Electron transport properties of graphene and graphene field-effect devices studied experimentally

Doktorsavhandling, 2013

This thesis contains experimental studies on electronic transport properties of graphene with the Aharonov-Bohm (AB) effect, thermopower (TEP) measurements, dual-gated graphene field effect devices, and quantum Hall effect (QHE). Firstly, in an effort to enhance the AB effect in graphene, we place superconducting-metal (aluminium) or normal-metal (gold) mirrors on the graphene rings. A significant enhancement of the phase coherence effect is conferred from the observation of the third harmonic of the AB oscillations. The superconducting contribution to the AB effect by the aluminium (Al) mirrors is unclear. Instead, we believe that a large mismatch of Fermi velocity between graphene and the mirror materials can account for the enhancement. Secondly, TEP measurement is performed on wrinkled inhomogeneous graphene grown by chemical vapour deposition (CVD). The gate-dependent TEP shows a large electron-hole asymmetry while resistance is symmetric. In high magnetic field and low temperature, we observe anomalously large TEP fluctuations and an insulating quantum Hall state near the Dirac point. We believe that such behaviors could be ascribed to the inhomogeneity of CVD-graphene. Thirdly, dual-gated graphene field effect devices are made using two gates, top- and back-gates. In particular, the top gate is made of Al deposited directly onto the middle part of the graphene channel. Naturally formed Al2O3 at the interface between Al and graphene can be facilitated for the dielectric layer. When the Al top-gate is floating, a double-peak structure accompanied by hysteresis appears in the graphene resistance versus back-gate voltage curve. This could indicate an Al doping effect and the coupling between the two gates. Lastly, we notice that the QHE is very robust in CVD-graphene grown on platinum. The effect is observed not only in high- but also low-mobility inhomogeneous graphene decorated with disordered multilayer patches.