Magnetotransport characterization of epitaxial graphene on SiC

Doktorsavhandling, 2012

Low-temperature magnetotransport is used to characterize graphene grown epitaxially on the silicon face of 4H silicon carbide (SiC/G). Observation of half-integer quantum Hall effect (QHE) in large Hall bars, patterned across several terraces of the SiC substrate, suggest that monolayer graphene grows continuously over defects. Complete characterization was possible using carrier density control technologies developed for SiC/G, including organic dielectrics, photochemical gating and a solid electrolyte. The photochemical gating with organic polymers, achieved by using a spacer layer directly in contact with graphene that protects its integrity, followed by a layer that responds to light, is envisioned as a prototypical architecture for the development of graphene-based sensors. Fine details of electron scattering were found through measurement of quantum corrections to the conductivity of SiC/G, arising from weak localization (WL) and electron-electron interactions (E-E). It was found that scattering is determined by charged impurities under graphene, while the effect of terraces is proposed to manifests as intervalley scattering. The extracted temperature dependence of the decoherence rate allowed to identify E-E interactions and to suggest spin-flip centers as sources of dephasing in the system. The analysis of WL provided an indirect measurement of the spin relaxation time in SiC/G, at the level of 50 ps. Altogether, this work contributed to develop the first application in which graphene outperforms conventional semiconducors, in the field of quantum metrology. The half integer QHE in SiC/G is proposed as standard for electrical resistance to replace GaAs heterostructures. A direct comparison with the QHE in GaAs, the most strict universality test of the QHE ever performed, supports the hypothesis that the electrical resistance is quantized in units of h/e2, with an uncertainty of 0.084 parts per billion. The accuracy of the comparison was limited by the critical current in the GaAs sample, 4 times lower than in the SiC/G sample.