1s spectra recorded at three photon energies from sample 1, with mainly 3 MLs of graphene. (b) The π-band collected at the K-point from this sample. A photon energy of 33 eV was used.

Figure 3

(a) LEEM image recorded from a second sample at 0.4 eV and a field of view of 25 μm. (b) Electron reflectivity curves extracted from the areas labeled A and E–G. (c) The π-band recorded at the K-point from this sample with a graphene coverage of mainly 2 MLs. A photon energy of 33 eV was used. This second sample was grown at 1800 °C for 20 min in an Ar pressure of 500 mbar.

Figure 4

(a) LEEM image recorded from a third sample at 1.3 eV and a field of view of 10 μm. This is the sample grown on a 4H-SiC substrate. (b) Electron reflectivity curves extracted from areas 1–8. (c) The two upper panels show selected-area LEED patterns collected at 45 eV using a probing area of 400 nm from positions 1 and 2 in (a). The lower panel shows the LEED pattern collected using a 5-μm aperture at position 2. (d) Two constant energy photoelectron angular distribution patterns ($E_i$,$k_x$,$k_y$) of the π-band collected using a probing area of 800 nm and a photon energy of 45 eV at an energy of ∼2 eV below the Fermi. One Dirac cone centered on each of the six K-points in the Brillouin zone is clearly observed. This third sample was grown at 1900 °C for 15 min in an Ar pressure of 850 mbar.

Figure 5

(a) LEEM image collected from a fourth sample at a voltage of 1.3 eV and a field of view of 50 μm. (b) XPEEM image collected from the same area at an energy of 1.4 eV using 130 eV photons, showing the variation in work function over the surface. The bright areas labeled 0 (which are the same as the dark gray areas labeled 0 in (a) correspond to regions where no graphene has formed. The gray areas show where graphene has formed, and the difference in darkness indicates that the number of graphene layers is different. (c) The upper curve shows the Si 2p spectrum collected from the bright areas in (b); the bottom curve shows the Si 2p spectrum obtained from the gray graphene-covered areas. (d) Selected-area LEED pattern collected from the bright areas, at 45 eV, showing a silicate $\sqrt{3}\times \sqrt{3}$ R30° diffraction pattern. This fourth sample was grown at 2000 °C for 10 min in an Ar pressure of 850 mbar.

Figure 6

(a) LEEM image recorded from a fifth sample at 0.4 eV and a field of view of 25 μm. (b) Electron reflectivity curves extracted from areas 1–8. (c) Selected-area LEED pattern collected at 45 eV from areas 1–3 using a probing area of 1.5 μm. This fifth sample was grown at 1900 °C for 15 min in an Ar pressure of 850 mbar.

Figure 7

(a) XPEEM image, collected from essentially the same area as in Fig. 6a, at an energy of 0.1 eV using 130 eV photons. (b) C 1s spectra recorded, using 450 eV photons, from the areas labeled 1, 6, and 2 in (a).