Fabrication Of Field-effect Transistors Research Articles

Electron transport tuning of graphene by helium ion irradiation

This article reviews charge carrier transport phenomena in single-layer graphene, in which crystalline defects are generated by helium-ion-beam irradiation using a helium-ion microscope. Crystalline defects work as electron scatterers, and the conductivity drastically decays as ion dose increases. Moreover, real-time conductivity monitoring during ion beam scans over the graphene surface is demonstrated. In cryogenic measurements under magnetic fields, defective graphene exhibits negative magnetoresistance, suggesting that strong localization occurred in this two-dimensional electron system, which survived even at room temperature. The localized state contributes to inducing a transport gap around the Dirac point, where the density of states is at its minimum, and it enables field-effect control of the carrier transport by tuning the carrier density. The fabrication and operation of field-effect transistors with defective graphene channels are demonstrated.

Precise synthesis of α,ω-chain-end-functionalized poly(dimethylsiloxane) with bromoaryl groups for incorporation in naphthalene-diimide-based N-type semiconducting polymers

To date, poly(dimethylsiloxane) (PDMS)-based materials have been developed to utilize their unique properties, such as backbone flexibility and good solubility in organic solvents, in many applications. In the present study, we successfully synthesized multiblock copolymers consisting of PDMS, naphthalene-diimide (NDI), and bithiophene units by statistical terpolymerization. First, α,ω-chain-end-functionalized PDMS with 4-bromophenyl groups (BrPh-PDMS-PhBr) was obtained by metal-free controlled ring-opening polymerization of cyclotrisiloxane using a strong organo-base catalyst, followed by an end-capping reaction with (4-bromophenyl)chlorodimethylsilane. BrPh-PDMS-PhBr was found to be a key material in the statistical terpolymerization of BrPh-PDMS-PhBr, NDI, and bithiophene monomers based on the Migita-Kosugi-Stille coupling reaction. Indeed, three PDMS-containing multiblock copolymers were successfully synthesized with different PDMS compositions of 10, 16, and 36 wt%. In practice, the multiblock copolymer with 10 wt% PDMS content showed a typical n-type semiconducting character (μe = 1.5 × 10−3 cm2 V−1 s−1), according to the results of organic field-effect transistor (OFET) fabrication and evaluation, despite the incorporation of insulating PDMS segments along the π-conjugated main chains.

Electric Transport in Few-Layer ReSe2 Transistors Modulated by Air Pressure and Light.

We report the fabrication and optoelectronic characterization of field-effect transistors (FETs) based on few-layer ReSe2. The devices show n-type conduction due to the Cr contacts that form low Schottky barriers with the ReSe2 nanosheet. We show that the optoelectronic performance of these FETs is strongly affected by air pressure, and it undergoes a dramatic increase in conductivity when the pressure is lowered below the atmospheric one. Surface-adsorbed oxygen and water molecules are very effective in doping ReSe2; hence, FETs based on this two-dimensional (2D) semiconductor can be used as an effective air pressure gauge. Finally, we report negative photoconductivity in the ReSe2 channel that we attribute to a back-gate-dependent trapping of the photo-excited charges.

Fabrication and characterization of field effect transistor based on single walled carbon nanotubes

ObjectivesSensor biology and sensor devices have been advancing since its inceptions. In this work, we report fabrication of carbon nanotubes filed-effect transistor (CNT-FET) sensor and its characterization. CNT intensively has been used in the construction of sensing layers due to their exceptional features, large surface area, stability, high mechanical strength, adaptability, and functional behavior. MethodsCarbon nanotubes (CNTs) as semiconductor were fabricated as an active nanomaterial between the source-drain electrodes. The fabrication of CNT-FETs performed by following conventional photolithography method and lift-off techniques. ResultsThe structural morphology of deposited CNT was confirmed by the scanning electron micrograph (SEM) imaging. The transfer curves between drain-source were considered as a function of the drain-source voltage (VDS) and gate-source voltage (VGS) from individual CNT-FET fabricated wafer. The characterized Ion/Ioff ratio was calculated for every CNT-FET device. The semiconductor properties of the fabricated CNT-FET device characterized by the source-drain current (IDS) versus gate voltage (VGS). ConclusionsCNT-FET based device have advantages of low cost fabrication, quick response, increased sensitivity, small size, and high flexibility. CNT-FETs have been used comprehensively in the biosensing of chemicals, proteins, nucleic acids, bacteria, and virus etc. This device could be used for SARS-CoV-2 and related variant detection in current scenario.

Graphene Nanoribbon Field-Effect Transistors with Top-Gate Polymer Dielectrics

Graphene nanoribbons (GNRs) have demonstrated great potential for nanoscale devices owing to their excellent electrical properties. However, the application of the GNRs in large-scale devices still remains elusive mainly due to the absence of facile, nonhazardous, and nondestructive transfer methods. Here, we develop a simple acid (HF)-free transfer method for fabricating field-effect transistors (FETs) with a monolayer composed of a random network of GNRs. A polymer layer that is typically used as mechanical support for transferring GNR films is utilized as the gate dielectric. The resultant GNR-FETs exhibit excellent FET characteristics with a large on/off switching current ratio of >104. The transfer process enables the demonstration of the first GNR-based nonvolatile memory. The process offers a simple route for GNRs to be utilized in various optoelectronic devices.

Polyethylene and Semiconducting Polymer Blends for the Fabrication of Organic Field-Effect Transistors: Balancing Charge Transport and Stretchability

Polyethylene is amongst the most used polymers, finding a plethora of applications in our lives owing to its high impact resistance, non-corrosive nature, light weight, cost effectiveness, and easy processing into various shapes from different sizes. Despite these outstanding features, the commodity polymer has been underexplored in the field of organic electronics. This work focuses on the development of new polymer blends based on a low molecular weight linear polyethylene (LPE) derivative with a high-performance diketopyrrolopyrrole-based semiconducting polymer. Physical blending of the polyethylene with semiconducting polymers was performed at ratios varying from 0 to 75 wt.%, and the resulting blends were carefully characterized to reveal their electronic and solid-state properties. The new polymer blends were also characterized to reveal the influence of polyethylene on the mechanical robustness and stretchability of the semiconducting polymer. Overall, the introduction of LPE was shown to have little to no effect on the solid-state properties of the materials, despite some influence on solid-state morphology through phase separation. Organic field-effect transistors prepared from the new blends showed good device characteristics, even at higher ratios of polyethylene, with an average mobility of 0.151 cm2 V−1 s−1 at a 25 wt.% blend ratio. The addition of polyethylene was shown to have a plasticizing effect on the semiconducting polymers, helping to reduce crack width upon strain and contributing to devices accommodating more strain without suffering from decreased performance. The new blends presented in this work provide a novel platform from which to access more mechanically robust organic electronics and show promising features for the utilization of polyethylene for the solution processing of advanced semiconducting materials toward novel soft electronics and sensors.

Recessed-Channel WSe2 Field-Effect Transistor via Self-Terminated Doping and Layer-by-Layer Etching.

Effective channel control with low contact resistance can be accomplished through selective ion implantation in Si and III-V semiconductor technologies; however, this approach cannot be adopted for ultrathin van der Waals materials. Herein, we demonstrate a self-aligned fabrication process based on self-terminated p-doping and layer-by-layer chemical etching to achieve low contact resistance as well as a high on/off current ratio in ultrathin tungsten diselenide (WSe2) field-effect transistors (FETs). Damage-free layer-by-layer thinning of the WSe2 channel is repeated up to a thickness of approximately 1.4 nm, while maintaining the selectively p-doped source/drain regions. The device characteristics of the recessed-channel WSe2 FET are systematically monitored during this layer-by-layer recess-channel process. The WSe2 etching rate is estimated to be 2-3 layers per cycle of oxidation and subsequent chemical etching. The self-terminated tungsten oxide (WOX) layer grown through ultraviolet-ozone treatment induces robust p-doping in the neighboring (or underlying) WSe2 through the electron withdrawal mechanism, which remains in the source/drain regions after channel oxide removal. The adopted self-terminated and self-aligned recess-channel process for ultrathin WSe2 FETs enables the realization of a high on/off output current ratio (>108) and field-effect mobility (∼190 cm2/V·s), while maintaining low contact resistance (0.9-6.1 kΩ·μm) without a postannealing process. The proposed facile and reproducible doping and atomic-layer-etching method for the fabrication of a recessed-channel FET with an ultrathin body can be helpful for high-performance two-dimensional semiconductor devices and is applicable to post-Si complementary metal-oxide semiconductor devices.

Uniform nucleation and epitaxy of bilayer molybdenum disulfide on sapphire

Two-dimensional transition-metal dichalcogenides (TMDs) are of interest for beyond-silicon electronics1,2. It has been suggested that bilayer TMDs, which combine good electrostatic control, smaller bandgap and higher mobility than monolayers, could potentially provide improvements in the energy-delay product of transistors3-5. However, despite advances in the growth of monolayer TMDs6-14, the controlled epitaxial growth of multilayers remains a challenge15. Here we report the uniform nucleation (>99%) of bilayer molybdenum disulfide (MoS2) on c-plane sapphire. In particular, we engineer the atomic terrace height on c-plane sapphire to enable an edge-nucleation mechanism and the coalescence of MoS2 domains into continuous, centimetre-scale films. Fabricated field-effect transistor (FET) devices based on bilayer MoS2 channels show substantial improvements in mobility (up to 122.6 cm2 V-1 s-1) and variation compared with FETs based on monolayer films. Furthermore, short-channel FETs exhibit an on-state current of 1.27 mA μm-1, which exceeds the 2028 roadmap target for high-performance FETs16.

Influence of h-BN Substrate on the Electrical Performance of MoSe2 FETs

The unique physical features and transmission characteristics of two-dimensional materials have been widely recognized by the scientific community and extensively explored in recent years to find an alternative to silicon semiconductors' physical size restrictions. Two-dimensional (2D) materials, such as Molybdenum Diselenide (MoSe2), show a promising role in the next generation of semiconductor devices for advanced electronics and optoelectronics. In this work, we reported the fabrication of field-effect transistor using mechanically exfoliated Molybdenum Diselenide. To explore the intrinsic behavior of Molybdenum Diselenide, layered hexagonal Boron Nitride (h-BN) was used as a substrate and dielectric. h-BN enhances the electron concentration and reduces the columbic scattering offered by SiO2 interface resultant enhancement in the mobility and on/off ratio by a factor of ~23 and ~200 times respectively. This study signifies the role of h-BN as a better substrate for two-dimensional material-based FETs.

Optoelectronic Properties of Hexagonal Boron Nitride Shielded Molybdenum Diselenide/Black-Phosphorus Based Heterojunction Field Effect Transistor

Herein, we report the fabrication of a novel heterojunction field-effect transistor (HJFET) based on two-dimensional graphene (Gr), molybdenum diselenide (MoSe2), and black phosphorus (BP) that is shielded using hexagonal boron nitride to prevent device degradation. We perform electrical and optoelectronic characterizations of Gr/n-MoSe2 and Gr/n-MoSe2/p-BP heterojunctions. Heterojunction n-MoSe2/p-BP exhibits a potential barrier at the interface, which allows the use of BP as a top-gate contact to adjust the electrical and optoelectronic performances of the Gr/n-MoSe2 heterojunction. In the absence of a gate voltage, the Gr/n-MoSe2 and Gr/n-MoSe2/p-BP heterojunctions indicate photoresponsivity (Rλ) and specific detectivity (D*) of 1.77 AW−1 and 1.4 × 1010 cmHz1/2W−1, and 0.8 AW−1 and 0.3 × 1010 cmHz1/2W−1, respectively. The Gr/n-MoSe2 junction field-effect transistor with p-BP as gate contact demonstrates the best optoelectronic performance with high stability in terms of photoresponsivity Rλmax = 3.37 AW−1 and specific detectivity D*max = 3.16 × 1010 cmHz1/2W−1, rendering it extremely promising for photodetection applications.

Atomic Layer Deposition of Ultrathin La2O3/Al2O3 Nanolaminates on MoS2 with Ultraviolet Ozone Treatment.

Due to the chemically inert surface of MoS2, uniform deposition of ultrathin high-κ dielectric using atomic layer deposition (ALD) is difficult. However, this is crucial for the fabrication of field-effect transistors (FETs). In this work, the atomic layer deposition growth of sub-5 nm La2O3/Al2O3 nanolaminates on MoS2 using different oxidants (H2O and O3) was investigated. To improve the deposition, the effects of ultraviolet ozone treatment on MoS2 surface are also evaluated. It is found that the physical properties and electrical characteristics of La2O3/Al2O3 nanolaminates change greatly for different oxidants and treatment processes. These changes are found to be associated with the residual of metal carbide caused by the insufficient interface reactions. Ultraviolet ozone pretreatment can substantially improve the initial growth of sub-5 nm H2O-based or O3-based La2O3/Al2O3 nanolaminates, resulting in a reduction of residual metal carbide. All results indicate that O3-based La2O3/Al2O3 nanolaminates on MoS2 with ultraviolet ozone treatment yielded good electrical performance with low leakage current and no leakage dot, revealing a straightforward approach for realizing sub-5 nm uniform La2O3/Al2O3 nanolaminates on MoS2.

Development and Functionalization of Electrolyte-Gated Graphene Field-Effect Transistor for Biomarker Detection.

In the current study, graphene and its derivatives have been investigated and used for many applications, including electronics, sensing, energy storage, and photocatalysis. Synthesis and fabrication of high quality, good uniformity, and low defects graphene are critical for high-performance and highly sensitive devices. Among many synthesis methods, chemical vapor deposition (CVD), considered a leading approach to manufacture graphene, can control the number of graphene layers and yield high-quality graphene. CVD graphene needs to be transferred from the metal substrates on which it is grown ontoinsulating substrates for practical applications. However, separation and transferring of graphene onto new substrates are challenging for a uniform layer without damaging or affecting graphene's structures and properties. Additionally, electrolyte-gated graphene field-effect transistor (EGGFET) has been demonstrated for its wide applications in various biomolecular detections because of its high sensitivity and standard device configuration. In this article, poly (methyl methacrylate) (PMMA)-assisted graphene transferring approach, fabrication of graphene field-effect transistor (GFET), and biomarker immunoglobulin G (IgG) detection are demonstrated. Raman spectroscopy and atomic force microscopy were applied to characterize the transferred graphene. The method is shown to be a practical approach for transferring clean and residue-free graphene while preserving the underlying graphene lattice onto an insulating substrate for electronics or biosensing applications.

Thermally stable and flexible substrate for high-performance organic field-effect transistors

Flexible substrates are susceptible to deformation above 180 °C. This is not favorable for the fabrication of high-performance organic field-effect transistors, as most of organic semiconducting films need thermal annealing above 200 °C. In this study, flexible willow glass was applied as substrate, which showed no deformation even at 500 °C. Herein, pentacene transistors were fabricated on willow glass with high hole mobility of 1.02 cm2 V−1 s−1. Compared to the commonly used poly(ethylene terephthalate) (PET) substrate, the willow glass exhibited higher hole mobility, better thermal stability, harder surface, and tolerable flexibility. The mobility decreased slightly under bending and recovered after the willow glass restored to its original state. The willow glass is speculated to be a good complement to the PET substrate, and shows great application prospects in the field of organic electronics.

Synthesis, Characterization, and Thin-Film Transistor Response of Benzo[i]pentahelicene-3,6-dione.

Organic semiconductors hold the promise of simple, large area solution deposition, low thermal budgets as well as compatibility with flexible substrates, thus emerging as viable alternatives for cost-effective (opto)-electronic devices. In this study, we report the optimized synthesis and characterization of a helically shaped polycyclic aromatic compound, namely benzo[i]pentahelicene-3,6-dione, and explored its use in the fabrication of organic field effect transistors. In addition, we investigated its thermal, optical absorption, and electrochemical properties. Finally, the single crystal X-ray characterization is reported.

Vertical short-channel MoS<sub>2</sub> field-effect transistors

Field effect transistors (FETs) based on two-dimensional (2D) materials have great potential applications in very large-scale integration technology, and high-performance short channel 2D semiconductor FETs are essential. Owing to the difficulty in obtaining channel lengths below 10 nm for 2D materials, there are few stable methods of fabricating short channel 2D semiconductor FETs. Here we report a method of stably fabricating vertical short-channel MoS<sub>2</sub> FETs by using graphene as the contact material and h-BN as the spacer. The 8-nm spacer transistor exhibits good switching characteristics. The on/off ratio is greater than 10<sup>7</sup> and the off-state current is less than 100 fA/μm under different source-drain voltages, which are immune well to the direct source-to-drain tunneling effect. This method can be used to rapidly screen two-dimensional materials that are immune to short-channel effects and also are suitable for the fabrication of high-performance FETs.

Fabrication of Flexible Organic Field Effect Transistors with High Carrier Mobility Via Sheath Gas-Assisted Direct Writing Poly(3-Hexylthiophene) Solution

Fabrication of Flexible Organic Field Effect Transistors with High Carrier Mobility Via Sheath Gas-Assisted Direct Writing Poly(3-Hexylthiophene) Solution

Fabrication of a Mesoporous Multimetallic Oxide-based Ion-Sensitive Field Effect Transistor for pH Sensing.

Sensitive and reliable noninvasive sensors are in demand to cope with an increasing need for robust working conditions and fast results. One of the leading potential technologies is field-effect transistor (FET)-based sensors to improve response time, sensitivity, and stability. Here, a sol–gel method fabricates an ion-sensitive field-effect transistor with a high current and output sensitivity for electrochemical sensing, solving binary device design, component regulating, and long-term stability, while maintaining the promoted sensitivity. Metal oxide-based devices with single and binary contents are fabricated and characterized for monitoring pH changes, with performance fitted to a Nernst–Poisson model. After detecting the performance, the result was compared with devices in different components and ratios to obtain excellent performance and high stability. In addition, these extended gate FETs with multimetallic oxide promise efficiency and stability optimization in terms of a flexible component design, demonstrating the feasibility of the novel sol–gel fabrication method to achieve efficient and reliable FET sensors.

TiO2 thin films derived by facile sol-gel method: Influence of spin rate and Al-doping on the optical and electronic properties

Titanium dioxide (TiO2) thin films with enhanced properties have been synthesized by sol-gel method and they can be used for various applications as in field-effect transistors (FETs), sensors, photovoltaics etc. The properties of TiO2 thin films were optimized by changing spin speed (1000 and 3000 rpm) and doping Al in the TiO2 lattice. The influence of spin speed and Al doping on the grain orientation and surface morphology of titania were studied using x-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Uniform, smooth and compact TiO2 thin films having tetragonal anatase phase and a band gap of 3.54 eV along with an optimum thickness and high dielectric constant of ~28 were obtained at a spin speed of 3000 rpm. These films have been previously reported as a gate dielectric layer in the fabrication of flexible ZnO:Al thin film transistor with a mobility of 4.96 cm2/Vs, therefore, the samples prepared at different spin speed could be useful for the fabrication of organic and inorganic FETs. Detailed study was performed on Al-doped TiO2 thin films. XRD and various vibrational modes noticed in the Raman spectra confirmed the formation of crystalline TiO2 films with a pure anatase tetragonal phase. A systematic shift in (101) plane in XRD and E1 g mode in Raman spectra confirms the substitutional doping of Al in the TiO2 matrix. A red shift in the band gap energy with increased transmittance in visible region was observed for the Al-doped TiO2 samples. The higher value of Urbach energy for the Al-doped samples reveals the presence of defects related to oxygen which were further studied by the photoluminescence spectra (PL). AFM images revealed compact, void free and uniform deposition of both TiO2 and TiO2: Al films. The particle size was found to be in the range of 30–40 nm and 40–100 nm for TiO2 and TiO2: Al, respectively. Enhanced electrical conductivity was observed for the Al-doped TiO2 thin films. The current Al-doped TiO2 system with enhanced optical, morphological and electrical properties could be a potential candidate in optoelectronic, sensing and photovoltaics applications.

Wafer-Scale Production of Transition Metal Dichalcogenides and Alloy Monolayers by Nanocrystal Conversion for Large-Scale Ultrathin Flexible Electronics.

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are unit-cell thick materials with tunable physical properties according to their size, morphology, and chemical composition. Their transition of lab-scale research to industrial-scale applications requires process development for the wafer-scale growth and scalable device fabrication. Herein, we report on a new type of atmospheric pressure chemical vapor deposition (APCVD) process that utilizes colloidal nanoparticles as process-scalable precursors for the wafer-scale production of TMD monolayers. Facile uniform distribution of nanoparticle precursors on the entire substrate leads to the wafer-scale uniform synthesis of TMD monolayers with the controlled size and morphology. Composition-controlled TMD alloy monolayers with tunable bandgaps can be produced by simply mixing dual nanoparticle precursor solutions in the desired ratio. We also demonstrate the fabrication of ultrathin field-effect transistors and flexible electronics with uniformly controlled performance by using TMD monolayers.

Graphene Suspension from Modified Electrochemical Exfoliation of Graphite

Graphene has emerged as one of the most exciting materials since its discovery in 2004. Graphene is a term used for one to ten layers of sp2 hybridized carbon atoms arranged in a hexagonal pattern. Monolayer graphene is the building block of all graphitic structures and has high electron mobility, high thermal conductivity, high specific surface area, extremely high resistance to gas permeation and high optical transmittance. These exceptional properties make monolayer graphene attractive for potential applications as a catalyst or catalyst support in direct methanol fuel cells (DMFC), proton exchange membrane fuel cells (PEMFC), electrode in lithium-ion batteries, supercapacitors, as well as in thermally and electrically conductive sensors. AB-stacked bilayer and trilayer graphene have applications in the electronic industry, such as fabrication of field-effect transistors (FEts) and light detectors, due to an induced bandgap in the presence of an applied electric field. In addition, graphene finds major applications in the optoelectronic industry, such as electrode materials in solar cells, supercapacitors and transparent LCDs.Graphene is traditionally synthesized using the simple mechanical exfoliation of graphite, which is not scalable, or from chemical vapour deposition of a carbon source on a suitable substrate, which is both a complex and an expensive method. Hence, a scalable and economic graphene synthesis method is needed to harness the remarkable properties of graphene. The electrochemical exfoliation of graphite provides a promising route for the bulk production of graphene because it provides graphene flakes with the largest size and a smaller defect density than the well-known chemical synthesis methods such as Hummer or modified Hummer processes. The large size flakes resulting from electrochemical exfoliation are suitable for scalable electronic device fabrication methods. Electrochemical exfoliation proceeds via intercalation of the ions present in the electrolyte and is followed by the subsequent expansion and later exfoliation of graphite. However, this strategy produces graphene with a scatter in the layer numbers. A polydisperse suspension of graphene is undesirable because the electronic properties of graphene change with the number of layers. The intermediate graphite intercalation compounds are formed by the insertion of intercalants between the layers of the graphite host material. The intercalants can be atoms, ions or molecules. Graphite intercalation occurs in stages where the stage number indicates the number of graphene layers between the two nearest intercalant layers. In the present study, a modified electrochemical exfoliation route of graphite is explored, which proceeds through the exfoliation of a stage pure graphite intercalation compound followed by low-intensity ultrasonication. The modified electrochemical exfoliation of graphite is capable of providing a monodisperse graphene suspension