外延石墨烯中的量子霍爾區域:載流子摻雜、傳輸和應用

Abstract

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Since the Nobel-prize-winning discovery of graphene in 2004, it has proven to be one of the most important and versatile material due to its extraordinary properties. From a metrological perspective, graphene grown on silicon carbide - known as epitaxial graphene - has been shown to be an ideal platform to build resistance standards. From its robust and precise ν = 2 plateau, to its tunable charge carrier density, and long-term stability, all of its properties make graphene an excellent candidate to build quantum Hall resistance standards. The experiments described in this thesis, further our understanding of graphene in the quantum Hall regime. We first attempted to use nitric acid as an adsorbent to understand the dynamics of desorption process in epitaxial graphene. The timescales associated with such a process are extracted from the data. This is important in order to fabricate devices with reversible hole doping without the use of a gate. Transport properties were measured on several devices post-nitric- acid-exposure at temperatures between 300 K and 1.5 K. In order to replicate the laboratory conditions, ambient conditions are applied. This would help in recreating similar conditions as to the ones in a laboratory when such devices are handled using this type of chemical vapor doping. Raman spectroscopy is used as a comparative tool to verify the timescales extracted from the transport measurements. Hot electrons are expected to cause local disruption of quantized conduction in two-dimensional systems and may lead to breakdown of the quantum Hall effect. The extent of local heating is generally limited to small regions near contacts where a large current enters and leaves the device, since the existence of the electrochemical potential that produces the Hall voltage is established at these points. We investigated the effect of current introduced at downstream points near where the Hall voltage is typically measured. For contact separation as small as a few micrometers we did not observe substantial interactions due to the internal distribution of current, even when a resistance of hundreds of ohms of is generated by dissipation at the side contacts. Using the above knowledge, the limiting factors for developing metrology grade epitaxial graphene quantum Hall arrays are lifted with a combination of the implementation of closely spaced superconducting contacts, high-quality material, and centimeter-scale growth. The ν = 2 Landau level has been the basis of a single quantized value that modern day resistance standard devices have been restricted to. Here, we demonstrate multiple quantized values from our devices, which could be used to disseminate the ohm all around the world. These devices are designed such that they give access to quantized resistances values from the standard 12.9 kΩ to more than three orders of magnitude, as high as 1.29MΩ. Several experimental methods such as Raman spectroscopy and standard electrical characterization using lock-in amplifier techniques are employed herein to verify the quality and demonstrate the versatility of these devices.