Quantized magnetotransport is observed in 5.6 × 5.6 mm2 epitaxial graphene devices, grown using highly constrained sublimation on the Si-face of SiC(0001) at high temperature (1900 °C). The precise quantized Hall resistance of Rxy=h2e2 is maintained up to record level of critical current Ixx = 0.72 mA at T = 3.1 K and 9 T in a device where Raman microscopy reveals low and homogeneous strain. Adsorption-induced molecular doping in a second device reduced the carrier concentration close to the Dirac point (n ≈ 1010 cm−2), where mobility of 18760 cm2/V is measured over an area of 10 mm2. Atomic force, confocal optical, and Raman microscopies are used to characterize the large-scale devices, and reveal improved SiC terrace topography and the structure of the graphene layer. Our results show that the structural uniformity of epitaxial graphene produced by face-to-graphite processing contributes to millimeter-scale transport homogeneity, and will prove useful for scientific and commercial applications.

Confocal laser scanning microscopy (CLSM) is a non-destructive, highly-efficient optical characterization method for large-area analysis of graphene on different substrates, which can be applied in ambient air, does not require additional sample preparation, and is insusceptible to surface charging and surface contamination. CLSM leverages optical properties of graphene and provides greatly enhanced optical contrast and mapping of thickness down to a single layer.We demonstrate the effectiveness of CLSM by measuring mechanically exfoliated and chemical vapor deposition graphene on Si/SiO2, and epitaxial graphene on SiC. In the case of graphene on Si/SiO2, both CLSM intensity and height mapping is powerful for analysis of 1-5 layers of graphene. For epitaxial graphene on SiC substrates, the CLSM intensity allows us to distinguish features such as dense, parallel 150 nm wide ribbons of graphene (associated with the early stages of the growth process) and large regions covered by the interfacial layer and 1-3 layers of graphene.In both cases, CLSM data shows excellent correlation with conventional optical microscopy, atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, influence on the layer growth and uniformity 5,32 . Recently, KPFM was shown to be a more reliable method for distinguishing the number of EG layers 33 . Nonetheless, Raman and SPM methods are time consuming and typically limit the scan size to a few tens of micrometers. Scaling up the production process requires fast and accurate characterization of the material quality at the wafer scale, while at the same time retaining sub-micrometer spatial resolution.In this report, we demonstrate that reflection mode CLSM is a superior tool for rapid characterization of large-area graphene and graphene nanostructures on Si/SiO2 and SiC, compared to conventional optical microscopy (OM), Raman spectroscopy, AFM, conductive atomic force microscopy (C-AFM), KPFM, and scanning electron microscope (SEM) methods.CLSM can simultaneously produce intensity and topography images as well as has a lateral resolution that can be pushed beyond the optical diffraction limit. The depth-of-field is enhanced by digitally selecting in-focus regions from multiple images at different focal planes, enabling high resolution over larger areas. First, we discuss CLSM results on various thicknesses of exfoliated graphene transferred to Si/SiO2 substrate. By comparing the results to AFM and Raman measurements, we present a method for assessing the CLSM intensity and height for graphene of different thicknesses. Next, we apply CLSM and Raman spectroscopy to CVD-grown graphene transferred to Si/SiO2 substrate to demonstrate fast and accurate, large-scale analysis. Finally, we apply CLSM to epitaxial graphene on SiC demonstrating the speed, accuracy, and versatility of CLSM compared to OM, SEM, AFM, KPFM, C-AFM and Raman microscopy. RESULTS Exfoliated graphene on Si/SiO2

Regarding the improvement of current quantized Hall resistance (QHR) standards, one promising avenue is the growth of homogeneous monolayer epitaxial graphene (EG). A clean and simple process was used to produce large, precise areas of EG. Properties like the surface conductivity and dielectric loss tangent remain unstable when EG is exposed to air due to doping from molecular adsorption. Experimental results are reported on the extraction of the surface conductivity and dielectric loss tangent from data taken with a non-contact resonance microwave cavity, assembled with an air-filled, standard R100 rectangular waveguide configuration. By using amorphous boron nitride (a-BN) as an encapsulation layer, stability of EG’s electrical properties under ambient laboratory conditions is greatly improved. Moreover, samples were exposed to a variety of environmental and chemical conditions. Both thicknesses of a-BN encapsulation thickness are sufficient to preserve surface conductivity and dielectric loss tangent to within 10 % of its previously-measured value, a result which has essential importance in the mass production of millimeter-scale graphene devices demonstrating electrical stability.