Stacking registry determination of graphene grown on the SiC(0001) by photoelectron holography
Graphical abstract
Introduction
Graphene, a single-layer C network in a honeycomb lattice, is a promising candidate for next generation devices and catalysts [1]. The thermal decomposition method of a SiC surface is expected for obtaining the large and uniform graphene in the industrial process. Early studies revealed the existence of the graphene precursor stage terminated with (6√3 × 6√3)-R30° periodicity of the surface C atom (hereafter the precursor layer) [2], [3], [4], [5]. This surface structure forms the graphene-like sp2 bond but partially bonded to the SiC substrate. Further Si elimination leads to the epitaxial growth of single-layer graphene (SLG) on the interface buffer layer. Interaction with the buffer layer and the substrate greatly affects the electronic properties of graphene [6]. Hence, comprehending the local structure of the graphene precursor and buffer layers and the relation in early growing stage are important for improving the performance of graphene devices. However, the subsurface atomic structure of SLG has been still controversial although various kinds of surface analyses have been attempted.
Two neighboring C atoms of a crystalline graphite with Bernal (AB) stacking are inequivalent, which leads to the observations of a triangular lattice by scanning tunneling microscopy (STM) and three-fold symmetric low-energy electron diffraction (LEED) patterns. The STM atomic images [7], [8], [9], [10] as well as the LEED patterns [11], [12] of the bilayer graphene on the SiC(0001) surfaces are also three-fold symmetric, suggesting that the two graphene sheets are in the relation of Bernal stacking. However, the observed STM images [7], [8], [9], [10] and LEED patterns [11], [12] were six-fold symmetric in the case of SLG on the buffer layer. These observations have been regarded as the indication of SLG decoupled from the substrate. Riedl et al. demonstrated that the precursor and buffer layers can be decoupled from the substrate and transformed to single and bilayer graphenes by hydrogen intercalation breaking SiC bonds in reversible manner [13]. This demonstration suggests the conservation of the Bernal-like registry of SLG on graphite-like buffer layer, while some STM and LEED works proposed AA stacking model [10], [12]. Most of theoretical works assumed or predicted AB stacking model as the favorable structure for the registry of SLG on the graphite-like buffer layer [14], [15], [16], [17], [18], but the direct experimental determination is not yet achieved.
Photoelectron from a localized core level is an excellent element-selective probe for surface structure analysis [19], [20]. Forward focusing peaks (FFPs) appearing in the photoelectron intensity angular distribution (PIAD) indicate the directions of surrounding atoms as seen from the photoelectron emitter atom [21]. We have studied the layer-specific atomic structure of the 4H-SiC(0001) surface at each stage of SLG formation using photoelectron diffraction (PED) [22]. The FFPs indicating the directions of the neighboring atoms seen from the C atoms directly bonded to the SiC substrate were observed for the precursor and the interface buffer layers. In this study, we succeeded in reconstructing the local atomic arrangement images of SLG as well as the buffer and the precursor layers by photoelectron holography. The stacking registry of SLG on the buffer layer was determined to be AB type.
Section snippets
Experimental methods
All experiments were performed at the circularly-polarized soft-X-ray beamline BL25SU of SPring-8, Japan [23]. The sample preparation attached directly to two-dimensional display-type spherical mirror analyzer (DIANA) [21], [24], [25], [26], [27], [28], [33]. Catalyst-referred etching (CARE) single-crystalline 4H-SiC(0001) wafer (N-doped, 0.02 Ω·cm) with an on-axis oriented Si face (0 ± 0.5° off) was used as the substrate [29], [30]. The wafer has an atomically flat and damage-free surface with a
Results and discussion
Fig. 1(a) and (b) shows the C 1s PIADs of the precursor layer and SLG with the buffer layer which were deduced by considering the dependence of escape depth on emission angle and subtracting the SiC substrate signal intensity. The measured 2π-sr PIADs were projected using equidistant cylindrical mapping and extended to 4π-sr by flipping the z-coordinate to prepare for the fitting process [36]. Note that the emission polar angle was corrected by considering the refraction effects due to the
Conclusion
In conclusion, we have characterized the subsurface structure on top of the 4H-SiC(0001) at two defined thermal graphitization stages, the graphene precursor terminated with (6√3 × 6√3)-R30° periodicity and SLG, by using photoelectron holography. The local atomic configurations in the precursor layer and SLG together with the buffer layer were reconstructed. The existence of local buckling and no significant structural difference were confirmed on both the precursor and buffer layers. The
Acknowledgements
This work was performed with the approval of the Japan Synchrotron Radiation Institute, Proposal No. 2009B1769, 2010A1469, and 2011A1471. The authors express deepest gratitude to Dr. Tetsuya Nakamura and Dr. Takayuki Muro for their experimental supports of the experiments. This study was supported by a Grant-in-Aid for JSPS Fellows, No. 11J09374 and JSPS Grant-in-Aid for Scientific Research on Innovative Areas “3D Active-site Science”: Grant Number 26105007 2604.
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