Hybrid Dielectrics Research Articles

Improving the Performance of Organic Thin-Film Transistors by Ion Doping of Ethylene-Glycol-Based Self-Assembled Monolayer Hybrid Dielectrics.

Tuning the electrostatics of ethylene-glycol-based self-assembled monolayers (SAMs) by doping with ions is shown. Molecular dynamics simulations unravel binding mechanisms and predict dipole strengths of the doped layers. Additionally, by applying such layers as dielectrics in organic thin-film transistors, the incorporated ions are proven to enhance device performance by lowering the threshold voltage and increasing conductivity.

Low Temperature Solution-Processed Gate Dielectrics for Low-Voltage Organic Field-Effect Transistors.

We report on the design, preparation, and electrical properties of novel solution-processed organic-inorganic hybrid dielectric films for the low-voltage operation of organic field-effect transistors (OFETs). Hybrid dielectric thin films (-20 nm thick) are easily fabricated by spin-coating a zirconium chloride precursor/organic additive reagent mixture, followed by annealing at low temperatures (-150 degrees C). The smooth and transparent hybrid dielectrics exhibit great insulating properties (leakage current densities -10(-7) A/cm2 at 2 MV/cm), high capacitance (170 nF/cm2). OFETs fabricated with hybrid dielectric and pentacene semiconductor function great at relatively low voltage (mobility: 1 cm2/V x s, on/off current ratio: 10(5)).

Polymer/metal oxide hybrid dielectrics for low voltage field-effect transistors with solution-processed, high-mobility semiconductors

Transistors for future flexible organic light-emitting diode (OLED) display backplanes should operate at low voltages and be able to sustain high currents over long times without degradation. Hence, high capacitance dielectrics with low surface trap densities are required that are compatible with solution-processable high-mobility semiconductors. Here, we combine poly(methyl methacrylate) (PMMA) and atomic layer deposition hafnium oxide (HfOx) into a bilayer hybrid dielectric for field-effect transistors with a donor-acceptor polymer (DPPT-TT) or single-walled carbon nanotubes (SWNTs) as the semiconductor and demonstrate substantially improved device performances for both. The ultra-thin PMMA layer ensures a low density of trap states at the semiconductor-dielectric interface while the metal oxide layer provides high capacitance, low gate leakage and superior barrier properties. Transistors with these thin (≤70 nm), high capacitance (100–300 nF/cm2) hybrid dielectrics enable low operating voltages (<5 V), balanced charge carrier mobilities and low threshold voltages. Moreover, the hybrid layers substantially improve the bias stress stability of the transistors compared to those with pure PMMA and HfOx dielectrics.

Polymer-sorted semiconducting carbon nanotube networks for high-performance ambipolar field-effect transistors.

Efficient selection of semiconducting single-walled carbon nanotubes (SWNTs) from as-grown nanotube samples is crucial for their application as printable and flexible semiconductors in field-effect transistors (FETs). In this study, we use atactic poly(9-dodecyl-9-methyl-fluorene) (a-PF-1-12), a polyfluorene derivative with asymmetric side-chains, for the selective dispersion of semiconducting SWNTs with large diameters (>1 nm) from plasma torch-grown SWNTs. Lowering the molecular weight of the dispersing polymer leads to a significant improvement of selectivity. Combining dense semiconducting SWNT networks deposited from an enriched SWNT dispersion with a polymer/metal-oxide hybrid dielectric enables transistors with balanced ambipolar, contact resistance-corrected mobilities of up to 50 cm2·V–1·s–1, low ohmic contact resistance, steep subthreshold swings (0.12–0.14 V/dec) and high on/off ratios (106) even for short channel lengths (<10 μm). These FETs operate at low voltages (<3 V) and show almost no current hysteresis. The resulting ambipolar complementary-like inverters exhibit gains up to 61.

The mutual influence of surface energy and substrate temperature on the saturation mobility in organic semiconductors

We report on a mutual correlation between the substrate temperature during semiconductor deposition and the surface energy of the gate dielectric on the charge carrier mobility in bottom gate top contact organic field effect transistors (OFETs) with N,N′-diphenyl-3,4,9,10-perylene tetracarboxylic diimide (DP-PDI) as organic semiconductor.The gate dielectric consists of a thin gate aluminum oxide and a self-assembled monolayer (SAM) with which the surface energy of the hybrid dielectric is tuned. The substrate temperature during evaporation of the DP-PDI semiconductor was systematically varied to obtain the best charge carrier mobility for each SAM. We found that the optimum substrate temperature correlates with the surface energy. Devices with low surface energy reach best mobility at higher substrate temperatures than devices with a higher surface energy.

Solution-Processable Low Voltage Organic Transistors of Thieno[3,2-b]thiophene-Based Conducting Polymer

A low-operating voltage and high performance polymeric field effect transistors using octadecylphosphonic acid-treated high-k AlOx and HfO2 hybrid dielectrics were demonstrated. High-k metal oxide hybrid dielectrics were prepared by oxygen plasma treatment of deposited Al film for AlOx and by spin coating of solution-processable HfO2 sol-gel solution for HfO2 in combination with phosphoric acid-based self-assembled monolayer (SAM), resulting in high capacitance (10 nF/cm2 for SiO2, 600 nF/cm2 for AlOx and 580 nF/cm2 for HfO2). With phosphoric acid-based SAM on high-k metal oxide and thermal annealing of thieno[3,2-b]thiophene-based conducting polymer, the device performance was significantly enhanced. The highest mobility of the transistors using ODPA-treated AlOx as a gate dielectric is 2.3 × 10−2 cm2 V−1 s−1 in the saturation region with the source-drain of −2 V. In ODPA-treated HfO2 hybrid dielectric, the saturated mobility is 1.1 × 10−2 cm2 V−1 s−1 and the threshold voltage was measured to be −0.31 V, which is at least one order lower than SiO2 hybrid dielectric (−3 V).

Effect of silane coupling agent chemistry on electrical breakdown across hybrid organic-inorganic insulating films.

Dielectric breakdown measurements were conducted on self-assembled monolayer (SAM)/native silicon oxide hybrid dielectrics using conductive atomic force microscopy (C-AFM). By depositing silane coupling agents (SCAs) through a diffusional barrier layer, SAM roughness was decoupled from chemistry to compare the chemical effects of exposed R-group functionality on dielectric breakdown. Using Weibull and current-voltage (I-V) analysis, the breakdown strength was observed to be independent of SCA R-group length, and the addition of a SAM was seen to improve the breakdown strength relative to native silicon oxide by up to 158%. Fluorinated SCAs were observed to suppress tunneling leakage and exhibited increased breakdown strength relative to their hydrocarbon analogs. Electron trapping, scattering, or attachment processes inherent to the fluorinated moieties are thought to be the origin of the improved breakdown properties.

Performance improvement of organic field-effect transistor ammonia gas sensor using ZnO/PMMA hybrid as dielectric layer

Ammonia (NH3) gas sensors based on organic field-effect transistor (OFET) using poly(methyl methacrylate) (PMMA) blending with zinc oxide (ZnO) nanoparticles as a gate dielectric layer were fabricated. Compared to those with the pure PMMA dielectric layer, the sensing properties of these devices using ZnO/PMMA hybrid as the gate dielectric layer were significantly improved when the sensors exposed to various concentrations of NH3, and the percentage response was nearly 10 folds higher than that using pure PMMA under 75ppm NH3. Also, the results showed that there was a remarkable shift in the threshold-voltage as well as a change in field-effect mobility after exposed to NH3 gas. By analyzing the morphologies of the dielectrics and pentacene films and the electrical characteristics of OFET, it was found that ZnO/PMMA hybrid gate dielectric layer was responsible for the enhanced sensing properties. Also, the decreased grain size of pentacene was formed on the ZnO/PMMA hybrid dielectric, facilitating NH3 to diffuse into the conducting channel and then interact with the ZnO nanoparticles. Moreover, the environmental stability of the OFET sensors was measured after storing the sensors under ambient atmosphere for 40 days.

Transparent capacitors based on nanolaminate Al2O3/TiO2/Al2O3 with H2O and O3 as oxidizers

Transparent capacitors with nanolaminate Al2O3/TiO2/Al2O3 (ATA) hybrid dielectrics have been prepared on quartz glass by atomic layer deposition. The maximal capacitance density of 14 fF/μm2 at 1 kHz was obtained. Moreover, an ultralow leakage current density of 2.1 × 10−9 A/cm2 at 1 V was realized by using O3 as the oxidizer. Fowler-Nordheim tunneling is the main mechanism of the leakage current at high fields, while several conduction mechanisms coexist at low fields. The AlZnO/ATA/AlZnO transparent capacitors exhibit an average optical transmittance of more than 80% in the visible range, which serve as good candidates for integration in transparent circuits.

Hybrid Nanodielectrics for Low‐Voltage Organic Electronics

Nanoscale hybrid dielectrics composed of an ultra‐thin polymeric low‐κ bottom layer and an ultra‐thin high‐κ oxide top layer, with high dielectric strength and capacitances up to 0.25 μFcm−2, compatible with low‐voltage, low‐power, organic electronic circuits are demonstrated. An efficient and reliable fabrication process, with 100% yield achieved on lab‐scale arrays, is demonstrated by means of pulsed laser deposition (PLD) for the fast growth of the oxide layer. With this strategy, high capacitance top gate (TG), n‐type and p‐type organic field effect transistors (OFETs) with high mobility, low leakage currents, and low subthreshold slopes are realized and employed in complementary‐like inverters, exhibiting ideal switching for supply voltages as low as 2 V. Importantly, the hybrid double‐layer allows for a neat decoupling between the need for a high capacitance, guaranteed by the nanoscale thickness of the double layer, and for an optimized semiconductor–dielectric interface, a crucial point in enabling high mobility OFETs, thanks to the low‐κ polymeric dielectric layer in direct contact with the polymer semiconductor. It is shown that such decoupling can be achieved already with a polymer dielectric as thin as 10 nm when the top oxide is deposited by PLD. This paves the way for a very versatile implementation of the proposed approach for the scaling of the operating voltages of TG OFETs with very low level of dielectric leakage currents to the fabrication of low‐voltage organic electronics with drastically reduced power consumption.

Deoxyribonucleic acid-ceramic hybrid dielectrics for potential application as gate insulators in organic field effect transistors

Hybrid films incorporating high dielectric constant k ceramics (BaTiO3 and TiO2) in deoxyribonucleic acid (DNA) were fabricated from highly stable dispersions of the ceramic nanoparticles in viscous, aqueous DNA solutions. Dielectric and electrical properties of the as-prepared nanocomposite films were investigated for potential use as gate insulators in organic field effect transistors. A k value as high as 14 was achieved with a 40 wt. % loading of ceramic nanoparticles in DNA. Electrical resistivities on the order of 1014 Ω cm with leakage current densities on the order of 10−9 A/cm2 were measured from current-voltage experiments under electric field biases up to 50 V/μm.

Paper No P32: Synthesis and Characterization of a Solution‐Processable Hybrid Organic–Inorganic High‐k Dielectric for Low‐Voltage OFET Applications

AbstractIn this paper, we describe preliminary steps in a new approach to make dielectric materials for low voltage monodomain organic field effect transistors to be used in backpanels for displays. A hybrid organic‐inorganic high‐k dielectric material based on oleic acid stabilized titanium oxide nanorods is synthesized and used to fabricate crossbar metal‐insulator‐metal capacitors in order to measure the dielectric properties of the hybrid dielectric. The hybrid dielectric has a high dielectric constant, of ∼8.1. A low parallel resistance is included in a model to explain the lossy behavior of the capacitors at frequencies below 1 kHz. Transistor characteristics and progress toward formation of a monodomain semiconductor will be presented.

Photoactive self-assembled monolayers for optically switchable organic thin-film transistors

We investigate the photoconductive and photovoltaic effects in organic thin-film transistors with thin hybrid dielectrics composed of aluminum oxide and self-assembled monolayers (SAMs). By using SAM molecules with an electro-optical functionality tuning of the photoinduced charge transfer at the interface of semiconductor and SAM upon illumination with laser light can be achieved. Control of the threshold voltage by the SAM composition enables the optical operation of the transistors without applying a gate voltage and affects the dynamics of photoinduced charge transfer.

Solid-state densification of spun-cast self-assembled monolayers for use in ultra-thin hybrid dielectrics

Ultra-thin self-assembled monolayer (SAM)-oxide hybrid dielectrics have gained significant interest for their application in low-voltage organic thin film transistors (OTFTs). A [8-(11-phenoxy-undecyloxy)-octyl]phosphonic acid (PhO-19-PA) SAM on ultrathin AlOx (2.5nm) has been developed to significantly enhance the dielectric performance of inorganic oxides through reduction of leakage current while maintaining similar capacitance to the underlying oxide structure. Rapid processing of this SAM in ambient conditions is achieved by spin coating, however, as-cast monolayer density is not sufficient for dielectric applications. Thermal annealing of a bulk spun-cast PhO-19-PA molecular film is explored as a mechanism for SAM densification. SAM density, or surface coverage, and order are examined as a function of annealing temperature. These SAM characteristics are probed through atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS). It is found that at temperatures sufficient to melt the as-cast bulk molecular film, SAM densification is achieved; leading to a rapid processing technique for high performance SAM-oxide hybrid dielectric systems utilizing a single wet processing step. To demonstrate low-voltage devices based on this hybrid dielectric (with leakage current density of 7.7×10−8Acm−2 and capacitance density of 0.62μFcm−2 at 3V), pentacene thin-film transistors (OTFTs) are fabricated and yield sub 2V operation and charge carrier mobilites of up to 1.1cm2V−1s−1.

Fabrication of Organic Thin-Film Transistors Based on Cross-Linked Hybrid Dielectric Materials

We report here the synthesis and dielectric properties of cross-linked hybrid dielectrics for organic field-effect transistor application. Hybrid thin films were first fabricated by hydrolysis of methacrylate-modified titanium and zirconium alkoxides and were further cross-linked via polymerization under UV. The dielectrics exhibit high dielectric constants (5.2–9) and good insulating properties with low leakage current densities (10-6–10-7 A/cm2) even though the dielectric film thickness was about 25–150 nm. Furthermore, the hybrid thin films had smooth and hydrophobic surfaces. The fabricated pentacene field-effect transistors using these dielectrics show good electric performance, including a carrier mobility as large as 0.29 cm2 V-1 s-1, a subthreshold swing as low as 0.13 V/decade, an on/off current ratio of ∼105, and a low threshold voltage of 0.3 V with TiO2 hybrid dielectrics. Organic thin-film transistors (OTFTs) fabricated with ZrO2 hybrid insulators exhibit a carrier mobility of 0.21 cm2 V-1 s-1 with an Ion/Ioff ratio of ∼104, a threshold voltage of -0.83 V, and a subthreshold swing of 0.39 V/decade.

UV-curable organic–inorganic hybrid gate dielectrics for organic thin film transistors

UV-curable hybrid thin films were prepared from ZrO2 hybrid sols containing the acrylic monomer, DPHA, on substrates. The prepared ZrO2 hybrid sols showed long-term storage stability. Hybrid dielectrics were prepared by sol–gel process and UV cross-linking below 160°C. Leakage currents of dielectric layers remained below 10−6A in 2MV/cm and dielectric constants were measured to be 3.85–4 at 1kHz. In addition, organic–inorganic hybrid thin films have smooth and hydrophobic surface. Pentacene OTFTs with thin hybrid dielectrics exhibit of mobility as large as 2.5cm2/Vs, subthreshold swing as low as 0.2V/decade, an on–off ratio of 105. These results demonstrated that UV-curable sol–gel hybrid systems are suitable for gate dielectrics in OTFTs.

Low-Voltage Oxide-Based Electric-Double-Layer TFTs Gated by Stacked $\hbox{SiO}_{2}$ Electrolyte/Chitosan Hybrid Dielectrics

Low-voltage oxide-based electric-double-layer (EDL) thin-film transistors (TFTs) gated by stacked SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> electrolyte/chitosan proton-conductor hybrid dielectric have been fabricated on glass substrates at room temperature. Such EDL TFTs exhibit a saturation mobility (μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) of 7.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , a subthreshold swing (S) of 100 mV/decade, a drain current on/off ratio (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on/off</sub> ) of 7.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> , and a threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ) of -0.48 V. After aging for one month in air ambient without surface passivation, such device shows μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> of 3.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on/off</sub> of 1.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> , and S of 120 mV/decade. Control experiments demonstrate that devices gated by stacked SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> electrolyte/chitosan hybrid dielectrics show improved stability compared with TFTs gated by single chitosan or single SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> electrolyte gate dielectric.

Nanolaminates: Increasing Dielectric Breakdown Strength of Composites

Processable, low-cost, high-performance hybrid dielectrics are enablers for a vast array of green technologies, including high-temperature electrical insulation and pulsed power capacitors for all-electric transportation vehicles. Maximizing the dielectric breakdown field (E(BD)), in conjunction with minimization of leakage current, directly impacts system performance because of the field's quadratic relationship with electrostatic energy storage density. On the basis of the extreme internal interfacial area and ultrafine morphology, polymer-inorganic nanocomposites (PNCs) have demonstrated modest increases in E(BD) at very low inorganic loadings, but because of insufficient control of the hierarchal morphology of the blend, have yielded a precipitous decline in E(BD) at intermediate and high inorganic volume fractions. Here in, we demonstrate that E(BD) can be increased up to these intermediate inorganic volume fractions by creating uniform one-dimensional nanocomposites (nanolaminates) rather than blends of spherical inorganic nanoparticles and polymers. Free standing nanolaminates of highly aligned and dispersed montmorillonite in polyvinyl butyral exhibited enhancements in E(BD) up to 30 vol % inorganic (70 wt % organically modified montmorillonite). These relative enhancements extend up to five times the inorganic fraction observed for random nanoparticle dispersions, and are anywhere from two to four times greater than observed at comparable volume fraction of nanoparticles. The breakdown characteristics of this model system suggested a trade-off between increased path tortuosity and polymer-deficient structural defects. This implies that an idealized PNC morphology to retard the breakdown cascade perpendicular to the electrodes will occur at intermediate volume fractions and resemble a discotic nematic phase where highly aligned, high-aspect ratio nanometer thick plates are uniformly surrounded by nanoscopic regions of polymer.

Hybrid Dielectric Resonator Antenna Composed of High-Permittivity Dielectric Resonator for Wireless Communications in WLAN and WiMAX

The-hybrid dielectric resonator antenna consisted of a cylindrical high-permittivity dielectric resonator, a rectangular slot, and two-rectangular patches were implemented. The hybrid dielectric resonator antenna had three resonant frequencies. The lower, middle, and higher resonant frequencies were associated with the rectangular slot, rectangular patches, and dielectric resonator, respectively. Parametric investigation was carried out using simulation software. The proposed hybrid dielectric resonator antenna had good agreement between the simulation results and the measurement results. The hybrid dielectric resonator antenna was implemented successfully for application in 2.4/5.2/5.8 GHz of WLAN and 2.5/3.5/5.5 GHz of WiMAX simultaneously.

Interface Engineering in High-Performance Low-Voltage Organic Thin-Film Transistors Based on 2,7-Dialkyl-[1]benzothieno[3,2-b][1]benzothiophenes

We investigated two different (2,7-dialkyl-[1]benzothieno[3,2-b][1]benzothiophenes; C(n)-BTBT-C(n), where n = 12 or 13) semiconductors in low-voltage operating thin-film transistors. By choosing functional molecules in nanoscaled hybrid dielectric layers, we were able to tune the surface energy and improve device characteristics, such as leakage current and hysteresis. The dipolar nature of the self-assembled molecules led to a shift in the threshold voltage. All devices exhibited high charge carrier mobilities of 0.6-7.0 cm(2) V(-1) s(-1). The thin-film morphology of BTBT was studied by means of atomic force microscopy (AFM), presented a dependency upon the surface energy of the self-assembled monolayer (SAM) hybrid dielectrics but not upon the device performance. The use of C(13)-BTBT-C(13) on hybrid dielectrics of AlO(x) and a F(15)C(18)-phosphonic acid monolayer led to devices with a hole mobility of 1.9 cm(2) V(-1) s(-1) at 3 V, on/off ratio of 10(5), small device-device variation of mobility, and a threshold voltage of only -0.9 V, thus providing excellent characteristics for further integration.