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

摘要

Principal structural defects in graphene layers, synthesized on acarbon-terminated face, i.e. the SiC(000) face of a 4H-SiC substrate, areinvestigated using microscopic methods. Results of high-resolution transmissionelectron microscopy (HRTEM) reveal their atomic arrangement. Mechanism of suchdefects creation, directly related to the underlying crystallographic structureof the SiC substrate, is elucidated. The connection between the 4H-SiC(000)surface morphology, including the presence of the single atomic steps, thesequences of atomic steps, and also the macrosteps, and the correspondingemergence of planar defective structure (discontinuities of carbon layers andwrinkles) is revealed. It is shown that disappearance of the multistep islandleads to the creation of wrinkles in the graphene layers. The densityfunctional theory (DFT) calculation results show that the diffusion of bothsilicon and carbon atoms is possible on a Si-terminated SiC surface at a hightemperature close to 1600{\deg}C. The creation of buffer layer at theSi-terminated surface effectively blocks horizontal diffusion, preventinggrowth of thick graphene layer at this face. At the carbon terminated SiCsurface, the buffer layer is absent leaving space for effective horizontaldiffusion of both silicon and carbon atoms. DFT results show that excess carbonatoms converts a topmost carbon layer to sp2 bonded configuration, liberatingSi atoms in barrierless process. The silicon atoms escape through the channelscreated at the bending layers defects, while the carbon atoms are incorporatedinto the growing graphene layers. These results explain growth of thickgraphene underneath existing graphene cover and also the creation of theprincipal defects at the C-terminated SiC(0001) surface