Graphene obtained by annealing of silicon carbide (SiC) substrates has recently attracted a lot of interest as a promising material for future carbon-based electronics. On the contrary to graphene made by micromechanical cleavage [1], epitaxial graphene [2] is  compatible with industrial processing techniques.  However, the electronic and optical properties of the epitaxial graphene can be substantially modified by interaction with the SiC substrate.

Raman spectroscopy has been proved to be very efficient and useful method to study single- and multi-layer graphene structures. In this communication Raman spectroscopy  of epitaxial graphene are presented. Investigated samples were prepared in Epigress VP508 SiC hot wall CVD reactor on 4H-SiC(0001) substrates with Si and C polarity as well as with exact and 4° off orientation. The appearance of single and/or multiple graphene layers on the SiC surface was verified by Scanning Tunneling Microscopy and Transmission Electron Microscopy. Micro-Raman experiments where performed at room temperature, using 532 nm line from Nd-YAG as a source of continuous wave excitation. Highly oriented pyrolitic Graphite (HOPG) was used as a reference sample.        

It is observed that the Raman shifts of characteristic 2D bands observed for layers deposited on Si-face of the SiC substrates are blue shifted with respect to the 2D peak observed for the graphene made by micromechanical cleavage [4] as well as the G’ band of reference HOPG sample. Interestingly the blue shift as well as the FWHM of the 2D transition observed for the 4⁰ off-axis substrate is larger than measured for on-axis sample. The observed effects could be explained in terms of strain induced by 4H-SiC substrates. Other mechanisms including unintentional doping are also discussed.

[1]  K.S. Novoselov et al. Science 306, 666 (2004)

[2] C. Berger et al. J. Phys. Chem. B 108, 19912 (2004)

[4] A. C. Ferrari, Solid State Commun. 143, 47 (2007)