The thermal decomposition of SiC(0001) surface is a promising industrial-scale graphene growth method; however, the crystalline quality of graphene still has considerable scope for improvement, for which it is essential to understand the growth process and clarify governing factors. In this study, we dedicated ourselves in theoretically investigating the C clustering (nucleation) process, with an aim to clarify the template effect of the SiC(0001) surface for the initial stage of graphene growth.

We studied the C clustering process at a single-bilayer SiC step tilting towards the [1120] or [1100] direction via the tight-binding total-energy calculation [1]. The clustering process was determined as follows: 1) a hexagonal C cluster with the lowest energy was placed on the SiC(0001) surface. 2) C atoms were added one-by-one to various sites around the cluster. 3) The most stable structure of the C cluster up to C₃₀ was determined.

Our numerical analysis revealed that penta-heptagonal/purely hexagonal structures appear at the C-terminated [1100] (C-[1100]) step or at the [1120] and Si-[1100] step. At the [1120] and Si-[1100] steps, C clusters bonded with the step edge and formed a 3D round structure, while those at the C-[1100] step extended to the terrace and formed the 2D island structure.

C clusters on the terrace exhibit penta-heptagonal structure so that they can relax the lattice mismatch between graphene and SiC(0001) surface [2]. This result on the terrace analogously explains the appearance of 2D penta-heptagonal structures at the C-[1100] step. In addition, the purely hexagonal configuration can be attributed to the 3D structure because the 3D structure is partly apart from the surface and free from the lattice mismatch. The dimension of the cluster (2D or 3D) is subject to the edge decollation by C clusters. This trend is determined by the SiC step structure, i.e. dangling bonds on the step edge. Hence, we revealed the structural controllability of C clusters by the template effect of SiC steps.

Acknowledgments

This work was partially supported by a Grant-in-Aid for JSPS Fellows (40-7700). We thank Prof. S. Tanaka of Kyushu University for useful discussions and comments.

References

[1] M. Yu et al., Physica E 42 (2009) 1-16.

[2] M. Inoue et al., Physical Review B 86 (2012) 085417-1-7.

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