Role of structure of C-terminated 4H-SiC(000(1)over-bar) surface in growth of graphene layers: Transmission electron microscopy and density functional theory studies

The principal structural defects in graphene layers, synthesized on a carbon-terminated face, i.e., the SiC(000 (1) over bar) face of a 4H-SiC substrate, are investigated using microscopic methods. Results of high-resolution transmission electron microscopy (HRTEM) reveal their atomic arrangement. The mechanism of such defects' creation, directly related to the underlying crystallographic structure of the SiC substrate, is proposed. The connection between the 4H-SiC(000 (1) over bar) surface morphology, including the presence of the single atomic steps, the sequence of atomic steps, and also the macrosteps, and the corresponding emergence of planar defective structure (discontinuities of carbon layers and wrinkles), is revealed. It is shown that the disappearance of the multistep island leads to the stress-related creation of wrinkles in the graphene layers. The density functional theory (DFT) calculation results show excess carbon atoms convert a topmost carbon layer to the sp(2)-bonded configuration, liberating Si atoms in the barrierless process. The DFT results show that the diffusion of carbon atoms is essentially impossible at the C-terminated SiC surface. On the contrary, DFT results prove that diffusion of the silicon atoms is possible on the C-terminated SiC surface at a high temperature close to 1600 degrees C. Because, according to TEM studies, at the carbon-terminated SiC surface, the buffer layer is absent, that creates a channel for effective horizontal diffusion of both silicon atoms under the graphene layer. Ultimately the silicon atoms escape could be facilitated by the channels created at the bending layer defects (wrinkles). The sp(2)-bonded carbon atoms are incorporated into the growing graphene layers, which contribute to stress in the growing layers, detachment from SiC support, and partial contribution to the creation of wrinkles. These results explain the phenomenon of the growth of thick undulated graphene layers by subsequent creation of the new layer underneath the existing graphene cover and also the creation of the principal defects in graphene at the C-terminated SiC(000 (1) over bar) surface.