Electron transport studies in epitaxial graphene on SiC

Licentiatavhandling, 2013

Furthermore, the dispersion in graphene k is light-like for graphene monolayers implying that electron transport would behave relativistically. If vsat = vf could be achieved in the material, then it is theoretically possible to achieve THz performance in long channel devices. Despite the nice theoretical picture, Nature is hardly so forthcoming. The prime objective of this work is to measure vsat in both as-grown and H-intercalated epitaxial graphene on 4H-SiC and 6H-SiC substrates. Hall measurements indicate that electron transport in Hintercalated material is found to be limited by impurity scattering. In the impurity scattering limit, one can infer a speed limit on the saturated electron velocity of vsat 2 · 107cm/s in epitaxial H-intercalated monolayers. This figure is definitive, as it sets epitaxial graphene on the same level as other semiconductor materials regarding the potential for frequency performance. In as-grown material it is possible to achieve a slightly higher vsat, but the material is generally very non-uniform. Both materials also suffer from the absence of a band-gap making device design intractable. In order to understand vsat in epitaxial graphene, theoretical and experimental approaches are needed. The consolidated theoretical model of ideal graphene is presented in detail. Graphene’s phonon spectrum, electronic band structure, and vf are derived from first principles. Band structure in bilayer graphene is also addressed and compared to the monolayer case. A possible solution to the band-gap problem is provided in the description of a graphene bilayer with an applied vertical electric field. Useful calculations are also shown regarding the density of states in monolayers and bilayers. The low vsat and high carrier density in epitaxial graphene motivate a semi-classical picture of electron transport via the Boltzmann Transport Equation. Special attention is directed towards the temperature dependence of phonon and long range and scattering mechanisms. Also described is an experimental characterization of epitaxial graphene layers on SiC. Since graphene monolayers and bilayers demonstrate very different physical properties, a method to accurately determine the number of layers is needed. Layer characterization via Raman spectroscopy is described in the context of theoretical and experimental results. Hall measurements are also shown for as-grown and H-intercalated layers providing valuable information about the mobility μ, sheet resistivity sh, and carrier density nsh. Experimental results obtained from pulsed IV measurements are also shown eventually bringing the discussion back to vsat. The nature of velocity saturation is then described in the context of the temperature dependent transport and scattering processes. In order to perform electrical measurements on graphene, a robust and minimally invasive fabrication strategy has been developed using both photolithography and electron beam lithography. These processes are designed in such a way as to preserve the quality of the epitaxial layer while providing outstanding contact resistance c < 0.2 · mm. Surface characterization is also performed via Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) to provide intuition regarding sense morphology. Finally, some potential solutions are motivated from a device context. Future experimental work will implement these with the aim of fabricating a high speed device.