National Physical Laboratory

Probing interface structure of quasi-free standing graphene

We investigate the surface and interface structure of quasi-free standing graphene (QFSG) on SiC, obtained by hydrogen intercalation using surface enhanced Raman scattering (SERS) and X-ray photoelectron spectroscopy (XPS). The structural changes of the QFSG are correlated with the changes in the electronic properties such as work function (Kelvin probe force microscopy) and carrier concentration and mobility (transport measurements in the van der Pauw geometry).

Graphene grown on SiC(0001) exhibits intrinsic electron doping due to charge transfer from the interfacial layer (IFL). IFL is a (6√3 x 6√3)R30 reconstructed carbon layer, topographically similar to graphene, but with a significant amount of carbon atoms still covalently bonded to the SiC(0001) surface, altering the graphene-like electronic properties and reducing the mobility in the first graphene layer. Hydrogen intercalation breaks the C-Si bonds and creates Si-H bonds, decoupling the IFL and converting it to a quasi-free standing one layer graphene (QFS 1LG).

The interface between SiC and graphene was investigated using SERS, which enhances the Si-H stretching mode (which is not measurable using conventional Raman spectroscopy) of the hydrogen bonded to the Si atoms of the SiC substrate and thus proves the passivation of substrate with hydrogen. Furthermore, the Raman analysis suggests that in QFS 2LG, hydrogen also penetrates in between the graphene layers to locally form C-H defects. The transport measurements in the van der Pauw geometry revealed change of the carrier type from n- to p-type, accompanied by a more than four times increase in carrier mobility. Using calibrated Kelvin probe force microscopy in vacuum, we were able to image the surface of the as-grown and intercalated graphene and resolve the graphene layers and work function distribution with nanometre resolution.

This work provides fundamental insights into the physics of intercalated graphene and highlights the superior quality and higher carrier mobility of QFS 1LG compared to as-grown and QFS 2LG, making QFS 1LG a preferred material for high-speed analogue electronics.

Probing interface structure of quasi-free standing graphene

Read the full paper online at 2D Materials

More on NPL's work on Graphene

For further details, please contact Christos Melios and Olga Kazakova

Last Updated: 23 May 2016
Created: 17 May 2016

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