Fabrication Of Field-effect Transistors Research Articles

Effects of Recess Depth Under the Gate Area on the Vth-Shift for Fabricating Normally-Off Field Effect Transistors on AlGaN/GaN Heterostructures.

Fabrication of normally-off field effect transistors (FETs) possessed uniform turn-on threshold voltage (Vth) is of special interests. In this work, they were fabricated using dry etching recess techniques under the gate region, with dry etching conditions of extremely low rate. We report how the recess depth under the gate area induced the Vth shift of normally-off FETs on AlGaN/GaN heterostructure, which were fabricated with a 1.5 nm/min etching rate. Chlorine-based inductively coupled plasma (ICP) was applied to perform the etching process for the AlGaN/GaN heterostructure. Devices were fabricated with different recess depths under the gate area, and examined to determine their performances, particularly the dependence of recess time and recess depth on Vth shift. The applied dry etching conditions resulted in a low-damaged and not-rough morphology on the etched surfaces of AlGaN/GaN. Fine controlled and well defined recess depth of the AlGaN/GaN heterostructure under the gate region was achieved with no etch-stop layers. Conventional fabrication processes were applied with the dry etching conditions of extremely low rate to fabricate normally-off MOSFETs of Al₂O₃/AlGaN/GaN. The achieved Vth of +5.64 V was high positive and the leakage current of off-state was measured as ~10-6 A/mm.

Dinaphthothiepine Bisimide and Its Sulfoxide: Soluble Precursors for Perylene Bisimide.

The synthesis and properties of dinaphtho[1,8-bc:1',8'-ef]thiepine bisimide (DNTBI) and its oxides are described. Their molecular design is conceptually based on the insertion of a sulfur atom into the perylene bisimide (PBI) core. These sulfur-inserted PBI derivatives adopt nonplanar structures, which significantly increases their solubility in common organic solvents. Upon electron injection, light irradiation, or heating, DNTBI and its sulfoxides undergo sulfur extrusion reactions to furnish PBI. The photoinduced and thermal sulfur extrusion reactions proceed almost quantitatively. This unique reactivity enabled the fabrication of a high-performance solution-processed n-type organic field-effect transistor with an electron mobility of up to 0.41 cm2 V-1 s-1.

Atomically flat HfO2 layer fabricated by mild oxidation HfS2 with controlled number of layers

In this work, homogeneous surface oxidation of hafnium disulfide (HfS2) is achieved by an extremely simple thermal treatment, i.e., mild oxidization in air. Due to the high thermal stability of hafnium dioxide (HfO2), atomically flat HfO2 is formed on top of unoxidized HfS2 and unlimited layer-by-layer oxidation is observed. The thickness of HfO2 can be controlled by oxidation temperature and time. The HfO2 layer fabricated by the oxidation of HfS2 is smooth and atomically flat with roughness comparable to that of pristine HfS2. Growth of a high-quality, uniform, atomically flat oxide film on top of semiconductor is the first step for the fabrication of field effect transistors and metal-insulator-semiconductor devices. Thus, our results will facilitate the future fabrication of HfO2 films with atomic-scale thickness.

Fabrication of carbon nanotube field-effect transistors in commercial silicon manufacturing facilities

Carbon nanotube field-effect transistors (CNFETs) are a promising nanotechnology for the development of energy-efficient computing. Despite rapid progress, CNFETs have only been fabricated in academic or research laboratories. A critical challenge in transferring this technology to commercial manufacturing facilities is developing a suitable method for depositing nanotubes uniformly over industry-standard large-area substrates. Such a deposition method needs to be manufacturable, compatible with today’s silicon-based technologies, and provide a path to achieving systems with energy efficiency benefits over silicon. Here, we show that a deposition technique in which the substrate is submerged within a nanotube solution can address these challenges and can allow CNFETs to be fabricated within industrial facilities. By elucidating the mechanisms driving nanotube deposition, we develop process modifications to standard solution-based methods that significantly improve throughput, accelerating the deposition process by more than 1,100 times, while simultaneously reducing cost. This allows us to fabricate CNFETs in a commercial silicon manufacturing facility and high-volume semiconductor foundry. We demonstrate uniform and reproducible CNFET fabrication across industry-standard 200 mm wafers, employing the same equipment currently being used to fabricate silicon product wafers. Using a solution-based deposition technique, carbon nanotube field-effect transistors can be fabricated in a commercial silicon manufacturing facility and a high-volume commercial foundry, demonstrating uniform and reproducible transistor fabrication across industry-standard 200 mm wafers.

Diamond Field-Effect Transistors With V2O5-Induced Transfer Doping: Scaling to 50-nm Gate Length

Wereport on the fabrication and measurement of hydrogen-terminated diamond field-effect transistors (FETs) incorporating V2O5 as a surface acceptor material to induce transfer doping. Comparing a range of gate lengths down to 50 nm, we observe inversely scaling peak output current and transconductance. Devices exhibited a peak drain current of ~700 mA/mm and a peak transconductance of ~150 mS/mm, some of the highest reported thus far for a diamond metal semiconductor FET (MESFET). Reduced sheet resistance of the diamond surface after V2O5 deposition was verified by four probe measurement. These results show great potential for improvement of diamond FET devices through scaling of critical dimensions and adoption of robust transition metal oxides such as V2O5.

The optimization of contact interface between metal/MoS2 FETs by oxygen plasma treatment

The quality of the contact is key for the performance enhancement of the MoS2 device. As the oxygen plasma treatment is developed in the fabrication of MoS2 field effect transistor (FET), we demonstrate that the tunneling layer (MoO3) is formed on the top of the MoS2 contact region and the process induced organic residues in the source and drain regions is effectively removed. The formation of MoO3 can degrade the Fermi level pinning effect at the MoS2/metal interface and lowering the Schottky barrier height. And the flatness of the interface has been greatly improved, with the roughness reduced from 0.53 to 0.166 nm. In this paper, we improve the device performance with an on-/off-current ratio increase by four orders of magnitude, a mobility increase by 10 times at room temperature and the 33-fold decrease in contact resistance. This research shows that oxygen plasma treatment is a promising method for the integration of MoS2 FET in the future.

Depletion induced depolarization field in Hf1−xZrxO2 metal-ferroelectric-semiconductor capacitors on germanium

Germanium Metal-Ferroelectric-Semiconductor (MFS) capacitors based on ferroelectric Hf1−xZrxO2 (HZO) with clean, oxide free Ge/HZO interfaces emerge as an interesting layer structure for the fabrication of ferroelectric field effect transistor (FeFET) non-volatile memory devices. It is shown that, at low temperature (<160 K), a semiconductor depletion forms in Ge near the interface, resulting in an increase in coercive voltage by about 2 V, accompanied by a distortion of the ferroelectric hysteresis with subloop asymmetric behavior, which becomes more severe at higher frequencies of measurement. At higher temperatures, the Ge surface near the ferroelectric is easily inverted due to the low energy gap of Ge, providing sufficient screening of the polarization charge by minority free carriers, in which case, nearly ideal, symmetric hysteresis curves are recovered. The depolarization field is experimentally extracted from the coercive voltage and the capacitance measurements, is found to be ∼ 2.2 MV/cm in the low temperature range, comparable to the coercive field, then rapidly decreases at higher temperatures, and effectively diminishes at room temperature. This makes Ge MFSs good candidates for FeFETs for low voltage non-volatile memory with improved reliability.

Architectures of Graphene-Based Field-Effect Transistors for Single-Molecule Experiments

With progress in the extreme miniaturisation of electronic components and the discovery of low-dimensional conductive materials, it is now possible to assemble field-effect transistors (FETs) that can incorporate single-molecule components as a channel or gate1,2. Such types of FETs have been recently used to detect and study various fundamental mechanisms at the single-molecule scale, among which the folding and unfolding of molecules, hybridization mechanisms, charge transport or chemical reactions1–5. In these experiments, devices were typically fabricated using architectures based on individual 1D materials, such as carbon nanotubes (CNTs) and silicon nanowires. The 1D topology facilitates the isolation of individual molecules in the circuit, but present drawbacks in scalability due to challenges in the growth, purification and/or assembling of such 1D materials into FET circuits. Here, we present new top-down approaches for the fabrication of single-molecule FETs, based on 2D graphene architectures. As CNTs, graphene is made of an hexagonal carbon lattice enabling excellent conductivity as well as carbon-based chemistry to anchor individual molecules, yet its 2D topology is more compatible with wafer-scale fabrication processes. First, we report the fabrication of large arrays of FETs based on graphene ribbons with controlled electrical properties. These arrays were built from high-quality large area graphene synthesized by chemical vapor deposition (CVD), followed by patterning steps using photolithography and plasma etching processes. Then, we report the design of two different architectures for single-molecule experiments: nanoconstrictions and nanogaps. Nanoconstrictions were achieved using electron-beam lithography (EBL), allowing to pattern high-resolution features (50nm) in the graphene channel. Nanogaps were obtained using the electroburning technique to open a gap of a few nanometers in the graphene channel6. We will present the design and fabrication process of these architectures, as well as their characterization using high-resolution microscopy (SEM/AFM) and transport measurements. Finally, we will discuss approached for the single-molecule functionalization for these architectures and their application in conductance-based single-molecule measurements.

Tuning the Ambipolar Character of Copolymers with Substituents: A Density Functional Theory Study.

Ambipolar copolymers play a vital role in reducing the cost and simplifying the fabrication of ambipolar organic field-effect transistors (OFETs). However, the development of them lags behind that of p-type and n-type copolymers. Herein, we aim to obtain ambipolar copolymers by introducing appropriate substituents into DPP-TVT [diketopyrrolopyrrole-(E)-1,2-di(2-thienyl)ethane]. We study the effects of substituents (-NH2, -OCH3, -F, -CF3, and -CN) on backbone planarity, electronic structures, and charge carrier mobility by density functional simulations. Electronic structure analyses show that the CN-substituted DPP-TVT lies in the trap-free energy window and has the narrowest band gap, suggesting promising ambipolar character. This was further confirmed by the results of its charge carrier mobility along both intra- and interchain directions. Besides the -CN group, the -NH2 group also proved to be effective in turning DPP-TVT into the ambipolar copolymer. Our study provides insights into modulating the performances of OFETs by introducing appropriate substituents into the copolymers to achieve ambipolar character.

Batch-Fabricated WSe₂-on-Sapphire Field-Effect Transistors Grown by Chemical Vapor Deposition

Stepper-based batch fabrication of top-gated field-effect transistors (FETs) is demonstrated on few-layer WSe2-on-sapphire grown by chemical vapor deposition. This article reports the electrical characterization of 94 transistors with three different transistor structures. ON-/ OFF-current ratios exceeding 105 at room temperature with noise-floor-limited gate current and contact-limited ON-current density of 15 nA/ $\mu \text{m}$ was achieved for FETs using a structure in which the gate overlaps the source and drain contacts. Two transistors with open channel access regions are used to allow ion doping through polyethylene oxide: cesium perchlorate (PEO: CsClO4) to induce electron and hole conductivity and allow characterization of the intrinsic FET characteristics. In these geometries, the ON-current increased by four orders of magnitude with ON-/ OFF-current ratio greater than 106 at ${V}_{\text {DS}} = -0.05$ V, compared to gate overlapped FETs. An analytic 2-D FET model was used to model the full current-voltage characteristics. This allowed the field-effect mobility to be extracted revealing the gate and drain field dependence for the first time. The fit of the measured characteristics from subthreshold to saturation is made with only four fitting parameters, revealing a field-dependent electron mobility ranging from 1 to 2.6 cm2/Vs.

Stable, low-resistance, 1.5 to 3.5 kΩ sq−1, diamond surface conduction with a mixed metal-oxide protective film

Semiconducting diamond has the potential for an order-of-magnitude increase in power handling over currently used semiconductors. This is made possible by diamond's higher thermal conductivity and a higher breakdown voltage than any other device-quality semiconductor. Diamond power devices have numerous potential applications in the power grid and in high-power high-frequency RF applications.One approach to leverage diamond's abilities is through the fabrication of field-effect transistors (FETs). The FET is made by forming a p-type surface conductive layer on the diamond surface. This is accomplished by terminating the diamond surface with hydrogen atoms and then coating the surface with a material that contains negative charges to compensate for the positive holes in the p-type layer. Impressive drain current (1.3 A mm−1), maximum operational voltages (>2000 V), and frequencies of unity current gain (fT of 75 GHz) have been demonstrated with this surface conductance method. This surface layer, however, is not stable and FET performance degrades over time on the scale of hours to days. This paper describes an encapsulating layer with a mixed oxide, Al2O3-SiO2, which maintains the resistance of the conductive layer in the range of 1.5 to 3.5 kΩ sq.−1 by protecting the diamond surface while maintaining a stable negative charge.

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.

Study on the Growth Parameters and the Electrical and Optical Behaviors of 2D Tungsten Disulfide.

Transition-metal dichalcogenides (TMDCs) with atomic thickness are promising materials for next-generation electronic and optoelectronic devices. Herein, we report uniform growth of triangular-shaped (∼40 μm) monolayer WS2 using the atmospheric-pressure chemical vapor deposition (APCVD) technique in a hydrogen-free environment. We have studied the optical and electrical behaviors of as-grown WS2 samples. The absorption spectrum of monolayer WS2 shows two intense excitonic absorption peaks, namely, A (∼630 nm) and B (∼530 nm), due to the direct gap transitions at the K point. Photoluminescence (PL) and fluorescence studies reveal that under the exposure of green light, monolayer WS2 gives very strong red emission at ∼663 nm. This corresponds to the direct band gap and strong excitonic effect in monolayer WS2. Furthermore, the efficacy of the synthesized WS2 crystals for electronic devices is also checked by fabricating field-effect transistors (FETs). FET devices exhibit an electron mobility of μ ∼ 6 cm2 V-1 s-1, current ON/OFF ratio of ∼106, and subthreshold swing (SS) of ∼641 mV decade-1, which are comparable to those of the exfoliated monolayer WS2 FETs. These findings suggest that our APCVD-grown WS2 has the potential to be used for next-generation nanoelectronic and optoelectronic applications.

Nonlinear optical performance of CdO/InSe Interfaces

In this article, the growth nature, structural and optical properties of CdO/InSe interfaces are investigated. The CdO/InSe interfaces are prepared by the thermal vacuum deposition technique. Structurally, while the CdO exhibited cubic structure, the InSe layer was amorphous in nature. The morphological analyses have shown that the interface is composed of randomly distributed circular grains of average sizes of ∼170 nm. The interfacing of the CdO and InSe resulted in enhancing the light absorbability of CdO by ∼21 times in the IR range. It also showed well aligned conduction bands and valence band offset of 0.72 eV. The Drude-Lorentz modeling of the imaginary part of the dielectric spectra of the CdO/InSe interfaces has shown that the device is suitable for the fabrication of field effect transistors. The drift mobility of free carriers at the interface reached 42.27 In addition, a quality factor larger than 103 is achieved in the IR range indicating the suitability of these optical interfaces to store electromagnetic energy. These properties are important as they shows the applicability of the CdO/InSe interface in solar cells and optoelectronics as optical signal receivers or converters.

Non-lithographic direct patterning of carbon nanomaterial electrodes via electrohydrodynamic-printed wettability patterns by polymer brush for fabrication of organic field-effect transistor

In this study, we report a new approach for patterning of carbon nanomaterials via the solution route through wettability patterning using electrohydrodynamic (EHD) printing. Differences in wettability were successfully obtained by treating dimethylchlorosilane-terminated polystyrene (PS-brush) with the micro-dripping mode of EHD printing up to 6-μm line width and scaled fine patterns with various shapes at 6-μm printing conditions. We obtained sub-micrometer scaled carbon nanomaterial patterns, which were utilized as source and drain (S/D) electrodes for organic field-effect transistors (OFETs), without using any lithographic process. In addition, the PS-brush treatment modified the surface hydrophobicity of the substrates that served as the PS-brush and SiO2 bilayer dielectrics to guarantee high stability during the OFET operation. As a result, the OFETs employing S/D electrodes based on carbon nanomaterial patterns exhibited the electrical performance of typical p-type OFETs with negligible hysteresis. Additionally, we confirmed that a solution processing organic semiconductor forms a more suitable crystal structure than a vacuum process based organic semiconductor by several tools like CPOM, AFM, and 2D-GIXD.

Fabrication of SiNWs-FET Nanostructure Via Atomic Force Microscopy Lithography

The unique electrical properties of silicon nanowires (SiNWs) is one of the reasons it become an attractive transducer for biosensor nowadays. Positive (holes) and negative (electron) charge carriers from SiNWs can simply interact with either positive or negative charge of sensing target. In this paper, we have studied the fabrication of silicon nanowires field effect transistor (SiNWs-FET) nanostructure patterned on 15 Ω resistivity of p-type silicon on insulator (SOI) wafer fabricated via atomic force microscopy lithography technique. To fabricate SiNWs-FET nanostructure, a conductive AFM tip, Cr/Pt cantilever tip, was used then various value of applied voltage, writing speed and relative humidity were studied. Subsequent, followed by wet etching processes, admixture of tetramethylammonium hydroxide (TMAH) and isopropyl alcohol (IPA) were used to remove the undesired of silicon layer and diluted hydrofluoric acid (HF) was used to remove the oxide layer. From the results, it shows that, cantilever tip at 9 V with 0.4 μm/s writing speed and relative humidity between 55% - 60% gives the best formation of silicon oxide to fabricate SiNWs-FET nanostructure.

Performance improvement in electrospun InGaZnO nanofibres field-effect-transistors using low thermal budget microwave calcination and Ar/O2 mixed-plasma surface treatment

In this study, we present a low thermal budget microwave annealing (MWA) method for calcination of electrospun In-Ga-ZnO (IGZO) nanofibres and demonstrate an improvement in the performance of IGZO nanofibre field-effect transistors (FETs) by Ar/O2 mixed-plasma surface treatment. The IGZO nanofibres were fabricated by electrospinning method and calcined using MWA method. This process allowed for a significant reduction in the heat treatment temperature and time. Subsequently, plasma surface treatment using various ratios of Ar/O2 gas mixtures was carried out. The surface morphology and chemical composition of MWA-calcined and plasma-treated IGZO nanofibres were studied by SEM and XPS analysis. In order to investigate the effects of MWA calcination combined with Ar/O2 mixed-plasma treatment on the electrical properties and the reliability of nanofibres-based transistors, IGZO nanofibres FETs were fabricated and applied to resistor-loaded inverters. Our results show that the O2 plasma treatment significantly improves the performance of IGZO nanofibres FETs and the resistor-loaded inverters based on IGZO nanofibres FETs, whereas Ar plasma treatment degrades the performance of these devices. The instability tests using positive bias temperature stress (PBTS) and negative bias temperature stress (NBTS) revealed that the O2 plasma treatment contributed to the stability of IGZO nanofibres FETs. Our results suggest that the MWA calcination combined with the Ar/O2 mixed-plasma surface treatment is a promising technique for the fabrication of high performance IGZO nanofibres FETs with low thermal budget processes.

Realizing an Omega-Shaped Gate MoS2 Field-Effect Transistor Based on a SiO2/MoS2 Core-Shell Heterostructure.

Substantial progress has been made in the experimental synthesis of large-area two-dimensional transition metal dichalcogenide (TMD) thin films in recent years. This has provided a solid basis to build non-planar structures to implement the unique electrical and mechanical properties of TMDs in various nanoelectronic and mechano-electric devices, which, however, has not yet been fully explored. In this work, we demonstrate the fabrication and characterization of MoS2 field-effect transistors (FETs) with an omega (Ω)-shaped gate. The FET is built based on the SiO2/MoS2 core-shell heterostructure integrated using atomic layer deposition (ALD) technique. The MoS2 thin film has been uniformly deposited by ALD as wrapping the SiO2 nanowire forming the channel region, which is further surrounded by the gate dielectric and the Ω-gate. The device has exhibited n-type behavior with effective switching comparable to the reference device with a planar MoS2 channel built on a SiO2/Si substrate. Our work opens up an attractive avenue to realize novel device structures utilizing synthetic TMDs, thereby broadening their potential application in future advanced nanoelectronics.

Charge transport mechanism in networks of armchair graphene nanoribbons

In graphene nanoribbons (GNRs), the lateral confinement of charge carriers opens a band gap, the key feature that enables novel graphene-based electronics. Despite great progress, reliable and reproducible fabrication of single-ribbon field-effect transistors (FETs) is still a challenge, impeding the understanding of the charge transport. Here, we present reproducible fabrication of armchair GNR-FETs based on networks of nanoribbons and analyze the charge transport mechanism using nine-atom wide and, in particular, five-atom-wide GNRs with large conductivity. We show formation of reliable Ohmic contacts and a yield of functional FETs close to unity by lamination of GNRs to electrodes. Modeling the charge transport in the networks reveals that transport is governed by inter-ribbon hopping mediated by nuclear tunneling, with a hopping length comparable to the physical GNR length. Overcoming the challenge of low-yield single-ribbon transistors by the networks and identifying the corresponding charge transport mechanism is a key step forward for functionalization of GNRs.

Surface-modified quantum-dot floating layer using novel thiol with large dipole moment for improved feasibility of light-erasable organic transistor memory applications

In this study, a new functional thiol with a pentafluorophenyl group was synthesized for the surface modification of CdSe quantum-dot floating layers; this was aimed at the fabrication of organic field-effect transistors (OFETs). The dipole moments and surface properties of the fluorinated thiols were used to control the transistor operations; these thiols acted as surface modifiers of the CdSe quantum-dot floating layers in the OFETs. Further, the new functional thiol exhibited a larger dipole moment than that of the commercial 2,3,4,5,6-pentafluorothiophenol. The OFET comprising the new functional thiol with the pentafluorophenyl group functioned as a normally ON transistor and exhibited bistable current states during nondestructive reading. In addition, it exhibited sensitive responses to electrical-only and light-only biases, which demonstrates its feasibility for light-responsive flash memory applications.