| Abstract Scope |
The thermal decomposition of silicon carbide (SiC) has proven itself as a feasible technique for producing high-quality graphene. Thanks to the efforts of researchers from around the globe, progress has been made towards better understanding and controlling epitaxial graphene formation on SiC. For instance, it is now well-known that SiC thermal decomposition begins at ledges, which are intrinsic to SiC substrates with a vicinal off-cut. This knowledge is due, in part, to the fact that all decomposition studies to date have begun with SiC substrates with varying degrees of vicinal off-cut. This raises a poignant question: How would a step-free surface affect thermal decomposition? For the first time, step-free SiC was homoepitaxially grown and subsequently thermally decomposed at various temperatures under vacuum (low-10^-5 mbar) to explore graphene formation in the absence of surface steps. Mesas with various percentages of step-free coverage were formed to create stepped and step-free regions next to each other on the exact same mesa. Atomic force microscopy (AFM), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) were used to study graphene formation on step-free surfaces and contrast the formation against vicinally stepped surfaces. Through AFM studies, it was found that graphene formation on step-free surfaces begins with the clustering of surface vacancies. These clusters form triangular pits that grow in area through lateral erosion along the SiC{1-100} planes. As erosion continues through Si desorption, the triangular pits merge and leave behind a graphitized surface. This formation is in contrast with stepped surfaces that erode along the existing ledges created by the vicinal cut of the substrate. Further differences in the graphene film on the step-free and stepped region are apparent from Raman spectroscopy and HRTEM. After thermal decomposition at 1475C for 10 min, Raman spectroscopy demonstrates that stepped regions have a blueshifted 2D peak position of 16 cm^-1 and a larger peak width by nearly 40 cm^-1 as compared to step-free regions. This difference is attributed to film thickness, which is measured by cross-sectional HRTEM. Stepped regions have a graphitic film nearly 2 nm thick as compared to less than 0.7 nm for step-free regions. This thickness variation is attributed to differences in surface energy of the stepped and step-free regions as described by the terrace-ledge-kink model. The vicinal ledges have a relatively higher surface energy as compared to the step-free surface. As such, the vicinal surface ledges are prone to faster Si desorption and faster graphene formation. |