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Quantum Transport in Misoriented Layers of Graphene: Physics and Device Applications

  • Author(s): Habib, K M Masum
  • Advisor(s): Lake, Roger K
  • et al.

Graphene is one of the promising candidates for the channel material of future electronic devices. Negligible spin-orbit coupling combined with high carrier mobility and long mean free path make graphene a very attractive material for post CMOS device applications. The individual layers in a misoriented or twisted stack of graphene behave as if they were electronically decoupled due to destructive quantum interference. The interlayer coupling is increased and the Fermi velocity is reduced in presence of a vertical electric field and negative differential conductance is predicted at small biases. These properties of misoriented graphene can potentially be exploited in novel switching mechanisms. In order to utilize these exceptional properties in device applications, it is important to understand if these phenomena still hold in the limit of nanoscaled device dimensions.

Our numerical simulations show that the coherent electronic decoupling between the layers of two-dimensional misoriented bilayer graphene is still present in lower dimensions when the misoriented region is reduced to the nanometer scale. We found a novel current switching mechanism in nanoscaled misoriented graphene layers that utilizes the voltage controlled quantum interference of electrons to achieve large, rapid modulation of the current with small voltage swings. Utilizing the voltage controlled quantum interference between standing electronic waves we demonstrated an oscillatory current voltage response suitable for multi-state switching. This switching mechanism does not rely on a bandgap or a potential barrier. Thus, it is not limited by the thermal limitation of 60 mV/dec.

The coherent, interlayer resistance of a misoriented, rotated interface in vertically stacked graphene is determined for a variety of misorientation angles. The fundamentally limiting quantum-resistance of the ideal interface with θ = 0o is on the order of 10−3 Ωμm2. For small rotations, the coherent interlayer resistance is a strong function of the Fermi energy, and it exponentially approaches the ideal quantum resistance at energies away from the charge neutral point. At room temperature, the total inter-layer resistance can still be sensitive to the rotation angle changing one to two orders of magnitude as the angle changes by a few degrees. Over a range of intermediate angles, the coherent resistance is much larger than the phonon-mediated resistance which results in a relatively constant total resistance on the order of 100 Ωμm2.

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