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Register a new userEvidence of structural strain in epitaxial graphene layers on 6H-SiC(0001)
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The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows to resolve submonolayer growth, including individual, localized C=C dimers in a diamond-like carbon matrix for AES C/Si ratio of $sim$3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio $>$6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.
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The thermal decomposition of SiC surface provides, perhaps, the most promising method for the epitaxial growth of graphene on a material useful in the electronics platform. Currently, efforts are focused on a reliable method for the growth of large-area, low-strain epitaxial graphene that is still lacking. We report here a novel method for the fast, single-step epitaxial growth of large-area homogeneous graphene film on the surface of SiC(0001) using an infrared CO2 laser (10.6 {mu}m) as the heating source. Apart from enabling extreme heating and cooling rates, which can control the stacking order of epitaxial graphene, this method is cost-effective in that it does not necessitate SiC pre-treatment and/or high vacuum, it operates at low temperature and proceeds in the second time scale, thus providing a green solution to EG fabrication and a means to engineering graphene patterns on SiC by focused laser beams. Uniform, low-strain graphene film is demonstrated by scanning electron microscopy and x-ray photoelectron, secondary ion mass, and Raman spectroscopies. Scalability to industrial level of the method described here appears to be realistic, in view of the high rate of CO2-laser induced graphene growth and the lack of strict sample-environment conditions.
The evolution in the surface morphology of epitaxial graphene films and 6H-SiC(0001) substrates is studied by electron channeling contrast imaging. Whereas film thickness is determined by growth temperature only, increasing growth times at constant temperature affect both internal stress and film morphology. Annealing times in excess of 8-10 minutes lead to an increase in the mean square roughness of SiC step edges to which graphene films are pinned, resulting in compressively stressed films at room temperature. Shorter annealing times produce minimal changes in the morphology of the terrace edges and result in nearly stress-free films upon cooling to room temperature.
We have analyzed by Scanning Tunnelling Microscopy (STM) thin films made of few (3-5) graphene layers grown on the C terminated face of 6H-SiC in order to identify the nature of the azimuthal disorder reported in this material. We observe superstructures which are interpreted as Moire patterns due to a misorientation angle between consecutive layers. The presence of stacking faults is expected to lead to electronic properties reminiscent of single layer graphene even for multilayer samples. Our results indicate that this apparent electronic decoupling of the layers can show up in STM data.
We present a structural analysis of the graphene-4HSiC(0001) interface using surface x-ray reflectivity. We find that the interface is composed of an extended reconstruction of two SiC bilayers. The interface directly below the first graphene sheet is an extended layer that is more than twice the thickness of a bulk SiC bilayer (~1.7A compared to 0.63A). The distance from this interface layer to the first graphene sheet is much smaller than the graphite interlayer spacing but larger than the same distance measured for graphene grown on the (000-1) surface, as predicted previously by ab intio calculations.
This paper has been withdrawn due to the adherance to the double submission policies of a refereed journal. Our apologies.
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