Abstract
Graphene grown on C-face SiC substrates using two procedures, high and low growth temperature and different ambients, was investigated using Low Energy Electron Microscopy (LEEM), X-ray Photo Electron Electron Microscopy (XPEEM), selected area Low Energy Electron Diffraction (μ-LEED) and selected area Photo Electron Spectroscopy (μ-PES). Both types of samples showed formation of μm-sized grains of graphene. The sharp (1 × 1) μ-LEED pattern and six Dirac cones observed in constant energy photoelectron angular distribution patterns from a grain showed that adjacent layers are not rotated relative to each other, but that adjacent grains in general have different azimuthal orientations. Diffraction spots from the SiC substrate appeared in μ-LEED patterns collected at higher energies, showing that the rotation angle between graphene and SiC varied. C 1s spectra collected did not show any hint of a carbon interface layer. A hydrogen treatment applied was found to have a detrimental effect on the graphene quality for both types of samples, since the graphene domain/grain size was drastically reduced. From hydrogen treated samples, μ-LEED showed at first a clear (1 × 1) pattern, but within minutes, a pattern containing strong superstructure spots, indicating the presence of twisted graphene layers. The LEED electron beam was found to induce local desorption of hydrogen. Heating a hydrogenated C-face graphene sample did not restore the quality of the original as-grown sample.
Highlights
Adjacent layers of graphene grown on C-face SiC have for many years been claimed to stack in a different way [1,2,3,4,5] than on Si-face SiC
The important point that we want to stress here is, that no superstructure spots are observed in these μ-LowEnergy Electron Diffraction (LEED) patterns and that this shows that there is no rotational disorder between adjacent layers on the 2 MLs and 3 MLs areas of this sample prepared by sublimation at 1500 °C in high vacuum
Before looking at how the μ-LEED pattern typically changed with time, it deserves to be mentioned that for this hydrogenated sample, in Figure 5c,d, the I(V) curves extracted outside the illuminated circular area did not show any periodic oscillations, but instead appeared similar to curve C in Figure 1b, irrespective of the size and position of the area selected
Summary
Adjacent layers of graphene grown on C-face SiC have for many years been claimed to stack in a different way [1,2,3,4,5] than on Si-face SiC. The results [9] showed formation of fairly large (some μm) grains (crystallographic domains) of graphene exhibiting sharp (1 × 1) spots in μ-LEED and adjacent grains with different azimuthal orientations. Our experimental results clearly showed the existence of fairly large graphene grains with different azimuthal orientations and that adjacent graphene layers were not rotationally disordered as earlier proposed for C-face graphene [1,2,3,4,5]. We prepared graphene on nominally on-axis C-face SiC samples using these two different growth methods and performed a comparative study utilizing the above mentioned surface science tools For both type of samples, we found formation of large grains of graphene exhibiting sharp (1 × 1) spots in μ-LEED and that adjacent grains showed different azimuthal orientations. Our findings concerning the differently prepared C-face graphene samples and the effects of the hydrogen treatments are reported
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