Graphene has emerged recently as a new material with outstanding electronic properties1. This includes mass-less Dirac fermions, ballistic transport properties at room temperature and good compatibility with silicon planar technology2. Different techniques have been developed in the last six years to fabricate mono or bi-layer graphene. They range from exfoliated graphite, either mechanically1 or in a liquid-phase solution3 to chemical vapor deposition on a metal surface4, and, more recently, to substrate-free synthesis when passing ethanol into an argon plasma5. The method investigated in this work consists in a controlled sublimation of few atomic layers of Si from a single crystal SiC substrate6. Such epitaxial growth of graphene (EG) seems to be the most suitable option for industrial applications but, for easy control, it necessitates either a large and homogeneous sheet of monolayer graphene (MLG) or few layers graphene (FLG) covering the full wafer surface. Basically, on both the Si and C faces of any SiC substrate, graphene grows selectively on some reconstructed parts of the surface. Controlling the growth means then controlling locally the surface reconstruction. At low pressure conditions (below 10-6 Torr), it remains challenging to grow FLG with homogeneous domains larger than few hundred nanometers on both faces7. The homogeneity can be increased by lowering the sublimation rate. It has been demonstrated on the Si face by working at high pressure under a noble gas atmosphere such as argon8,9. In this work10, the surface reconstruction of the C face during the Si sublimation is modified by covering the SiC substrate with a graphite cap. It leads to a strongly step-bunched morphology with on few selected terraces the growth of long anisotropic graphene ribbons (5 μm wide and up to 600 μm long). Since the Raman fingerprint of Bernal stacked FLG depends strongly of the number of graphene layers11 and the absorbtance of FLG is almost independent of the wavelength and proportional to the number of graphene layers12, we combine micro-Raman spectroscopy with micro-transmission measurements to study the quality and thickness uniformity of these ribbons. We find that most of these ribbons are homogeneous monolayers or bilayers of graphene and that the thermal stress between the graphene layer and the 6H-SiC substrate is relaxed by the formation of wrinkles. This combination of techniques is especially useful to discriminate without any ambiguity between a monolayer graphene and a misoriented bilayer because their Raman fingerprint are identical. The spectra and extinction coefficient of a monolayer, a Bernal stacked bilayer noted AB, and a misoriented bilayer noted AA' are shown in Figure 1.

Epitaxial graphene growth is significantly different depending on the polarity of the 6H-SiC surface: Si- or C-face. On the Si-face, a uniform coverage of few layers on the whole sample can be obtained, but with electrical properties disturbed by the presence of a Carbon-rich buffer layer at the interface. On the contrary, on the C-face, we demonstrated that almost free-standing very large monolayers of graphene can be obtained by covering the sample with a graphitic cap during the growth.

We studied the in-plane magnetoresistance R(B, T) anisotropy in epitaxial multilayer graphene films grown on the Si face of a 6H-SiC substrate that originates from steplike morphology of the SiC substrate. To enhance the anisotropy, a combination of argon atmosphere with graphite capping was used during the film growth. The obtained micro-Raman spectra demonstrated a complex multilayer graphene structure with the smaller film thickness on terraces as compared to the step edges. Several Hall bars with different current/steps mutual orientations have been measured. A clear anisotropy in the magnetoresistance has been observed, and attributed to variations in electron mobility governed by the steplike structure. Our data also revealed that (i) the graphene-layer stacking is mostly Bernal type, (ii) the carriers are massive, and (iii) the carriers are confined to the first 2-4 graphene layers following the buffer layer.

The current status of long, self-organized, epitaxial graphene ribbons grown on the (0 0 0 - 1) face of 6H-SiC substrates is reviewed. First, starting from the early stage of growth it is shown that on the C face of 6H-SiC substrates the sublimation process is not homogeneous. Most of the time it starts from defective sites, dislocations or point defects, that define nearly circular flakes surrounded by bare SiC. These flakes have a volcano-like shape with a graphite chimney at the centre, where the original defect was located. At higher temperatures a complete conversion occurs, which is not yet homogeneous on the whole sample. This growth process can be modified by covering the sample with a graphite cap. It changes the physics of the surface reconstruction during the Si-sublimation process and, on the C face, makes more efficient the reconstruction of few selected terraces with respect to the others. The net result is the formation of strongly step-bunched areas with, in between, long and large reconstructed terraces covered by graphitic material. Despite the low intrinsic optical absorption of a few graphene layers on SiC, micro-transmission experiments, complemented by micro-Raman spectroscopy, demonstrate that most of this graphitic coverage is made of one or two homogeneous graphene layers. We show also that most of the thermal stress between the graphene layer and the 6H-SiC substrate is relaxed by pleats or wrinkles which are clearly visible on the AFM images. Finally, the results of transport experiments performed on the graphitic ribbons reveal the p-type character of the ribbons.

Using high temperature annealing conditions with a graphite cap covering the C-face of an 8 off-axis 4H-SiC sample, large and homogeneous single epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magnetotransport measurements. We find a moderate p-type doping, high carrier mobility, and half integer quantum Hall effect typical of high quality graphene samples. This opens the way to a fully compatible integration of graphene with SiC devices on the wafers that constitute the standard in today's SiC industry. (C) 2010 American Institute of Physics. [doi:10.1063/1.3480610]

Using a graphite cap to cover the silicon carbide (SiC) sample, it is shown that large isolated graphene anisotropic ribbons can be grown on the C face of on-axis, semi-insulating, 6H-SiC wafers. The role of the cap is to modify the physics of the surface reconstruction process during Si sublimation, making more efficient the reconstruction of few selected terraces with respect to the others. The net result is the formation of a strongly step-bunched morphology with, in between, long (up to 600 mu m) and large (up to 5 mu m) homogeneous monolayers of graphene ribbons. This is shown by optical and scanning electron microscopy, while a closer view is provided by atomic force microscopy (AFM). From Raman spectroscopy, it is shown that most of the ribbons are homogeneous monolayers or bilayers of graphene. It is also shown that most of the thermal stress between the graphene layer and the 6H-SiC substrate is relaxed by wrinkles. The wrinkles can be easily displaced by an AFM tip, which demonstrates evidence of graphene ironing at the nanoscale. Finally and despite the very low optical absorption of a single graphene layer, one shows that differential optical microtransmission can be combined to the micro-Raman analysis to confirm the monolayer character of the thinnest ribbons.

We report a comparative investigation of Few layers graphene grown on 6H, 4H, and 3C-SiC substrates. We show that the size of the graphitic domains depends more on the < 0001 > SiC Surface orientation than the polytypism.