Variable range hopping conduction in the epitaxial graphene buffer layer on SiC(0001)
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The properties of epitaxial graphene grown by thermal decomposition of hexagonal silicon carbide (SiC) have been the focus of extensive research for several decades now. In this thesis I am interested in the electronic transport properties of the first graphene layer grown on the (0001) of SiC, referred to as the buffer layer. It has been shown previously that the buffer layer is structurally a continuous graphene layer subject to periodic interactions with the underlying substrate. Electronically, the band structure presents a gap around the Fermi level. While much effort has been devoted to the surface science of the buffer layer, little is known about the actual dynamics of its charge carriers. To shed light on its properties as an electronic material, I performed temperature and bias voltage dependent electronic transport measurements on buffer layer devices. The buffer layer is found to display an insulating behavior with the conductivity following a 2D Mott variable range hopping model between 80K and 420K. At lower temperatures, the hopping can also be activated by a strong bias electric field. An effective temperature can be defined to take into account both temperature and electric field, which gives an estimate for the localization length of electrons in the buffer layer between 1 and 2 nm. A model of localization based on the image potential experienced by charges in the corrugation of the buffer layer is presented to explain the observed transport properties.