Electrical quantum standards, e.g., quantum resistance, current, and voltage, have played a decisive role in modern metrology, particularly since the International System of Units (SI) revision in 2019. The SI unit of electrical resistance, the ohm, is implemented by a quantum Hall resistance (QHR) based on the quantum Hall effect. Furthermore, other SI units, such as the farad, the ampere, and the kilogram (realized by Kibble balance), essentially require QHR standards in their traceability route to realize their SI units. Therefore, QHR standards play an essential role in implementing the new SI. Conventional GaAs heterostructure QHR standards with the quantized resistance value of RH = h/2e2 (filling factor ν = 2) are operating under the extreme conditions of high magnetic flux density B > 10 T, limited current I < 50 µA, and low temperature T < 1.5 K. These extreme operating conditions significantly hinder the dissemination of primary resistance standards, let alone the commercialization of quantum resistance standards. Simplifying the operation conditions of QHR standards can shorten the calibration chain from primary resistance standards to the final product, resulting in higher accuracy for the end users in science, technology, and industry. Moreover, the other SI units correlated with the unit ohm can also benefit from the improved QHR standard in their traceability route. Developing a new generation of QHR standards that can operate under relaxed conditions of the lower B (< 6 T), higher I (> 100 µA), and higher T (≥ 4.2 K) is the cornerstone of the electrical quantum standards.

In the past decade, epitaxial graphene on SiC has emerged as a promising alternative to GaAs heterostructures for primary QHR standards with a 10-9 accuracy (nΩ/Ω) because epitaxial graphene is promising to realize the SI unit of resistance under relaxed conditions of B, I, and T. In this study, large-scale, high-quality single-layer epitaxial graphene is grown on semi-insulating SiC substrates. A simple, efficient, and cost-effective fabrication process based on optical lithography is successfully developed to fabricate twelve identical graphene QHR devices into a centimeter graphene chip. The intrinsic high carrier density of the as-grown epitaxial graphene is reduced and tailored by F4-TCNQ molecular doping. By precisely adjusting the F4-TCNQ dopant concentration, the carrier density is tuned to the desired values spanning from intrinsic n-type to the p-type regimes for developing QHR standards. A physical model in terms of the energy band diagram is established to explain the electron transfer mechanism between the F4-TCNQ and epitaxial graphene/SiC surface.

Furthermore, the n- and p-type epitaxial graphene is used to successfully develop the primary QHR standards, which can operate under relaxed conditions. The n-type graphene QHR standards achieved a high accuracy of less than 2 nΩ/Ω (two parts per billion, k = 2) at a moderate magnetic flux density of B = 4.5 T, high current of I = 232.5 μA, and easier to access temperature of T = 4.2 K, simultaneously. To our knowledge, this is the best performance of graphene QHR standards achieved in literature so far. The graphene QHR standards have maintained the 2 nΩ/Ω accuracy in terms of time over 2.5 years, multiple cool-down cycles over fifteen times, and long-distance shipment over 800 km between two metrology institutes. More importantly, this dissertation first systematically demonstrated that p-type epitaxial graphene can also be used for primary resistance standards, as accurate (10-9 accuracy) as GaAs and n-type graphene counterparts for realizing the SI unit ohm in quantum resistance metrology. Furthermore, a contour plot of graphene QHR standards is proposed to reveal the correlation of the quantization regime (10-9 accuracy) with magnetic field and carrier density. The contour plots serve to benchmark the graphene QHR device according to end users’ specifications. When implemented, graphene QHR standards may lead to broader dissemination of primary resistance standards beyond national metrology institutes, extending to commercial calibration laboratories and industry on-site.