Ref HAL: hal-03037523_v1
DOI: 10.4028/www.scientific.net/MSF.711.235
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We present the growth and characterization of epitaxial Graphene on the (000-1) and (11-20) planes. In both cases, the growth was carried out in a RF furnace, by implementing our technique of confined atmosphere, covering the SiC substrate with a graphitic cap during the growth. The grown material was investigated by means of AFM, SEM, Raman spectroscopy and magneto transport. Contrary to the (0001) face, in both faces (000-1) and (11-20), almost free standing Graphene monolayers of very high quality are grown. These Graphene sheet are uniform, continuous, almost strain-free and lightly doped. In both faces, Hall bars were fabricated and Shubnikov-de Haas oscillations typical of Graphene, as well as the Half Integer Quantum Hall Effect are observed.

Commentaires: 4th Workshop on Advanced Semiconductor Materials and Devices for Power Electronics Applications (HeteroSiC & WASMPE 2011), Tours, FRANCE, JUN 27-30, 2011

Résumé:

The Raman bands intensities, frequencies, widths and profiles of few layer graphene (FLG) are extremely sensitive to doping, strain, stacking order, number of graphene sheets, crystalline quality and laser wavelength. For samples of epitaxial graphene grown on silicon carbide, the laser wavelength is basically the only factor known a priori. Furthermore, the concomitance of second order SiC Raman modes in the graphene G and D bands region complicates the extraction of the graphene signal. Developing a robust characterization tool enabling to disentangle all these effects and to provide reliable information is thus highly needed and challenging. We propose an approach where micro-Raman spectroscopy is combined with reflectivity and transmission measurements to discriminate between the different contributions in the measured Raman signatures and to access to the detailed characteristics of the samples. Here, we demonstrate the efficiency of this method in order to determine the number of graphene layers. We address then the question of using the 2D band width or ratio of the 2D and G bands intensities as a criteria to distinguish between mono and multi layer(s) graphene.

Ref HAL: hal-00803575_v1
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A detailed comparison of true epitaxial graphene monolayers grown on both faces of 6H SiC substrates (Si and C faces) is made by combining micro-Raman spectroscopy with transmission measurements. We have already shown that such combination allows to discriminate without any ambiguity between a graphene monolayer and a twisted or folded graphene bilayer (AA') grown on the C face of 6H SiC [1]. In this presentation, we will focus on the Raman spectra and the transmission measurements performed on graphene monolayers grown on the Si face of 6H SiC. The graphene growth was tuned to get a mixed surface at the early stage of graphitization with i) bare SiC, ii) buffer layer and iii) in some localized areas small monolayers flakes on top of the buffer layer. These unique samples enabled to measure properly the Raman spectrum of the buffer layer (carbon layer with a large number of sp3 bonds) and its optical extinction at 514.5 nm. The Raman spectrum of this buffer layer remains visible after the growth of one monolayer on top but the Raman intensity is strongly reduced (typically divided by a factor of 3). This cannot be attributed to the absorption coefficient of graphene which is relatively low (few percents). Finally, several Raman mapping reveal the uniformity of the graphene monolayers in terms of thickness and crystalline quality, but also that they are subjected to a non uniform compressive strain.

Ref HAL: hal-03037526_v1
DOI: 10.4028/www.scientific.net/AMR.324.269
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We report magnetotransport measurements in single epitaxial graphene layers grown on the C-face of an 8 degrees off-axis 4H-SiC substrate using high temperature annealing conditions with a graphite cap covering the sample. The graphene sheets were found p-type doped, with mobilities varying between 1000 and 11000 cm(2/)V.s from device to device at 1.6 K. We examine the signature of weak localization and universal conductance fluctuations at weak magnetic field and we show that the phase coherence lengths extracted from the two phenomena are in satisfactory agreement.

Commentaires: 1st Mediterranean Conference on Innovative Materials and Applications (CIMA 2011), Beirut, LEBANON, MAR 15-17, 2011

Ref HAL: hal-00666145_v1
PMID 21711670
DOI: 10.1186/1556-276X-6-141
WoS: 000290525700050
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4 Citations
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Using high-temperature annealing conditions with a graphite cap covering the C-face of, both, on axis and 8 degrees off axis 4H-SiC samples, 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 magneto-transport measurements. On the best samples, we find a moderate p-type doping, a high-carrier mobility and resolve the half-integer quantum Hall effect typical of high-quality graphene samples. A rough estimation of the density of states is given from temperature measurements.

Ref HAL: hal-00666151_v1
DOI: 10.1166/nnl.2011.1118
WoS: 000293211200009
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11 Citations
Résumé:

We review the results of growing few layer graphene on alpha-SiC with different surface orientations. To this end we have used successively a pure {000-1} C face, a 8 degrees-off one, a nonpolar {11-20} surface and, finally, a 8 degrees-off Si face and a pure (exactly {0001}-oriented) Si one. Without any need for complex hydrogen annealing, we have shown that a pure C or a 8 degrees-off C or even a pure {11-20} surface allows the growth of single layer epitaxial graphene sheets that are strain-free, p-type, low doped and exhibit reasonably high carrier mobility. This is not the case on pure Si or 8 degrees-off Si faces on which strained material is constantly found with heavy n-type doping and low carrier mobility.

Ref HAL: hal-00666152_v1
PMID 21801347
DOI: 10.1186/1556-276X-6-478
WoS: 000296252700001
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21 Citations
Résumé:

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 mu m) 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.