We present a new fabrication method for epitaxial graphene on SiC which enables the growth of ultrasmooth defect-and bilayer-free graphene sheets with an unprecedented reproducibility, a necessary prerequisite for wafer-scale fabrication of high quality graphene-based electronic devices. The inherent but unfavorable formation of high SiC surface terrace steps during high temperature sublimation growth is suppressed by rapid formation of the graphene buffer layer which stabilizes the SiC surface. The enhanced nucleation is enforced by decomposition of polymer adsorbates which act as a carbon source. With most of the steps well below 0.75 nm pure monolayer graphene without bilayer inclusions is formed with lateral dimensions only limited by the size of the substrate. This makes the polymer assisted sublimation growth technique the most promising method for commercial wafer scale epitaxial graphene fabrication. The extraordinary electronic quality is evidenced by quantum resistance metrology at 4.2 K with until now unreached precision and high electron mobilities on mm scale devices. Main TextThe success of graphene as a basis for new applications depends crucially on the reliability of the available technologies to fabricate large areas of homogenous high quality graphene layers. Epitaxial growth on metals as well as on SiC substrates is employed with specific benefits and drawbacks.Single graphene layers epitaxially grown on SiC offer a high potential for electronic device applications. They combine excellent properties, e.g. high electron mobilities, with the opportunity for wafer-scale fabrication and direct processing on semi-insulating substrates without the need to transfer the graphene to a suitable substrate (Avouris & Dimitrakopoulos 2012). Some progress has been achieved during the recent years. In particular, high temperature sublimation growth under Ar atmosphere (Virojanadara et al. 2008),(Emtsev et al. 2009 or by confinement control (Heer et al. 2011), (Real et al. 2012) was a breakthrough for synthesizing large-area graphene on SiC substrates.The coverage of graphene bilayers could be reduced from wide stripes formed along the terraces to small micrometer-sized bilayer patches (Virojanadara et al. 2009). Further it was found that beyond pure sublimation growth from SiC graphene formation can be assisted by additional carbon supply from external sources (Al-Temimy et al. 2009;Moreau et al. 2010). In particular, by using propane in

We report on electronic transport measurements in rotational square probe configuration in combination with scanning tunneling potentiometry of epitaxial graphene monolayers which were fabricated by polymer-assisted sublimation growth on SiC substrates. The absence of bilayer graphene on the ultralow step edges of below 0.75 nm scrutinized by atomic force microscopy and scanning tunneling microscopy result in a not yet observed resistance isotropy of graphene on 4H- and 6H-SiC(0001) substrates as low as 2%. We combine microscopic electronic properties with nanoscale transport experiments and thereby disentangle the underlying microscopic scattering mechanism to explain the remaining resistance anisotropy. Eventually, this can be entirely attributed to the resistance and the number of substrate steps which induce local scattering. Thereby, our data represent the ultimate limit for resistance isotropy of epitaxial graphene on SiC for the given miscut of the substrate.

Graphene-based quantised Hall resistance standards promise high precision for the unit ohm under less exclusive measurement conditions, enabling the use of compact measurement systems. To meet the requirements of metrological applications, national metrology institutes developed large-area monolayer graphene growth methods for uniform material properties and optimized device fabrication techniques. Precision measurements of the quantized Hall resistance showing the advantage of graphene over GaAs-based resistance standards demonstrate the remarkable achievements realized by the research community. This work provides an overview over the state-of-the-art technologies in this field.

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