Dual-gated Graphene Research Articles

Criterion for vanishing valley asymmetric transmission in dual-gated bilayer graphene

Abstract Realizing valley asymmetric transmission(VAT) for incoming valley unpolarized Dirac carriers across an electrostatically modulated confinement potential(ECP) step, which is located at the interface of the unipolar ($n-n'$) junction in dual-gated Bernal-bilayer graphene(BBG), has caught growing attention because it provides a platform to study the valley polarizer. Such a junction, however, seems to preclude from supporting vanishing VAT. Based on the effective two-band Hamiltonian of BBG with wave matching approach, we demonstrate theoretically that VAT through the $n$/$ n'$ interface vanishes in the following conditions: (i) the disappearing electrostatically modulated interlayer potential difference(EPD), or (ii) the invariance condition of the sum or difference of ECPs and EPDs, which physically corresponds to the zero band offset of the conduction or valence band in the band alignment of $n-n'$ junction. Guided by this, it is worth mentioning that the we can derive simple, analytical valley-independent expressions for the reflection and transmission probabilities. We further find the appearance of the nearly vanishing VAT for the four-band model in bilayer graphene with 'Mexican hat'-like band. As a byproduct, we apply these findings to suggest the best design for the valley-dependent selection of momenta based on dual-gated BBG connected to two pristine BBG electrodes in which ECPs of the two outer regions are distinguishable and show that the sign of valley polarization can be switched via varying EPD. We expect our results can provide guidance for future device applications relying on ballistic and valley-selective transport.

Emergent Inhomogeneity and Nonlocality in a Graphene Field-Effect Transistor on a Near-Parallel Moiré Superlattice of Transition Metal Dichalcogenides.

At near-parallel orientation, twisted bilayers of transition metal dichalcogenides exhibit interlayer charge transfer-driven out-of-plane ferroelectricity. Here, we report detailed electrical transport in a dual-gated graphene field-effect transistor placed on a 2.1° twisted bilayer WSe2. We observe hysteretic transfer characteristics and an emergent charge inhomogeneity with multiple local Dirac points evolving with an increasing electric displacement field (D). Concomitantly, we also observe a strong nonlocal voltage signal at D ∼ 0 V/nm that decreases rapidly with increasing D. A linear scaling of the nonlocal signal with longitudinal resistance suggests edge mode transport, which we attribute to the breaking of valley symmetry of graphene due to the spatially fluctuating electric field from the underlying polarized moiré domains. A quantitative analysis suggests the emergence of finite-size domains in graphene that modulate the charge and the valley currents simultaneously. This work underlines the impact of interfacial ferroelectricity that can trigger a new generation of devices.

Performance analysis of graphene field effect transistor at nanoscale regime

In this paper, we examine the response of charge carriers in a dual-gate graphene field-effect transistor (GFET). We have conducted an investigation into the input-output properties of both monolayer and bilayer graphene channels. Additionally, we demonstrated the relationship between surface potential and quantum capacitance using equivalent circuit modelling. We offer precise mathematical equations that define the border points separating distinct regions. These equations guarantee the continuity of the Jacobian matrix, which is crucial for the successful implementation of a circuit simulator. We can classify the current-voltage (I-V) characteristics of bilayer graphene into three distinct regions: triode, unipolar, and ambipolar. The equivalent circuit modelling findings indicated that GFETs had superior output characteristics and transconductance behavior compared to FETs. We observed a direct correlation between transconductance and gate voltage across various drain-to-source voltages.Nevertheless, with a higher gate voltage, there was a corresponding drop in the transconductance value. The threshold voltage for this dual-gate GFET is dependent on the back gate voltage. The GFET's threshold value decreased as the back gate voltage increased.

Gate-tunable anomalous Hall effect in Bernal tetralayer graphene

Large spin-orbit coupling is often thought to be critical in realizing magnetic order-locked charge transport such as the anomalous Hall effect (AHE). Recently, artificial stacks of two-dimensional materials, e.g., magic-angle twisted bilayer graphene on hexagonal boron-nitride heterostructures and dual-gated rhombohedral trilayer graphene, have become platforms for realizing AHE without spin-orbit coupling. However, these stacking arrangements are not energetically favorable, impeding experiments and further device engineering. Here we report an anomalous Hall effect in Bernal-stacked tetralayer graphene devices (BTG), the most stable configuration of four-layer graphene. BTG AHE is switched on by a displacement field and is most pronounced at low carrier densities. The onset of AHE occurs in tandem with a full metal to a broken isospin transition indicating an orbital origin of the itinerant ferromagnetism. At lowest densities, BTG exhibits an unconventional hysteresis with step-like anomalous Hall plateaus. Persisting to several tens of kelvin, AHE in BTG demonstrates the ubiquity and robustness of magnetic order in readily available and stable multilayer Bernal graphene stacks—a new venue for intrinsic non-reciprocal responses.

Impact analysis of vacancy defects on Analog/RF performance parameters of GNR FET

The effects of vacancy defects, such as single vacancy defect (SVD) and di-vacancy defect (DVD) for low voltage, on the performance parameters of a dual-gated graphene nanoribbon (GNR) field-effect transistor (FET) are investigated and compared to an ideal GNR FET with no vacancy defect (zero vacancy). Self-consistent atomistic simulations are used to evaluate the performance parameters of the device based on the non-equilibrium Green's function (NEGF) formalism. Initially, the impact of vacancy defects on the analog and RF performance parameters is examined, and then a trade-off analysis is conducted between transistor efficiency and frequency. The results indicate that the di-vacancy device has the highest transconductance generation factor (TGF), output resistance, intrinsic gain, gate capacitance, power delay product, gain frequency product (GFP), and gain transfer frequency product (GTFP). In contrast, the highest transconductance value, transconductance frequency product (TFP) and cut-off frequency are obtained with the device without any vacancy defect.

High-performance dual-gate graphene pH sensors

Field-effect transistors (FETs) are versatile tools for high-precision biophysical measurements, and their measurement sensitivity and resolution can be improved by using innovative materials and device designs. Here, we report on the sensitivity and noise performance of dual-gated graphene FETs. When measuring pH, our devices exhibit a sensitivity of up to 30 V per unit change in pH, ≈500-fold greater than the Nernst value at room temperature, and noise-limited resolution of 2 × 10−4 in the biomedically relevant 0.1–10 Hz bandwidth. This level of performance is obtained due to a highly asymmetric dual-gate design utilizing an ionic liquid top-gate dielectric coupled with graphene's large intrinsic quantum capacitance (≈15 μC/cm2). Our results improve upon the sensitivity and resolution of previously demonstrated Si- and MoS2-channel FET biosensors.

Simulation insights of a new dual gate graphene nano-ribbon tunnel field-effect transistors for THz applications

This paper introduces a dual gate Graphene Nano-Ribbon Tunnel Field Effect Transistor (DG GNR TFET) structure and its performance characteristics are analysed. Technology Computer-Aided Design (TCAD) tool is used to simulate this novel non-silicon GNR TFET and to characterize various parameters of the device. A DG GNR TFET shows better device performance in terms of its surface potential, electric field and ON current (ION). The simulation results show that the proposed structure exhibits better ION/IOFF (109), average sub-threshold swing (43.16 mv/Dec) and threshold (0.3 V) when compared to a conventional Silicon TFET. Design parameters for the device such as dielectric thickness, body thickness, work function, source doping concentration, and channel length are analysed to deduce the fact that GNR can be used as an effective material in TFETs for low power electronics applications. Moreover, key parameters such as transconductance, transconductance generation efficiency and cut-off frequency (THz) are greatly improved in this proposed structure, which is highly desirable for analog/RF applications.

Charge carrier modulation in dual-gated graphene field effect transistor using honey as polar organic gate dielectric

Charge carrier modulation of graphene-based field effect transistors (GFETs) is the key factor to utilize and enhance its fascinating properties for technological applicability. Here, we have demonstrated the gate-dependent tweaking of electrical properties of graphene devices by application of honey as a top gate dielectric. Electrical characterization of dual-gated GFET is elucidated at different top and back-gate voltages. A charge neutrality point is fine-tuned by varying the top gate voltage (Vtg) from + 3 to − 4 V. The change in carrier density is clearly observed from 3.66 × 1012 to 2.15 × 1011 cm−2 at + 3 to − 4 Vtg. The charge carrier mobility of gel-gated GFET is increased significantly to 5376 cm2/V ⋅ sec by increasing top-gate voltages up to − 4 V. Result demonstrates a cost-effective, facile and rapid fabrication of top-gated devices and suggest natural dielectric materials as good candidate to replace conventionally available gate dielectrics in FET technology.

Dual-Gated Graphene Devices for Near-Field Nano-imaging

Graphene-based heterostructures display a variety of phenomena that are strongly tunable by electrostatic local gates. Monolayer graphene (MLG) exhibits tunable surface plasmon polaritons, as revealed by scanning nano-infrared experiments. In bilayer graphene (BLG), an electronic gap is induced by a perpendicular displacement field. Gapped BLG is predicted to display unusual effects such as plasmon amplification and domain wall plasmons with significantly larger lifetime than MLG. Furthermore, a variety of correlated electronic phases highly sensitive to displacement fields have been observed in twisted graphene structures. However, applying perpendicular displacement fields in nano-infrared experiments has only recently become possible [Li, H.; Nano Lett. 2020, 20, 3106-3112]. In this work, we fully characterize two approaches to realizing nano-optics compatible top gates: bilayer MoS2 and MLG. We perform nano-infrared imaging on both types of structures and evaluate their strengths and weaknesses. Our work paves the way for comprehensive near-field experiments of correlated phenomena and plasmonic effects in graphene-based heterostructures.

Nonconventional Analog Comparators Based on Graphene and Ferroelectric Hafnium Zirconium Oxide

Unlike transitional semiconductors, graphene has zero bandgap and symmetric electron/hole transport, which leads to unique V-shaped transfer characteristics. Using this property, we design and demonstrate a new type of comparator, which can calculate the absolute distance between two signals, |A - B|, directly. Dual-gate graphene transistors with ferroelectric hafnium zirconium oxide are fabricated to serve as the basic units of the comparators. We show that the remanent polarization of the ferroelectric hafnium oxide can reach ~30 μC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the output current of the comparator can serve as a scalar indicator of the similarity level between two signals. The embedded ferroelectric layer can store the reference signal in situ, which will reduce the energy consumption and latency related to the data transport. Furthermore, we demonstrate the feasibility of using ferroelectric graphene comparator in image classification and motion detection. Using the k-nearest neighbors (KNNs) algorithm, we show that the graphene comparator arrays can recognize the handwritten digits in the modified national institute of standards and technology (MNIST) data set with over 80% accuracy. These ferroelectric graphene comparators will have broad applications in robotics, security system, self-driving vehicles, and sensor networks.

Equivalent Circuit Modeling of a Dual-Gate Graphene FET

This paper presents a simple and comprehensive model of a dual-gate graphene field effect transistor (FET). The quantum capacitance and surface potential dependence on the top-gate-to-source voltage were studied for monolayer and bilayer graphene channel by using equivalent circuit modeling. Additionally, the closed-form analytical equations for the drain current and drain-to-source voltage dependence on the drain current were investigated. The distribution of drain current with voltages in three regions (triode, unipolar saturation, and ambipolar) was plotted. The modeling results exhibited better output characteristics, transfer function, and transconductance behavior for GFET compared to FETs. The transconductance estimation as a function of gate voltage for different drain-to-source voltages depicted a proportional relationship; however, with the increase of gate voltage this value tended to decline. In the case of transit frequency response, a decrease in channel length resulted in an increase in transit frequency. The threshold voltage dependence on back-gate-source voltage for different dielectrics demonstrated an inverse relationship between the two. The analytical expressions and their implementation through graphical representation for a bilayer graphene channel will be extended to a multilayer channel in the future to improve the device performance.

Graphene Mobility Dependence on the Resistivity of Si Wafer

Graphene field effect transistors (GFETs) are typically benchmarked based on their field effect mobility. Constant mobility model proposed by Kim1 et.al is extensively used to derive this mobility parameter by directly fitting the transfer curves of graphene devices. Various methodologies were suggested to supplant and improve this existing mobility model. Among them Zhang2 et.al suggested the need for using measured gate oxide capacitance in order to extract mobility for top-gated GFETs. One of the commonly overlooked features, in the case of back gated GEFTs is the dopant concentration or the resistivity of Si substrate used. Herein, we explore the dependence of graphene carrier mobility on the induced carrier concentration and depletion capacitance of underlying Si substrate. Si with resistivities 0.01-0.02 Ω-cm and 0.001-0.005 Ω-cm were used to grow 10 nm thermal SiO2. GFETs were fabricated using CVD graphene on top of this thermal oxide substrate and the corresponding transfer curves are co-related with the capacitance density measured from corresponding MOSCAP structure. Detailed carrier mobility extraction and results are presented.(1) Kim, S.; Nah, J.; Jo, I.; Shahrjerdi, D.; Colombo, L.; Yao, Z.; Tutuc, E.; Banerjee, S. K. Realization of a High Mobility Dual-Gated Graphene Field-Effect Transistor with Al2O3 Dielectric. Appl. Phys. Lett. 2009, 94 (6). https://doi.org/10.1063/1.3077021.(2) Zhang, Z.; Xu, H.; Zhong, H.; Peng, L. M. Direct Extraction of Carrier Mobility in Graphene Field-Effect Transistor Using Current-Voltage and Capacitance-Voltage Measurements. Appl. Phys. Lett. 2012, 101 (21). https://doi.org/10.1063/1.4768690.

Feasibility of a Dual-Gate Graphene Transistor to Test Various Gate Dielectrics for Two-Dimensional Device Application

Graphene transistors with a dual-gate structure have been fabricated to study the effects of a functional dielectric substrate. From graphene’s conductance modulation by using dual-gate voltage control in which a polymethyl methacrylate (PMMA) layer is employed as a top-gate dielectric, we propose that the interaction between the graphene and the functional material working as a back-gate dielectric substrate can be understood. We evaluated two graphene devices, one having the standard structure of Si/SiO2/graphene/PMMA/top-gate and the other employing periodically poled lithium niobate (PPLN) with the structure of PPLN/graphene/PMMA/top-gate. The feasibility of this approach is discussed in terms of the effect of the alternating polarization domains on graphene’s charge transport.

A new pressure microsensor based on dual-gate graphene field-effect transistor with a vertically movable top-gate: Proposal, analysis, and optimization

The combination of modern electronic devices and nanoelectromechanical systems (NEMS) has given new impulses to the development of sensors and biosensors. A new pressure microsensor based on graphene field-effect transistor (GFET) endowed with dual-gate design is proposed herein. The movable top-gate concept is used as sensing principle to detect the pressure-induced nanoscale displacement. A compact GFET drain-current model based on drift-diffusion carrier transport and field-effect principle is employed for the performance assessment of the proposed pressure microsensor. The investigation includes the top-gate and dual-gate configuration, transfer characteristic behavior, sensitivity analysis, operating regime, and the impact of gates' biases on the microsensor sensitivity. The performance analysis reveals that the dual-gate GFET design outperforms the top-gated GFET in terms of sensitivity. In addition, the high back-gate voltage and the low drain-source voltage have been found useful in improving the sensitivity of the DG GFET-based pressure sensor. Moreover, a metaheuristic optimization technique based on genetic algorithm (GA) has been adopted to find the optimal sensor parameters values that lead to the best sensitivity performance. The merits of the proposed dual-gate GFET-based pressure microsensor, namely small-size, high-sensitivity, graphene cost, and CMOS compatibility, make it a promising candidate for high-performance pressure sensing applications.

Characteristics of Dual‐Gate Graphene Thermoelectric Devices Based on Voltage Regulation

A dual-gate graphene thermoelectric device structure is proposed. Under the control of back-gate and top-gate voltage, the type and degree of carrier transport in graphene channels can be changed, where the absorption and release of heat are controlled at the contact between graphene and metal electrodes. More details can be found in article number 1901466 by Xiumin Gao and co-workers.

Tunable 1/f Noise in CVD Bernal-Stacked Bilayer Graphene Transistors.

Low-frequency noise is a key performance-limiting factor in almost all electronic systems. Thanks to its excellent characteristics such as exceptionally high electron mobility, graphene has high potential for future low-noise electronic applications. Here, we present an experimental analysis of low-frequency noise in dual-gate graphene transistors based on chemical vapor-deposited Bernal-stacked bilayer graphene. The fabricated dual-gate bilayer graphene transistors adopt atomic layer-deposited Al2O3 and HfSiO as top-gate and back-gate dielectric, respectively. Our results reveal an obvious M-shape gate-dependent noise behavior which can be well described by a quantitative charge-noise model. The minimal area normalized noise spectral density at 10 Hz reaches as low as about 3 × 10-10 μm2·Hz-1 at room temperature, much lower than the best results reported previously for graphene devices. In addition, the observed noise level further decreases by more than 10 times at temperature of 20 K. Meanwhile, the noise spectral density amplitude can be tuned by more than 2 orders of magnitude at 20 K by dual-gate voltages.

Quantum Capacitance in Dual-gated Graphene FETs with AlOx Insulating Layer

Quantum capacitance (Cq), which arises from the lack of charge carriers in low dimensional metallic materials, can be used to explore intrinsic properties of materials such as their density of states or residual carrier density when properly extracted. Here, Cq is extracted from the measured total capacitance of graphene-based devices using a dual-gated FET configuration with an oxidized aluminum/aluminum top-gate at room temperature. Acquired Cq is well in agreement with a theory for ideal graphene except for near Dirac point. The discrepancy in values near Dirac point is then adjusted by considering extra residual carrier density induced from charged impurities at finite temperature. The modified theory with residual carriers shows a great agreement with Cq measurements with the extracted residual carrier density of n* = 3.58 × 1011 cm−2.

Modelling of transport properties of graphene field-effect transistor for sensor application

A dual-gated ultra-thin graphene field effect transistor (GFET) suitable for electronic sensing application is modelled. The applied simulation approach reproduces accurately the transport properties of the GFET characteristics and enables investigation of the influence of the different physical, biological and chemical factors. The simulation readouts and additional charges in the system are interpreted in the form of current-voltage characteristics and shift in Dirac peaks. These features could be extracted to predict the sensing mechanism of the GFET.

Electrical Control of Cyclotron Resonance in Dual-Gated Trilayer Graphene.

Landau levels (LLs) of ABA-stacked trilayer graphene (TLG) are described as the combination of monolayer graphene-like LLs and bilayer graphene-like LLs. They are extremely sensitive to the applied perpendicular electric displacement field D. Here, we demonstrate the electrical control of cyclotron resonance (CR) in a dual-gated ABA-stacked TLG. Under the irradiation of mid-infrared light, we observed the photovoltage induced by the CR absorption through the photothermoelectric effect. The resonant magnetic field in CR is changed by applying D while keeping the carrier density constant. Numerical simulations based on the tight-binding model complement the experimental observations. We believe that the present study provides a boost to graphene-based photodetectors and photoemitters with an electrically tunable wavelength in mid-infrared to terahertz spectral ranges.

Fractional and Symmetry-Broken Chern Insulators in Tunable Moiré Superlattices.

We study dual-gated graphene bilayer/hBN moiré superlattices. Under zero magnetic field, we observe additional resistance peaks as the charge density varies. The peaks' resistivities vary approximately quadratically with an applied perpendicular displacement field D. Data fit to a continuum model yield a bilayer/hBN interaction energy scale ∼30 ± 10 meV. Under a perpendicular magnetic field, we observe Hofstadter butterfly spectra as well as symmetry-broken and fractional Chern insulator states. Their topology and lattice symmetry breaking is D-tunable, enabling the realization of new topological states in this system.