Back-gated Graphene Field-effect Transistors Research Articles

Al back-gated graphene field-effect transistors for capacitive sensing applications based on quantum capacitance effect

We report a new device design of a graphene field-effect transistor (G-FET) for capacitive sensing application. A channel area exposed to ambient conditions in G-FET is known to be a promising candidate for molecular level sensing applications because graphene can attract certain molecules with its freely hanging sigma bonds. In addition, molecules that adhere to graphene act as impurities that affect the electron transport within graphene. Two of the most common ways to evaluate such a change are measuring the changes in resistance and in quantum capacitance. Previous research studies have been largely focused on using resistive measurement due to restrictions from device design even though capacitive measurement can be cost-effective. To overcome the obstacles, we developed G-FET with high capacitance and a large exposed channel area by incorporating Al back-gate electrodes with naturally oxidized AlOx surface as an insulating layer. The measured capacitance was well-modulated in vacuum by the gate voltage due to the quantum capacitance effect. Also, the capacitance curve was shifted up to the right in the 100 ppm NO2 environment. The capacitance at zero gate bias was increased by 56.6% from the vacuum to the 100 ppm NO2 environment. These results indicate that the proposed device can be used for capacitive sensing applications.

High-Frequency Noise Characterization and Modeling of Graphene Field-Effect Transistors

This article presents the small-signal and noise characterization of different technologies based on chemical vapor deposition (CVD) and silicon-carbide (SiC) graphene field-effect transistors (GFETs). The noise model, built on noise figure measurements under $50~\Omega $ using the F50 method, was verified by additional source-pull measurements, with special care for the GFET stability. The four noise parameters were then extracted by using the validated F50 model up to 18 GHz, and the correlations between noise and small-signal parameters were shown for two different configurations: top-gated and back-gated GFETs.

Growth and carrier transport performance of single-crystalline monolayer graphene over electrodeposited copper film on quartz glass

The synthesis of single crystal graphene domains on cold rolling Cu foil has recently been reported. However, the cold rolling Cu foils have many rolling lines that increase the roughness of the foil. In this work, we developed an electrodeposited method to achieve high-quality Cu film on which the nucleation density of graphene decreased greatly. The Cu film was electrodeposited over chemical plated silver film on quartz glass. The roughness and thickness of the Cu film were positively correlated with the deposition temperature and current density, respectively. High-quality single crystal graphene with low nucleation density was achieved on electrodeposited Cu film. With increasing deposition temperature of Cu film from 20 to 65 °C, the nucleation density of graphene increased from 124 to 448 mm−2. The nucleation density of graphene increased from 124 to 192 mm−2 with increasing current density from 0.03 to 0.07 A/cm−2. Finally, a back-gated graphene field effect transistor (FET) was fabricated with a carrier transport performance of μh = ~4041 cm2V−1s−1 and μe = ~2580 cm2V−1s−1 at room temperature.

Stable hydroxyl functionalization and p-type doping of graphene by a non-destructive photo-chemical method

A photon-initiated chemical route for hydroxylation of CVD graphene and exfoliated graphene is presented. Hydroxyl functionalities have been bonded covalently to exfoliated and CVD graphene by treating the graphene with H2O2 in the presence of UV light. Back-gated graphene field effect transistors fabricated from hydroxyl functionalized graphene shows p-type doping and mobility in the range of 1000 cm2 V−1 s−1 which is much higher than reported in graphene oxide and reduced graphene oxide. Vacuum stability of hydroxyl functional groups is demonstrated by keeping hydroxyl functionalized graphene in vacuum for 22 h and comparing the electrical characteristics. As a further proof of –OH functionalization, we demonstrate the deposition of Al2O3 by atomic layer deposition, which is not possible on pristine graphene.

Frequency multiplier based on back-gated graphene FETs with M-shaped resistance characteristics

The ambipolar graphene field-effect transistors (GFETs) usually exhibit Λ-shaped resistance versus gate voltage characteristics (R-Vg curve) with the n- and p-type regions switching at the neutrality points of graphene. However, M-shaped R-Vg curves were frequently observed in our back-gated GFETs without intentional doping. Here, we proposed an implementation of a frequency multiplier using the M-like shape of the R-Vg curve. We first investigated the effect of the channel length and the contact transfer length in a GFET on the shape of the R-Vg curve and then evaluated the influence of the various shapes of R-Vg curves on the performance of the frequency multiplier (including tripler and quadrupler). Finally, a frequency tripler based on a single GFET has been experimentally demonstrated. When applying a sinusoid input signal with a fundamental frequency at a suitable operation area, around 80% output signal power is concentrated at the third harmonic. The excellent output spectral purity makes GFETs with the M-shaped R-Vg curve promising candidates for the frequency multiplier.

Three-Dimensional Graphene Field-Effect Transistors as High-Performance Photodetectors.

Graphene is an ideal material for high-performance photodetectors because of its superior electronic and optical properties. However, graphene's weak optical absorption limits the photoresponsivity of conventional photodetectors based on planar (two-dimensional or 2D) back-gated graphene field-effect transistors (GFETs). Here, we report a self-rolled-up method to turn 2D buried-gate GFETs into three-dimensional (3D) tubular GFETs. Because the optical field inside the tubular resonant microcavity is enhanced and the light-graphene interaction area is increased, the photoresponsivity of the resulting 3D GFETs is significantly improved. The 3D GFET photodetectors demonstrated room-temperature photodetection at ultraviolet, visible, mid-infrared, and terahertz (THz) regions, with both ultraviolet and visible photoresponsivities of more than 1 A W-1 and photoresponsivity of 0.232 A W-1 at 3.11 THz. The electrical bandwidth of these devices exceeds 1 MHz. This combination of high photoresponsivity, a broad spectral range, and high speed will lead to new opportunities for 3D graphene optoelectronic devices and systems.

Long-term stability of photodetectors based on graphene field-effect transistors encapsulated with Si3N4 layers

Graphene photodetectors have drawn a lot of interest due to their superior properties. For entering real applications, the existing open-face graphene photodetectors need to be properly protected to obtain long term stability. Here we demonstrate a facile method to obtain air-stable graphene photodetectors by directly depositing Si3N4 layer on the surface of buried-gate graphene field-effect transistors (GFETs). The electrical and photoelectrical properties of the Si3N4-encapsulated GFETs were carefully investigated and compared with that of the reference open-face GFETs. A photoresponsivity of 6.3 mA/W was obtained under a gate bias of 6 V, which was over 4 times higher than the photoresponsivities of typical back-gated GFET photodetectors under gate biases up to 80 V. Furthermore, the Si3N4-encapsulated GFET photodetectors demonstrated good stability for several months.

Wafer Scale Graphene Field Effect Transistors on Thin Thermal Oxide

In this study, we present the feasibility to fabricate back-gated graphene field-effect transistors (GFETs) on 10nm thermal SiO2 substrate. Here, we compare the mobility of graphene devices at different locations of the transferred CVD graphene. We observed that there is a n-type doping of the graphene devices with Dirac points within ±0.5V from an ideal value of 0V. The downscaling of the back-gate dielectric thickness reduces the operating voltage range, commonly required for low power electronics, and the devices are stable during operation in air under ambient conditions. The extracted contact resistance is comparable to the earlier reports found in literature and this provides a feasibility to fabricate low power futuristic graphene based nanoelectronics.

Position-dependent and millimetre-range photodetection in phototransistors with micrometre-scale graphene on SiC.

The extraordinary optical and electronic properties of graphene make it a promising component of high-performance photodetectors. However, in typical graphene-based photodetectors demonstrated to date, the photoresponse only comes from specific locations near graphene over an area much smaller than the device size. For many optoelectronic device applications, it is desirable to obtain the photoresponse and positional sensitivity over a much larger area. Here, we report the spatial dependence of the photoresponse in backgated graphene field-effect transistors (GFET) on silicon carbide (SiC) substrates by scanning a focused laser beam across the GFET. The GFET shows a nonlocal photoresponse even when the SiC substrate is illuminated at distances greater than 500 µm from the graphene. The photoresponsivity and photocurrent can be varied by more than one order of magnitude depending on the illumination position. Our observations are explained with a numerical model based on charge transport of photoexcited carriers in the substrate.

Improving the radiation hardness of graphene field effect transistors

Ionizing radiation poses a significant challenge to the operation and reliability of conventional silicon-based devices. Here, we report the effects of gamma radiation on graphene field-effect transistors (GFETs), along with a method to mitigate those effects by developing a radiation-hardened version of our back-gated GFETs. We demonstrate that activated atmospheric oxygen from the gamma ray interaction with air damages the semiconductor device, and damage to the substrate contributes additional threshold voltage instability. Our radiation-hardened devices, which have protection against these two effects, exhibit minimal performance degradation, improved stability, and significantly reduced hysteresis after prolonged gamma radiation exposure. We believe this work provides an insight into graphene's interactions with ionizing radiation that could enable future graphene-based electronic devices to be used for space, military, and other radiation-sensitive applications.

Scalable Production of High-Sensitivity, Label-Free DNA Biosensors Based on Back-Gated Graphene Field Effect Transistors.

Scalable production of all-electronic DNA biosensors with high sensitivity and selectivity is a critical enabling step for research and applications associated with detection of DNA hybridization. We have developed a scalable and very reproducible (>90% yield) fabrication process for label-free DNA biosensors based upon graphene field effect transistors (GFETs) functionalized with single-stranded probe DNA. The shift of the GFET sensor Dirac point voltage varied systematically with the concentration of target DNA. The biosensors demonstrated a broad analytical range and limit of detection of 1 fM for 60-mer DNA oligonucleotide. In control experiments with mismatched DNA oligomers, the impact of the mismatch position on the DNA hybridization strength was confirmed. This class of highly sensitive DNA biosensors offers the prospect of detection of DNA hybridization and sequencing in a rapid, inexpensive, and accurate way.

Surface treatment process applicable to next generation graphene-based electronics

The polymer residue remaining on chemical-vapor-deposited graphene after its transfer to the substrate and subsequent lithographic patterning tends to cause problems such as decrease in electron mobility, and unwanted doping. In this study, by using a controllable low-energy Ar+ ion beam (9.5 eV), the residue was cleaned perfectly without damaging the graphene surface. Further, a back-gate graphene field-effect transistor fabricated on the Ar+-ion-cleaned graphene surface showed about 4 times higher drain current than that showed by a similar transistor fabricated on pristine graphene. We believe that the technique used in this study can be useful in preventing the problems caused by the residue remaining on the graphene surface and can be applied not only to the processing of next-generation graphene-based electronics but also to other 2D materials-based electronic material processing.

HfO2 dielectric thickness dependence of electrical properties in graphene field effect transistors with double conductance minima

We investigate the electrical properties in back-gated graphene field effect transistors (GFETs) with SiO2 dielectric and different thickness of high-k HfO2 dielectric. The results show that transform characteristic (Ids–Vgs) curves of GFETs are uniquely W-shaped with two charge neutrality point (left and right) in both SiO2 and HfO2 dielectric (SiO2-GFETs and HfO2-GFETs). The gate voltage reduces drastically in HfO2-GFETs compared with that in SiO2-GFETs, and it becomes much smaller with the decline of HfO2 thickness. The left charge neutrality point in Id–Vg curves of all HfO2-GFETs is negative, compared to the positive ones in SiO2-GFETs, which means that there exists n-doping in graphene with HfO2 as bottom dielectric. We speculate that this n-doping comes from the HfO2 layer, which brings fixed charged impurities in close proximity to graphene. The carrier mobility is also researched, demonstrating a decreasing trend of hole mobility in HfO2-GFETs contrast to that in SiO2-GFETs. In a series of HfO2-GFETs with different HfO2 dielectric thickness, the hole mobility shows a tendency of rise when the thickness decreases to 7 nm. The possible reason might be due to the introduced impurities into HfO2 film from atomic layer deposition process, the concentration of which varies from the thickness of HfO2 layer.

Hysteresis analysis of graphene transistor under repeated test and gate voltage stress

The current transport characteristic is studied systematically based on a back-gate graphene field effect transistor, under repeated test and gate voltage stress. The interface trapped charges caused by the gate voltage sweep process screens the gate electric field, and results in the neutral point voltage shift between the forth and back sweep direction. In the repeated test process, the neutral point voltage keeps increasing with test times in both forth and back sweeps, which indicates the existence of interface trapped electrons residual and accumulation. In gate voltage stress experiment, the relative neutral point voltage significantly decreases with the reducing of stress voltage, especially in −40 V, which illustrates the driven-out phenomenon of trapped electrons under negative voltage stress.

Induction heating-assisted repeated growth and electrochemical transfer of graphene on millimeter-thick metal substrates

Chemical vapor deposition in a hot wall reactor is the most common technique for the production of large area single layer graphene. However, growth in this type of reactors is time consuming and the results are limited by the surface quality of the widely used catalytic metal foils as growth substrates. In this work we demonstrate the use of millimeter-thick Cu and Pt substrates for graphene growth via inductive magnetic heating, which allows for fast temperature ramps during heat up and cooling. Based on a detailed growth study, a two-step growth process resulting in continuous monolayer graphene films of high crystal quality with grain size of larger than 90μm is established. An electrochemical transfer process is used to separate the graphene film from the metallic substrate, yielding excellent results in terms of defect density, doping and residual contamination. Back-gated graphene field-effect transistors fabricated on Si/SiO2 structures exhibit a high reproducibility with a peak mobility higher than 4000cm2/Vs. The combination of the highly time efficient graphene growth and electrochemical transfer together with the reusability of the growth substrates and the possibility of applying novel surface pretreatments pave the way for the use of high quality substrates in industrial applications.

Plasmonic and bolometric terahertz detection by graphene field-effect transistor

Polarization dependence analysis of back-gated graphene field-effect transistor terahertz responsivity at frequencies ranging from 1.63 to 3.11 THz reveals two independent mechanisms of THz detection by graphene transistor: plasmonic, associated with the transistor nonlinearity, and bolometric, caused by graphene sheet temperature increase due to THz radiation absorption. In the bolometric regime, electron and hole branches demonstrate a very different response to THz radiation, which we link to the asymmetry of the current-voltage characteristics temperature dependence with respect to the Dirac point. Obtained results are important for development of high-efficiency graphene THz detectors.

Observation of the Ambient Effect in BTI Characteristics of Back-Gated Single Layer Graphene Field Effect Transistors

In this paper, the ambient doping effect on negative/positive bias temperature instability (NBTI/PBTI) of single layer graphene field effect transistors (FETs) is investigated. In ambient air, the ΔVth of NBTI is comparable with that of PBTI under the same stress voltage at room temperature. The ΔVth of NBTI appears insensitive to temperature, while the ΔVth of PBTI increases significantly with rising temperatures, due to the thermally activated charging of ambient doped defects at the graphene/SiO2 interface. This effect also results in an abnormal recovery of NBTI. In an ambient vacuum, the ΔVth is much less than that in ambient air. In addition, the ΔVth of both NBTI and PBTI decreases substantially for higher temperatures in the vacuum. The adsorbed molecules are mainly responsible for the ΔVth under BTI stress and the back-gated graphene FETs in the air.

Stability analysis of a back-gate graphene transistor in air environment

The stability of a graphene field effect transistor (GFET) is important to its performance optimization, and study of hysteresis behavior can propose useful suggestions for GFET fabrication and optimization. In this work, a back-gate GFET has been fabricated and characterized, which is compatible with the CMOS process. The stability of a GFET in air has been studied and it is found that a GFET's electrical performance dramatically changes when exposed to air. The hysteresis characteristic of a GFET depending on time has been observed and analyzed systematically. Hysteresis behavior is reversed at room temperature with the Dirac point positive shifted when the GFET is exposed to air after annealing.

In Situ Electrical Studies of Ozone Based Atomic Layer Deposition on Graphene

In this study, we present in-situ electrical studies of back-gated graphene field-effect transistors (GFETs) exposed to ozone at room temperature. Here, we compare the effect of the ozone exposure on transport characteristics in two different environments using a vacuum probe station with static vacuum and an atomic layer deposition (ALD) chamber having a dynamic environment with continuous nitrogen purge. We observed that in both the cases there is a p-type doping in graphene upon exposure to ozone, but, there is a significant difference in the ozone exposure time required to witness a noticeable effect on the transport properties, with the dynamic environment requiring a longer ozone treatment period. The observed effect on graphene devices was found to be reversible under vacuum suggesting a physisorption process. We also demonstrate top-gate GFETs on chemical vapor deposited (CVD) graphene using 10 nm ALD Al2O3 as gate dielectric deposited with the ozone functionalization approach.

Atomic Layer Deposition of Dielectrics on Graphene Using Reversibly Physisorbed Ozone

Integration of graphene field-effect transistors (GFETs) requires the ability to grow or deposit high-quality, ultrathin dielectric insulators on graphene to modulate the channel potential. Here, we study a novel and facile approach based on atomic layer deposition through ozone functionalization to deposit high-κ dielectrics (such as Al(2)O(3)) without breaking vacuum. The underlying mechanisms of functionalization have been studied theoretically using ab initio calculations and experimentally using in situ monitoring of transport properties. It is found that ozone molecules are physisorbed on the surface of graphene, which act as nucleation sites for dielectric deposition. The physisorbed ozone molecules eventually react with the metal precursor, trimethylaluminum to form Al(2)O(3). Additionally, we successfully demonstrate the performance of dual-gated GFETs with Al(2)O(3) of sub-5 nm physical thickness as a gate dielectric. Back-gated GFETs with mobilities of ~19,000 cm(2)/(V·s) are also achieved after Al(2)O(3) deposition. These results indicate that ozone functionalization is a promising pathway to achieve scaled gate dielectrics on graphene without leaving a residual nucleation layer.