Physics
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Item Surface Studies of Graphene and Graphene Substrates(2013) Burson, Kristen M.; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene has attracted a great deal of attention for its exceptional electronic and mechanical properties. As graphene, a two-dimensional lattice of carbon atoms, is an `all surface' material, its interactions with the underlying substrate play a crucial role in determining graphene device behavior. In order to tailor graphene device properties, the interaction between graphene and the underlying substrate must be clearly understood. This thesis addresses the question of the relationship between graphene and graphene substrates by considering both the substrate topography and the impact of charged impurities in the substrate. Utilizing scanning tunneling microscopy and high-resolution atomic force microscopy, we measure the topography of silicon dioxide (SiO2) supported graphene and the underlying SiO2(300nm)/Si substrates. We conclude that the graphene adheres conformally to the substrate with 99% fidelity and resolve finer substrate features by atomic force microscopy than previously reported. To quantify the density of charged impurities, simultaneous atomic force microscopy (AFM) and Kelvin probe microscopy are used to measure the potential and topographic landscape of graphene substrates, SiO2 and hexagonal boron nitride (h-BN). We find that the surface potential of SiO2 is well described by a random two-dimensional surface charge distribution with charge densities of ~1011 cm-2, while BN exhibits charge fluctuations that are an order of magnitude lower than this. Charged impurities in the substrate present a scattering source for transport through graphene transistors, and the difference in magnitude in measured substrate charged impurities densities for SiO2 and BN is consistent with the observed improvement in charged carrier mobility in graphene devices on h-BN over graphene devices on SiO2. Finally, this thesis presents a theoretical model elucidating the challenges of imaging corrugated substrates by non-contact AFM and an experimental work using Kelvin probe microscopy to characterize the electrostatic potential steps at interfaces of small-molecule organic heterojunctions.Item Kelvin Probe Microscopy Studies of Epitaxial Graphene on SiC(0001)(2011) Curtin, Alexandra E.; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Epitaxial graphene on SiC(0001) presents a promising platform for device applications and fundamental investigations. Graphene growth on SiC(0001) can produce consistent monolayer thickness on terraces and good electronic properties. In exfoliated graphene on SiO2, random charged impurities in the SiO2 surface are thought to be the dominant scatterers, explaining the observed transport properties as well as the spatial charge inhomogeneity seen in scanned-probe experiments. In contrast, the scattering mechanisms and charge distribution in epitaxial graphene remain relatively unexplored. Here I use Kelvin probe microscopy (KPM) in ambient and UHV conditions to directly measure the surface potential of epitaxial graphene on SiC(0001). Ambient-environment KPM on graphene/SiC(0001) shows surface potential variations of only 12 meV. Taken together with transport measurements, the data suggest that the graphene samples in ambient are in the low-doped regime, near the minimum conductivity of roughly 4e2/h. I am also able to use UHV KPM of graphene/ SiC(0001) to identify the discrete surface potentials of monolayer and bilayer graphene as well as the insulating interfacial carbon layer and bare SiC, correlated with scanning electron micrographs of the same location. The surface potential differences between monolayer and bilayer graphene and between IFL and monolayer graphene are both suggestive of low doping (≤1012 cm-2). The surface potentials of monolayer and bilayer graphene are relatively smooth, while the IFL and bare SiC, in contrast, showed larger variations in surface potential suggesting the presence of unscreened charged impurities present on the IFL that are later screened by the overgrown graphene. I model the potential variations for unscreened and graphene-screened charged impurities using the self-consistent theory of graphene developed by Adam et al. The results show that although surface potential variations are, as expected, larger in the IFL than in graphene, both surfaces display surface potential variations 10-40 times smaller than predicted by theory. While ambient electronic transport data and surface potential steps suggest our samples are only lightly doped (≤1012 cm-2), in a regime dominated by electron-hole puddles, we do not observe these puddles in UHV. The absence of puddles in UHV leaves the source of doping in these samples an open question.