Gated Graphene Devices Research Articles

Simultaneous dual-configuration van der Pauw measurements of gated graphene devices

The van der Pauw technique is the gold standard for measuring the sheet resistance of thin conductors by relying on the combination of two 4-point resistance measurements obtained in different current–voltage probe configurations. The presence of hysteresis and/or time-dependent effects will result in the van der Pauw technique producing inaccurate and misleading results. This limitation can be overcome by simultaneously measuring the two 4-point resistances at different frequencies with two lock-in amplifiers. In this work, we discuss in detail the fundamental requirements and practical considerations for the proper implementation of this measurement technique. Furthermore, we show that the relative deviation between sequential and simultaneous measurements is below 1% in the region of interest for a graphene-based field-effect transistor and 0.04% on average for resistive test structures. The simultaneous van der Pauw technique could therefore become a new standard for reliably assessing the electrical properties of 2D materials and devices.

Origin of electrical noise near charge neutrality in dual gated graphene device

This Letter investigates low frequency 1/f noise in an hBN encapsulated graphene device in a dual gated geometry. The noise study is performed as a function of top gate carrier density (nTG) at different back gate density (nBG). The noise at low nBG is found to be independent of top gate carrier density. With increasing nBG, noise value increases, and a noise peak is observed near charge inhomogeneity of the device. A further increase in nBG leads to a decrease in noise magnitude. The shape of the noise is found to be closely related to a charge inhomogeneity region of the device. Moreover, the noise and conductivity data near charge neutrality show clear evidence of noise emanating from a combination of charge number and mobility fluctuation.

Temperature dependent charge transport in ferroelectrically gated graphene far from the Dirac point

Charge transport in ferroelectric (FE) gated graphene far from the Dirac point (DP) was studied in the temperature range 300 K < T < 350 K. A non-monotonic/monotonic/non-monotonic behavior in the conductivity [σ(T)] was observed as one moved away from the DP. As the gate polarization increased, additional impurity charges were compensated, which reduced charge scattering. The uncompensated charges doped graphene and σ(T) switched to a monotonic increase with increasing T. However, far from the DP, the polarization reached saturation, which resulted in still lower impurity charge scattering. The carrier concentration increased, and a non-monotonic response in σ(T) reappeared, which was attributed to phonon scattering. A theoretical model is presented that combined impurity charge and phonon scattering conduction mechanisms. The top gate polarizable FE provided a novel approach to investigate charge transport in graphene via controlled compensation of impurity charges, and in the process revealed non-monotonic behavior in σ(T) not previously seen in SiO2 back gated graphene devices.

Deep THz modulation at Fabry-Perot resonances using graphene in periodic microslits.

Potential applications of terahertz (THz) radiation are constantly being investigated for high-speed communication due to its large bandwidth. For example, frequency hopping communication technology would benefit from the large bandwidth. To attach the information to the carrier wave, THz modulators with deep and stable modulation at different frequencies are crucial, yet are still lacking. Here a THz modulator, designed by integrating a non-resonant field enhancement effect of periodic metal microslits to assist a Fabry-Perot resonance structure (MS-FP) is proposed and demonstrated. New equations are developed to describe the superior performance of the novel design. The >95% modulation depth is achieved by a SiO2/Si gated graphene device at 14 Fabry-Perot resonant frequencies across 1.4 THz bandwidth, outperforming the recently reported 75% modulation depth THz modulator with a similar Fabry-Perot structure.

Electrostatics of metal-graphene interfaces: sharp p-n junctions for electron-optical applications.

Creation of sharp lateral p-n junctions in graphene devices, with transition widths w well below the Fermi wavelength λF of graphene's charge carriers, is vital to study and exploit these electronic systems for electron-optical applications. The achievement of such junctions is, however, not trivial due to the presence of a considerable out-of-plane electric field in lateral p-n junctions, resulting in large widths. Metal-graphene interfaces represent a novel, promising and easy to implement technique to engineer such sharp lateral p-n junctions in graphene field-effect devices, in clear contrast to the much wider (i.e. smooth) junctions achieved via conventional local gating. In this work, we present a systematic and robust investigation of the electrostatic problem of metal-induced lateral p-n junctions in gated graphene devices for electron-optics applications, systems where the width w of the created junctions is not only determined by the metal used but also depends on external factors such as device geometries, dielectric environment and different operational parameters such as carrier density and temperature. Our calculations demonstrate that sharp junctions (w ≪ λF) can be achieved via metal-graphene interfaces at room temperature in devices surrounded by dielectric media with low relative permittivity (<10). In addition, we show how specific details such as the separation distance between metal and graphene and the permittivity of the gap in-between plays a critical role when defining the p-n junction, not only defining its width w but also the energy shift of graphene underneath the metal. These results can be extended to any two-dimensional (2D) electronic system doped by the presence of metal clusters and thus are relevant for understanding interfaces between metals and other 2D materials.

Graphene-loaded metal wire grating for deep and broadband THz modulation in total internal reflection geometry

We employed a metallic wire grating loaded with graphene and operating in total internal reflection (TIR) geometry to realize deep and broadband THz modulation. The non-resonant field enhancement effect of the evanescent wave in TIR geometry and in the subwavelength wire grating was combined to demonstrate a ∼77% modulation depth (MD) in the frequency range of 0.2–1.4 THz. This MD, achieved electrically with a SiO2/Si gated graphene device, was 4.5 times higher than that of the device without a metal grating in transmission geometry. By optimizing the parameters of the metallic wire grating, the required sheet conductivity of graphene for deep modulation was lowered to 0.87 mS. This work has potential applications in THz communication and real-time THz imaging.

Unconventional Correlation between Quantum Hall Transport Quantization and Bulk State Filling in Gated Graphene Devices.

We report simultaneous transport and scanning microwave impedance microscopy to examine the correlation between transport quantization and filling of the bulk Landau levels in the quantum Hall regime in gated graphene devices. Surprisingly, a comparison of these measurements reveals that quantized transport typically occurs below the complete filling of bulk Landau levels, when the bulk is still conductive. This result points to a revised understanding of transport quantization when carriers are accumulated by gating. We discuss the implications on transport study of the quantum Hall effect in graphene and related topological states in other two-dimensional electron systems.

Imaging and tuning molecular levels at the surface of a gated graphene device.

Gate-controlled tuning of the charge carrier density in graphene devices provides new opportunities to control the behavior of molecular adsorbates. We have used scanning tunneling microscopy (STM) and spectroscopy (STS) to show how the vibronic electronic levels of 1,3,5-tris(2,2-dicyanovinyl)benzene molecules adsorbed onto a graphene/BN/SiO2 device can be tuned via application of a backgate voltage. The molecules are observed to electronically decouple from the graphene layer, giving rise to well-resolved vibronic states in dI/dV spectroscopy at the single-molecule level. Density functional theory (DFT) and many-body spectral function calculations show that these states arise from molecular orbitals coupled strongly to carbon–hydrogen rocking modes. Application of a back-gate voltage allows switching between different electronic states of the molecules for fixed sample bias.

Mapping Dirac quasiparticles near a single Coulomb impurity on graphene

The response of Dirac fermions to a Coulomb potential is predicted to differ significantly from the behavior of non-relativistic electrons seen in traditional atomic and impurity systems. Surprisingly, many key theoretical predictions for this ultra-relativistic regime have yet to be tested in a laboratory. Graphene, a 2D material in which electrons behave like massless Dirac fermions, provides a unique opportunity to experimentally test such predictions. The response of Dirac fermions to a Coulomb potential in graphene is central to a wide range of electronic phenomena and can serve as a sensitive probe of graphene's intrinsic dielectric constant, the primary factor determining the strength of electron-electron interactions in this material. Here we present a direct measurement of the nanoscale response of Dirac fermions to a single Coulomb potential placed on a gated graphene device. Scanning tunneling microscopy and spectroscopy were used to fabricate tunable charge impurities on graphene and to measure how they are screened by Dirac fermions for a Q = +1|e| impurity charge state. Electron-like and hole-like Dirac fermions were observed to respond very differently to tunable Coulomb potentials. Comparison of this electron-hole asymmetry to theoretical simulations has allowed us to test basic predictions for the behavior of Dirac fermions near a Coulomb potential and to extract the intrinsic dielectric constant of graphene: {\epsilon}_g= 3.0 \pm 1.0. This small value of {\epsilon}_g indicates that microscopic electron-electron interactions can contribute significantly to graphene properties.

Local gating of decoupled graphene monolayers

We study transport properties of a locally gated graphene device. The samples are made by exfoliation of natural graphite onto a SiO 2 substrate such that single layers of graphene can be contacted and the potential can be tuned by applying a voltage to the SiO 2 . Additional top gates are fabricated to control the carrier concentration locally. In doing so we find that induced electrons and holes are distributed in two 2D systems that are decoupled. The characteristic of the Shubnikov–de Haas oscillations shows that these systems behave like two monolayers of graphene, which has to be decoupled due to its magneotransport characteristic.

Room temperature carrier transport in graphene

Graphene is a novel new material with an unusual zero-gap band structure, where electrons and holes are closely connected through a relativistic Dirac equation. It is of interest to study the various scattering mechanisms and the transport through device structures fabricated on this new material. Here, we use Rode’s method to study the transport through gated graphene devices. The results are compared with recent results obtained for both back-gates and electrochemical gates. The transport is dominated by the trapped charge at the graphene-SiO2, but phonon scattering is shown to be important.

Quantum Hall resistances of a multiterminal top-gated graphene device

Four-terminal resistances, both longitudinal and diagonal, of a locally gated graphene device are measured in the quantum-Hall (QH) regime. In sharp distinction from previous two-terminal studies [J. R. Williams et al., Science 317, 638 (2007); B. \"Ozyilmaz et al., Phys. Rev. Lett. 99, 166804 (2007)], asymmetric QH resistances are observed, which provide information on reflection as well as transmission of the QH edge states. Most quantized values of resistances are well analyzed by the assumption that all edge states are equally populated. Contrary to the expectation, however, a 5/2 transmission of the edge states is also found, which may be caused by incomplete mode mixing and/or by the presence of counterpropagating edge states. This four-terminal scheme can be conveniently used to study the edge-state equilibration in locally gated graphene devices as well as monolayer and multilayer graphene hybrid structures.