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

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 us to resolve submonolayer growth, including individual, localized $\mathrm{C}=\mathrm{C}$ dimers in a diamondlike carbon matrix for AES $\mathrm{C}/\mathrm{Si}$ ratio of $\sim 3$, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES $\mathrm{C}/\mathrm{Si}$ 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.

Figure 1

Effects of the scattering depolarization on the second order scattering from the 6H-SiC (0001) bulk, and with 1.5L epigraphene. The vertical dotted line indicates the position of the epigraphene $G$ peak, $\sim 1592{\mathrm{cm}}^{-1}$.

Figure 2

Depolarized Raman spectra of the (a) $D$ and $G$ peak and (b) $D^{\prime }$ peak regions. The average thicknesses of the epigraphene films (grown for the same annealing time, 8 min) are determined by the C:Si AES intensity ratio. Dotted lines in panels (a) and (b) correspond to the position of the $G$ and $D^{\prime }$ peak, respectively, for 1 layer exfoliated graphene [12]. The evolution of $G$ and $D^{\prime }$ Raman shift lines corresponding to $\sim 1$ layer are plotted as functions of the annealing times [panel (c)].