Multiscale investigation of graphene layers on 6H-SiC(000-1)
© Tiberj et al; licensee Springer. 2011
Received: 6 September 2010
Accepted: 24 February 2011
Published: 24 February 2011
In this article, a multiscale investigation of few graphene layers grown on 6H-SiC(000-1) under ultrahigh vacuum (UHV) conditions is presented. At 100-μm scale, the authors show that the UHV growth yields few layer graphene (FLG) with an average thickness given by Auger spectroscopy between 1 and 2 graphene planes. At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist. This is confirmed at the nanometer scale by scanning tunneling microscopy (STM). Finally, STM (at the nm scale) and Raman spectroscopy (at the μm scale) show that the FLG stacking is turbostratic, and that the domain size of the crystallites ranges from 10 to 100 nm. The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.
The unique electronic, optical, and mechanical properties of graphene [1–3] give rise to an intense research activity for both scientific and technological purposes. Among these research activities, special effort is devoted to develop preparation techniques [4–12] which yield large-scale graphene wafers of high quality and uniformity. Today, one of the most promising methods for microelectronic applications consists in a controlled sublimation of a few Si atomic layers from a single crystalline SiC surface [9–20]. The remaining C atoms rearrange themselves and form few layer graphene (FLG), often called "epitaxial graphene." Such FLG samples can be grown either on the Si face (0001) of a SiC substrate or on the C face (000-1). Graphene growth on the Si face has been extensively studied in the last few years [9–12]. It has been shown that large, homogeneous graphene monolayers and bilayers can be obtained on top of a SiC surface reconstruction [13–15]. The graphene planes are Bernal (AB) stacked. The interface between the first graphene plane and the SiC surface is composed of an intermediate C-rich layer having covalent bonds with Si atoms of the substrate [13–15]. Epitaxial graphene on the Si-face is usually highly n-type doped (around 1013 cm-2) with a low carrier mobility (usually few thousands cm2 V-1 s-1).
On the C face, the situation is completely different. There is no need for a buffer layer anymore but two different pristine surface reconstructions exist below the graphene layers: (2 × 2)C and (3 × 3) SiC reconstructions, and the graphene layers have several orientations on top of each surface reconstruction [16–18]. The interaction between graphene layers and the C face of SiC substrate is reduced compared to the one existing on the Si face. Graphene grown on (3 × 3) SiC surfaces experience the weakest interaction with the underlying substrate. This weaker interaction between the graphene layers and the SiC substrate may be one of the reason for the better carrier mobility measured on epitaxial graphene on C-face (27000 cm2 V-1 s-1) . It also explains why, long, self-ordered, strain-free graphene ribbons can be grown on large reconstructed terraces . Besides, it has been shown that the interaction with the environment impacts also the transport properties of exfoliated graphene . It is therefore of primary importance to study the graphene/SiC interaction. The focus of this article will be on FLG grown in ultrahigh vacuum (UHV) on 6H-SiC (000-1). Surface reconstruction will be probed by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). Thanks to previous studies [17, 18], SiC surface reconstruction, graphene orientation, and stacking can be determined from the Moiré patterns observed in STM. The observed crystallite size and stacking will be compared to those from Raman spectroscopy performed on the same sample. Despite the different sizes of the probed area between STM (microscopic) and Raman spectroscopy (macroscopic), a very good agreement has been found.
Scanning tunneling microscopy
The in-plane sizes of the crystallites L a ranges from 20 to 60 nm, which are in excellent agreement with previous STM observations.
Laser power fluctuations are corrected thanks to an additional low-noise photodiode that measures the laser power during the acquisition of the Raman map, and the estimated thickness found is shown in Figure 4a. At the scale of the Raman probe, the FLG coverage ranges from two to seven graphene planes, with an average of two to three graphene planes. This is one monolayer thicker than the value deduced from AES and STM experiments. Such discrepancy occurs because of the different sizes of areas probed by Raman spectroscopy (1 μm), AES (100 μm), and STM (few nm up to 300 nm). Moreover, STM focuses always on the most interesting area of the sample (i.e., the thinnest FLG) where bare SiC surface, mono, and few layers can be measured. However, it has already been shown in Figure 2a,b that thick multilayer flakes grow close to the edges of SiC steps. This is not peculiar to these images. The growth rate is always higher at step edges. It can also be stated from Figure 4a that the thickest FLGs measured by Raman spectroscopy are located at discrete spots with a small lateral extension (< 2 μm). This is the case of multilayers seen in Figure 2a, while the majority of the probed area corresponds to FLG with less than three graphene planes. As previously said, using only the G band-integrated intensity, one can have an error of one graphene plane. The thickness estimated from Raman mapping is then consistent with STM results. Concerning AES, the agreement is poorer, and it would be interesting to perform a more detailed cross calibration of AES vs Raman spectroscopy to better understand the correlation between these two different techniques.
The FLG has been grown on 6H-SiC (000-1) in UHV conditions. An average thickness of one to two graphene planes was found from AES, whereas Raman spectroscopy results indicate an average thickness of two to three graphene planes. This small discrepancy might occur because of the uncertainty of one to two layers for thickness determined from Raman spectroscopy and from the difference of the spot size between these two techniques. LEED and STM experiments show (i) a (3 × 3) SiC surface reconstruction, (ii) a wide distribution of disorientation between the first graphene sheet and the SiC surface, and (iii) rotational stacking fault between the graphene layers corresponding to a turbostratic stacking for the multilayers. The single Lorentzian shape of the 2D Raman band measured on these FLG confirms this turbostratic stacking. The FLG domain size (deduced from the I D/I G ratio) ranges from 20 to 60 nm, which is in excellent agreement with the graphene crystallites size probed by STM. Finally, the most striking result is that the D, G, and 2D bands are highly blue shifted (+30, +28, and +60 cm-1, respectively). This means that the graphene lattice is highly compressively strained (around -0.4/-0.5%). Usually, FLG grown on the C face of SiC are fully relaxed by forming wrinkles to release the thermal stress during the cooling down of the sample after the growth. For this particular case, the strain might arise because graphene crystallite edges are bound to the SiC surface.
Auger electron spectroscopy
few layer graphene
highly oriented pyrolytic graphite
low energy electron diffraction
scanning tunneling microscopy
This study was financially supported by the French ANR ("GraphSiC" Project No. ANR-07-BLAN-0161), and by the Région Rhône-Alpes ("Cible07" and "Cible08" programs). F.H. holds a doctoral support from la Région Rhône-Alpes.
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