Graphene Field Effect Devices Research Articles

Enhanced Mobility in Suspended Chemical Vapor-Deposited Graphene Field-Effect Devices in Ambient Conditions.

High-field-effect mobility and the two-dimensional nature of graphene films make it an interesting material for developing sensing applications with high sensitivity and low power consumption. The chemical vapor deposition process allows for producing high-quality graphene films in a scalable manner. Considering the significant impact of the underlying substrate on the graphene device performance, methods to enhance the field-effect mobility are highly desired. This work demonstrates a simplified fabrication process to develop suspended, two-terminal chemical vapor deposition (CVD) graphene devices with enhanced field-effect mobility operating at room temperature. Enhanced hole field-effect mobility of up to ∼4.8 × 104 cm2/Vs and average hole mobility >1 × 104 cm2/Vs across all of the devices is demonstrated. A gradual increase in the width of the graphene device resulted in the increase of the full width at half-maximum (FWHM) of field-effect characteristics and a decrease in the field-effect mobility. Our work presents a simplified fabrication approach to realize high-mobility suspended CVD graphene devices, beneficial for developing CVD graphene-related applications.

Low-leakage epitaxial graphene field-effect transistors on cubic silicon carbide on silicon

Epitaxial graphene (EG) on cubic silicon carbide (3C-SiC) on silicon holds the promise of tunable nanoelectronic and nanophotonic devices, some uniquely unlocked by the graphene/cubic silicon carbide combination, directly integrated with the current well-established silicon technologies. Yet, the development of graphene field-effect devices based on the 3C-SiC/Si substrate system has been historically hindered by poor graphene quality and coverage, as well as substantial leakage issues of the heteroepitaxial system. We address these issues by growing EG on 3C-SiC on highly resistive silicon substrates using an alloy-mediated approach. In this work, we demonstrate a field-effect transistor based on EG/3C-SiC/Si with gate leakage current 6 orders of magnitude lower than the drain current at room temperature, which is a vast improvement on the current literature, opening the possibility for dynamically tunable nanoelectronic and nanophotonic devices on silicon at the wafer level.

Sensing Remote Bulk Defects through Resistance Noise in a Large-Area Graphene Field-Effect Transistor

The substrate plays a crucial role in determining the transport and low-frequency noise behavior of graphene field-effect devices. Typically, a heavily doped Si/SiO2 substrate is used to fabricate these devices for efficient gating. Trapping-detrapping processes close to the graphene/substrate interface are the dominant sources of resistance fluctuations in the graphene channel, while Coulomb fluctuations arising due to any remote charge fluctuations inside the bulk of the substrate are effectively screened by the heavily doped substrate. Here, we present the electronic transport and low-frequency noise characteristics of a large-area CVD graphene field-effect transistor (FET) prepared on a lightly doped Si/SiO2 substrate (NA ≈ 1015 cm-3). Through a systematic characterization of transport, noise, and capacitance at various temperatures, we reveal that the remote Si/SiO2 interface can affect the charge transport in graphene severely and any charge fluctuations inside the bulk of the silicon substrate can be sensed by the graphene channel. The resistance (R) vs back-gate voltage (Vbg) characteristics of the device show a hump around the depletion region formed at the SiO2/Si interface, confirmed by the capacitance (C)-voltage (V) measurement. A low-frequency noise measurement on these fabricated devices shows a peak in the noise amplitude close to the depletion region. This indicates that due to the absence of any charge layer at the Si/SiO2 interface, the screening ability decreases, and as a consequence, any fluctuations in the deep-level Coulomb impurities inside the silicon substrate can be observed as noise in resistance in the graphene channel via mobility fluctuations. The noise behavior on ionic liquid-gated graphene on the same substrate exhibits no such peak in noise and can be explained by the interfacial trapping-detrapping processes close to the graphene channel. Our study will definitely be useful for integrating graphene with the existing silicon technology, in particular, for high-frequency applications.

PH Sensitivity of Edge-Gated Graphene Field-Effect Devices with Covalent Edge Functionalization

pH Sensitivity of Edge-Gated Graphene Field-Effect Devices with Covalent Edge Functionalization

Correlation-driven electron-hole asymmetry in graphene field effect devices

Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in twisted graphene and moiré heterostructures has spurred interest into whether it stems from single-particle effects or from correlations, which are core to the emergence of intriguing phases in moiré systems. Here, we report an effective way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field-effect devices. As the chemical potential moves from the hole to the electron-doped side, we see an increased strength of electronic correlations manifested by an increase in the band velocity and inverse quasiparticle lifetime. These results suggest that electronic correlations intrinsically drive the electron-hole asymmetry in graphene and by leveraging this asymmetry can provide alternative avenues to generate exotic phases in twisted moiré heterostructures.

Plasma‐Enhanced Atomic Layer Deposition of Al2O3 on Graphene Using Monolayer hBN as Interfacial Layer

AbstractThe deposition of dielectric materials on graphene is one of the bottlenecks for unlocking the potential of graphene in electronic applications. The plasma enhanced atomic layer deposition of 10 nm thin high quality aluminum oxide (Al2O3) on graphene is demonstrated using a monolayer of hexagonal boron nitride (hBN) as protection layer. Raman spectroscopy is performed to analyze possible structural changes of the graphene lattice caused by the plasma deposition. The results show that a monolayer of hBN in combination with an optimized deposition process can effectively protect graphene from damage, while significant damage is observed without an hBN layer. Electrical characterization of double gated graphene field effect devices confirms that the graphene does not degrade during the plasma deposition of Al2O3. The leakage current densities are consistently below 1 pA µm−2 for electric fields across the insulators of up to 8 MV cm−1, with irreversible breakdown happening above. Such breakdown electric fields are typical for Al2O3 and can be seen as an indicator for high quality dielectric films.

Room-temperature negative magnetoresistance of helium-ion-irradiated defective graphene in the strong Anderson localization regime

Anderson localization (AL), a major topic in condensed matter physics, has been extensively studied. Since the discovery of graphene, its unique AL in Dirac materials, particularly weak localization (WL), has been studied intensively. Strong localization (SL) has also been investigated in graphene with intentionally introduced defects. The precise control of spacing/strength of defects using conventional methods is challenging; thus, He-ion-irradiation is a promising technology. However, magnetotransport, a sensitive tool to probe AL, has not yet been studied for He-ion-irradiated graphene. Herein, we systematically investigate the magnetotransport of He-ion-irradiated graphene. We observe negative magnetoresistance (MR) due to SL-dominated hopping transport. By systematically tuning the device parameters, negative MR at room temperature is first revealed for graphene field-effect transistor devices. Our study reveals carrier localization via searching in the multidimensional parameter space of Dirac materials, contributing to the development of fundamental AL physics and magnetoelectronic device applications.

Inhomogeneous work-function hysteresis in chemical vapor deposition-grown graphene field-effect devices

The work function of graphene is known to be tuned by gate voltage on SiO2 substrates. For a graphene field-effect transistor (FET), chemical species such as oxygen and water molecules induce electrostatic interactions between the graphene and SiO2 substrate, and consequently, the conductance of graphene can be shifted with respect to the applied field effect. Furthermore, these shifts cause an inevitable hysteresis in the I–V characteristics of the graphene device, which degrades its performance. Herein, we study the work-function of graphene devices on SiO2 substrates with chemical vapor deposition-grown graphene using Kelvin probe force microscopy and report the gate voltage-dependent work-function hysteresis, which is analogous to the hysteresis in the electronic transport of graphene FETs. The degree of work-function hysteresis is inhomogeneous depending on the positions on the graphene, and it originates from the inhomogeneous distribution of the chemical species such as H2O and O2 molecules at the interface of a graphene/SiO2 substrate. This inhomogeneity of chemical species seems to be moderated by the gate voltage sweeping during the gate-voltage-dependent work-function measurements. The proposed study provides an advanced understanding of hysteresis phenomena in graphene devices and the guidance for developing controlled graphene devices with minimal influence from ambient conditions.

High-performance portable graphene field-effect transistor device for detecting Gram-positive and -negative bacteria

Current techniques for Gram-typing and for diagnosing a pathogen at the early infection stage rely on Gram stains, cultures, Enzyme linked immunosorbent assay (ELISA), polymerase chain reaction (PCR), and gene microarrays, which are labor-intensive and time-consuming approaches. In addition, a delayed or imprecise diagnosis of clinical pathogenic bacteria leads to a life-threatening emergency or overuse of antibiotics and a high-rate occurrence of antimicrobial-resistance microbes. Herein, we report high-performance antibiotics (as bioprobes) conjugated graphene micropattern field-effect transistors (ABX-GMFETs) to facilitate on-site Gram-typing and help in the detection of the presence or absence of Gram-negative and -positive bacteria in the samples. The ABX-GMFET platform, which consists of recognition probes and GM transistors conjugated with novel interfacing chemical compounds, was integrated into the microfluidics to minimize the required human intervention and facilitate automation. The mechanism of binding of ABX-GMFET was based on a charge or chemical moiety interaction between the bioprobes and target bacteria. Subsequently, ABX-GMFETs exhibited unprecedented high sensitivity with a limit of detection (LOD) of 100 CFU/mL (1–9 CFU/mL), real-time target specificity.

Giant-Capacitance-Induced Wide Quantum Hall Plateaus in Graphene on LaAlO3/SrTiO3 HeterostructuresSupported by the National Natural Science Foundation of China (Grant Nos. 11974324, 11804326 and U1832151), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDC07010000), the National Key Research and Development Program of China (Grant No. 2017YFA0403600), Anhui Initiative in Quantum Information Technologies (Grant No. AHY170000), and Hefei

Hybrid structures of two distinct materials provide an excellent opportunity to optimize functionalities. We report the realization of wide quantum Hall plateaus in graphene field-effect devices on the LaAlO3/SrTiO3 heterostructures. Well-defined quantized Hall resistance plateaus at filling factors v = ± 2 can be obtained over wide ranges of the magnetic field and gate voltage, e.g., extending from 2 T to a maximum available magnetic field of 9 T. By using a simple band diagram model, it is revealed that these wide plateaus arise from the ultra-large capacitance of the ultra-thin LAO layer acting as the dielectric layer. This is distinctly different from the case of epitaxial graphene on SiC substrates, where the realization of giant Hall plateaus relies on the charge transfer between the graphene layer and interface states in SiC. Our results offer an alternative route towards optimizing the quantum Hall performance of graphene, which may find its applications in the further development of quantum resistance metrology.

Monitoring the hemostasis process through the electrical characteristics of a graphene-based field-effect transistor

Graphene-based transistors are promising devices in the evaluation of carrier density in biological analytes. We report on the design and fabrication of a graphene-based field-effect transistor for monitoring and assessing the interaction between the coagulation factors based on the charge carrier density in a blood sample. When biochemical reactions occurred during the coagulation cascade process, a dopant effect was noticed on the graphene surface by the change in Dirac point voltage values. Additional experiments were performed using blood samples treated with activators (vitamin K, calcium chloride, and thromboplastin reagent) and inhibitors (heparin drugs) to evaluate the selectivity of the graphene field-effect transistor devices. Since the transfer characteristic curves presented divergent behaviours for different levels of procoagulants and anticoagulants, the measurements showed that the devices can assess changes in the concentrations of factors that inhibit or accelerate the cascade process when using untreated and treated samples. Reproducibility was verified by testing samples from different sources. To the best of our knowledge, this study is the first to demonstrate the potential of graphene in monitoring the hemostasis process through the analysis of the electrical properties of human whole blood.

Biomolecular Sensors Based on Microfluidics-Embedded Graphene Field-Effect Transistors

With the rapid pace of discovery of molecular biomarkers associated with cancer and other diseases, there is a need for bioanalytical techniques able to detect and quantify large panels of biomolecules, with high throughput and accuracy. Nanoelectronic sensors are emerging in this field due to their potential for improved sensitivity, parallelization and portability. In this presentation, I will introduce our recent progress in assembling nanoelectronic biomolecular sensors based on graphene field-effect transistor device arrays combined with microfluidic circuitry. In particular, I will present two designs targeting either DNA oligonucleotide sequences or antibody-antigen coupling, and I will describe their electrical signatures for specific and non-specific biomolecular interactions with the graphene material. Finally, I will discuss the electronic detection of breast cancer and leukemia biomarkers for clinical diagnostics applications.

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.

Impact of Extrinsic Interface Traps and Doping Atoms on Conductivity of Graphene Field Effect Devices

Near-interfacial oxide traps and chemical impurities on the graphene surface or at the graphene-dielectric interface can be a source of intentional or unintentional doping of graphene sheet. The efficiency of such chemical doping can vary in a wide range depending on parameters of graphene field effect devices. Mechanisms of such sensitivity of doping efficiency to the device characteristics need to be understood. The objective of this paper is to theoretically derive the analytical relations, adapted to the explicit calculation of graphene chemical doping.

Graphene devices with bottom-up contacts by area-selective atomic layer deposition

Graphene field-effect transistor devices were fabricated using a bottom-up and resist-free method, avoiding common compatibility issues such as contamination by resist residues. Large-area CVD graphene sheets were structured into device channels by patterning with a focused ion beam. Platinum contacts were then deposited by direct-write atomic layer deposition (ALD), which is a combination between electron beam induced deposition (EBID) and bottom-up area-selective ALD. This is a unique approach that enables nucleation of Pt ALD on graphene, and therefore these devices are the first reported graphene devices with contacts deposited by ALD. Electrical characterization of the devices confirms ambipolar transistor behaviour with typical field-effect mobilities in the range of 1000–1800 cm2 V−1 s−1. We observe clear signs of strong Pt-graphene coupling and contact induced hole doping, implying good contact properties in contrast to the conventionally weak bonding between Pt and graphene. We attribute these observations to the reduced amount of resist residue under the contacts, the improved wettability of the Pt due to the use of ALD, and the formation of a graphitic interlayer that bonds the Pt more strongly to the graphene. We conclude that direct-write ALD is a very suitable technique for metallization of graphene devices and to study the intrinsic properties of metal-graphene contacts in more detail. In addition, it offers unique opportunities to control the metal-graphene coupling strength.

The effects of lithographic residues and humidity on graphene field effect devices

Recently, unknown-manner changes in charge neutrality point (CNP) positioning were ascribed to humidity at graphene field effect transistors (GFETs). While the exact means of humidity interacting with hydrophobic graphene remains unknown, this work examines pristine and lithographic-process-applied graphene surfaces with surface enhanced Raman spectra (SERS). SERS analysis shows that the lithographic-process-applied graphene does not have the same properties as those of pristine graphene. Furthermore, this study has experimentally investigated the effect of humidity on the transfer characteristics of GFET and proposed a model to explain the formation of asymmetric I DS–V bg branches in accordance with the SERS results and humidity responses.

Room‐Temperature Quantum Transport Signatures in Graphene/LaAlO3/SrTiO3 Heterostructures

High mobility graphene field-effect devices, fabricated on the complex-oxide heterostructure LaAlO3 /SrTiO3 , exhibit quantum interference signatures up to room temperature. The oxide material is believed to play a critical role in suppressing short-range and phonon contributions to scattering. The ability to maintain pseudospin coherence at room temperature holds promise for the realization of new classical and quantum information technologies.

Tomography of a Probe Potential Using Atomic Sensors on Graphene.

Our ability to access and explore the quantum world has been greatly advanced by the power of atomic manipulation and local spectroscopy with scanning tunneling and atomic force microscopes, where the key technique is the use of atomically sharp probe tips to interact with an underlying substrate. Here we employ atomic manipulation to modify and quantify the interaction between the probe and the system under study that can strongly affect any measurement in low charge density systems, such as graphene. We transfer Co atoms from a graphene surface onto a probe tip to change and control the probe's physical structure, enabling us to modify the induced potential at a graphene surface. We utilize single Co atoms on a graphene field-effect device as atomic scale sensors to quantitatively map the modified potential exerted by the scanning probe over the whole relevant spatial and energy range.

Tunable doping of graphene by using physisorbed self-assembled networks.

One current key challenge in graphene research is to tune its charge carrier concentration, i.e., p- and n-type doping of graphene. An attractive approach in this respect is offered by controlled doping via well-ordered self-assembled networks physisorbed on the graphene surface. We report on tunable n-type doping of graphene using self-assembled networks of alkyl-amines that have varying chain lengths. The doping magnitude is modulated by controlling the density of the strong n-type doping amine groups on the surface. As revealed by scanning tunneling and atomic force microscopy, this density is governed by the length of the alkyl chain which acts as a spacer within the self-assembled network. The modulation of the doping magnitude depending on the chain length was demonstrated using Raman spectroscopy and electrical measurements on graphene field effect devices. This supramolecular functionalization approach offers new possibilities for controlling the properties of graphene and other two-dimensional materials at the nanoscale.

Tunable field effect properties in solid state and flexible graphene electronics on composite high – low k dielectric

We demonstrate tunable field effect properties in solid state and flexible graphene field effect devices (FEDs) fabricated using a poly(methylmethacrylate) (PMMA) and lithium fluoride (LiF) composite dielectric. Increasing the concentration of LiF in the composite dielectric increases the capacitance, which thereby reduces the operating gate voltages of FEDs significantly from 10 V to 1 V to achieve similar conductivity. Electron and hole mobility of 350 and 310 cm2/V at VD = −5 V are obtained for graphene FEDs with 10% LiF concentration in the composite. Composite dielectric also enabled excellent FEDs on flexible substrates without any significant change in mobility and resistance. Flexible FEDs with only 5% and 12% variation in mobility for 300 and 750 bending are obtained.