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Micro-Raman and micro-transmission imaging of epitaxial graphene grown on the Si and C faces of 6H-SiC.

Tiberj A, Camara N, Godignon P, Camassel J - Nanoscale Res Lett (2011)

Bottom Line: On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped.A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction.While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratoire Charles Coulomb, UMR5221 CNRS-Université Montpellier II, Place Eugène Bataillon - cc074, 34095 Montpellier Cedex 5, France. Antoine.Tiberj@univ-montp2.fr.

ABSTRACT
Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping.

No MeSH data available.


Related in: MedlinePlus

Plot of the normalized integrated intensities of the G and 2D band against the relative extinction for several mono- and bilayer graphene. The G and 2D band integrated intensities are normalized with the integrated intensities of an HOPG reference sample. Theoretical values of the relative extinction are indicated by two vertical dashed lines. A correlation exists between the G band intensity and the relative extinction. Two cloud of points can be easily distinguished between mono- and bilayers thanks to the relative extinction. The scattering of the G band intensity is too large to use it as an absolute thickness measurement. The 2D band scattering is even higher especially for bilayer. 2D band of AB bilayer is as intense as monolayers whereas misoriented bilayers are twice intense than a monolayer.
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Figure 4: Plot of the normalized integrated intensities of the G and 2D band against the relative extinction for several mono- and bilayer graphene. The G and 2D band integrated intensities are normalized with the integrated intensities of an HOPG reference sample. Theoretical values of the relative extinction are indicated by two vertical dashed lines. A correlation exists between the G band intensity and the relative extinction. Two cloud of points can be easily distinguished between mono- and bilayers thanks to the relative extinction. The scattering of the G band intensity is too large to use it as an absolute thickness measurement. The 2D band scattering is even higher especially for bilayer. 2D band of AB bilayer is as intense as monolayers whereas misoriented bilayers are twice intense than a monolayer.

Mentions: Coming back to the maps in Figure 1, some correlation exists between the extinction and the G band intensity. From this observation one could conclude that the G band intensity can be used to evaluate the number of graphene layers. This is not that simple. In Figure 4, we plot the normalized integrated intensities of the G and 2D band measured on many mono and bilayer graphene (not shown here) versus the corresponding extinction values. The theoretical extinctions values are indicated as vertical dashed lines. Despite the correlation existing between the G band intensity and the relative extinction, the scattering of the G band intensity is too large when compared to the difference between a mono- and bilayer. We can even find bilayers that has the same G band intensity than monolayers. Therefore, the G band intensity cannot be used as an absolute thickness measurement but rather as a first guess if the relative extinction cannot be measured. The relative extinction is indeed not measurable in several cases: (i) if no bare SiC exists on the sample (for instance as graphene fully covers the SiC surface), (ii) if impurities (dust) or metal contacts cover the graphene and/or the SiC. In this particular case, we can evaluate the thickness by assuming that the average G band normalized intensity of a monolayer is between 0.025 and 0.03. Beware that these values depend on the experimental configuration and must be calibrated. For thin FLG (¡5 layers) the error can be of 1 layer and, for thicker FLGs, the estimated thickness can have a factor two error. Unlike the G band, the 2D band intensity cannot be used to evaluate the thickness for several reasons. For monolayers, the 2D band intensity strongly depends on the Fermi Level [34]. We used this dependance to evaluate the absolute value of the doping in the monolayer ribbon. Moreover in Figure 4, we can clearly see that the 2D band of an AB bilayer is as intense as the one of a monolayer.


Micro-Raman and micro-transmission imaging of epitaxial graphene grown on the Si and C faces of 6H-SiC.

Tiberj A, Camara N, Godignon P, Camassel J - Nanoscale Res Lett (2011)

Plot of the normalized integrated intensities of the G and 2D band against the relative extinction for several mono- and bilayer graphene. The G and 2D band integrated intensities are normalized with the integrated intensities of an HOPG reference sample. Theoretical values of the relative extinction are indicated by two vertical dashed lines. A correlation exists between the G band intensity and the relative extinction. Two cloud of points can be easily distinguished between mono- and bilayers thanks to the relative extinction. The scattering of the G band intensity is too large to use it as an absolute thickness measurement. The 2D band scattering is even higher especially for bilayer. 2D band of AB bilayer is as intense as monolayers whereas misoriented bilayers are twice intense than a monolayer.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211991&req=5

Figure 4: Plot of the normalized integrated intensities of the G and 2D band against the relative extinction for several mono- and bilayer graphene. The G and 2D band integrated intensities are normalized with the integrated intensities of an HOPG reference sample. Theoretical values of the relative extinction are indicated by two vertical dashed lines. A correlation exists between the G band intensity and the relative extinction. Two cloud of points can be easily distinguished between mono- and bilayers thanks to the relative extinction. The scattering of the G band intensity is too large to use it as an absolute thickness measurement. The 2D band scattering is even higher especially for bilayer. 2D band of AB bilayer is as intense as monolayers whereas misoriented bilayers are twice intense than a monolayer.
Mentions: Coming back to the maps in Figure 1, some correlation exists between the extinction and the G band intensity. From this observation one could conclude that the G band intensity can be used to evaluate the number of graphene layers. This is not that simple. In Figure 4, we plot the normalized integrated intensities of the G and 2D band measured on many mono and bilayer graphene (not shown here) versus the corresponding extinction values. The theoretical extinctions values are indicated as vertical dashed lines. Despite the correlation existing between the G band intensity and the relative extinction, the scattering of the G band intensity is too large when compared to the difference between a mono- and bilayer. We can even find bilayers that has the same G band intensity than monolayers. Therefore, the G band intensity cannot be used as an absolute thickness measurement but rather as a first guess if the relative extinction cannot be measured. The relative extinction is indeed not measurable in several cases: (i) if no bare SiC exists on the sample (for instance as graphene fully covers the SiC surface), (ii) if impurities (dust) or metal contacts cover the graphene and/or the SiC. In this particular case, we can evaluate the thickness by assuming that the average G band normalized intensity of a monolayer is between 0.025 and 0.03. Beware that these values depend on the experimental configuration and must be calibrated. For thin FLG (¡5 layers) the error can be of 1 layer and, for thicker FLGs, the estimated thickness can have a factor two error. Unlike the G band, the 2D band intensity cannot be used to evaluate the thickness for several reasons. For monolayers, the 2D band intensity strongly depends on the Fermi Level [34]. We used this dependance to evaluate the absolute value of the doping in the monolayer ribbon. Moreover in Figure 4, we can clearly see that the 2D band of an AB bilayer is as intense as the one of a monolayer.

Bottom Line: On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped.A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction.While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratoire Charles Coulomb, UMR5221 CNRS-Université Montpellier II, Place Eugène Bataillon - cc074, 34095 Montpellier Cedex 5, France. Antoine.Tiberj@univ-montp2.fr.

ABSTRACT
Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping.

No MeSH data available.


Related in: MedlinePlus