Formation of High-Quality Quasi-Free-Standing Bilayer Graphene on SiC(0001) by Oxygen Intercalation upon Annealing in Air

Bilayer graphene appears as a promising candidate for future electronic applications, since it offers not only high carrier mobility and ballistic transport at room temperature, but also an electric field tunable band gap. However, even though the large-scale synthesis of a monolayer (MG) of graphene on SiC(0001) surfaces by temperature-induced sublimation of Si atoms is an established process, the preparation of homogeneous bilayer graphene is very challenging and has yet not been demonstrated. Furthermore, the graphene produced on SiC(0001) by this technique has a strong interaction with the substrate, which is mediated by a carbon monolayer (also known as buffer layer) with an atomic arrangement similar to the graphene, but covalently bonded to the SiC. Here we show that it is possible to convert epitaxial MG on SiC(0001) into decoupled bilayer graphene via oxygen intercalation upon annealing in air. During this process the buffer layer underneath the MG loses its covalent bonding to the substrate and is converted into a graphene layer due to the oxidation of the SiC surface.

Figure 1 Raman spectra of: (a) pristine monolayer graphene and (b) quasi-free-standing bilayer graphene after thermal treatment in air. The 2D peaks of the spectra in (a) and (b) are shown in detail in (c) and (d) , respectively.

Raman spectroscopy was used to investigate the samples. Spectra of graphene obtained before and after the oxygen intercalation are displayed in Fig. 1(a) and (b), respectively. The spectrum of Fig. 1(a) shows the typical features of high-quality MG, with narrow G (1590 cm-1) and 2D peaks (2727 cm-1) [see also Fig. 1(c)]. In addition, spectral structures related to the underlying buffer layer are always observed in the spectrum of the pristine sample in the range between 1200 and 1650 cm-1 [Fig. 1(a)]. The spectra collected after annealing in air [Fig. 1(b)] show typical characteristics of free-standing bilayer graphene. The 2D peak cannot be fitted by a single Lorentzian, but is well described by four of them [Fig. 1(d)]. The high quality of the material is confirmed by the absence of a defect-related D peak (1375 cm-1). Moreover, the buffer layer-related Raman features are no longer observed, indicating that indeed it has been converted into an atomic layer of graphene [Fig. 1(b)]. The high-quality bilayer graphene obtained upon air annealing is homogeneous over the entire sample surface area (1x1 cm2). Angle resolved photoelectron spectroscopy confirms the results.

 

In Fig. 2 the two π-bands of the bilayer graphene can be observed around the M-Point as well as along the Γ-Κ direction in the vicinity of the Κ-Point in the Brillouin zone. Finally, the presence of an oxided SiC interface in the thermally treated sample was confirmed by XPS (not shown). Thus, the formation of bilayer graphene occurs as follows: oxygen precursors (O2 and/or H2O) diffuse underneath the buffer layer, break the covalent bonding between this layer and the SiC, and build up an oxide-terminated SiC surface. Such a process converts the system from MG/buffer layer to a decoupled bilayer graphene. 


In contrast to earlier attempts to intercalate graphene on SiC by oxygen, the quasi-free-standing bilayer graphene produced in this work offers high-quality and is homogeneous over a large area. Most strikingly, the thermal annealing in air provides a simple and easy way to produce bilayer graphene on SiC(0001) without the need of well-controlled processes requiring either high purity atmospheres or UHV thin film deposition followed by annealing, as it is used in the other approaches for decoupling graphene.

 

The corresponding article has recently been selected by the new Editor-in-Chief of Carbon as a research highlight (link: www.journals.elsevier.com/carbon/editors-picks-carbon/).

Publication
1 Autor M. H. Oliveira Jr. , T. Schumann , F. Fromm , R. Koch , M. Ostler , M. Ramsteiner , T. Seyller , J. M. J. Lopes , H. Riechert
Titel

Formation of high-quality quasi-free-standing bilayer graphene on SiC(0001) by oxygen intercalation upon annealing in air

Source Carbon , 52 , 83 ( 2013 )
DOI : 10.1016/j.carbon.2012.09.008 | 2314 Cite : Bibtex RIS
M. H. Oliveira Jr., T. Schumann, F. Fromm, R. Koch, M. Ostler, M. Ramsteiner, T. Seyller, J. M. J. Lopes, and H. Riechert