Atomic and electronic structure of few-layer graphene on SiC(0001) studied with scanning tunneling microscopy and spectroscopy

Epitaxial growth of graphene on SiC surfaces by solid state graphitization is a promising route for future development of graphene based electronics. In the present work, we have studied the morphology, atomic scale structure, and electronic structure of thin films of few-layer graphene (FLG) on SiC(0001) by scanning tunneling microscopy and spectroscopy (STS). We show that a quantitative evaluation of the roughness induced by the interface is a tool for determining the layer thickness of FLG. We present and interpret thickness dependent tunneling spectra, which can serve as an additional fingerprint for the determination of the layer thickness. By performing spatially resolved STS, we find evidence that the charge distribution in bilayer graphene is inhomogeneous.

Figure 1

(Color online) STM images of SiC(0001) covered with 1 ML graphene taken at a large bias voltage of $U_T=-1.85\mathrm{V}$ $(I_T=7\mathrm{pA})$. The $z$ scale is $0.00–0.866\mathrm{\AA }$ in (a) and $0.00–0.83\mathrm{\AA }$ in (b).

Figure 2

(Color online) STM images of FLG films with well-defined thicknesses. (a) 0 ML ($U_T=-540\mathrm{mV}$, $I_T=8\mathrm{pA}$); $z$ scale is $0–191\mathrm{pm}$. (b) 1 ML ($U_T=-114\mathrm{mV}$, $I_T=7\mathrm{pA}$); $z$ scale is $0–96\mathrm{pm}$. (c) 2 ML ($U_T=-0.114\mathrm{mV}$, $I_T=5\mathrm{pA}$); $z$ scale is $0–78\mathrm{pm}$. (d) 3 ML ($U_T=-110\mathrm{mV}$, $I_T=5\mathrm{pA}$); $z$ scale is $0–28\mathrm{pm}$. The roughness caused by the interface layer decreases with increasing layer thickness (see text).

Figure 3

Interface-induced roughness $R$ as a function of layer thickness $t$ in ML. For explanation, see text.

Figure 4

(Color online) (a) Atomically resolved structure of 1 ML graphene imaged at $U_T=-114\mathrm{mV}$ and $I_T=7\mathrm{pA}$ ($z$ scale is $0–82\mathrm{pm}$). (b) 3 ML graphene imaged at $U_T=-110\mathrm{mV}$ and $I_T=5\mathrm{pA}$ ($z$ scale is $0–10\mathrm{pm}$). (c) Bilayer graphene imaged at $U_T=-0.114\mathrm{mV}$, $I_T=5\mathrm{pA}$ ($z$ scale is $0–75\mathrm{pm}$). The honeycomb structure of graphene is indicated by hexagons showing equivalent atoms for a monolayer case and inequivalent atoms for the trilayer case and part of the bilayer case.

Figure 5

(Color online) Creation of an $AB∕A{A}^{\prime }$ transition. The bilayer in the top graph follows the natural $AB$ stacking. Shifting the bottom layer by the SiC surface unit vector ${\vec{s}}_2$ results in a stacking close to an $AA$ configuration (therefore $A{A}^{\prime }$) as shown in the bilayer on the left hand side. The STM image of such a configuration is expected to look more like that of a graphene monolayer.

Figure 6

(Color online) (a) Atomically resolved STM image of a graphene-covered SiC step imaged at $U_T=-0.159\mathrm{mV}$ and $I_T=5\mathrm{pA}$. The $z$ scale is $0–0.52\mathrm{nm}$. (b) 3D visualization of the image shown in (a). (c) Close-up of the upper terrace. (d) Close-up of the lower terrace.

Figure 7

(Color online) Height distribution of the image shown in Fig. 6a. The left and the right maxima correspond to the lower and upper terraces of the step, respectively. The step height is $\Delta h=0.40\mathrm{nm}$. The small maximum corresponds to the small terrace on the side of the step in the lower half of the image in Fig. 6a.

Figure 8

(Color online) Explanation of the step height observed in Fig. 6a and quantified in Fig. 7. For details, see text.

Figure 9

(Color online) Representative $dI∕dV$ spectra of FLG with different thicknesses. For clarity, the spectra are vertically offset from each other and their zeros indicated by tick marks.

Figure 10

(Color) (a) Experimental $dI∕dV$ curves (black) of FLG films with 2 ML thickness together with the calculated DOS (red) before (solid) and after (dashed) Gaussian broadening with ${\omega }_G=0.2\mathrm{eV}$. The top panel shows the details of the band structure at the $K$ point. [(b) and (c)] Same as (a) for $d=3$ and 4 ML, respectively. For details about the calculation, see text. Corresponding features in the broadened DOS and experimental $dI∕dV$ spectra are marked by arrows.

Figure 11

(Color) Spatially resolved STS spectra of 2 ML graphene on SiC(0001) in a color scale image. The positions corresponding to minima in the interface-induced height variation and to the maximum in between are indicated by arrows on the right hand side and are labeled depression and rim, respectively. The vertical dashed line indicates the bias voltage at which atomically resolved images were taken.