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Kelvin Probe Force Microscopy is a method to assess the contact potential difference between a sample and the probe tip. It remains a relative tool unless a reference standard with a known work function is applied, typically bulk gold or cleaved highly oriented pyrolytic graphite. In this report, we suggest a verifiable, two-dimensional standard in the form of a photolithographically patterned, wire-bonded structure manufactured in the technology of transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating high-purity nominally on-axis 4H-SiC(0001). The particular structure has its hole density p S = 1.61 × 1013 cm−2 measured through a classical Hall effect, its number of the graphene layers N = 1.74 extracted from the distribution of the ellipsometric angle Ψ , measured at the angle of incidence AOI = 50 ° and the wavelength λ = 490 nm, and its work function ϕ G R = 4.79 eV postulated by a Density Functional Theory model for the specific p S and N. Following the algorithm, the contact potential difference between the structure and a silicon tip, verified at Δ V G R − Si = 0. 64 V , ought to be associated with ϕ G R = 4.79 eV and applied as a precise reference value to calculate the work function of an arbitrary material. [Display omitted] • QFS graphene on SiC as a KPFM reference standard. • Electrically verifiable and updatable. • A likely complement to HOPG and bulk gold. [ABSTRACT FROM AUTHOR]

The ability to precisely measure magnetic fields under extreme operating conditions is becoming increasingly important as a result of the advent of modern diagnostics for future magnetic-confinement fusion devices. These conditions are recognized as strong neutron radiation and high temperatures (up to 350 °C). We report on the first experimental comparison of the impact of neutron radiation on graphene and indium antimonide thin films. For this purpose, a 2D-material-based structure was fabricated in the form of hydrogen-intercalated quasi-free-standing graphene on semi-insulating high-purity on-axis 4H-SiC(0001), passivated with an Al2O3 layer. InSb-based thin films, donor doped to varying degrees, were deposited on a monocrystalline gallium arsenide or a polycrystalline ceramic substrate. The thin films were covered with a SiO2 insulating layer. All samples were exposed to a fast-neutron fluence of ≈ 7 × 10 17 cm−2. The results have shown that the graphene sheet is only moderately affected by neutron radiation compared to the InSb-based structures. The low structural damage allowed the graphene/SiC system to retain its electrical properties and excellent sensitivity to magnetic fields. However, InSb-based structures proved to have significantly more post-irradiation self-healing capabilities when subject to proper temperature treatment. This property has been tested depending on the doping level and type of the substrate. [ABSTRACT FROM AUTHOR]

The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps ~10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be ~190 Ωµm for step height hS = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications. [ABSTRACT FROM AUTHOR]

This paper presents recent progress in the development of the new type of Hall-effect sensor with the graphene layer acting as sensing material. Newly developed Hall-effect sensor is made of quasi-free-standing bilayer graphene structure. The graphene structure is placed inside the standard QFN-32 package for integrated circuits and mounted on the miniature printed circuit board with four leads, which allowed obtaining operational magnetic field sensor. Basic functional properties of the developed sensor were investigated as well as time drift and temperature dependence of these parameters. Performed investigation indicates high linearity of the sensor within the tested range of an external magnetic field. The measured current-related sensitivity of the sensor is about 50 V/AT, without any amplification of the output signal from the graphene structure. Also, long-term tests of sensitivity and offset voltage were performed. The results indicate high stability of the investigated parameters within long time. Obtained results are very promising and indicate the possibility of utilization of the developed sensor in measurement application. [ABSTRACT FROM AUTHOR]

Abstract In this report we demonstrate the results of charge carriers transport studies in graphene using a Hall effect sensor fabricated on quasi-free-standing monolayer graphene grown on a semi-insulating on-axis vanadium-compensated 6H-SiC(0001) substrate in an epitaxial Chemical Vapor Deposition process. The sensor is passivated with aluminum oxide through atomic layer deposition and offers current-mode sensitivity of 140 V/AT with thermal stability of − 0.02%/K within the range between 80 and 573 K. The electrical properties of the graphene layer are determined as a function of temperature ranging from 300 to 770 K. High-temperature characteristics of passivated and not passivated graphene are compared and their profiles explained through a double-carrier transport involving the spontaneous-polarization-induced holes in the graphene layer and the thermally activated electrons from a shallow donor level of nitrogen in the quasi-cubic (k 1) site and a deep acceptor level of vanadium in the hexagonal (h) site both present in the bulk of the vanadium-compensated SiC substrate. Finally, we conclude that this mechanism is directly responsible for the limitation of the thermal stability of the sensor's current-mode sensitivity. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

In this report, we demonstrate the preparation method of a multi-layer stack with a pre-defined number of graphene layers, which was obtained using chemical vapor deposition graphene deposited on a copper substrate and subsequently transferred onto a poly(methyl methacrylate) (PMMA) substrate. The prepared multi-layer stack can also be transferred onto an arbitrary substrate and in the end, the polymer can be removed, which in consequence significantly increases the range of possible graphene applications. The multi-layer character was confirmed by optical transmittance measurements and Raman spectroscopy, whereas the microstructure of the multi-layer graphene stack was investigated using Scanning Electron Microscopy. The electrical properties in the function of the number of graphene layers were assessed with standard Hall Effect measurements. Finally, we showed the practical application of the multi-layer graphene stack as a saturable absorber of a mode-locked Er-doped fiber laser. [ABSTRACT FROM AUTHOR]

In this report, we demonstrate a complete Hall effect element that is based on quasi-free-standing monolayer graphene synthesized on a semi-insulating on-axis Si-terminated 6H-SiC substrate in an epitaxial Chemical Vapor Deposition process. The device offers the current-mode sensitivity of 87 V/AT and low excess noise (Hooge's parameter aH<2α10-3) enabling room-temperature magnetic resolution of 650 nT/Hz0.5 at 10 Hz, 95 nT/Hz0.5 at 1 kHz, and 14 nT/Hz0.5 at 100 kHz at the total active area of 0.1275mm². The element is passivated with a silicone encapsulant to ensure its electrical stability and environmental resistance. Its processing cycle is suitable for large-scale commercial production and it is available in large quantities through a single growth run on an up to 4-in SiC wafer. [ABSTRACT FROM AUTHOR]

Epitaxial Chemical Vapor Deposition growth of graphene on silicon carbide offers the maturity and reliability expected for large-scale fabrication of graphene-based devices. In particular, the ultimate challenge of graphene synthesis on SiC, i.e. quasi-free-standing monolayer graphene which comes through hydrogen atom intercalation of the sole buffer layer grown on the Si-face of SiC, offers high carrier mobility (as high as 6600 [cm 2 /Vs]) and electrical stability throughout the device processing cycle. In this report, we present extensive statistics of the electrical properties of QFS-monolayer graphene grown on 4H(0001) and 6H(0001) semi-insulating 10 mm × 10 mm substrates, being a result of 110 individual processes. The adopted explanation for the origin of the as-grown doping level in epitaxial graphene based on the spontaneous polarization of hexagonal SiC and its most up-to-date values is reaffirmed. We introduce the issue of the step-edge-induced offset voltage radial dependence and confront it with the morphological analysis of the average step edge height and terrace width, all related to the place of origin of a specific sample within a 4-inch SiC wafer. Finally, we conclude that within the step edge area QFS-monolayer graphene is statistically nearly half as resistive as the previously reported QFS-bilayer graphene. [ABSTRACT FROM AUTHOR]

We demonstrate the demodulation of a multi-Gb/s ON–OFF keying (OOK) signal on a 96 GHz carrier by utilizing a 250-nm graphene field-effect transistor as a zero bias power detector. From the eye diagram, we can conclude that the devices can demodulate the OOK signals up to 4 Gb/s. [ABSTRACT FROM PUBLISHER]

Epitaxial Chemical Vapor Deposition growth of graphene on silicon carbide is one of the most promising technologies to realize graphene-based electronics. Particularly, the quasi-free-standing bilayer which comes through hydrogen atom intercalation of monolayer graphene grown on the Si-face of SiC offers high carrier mobility (as high as 5000 [cm 2 /Vs]) and electrical stability throughout the device processing cycle. In this report, we present a statistical perspective on transport properties of QFS-bilayer graphene grown on 4H(0 0 0 1) and 6H(0 0 0 1) SiC, being a result of 460 individual processes on semi-insulating 10 mm × 10 mm substrates. The mutual relation between charge carrier concentration and mobility is determined and analyzed to raise the awareness of the effect of SiC hexagonality on charge transport in graphene. Special attention is paid to the applicability of quasi-free-standing bilayer graphene on SiC, primarily in magnetic field detection. The issue of the step-edge-induced offset voltage anisotropy in Hall effect sensors is introduced and a method to minimize it is presented. [ABSTRACT FROM AUTHOR]

This letter presents the design, fabrication and characterization of the first graphene based monolithic microwave integrated circuit (MMIC) in microstrip technology operating in W-band. The circuit is a resistive mixer in a 250 nm graphene field effect transistor (G-FET) technology on a SiC substrate. A conversion loss of 18 dB is achieved which is limited by the available local oscillator (LO) power. The mixer exhibits a flat response over radio frequency (RF) range of 90-100 GHz. The RF bandwidth is also limited by the measurement setup. [ABSTRACT FROM PUBLISHER]

In this report, we demonstrate an applied protocol for layer-resolved Raman imaging and analysis of undesirable ad-layers found in Chemical Vapor Deposition graphene grown on copper foil and transferred onto an oxidized silicon substrate. The method assumes that the intensity of the silicon-related Raman-active mode at 520 cm-1 is attenuated by 2.3 % each time the light passes through a single graphene layer. Upon normalization with respect to a reference graphene-free area, the 520 cm-1 mode relative intensity r-I Si measured in a back-scatter mode follows a univalent function of the number of the graphene layers N. Since N is treated as a continuous argument, it can be ascribed a fractional value and considered statistically. Importantly, the r-I Si offers higher layer differentiation capability and unambiguity than non-functional indicators, including the 2D band width or the 2D-to-G band intensity ratio, thus providing unequivocal evaluation. [Display omitted] • Method for high-resolution Raman imaging of CVD graphene. • Truly functional and univalent protocol for layer-resolved analysis. • Applicable to multilayer graphene. • Useful extension for the Graphene Flagship technical standard. [ABSTRACT FROM AUTHOR]

In this paper, we report on the first experimental study on the impact of neutron radiation on quasi-free-standing (QFS) graphene. For this purpose, we have fabricated hydrogen-intercalated QFS graphene on semiinsulating high-purity 4H-SiC(0001), passivated it with an Al 2 O 3 layer, and exposed it to a fast-neutron fluence of ≈ 6. 6 × 1 0 17 cm−2. The results have shown that the graphene sheet is only moderately affected by the neutron radiation with the estimated defect density of ≈ 4 × 1 0 10 cm−2. The low structural damage allowed the Al 2 O 3 /graphene/SiC system to maintain its electrical properties and an excellent sensitivity to magnetic fields characteristic of QFS graphene. Consequently, our findings suggest that the system may be a promising platform for magnetic diagnostics in magnetic-confinement fusion reactors. However, the scope of its use should be a subject of further study. In this context, we have explored possible modes of damage and have concluded that the main factor that affects the electrical parameters of the structure is the impact of neutrons on the layer of hydrogen atoms saturating the SiC(0001) surface. We have shown, employing density functional theory (DFT) computations, that damage to the intercalating layer could lower hole concentration in graphene via reduced charge polarization and local coupling on the interface. [Display omitted] • Quasi-free-standing properties of graphene persist after neutron irradiation. • Post-radiation damage to the H intercalation lowers the carrier density in graphene. • A degree of self-healing in the system via surface diffusion of hydrogen. • Temperature above 200 °C facilitates the reconstruction of damaged H intercalation. [ABSTRACT FROM AUTHOR]

• Partial H interaction impacts properties of quasi-free-standing graphene (QFG). • Reduced H coverage results in effective n-type doping of the graphene sheet. • Reduced H coverage degrades the linear energy dispersion of bands π and π ∗ . • At low coverage of H strong chemical bonds are observed at the interface. • Graphene at 0.25 and 0.5 H interaction retains qualitative characteristics of QFG. Graphene has attracted huge attention due to its unique electronic properties, however, when supported those are significantly dependent on the interface interactions. One of the methods of decoupling graphene sheets from a substrate is hydrogen intercalation, which has been shown to produce quasi-free-standing (QFS) layers on a SiC(0001) surface. Still, the effects of incomplete H termination of SiC remain mostly unknown. This work investigates, employing density functional theory calculations, the impact of partial termination on the structural, and electronic properties of graphene. It is predicted that interfaces with partially damaged H layer or produced under a lower technological standard could still benefit from the intrinsic, however, quantitatively reduced, properties of QFS graphene. [ABSTRACT FROM AUTHOR]

In this report, we present transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on 15-mm × 15-mm semi-insulating vanadium-compensated on-axis 6H SiC(0001), characterized in that its room-temperature direct-current Hall-effect-derived hole mobility μ p = 5019 cm2/Vs, and its statistical number of layers (N), as indicated by the relative intensity of the SiC-related Raman-active longitudinal optical A 1 mode at 964 cm−1, equals N = 1.05. The distribution of the ellipsometric angle Ψ measured at an angle of incidence of 50 ° and λ = 490 nm points out to N = 0.97. The close-to-unity value of N implies that the material under study is a close-to-perfect quasi-free-standing monolayer, which is further confirmed by High-Resolution Transmission Electron Microscopy. Therefore, its spectroscopic properties, which include the Si H peak at 2131 cm−1, the histograms of Ψ and Δ , and the Raman G and 2D band positions, widths, and the 2D-to-G band intensity ratios, constitute a valuable reference for this class of materials. • Quasi-free-standing single-layer graphene in an industrial technology • Direct observation of the hydrogen-related Si-H peak at 2131 cm-1 • Parameters for effective ellipsometric characterization of QFS graphene on SiC • Referential database of graphene spectroscopic properties • Novel approach to graphene certification [Display omitted] [ABSTRACT FROM AUTHOR]

We show that monolayer graphene can be grown isothermally on polycrystalline copper foils via ultra-high vacuum chemical vapor deposition (UHV-CVD), using acetylene as a carbon precursor. The growth is self-limiting, yielding monolayer graphene with a quality comparable to that of graphene grown by atmospheric- or low-pressure chemical vapor deposition. Copper sublimation, a typical concern for UHV-CVD, is shown to be suppressed by growing graphene domains. Further, the roughness of the copper surface after growth is similar to that of copper foils after growth processes at higher pressures. A dependency of the growth kinetics on the surface orientation of the copper grains is observed and a growth model including all stages of growth is presented and discussed. Similar to observations at higher growth pressures, the graphene domains possess sigmoidal growth, however the overall growth behavior is more complicated with two subsequent growth modes. The role of hydrogen is investigated and shows that, contrary to reports for higher growth pressures, dissolved hydrogen in the copper foil plays an essential role for graphene growth whereas ambient hydrogen does not have a noticeable influence. [ABSTRACT FROM AUTHOR]