Effect of buffer layer coupling on the lattice parameter of epitaxial graphene on SiC(0001)

Graphene with its broad range of superlative properties is promising for several applications, and thus anticipated to play a major role in future technologies. The practical utilization of this material will require the development of synthesis processes which will allow for its fabrication on an industrial scale. One of the most promising preparation methods is the surface graphitization of silicon carbide (SiC) substrates. It makes use of high temperature thermal treatments (usually above 1400 °C) to sublimate Si atoms and create a carbon-rich SiC surface where graphene is formed. In particular, epitaxial growth on the Si-terminated (0001) face of hexagonal SiC is extremely attractive as it permits to form a single layer of graphene which is continuous over large areas. Nevertheless, in this case the graphene suffers from a strong interaction with the substrate, due to the existence of another atomic layer between it and the SiC. This interfacial layer, also known as buffer layer (BL), is expected to possess an atomic arrangement similar to that of graphene, but it is strongly coupled to the SiC substrate via covalent bonding. It is currently not well understood how the structure and morphology of the BL changes during the growth and post-growth processes and, most importantly, how such modifications will finally affect the properties of the monolayer graphene on top.

Figure 1: Reciprocal space map of monolayer graphene/BL on SiC(0001). The axes are scaled with the reciprocal lattice units (rlu) of SiC, qa and qr mark the radial and angular directions. In addition to the SiC-related reflections, two graphene-related reflections are observed (assigned as G(10-10) and G(11-20)). The appearance of these isolated reflections reveals that the graphene layer possesses a single epitaxial orientation with respect to the substrate.

Figure 2: Linescans through the G(11-20) reflection along qr, transformed to real space, performed for three different type of carbon-coverages on SiC(0001): a bare BL, monolayer graphene/BL, and a graphene bilayer. The dotted line indicates the value for the lattice constant of graphite. The x-axis is scaled to the lattice constant of graphene. Note that the graphene bilayer is strain-free, i.e. it possesses an in-plane lattice constant almost identical (2.460 Å) to that of graphite.

In order to shine further light on this issue, we have systematically investigated different types of epitaxial graphene/SiC(0001) samples using synchrotron-based grazing-incidence x-ray diffraction (GID). The GID analysis was complemented by Raman spectroscopy.

 

GID allowed us to measure the in-plane lattice parameters of single graphene layers, as well as of the BL (bare or covered by a monolayer graphene), with very high precision. Based on this, detailed information about the average strain level in each atomic layer could be obtained. We focused our attention on understanding the role played by the BL. We experimentally observed that its in-plane lattice parameter and corrugation changes when it is uncovered or beneath a monolayer of graphene. In the latter case, we made use of the GID results to unambiguously demonstrate that the BL is indeed the main responsible for the compressive strain generally measured in epitaxial monolayer graphene on SiC(0001).

 

By promoting its decoupling from the substrate via oxygen intercalation (thermal treatments in an O2-containing atmosphere leads to the oxidation of the SiC surface and breaking of the covalent bonds which connects the BL to SiC), the BL turns into a graphene monolayer.

 

Strikingly, this effect leads to a simultaneous relaxation of the uppermost monolayer graphene and formation of a strain-free graphene bilayer. The present results are of general relevance as they show that GID is a powerful tool for precise structural studies of purely two-dimensional (2D) atomic crystals.

1 Autor T. Schumann , M. Dubslaff , M. H. Oliveira Jr. , M. Hanke , J. M. J. Lopes , H. Riechert
Titel

The effect of the buffer layer coupling on the lattice parameter of epitaxial graphene on SiC(0001)

Source Phys. Rev. B , 90 , 041403(R) ( 2014 )
DOI : 10.1103/PhysRevB.90.041403 | Download: PDF | 2587 Cite : Bibtex RIS
T. Schumann, M. Dubslaff, M. H. Oliveira Jr., M. Hanke, J. M. J. Lopes, and H. Riechert