Materials Science Forum Vols. 645-648

Abstract: Transmission Electron Microscopy (TEM) investigations of graphene layers synthesized on Si and C-terminated on-axis oriented 4H-SiC are presented. The high-resolution TEM (HRTEM) revealed distinctive distance differences between the first carbon graphene layer and SiC surface for both polarities. The prolonged annealing of SiC with carbon face shows, that in addition to the increase of number of graphene layers, there is also observed splitting between stack of graphene layers and the surface of SiC substrate. In addition, the density of so called “puckers” increases.

Abstract: 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.

Abstract: This article explores the formation of graphene layers on 3C-SiC(111) epilayers grown on silicon substrates using thermal annealing under Ultra High Vaccum (UHV) environment. The formation of graphene is demonstrated by use of near field microscopy (STM and AFM) and X-ray Photoelectron Spectroscopy (XPS). The evolution of the surface stoichiometry of the 3C-SiC(111) pseudo substrates during the graphitization process is similar to that of the commonly used Si terminated -SiC bulk substrates, starting from a Si rich to the C rich surface characterized by a diffraction pattern. Graphitization process leads to a strong modification of the surface at a microscopic scale which is compared to that reported in case of 6H-SiC substrates. XPS spectra reveal the presence of typical C-C bonds related to a graphitic arrangement. Its high level of ordering is attested by the observation both of (66)SiC and (11)graphene surface reconstructions by STM. These results demonstrate the formation of graphene on 3C-SiC(111)/Si pseudo substrates. They open perspectives for developing novel C/SiC/Si heterostructures and put light on the ability of 3C-SiC/Si templates to become a low cost alternative of onerous -SiC substrates.

Abstract: Few Layers Graphene (FLG) films were grown on the carbon-terminated surface of 4H-SiC from nickel silicide supersaturated with carbon. The process was realised by annealing of thin Ni films deposited on silicon carbide followed by wet processing to remove the nickel silicide. To identify and characterize the fabricated FLG films, micro-Raman scattering spectroscopy, AFM and optical microscopy have been used. The films grown on samples with initially deposited nickel thinner than 20 nm show clear graphene footprints in micro-Raman scattering spectra, namely a single component, Lorentzian shape 2D band with FWHM remarkably lower than that of the 2D peak of graphite.

Abstract: By carbon evaporation under ultrahigh vacuum (UHV) conditions, epitaxial graphene can be grown on SiC(0001) at significantly lower temperatures than with conventional Si sublimation. Therefore, the degradation of the initial SiC surface morphology can be avoided. The layers of graphene are characterized by low energy electron diffraction (LEED), angle resolved ultraviolet photoelectron spectroscopy (ARUPS), and atomic force microscopy (AFM). On SiC the graphene lattice is rotated by 30o in comparison to preparation by annealing in UHV alone.

Abstract: Epitaxial graphene growth on SiC is investigated using low-energy electron microscopy (LEEM) and first-principles calculations. LEEM is one of the most powerful tools to identify the thickness of graphene on SiC with a good spatial resolution. With the help of such LEEM, the thickness-dependent physical properties are identified by various experiments. It is shown that epitaxial graphene sheets continue even over steps of the substrate, and that a new graphene sheet often grows from step edges while the surface morphology changes drastically. Furthermore, the first-principles calculations also show the energetics of the epitaxial graphene growth on SiC. It is expected that the fine control of epitaxial graphene growth on SiC will open the way to novel graphene devices in the post-scaling era of the ultra-large-scale integrations (ULSI).

Abstract: The phonon frequencies of epitaxial graphene on silicon carbide (SiC) depend on mechanical strain and charge transfer from the substrate to the epitaxial layer. Strain and doping depend on the preparation process and on the number of graphene layers. We measured the phonon frequencies by Raman spectroscopy and compare the results between epitaxial layers fabricated by high temperature annealing and by hydrogen intercalation of the covalently bound graphene layer of the 6 p 3  6 p 3 reconstructed SiC surface. Only the latter graphene layer shows tensile strain, which can partly be explained by lattice mismatch between substrate and epitaxial graphene.

Abstract: Epitaxial graphene was grown on the surface of on-axis and off-axis SiC (0001) by solid state graphitization at high temperatures (2000 °C) in Ar ambient. The effect of the miscut angle on the lateral uniformity of the few layers of graphene (FLG) was investigated by combined application of micro-Raman spectroscopy and Torsion Resonance Conductive Atomic Force Microscopy, the latter method enabling a quantification of the FLG coverage on SiC with submicrometer lateral resolution. While the on-axis samples result in uniform coverage by thin (~ 3 monolayers) FLG, the coverage for off-axis samples is much less uniform, following closely the step bunching morphology of the SiC surface.

Abstract: 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.

Abstract: Optical transmission and transmission electron microscopy studies of epitaxial graphene structures grown on the carbon terminated face of 4H-SiC(000-1) on-axis substrates are presented. Several samples obtained using different growth conditions were studied. Optical microscope showed regions of micrometer size with different layer number. The exact number of layers was obtained from transmission electron microscope studies. Optical transmission spectra showed no wavelength dependence and allowed us to obtain the average number of graphene layers.