Layer In Field-effect Transistors Research Articles

Gradual improvements of charge carrier mobility at ionic liquid/rubrene single crystal interfaces

We report evolution of electric characteristics of an electric double layer field-effect transistor based on the ionic liquid/rubrene single crystal interfaces. In contrast to usual devices, the field effect mobility was found to gradually increase with time for a day, followed by minor long-term fluctuations. Although the details of the evolution were somewhat device dependent, the final values of the mobility turned out to be 3–4 times larger irrespective of the initial values. These observations are explained by the evolution of the flat interface by defect-induced spontaneous dissolution of rubrene molecules at the ionic liquid/rubrene single crystal interfaces, revealed by frequency modulation atomic force microscopy.

Narrow band-gap donor–acceptor copolymers based on diketopyrrolopyrrole and diphenylethene: Synthesis, characterization and application in field effect transistor

Three low band-gap diketopyrrolopyrrole based polymers with varying donor groups of furan, thiophene and phenyl were synthesized and then copolymerized with diphenylethene. We investigate the influence of different donor groups and comonomers on the band-gap and field effect transistors. The efficient synthesis of the diketopyrrolopyrrole based copolymers was clearly characterized by a variety of measurements. Two dimensional Grazing Incident X-ray Diffraction was measured to prove that furan and thiophene based copolymers have ordered edge-on structure. These copolymers exhibited strong π–π stacking and excellent hole mobilities when applied in the electric double layer field effect transistors. The high mobility of 2.36 cm2 V−1 s−1 with an on/off ratio of 103 was achieved.

Molecules on Semiconductors: From Chromogenic Interactions to Information Processing Devices

Rapid development of electronic technologies requires new semiconducting and dielectric materials of superior quality. Recently new applications (like LCD and OLED displays) initiated the employment of various organic materials (pentacene, oligothiophenes) as semiconductors for thin layer field effect transistors. Micro- and nanoelectronics also search for new materials of unusual properties. One of the recent examples is a memristor - a memory resistor. This fourth fundamental passive element (like resistor, capacitor and inductor) was predicted theoretically in early seventies and demonstrated experimentally 37 years later. Memristors have opened new fields of electronics and enabled fabrication of circuits with synaptic properties and learning abilities. Not surprisingly the prototype of this amazing device was based on very popular wide band gap semiconductor: titanium dioxide. This material is widely used as photocatalyst and in dye-sensitized solar cells, but its applications in electronic elements are relatively rare. However, TiO2 is the first material for which the photoelectrochemical photocurrent switching effect was described, which further resulted in new materials for unconventional logic devices. Other metal oxides, especially binary ones (e.g. PbMoO4, BiVO4) exhibit similar semiconducting properties (the same n-type conductivity, similar band gap values), but their properties as active materials in micro/nanoelectronic devices are almost not recognized. The role of these oxide semiconductors may be two-fold. Firstly, they can provide a solid substrate and a scaffold for self-assembly of molecular layers. Transition metal oxides should provide self-assembly template for organic molecules. Secondly, due to their non-innocent interactions with organic ligands may substantially modify properties of organic materials. These interactions may be strong enough to generate new (emergent) optical and electronic features which are not present in parent compounds. Photoinduced electron transfer (PET), optical electron transfer (OET) and photoelectrochemical photocurrent switching effect (PEPS) are such phenomena. This contribution presents the nature of chromogenic interactions from the quantum-chemical point of view on the example of aromatic hydrocarbons anchored to various d 0 ions in the coordination environment present in semiconducting oxide phases. The consequences of these interactions are related to the functional properties which are shown from experimental perspective in the second part. Optoelectronic switches, logic gates, binary computing circuits and artificial synaptic devices will be discussed in detail.

Mobility enhancement in electric double layer gated n-ZnO ultraviolet photodetector by synergy of gate and illumination: A photo Hall study

We report a large enhancement of the Hall mobility of a ZnO film (channel) by simultaneously application of an ultraviolet illumination along with a gate bias in an electric double layer field effect transistor configuration. The effect arises from a synergy between the illumination and the field effect (FE), leading to large enhancement of the channel conductivity and the photo response. We propose that large carrier density created by the simultaneous presence of the illumination and the FE leads to neutralization of some of the oxygen charged vacancies which in turn reduce potential scattering leading to the enhanced mobility.

Contorted polycyclic aromatics.

CONSPECTUS: This Account describes a body of research in the design, synthesis, and assembly of molecular materials made from strained polycyclic aromatic molecules. The strain in the molecular subunits severely distorts the aromatic molecules away from planarity. We coined the term "contorted aromatics" to describe this class of molecules. Using these molecules, we demonstrate that the curved pi-surfaces are useful as subunits to make self-assembled electronic materials. We have created and continue to study two broad classes of these "contorted aromatics": discs and ribbons. The figure that accompanies this conspectus displays the three-dimensional surfaces of a selection of these "contorted aromatics". The disc-shaped contorted molecules have well-defined conformations that create concave pi-surfaces. When these disc-shaped molecules are substituted with hydrocarbon side chains, they self-assemble into columnar superstructures. Depending on the hydrocarbon substitution, they form either liquid crystalline films or macroscopic cables. In both cases, the columnar structures are photoconductive and form p-type, hole- transporting materials in field effect transistor devices. This columnar motif is robust, allowing us to form monolayers of these columns attached to the surface of dielectrics such as silicon oxide. We use ultrathin point contacts made from individual single-walled carbon nanotubes that are separated by a few nanometers to probe the electronic properties of short stacks of a few contorted discs. We find that these materials have high mobility and can sense electron-deficient aromatic molecules. The concave surfaces of these disc-shaped contorted molecules form ideal receptors for the molecular recognition and assembly with spherical molecules such as fullerenes. These interfaces resemble ball-and-socket joints, where the fullerene nests itself in the concave surface of the contorted disc. The tightness of the binding between the two partners can be increased by creating more hemispherically shaped contorted molecules. Given the electronic structure of these contorted discs and the fullerenes, this junction is a molecular version of a p-n junction. These ball-and-socket interfaces are ideal for photoinduced charge separation. Photovoltaic devices containing these molecular recognition elements demonstrate approximately two orders of magnitude increase in charge separation. The ribbon-shaped, contorted molecules can be conceptualized as ultranarrow pieces of graphene. The contortion causes them to wind into helical ribbons. These ribbons can be formed into the active layer of field effect transistors. We substitute the ribbons with di-imides and therefore are able to transport electrons. Furthermore, these materials absorb light strongly and have ideal energetic alignment of their orbitals with conventional p-type electronic polymers. In solar cells, these contorted ribbons with commercial donor polymers have record efficiencies for non-fullerene-based solar cells. An area of interest for future exploration is the merger of these highly efficient contorted ribbons with the well-defined interfaces of the ball-and-socket materials.

Graphene oxide-zinc oxide nanocomposite as channel layer for field effect transistors: effect of ZnO loading on field effect transport.

The effects of ZnO on graphene oxide (GO)-ZnO nanocomposites are investigated to tune the conductivity in GO under field effect regime. Zinc oxides with different concentrations from 5 wt % to 25 wt % are used in a GO matrix to increase the conductivity in the composite. Six sets of field effect transistors with pristine GO and GO-ZnO as the channel layer at varying ZnO concentrations were fabricated. From the transfer characteristics, it is observed that GO exhibited an insulating behavior and the transistors with low ZnO (5 wt %) concentration initially showed p-type conductivity that changes to n-type with increases in ZnO loading. This n-type dominance in conductivity is a consequence of the transfer of electrons from ZnO to the GO matrix. From X-ray photoelectron spectroscopic measurements, it is observed that the progressive reduction in the C-OH oxygen group took place with increases in ZnO loading. Thus, from insulating GO to p- and then n-type, conductivity in GO could be achieved with reduction in the C-OH oxygen group by photocatalytic reduction of GO with varying degrees of ZnO. The restoration of sp(2) electron network in the GO matrix with the anchoring of ZnO nanostructures was observed from Raman spectra. From UV-visible spectra, the band gap in pristine GO was found to be 3.98 eV and reduced to 2.8 eV with increase in ZnO attachment.

Pulsed direct flame deposition and thermal annealing of transparent amorphous indium zinc oxide films as active layers in field effect transistors.

Indium-zinc oxide (IZO) films were deposited via flame spray pyrolysis (FSP) by pulsewise shooting a Si/SiO2 substrate directly into the combustion area of the flame. Based on UV-vis measurements of thin-films deposited on glass substrates, the optimal deposition parameters with respect to low haze values and film thicknesses of around 100 nm were determined. Thermal annealing of the deposited films at temperatures between 300 and 700 °C was carried out and staggered bottom gate thin-film transistors (TFT) were fabricated. The thin films were investigated by scanning electron microscopy, atomic force microscopy, X-ray diffraction, Fourier transformed infrared spectroscopy, and room-temperature photoluminescence measurements. The outcome of these investigations lead to two major requirements in order to implement a working TFT: (i) organic residues from the deposition process need to be removed and (ii) the net free charge carrier concentration has to be minimized by controlling the trap states in the semiconductor. The optimal annealing temperature was 300 °C as both requirements are fulfilled best in this case. This leads to field effect transistors with a low hysteresis, a saturation mobility of μSat = 0.1 cm(2)/(V s), a threshold voltage of Vth = -18.9 V, and an Ion/Ioff ratio on the order of 10(7). Depending on thermal treatment, the defect density changes significantly strongly influencing the transfer characteristics of the device.

Enhancing photoresponse by synergy of gate and illumination in electric double layer field effect transistors fabricated on n-ZnO

We report that photoresponse of ZnO in ultraviolet (UV) can be enhanced substantially by simultaneously applying a gate bias in an Electric Double Layer Field Effect Transistor configuration fabricated on ZnO as a channel. The effect arises from synergy between UV illumination and applied gate bias, which leads to a substantial enhancement in the device current. We propose that large carrier density created by the illumination and the gate leads to neutralization of some of the oxygen charged vacancies which in turn reduce potential scattering leading to enhanced field effect mobility. This is verified by gate bias controlled Photo Luminescence experiment.

Microstructure Evolution and Device Performance in Solution-Processed Polymeric Field-Effect Transistors: The Key Role of the First Monolayer

Probing the role of the first monolayer in the evolution of the film polymer microstructure is essential for the fundamental understanding of the charge carrier transport in polymeric field-effect transistors (FETs). The monolayer and its subsequent microstructure of a conjugated polymer [poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT] film were fabricated via solution deposition by tuning the dip-coating speed and were then studied as accumulation and transporting layers in FETs. Investigation of the microstructure of the layers prepared at different coating velocities revealed that the monolayer serves as an important base for further development of the film. Significant improvement of the charge carrier transport occurs only at a critical multilayer network density that establishes the required percolation pathways for the charge carriers. Finally, at a low dip-coating speed, the polymer chains are uniaxially oriented, yielding pronounced structural anisotropy and high charge carrier mobilities of 1.3 cm(2) V(-1) s(-1) in the alignment direction.

Improving charge transport in poly(3-hexylthiophene) transistors via blending with an alkyl-substituted phenylene-thiophene-thiophene-phenylene molecule

A prototypical semiconducting bicomponent system consisting of a conjugated polymer, that is, poly(3-hexylthiophene) (P3HT), blended with a small thiophene containing conjugated molecule, that is, an alkyl-substituted bisphenyl-bithiophene [phenylene–thiophene–thiophene–phenylene (PTTP)], has been used as an electroactive active layer in field-effect transistors (FETs). The self-assembly of this bicomponent system at surfaces has been studied at different length scales, from the nanoscale to the macroscale, and compared with the behavior of monocomponent films of PTTP and P3HT. The correlation between morphology and electric properties of the semiconducting material is explored by fabricating prototypes of FETs varying the relative concentrations of the two-component blend. The maximum charge carrier mobility value, achieved with a few percent of PTTP component, is not simply due to a uniform dispersion of the molecules in the polymer matrix, but rather to the generation of very long percolation paths, whose composition and electrical properties can be tuned with the PTTP concentration. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Mixed selenium-sulfur fused ring systems as building blocks for novel polymers used in field effect transistors

Eight novel polymer materials for transistor applications were prepared based on the fused ring building block selenolo[3,2-b]thiophene. The molecular structure of selenolo[3,2-b]thiophene was determined by single-crystal X-ray diffraction and was further modified for use as a monomer during the Stille synthesis of various copolymers containing substituted and unsubstituted thiophenes, thiazoles, dithienopyrroles (DTP) and benzothiadiazole subunits. The resultant polymers were analyzed for UV-Vis absorption and show a significant red shift when compared to regioregular poly(3-hexylthiophenes) (P3HT), attributed to the increased planarity and conjugation of the polymer backbone. Cyclic voltammetry experiments on the new polymers show that almost all of the new structures have a higher oxidation potential compared to P3HT, making them more stable toward oxidative doping under ambient atmospheres. The polymers were also evaluated for their electrical properties through their performance as the active layer in field effect transistors. The polymers showed relatively high mobility values, of which the highest observed value is 0.11 cm2 V−1s−1, with on/off ratios of about 105.

Polymer bilayer films with semi-interpenetrating semiconducting/insulating microstructure for field-effect transistor applications

We report an unexpected improvement in the microstructural and electrical properties of regioregular poly(3-hexylthiophene) (P3HT) as an active layer in field-effect transistors (FETs) by introducing soft insulating chains from poly(methyl methacrylate) (PMMA) as a cover layer in a solvent mode-dependent manner. A joint experimental and theoretical spectroscopic and electrical analysis is provided to study the P3HT/PMMA bilayer films with semi-interpenetrating semiconducting/insulating interlocked interdiffusion microstructures. Absorption and Raman studies reveal that disordered P3HT chains in films cast from a low-boiling point (bp) solvent favor reorganization into more ordered structures during PMMA covering, thus enhancing the electrical and switch behaviors of FETs. Although high-bp solvents initially create highly crystalline P3HT films, the films are significantly destroyed after the deposition of the PMMA layer, thus dramatically degrading FET characteristics. The semi-interpenetrating semiconducting/insulating microstructures in the designed bilayer films indicate the possibility of ameliorating field-effects in polymer FETs, especially in subthreshold swing and switch on–off behaviors.

Creating Novel Transport Properties in Electric Double Layer Field Effect Transistors Based on Layered Materials

ABSTRACTWe present a study on the liquid/solid interface, which can be electrostatically doped to a high carrier density (n~1014 cm-2) by electric-double-layer gating. Using micro-cleavage technique on the layered materials: ZrNCl and graphene, atomically flat channel surfaces can be easily prepared. Intrinsic high carrier density transport regime is accessed at the channel interface of electric double-layer field effect transistor, where novel transport properties are unveiled as the field-induced superconductivity on the ZrNCl with high transition temperature at 15 K, and accessing a high carrier density up to 2×1014 cm-2 in graphene and its multi-layers.

Field effect transistor behavior of organic light-emitting diodes with a modified configuration of ITO anode

We have developed the organic light-emitting diodes (OLEDs) with a modified configuration of ITO anode in which a thin channel was etched to form a bottom-contact field effect transistor (FET) using ITO and MgAg as a source/drain electrode and a gate electrode, respectively. The hole injection layer in OLEDs functioned as an active layer of FET and the other organic layers as insulator-like layer. The devices were found to exhibit a behavior of FET due to horizontal charge migration between source and drain, and an electro-optical transfer characteristic due to vertical charge transport and recombination. We have investigated the dependence of drain current on the channel length from 5 to 30 μm and found that the modified channel length could change drain current directionally and quantitatively.

Electric Double Layer Gate Field-Effect Transistors Based on Si

Electric double layer field-effect transistors (EDL-FETs) were fabricated using single crystal Si wafer as the active semiconductor and various characteristics were studied including dynamic response against step-function gate bias. The static FET mobility was more than 100 cm2 V-1 s-1. The response time of the drain current was 20 µs for ionic liquid and 3 ms for poly(ethylene glycol) (PEG) solution of LiBF4. Unexpected fast response was observed at a certain “speed up bias” condition. This effect will be useful to switching circuits using EDL-FETs.

Luminescent Ethynyl−Pyrene Liquid Crystals and Gels for Optoelectronic Devices

Two functional ethynyl-pyrene derivatives have been designed and synthesized by di- and tetra-substitutions of bromo pyrene derivatives with N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide fragments. The photoluminescence wavelength of the pyrene core can be tuned by the substitution pattern and the state of matter (solid, solution, gel, or liquid crystal). The disubstituted pyrene derivative 1 is not mesomorphic but produces robust and highly fluorescent gels in DMF, toluene, and cyclohexane. The well-defined fibers and ropes of the gel states were characterized by SEM and laser scanning confocal microscopy, and extended over several micrometers. The gels were integrated as active layers in field-effect transistors, which provided good bulk electron and hole charge mobilities as well as light emission generation. The tetra-substituted pyrene derivative is not a gelator but displays a stable liquid crystalline phase with 2D hexagonal symmetry between 20 and 200 degrees C. The pronounced luminescence properties of the mesophase allow one to observe original mesophase textures with flower-like patterns directly by fluorescence microscopy without crossed-polarizers.

Gate induced superconductivity in layered material based electronic double layer field effect transistors

Applying the principle of field effect transistor to layered materials provides new opportunities to manipulate their electronic properties for interesting sciences and applications. Novel gate dielectrics like electronic double layer (EDL) formed by ionic liquids are demonstrated to achieve an electrostatic surface charge accumulation on the order of 1014cm−2. To realize electric field-induced superconductivity, we chose a layered compound: ZrNCl, which is known to be superconducting by introducing electrons through intercalation of alkali metals into the van der Waals gaps. A ZrNCl-based EDL transistor was micro fabricated on a thin ZrNCl single crystal made by mechanical micro-cleavage. Accumulating charges using EDL gate dielectrics onto the channel surface of ZrNCl shows effective field effect modulation of its electronic properties. Sheet resistance of ZrNCl EDL transistor is reduced by applying a gate voltage from 0 to 4.5V. Temperature dependence of sheet resistance showed clear evidence of metal–insulator transition upon gating, observed at a gate voltage higher than 3.5V. Furthermore, gate-induced superconductivity took place after metal–insulator transition when the transistor is cooled down to about 15K.

Oligothiophene Self‐Assembly on the Surface of ZnO Nanorods: Toward Coaxial p–n Hybrid Heterojunctions

We describe herein the design, synthesis and detailed structural characterization of hybrid 1D nanostructures. They are prepared by supramolecular self-assembly of oligothiophene molecules on the surface of zinc oxide nanorods in solution at room temperature. Electronic absorption spectroscopy and X-ray diffraction show that both organic and inorganic components in the coaxial p-n heterojunctions are crystalline. Especially, it is demonstrated that the organic compounds form a self-assembled monolayer at the surface of the nanorods, which is not the case when zinc oxide quantum dots are instead used. As a result of their hybrid nature, the 1D nanostructures lead to ambipolar semiconducting nanostructured materials as active layers in field-effect transistors.

Surface modification of hafnia with polyelectrolytes based on the spin-coating electrostatic self-assembly method

In this paper, we describe organic surface modification and functionalization of a hafnia substrate, which has been extensively investigated as a replacement of the gate insulting SiO 2 layer in field effect transistors. The surface state of the hafnia was assessed by water contact angle ( θ water) measurement with comparison to that of the silicone during the layer-by-layer (LBL) deposition of poly(allyamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) bilayers by means of the spin-coating electrostatic self-assembly, SCESA, method. The surface state of virgin hafnia ( θ water = 73 ± 1°) turned hydrophilic ( θ water = 8 ± 2°) after submission to the standard RCA cleaning process of silicon. The thickness of the multilayer films on the cleaned hafnia surface was found to grow linearly with an increase in the number of PAH/PSS bilayers ( d = 2.2 ± 0.1 nm), indicating the consistency in the formation of uniform films. The average water contact angle of the PAH and PSS layers on hafnia alternately switched between 36.0 ± 0.7° and 29.7 ± 0.4° during the nine deposition cycles. The analysis of the surface topography by means of atomic force microscopy (AFM) indicated that the surface roughness of the first PAH layer deposited on the hafnia was strongly smoothed from 1.54 to 0.44 nm with increasing the LBL deposition of polyelectrolytes.

Nonlinear transport in semiconducting polymers at high carrier densities

Conducting and semiconducting polymers are important materials in the development of printed, flexible, large-area electronics such as flat-panel displays and photovoltaic cells. There has been rapid progress in developing conjugated polymers with high transport mobility required for high-performance field-effect transistors (FETs), beginning with mobilities around 10(-4) cm(2) V(-1) s(-1) to a recent report of 1 cm(2) V(-1) s(-1) for poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). Here, the electrical properties of PBTTT are studied at high charge densities both as the semiconductor layer in FETs and in electrochemically doped films to determine the transport mechanism. We show that data obtained using a wide range of parameters (temperature, gate-induced carrier density, source-drain voltage and doping level) scale onto the universal curve predicted for transport in the Luttinger liquid description of the one-dimensional 'metal'.