Evidence of Structural Strain in Epitaxial Graphene Layers on 6H-SiC(0001)

Abstract

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 ∼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.PACS numbers: 81.05. Uw, 65.40.De, Graphene has been shown recently to possess many of the remarkable electronic properties of carbon nanotubes [1], while lending itself more readily to the planar paradigm of integrated-circuit fabrication processes. Epitaxial graphene (epigraphene) on silicon carbide (SiC) surfaces is emerging as an attractive process alternative to the painstaking layer-by-layer exfoliation of graphite crystals [1,2,3,4,5]. Epigraphene shares the key electron transport properties of free-standing exfoliated films [1,2,3]. However, in contrast to the exfoliated films, new features in the electronic structures appear in epitaxial graphene grown on the (0001) surface of 6H-SiC, whose origins remain controversial [6,7,8,9]. The novel electronic properties and the choice of the type of substrate are significant for the design of nonlinear devices and underscores the importance of substrate interactions in epitaxial films. This Letter is focussed on Si-terminated 6H-SiC(0001).The interaction of epigraphene with SiC substrate is mediated by a monolayer of C atoms, so called "buffer layer", arranged in a honeycomb lattice, like graphene, but bonded in sp 3 configuration, with each atom forming a covalent bond to a Si atom beneath [5]. This buffer layer evolves from C-rich, high temperature surface reconstructions of 6H-SiC (0001) upon thermal desorption of Si atoms around T=1100• C. Annealing to higher temperature (1250• C) results in further desorption of Si, which promotes the formation of a second carbon layer, and deprives the original (topmost) carbon atoms of their covalent bonds to Si atoms, inducing sp 2 bonding configuration, i.e., into that of a graphene layer [2,10]. This paper presents a detailed investigation of the processes by which Si loss occurs, and its effect on the bonding, electronic and mechanical properties of the resulting carbon film. Our observations are made possible by the polarization analysis of the Raman signal from the sample surface. Although unpolarized Raman spectroscopy has been widely used in characterizing graphene layers [11,12,13,14,15,16,17,18,19], this is, to the best our k...

Uniaxial strain in graphene by Raman spectroscopy:Gpeak splitting, Grüneisen parameters, and sample orientation

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