Comparison of Epitaxial Graphene Growth on Polar and Nonpolar 6H-SiC Faces: On the Growth of Multilayer Films

We present epitaxial graphene (EG) growth on nonpolar 6H-SiC-faces by solid-state decomposition of the SiC substrate in the Knudsen flow regime in vacuum. The material characteristics are compared with those known for EG grown on polar SiC-faces under similar growth conditions. X-ray photoelectron spectroscopy (XPS) measurements indicate that nonpolar faces have thicker layers than polar faces. Among nonpolar faces, the m-plane (1 (1) over bar 00) has thicker layers than the a-plane (11 (2) over bar0). Atomic force microscopy (AFM) shows nano-crystalline graphite features for nonpolar faces, consistent with the small grain size measured by Raman spectroscopy. This is attributed to the lack of a hexagonal template, unlike on the polar Si- and C-faces. These nonpolar face EG films exhibited stress decreasing with increasing growth temperature. These variations are interpreted on the basis of different growth mechanisms on the various faces, as expected from the large differences in surface energy and step dynamics on the various SiC surfaces. Surfaces with smaller grain sizes systematically exhibited thicker layers. Using this observation, we argue that multilayer EG growth, after the nucleation of the first layers, is determined primarily by Si diffusion through grain boundaries and defects, as Si cannot diffuse through a perfect graphene lattice. A greater density of grain boundaries allows more Si to escape during growth, allowing thicker layers of carbon to be grown.