Effect of oxygen adsorption on the local properties of epitaxial graphene on SiC (0001)

The effect of oxygen adsorption on the local structure and electronic properties of monolayer graphene grown on SiC (0001) has been studied by means of low-energy electron microscopy (LEEM), microprobe low-energy electron diffraction (μLEED), and microprobe angle resolved photoemission (μARPES). We show that the buffer layer of epitaxial graphene on SiC (0001) is partially decoupled after oxidation. The monitoring of the oxidation process demonstrates that the oxygen saturates the Si dangling bonds, breaks some Si–C bonds at the interface, and intercalates the graphene layer. Accurate control over the oxidation parameters enables us to tune the charge density modulation in the layer.

Figure 1

(a) LEEM image of a clean and unexposed graphene/4H-SiC (0001) surface, with an electron beam energy of 2.50 eV. (b) Typical reflectivity curves extracted from the LEEM data before and after oxidation. A typical reflectivity curve of a graphene bilayer obtained before oxidation is also presented. The local $IV$ curves have been obtained by integrating the intensity signal on a small selected area of LEEM image, while changing the primary electron energy. These curves correspond roughly to the classical (0,0) LEED spot $IV$ curve, but measured over a small micron-sized area. (c) and (d) μLEED images, taken at 45 eV, before and after the second oxidation, respectively. The images present the SiC, graphene, and buffer structures, noted SiC, G, and B, respectively.

Figure 2

(a) μLEED pattern, obtained at 36 eV, of a ($\sqrt{3}\times \sqrt{3}$)$R$30° superstructure. (b) Schematic representation of the oxygen intercalation between graphene layers, which explains this superstructure detected in LEED.

Figure 3

(a)–(c) Band dispersion as a function of $k_{//}$ around the $K$ point of the first Brillouin zone, obtained by μARPES at $h$ν $=$ 40 eV, performed before oxidation and after each oxidation step. The Fermi level and the Dirac point are superimposed on the images.

Figure 4

Sketch of the proposed oxidation process model. After the oxidation process, the oxygen penetrates the surface layer to saturate the Si dangling bonds, breaks some Si–C bonds and intercalates between the buffer and graphene layer.