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In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasi-free-standing graphene on an unintentionally-doped homoepitaxial layer grown on a conducting 4H–SiC substrate 4° off-axis from the basal [0001] direction towards [11-20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. Its morphology is studied with atomic force and scanning electron microscopy. Its few-layer character and p-type conductance are deduced from a Raman map and its layers structure determined from a high-resolution X-ray diffraction pattern. Transport properties of the graphene are estimated through Hall effect measurements between 100 and 350 K. The results reveal an unusually low sheet resistance below 100 Ω/sq and high hole concentration of the order of 1015 cm−2. We find that the material's electrical properties resemble those of an epitaxially-grown SiC PIN diode, making it an attractive platform for the semiconductor devices technology.

State-of-the-art secondary ion mass spectrometry (SIMS), X-ray reflectivity (XRR) and atomic force microscopy (AFM) have been used to determine the effect of oxidation temperature on the inhomogeneity of chemical composition and density in nanometric SiO2 films grown on n-type 4H-SiC by thermal oxidation in dry oxygen. Within the films grown at 1000–1100 °C with thicknesses of 7.6–21.1 nm, we have revealed three regions with various distributions of the Si, O and C concentrations: the transition region, adjacent to the substrate surface, where the O concentration steeply rises and the Si and C concentrations go down, the middle region, where the O, Si and C concentrations are nearly constant, and the near-surface region, adjacent to the outer surface of the oxide film, where the O concentration increases towards the surface. Based on the images of the two-dimensional distributions of the O and C concentrations and the density profiles in the oxide films, we postulate that in the transition and middle regions the density depends on the ratio of the volume of SiOxCy and SiO2 phases. In the near-surface regions, the density is mainly determined by SiO2 porosity.

In this report, we demonstrate a method for the enhancement of Raman active modes of hydrogen-intercalated quasi-free-standingepitaxial chemical vapor deposition graphene and the underlying semi-insulating 6H–SiC(0001) substrate through constructivesignal interference within atomic-layer-deposited amorphous Al2O3passivation. We find that an optimum Al2O3thicknessof 85 nm for the graphene 2D mode and one of 82 nm for the SiC longitudinal opticalA1mode at 964 cm–1enable a60% increase in their spectra intensities. We demonstrate the method's efficiency in Raman-based determination of the dielectricthickness and high-resolution topographic imaging of a graphene surface

In this report, we demonstrate a novel high-temperature Hall effect sensor that is based on quasi-free-standing monolayer graphene epitaxially grown on high-purity semiinsulating (SI) on-axis 4H-SiC(0001) substrate in a chemical vapor deposition process. To ensure statistical perspective, characteristics of 23 elements are determined as a function of temperature ranging from 300 to 770 K. Passivated with a 100-nm-thick atomic-layer-deposited aluminum oxide, the sensor offers current-mode sensitivity of 80 V/AT with thermal stability of −0.02%/K within the range between 300 and 573 K, and −0.06%/K between 573 and 770 K. The sensor’s room-temperature output voltage is monitored in the magnetic field from −300 to +300 mT and its offset voltage at 0 T is assessed. Its high-temperature electrical properties are explained through a double-carrier transport involving spontaneous-polarization-induced holes in the graphene layer and thermally activated electrons emitted from a deep acceptor level related to silicon vacancy $\text{V}_{\textsf {Si}}^{\textsf {1-/2-}}$ occupying the ${k}$ site of the 4H-SiC lattice. The sensor is compared with a previously reported one on vanadium-compensated SI on-axis 6H-SiC(0001). The new sensor’s applicability to magnetic field detection at high temperatures is verified.

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.

Graphene is considered a record-performance nonlinear-optical material on the basis of numerous experiments. The observed strong nonlinear response ascribed to the refractive part of graphene’s electronic third-order susceptibility χ(3) cannot, however, be explained using the relatively modest χ(3) value theoretically predicted for the 2D material. Here we solve this long-standing paradox and demonstrate that, rather than χ(3)-based refraction, a complex phenomenon which we call saturable photoexcited-carrier refraction is at the heart of nonlinear-optical interactions in graphene such as self-phase modulation. Saturable photoexcited-carrier refraction is found to enable self-phase modulation of picosecond optical pulses with exponential-like bandwidth growth along graphene-covered waveguides. Our theory allows explanation of these extraordinary experimental results both qualitatively and quantitatively. It also supports the graphene nonlinearities measured in previous self-phase modulation and self-(de)focusing (Z-scan) experiments. This work signifies a paradigm shift in the understanding of 2D-material nonlinearities and finally enables their full exploitation in next-generation nonlinear-optical devices., Graphene enables extraordinary nonlinear-optical refraction, far exceeding predictions based on conventional nonlinear-susceptibility theory. Here, Vermeulen et al. show that rather than the nonlinear susceptibility, a complex saturable refraction process is central to graphene’s unusual behavior.

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.

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 [cm2/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.

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.

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 [cm2/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.

The use of graphene in optical and photonic applications has gained much attention in recent years. To maximize the exploitation of graphene's extraordinary optical properties, precise control over its Fermi level (e.g. by means of chemical doping) will be of vital importance. In this work, we show the usage of a versatile p-doping strategy based on the incorporation of bis(trifluoromethanesulfonyl)amide (TFSA), functioning as an active p-dopant molecule, into a poly(2,2,3,3,4,4,5,5-octafluoropentyl methacrylate) (POFPMA) polymer matrix. The TFSA/POFPMA dopant can be utilized both onto large size graphene regions via spin coating and on small predefined spatial zones of micrometer dimension by localized inkjet printing. Whereas pure TFSA suffers from a clustered layer deposition combined with environmental instability, the application of the POFPMA polymer matrix yields doping layers revealing superior properties counteracting the existing shortcomings of pure TFSA. A first key finding relates to the optical quality of the dopant layer. We obtain a layer with an extremely low surface roughness (0.4-0.8 nm/25 μm 2) while exhibiting very high transparency (absorbance

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 (k1) 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.

We solve the long-standing discrepancy between the theoretically predicted and experimentally observed performance of nonlinear-optical refraction in graphene. Hereto we study self-phase modulation in silica-core waveguides covered with graphene.

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.

We report on the electrical characterization and Raman spectroscopy of chemical vapor deposition copper-grown graphene transferred onto a Si/SiO2 substrate by high-speed (1 mm/s) electrochemical delamination. We determine graphene’s sheet resistance, carrier mobility, and concentration as well as its physical quality as a function of the electolyte concentration. Graphene’s electrical properties are investigated with standard Hall measurements in van der Pauw geometry and a contactless method that employs a single-post dielectric resonator operating at microwave frequencies. These properties are related to the widely used copper etching technique. The results prove that the high-speed electrochemical delamination provides good-quality graphene within a short time scale.

We experimentally demonstrate a negative Kerr nonlinearity for quasiundoped graphene. Hereto, we introduce the method of chirped-pulse-pumped self-phase modulation and apply it to graphene-covered silicon waveguides at telecom wavelengths. The extracted Kerr-nonlinear index for graphene equals n(2,gr) = -10(-13) m(2) = W. Whereas the sign of n(2,gr) turns out to be negative in contrast to what has been assumed so far, its magnitude is in correspondence with that observed in earlier experiments. Graphene's negative Kerr nonlinearity strongly impacts how graphene should be exploited for enhancing the nonlinear response of photonic (integrated) devices exhibiting a positive nonlinearity. It also opens up the possibility of using graphene to annihilate unwanted nonlinear effects in such devices, to develop unexplored approaches for establishing Kerr processes, and to extend the scope of the "periodic poling" method often used for second-order nonlinearities towards third-order Kerr processes. Because of the generic nature of the chirped-pulse-pumped self-phase modulation method, it will allow fully characterizing the Kerr nonlinearity of essentially any novel (2D) material.

We present the use of ferntosecond laser ablation for the removal of monolayer grapheme from silicon-on-insulator (SOI) waveguides, and the use of oxygen plasma etching through a metal mask to peel off graphene from the grating couplers attached to the waveguides. Through Raman spectroscopy and atomic force microscopy, we show that the removal of graphene is successful with minimal damage to the underlying SOI waveguides. Finally, we employ both removal techniques to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides using the cut-back method. This loss contribution is measured to he 0.132 dB/mu m.

Controllable chemical doping of graphene has already proven very useful for electronic applications, but when turning to optical and photonic applications, the additional requirement of having both a high transparency and a low surface roughness has, to our knowledge, not yet been fulfilled by any chemical dopant system reported so far. In this work, a new method that meets for the first time this optical-quality requirement while also providing efficient, controllable doping is presented. The method relies on F4-TCNQ dissolved in methyl ethyl ketone (MEK) yielding a uniform deposition after spin coating because of an extraordinary charge transfer interaction between the F4-TCNQ and MEK molecules. The formed F4-TCNQ/MEK layer exhibits a very high surface quality and optical transparency over the visible-infrared wavelength range between 550 and 1900 nm. By varying the dopant concentration of F4-TCNQ from 2.5 to 40 mg ml(-1) MEK, the doping effect can be controlled between Delta n - + 5.73 x 10(12) cm(-2) and + 1.09 x 10(13) cm(-2) for initially strongly p-type hydrogen- intercalated graphene grown on 6H-silicon-carbide substrates, and between Delta n = + 5.56 x 10(12) cm(-2) and + 1.04 x 10(13) cm(-2) for initially weakly p-type graphene transferred on silicon samples. This is the first time that truly optical-quality chemical doping of graphene is demonstrated, and the obtained doping values exceed those reported before for F4-TCNQ-based graphene doping by as much as 50%. This work was supported by ERC-FP7/2007-2013 grant 336940, EU-FET GRAPHENICS (grant agreement no. 618086), EU-FP7 Graphene Flagship (grant agreement no. 604391), the Polish National Science Centre UMO-2013/09/N/ST5/02481, BELSPO-IAP, VUB-OZR and Methusalem.

We present a new approach to remove monolayer graphene transferred on top of a silicon-on-insulator (SOI) photonic integrated chip. Femtosecond laser ablation is used for the first time to remove graphene from SOI waveguides, whereas oxygen plasma etching through a metal mask is employed to peel off graphene from the grating couplers attached to the waveguides. We show by means of Raman spectroscopy and atomic force microscopy that the removal of graphene is successful with minimal damage to the underlying SOI waveguides. Finally, we employ both removal techniques to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides using the cut-back method. (C) 2015 Optical Society of America

Paper presents the results of investigation of the influence of protective layer on the basic functional properties of experimental graphene Hall-effect sensors. Both monolayer and bilayer type of graphene structure was investigated under external magnetic field. Measurement system for obtaining U H(B) characteristics of Hall-effect sensors was developed using Helmholtz coils as a source of magnetic field. Results of executed tests are presented in the paper as charts, which were analyzed and discussed. Finally, the conclusions were formulated, which are included in the last section of the paper.

Paper presents the results of investigation of the temperature influence on the basic functional properties of graphene Hall-effect sensors. The measurement system utilizing Helmholtz coils as a source of external magnetic field and environmental chamber for setting temperature was developed. Two types of monolayer graphene structures grown on both Si and C face of SiC substrate were investigated in the room temperature (about 20 oC) and their functional properties were compared. Next, the temperature influence on functional properties of both types of graphene structures was investigated using environmental chamber. The results of measurements are presented as charts and analyzed in the paper. On the basis of the results, conclusions were formulated, which are included in the last section of the paper.

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.

Paper presents the results of the hall effect testing in the graphene structures. Special hall effect structures were designed and build, using large graphene sheets. Laboratory testing stand was developed to test sensitivity and offset voltage in hall effect structures under external magnetic field. Characteristics of investigated structures were measured, including such impacting factors as structure size, external magnetic field strength, temperature and time.

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 similar to 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 similar to 190 Omega mu m for step height h(S) = 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. (C) 2014 AIP Publishing LLC.

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 αH

A sapphire rod resonator operating at microwave frequencies was used to determine the electric properties of 600 nm thick YBCO films in the superconducting state. The rigorous electromagnetic modelling was applied to transform the measured Q-factor and the resonant frequency to the complex conductivity of high accuracy, which was previously shown to describe the intrinsic properties of superconductor thin films in more precise manner than the complex impedance. Static external magnetic field induces typical transition to normal state due to introduction of magnetic vortices into the sample. Observed magnetic hysteresis has the origin in the strong temperature dependent pinning. Additional energy absorption at about 1.5 T was observed.

We report on microwave frequency characterization of yttrium barium copper oxide (YBCO) thin films in both normal and superconducting states. A microwave single-post dielectric resonator technique using two different resonators was used for the complex conductivity determination of YBCO samples deposited on dielectric substrates. The intrinsic complex conductivity of YBCO films was determined from the measured resonator quality factor Q and resonant frequency values employing rigorous electromagnetic modeling of the resonant structures with the mode-matching and Rayleigh-Ritz techniques. Such approach allowed us to determine the intrinsic properties of the films (conductivity, permittivity, and penetration depth) without any simplifications and errors associated with approximate modeling employing perturbation theory. We describe the superconducting material only through its intrinsic material properties, such as the complex conductivity, which does not depend on the thickness of the sample and other parameters. From both simulations and experimental results, we show that the proposed method of intrinsic complex conductivity determination is particularly useful for the characterization of very thin YBCO layers. To support the novelty of our approach, it is shown that significant differences appear between the rigorous and perturbation computations for thin superconducting films (below 50 nm).

The physics of nonlinear-optical refraction in graphene has remained unclear for almost a decade now. We solve this issue through self-phase modulation experiments in graphene-covered waveguides, revealing the extra-ordinary free-carrier interactions that underpin the refraction process.