Paper 2D+EM+NS+PS+SS+TF-MoM6
Formation of Graphene on the C-face of SiC{0001}: Experiment and Theory

Monday, November 10, 2014, 10:00 am, Room 310

There are two {0001} surfaces of SiC, the (0001) surface known as the “Si-face”, and the (000-1) surface or “C-face”. The formation of graphene (by heating the SiC to 1100 – 1600 ° C in various gaseous environments) has been studied for both surfaces, although it is much better understood on the Si-face. In that case, an intermediate C-rich layer, or “buffer layer” forms between the graphene and the SiC crystal. This buffer layer has 6√3x6√3-R30° symmetry; its structure is well established,¹ and it acts as a template for the formation of subsequent graphene layers. In contrast, graphene formation on the C-face is much less well understood. More than one interface structure between the graphene and the SiC has been observed,^2,3 and, with one notable exception,⁴ the quality of the graphene formed on the C-face is generally lower than that for the Si-face.

In this work we provide new experimental and theoretical results that allow us to understand graphene formation on the C-face of SiC. Experimentally, by heating the SiC in a disilane environment, we map out the phase diagram of different surface and interface structures that form on the SiC as a function of disilane pressure and sample temperature. New surface structures that develop just prior to the graphene formation are observed. With additional heating, graphene forms on the surface, but some remnant of the surface structure prior to the graphene formation is believed to persist at the graphene/SiC interface. From first-principles theory, we find that the hydrogen in the disilane environment plays a critical role in the surface/interface structures that form. Experimentally, for disilane pressures below 5x10^-5 Torr, we find a 2x2 surface structure forming prior to graphene formation. From theory we identify this structure as consisting of a silicon adatom together with a carbon restatom on the surface, with H-termination of those atoms being possible but not necessary. At higher disilane pressures we observe a 4x4 structure, and we identify that as consisting of a lower density of Si adatoms than the 2x2, now with at least some of the adatoms and restatoms being H-terminated. With graphene formation, this structure converts to the observed √43x√43-R± 7.6° interface structure. At higher disilane pressures we theoretically predict the formation of a surface consisting simply of H-terminated carbon restatoms. Experiments are underway to observe that surface, along with subsequent graphene formation on the surface.

(1) Y. Qi et al., PRL 105, 085502 (2010).

(2) F. Hiebel et al., PRB 78, 153412 (2008).

(3) N. Srivastava et al., PRB 85, 041404 (2012).

(4) W. A. de Heer et al., PNAS 108, 16900 (2011).