Fabrication and charge transport measurements on graphene-based nanostructures in the quantum hall regime
Yanık, Cenk (2016) Fabrication and charge transport measurements on graphene-based nanostructures in the quantum hall regime. [Thesis]
Quantum Hall effect(QHE) is not only important from fundamental physics point of view but also it provides the international resistance standard. Therefore, it has a direct impact on the whole electronics industry in terms of reaching the ultimate precision in any application. Achieving QHE at higher currents near the breakdown regime is crucial for improving the resistance standard. Graphene seems to be a good candidate for the resistance metrology towards better precision and wider application under less strict conditions due to its unique electronic properties. In this thesis, we first investigated the breakdown of the QHE in mechanically exfoliated single layer graphene samples on SiOx substrates. We found that the breakdown emerges as a gradual increase in the longitudinal resistivity rather than an abrupt jump. We have also observed that the deviation of the Hall resistance with current remains very small until an abrupt increase around jx = 5A=m. The exponential dependence of the conductivity on the current is attributed to impurity mediated inter-Landau level tunnelling of carriers. As a second study, graphene samples were suspended and electrically characterized at temperatures ranging from room temperature to 20 mK at magnetic fields between 0-12 Tesla. Various techniques were developed to fabricate suspended devices and treated them to reach ultra-high cleanliness. These techniques lead us to produce devices with charge mobility values in excess of 10⁶ cm²V⁻¹s⁻¹. We observed that in these devices, the minimum conductivity around the Dirac point can exceed the theoretically predicted value of 4e²/πh. In such monolayer graphene devices, quantum Hall filling factors v= 0, ∓1 can also emerge in the magneto-transport measurements in addition to the expected 2(2n+1) plateaus. The presence of these plateaus in these ultra high quality suspended samples indicate the lifting of the valley and spin degeneracy.
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