Structure and properties of incommensurate and commensurate phases of graphene on SiC(0001)
Conrad, Matthew David
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Realizing a technologically relevant graphene semiconductor has been one of the key challenges for advancing graphene electronics. Recently, a semiconducting form of graphene was discovered in the first graphene layer that grows on SiC, often called the ``buffer layer." Its semiconductor character has been attributed to functionalization through sp3 bonding to the SiC interface. As such, the buffer layer is the only known highly ordered functionalized graphene system and it has correspondingly gone from being a ``dead layer" and a nuisance in the production of monolayer graphene to one of the most important examples of functionalized graphene. However, the mechanism (bonding geometry, strain, confinement, etc.) causing the buffer layer's semiconducting properties has remained illusive due to the dual challenge of a large, computationally demanding, interface structure and the experimental difficulty of uniform buffer layer growth. In this thesis I present the first surface x-ray diffraction (SXRD) measurements of the interface structure using improved buffer growth conditions. SXRD measurements reveal a new interface system with an incommensurate mutual modulation of the graphene and SiC interface. For the first time, electronic structure calculations using the SXRD derived interface structure provide an explanation for the semiconducting buffer. The structure of the buffer layer and SiC interface is investigated further through a combined x-ray standing wave (XSW) and x-ray reflectivity (XRR) analysis to reveal possible origins of the incommensurate structure resulting from a depleted Si interface. Additionally, I compare compare films (buffer only and buffer+ML) formed under different growth conditions. This work demonstrates that the properties of the buffer layer are malleable and can be altered. These results indicate that controlling the buffer layer interface is a viable platform for graphene band gap engineering and paves the way for advancing graphene electronics.