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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Atmospheric and Long-term Aging Effects on the Electrical Properties of Variable Thickness WSe 2 Transistors

Atmospheric and long-term aging effects on electrical properties of WSe 2 transistors with various thicknesses are examined. While countless published studies report electrical properties of transition-metal dichalcogenide materials, many are not attentive to testing environment or to age of samples, which we have found significantly impacts results. Our as-fabricated exfoliated WSe 2 pristine devices are predominantly n-type, which is attributed to selenium vacancies. Transfer characteristics of as-fabricated devices measured in air then vacuum reveal physisorbed atmospheric molecules significantly reduced n-type conduction in air. First-principles calculations suggest this short-term reversible atmospheric effect can be attributed primarily to physisorbed H 2 O on pristine WSe 2 , which is easily removed from the pristine surface in vacuum due to the low adsorption energy. Devices aged in air for over 300 h demonstrate irreversibly increased p-type conduction and decreased n-type conduction. Moreover, they develop an extended time constant for recovery of the atmospheric adsorbents effect. Short-term atmospheric aging (up to approximately 900 h) is attributed to O 2 and H 2 O molecules physisorbed to selenium vacancies where electron transfer from the bulk and adsorbed binding energies are higher than the H 2 O-pristine WSe2. The residual/permanent aging component is attributed to electron trapping molecular O 2 and isoelectronic O chemisorption at selenium vacancies, which also passivates the near-conduction band gap state, p-doping the material, with very high binding energy. All effects reflected have the expected thickness dependence, namely, thinner devices are more sensitive to atmospheric and long-term aging effects.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Improving the Electronic Transporting Property for Flexible Field-Effect Transistors with Naphthalene Diimide-Based Conjugated Polymer through Branching/Linear Side-Chain Engineering Strategy

n-Type organic/polymeric semiconductors with high electron mobilities are highly demanded for future flexible organic circuits. Except for developing a new conjugated backbone, recent studies show that side-chain engineering also plays an indispensable role in boosting the charge-transporting property. In this paper, we report a new polymer PNDI2T-DTD with a representative n-type naphthalene diimide (NDI)–bithiophene backbone for high-performance n-type flexible thin-film transistors through branching/linear side-chain engineering strategy. Serving as the flexible side chains, the linear/branching side-chain pattern is found to be effective in tuning the preaggregation behavior in solution and the packing ordering of polymeric chains, resulting in the improvement of thin-film crystallinity. We report the electron mobility of the thin film of PNDI2T-DTD on flexible substrates can reach 1.52 cm 2 V –1 s –1 , which is approximately five times higher than that of PNDI2T-DT with the same conjugated backbone and only branching alkyl chains.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Accumulation-Type Ohmic van der Waals Contacts to Nearly Intrinsic WSe 2 Nanosheet-Based Channels: Implications for Field-Effect Transistors

We report the fabrication of ohmic van der Waals (vdW) contacts to nearly intrinsic WSe 2 nanosheet-based channels in field-effect transistors (FETs) using degenerately p-doped MoS2 (p + -MoS2) as a contact metal. We demonstrate that accumulation-type ohmic contacts and the high device performance are achievable without electrostatically gating the drain/source contact regions despite the nearly intrinsic nature of WSe 2 . Back-gated WSe 2 FETs with p + -MoS 2 bottom contacts (which screen the back-gate electric field in the drain/source regions) exhibit linear output characteristics, a high on/off ratio of 10 8 , and a high two-terminal field-effect mobility up to ~200 cm2 V –1 s –1 at room temperature. Our theoretical modeling reveals that the p + -MoS 2 /WSe 2 vdW junction behaves like a metal/semiconductor ohmic contact signified by a vanishingly thin space-charge region of ~1 nm on the p + -MoS 2 side and a substantial accumulation layer of free holes on the WSe 2 side, which is further verified by additional temperature-dependent and dual-gated measurements of WSe 2 FETs. We attribute the formation of accumulation-type ohmic contacts free of a Schottky barrier to the near absence of Fermi-level pinning at the vdW interface and the work function of p + -MoS 2 being larger than the ionization energy of WSe 2 . This study represents an important step toward achieving low-resistance ohmic contacts to two-dimensional (2D) semiconductors by eliminating the Fermi-level pinning effects, which is expected to have significant implications for next-generation 2D semiconductor-based nanoelectronics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Radical Polymer-Based Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are an emerging platform for bioelectronic applications. Significant effort has been placed in designing advanced polymers that simultaneously transport both charge and ions (i.e., macromolecules that are mixed conductors). Furthermore, the considerations for mixed organic conductors are often different from the established principles that are well-known in the solid-state organic electronics field; thus, the discovery of new OECT macromolecular systems is highly desired. Here, we demonstrate a new materials system by blending a radical polymer (i.e., a macromolecule with a nonconjugated backbone and with stable open-shell sites at its pendant group) with a frequently used conjugated polymer. Specifically, poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) was blended with poly(3-hexylthiophene) (P 3 HT) to create thin films with distinct closed-shell and open-shell domains. Importantly, the sharp and unique oxidation–reduction (redox) potential associated with the radical moieties of the PTEO chain provided a distinct actuation feature to the blended films that modulated the ionic transport of the OECT devices. In turn, this led to controlled regulation of the doping of the P 3 HT phase in the composite film. By decoupling the ionic and electronic transport into two distinct phases and by using an ion transport phase with well-controlled redox activity, never-before-seen performance for a P3HT-based OECT was observed. That is, at loadings as low as 5% PTEO (by weight) OECTs achieved figure-of-merit (i.e., μC*) values >150 F V –1 cm –1 s –1 , which place the performance on the same order as state-of-the-art conjugated polymers despite the relatively common conjugated macromolecular moiety implemented. As such, this effort presents a design platform by which to readily create a tailored OECT response through strategic macromolecular selection and polymer processing.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Salts as Additives: A Route to Improve Performance and Stability of n-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are becoming increasingly ubiquitous in various applications at the interface with biological systems. However, their widespread use is hampered by the scarcity of electron-conducting (n-type) backbones and the poor performance and stability of the existing n-OECTs. Here, we introduce organic salts as a solution additive to improve the transduction capability, shelf life, and operational stability of n-OECTs. We demonstrate that the salt-cast devices present a 10-fold increase in transconductance and achieve at least one year-long stability, while the pristine devices degrade within four months of storage. The salt-added films show improved backbone planarity and greater charge delocalization, leading to higher electronic charge carrier mobility. These films show a distinctly porous morphology where the interconnectivity is affected by the salt type, responsible for OECT speed. The salt-based films display limited changes in morphology and show lower water uptake upon electrochemical doping, a possible reason for the improved device cycling stability. Our work provides a new and easy route to improve n-type OECT performance and stability, which can be adapted for other electrochemical devices with n-type films operating at the aqueous electrolyte interface.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Revealing the Full Potential of Glycolated Mixed Ionic-Electronic Semiconductors – Symmetric Monomer Polymerization to Boost Electrochemical Transistor Performance

Organic electrochemical transistors (OECTs) enable the transduction of ionic signals into electronic outputs, positioning them as ideal candidates for next-generation sensing and (bio)signal processing applications. Recent years have witnessed the development of various OECT channel materials, affording insights into structural fine-tuning to achieve optimal performance and/or stability. However, homocouplings, commonly present in alternating conjugated polymers, have largely been overlooked. This study investigates the effect of homocoupling on OECT materials by employing two synthesis methods – standard Stille polymerization and an alternative symmetric approach – to create the p-type enhancement-mode benchmark polymer pgBTTT. The impact of homocoupling, and its absence, is studied by comparing the bulk properties of the two polymers and evaluating their respective OECT metrics. The new, homocoupling-free polymer exhibits a notably improved OECT performance ( μC *), mainly due to an average 3-fold increase in electronic mobility (μ).

defects↗
DOE OSTI2021

An unprecedented quinoid–donor–acceptor strategy to boost the carrier mobilities of semiconducting polymers for organic field-effect transistors

Quinoidal–aromatic conjugated polymers hold great application potential in organic field-effect transistors (OFETs). However, the development of high mobility quinoidal–aromatic conjugated polymers still lags behind the more popular donor–acceptor (D–A) conjugated polymers, mainly owing to the lack of a rational design strategy and efficient building block. Herein, a novel quinoid–donor–acceptor (Q–D–A) strategy is demonstrated to modulate the energy-level and boost the charge carrier transport mobility of conjugated polymers as opposed to the D–A system. On the basis of this strategy, a quinoidal–aromatic conjugated polymer, namely PAQM-BT, is designed and synthesized. With the combined use of quinoid, donor and acceptor units in the backbone, the resulting Q–D–A polymer PAQM-BT displays the narrowest bandgap with the deepest-lying lowest unoccupied molecular orbital (LUMO) energy level, highest backbone coplanarity, enhanced thin-film crystallinity and smallest effective hole mass ($m^*_h$) in comparison with the corresponding D–A polymer PT3B1 and quinoid–donor (Q–D) polymer PAQM-3T. Benefitting from the more effective intra- and inter-chain charge transport, as corroborated by experiment and theoretical calculations, OFET devices based on PAQM-BT exhibit a highly boosted hole mobility of up to 5.10 cm 2 V -1 s -1 , which is one and four orders of magnitude higher than that of PAQM-3T and PT3B1, respectively, and is among the highest for quinoidal–aromatic conjugated polymers. Finally, the potent Q–D–A strategy not only allows the energy level to be modulated but also leads to effective charge carrier transport, opening up possibilities to the development of high mobility quinoidal–aromatic conjugated polymers based on a variety of quinoids, donors, and acceptors.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Insulator-to-metal phase transition in a few-layered MoSe 2 field effect transistor

The metal-to-insulator phase transition (MIT) in low-dimensional materials and particularly two-dimensional layered semiconductors is exciting to explore due to the fact that it challenges the prediction that a two-dimensional system must be insulating at low temperatures. Thus, the exploration of MITs in 2D layered semiconductors expands the understanding of the underlying physics. Here we report the MIT of a few-layered MoSe 2 field effect transistor under a gate bias (electric field) applied perpendicular to the MoSe 2 layers. With low applied gate voltage, the conductivity as a function of temperature from 150 K to 4 K shows typical semiconducting to insulating character. Above a critical applied gate voltage, V c , the conductivity becomes metallic (i.e., the conductivity increases continuously as a function of decreasing temperature). Evidence of a metallic state was observed using an applied gate voltage or, equivalently, increasing the density of charge carriers within the 2D channel. We analyzed the nature of the phase transition using percolation theory, where conductivity scales with the density of charge carriers as σ ∝ (n - n c ) δ . The critical exponent for a percolative phase transition, δ(T), has values ranging from 1.34 (at T = 150 K) to 2 (T = 20 K), which is close to the theoretical value of 1.33 for percolation to occur. Thus we conclude that the MIT in few-layered MoSe 2 is driven by charge carrier percolation. Finally, the conductivity does not scale with temperature, which is a hallmark of a quantum critical phase transition.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Reliability of AlGaN/GaN high electron mobility transistors on low dislocation density bulk GaN substrate: Implications of surface step edges

To enable gaining insight into degradation mechanisms of AlGaN/GaN high electron mobility transistors, devices grown on a low-dislocation-density bulk-GaN substrate were studied. Gate leakage current and electroluminescence (EL) monitoring revealed a progressive appearance of EL spots during off-state stress which signify the generation of gate current leakage paths. Atomic force microscopy evidenced the formation of semiconductor surface pits at the failure location, which corresponds to the interaction region of the gate contact edge and the edges of surface steps.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Large sensitivity enhancement in semiconducting organic field effect transistor sensors through incorporation of ultra-fine platinum nanoparticles

We report remarkable improvement in sensitivity of pentacene-based field effect transistor devices towards trace nitro-aromatic explosive vapors through the incorporation of high density, sub-2 nm platinum nanoparticles (NPs) within these structures. Exploiting the unique electronic properties of these NPs, we have demonstrated a detection limit of 56.6 parts per billion of 2,4-dinitrotoluene (DNT) vapor while control samples without any embedded NPs showed no observable sensitivity to DNT vapor. We attribute this remarkable enhancement in sensitivity to the ability of these NPs to function as discrete nodes, participating in the charge transfer with adsorbed nitro-aromatic molecules.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Recovery from ultraviolet-induced threshold voltage shift in indium gallium zinc oxide thin film transistors by positive gate bias

The effect of short-duration ultraviolet (UV) exposure on the threshold voltage (V{sub th}) of amorphous indium gallium zinc oxide thin film transistors (TFTs) and its recovery characteristics were investigated. The V{sub th} exhibited a significant negative shift after UV exposure. The V{sub th} instability caused by UV illumination is attributed to the positive charge trapping in the dielectric layer and/or at the channel/dielectric interface. The illuminated devices showed a slow recovery in threshold voltage without external bias. However, an instant recovery can be achieved by the application of positive gate pulses, which is due to the elimination of the positive trapped charges as a result of the presence of a large amount of field-induced electrons in the interface region.

36 MATERIALS SCIENCE↗
DOE OSTI2013

AlGaN/GaN field effect transistors for power electronics—Effect of finite GaN layer thickness on thermal characteristics

AlGaN/GaN heterostructure field effect transistors with a 150 nm thick GaN channel within stacked Al{sub x}Ga{sub 1−x}N layers were investigated using Raman thermography. By fitting a thermal simulation to the measured temperatures, the thermal conductivity of the GaN channel was determined to be 60 W m{sup −1} K{sup −1}, over 50% less than typical GaN epilayers, causing an increased peak channel temperature. This agrees with a nanoscale model. A low thermal conductivity AlGaN buffer means the GaN spreads heat; its properties are important for device thermal characteristics. When designing power devices with thin GaN layers, as well as electrical considerations, the reduced channel thermal conductivity must be considered.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

The effect of Ta doping in polycrystalline TiO{sub x} and the associated thin film transistor properties

Tantalum (Ta) is suggested to act as an electron donor and crystal phase stabilizer in titanium oxide (TiO{sub x}). A transition occurs from an amorphous state to a crystalline phase at an annealing temperature above 300 °C in a vacuum ambient. As the annealing temperature increases from 300 °C to 450 °C, the mobility increases drastically from 0.07 cm{sup 2}/Vs to 0.61 cm{sup 2}/Vs. The remarkable enhancement of thin film transistor performance is suggested to be due to the splitting of Ti 3d band orbitals as well as the increase in Ta{sup 5+} ions that can act as electron donors.

36 MATERIALS SCIENCE↗
DOE OSTI2013

High-mobility, air stable bottom-contact n-channel thin film transistors based on N,N′-ditridecyl perylene diimide

Bottom-gate bottom-contact (BGBC) organic thin film transistors (OTFTs) based on N,N′-ditridecyl perylene diimide exhibit electron mobility as high as 3.54 cm{sup 2} V{sup −1} s{sup −1} in nitrogen, higher than that (1 cm{sup 2} V{sup −1} s{sup −1}) of bottom-gate top-contact devices. The better performance of BGBC configuration in N{sub 2} is attributed to lower contact resistance, which is further reduced by thermal annealing. After thermally annealing the BGBC OTFTs at 180 °C, electron mobility as high as 3.5 cm{sup 2} V{sup −1} s{sup −1}, current on/off ratio of 10{sup 6} and threshold voltage of 9 V are achieved in air, and the mobility retains above 1 cm{sup 2} V{sup −1} s{sup −1} after storage for two months in air. Thermal treatment enhanced crystalline grains, reduced grain boundaries, and suppressed the adsorption of H{sub 2}O and O{sub 2}, leading to excellent performance in air.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Encapsulated gate-all-around InAs nanowire field-effect transistors

We report the fabrication of lateral gate-all-around InAs nanowire field-effect transistors whose gate overlaps the source and drain electrodes and thus fully encapsulates the nanowire channel. They feature large drive current and transconductance that surpass those of conventional non-gate-overlap devices. The improved device characteristics can be attributed to the elimination of access resistance associated with ungated segments between the gate and source/drain electrodes. Our data also reveal a correlation between the normalized transconductance and the threshold voltage, which points to a beneficial effect of our wet-etching procedure performed prior to the atomic-layer-deposition of the gate dielectric.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Double-metal-gate nanocrystalline Si thin film transistors with flexible threshold voltage controllability

We fabricated nano-crystalline Si (nc-Si:H) thin-film transistors (TFTs) with a double-metal-gate structure, which showed a high electron-mobility (μ{sub FE}) and adjustable threshold voltages (V{sub th}). The nc-Si:H channel and source/drain (S/D) of the multilayered TFT were deposited at 375 °C by inductively coupled plasma chemical vapor deposition. The low grain-boundary defect density of the channel layer is responsible for the high μ{sub FE} of 370 cm{sup 2}/V-s, a steep subthreshold slope of 90 mV/decade, and a low V{sub th} of −0.64 V. When biased with the double-gate driving mode, the device shows a tunable V{sub th} value extending from −1 V up to 2.7 V.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Hydrogen passivation of electron trap in amorphous In-Ga-Zn-O thin-film transistors

We report an experimental evidence that some hydrogens passivate electron traps in an amorphous oxide semiconductor, a-In-Ga-Zn-O (a-IGZO). The a-IGZO thin-film transistors (TFTs) annealed at 300 °C exhibit good operation characteristics; while those annealed at ≥400 °C show deteriorated ones. Thermal desorption spectra (TDS) of H{sub 2}O indicate that this threshold annealing temperature corresponds to depletion of H{sub 2}O desorption from the a-IGZO layer. Hydrogen re-doping by wet oxygen annealing recovers the good TFT characteristic. The hydrogens responsible for this passivation have specific binding energies corresponding to the desorption temperatures of 300–430 °C. A plausible structural model is suggested.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Reduction of defect formation in amorphous indium-gallium-zinc-oxide thin film transistors by N{sub 2}O plasma treatment

An abnormal sub-threshold leakage current is observed at high temperature in amorphous indium-gallium-zinc-oxide thin film transistors (a-IGZO TFTs). This phenomenon occurs due to a reduced number of defects in the device's a-IGZO active layer after the device has undergone N{sub 2}O plasma treatment. Experimental verification shows that the N{sub 2}O plasma treatment enhances the thin film bonding strength, thereby suppressing the formation of temperature-dependent holes, which are generated above 400 K by oxygen atoms leaving their original sites. The N{sub 2}O plasma treatment devices have better stability performance than as-fabricated devices. The results suggest that the density of defects for a-IGZO TFTs with N{sub 2}O plasma treatment is much lower than that in as-fabricated devices. The N{sub 2}O plasma treatment repairs the defects and suppresses temperature-dependent sub-threshold leakage current.

36 MATERIALS SCIENCE↗