Source-drain Electrodes Research Articles

Evaluating the use of graphene electrodes in sub-micrometric, high-frequency n-type organic transistors

In this work we report on fully operational sub-micrometric low voltage OFETs by using graphene as the source-drain electrodes pair and a high-κ ultra-thin dielectric in a local gate architecture. The impact of the graphene electrodes on the miniaturization of the organic devices has been assessed, with particular attention to the influence of the contact resistances as well as the parasitic overlap gate capacitance on the device bandwidth. By the use of a modified Transmission-Line-Method, contact resistances have been analyzed as function of the applied voltages, revealing characteristic functional trends that follow the doping state of graphene electrodes. Through impedance spectroscopy of the electrodes, cut-off frequencies as high as 105 Hz have been estimated, highlighting the peculiar role of quantum capacitance of graphene in such architectures.

Highly Sensitive Hydrazine Detection Using a Vertically Oriented ZnO Nanosheet-based Field-Effect Transistor

Hydrazine, a highly toxic compound, is used worldwide in industries as a reducing agent for numerous applications. It has a harmful effect on humans and the environment; therefore, advanced analytical techniques are utilized to monitor hydrazine levels to minimize exposure. In this work, we report the fabrication of vertically oriented ZnO nanosheets based solution-gated field-effect transistor (FET) sensor for the low concentration detection of hydrazine. The ZnO nanosheets between source-drain electrodes were encapsulated with Nafion matrix to enhance stability and selectivity during hydrazine detection. The ZnO nanosheets-based solution-gated FET sensor’s sensing properties are compared with and without different concentrations of hydrazine in buffer solution. Conductance of the solution-gated FET hydrazine sensor showed substantial change upon addition of different concentrations of hydrazine. The fabricated hydrazine sensor exhibits high sensitivity (12150 μA μM−1 cm−2) in the linear response range of 5–110 nM. The low detection limit (∼1.2 nM; signal-to-noise (S/N) ratio of 3), high sensitivity, stability, and reproducibility are due to the direct growth of nanosheets that provide the controlled morphology and enhanced surface area. Besides, solution-gated FET hydrazine sensor was successfully applied to detect hydrazine in water samples for practical validation of developed hydrazine sensors.

Fabrication of Recessed-Gate AlGaN/GaN Hemts Utilizing Contactless Photo-Electrochemical (CL-PEC) Etching

AlGaN/GaN HEMTs are promising devices to attain both high-power and high-speed switching operation for the use in next-generation power electronics circuits. From the viewpoint of fail-safe and low-power consumption design, however it is necessary to attain normally-off operation with a positive threshold voltage (V th). One of the promising approaches to attain such operation is adopting a recessed-gate structure, which can be fabricated by thinning the AlGaN layer beneath the gate electrode. We have reported the photo-electrochemical (PEC) etching process for the fabrication of the recessed-gate AlGaN/GaN HEMTs was promising to improvement of transfer characteristics of the device and control of V th[1]. In this study, we developed the contactless PEC (CL-PEC) etching process[2-5], whose experimental system was simpler than that of conventional PEC etching process, for the fabrication of the recessed-gate structure. Consequently, for the devices fabricated by CL-PEC, the etched surface was very smooth such as the as-grown surface. The device characteristics improved to be compared with those without the recessed-gate fabrication process.We used an i-Al0.24Ga0.76N (28.4 nm)/i-GaN heterostructure grown by metal organic vapor phase epitaxy on a semi-insulating SiC substrate as a sample. A Ti ohmic contact was formed on the edge of the sample as a cathode pad which was not connected to the outer circuit. The CL-PEC etching process was conducted by only dipped the sample into 0.025 M K2S2O8 aqueous solution (pH = 3.4) under illumination of UVC light, whose center wavelength was 260 nm and the intensity was 4 mW/cm2. The distance between UVC light source and the surface of sample was set to 4 cm, where the optical path in the solution was fixed at 0.5 cm. As an etching mask, the lamination film of SiN (100 nm) and photoresist was formed by sputtering and photolithography process. After the etching, TMAH treatment was conducted in recessed-gate region. An inductive-coupled-plasma reactive-ion-etching (ICP-RIE) was conducted for the device isolation. The source-drain electrodes (Ti/Al/Ti/Au), Schottky-gate and MIS-gate electrodes were fabricated by a standard evaporation process. The various devices with different gate lengths (3, 5 and 10 µm) were fabricated on the same chip. The gate width for all device was designed to be 100 μm.The etching depth and the surface morphology were evaluated by AFM technique after the CL-PEC etching. An etching rate of AlGaN layer was very slow (< 1.0 nm/min) under the optimal condition, leading to the precise control of the residual thickness of the AlGaN layer. In addition, the self-termination of the etching was observed. The etching was stopped in the AlGaN layer, leaving a thickness of 6 nm. The surface after the etching was very smooth, where the root-mean-square (RMS) value of roughness was 0.4 nm (in 7 × 7 μm2 area). The drain current-voltage (I DS-V DS) characteristics of Schottky recessed-gate HEMTs showed good pinch-off behavior. The drain leakage currents with a reverse bias decreased and the V th was shifted in the positive direction from -3.4V (planar-gate devices) to 0 V. The value of sub-threshold slope (SS), obtained from the transfer characteristics, was smaller than that obtained for the planar-gate HEMTs. In addition, the standard deviation, σ, of the V th was 5.7 mV which was much smaller than that of the planar-gate devices. These results show that the uniformity of device performance enhanced after the CL-PEC process. The MIS recessed-gate HEMTs also showed good I DS-V DS characteristics with constant saturation currents and pinch-off behavior. As similar to the Schottky HEMTs, shift of V th in the positive direction, and the improved V th uniformity were observed. In addition to this, the MIS gate structure has the advantage that the device operates under positive bias by reducing gate leakage currents. This feature is promising for increasing the drain current reduced by the recess gate structure. In conclusion, excellent electrical properties such as low-leakage currents and precise control of V th for recessed-gate AlGaN/GaN HEMTs were achieved by the low-damage CL-PEC etching with the self-terminated nature.Acknowledgements: This work was supported by JSPS KAKENHI-JP16H06421, 20H02175.[1] Y.Kumazaki et al., JJAP, 121, 184501 (2017). [2] F.Horikiri et al., APEX, 12, 031003 (2019) [3] M.Toguchi et al., APEX, 12, 066504 (2019). [4] F.Horikiri et al., IEEE Trans. Semicond. Manuf., 32, 489 (2019). [5] K.Miwa et al., APEX, 13, 026508 (2020). Figure 1

Lithographical Fabrication of Organic Single-Crystal Arrays by Area-Selective Growth and Solvent Vapor Annealing.

Miniaturized organic single-crystal arrays that are addressed by reading-out circuits are crucial for high performance and high-level integration organic electronics. Here, we report a lithography compatible strategy to fabricate organic single-crystal arrays via area-selective growth and solvent vapor annealing (SVA). The organic semiconducting molecules can first selectively grow on photographically patterned drain-source electrodes, forming ordered amorphous aggregates that can further be converted to discrete single-crystal arrays by SVA. This strategy can be applied to self-align the microsized organic single crystals on predesigned locations. With this method, suppression of cross-talk among devices, organic field-effect transistors, and basic logic gate arrays with reading-out electrodes are further demonstrated.

Substrate-free, stretchable electrolyte gated transistors

Abstract Improving device deformability under mechanical stress is an essential requirement for the realization of next-generation wearable electronic applications. Herein, we report a new concept of stretchable thin-film transistors (TFTs) that can be fabricated without substrate components. Instead of using plastic substrates, we employ an elastic (and also free-standing) solid-state electrolyte as a gate insulator. The strategy not only realizes stretchable TFTs, but also enhances the device performance (e.g. mobility, operation voltage) by the formation of high-capacitance electric double layers. As source-drain electrodes, semi-circular shaped gold (Au) thin-film was employed to enhance the device stretchability. By the combination of a solid electrolyte (gate insulator), metal film (electrodes), and polymer semiconductor (channel layer), the device is stretchable up to around 10%, and shows typical p-channel switching performance with low operation voltage of 1 V and TFT mobility of about 0.81 cm2/V s. The devices were processed by a simple solution process and thermal evaporation under low temperature (

Lithography-free fabrication of low operating voltage and large channel length graphene transistor with current saturation by utilizing Li+ of ion-conducting-oxide gate dielectric

The large channel length graphene field-effect transistor (GFET) can outperform its competitors due to its larger active area and lower noise. Such long channel length devices have numerous applications, e.g., in photodetectors, biosensors, etc. However, long channel length graphene devices are not common due to their semi-metallic nature. Here, we fabricate large channel length (up to 5.7 mm) GFETs through a simple, cost-effective method that requires thermally evaporated source-drain electrode deposition, which is less cumbersome than the conventional wet-chemistry based photolithography. The semiconducting nature of graphene has been achieved by utilizing the Li+ ion of the Li5AlO4 gate dielectric, which shows current saturation at a low operating voltage (∼2 V). The length scaling of these GFETs has been studied with respect to channel length variation within a range from 0.2 mm to 5.7 mm. It is observed that a GFET of 1.65 mm channel length shows optimum device performance with good current saturation. This particular GFET shows a “hole” mobility of 312 cm2 V−1 s−1 with an on/off ratio of 3. For comparison, another GFET has been fabricated in the same geometry by using a conventional SiO2 dielectric that does not show any gate-dependent transport property, which indicates the superior effect of Li+ of the ionic gate dielectric on current saturation.

Organic transistors based on oxidized single-walled carbon nanotube electrodes patterned via simple spray coating

We developed a facile spray-printing method to fabricate oxidized single-walled carbon nanotube (Oxy-SWCNT) source-drain electrodes for use in organic field-effect transistors (OFETs). Surface oxidized SWCNT bundles enables the solution processability of Oxy-SWCNT without dispersant. The spray printing of an Oxy-SWCNT solution via a shadow mask led to electrode patterns with excellent transparency and electrical conductivity. Pattern resolution was optimized by adjusting printing volume and substrate temperature. The performance of the pentacene FETs with Oxy-SWCNT electrodes was compared with that of conventional Au electrodes. The FETs with Oxy-SWCNT electrodes exhibited higher charge carrier mobilities, on/off current ratios, and optical transmittances than those of FETs with Au electrodes. The increased device performance of the FETs with Oxy-SWCNT electrodes is due to the low contact resistance resulting from the efficient charge injection and transport across the electrode-channel interface. High work function (−5.1 eV) of the Oxy-SWCNT facilitated the charge injection. Our study demonstrates that Oxy-SWCNTs exhibit exceptional potential as a printable electrode material for OFETs and promise a better replacement for electrodes in transparent and flexible electronics.

Graphene-based vertical thin film transistors

Vertical field effect transistors (VFETs), where the channel material is sandwiched between source-drain electrodes and the channel length is simply determined by its body thickness, have attracted considerable interest for high performance electronics owning to their intrinsic short channel length. To enable the effective gate modulation and current switching behavior, the electrode of conventional VFET is largely based on perforated metals, in which the gate electrical field could penetrate through. Recently, with the emerge of graphene, a new type of graphene based VFETs has been developed. With finite density of states and the weak electrostatic screening effect, graphene exhibits a field-tunable work-function and partial electrostatic transparency, it can thus function as an “active" contact with tunable graphene-channel junction, enabling entirely new transistor functions or higher device performance not previously possible. In this review, we discuss the research progresses of graphene-based VFET, including the its basic device structure, carrier transport mechanism, device performance and novel properties demonstrated.

Organic electrochemical transistor for sensing of sialic acid in serum samples

Sialic acid usually locates at the terminal of glycoproteins and glycolipids on cell surface. Compared to normal cells, cancer cells generally express much more sialic acid residues, and the sialylation of cell surface proteins or lipids is related to the progression of tumors, which leads to high expression of serum sialic acid in cancer patients. This work used an organic electrochemical transistor as the sensing platform to design a simple and suitable device for sensitive and convenient detection of sialic acid level in serum samples. The transistor-based biosensor consisted of three typical drain/source/gate Au electrodes on a glass substrate and a polymer membrane to serve as conducting channel between source electrode and drain electrode. The gate electrode was modified with carboxylated multi-wall carbon nanotubes to covalently bind 3-aminobenzeneboronic acid, which specifically recognized sialic acid to change the effect gate voltage of the transistor, and thus produced the signal of drain-source channel current for sensitive detection of sialic acid ranging from 0.1 to 7 mM. The novel biosensor possessed excellent specificity for distinguishing normal and cancer people. The detection results of serum samples from lung cancer patients demonstrated the excellent performance of the transistor-based biosensor, showing the potential application in clinical diagnose.

High Resolution Patterning of Functional Inks using Wettability and Repellency Control Method and Its Application to Electronic Devices

High performance and easily processable wettable/repellent patterns formation were achieved using photo-induced surface control materials (PISC) by conventional i-line exposure and annealing. Functional inks can be patterned spontaneously on exposed surface which realize high resolution patterning by drop-coating methods. Fine silver lines of 10-μm width were achieved by inkjet printing with Ag nanoparticle (AgNP) inks on this pattern. Using this technique, silver source-drain electrodes of organic thin film transistor (OTFT) were successfully fabricated. A novel patterning method using alkali-developable type PISC was also devised for formation of high resolution, aspect patterns and high thickness microlenses, which difficult to be formed by photolithography technique.

Electron irradiation of multilayer PdSe2 field effect transistors

Palladium diselenide () is a recently isolated layered material that has attracted a lot of interest for its pentagonal structure, air stability and electrical properties that are largely tunable by the number of layers. In this work, multilayer is used as the channel of back-gate field-effect transistors, which are studied under repeated electron irradiations. Source-drain electrodes enable contacts with resistance below . The transistors exhibit a prevailing n-type conduction in high vacuum, which reversibly turns into ambipolar electric transport at atmospheric pressure. Irradiation by electrons suppresses the channel conductance and promptly transforms the device from n-type to p-type. An electron fluence as low as dramatically changes the transistor behavior, demonstrating a high sensitivity of to electron irradiation. The sensitivity is lost after a few exposures, with a saturation condition being reached for fluence higher than . The damage induced by high electron fluence is irreversible as the device persists in the radiation-modified state for several hours, if kept in vacuum and at room temperature. With the support of numerical simulation, we explain such a behavior by electron-induced Se atom vacancy formation and charge trapping in slow trap states at the interface.

Thiazolothiazole‐Based Quinoidal Compounds for High‐Performance n‐Channel Organic Field‐Effect Transistors with Low‐Cost Metal Electrodes

AbstractThiazolothiazole‐based quinoidal molecules B1 and B2 are designed and synthesized. B1 and B2 have low‐lying lowest unoccupied molecular orbital energy levels of −4.47 and −4.41 eV, respectively. In thin films, B1 forms J‐type aggregation while B2 adopts H‐type aggregation. B1 and B2 can react with copper and silver metals and form charge transfer salts. Thin‐film transistor characteristics show both compounds display n‐channel field‐effect behavior, and the Ag and Cu source–drain electrode‐based devices exhibit similar mobility as Au source–drain electrode‐based ones. B1‐based transistors with Au, Cu, and Ag electrodes exhibit electron mobilities of 0.54, 0.45, and 0.39 cm2 V−1 s−1, respectively. The mobility of B2 is 0.061 cm2 V−1 s−1 for Au electrode‐based devices, 0.081 cm2 V−1 s−1 for Cu electrode‐based devices, and 0.09 cm2 V−1 s−1 for Ag electrode‐based devices. The self‐doping layer formed by the reaction of electrodes (Ag, Cu) and organic semiconductors (B1 and B2), at the interface of electrode/organic semiconductor is responsible for the high performance of Ag and Cu electrode‐based devices. All these results demonstrate the introduction of self‐doping layer at electrode/organic semiconductor is a general strategy to fabricate high performance transistors with low cost metals Ag and Cu as source–drain electrodes.

Thin-Film Transistor (TFT) Gas Sensor Based on Quantum Dots-Sensitized Molybdenum Disulfide

Introduction NO2 primarily gets in the air from the burning of fuel. Exposure to NO2 may potentially increase susceptibility to respiratory infections, and a 5 min emergency exposure limit of 35 ppm NO2 exposure has been proposed by the American Industrial Hygiene Association [1]. There is an increasing demand to develop large-scale networking of gas sensors for achieving online NO2 monitoring with low power consumption. Semiconductor gas sensors employing high-temperature ceramics technology have attracted intense research and industry interests because of their high sensitivity and low cost. However, the high operating temperature of these sensors still sets a limit to their compatibility with CMOS technology, making it challenging to achieve gas sensor array with large-scale manufacture and application level. On the other hand, electronic gas sensors based on TFTs have shown promising results, because of the control over gate voltage can modulate the carrier concentration in the semiconductor channel by orders of magnitude, which allows the TFT gas sensors to be more sensitive and selective. 2D transition metal dichalcogenides (TMDs), typically molybdenum disulfide (MoS2) nanoflakes, are popular channel materials used for TFT-based gas sensors [2, 3], due to their unique electronic properties and large specific surface area. However, MoS2 TFT gas sensors always suffer the sluggish and weak response as well as incomplete recovery during operated at room temperature, significantly limiting their practical applications.Herein, we proposed to construct MoS2 TFT gas sensor by sensitizing with PbS quantum dots (QDs), a highly tunable 0D nanomaterials with size-dependent bandgap and excellent solution processability [4], aiming to improve the room-temperature response and recovery kinetics. The huge amount of surface dangling bonds of QDs make them sensitive receptors for gas molecules. The hybrid PbS/MoS2 architecture benefited from excellent adsorption of the NO2 molecules to the highly active surface of PbS QDs, combined with the high mobility MoS2 nanoflakes-based TFT, thereby achieve the optimal NO2-sensing performance at room temperature. Experimental details Fabrication of MoS2 TFT sensors. The few-layer MoS2 nanoflakes were mechanically exfoliated from bulk crystal MoS2 onto the Si substrates covered by a 300 nm thick SiO2 layer. The substrates with interdigitated 5 nm Cr/100 nm Au as source-drain electrodes were pre-patterned by photolithography, followed by electron beam evaporation and the lift-off process. Alternatively, high quality MoS2 monolayer can be also synthesized on the above SiO2/Si substrates in a tube furnace by high purity sulfur powder and MoO3 powder, as S and Mo precursor, respectively, and then obtained MoS2 TFT with a bottom-gate bottom-contact configuration. Sensitization of MoS2 channel with PbS QDs. Organo-hot injection method were typically employed for the synthesis of PbS QDs. Ascribe to the excellent solution processability and stable dispersibility of QDs, we use programmable electrohydrodynamic jet printing technologies as well as spin-coating methods to directly deposit QDs on the surface of MoS2 nanoflakes. Finally, we successfully fabricated the TFT sensors based on 0D PbS/2D MoS2. Gas sensing experiments. As-fabricated PbS/MoS2 TFT sensors were measured using probe station connected with semiconductor parameter analyzer (Model B1500A, Keysight) to monitor the real-time current change, and electrical properties including current-voltage, transfer and output characteristics. All the electrical and gas sensing characterizations had been carried out in an ambient air atmosphere or inert atmosphere of N2 at room temperature (25°C). The concentration of NO2 was controlled by a mixture of NO2 (50 ppm in N2) and N2 (99.99%) flow measured by the mass flow controller. Results and Conclusions In conclusion, we have proposed a novel strategy to construct PbS/MoS2 TFT gas sensors for room temperature NO2 detection. Compared to the pristine MoS2 TFT sensors, the PbS/MoS2 TFT sensors exhibited higher sensitivity with fast response/recovery kinetics toward NO2. The sensing mechanism was attributed to the synergistic effect by the favorable 0D-2D interface, making combined efforts for excellent access of gas molecules to highly active PbS QDs, as well as the superb charge transport in MoS2 nanoflakes that made a great enhancement of the gas-sensing performance. The 0D-QDs with tunable bandgaps will further promote progress in the engineering of the band alignment at the 0D-2D heterojunction interface, paving a promising way to develop gas-sensing performance of 2D layered materials.

Photogating for small high-responsivity graphene middle-wavelength infrared photodetectors

We demonstrated a middle-wavelength infrared (MWIR) graphene photodetector using the photogating effect. This effect was induced by photosensitizers situated around a graphene channel that coupled incident light and generated a large electrical charge. The graphene-based MWIR photodetector consisted of a top graphene channel, source–drain electrodes, an insulator layer, and a photosensitizer, and its photoresponse characteristics were determined by current measurements. Irradiation of the graphene channel of the vacuum cooled device by an MWIR laser generated a clear photoresponse, as evidenced by modulation of the output current during irradiation. The MWIR photoresponse with the photogating effect was 100 times greater than that obtained from conventional graphene photodetectors without the photogating effect. The device maintained its MWIR photoresponse at temperatures up to 150 K. The effect of the graphene channel size on the responsivity was evaluated to assess the feasibility of reducing the photodetector area, and decreasing the channel area from 100 to 25 μm2 improved the responsivity from 61.7 to 321.0 AW − 1. The results obtained in our study will contribute to the development of high-performance graphene-based IR imaging sensors.

A mathematical model of OECTs with variable internal geometry

Organic Electrochemical Transistor (OECTs) are devices that can measure the ionic content of liquid samples and biological systems. The response of an OECT can provide information on the physiological conditions and characteristics of a biological system. In a typical OECT configuration, the system or sample is connected to a reference electrode (the gate) and to a semiconducting material, typically PEDOT:PSS, with two other terminals (the drain and the source) for connection to an external circuit. The transistor architecture of OECTs enables signal control and amplification. Upon application of an external electromagnetic field at the electrodes, ions are driven from the liquid sample towards the PEDOT:PSS channel, where they modify the conductivity of the channel and generate a continuous current as a function of time. The intensity of that current and the time to the steady state can be correlated to the characteristics of the ions in solution. In most of the existing theories that model the behavior of OECTs, the internal configuration and geometrical parameters of the device are assumed to be constant over time. This simplifying assumption breaks down in living systems and in all those soft devices with elevated value of compliance and absorption (such as devices on paper, textile or polymeric sponges). Similar simplified models may fail to predict the behavior of real systems within acceptable bounds. Here, we present a mathematical model that describes the behavior of OECTs in which the geometry of the internal fluidic circuits of the system can change over time. These circuits represent the network of chambers and channels through which the liquid solution flows from the gate to the drain-source electrodes, enabling the transport of ions. At a certain time, the liquid solution shall be spread throughout a fraction only of the entire network available for liquid transport, i.e. the wet fraction p. The mathematical model that we have developed in this work uses the data generated by OECTs to determine the wet fraction p and the concentration C of ions of a system. The model enables quantification of a system without calibration of the device, which may be of interest for those working in the fields of bioengineering, biomedical sensors, wearable electronics, flexible electronics. In experiments where the variables of system were varied over large intervals, the model achieved an excellent performance and a precision up to 92%.

Electron Charge Transport in Non-Peripherally Substituted Copper Phthalocyanine

Bottom-gate, bottom-contact organic thin film transistors (OTFTs) were fabricated using solvent soluble copper-1,4,8,11,15,18,22,25-octakis(hexyl)phthalocyanine as the active semiconductor layer. The compound was deposited as 70 nm thick spin-coated films onto gold source-drain electrodes supported on octadecyltrichlorosilane treated 250 nm thick SiO2 gate insulator. The analysis of experimental results showed the n-type field effect behaviour. Devices annealed at 100 oC under vacuum were found to exhibit the field-effect mobility of 0.0989 cm2 V−1 s−1, with an on/off current modulation ratio of ∼106, a reduced threshold voltage of 0.7 V and a sub-threshold swing of 2.12 V decade−1. The variations in surface morphology of the devices are found reflected considerably in the electrical measurements. The device contact resistance was found to be decreased as the gate bias increased and also with the annealing.

Clamping effects on mechanical stability and energy dissipation in nanoresonators based on carbon nanotubes

With continuous downscaling of resonators, clamping is expected to significantly impact the mechanical stability as well as the energy dissipation mechanisms, especially at the nanoscale. To understand the clamping effects at the nanoscale, we here report on an experimental investigation of a same nanotube based resonator subjected to two different clamping configurations. We investigate clamping associated stability and damping mechanisms by pushing the resonator into the nonlinear regime. The nanotube was first dry-transferred and suspended between source-drain palladium electrodes resulting in a bottom clamped configuration. A selective top-metallization process by platinum atomic layer deposition applied later resulted in a top-bottom clamped configuration. Large nanotube motional amplitude leading to a nonlinear Duffing response initiated small slippage of the nanotube. This instability in clamping was seen in both clamping configurations and was measured as an irreversible resonance frequency downshift. For the measured resonator devices, a gate induced nanotube tension in the range of 58–71 pN was estimated to overcome clamping forces and initiate slipping. In terms of energy dissipation, the top-metallization process was accompanied by a reduction in amplitude dependent nonlinear damping and Q-factor enhancement. Subjecting the same nanotube to both clamping configurations allowed for a direct comparison of clamping and quantification of nonlinear damping. In the present case, nonlinear damping was observed at an estimated nanotube motional amplitude of 11 nm (and higher), being dominant in bottom clamped configuration, suggesting the origin of this nonlinear damping to partially stem from external mechanisms in addition to other possible internal dissipation paths reported such as viscoelastic effects.

Ultra-sensitive NO2 gas sensors based on single-wall carbon nanotube field effect transistors: Monitoring from ppm to ppb level

Owing to their sensitive chemical-to-electrical transducer capabilities and compatibility with device miniaturization and low operation temperature, single-walled carbon nanotube field-effect transistor (SWCNT-FET) represents an attractive platform to provide solutions in the gas sensing field. In this work, SWCNT-FETs were fabricated and their performances for detecting low NO2 concentrations were evaluated. Outstanding devices response was obtained, which was shown to follow a 2 power law dependence between the response and the NO2 concentration in the range of 100 ppb up to 10 ppm. Such ultra-high response is attributed to an enhancement of the Schottky barrier modulation triggered by the specific device configuration. The device configuration is based on individual semiconducting SWCNTs directly connecting the interdigitated Source-Drain electrodes. To the best of our knowledge, the results reported here correspond to the most sensitive device among the devices based on non-functionalized carbon materials and operational at low temperatures. Furthermore, the obtained results are supported by a deep SWCNT characterization, and the changes in Schottky barrier's height induced by the presence of gas molecules are estimated and discussed. Overall, the present reported results provide useful information for establishing a robust process for the fabrication of the next generation of CNT based gas-sensing devices.

Silver film etching processing using the inkjet method for the organic thin film transistor

Abstract Thin-film transistors (TFT) are an important component of many printed electronics, as they greatly influence the performance of the device. Thus, optimum performance of a TFT is considered to be the most important factor in an electronic application field. This paper reports the enhanced performance of an organic TFT (OTFT) by a facile inkjet etching method for the first time. Thermally evaporated Ag with a thickness of 20–80 nm is used for the source–drain electrode. By employing temperature control and adopting a proper thickness of silver electrode, the source-drain patterning is improved and optimized for the fine pattern with accuracy of 50 μm-channel without residue. Accordingly, the field-effect mobility is enhanced to 0.454 cm2/V and the on/off current ratio is greater than 105. Furthermore, the fabricated OTFT with etched Ag electrode demonstrates more uniform electrical stability than printed Ag electrode.

Characteristics of Oxide TFT Using Carbon-Doped Ιn2O3 Thin Film Fabricated by Low-Temperature ALD Using Ethylcyclopentadienyl Indium (Ιn-EtCp) and H2O & O3

Introduction In2O3-based metal oxide semiconductors have been widely investigated as active channel materials for oxide thin film transistors (TFTs) in flexible devices. The bond dissociation energy (BDE) between metal and oxygen element in the In2O3-based metal oxide has been recently focused because a low BDE easily occurs excess oxygen vacancy (Vo) and results in metal like conductor. In-Ti-O, In-Si-O, and In-Si-C-O have been studied in attempts to suppress Vo by a high BDE such as Ti-O (667 kJmol-1), Si-O (779 kJmol-1), and C-O (1076 kJmol-1) [1, 2]. In2O3 is an especially promising material because of a simple binary metal oxide and a high carrier mobility (~ 65 cm2V-1s-1). PET film was generally employed as substrate for flexible devices and limited to a fabrication process temperature below 200 °C. Considering to this process temperature, atomic layer deposition (ALD) technique is essentially suitable to produce the In2O3 films, and many In compounds such as InCl3, TMIn, In(tmhd)3, InCp, and InEtCp have been employed as precursors [3]. As-grown In2O3 film generally exhibited metal-like conductor because of excess Vo into the In2O3 film [4]. To compensate Vo sites in as-grown In2O3 film, the post-metal deposition annealing (PMA) process in air, O2, and O3 was employed during final process of TFT fabrication. However, the PMA was generally carried out at a high temperature above 300 °C [4]. In this paper, we studied growth of In2O3 films as a function of InEtCp and H2O/O3 doses using ALD at 150 °C with InEtCp precursor and H2O/O3 gas. We also demonstrate transistor performance of carbon-doped In2O3 (In-O-C) TFT with SiO2/Al2O3 dielectric after PMA at 150 °C in O3 gas. Experiment The In2O3 films were typically deposited on Si/SiO2 substrates by ALD at 150 °C using InEtCp and H2O/O3. The thickness of the In2O3 films as a function of InEtCp and H2O/O3 doses were measured using a spectroscopic ellipsometer. The chemical bonding state, carbon content and O/In ratio of the In2O3 films were also analyzed by XPS. Back-gate-type In-O-C TFTs were fabricated as follows: Firstly, a 5-nm-thick Al2O3 films deposited on p++Si/SiO2 (250 nm) by ALD using trimethylaluminium precursor and O3 gas. Next, a 5-nm-thick In-O-C channel layer was deposited on the Al2O3 films by the same ALD condition at 150 °C, and subsequently patterned by lithographic process. Ti/Au source and drain electrodes were deposited. Finally, PMA was performed at 150 °C in O3 for 0-150 min. Results and Discussion At first the In2O3 film thickness as a function of H2O/O3 dose per pulse was measured while the InEtCp dose was 2.8×10-5 mol by ALD at 150 °C. The thickness of the In2O3 film was saturated when H2O/O3 was 0.9×10-4/2.9×10-3 mol (an inset graph of Fig. 1). According to this ALD condition, the film thickness as a function of number of ALD cycle was examined and the estimated growth per cycle (GPC) was 0.11 nm/cycle as shown in Fig. 1. The In2O3 films had a carbon content and O/In ratio were about 1.7 at.% and 1.2/1 (InO1.16C0.04), respectively, by XPS (not shown). The electrical properties of InO1.16C0.04 TFT changed dramatically from metal like conductor to semiconductor for PMA time between 0 and 90 min. Threshold voltage (V th) shifted from 4.1 to 4.3 V as PMA time increases from 90 to 150 min. Figure 2 shows the I d-V d characteristics of InO1.16C0.04 TFTs (L/W = 50/5 μm) after PMA for 150 min. The I d was linearly increased at low V d, indicating that ohmic contacts were consistently formed between the InO1.16C0.04 films and the source-drain electrodes. I d also shows saturation at a high V d, which indicates that the electron transport in the channel is well controlled by the V g. The TFT exhibited superior transistor performance such as relatively small S.S. of 0.37 V/decade, a small V th shift of +0.3 V, and a large μ FE of 20.4 cm2/Vs. This is due to the suppression of excess Vo in InO1.16C0.04 channel layer. Conclusions We studied characteristics of as-grown In2O3 films and its InO1.16C0.04 TFT. The excellent characteristics of InO1.16C0.04 TFT, which was fabricated even in a low temperature fabrication process at 150 °C, could be obtained due to the carbon doping into In2O3 film and relatively strong oxidation strength of O3 during PMA.