We report the first observation of the quantum Hall effect in epitaxial graphene. The result described in the submitted manuscript fills the yawning gap in the understanding of the electronic properties of this truly remarkable material and demonstrate suitability of the silicon carbide technology for manufactiring large area high quality graphene. Having found the quantum Hall effect in several devices produced on distant parts of a single large-area wafer, we can confirm that material synthesized on the Si-terminated face of SiC promises a suitable platform for the implementations of quantum resistance metrology at elevated temperatures and, in the longer term, opens bright prospects for scalable electronics based on graphene.
In this paper we review the recent extraordinary progress in the development of a new quantum standard for resistance based on graphene. We discuss the unique properties of this material system relating to resistance metrology and discuss results of the recent highest-ever precision direct comparison of the Hall resistance between graphene and traditional GaAs. We mainly focus our review on graphene expitaxially grown on SiC, a system which so far resulted in the best results. We also brie y discuss progress in the two other graphene material systems, exfoliated graphene and chemical vapour de…
We demonstrate that the carrier concentration of epitaxial graphene devices grown on the C-face of a SiC substrate is efficiently modulated by a buried gate. The gate is fabricated via the implantation of nitrogen atoms in the SiC crystal, 200 nm below the surface, and works well at intermediate temperatures: 40K-80K. The Dirac point is observed at moderate gate voltages (1-20V) depending upon the surface preparation. For temperatures below 40K, the gate is inefficient as the buried channel is frozen out. However, the carrier concentration in graphene remains very close to the value set at T\s…
The field of Quantum Hall metrology had a strong start with the implemntation of GaAs-based devices, given that 2D materials systems provided access to interesting quantum phenomena, including the infrastructure associated with making relevant measurements. With the technology laid out, further improvements in both infrastructure and standards were achieved in the previous two decades as EG-based quantized Hall resistance (QHR) devices became established as national standards. Since the metrology community has reached some understanding that a comparison against GaAs-based QHR devices had been…
Epitaxial graphene films were formed on the Si-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. A high-k gate stack on epitaxial graphene is realized by inserting a fully oxidized nanometer thin aluminum film as a seeding layer followed by an atomic-layer deposition process. The electrical properties of epitaxial graphene films are sustained after gate stack formation without significant degradation. At low temperatures, the quantum-Hall effect in Hall resistance is observed along with pronounced Shubnikov-de Hass oscillations in diagonal magneto-resistance …
We have performed a metrological characterization of the quantum Hall resistance in a 1 $μ$m wide graphene Hall-bar. The longitudinal resistivity in the center of the $ν=\pm 2$ quantum Hall plateaus vanishes within the measurement noise of 20 m$Ω$ upto 2 $μ$A. Our results show that the quantization of these plateaus is within the experimental uncertainty (15 ppm for 1.5$ μ$A current) equal to that in conventional semiconductors. The principal limitation of the present experiments are the relatively high contact resistances in the quantum Hall regime, leading to a significantly increased noise …
The electrical properties of graphene depend sensitively on the substrate. For example, recent measurements of epitaxial graphene on SiC show resistance arising from steps on the substrate. Here we calculate the deformation of graphene at substrate steps, and the resulting electrical resistance, over a wide range of step heights. The elastic deformations contribute only a very small resistance at the step. However, for graphene on SiC(0001) there is strong substrate-induced doping, and this is substantially reduced on the lower side of the step where graphene pulls away from the substrate. The…
We have studied the transport properties of graphene grown on silicon side of SiC. Samples under study have been prepared by two different growth methods in two different laboratories. Magnetoresistance and Hall resistance have been measured at temperatures between 4 and 100 K in resistive magnet in magnetic fields up to 22 T. In spite of differences in sample preparation, the field dependence of resistances measured on both sets of samples exhibits two periods of magneto-oscillations indicating two different parallel conducting channels with different concentrations of carriers. The semi-quan…
Disentangling Measurement Protocol from Material Physics in Isotypic and Anisotypic SiC/Si Heterojunctions: A Metrology-Driven Study for Betavoltaic Energy Conversion
Robust quantum Hall resistance standard from uniform wafer-scale epitaxial graphene on SiC
Laboratoire National de Métrologie et d’Essais (LNE)（法国国家计量与测试实验室）；Université Côte d’Azur（蔚蓝海岸大学）；Centre de Recherche sur l’Hétéroépitaxie et ses applications (CRHEA)（异质外延及其应用研究中心）；CERFACS（欧洲计算流体力学与空气动力学中心）；Université Paris-Saclay（巴黎萨克雷大学）；Centre de Nanosciences et de Nanotechnologies (C2N)（纳米科学与纳米技术研究中心）
We demonstrate a programmable quantum Hall circuit that implements a novel iterative voltage bisection scheme and allows obtaining any binary fraction $(k/2^n)$ of the fundamental resistance quantum $R_K/2=h/2e^2$. The circuit requires a number $n$ of bisection stages that only scales logarithmically with the precision of the fraction. The value of $k$ can be set to any integer between 1 and $2^n$ by proper gate configuration. The architecture exploits gate-controlled routing, mixing and equilibration of edge modes of robust quantum Hall states. The device does not contain {\em any} internal o…
The quantum Hall effect (QHE) theoretically provides a universal standard of electrical resistance in terms of the Planck constant $h$ and the electron charge $e$. In graphene, the spacing between the lowest discrete energy levels occupied by the charge carriers under magnetic field is exceptionally large. This is promising for a quantum Hall resistance standard more practical in graphene than in the GaAs/AlGaAs devices currently used in national metrology institutes. Here, we demonstrate that large QHE devices, made of high quality graphene grown by propane/hydrogen chemical vapour deposition…
Pressure, compressibility, and Hall conductance of anisotropic states at higher Landau levels are computed. Pressure and compressibility become negative. Hall conductance is unquantized and varies with filling factor. These facts agree with the recent experimental observations of highly anisotropic compressible states at higher Landau levels. It is shown, as an implication of negative pressure, that the quantum Hall effect has extraordinary stability, that is, Hall resistance is quantized even when the longitudinal resistance does not vanish.
In the quantum anomalous Hall effect, the edge states of a ferromagnetically doped topological insulator exhibit quantized Hall resistance and dissipationless transport at zero magnetic field. Up to now, however, the resistance was experimentally assessed with standard transport measurement techniques which are difficult to trace to the von-Klitzing constant R$_K$ with high precision. Here, we present a metrologically comprehensive measurement, including a full uncertainty budget, of the resistance quantization of V-doped (Bi,Sb)$_2$Te$_3$ devices without external magnetic field. We establishe…
In this letter we report on transport measurements of epitaxial graphene on SiC(0001) with oxygen adsorption. In a $50\times 50 μ\mathrm{m^2}$ size Hall bar we observe the half-integer quantum Hall effect with a transverse resistance plateau quantized at filling factor around $ν= 2$, an evidence of monolayer graphene. We find low electron concentration of $9\times 10^{11} \textrm{cm}^{-2}$ and we show that a doping of $10^{13}\textrm{cm}^{-2}$ which is characteristic of intrinsic epitaxial graphene can be restored by vacuum annealing. The effect of oxygen adsorption on carrier density is confi…
We report the studies of high-quality HgTe/(Cd,Hg)Te quantum wells (QWs) with a width close to the critical one $d_c$, corresponding to the topological phase transition and graphene like band structure in view of their applications for Quantum Hall Effect (QHE) resistance standards. We show that in the case of inverted band ordering, the coexistence of conducting topological helical edge states together with QHE chiral states degrades the precision of the resistance quantization. By experimental and theoretical studies we demonstrate how one may reach very favorable conditions for the QHE resi…
Replacing GaAs by graphene to realize more practical quantum Hall resistance standards (QHRS), accurate to within $10^{-9}$ in relative value, but operating at lower magnetic fields than 10 T, is an ongoing goal in metrology. To date, the required accuracy has been reported, only few times, in graphene grown on SiC by sublimation of Si, under higher magnetic fields. Here, we report on a device made of graphene grown by chemical vapour deposition on SiC which demonstrates such accuracies of the Hall resistance from 10 T up to 19 T at 1.4 K. This is explained by a quantum Hall effect with low di…
Defects in silicon carbide are of intense and increasing interest for quantum-based applications due to this material's properties and technological maturity. We calculate the multi-particle symmetry adapted wave functions of the negatively charged silicon vacancy defect in hexagonal silicon carbide via use of group theory and density functional theory and find the effects of spin-orbit and spin-spin interactions on these states. Although we focused on $\textrm{V}_{\textrm{Si}}^-$ in 4H-SiC, because of its unique fine structure due to odd number of active electrons, our methods can be easily a…
We demonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8K and magnetic fields below 5T. Operating this system requires little experimental knowledge or laboratory infrastructure, thereby greatly advancing the proliferation of primary quantum standards for precision electrical metrology. This significant advance in technology has come about as a result of the unique properties of epitaxial graphene on SiC.
We show that quantum resistance standards made of transferred graphene reach the uncertainty of semiconductor devices, the current reference system in metrology. A large graphene device (150 \times 30 \mum2), exfoliated and transferred onto GaAs, revealed a quantization with a precision of (-5.1 \pm 6.3) \times 10-9 accompanied by a vanishing longitudinal resistance at current levels exceeding 10 \muA. While such performance had previously only been achieved with epitaxially grown graphene, our experiments demonstrate that transfer steps, inevitable for exfoliated graphene or graphene grown by…
We investigate the magneto-transport properties of epitaxial graphene single-layer on 4H-SiC(0001), grown by atmospheric pressure graphitization in Ar, followed by H2 intercalation. We directly demonstrate the importance of saturating the Si dangling bonds at the graphene/SiC(0001) interface to achieve high carrier mobility. Upon successful Si dangling bonds elimination, carrier mobility increases from 3 000 cm^2/Vs to > 11 000 cm^2/Vs at 0.3 K. Additionally, graphene electron concentration tends to decrease from a few 10^12 cm^-2 to less than 10^12 cm^-2. For a typical large (30x280 um^2) Hal…
The electrical transport properties of epitaxial graphene layers are correlated with the SiC surface morphology. In this study we show by atomic force microscopy and Raman measurements that the surface morphology and the structure of the epitaxial graphene layers change significantly when different pretreatment procedures are applied to nearly on-axis 6H-SiC(0001) substrates. It turns out that the often used hydrogen etching of the substrate is responsible for undesirable high macro steps evolving during graphene growth. A more advantageous type of sub-nanometer stepped graphene layers is obta…
We report development of a quantum Hall resistance standard accurate to a few parts in a billion at 300 mK and based on large area epitaxial graphene. The remarkable precision constitutes an improvement of four orders of magnitude over the best results obtained in exfoliated graphene and is similar to the accuracy achieved in well-established semiconductor standards. Unlike the traditional resistance standards the novel graphene device is still accurately quantized at 4.2 K, vastly simplifying practical metrology. This breakthrough was made possible by exceptional graphene quality achieved wit…
Physical properties of anisotropic compressible quantum Hall states and their implications to integer quantum Hall effect are studied based on a mean field theory on the von Neumann lattice. It is found that the Hall gas has unusual thermodynamic properties such as negative pressure and negative compressibility and unusual transport properties. Transport properties and density profile of Hall gas states at half fillings agree with those of anisotropic states discovered experimentally in higher Landau levels. Hall gas formed in the bulk does not spread but shrinks, owing to negative pressure, a…
We present precision measurements of the fractional quantized Hall effect where the quantized resistance $R^{[1/3]}$ in the fractional quantum Hall state at filling factor 1/3 was compared with a quantized resistance $R^{[2]}$, represented by an integer quantum Hall state at filling factor 2. A cryogenic current comparator bridge capable of currents down to the nanoampere range was used to directly compare two resistance values of two GaAs-based devices located in two cryostats. A value of $1 - (5.3 \pm 6.3) 10^{-8}$ (95% confidence level) was obtained for the ratio $(R^{[1/3]}/6R^{[2]})$. Thi…
Monolayer epitaxial graphene (EG) grown on hexagonal Si-terminated SiC substrates is intrinsically electron-doped (carrier density is about 10^13 cm^(-2)). We demonstrate a clean device fabrication process using a precious-metal protective layer, and show that etching with aqua regia results in p-type (hole) molecular doping of our un-gated, contamination-free EG. Devices fabricated by this simple process can reach a carrier density in the range of 10^10 cm^(-2) to 10^11 cm^(-2) with mobility about 8000 cm^2/V/s or higher. In a moderately doped device with a carrier density n = 2.4 x 10^11 cm^…
We investigate the magnetotransport properties of quasi-free standing epitaxial graphene bilayer on SiC, grown by atmospheric pressure graphitization in Ar, followed by H$_2$ intercalation. At the charge neutrality point the longitudinal resistance shows an insulating behavior, which follows a temperature dependence consistent with variable range hopping transport in a gapped state. In a perpendicular magnetic field, we observe quantum Hall states (QHSs) both at filling factors ($ν$) multiple of four ($ν=4, 8, 12$), as well as broken valley symmetry QHSs at $ν=0$ and $ν=6$. These results unamb…
We show that implementations for quantum sensing with exceptional sensitivity and spatial resolution can be made using spin-3/2 semiconductor defect states. We illustrate this using the silicon monovacancy deep center in hexagonal SiC based on our rigorous derivation of this defect's ground state and of its electronic and optical properties. For a single $\textrm{V}_{\textrm{Si}}^-$ defect, we obtain magnetic field sensitivities capable of detecting individual nuclear magnetic moments. We also show that its zero-field splitting has an exceptional strain and temperature sensitivity within the t…
Just a few of the promising applications of graphene Corbino pnJ devices include two-dimensional Dirac fermion microscopes, custom programmable quantized resistors, and mesoscopic valley filters. In some cases, device scalability is crucial, as seen in fields like resistance metrology, where graphene devices are required to accommodate currents of the order 100 μA to be compatible with existing infrastructure. However, fabrication of these devices still poses many difficulties. In this work, unusual quantized resistances are observed in epitaxial graphene Corbino p-n junction devices held at t…
We report on the stability of the quantum Hall plateau in wide Hall bars made from a chemically gated graphene film grown on SiC. The $ν=2$ quantized plateau appears from fields $B \simeq 5$ T and persists up to $B \simeq 80$ T. At high current density, in the breakdown regime, the longitudinal resistance oscillates with a $1/B$ periodicity and an anomalous phase, which we relate to the presence of additional electron reservoirs. The high field experimental data suggest that these reservoirs induce a continuous increase of the carrier density up to the highest available magnetic field, thus en…
Quantum Hall effect (QHE) is the basis of modern resistance metrology. In Quantum Hall Array Resistance Standards (QHARS), several individual QHE elements, each one having the same QHE resistance (typically half of the von Klitzing constant), are arranged in networks that realize resistance values close to decadic values (such as 1 kiloohm or 100 kiloohm), of direct interest for dissemination. The same decadic value can be approximated with different grades of precision, and even for the same approximation several networks of QHE elements can be conceived. The paper investigates the design of …
In this work, limiting factors for developing metrologically useful arrays from epitaxial graphene on SiC are lifted with a combination of centimeter-scale, high-quality material growth and the implementation of superconducting contacts. Standard devices for metrology have been restricted to having a single quantized value output based on the $ν$ = 2 Landau level. With the demonstrations herein of devices having multiple outputs of quantized values available simultaneously, these versatile devices can be used to disseminate the ohm globally. Such devices are designed to give access to quantize…
We explore the robust quantization of the Hall resistance in epitaxial graphene grown on Si-terminated SiC. Uniquely to this system, the dominance of quantum over classical capacitance in the charge transfer between the substrate and graphene is such that Landau levels (in particular, the one at exactly zero energy) remain completely filled over an extraordinarily broad range of magnetic fields. One important implication of this pinning of the filling factor is that the system can sustain a very high nondissipative current. This makes epitaxial graphene ideally suited for quantum resistance me…
Graphene, the first true two-dimensional material still reveals the most remarkable transport properties among the growing class of two-dimensional materials. Although many studies have investigated fundamental scattering processes, the surprisingly large variation in the experimentally determined resistances associated with a localized defect is still an open issue. Here, we quantitatively investigate the local transport properties of graphene prepared by polymer assisted sublimation growth (PASG) using scanning tunneling potentiometry. PASG graphene is characterized by a spatially homogeneou…
In view of the progress achieved in the field of the ac quantum Hall effect, the Working Group of the Comite Consultatif d'Electricite et Magnetisme (CCEM) on the AC Quantum Hall Effect asked the authors of this paper to write a compendium which integrates their experiences with ac measurements of the quantum Hall resistance. In addition to the important early work performed at the Bureau International des Poids et Mesures and the National Physical Laboratory, UK, further experience has been gained during a collaboration of the authors' institutes NRC, METAS, and PTB, and excellent agreement b…
Electron pumps capable of delivering a current higher than 100pA with sufficient accuracy are likely to become the direct mise en pratique of the possible new quantum definition of the ampere. Furthermore, they are essential for closing the quantum metrological triangle experiment which tests for possible corrections to the quantum relations linking e and h, the electron charge and the Planck constant, to voltage, resistance and current. We present here single-island hybrid metal/semiconductor transistor pumps which combine the simplicity and efficiency of Coulomb blockade in metals with the u…
Graphene-metal contact resistance is governed by both intrinsic and extrinsic factors. Intrinsically, both the density of states bottleneck near the Dirac point and carrier reflection at the graphene-metal interface lead to a high contact resistance. Moreover, graphene exhibits insulating behavior for out-of-the-plane conduction. Extrinsically, surface contamination introduced by photoresist residue or different adsorbed species during standard lithography processing alters graphene's intrinsic properties by uncontrolled doping and increased scattering which results in high and inconsistent co…
The von Neumann lattice representation is a convenient representation for studying several intriguing physics of quantum Hall systems. In this formalism, electrons are mapped to lattice fermions. A topological invariant expression of the Hall conductance is derived and is used for the proof of the integer quantum Hall effect in the realistic situation. Anisotropic quantum Hall gas is investigated based on the Hartree-Fock approximation in the same formalism. Thermodynamic properties, transport properties, and unusual response under external modulations are found. Implications for the integer q…
We report on a comparison of four GaAs/AlGaAs-based quantum resistance standards using an original technique adapted from the well-known Wheatstone bridge. This work shows that the quantized Hall resistance at Landau level filling factor $ν=2$ can be reproducible with a relative uncertainty of $32\times 10^{-12}$ in the dissipationless limit of the quantum Hall effect regime. In the presence of a very small dissipation characterized by a mean macroscopic longitudinal resistivity $\bar{R_{xx}(B)}$ of a few $μΩ$, the discrepancy $ΔR_{\mathrm{H}}(B)$ measured on the Hall plateau between quantum H…
Establishing good electrical contacts to nanoscale devices is a major issue for modern technology and contacting 2D materials is no exception to the rule. One-dimensional edge-contacts to graphene were recently shown to outperform surface contacts but the method remains difficult to scale up. We report a resist-free and scalable method to fabricate few graphene layers with electrical contacts in a single growth step. This method derives from the discovery reported here of the growth of few graphene layers on a metallic carbide by thermal annealing of a carbide forming metallic film on SiC in h…
HgTe quantum wells with a thickness of ${\sim}$7 nm may have a graphene-like band structure and have been recently proposed to be potential candidates for quantum Hall effect (QHE) resistance standards under the condition of operation in the fields above certain critical field $B_c$, above which the topological phase (with parasitic edge conduction) disappears. We present experimental studies of the magnetoresistance of different of HgTe quantum wells as a function temperature and magnetic field, determining the critical magnetic field $B_c$. We demonstrate that for QWs of specific width $B_c$…
In this study, we first show that the argon flow during epitaxial graphene growth is an important parameter to control the quality of the buffer and the graphene layer. Atomic force microscopy (AFM) and low-energy electron diffraction (LEED) measurements reveal that the decomposition of the SiC substrate strongly depends on the Ar mass flow rate while pressure and temperature are kept constant. Our data are interpreted by a model based on the competition of the SiC decomposition rate, controlled by the Ar flow, with a uniform graphene buffer layer formation under the equilibrium process at the…
Quantum Hall effect (QHE) devices based on epitaxial graphene films grown on SiC were fabricated and studied for development of the QHE resistance standard. The graphene-metal contacting area in the Hall devices has been improved and fabricated using a double metalization process. The tested devices had an initial carrier concentration of (0.6 - 10)*10^11 1/cm^2 and showed half-integer quantum Hall effect at a relatively low (3 T) magnetic field. Application of the photochemical gating method and annealing of the sample provides a convenient way for tuning the carrier density to the optimum va…
Graphene grown epitaxially on SiC, close to the charge neutrality point (CNP), in an orthogonal magnetic field shows an ambipolar behavior of the transverse resistance accompanied by a puzzling longitudinal magnetoresistance. When injecting a transverse current at one end of the Hall bar, a sizeable non local transverse magnetoresistance is measured at low temperature. While Zeeman spin effect seems not to be able to justify these phenomena, some dissipation involving edge states at the boundaries could explain the order of magnitude of the non local transverse magnetoresistance, but not the a…
Single-photon sources are of paramount importance in quantum communication, quantum computation, and quantum metrology. In particular, there is great interest in realizing scalable solid-state platforms that can emit triggered photons on demand to achieve scalable nanophotonic networks. We report on a visible-spectrum single-photon emitter in 4H silicon carbide (SiC). The emitter is photostable at room and low temperatures, enabling photon counts per second in excess of 2$\times$10$^6$ from unpatterned bulk SiC. It exists in two orthogonally polarized states, which have parallel absorption and…
Since 2017, epitaxial graphene has been the base material for the US national standard for resistance. A future avenue of research within electrical metrology is to remove the need for strong magnetic fields, as is currently the case for devices exhibiting the quantum Hall effect. The quantum Hall effect is just one of many research endeavours that revolve around recent quantum physical phenomena like composite fermions, charge density waves, and topological properties [1-2]. New materials, like magnetically doped topological insulators (MTIs), offer access to the quantum anomalous Hall effect…
Light absorption in graphene causes a large change in electron temperature, due to low electronic heat capacity and weak electron phonon coupling [1-3], making it very attractive as a hot-electron bolometer material. Unfortunately, the weak variation of electrical resistance with temperature has substantially limited the responsivity of graphene bolometers. Here we show that quantum dots of epitaxial graphene on SiC exhibit an extraordinarily high variation of resistance with temperature due to quantum confinement, higher than 430 Mohm/K at 2.5 K, leading to responsivities for absorbed THz pow…
We propose a way to realize a programmable quantum current standard (PQCS) from the Josephson voltage standard and the quantum Hall resistance standard (QHR) exploiting the multiple connection technique provided by the quantum Hall effect (QHE) and the exactness of the cryogenic current comparator. The PQCS could lead to breakthroughs in electrical metrology like the realization of a programmable quantum current source, a quantum ampere-meter and a simplified closure of the Quantum Metrological Triangle. Moreover, very accurate universality tests of the QHE could be performed by comparing PQCS…
The superconducting circuit community has recently discovered the promising potential of superinductors. These circuit elements have a characteristic impedance exceeding the resistance quantum $R_\text{Q} \approx 6.45~\text{k}Ω$ which leads to a suppression of ground state charge fluctuations. Applications include the realization of hardware protected qubits for fault tolerant quantum computing, improved coupling to small dipole moment objects and defining a new quantum metrology standard for the ampere. In this work we refute the widespread notion that superinductors can only be implemented b…
The Josephson effect in superconductors links a quantized output voltage Vout = f \cdot(h/2e) to the natural constants of the electron's charge e, Planck's constant h, and to an excitation frequency f with important applications in electrical quantum metrology. Also semiconductors are routinely applied in electrical quantum metrology making use of the quantum Hall effect. However, despite their broad range of further applications e.g. in integrated circuits, quantized voltage generation by a semiconductor device has never been obtained. Here we report a semiconductor quantized voltage source g…
The quantum Hall effect (QHE) is a cornerstone in the new International System of Units (SI), wherein the base units are derived from seven fundamental constants such as Planck's constant h and elementary charge e. Graphene has revolutionized practical resistance metrology by enabling the realization of quantized resistance h/2e^2 = 12.9... kOhm under relaxed experimental conditions. Looking ahead, graphene also has the potential to improve realizations of the electronic kilogram using the Kibble balance, and the quantum Ampere in wide current ranges. However, these prospects require different…
We report on the observation of inertial-ballistic transport in nanoscale cross junctions fabricated from epitaxial graphene grown on SiC(0001). Ballistic transport is indicated by a negative bend resistance of R12,43 ~ 170 ohm which is measured in a non-local, four-terminal configuration at 4.2 K and which vanishes as the temperature is increased above 80 K.
We report measurement of the radio-frequency (RF) transport electromagnetic properties of chemical vapour deposition (CVD) graphene over the DC to 110 MHz frequency range at room temperature. Graphene on Si/SiO2 substrate was mounted in a shielded four terminal-pair (4TP) adaptor which enabled direct connection to a calibrated precision impedance analyser for measurements. Good agreement is observed for the DC four-probe resistance and the 4TP resistance at 40 Hz, both yielding R ~ 104 Ω. In general the apparent graphene channel electromagnetic properties are found to be strongly influenced by…
We have produced ultrathin epitaxial graphite films which show remarkable 2D electron gas (2DEG) behavior. The films, composed of typically 3 graphene sheets, were grown by thermal decomposition on the (0001) surface of 6H-SiC, and characterized by surface-science techniques. The low-temperature conductance spans a range of localization regimes according to the structural state (square resistance 1.5 kOhm to 225 kOhm at 4 K, with positive magnetoconductance). Low resistance samples show characteristics of weak-localization in two dimensions, from which we estimate elastic and inelastic mean fr…
