Source-drain Electrodes Research Articles

Ultrathin Electronic Composite Sheets of Metallic/Semiconducting Carbon Nanotubes Embedded in Conjugated Block Copolymers

Ultrathin composite films consisting of mixtures of metallic (m-) and semiconducting (s-) single-walled carbon nanotubes (SWNTs) with a conjugated block copolymer are developed from a solution-based process. The electronic properties of the films are precisely controlled from metallic to semiconducting to insulating. The tunability of the electronic composite sheets is mainly attributed to (1) the efficient dispersion of SWNTs with a conjugated block copolymer in solution, (2) the control of the number of nanotubes by centrifugation, and (3) the individually networked deposition of SWNTs embedded in the conjugated block copolymer on the target substrate by spin-coating. A highly reliable field-effect transistor with a networked composite film is realized with a specific range of tube density and a high on/off current ratio of approximately 106 which resulted from the Schottky barriers evolved between the individual m- and s-SWNTs in the network. There is also great freedom when choosing both the gate dielectrics and source-drain electrodes for transistors containing the composite films. Furthermore, the fabricated electronic composites are highly transparent, flexible, and chemically robust and thus, they can be conveniently micropatterned by photolithography, as well as by unconventional transfer printing techniques.

High-performance organic field-effect transistors based on copper/copper sulphide bilayer source-drain electrodes

We demonstrate an organic field-effect transistor (OFET) using copper (Cu) and copper sulfide (CuxS) as the source-drain electrodes. The OFET using Cu/CuxS results in a fivefold higher field-effect mobility and a 12 V reduction in threshold voltage as compared with the conventional devices with gold electrodes. The improvements of device performances are attributed to the enhancement of charge-injection. X-ray photoelectron spectroscopy revealed that sulfate ions were formed during deposition, which is considered to be responsible for the improvements.

Interfaces and traps in pentacene field-effect transistor

The equivalent circuit parameters for a pentacene organic field-effect transistor are determined from low frequency impedance measurements in the dark as well as under light illumination. The source-drain channel impedance parameters are obtained from Bode plot analysis and the deviations at low frequency are mainly due to the contact impedance. The charge accumulation at organic semiconductor–metal interface and dielectric–semiconductor interface is monitored from the response to light as an additional parameter to find out the contributions arising from photovoltaic and photoconductive effects. The shift in threshold voltage is due to the accumulation of photogenerated carriers under source-drain electrodes and at dielectric–semiconductor interface, and also this dominates the carrier transport. The charge carrier trapping at various interfaces and in the semiconductor is estimated from the dc and ac impedance measurements under illumination.

Effect of Oxygen Plasma Treatment on Crystal Growth Mode at Pentacene/Ni Interface in Organic Thin-Film Transistors

We report how treatment of nickel (Ni) with O(2) plasma affects the polarity of Ni surface, crystallinity of pentacene film on the Ni, and electrical properties of pentacene organic thin-film transistors (OTFTs) that use Ni as source-drain electrodes. The polar component of surface energy in Ni surface increased from 8.1 to 43.3 mJ/m(2) after O(2)-plasma treatment for 10 s. From X-ray photoelectron spectra and secondary electron emission spectra, we found that NiO(x) was formed on the O(2)-plasma-treated Ni surface and the work function of O(2)-plasma-treated Ni was 0.85 eV higher than that of untreated Ni. X-ray diffraction and atomic force microscopy measurements showed that pentacene molecules are well aligned as a thin-film and grains grow much larger on O(2)-plasma-treated Ni than on untreated Ni. This change in the growth mode is attributed to the reduction of interaction energy between pentacene and Ni due to formation of oxide at the Ni/pentacene interface. Thus, O(2)-plasma treatment promoted the growth of well-ordered pentacene film and lowered both the hole injection barrier and the contact resistance between Ni and pentacene by forming NiO(x), enhancing the electrical property of bottom-contact OTFTs.

Simulation based performance comparison of transistors designed using standard photolithographic and coarse printing design specifications

In this work a simulation based comparative study of organic field effect transistors designed using standard lithographic and printing designs is presented. The device simulations were performed using two-dimensional drift-diffusion equations with a Poole–Frenkel field dependent mobility model. Both photolithographic and coarse printing transistor designs employed common materials such as 150nm thick pentacene, 150nm thick parylene gate insulator, gold source-drain electrodes and aluminum gate electrodes. The major differences between the two fabrication specifications are the minimum source/drain line width and the transistor channel length. The typical specifications for the minimum line width and channel length were 2μm and 5μm for photolithography and 25μm and 20μm for coarse printing techniques, respectively. The gate, source, and drain capacitances and channel on-resistances at various channel lengths and gate overlaps have been extracted and presented specifically for both process schemes. Due to increased channel length and gate-source/drain overlap of printed electrodes relative to lithographic design, the resulting on-resistance and capacitances for coarse printing are significantly higher. These results demonstrate a substantial operating frequency reduction for printing design relative to photolithographic design. For the tested materials and designs it is shown that the cut-off frequency for the photolithographic process was 400kHz but decreased to a much lower 26kHz for the coarse printing process. Since printing technology uses various other materials, which typically have less performance than the ones used for this simulation, the actual printed device might have even lower performance than predicted here.

Device Configurations for Ambipolar Transport in Flexible, Pentacene Transistors

Ambipolar pentacene transistors in bottom contact-bottom gate geometry are fabricated on flexible substrates using parylene as a dielectric and self-assembled monolayer treatment of the source-drain electrodes to improve charge injection. Hole and electron mobilities of 0.07-0.1 cm2 V−1 s−1 and 0.01-0.04 cm2 V−1 s−1 are achieved. CMOS like inverters are built and gains of up to 110 are reported.

Ultrathin Electronic Composite Sheets of Metallic/Semiconducting Carbon Nanotubes Embedded in Conjugated Block Copolymers

AbstractUltrathin composite films consisting of mixtures of metallic (m‐) and semiconducting (s‐) single‐walled carbon nanotubes (SWNTs) with a conjugated block copolymer are developed from a solution‐based process. The electronic properties of the films are precisely controlled from metallic to semiconducting to insulating. The tunability of the electronic composite sheets is mainly attributed to (1) the efficient dispersion of SWNTs with a conjugated block copolymer in solution, (2) the control of the number of nanotubes by centrifugation, and (3) the individually networked deposition of SWNTs embedded in the conjugated block copolymer on the target substrate by spin‐coating. A highly reliable field‐effect transistor with a networked composite film is realized with a specific range of tube density and a high on/off current ratio of approximately 106 which resulted from the Schottky barriers evolved between the individual m‐ and s‐SWNTs in the network. There is also great freedom when choosing both the gate dielectrics and source‐drain electrodes for transistors containing the composite films. Furthermore, the fabricated electronic composites are highly transparent, flexible, and chemically robust and thus, they can be conveniently micropatterned by photolithography, as well as by unconventional transfer printing techniques.

Implementation of Molecular Transistor Electrodes by Electromigration

Gaps with a size of less than 5 nm have been fabricated in 15-nm-thick and 200-nm-wide gold strips deposited on sapphire substrates. Preparation conditions providing a sufficient adhesion of such electrodes as well as the parameters for the electromigration process used to fabricate the gaps have been found which allow a successful gap implementation. Such gaps transform gold strips to the source-drain electrodes of planar single electron transistors based on nanoparticles or molecules.

The performance improvement in pentacene organic thin film transistors by inserting C 60/MoO 3 ultrathin layers

A series of bottom-gated top-contact pentacene organic thin film transistors are fabricated. C 60 and/or MoO 3 ultrathin layers are inserted between the pentacene and Al source-drain electrodes to reduce the contact resistance. With proper order and thickness of the two layers modification, the injection barrier is greatly lowered down and the field-effect mobility increases from 0.095 cm 2/(V s) to 0.65 cm 2/(V s). The threshold voltage decreases from −11.3 V to −6.4 V. It means that the injection barrier plays an important role in the contact resistance without modification and multiple ultrathin layers modification is an effective method to improve the performance of the OTFTs. Then the output curve of the devices with better modification is simulated by a charge drift model. Taking into account of the contact effect, the field-effect mobility is improved to 1.05 cm 2/(V s). It indicates that after modification, the injection barrier is lowered down, but the contact resistance caused by the charge drift in the contact region become the more important role and still affect the performance.

Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

AbstractElectron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.

The polymer gate dielectrics and source-drain electrodes on n-type pentacene-based organic field-effect transistors

Pentacene-based organic field-effect transistors (OFETs) with different polymer gate dielectrics, such as polyvinyl alcohol (PVA), poly 4-vinyl phenol (PVP), and polystyrene (PS), are fabricated to study the influence of polymer dielectrics on the formation of the n-type conduction (electron) channel in the pentacene active layer. The output characteristics of OFETs and capacitance–voltage measurements indicate that the formation of n-type conduction channel in the active layer is hindered by the electron traps at the contact interface with PVP dielectric layers, probably due to the high dissociation constant of protons of the hydroxyl groups in PVP. The dissociated protons at PVP dielectric layer form the electron traps and restrict the formation of n-type conduction channel. In comparison, OFETs applying PVA of relatively lower dissociation constant than that of PVP as the gate dielectric present the decent n-type output characteristics. The appropriate work function of source-drain electrodes as well as a trap-free dielectric layer are essentially important to determine the performance of pentacene-based n-type OFETs. The pentacene-based OFETs applying calcium as the source-drain electrodes and PS as the dielectric layer has the electron mobility of 0.077 cm 2s −1V −1 in this study.

Sandwich-type gated mechanical break junctions

We introduce a new device architecture for the independent mechanical and electrostatictuning of nanoscale charge transport. In contrast to previous gated mechanicalbreak junctions with suspended source–drain electrodes, the devices presentedhere prevent an electromechanical tuning of the electrode gap by the gate. Thissignificant improvement originates from a direct deposition of the source and the drainelectrodes on the gate dielectric. The plasma-enhanced native oxide on the aluminumgate electrode enables measurements at gate voltages up to 1.8 V at cryogenictemperatures. Throughout the bending-controlled tuning of the source–drain distance, theelectrical continuity of the gate electrode is maintained. A nanoscale island in theCoulomb blockade regime serves as a first experimental test system for the devices, inwhich the mechanical and electrical control of charge transport is demonstrated.

Enhancement of ZnO thin film transistor performance by high-dose proton irradiation

Abstract The object of this study was to improve the performance of ZnO thin film transistors (TFTs) by exposing them to high-dose proton irradiation. A rapid thermal annealing (RTA) process was necessary to improve the interface characteristics between the source–drain electrodes and the channel layer for the high performance of ZnO-TFTs. However, this affected the resistivity of the ZnO channel layer; it was dramatically decreased during the RTA process. As a result, the RTA-treated ZnO-TFTs did not show the proper off-state characteristics. In order to control the electrical properties of the channel layer, we exposed the RTA-treated ZnO-TFTs to 6.1 MeV of proton irradiation beam energy at fluences from 6.7 × 1012 cm−2 to 6.5 × 1014 protons-cm−2. The resulting resistivity of the ZnO thin film increased after the high-dose proton irradiation. In addition, we studied the structural and electrical properties and the variations in the native defects of ZnO thin films. The field effective mobility of ZnO-TFTs increased from 1.65 to 4.12 cm2/V s after both the RTA and the high-dose proton irradiation. We obtained an enhancement of ZnO-TFT performance using high-dose proton irradiation.

P‐15: OTFT Backplane for Electrophoretic Display Fabricated by Combined Printing Technology

AbstractIn this paper, The OTFT backplane with bottom contact structure on plastic substrate using combined process will be discussed. The active size of backplane was 6″; in diagonal direction and consisted of 192×150 pixels, containing 1 OTFT and 1 capacitance in each pixel. Each layer of OTFT was used combining process to overcome the problem of printing technique. The gate electrodes were fabricated by screen printing with wet etching. The source drain electrodes were made by ink jet printing combining plasma treatment. Finally, The EPD panel successfully displayed test patterns.

Interface Engineering for Organic Electronics

AbstractThe field of organic electronics has been developed vastly in the past two decades due to its promise for low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, and ease of processing. The performance and lifetime of these devices, such as organic light‐emitting diodes (OLEDs), photovoltaics (OPVs), and field‐effect transistors (OFETs), are critically dependent on the properties of both active materials and their interfaces. Interfacial properties can be controlled ranging from simple wettability or adhesion between different materials to direct modifications of the electronic structure of the materials. In this Feature Article, the strategies of utilizing surfactant‐modified cathodes, hole‐transporting buffer layers, and self‐assembled monolayer (SAM)‐modified anodes are highlighted. In addition to enabling the production of high‐efficiency OLEDs, control of interfaces in both conventional and inverted polymer solar cells is shown to enhance their efficiency and stability; and the tailoring of source–drain electrode–semiconductor interfaces, dielectric–semiconductor interfaces, and ultrathin dielectrics is shown to allow for high‐performance OFETs.

Uniformity of fully gravure printed organic field-effect transistors

Abstract Fully mass-printed organic field-effect transistors were made completely by means of gravure printing. Therefore a special printing layout was developed in order to avoid register problems in print direction. Upon using this layout, contact pads for source–drain electrodes of the transistors are printed together with the gate electrodes in one and the same printing run. More than 50,000 transistors have been produced and by random tests a yield of approximately 75% has been determined. The principle suitability of the gravure printed transistors for integrated circuits has been shown by the realization of ring oscillators.

Low-voltage indium gallium zinc oxide thin film transistors on paper substrates

We have fabricated bottom-gate amorphous (α-) indium-gallium-zinc-oxide (InGaZnO4) thin film transistors (TFTs) on both paper and glass substrates at low processing temperature (≤100 °C). As a water and solvent barrier layer, cyclotene (BCB 3022–35 from Dow Chemical) was spin-coated on the entire paper substrate. TFTs on the paper substrates exhibited saturation mobility (μsat) of 1.2 cm2 V−1 s−1, threshold voltage (VTH) of 1.9 V, subthreshold gate-voltage swing (S) of 0.65 V decade−1, and drain current on-to-off ratio (ION/IOFF) of ∼104. These values were only slightly inferior to those obtained from devices on glass substrates (μsat∼2.1 cm2 V−1 s−1, VTH∼0 V, S∼0.74 V decade−1, and ION/IOFF=105–106). The uneven surface of the paper sheet led to relatively poor contact resistance between source-drain electrodes and channel layer. The ability to achieve InGaZnO TFTs on cyclotene-coated paper substrates demonstrates the enormous potential for applications such as low-cost and large area electronics.

Semiconductor Properties of Perylene Diimide Derivatives

Density functional theory (DFT) calculations were carried out to study the charge transfer properties of N, N'-bisperylene-3, 4, 9, 10-tetracarboxylic diimide (1), N,N'-bis (3-chlorobenzyl) peiylene-3, 4, 9, 10-tetracarboxylic diimide (2), N,N'-bis (3-fluorobenzyl) perylene-3, 4, 9, 10-tetracarboxylic diimide (3), and N,N'-bis (3, 3-difluorobenzyl) perylene 3, 4, 9, 10-tetracarboxylic diimide (4) for use as organic field effect transistors (OFETs). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies, ionization energy (IE), electron affinity (EA), and reorganization energy (A) were investigated for these compounds. Corresponding to energy changes in the HOMO and LUMO for 2-4, the introduction of chlorobenzyl or fluorobenzyl groups leads to high adiabatic electron affinity (EA(subscript a)) values for 2-4. The transfer integral (V) and field effect transistor (FET) properties for the four compounds with known crystal structures were calculated based on Marcus electron transfer theory. The calculated results reveal that a very small injection barrier exists relative to the Au source-drain electrode of 1-4 for electrons, producing good potential n-type semiconductors. Intrinsic electron transfer mobilities (μ(subscript -)) of 5.39, 0.59, 0.023, and 0.17 cm^2 were calculated for compounds 1-4, respectively. The high intrinsic electron mobilities for 1-4 were rationalized by examining the changes in geometric structure upon reduction and charge transfer integral for the different transfer modes in crystal 3.

Selectively patterned highly conductive poly(3,4-ethylenedioxythiophene)-tosylate electrodes for high performance organic field-effect transistors

We have improved the performance of pentacene field-effect transistors by using highly conductive poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) source-drain electrodes (∼103 S/cm) formed by a simple solution-based process. A high field-effect mobility of 0.25 cm2/Vs and an ON/OFF current ratio of 107 were obtained in pentacene-based bottom contact organic field-effect transistors (OFETs), which constitutes an improvement over OFETs based on Au and PEDOT:PSS electrodes. Two-dimensional grazing incidence x-ray diffraction and ultraviolet photoemission spectroscopy results confirmed that the crystalline properties of the pentacene film and the hole injection from the PEDOT-Tos electrode to the pentacene layer are more efficient than those from Au and PEDOT:PSS electrodes.

Patterned ZnO nanorods network transistor fabricated by low-temperature hydrothermal process

The ZnO nanorods network transistor fabricated by the hydrothermal process has been demonstrated. Bottom-gate structure was selected and the source-drain electrodes were formed by the conventional lift off process. After the deposition of ZnO seed layer by the sol–gel method, the channel region was defined by the HCl etching. Then the ZnO nanorods selectively grew on the channel region with the help of patterned seed layer. The device obeys the n-type depletion mode with the I on/ I off ratio of 10 3 and field-effect mobility of 6.7 cm 2/V s. The fabrication temperature of the whole process is below 300 °C which has great potential in the application on the transparent substrate.