Flexible Metal Oxide Semiconductor Devices Made by Solution Methods.

For the fabrication of next-generation flexible metal oxide devices, solution-based methods are considered as a promising approach because of their potential advantages, such as high-throughput, large-area scalability, low-cost processing, and easy control over the chemical composition. However, to obtain certain levels of electrical performance, a high process temperature is essential, which can significantly limit its application in flexible electronics. Therefore, this article discusses recent research conducted on developing low-temperature, solution-processed, flexible, metal oxide semiconductor devices, from a single thin-film transistor device to fully integrated circuits and systems. The main challenges of solution-processed metal oxide semiconductors are introduced. Recent advances in materials, processes, and semiconductor structures are then presented, followed by recent advances in electronic circuits and systems based on these semiconductors, including emerging flexible energy-harvesting devices for self-powered systems that integrate displays, sensors, data-storage units, and information processing functions.

Locally Controlled Sensing Properties of Stretchable Pressure Sensors Enabled by Micro-Patterned Piezoresistive Device Architecture.

For wearable health monitoring systems and soft robotics, stretchable/flexible pressure sensors have continuously drawn attention owing to a wide range of potential applications such as the detection of human physiological and activity signals, and electronic skin (e-skin). Here, we demonstrated a highly stretchable pressure sensor using silver nanowires (AgNWs) and photo-patternable polyurethane acrylate (PUA). In particular, the characteristics of the pressure sensors could be moderately controlled through a micro-patterned hole structure in the PUA spacer and size-designs of the patterned hole area. With the structural-tuning strategies, adequate control of the site-specific sensitivity in the range of 47~83 kPa-1 and in the sensing range from 0.1 to 20 kPa was achieved. Moreover, stacked AgNW/PUA/AgNW (APA) structural designed pressure sensors with mixed hole sizes of 10/200 µm and spacer thickness of 800 µm exhibited high sensitivity (~171.5 kPa-1) in the pressure sensing range of 0~20 kPa, fast response (100~110 ms), and high stretchability (40%). From the results, we envision that the effective structural-tuning strategy capable of controlling the sensing properties of the APA pressure sensor would be employed in a large-area stretchable pressure sensor system, which needs site-specific sensing properties, providing monolithic implementation by simply arranging appropriate micro-patterned hole architectures.

Photochemical Molecular Tailoring for Efficient Diffusion and Reorganization of Organic Nanocrystals for Ultra-Flexible Organic Semiconductor Arrays.

Solution-processed organic single crystals with high carrier mobility have been actively investigated for diverse applications such as displays, sensors, and next generation electronics on a flexible platform. However, the lack of precise alignment and growth control of organic single crystals impedes the widespread adoption of organic materials in an industrial perspective. Here, a photochemical modification approach is reported tailoring the solubility and molecular diffusivity of polymeric sacrificial layer and sequential batch-type vapor annealing to implement high-performance (average saturation mobility: 8.01 cm2 V-1 s-1 ) organic single-crystal thin film transistors with large channel width including multiple aligned single crystals. Additionally, the mechanical properties of the organic single crystals are systematically investigated with extreme strain conditions such as bending radius of 150 µm.

Autonomous Artificial Olfactory Sensor Systems with Homeostasis Recovery via a Seamless Neuromorphic Architecture.

Neuromorphic olfactory systems have been actively studied in recent years owing to their considerable potential in electronic noses, robotics, and neuromorphic data processing systems. However, conventional gas sensors typically have the ability to detect hazardous gas levels but lack synaptic functions such as memory and recognition of gas accumulation, which are essential for realizing human-like neuromorphic sensory system. In this study, a seamless architecture for a neuromorphic olfactory system capable of detecting and memorizing the present level and accumulation status of nitrogen dioxide (NO2) during continuous gas exposure, regulating a self-alarm implementation triggered after 147 and 85 s at a continuous gas exposure of 20 and 40 ppm, respectively. Thin-film-transistor type gas sensors utilizing carbon nanotube semiconductors detect NO2 gas molecules through carrier trapping and exhibit long-term retention properties, which are compatible with neuromorphic excitatory applications. Additionally, the neuromorphic inhibitory performance is also characterized via gas desorption with programmable ultraviolet light exposure, demonstrating homeostasis recovery. These results provide a promising strategy for developing a facile artificial olfactory system that demonstrates complicated biological synaptic functions with a seamless and simplified system architecture.

Full integration of highly stretchable inorganic transistors and circuits within molecular-tailored elastic substrates on a large scale.

The emergence of high-form-factor electronics has led to a demand for high-density integration of inorganic thin-film devices and circuits with full stretchability. However, the intrinsic stiffness and brittleness of inorganic materials have impeded their utilization in free-form electronics. Here, we demonstrate highly integrated strain-insensitive stretchable metal-oxide transistors and circuitry (442 transistors/cm2) via a photolithography-based bottom-up approach, where transistors with fluidic liquid metal interconnection are embedded in large-area molecular-tailored heterogeneous elastic substrates (5 × 5 cm2). Amorphous indium-gallium-zinc-oxide transistor arrays (7 × 7), various logic gates, and ring-oscillator circuits exhibited strain-resilient properties with performance variation less than 20% when stretched up to 50% and 30% strain (10,000 cycles) for unit transistor and circuits, respectively. The transistors operate with an average mobility of 12.7 ( ± 1.7) cm2 V-1s-1, on/off current ratio of > 107, and the inverter, NAND, NOR circuits operate quite logically. Moreover, a ring oscillator comprising 14 cross-wired transistors validated the cascading of the multiple stages and device uniformity, indicating an oscillation frequency of ~70 kHz.

InP/ZnS quantum dot photoluminescence modulation via in situ H2S interface engineering.

InP quantum dots (QDs) are attracting significant interest as a potentially less toxic alternative to Cd-based QDs in many research areas. Although InP-based core/shell QDs with excellent photoluminescence properties have been reported so far, sophisticated interface treatment to eliminate defects is often necessary. Herein, using aminophosphine as a seeding source of phosphorus, we find that H2S can be efficiently generated from the reaction between a thiol and an alkylamine at high temperatures. Apart from general comprehension that H2S acts as a S precursor, it is revealed that with core etching by H2S, the interface between InP and ZnS can be reconstructed with S2- incorporation. Such a transition layer can reduce inherent defects at the interface, resulting in significant photoluminescence (PL) enhancement. Meanwhile, the size of the InP core could be further controlled by H2S etching, which offers a feasible process to obtain wide band gap InP-based QDs with blue emission.

Truly form-factor-free industrially scalable system integration for electronic textile architectures with multifunctional fiber devices.

An integrated textile electronic system is reported here, enabling a truly free form factor system via textile manufacturing integration of fiber-based electronic components. Intelligent and smart systems require freedom of form factor, unrestricted design, and unlimited scale. Initial attempts to develop conductive fibers and textile electronics failed to achieve reliable integration and performance required for industrial-scale manufacturing of technical textiles by standard weaving technologies. Here, we present a textile electronic system with functional one-dimensional devices, including fiber photodetectors (as an input device), fiber supercapacitors (as an energy storage device), fiber field-effect transistors (as an electronic driving device), and fiber quantum dot light-emitting diodes (as an output device). As a proof of concept applicable to smart homes, a textile electronic system composed of multiple functional fiber components is demonstrated, enabling luminance modulation and letter indication depending on sunlight intensity.

Mechanically robust textile-based strain and pressure multimodal sensors using metal nanowire/polymer conducting fibers.

Recently, multifunctional textile-based sensory systems have attracted a lot of attention because of the growing demand for wearable electronics performing real-time monitoring of various body signals and movements. In particular, textile-based physical sensors often require multimodal sensing capabilities to accurately detect and identify multiple mixed stimuli simultaneously. Here, we demonstrate a textile-based strain/pressure multimodal sensor using high-k poly(vinylidene fluoride)-co-hexafluoropropylene ion-gel film and silver nanowire/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate-coated conducting fibers. The multimodal sensors exhibited reliable strain and pressure-sensing characteristics for strain ranges up to 25% and pressures up to 50 kPa, respectively, with a relatively high strain gauge factor (up to 2.74) and pressure sensitivity (0.32 kPa-1). More importantly, the textile-based multimodal sensor was able to detect the strain and pressure independently, allowing facile discrimination of strain and pressure. Using this approach, we demonstrated a textile-based multimodal sensor that incorporates one strain sensor and two pressure sensors detecting multiple weights simultaneously.

Optoelectronic system and device integration for quantum-dot light-emitting diode white lighting with computational design framework.

We propose a computational design framework to design the architecture of a white lighting system having multiple pixelated patterns of electric-field-driven quantum dot light-emitting diodes. The quantum dot of the white lighting system has been optimised by a system-level combinatorial colour optimisation process with the Nelder-Mead algorithm used for machine learning. The layout of quantum dot patterns is designed precisely using rigorous device-level charge transport simulation with an electric-field dependent charge injection model. A theoretical maximum of 97% colour rendering index has been achieved with red, green, cyan, and blue quantum dot light-emitting diodes as primary colours. The white lighting system has been fabricated using the transfer printing technique to validate the computational design framework. It exhibits excellent lighting performance of 92% colour rendering index and wide colour temperature variation from 1612 K to 8903 K with only the four pixelated quantum dots as primary.

In-Plane Amorphous Oxide Ionotronic Devices and Circuits with Photochemically Enabled Favorable Interfaces.

Here, we demonstrate a side-gated in-plane structure of solution-processed amorphous oxide semiconductor ionotronic devices and logic circuits enabled by ion gel gate dielectrics with a monolithically integrated nanoscale passivation architecture. The large capacitance of the electric double layer (EDL) in the ion gel allows a device structure to be a side gate geometry, forming an in-plane structured amorphous In-Ga-Zn-O (a-IGZO) ionotronic transistor, which can be translated into a simplified logic gate configuration with a low operation voltage. Particularly, the monolithic passivation of the coplanar electrodes offers advantages over conventional inhomogeneous passivation, mitigating unintentional parasitic leakage current through the ion gel dielectric layer. More importantly, the monolithically integrated passivation over electrodes was readily obtained with a complementary metal-oxide semiconductor-compatible photochemical process by employing a controlled ultraviolet light manipulation under ozone ambient, which introduced not only much enhanced electrical characteristics but also a scalable device architecture. We investigated various electrical behaviors of the side-gated a-IGZO ionotronic transistor based on EDL, which is called an electric double layer transistor (EDLT), and logic circuits enabled by photochemically realized monolithic aluminum oxide (AlOX) passivation comparing to the native or polymerized passivation layer, which reveals that the photoassisted AlOX secures high-performance a-IGZO EDLTs with a low off current (<5.23 × 10-8 A), high on/off ratio (>1.87 × 105), and exceptional high carrier mobility (>14.5 cm2 V-1 s-1). Owing to the significantly improved electrical characteristics, an inverter circuit was successfully achieved with broad operation voltages from an ultralow VDD of 1 mV to 1.5 V, showing a fully logical voltage transfer characteristic with a gain of more than 4 V V-1.

Self-Aligned Top-Gate Metal-Oxide Thin-Film Transistors Using a Solution-Processed Polymer Gate Dielectric.

For high-speed and large-area active-matrix displays, metal-oxide thin-film transistors (TFTs) with high field-effect mobility, stability, and good uniformity are essential. Moreover, reducing the RC delay is also important to achieve high-speed operation, which is induced by the parasitic capacitance formed between the source/drain (S/D) and the gate electrodes. From this perspective, self-aligned top-gate oxide TFTs can provide advantages such as a low parasitic capacitance for high-speed displays due to minimized overlap between the S/D and the gate electrodes. Here, we demonstrate self-aligned top-gate oxide TFTs using a solution-processed indium-gallium-zinc-oxide (IGZO) channel and crosslinked poly(4-vinylphenol) (PVP) gate dielectric layers. By applying a selective Ar plasma treatment on the IGZO channel, low-resistance IGZO regions could be formed, having a sheet resistance value of ~20.6 kΩ/sq., which can act as the homojunction S/D contacts in the top-gate IGZO TFTs. The fabricated self-aligned top-gate IGZO TFTs exhibited a field-effect mobility of 3.93 cm2/Vs and on/off ratio of ~106, which are comparable to those fabricated using a bottom-gate structure. Furthermore, we also demonstrated self-aligned top-gate TFTs using electrospun indium-gallium-oxide (IGO) nanowires (NWs) as a channel layer. The IGO NW TFTs exhibited a field-effect mobility of 0.03 cm2/Vs and an on/off ratio of >105. The results demonstrate that the Ar plasma treatment for S/D contact formation and the solution-processed PVP gate dielectric can be implemented in realizing self-aligned top-gate oxide TFTs.

Highly Scalable and Robust Mesa-Island-Structure Metal-Oxide Thin-Film Transistors and Integrated Circuits Enabled by Stress-Diffusive Manipulation.

The increasing interest in flexible and wearable electronics has demanded a dramatic improvement of mechanical robustness in electronic devices along with high-resolution implemented architectures. In this study, a site-specific stress-diffusive manipulation is demonstrated to fulfill highly robust and ultraflexible amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) and integrated circuits. The photochemically activated combustion sol-gel a-IGZO TFTs on a mesa-structured polyimide show an average saturation mobility of 6.06 cm2 V-1 s-1 and a threshold voltage of -0.99 V with less than 9% variation, followed by 10 000 bending cycles with a radius of 125 µm. More importantly, the site-specific monolithic formation of mesa pillar-structured devices can provide fully integrated logic circuits such as seven-stage ring-oscillators, meeting the industrially needed device density and scalability. To exploit the underlying stress-diffusive mechanism, a physical model is provided by using a variety of chemical, structural, and electrical characterizations along with multidomain finite-element analysis simulation. The physical models reveal that a highly scalable and robust device can be achieved via the site-specific mesa architecture, by enabling generation of multineutral layers and fine-tuning the accumulated stresses on specific element of devices with their diffusion out into the boundary of the mesa regions.

Highly-Sensitive Textile Pressure Sensors Enabled by Suspended-Type All Carbon Nanotube Fiber Transistor Architecture.

Among various wearable health-monitoring electronics, electronic textiles (e-textiles) have been considered as an appropriate alternative for a convenient self-diagnosis approach. However, for the realization of the wearable e-textiles capable of detecting subtle human physiological signals, the low-sensing performances still remain as a challenge. In this study, a fiber transistor-type ultra-sensitive pressure sensor (FTPS) with a new architecture that is thread-like suspended dry-spun carbon nanotube (CNT) fiber source (S)/drain (D) electrodes is proposed as the first proof of concept for the detection of very low-pressure stimuli. As a result, the pressure sensor shows an ultra-high sensitivity of ~3050 Pa-1 and a response/recovery time of 258/114 ms in the very low-pressure range of <300 Pa as the fiber transistor was operated in the linear region (VDS = -0.1 V). Also, it was observed that the pressure-sensing characteristics are highly dependent on the contact pressure between the top CNT fiber S/D electrodes and the single-walled carbon nanotubes (SWCNTs) channel layer due to the air-gap made by the suspended S/D electrode fibers on the channel layers of fiber transistors. Furthermore, due to their remarkable sensitivity in the low-pressure range, an acoustic wave that has a very tiny pressure could be detected using the FTPS.

Nanocluster-Based Ultralow-Temperature Driven Oxide Gate Dielectrics for High-Performance Organic Electronic Devices.

The development of novel dielectric materials with reliable dielectric properties and low-temperature processibility is crucial to manufacturing flexible and high-performance organic thin-film transistors (OTFTs) for next-generation roll-to-roll organic electronics. Here, we investigate the solution-based fabrication of high-k aluminum oxide (Al2O3) thin films for high-performance OTFTs. Nanocluster-based Al2O3 films fabricated by highly energetic photochemical activation, which allows low-temperature processing, are compared to the conventional nitrate-based Al2O3 films. A wide array of spectroscopic and surface analyses show that ultralow-temperature photochemical activation (<60 °C) induces the decomposition of chemical impurities and causes the densification of the metal-oxide film, resulting in a highly dense high-k Al2O3 dielectric layer from Al-13 nanocluster-based solutions. The fabricated nanocluster-based Al2O3 films exhibit a low leakage current density (<10-7 A/cm2) at 2 MV/cm and high dielectric breakdown strength (>6 MV/cm). Using this dielectric layer, precisely aligned microrod-shaped 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C8-BTBT) single-crystal OTFTs were fabricated via solvent vapor annealing and photochemical patterning of the sacrificial layer.

High-Mobility and Hysteresis-Free Flexible Oxide Thin-Film Transistors and Circuits by Using Bilayer Sol-Gel Gate Dielectrics.

In this paper, we demonstrate high-performance and hysteresis-free solution-processed indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) and high-frequency-operating seven-stage ring oscillators using a low-temperature photochemically activated Al2O3/ZrO2 bilayer gate dielectric. It was found that the IGZO TFTs with single-layer gate dielectrics such as Al2O3, ZrO2, or sodium-doped Al2O3 exhibited large hysteresis, low field-effect mobility, or unstable device operation owing to the interfacial/bulk trap states, insufficient band offset, or a substantial number of mobile ions present in the gate dielectric layer, respectively. To resolve these issues and to explain the underlying physical mechanisms, a series of electrical analyses for various single- and bilayer gate dielectrics was carried out. It is shown that compared to single-layer gate dielectrics, the Al2O3/ZrO2 gate dielectric exhibited a high dielectric constant of 8.53, low leakage current density (∼10-9 A cm-2 at 1 MV cm-1), and stable operation at high frequencies. Using the photochemically activated Al2O3/ZrO2 gate dielectric, the seven-stage ring oscillators operating at an oscillation frequency of ∼334 kHz with a propagation delay of <216 ns per stage were successfully demonstrated on a polymeric substrate.

Corrugated Heterojunction Metal-Oxide Thin-Film Transistors with High Electron Mobility via Vertical Interface Manipulation.

A new strategy is reported to achieve high-mobility, low-off-current, and operationally stable solution-processable metal-oxide thin-film transistors (TFTs) using a corrugated heterojunction channel structure. The corrugated heterojunction channel, having alternating thin-indium-tin-zinc-oxide (ITZO)/indium-gallium-zinc-oxide (IGZO) and thick-ITZO/IGZO film regions, enables the accumulated electron concentration to be tuned in the TFT off- and on-states via charge modulation at the vertical regions of the heterojunction. The ITZO/IGZO TFTs with optimized corrugated structure exhibit a maximum field-effect mobility >50 cm2 V-1 s-1 with an on/off current ratio of >108 and good operational stability (threshold voltage shift <1 V for a positive-gate-bias stress of 10 ks, without passivation). To exploit the underlying conduction mechanism of the corrugated heterojunction TFTs, a physical model is implemented by using a variety of chemical, structural, and electrical characterization tools and Technology Computer-Aided Design simulations. The physical model reveals that efficient charge manipulation is possible via the corrugated structure, by inducing an extremely high carrier concentration at the nanoscale vertical channel regions, enabling low off-currents and high on-currents depending on the applied gate bias.

High-performance and scalable metal-chalcogenide semiconductors and devices via chalco-gel routes.

We report a general strategy for obtaining high-quality, large-area metal-chalcogenide semiconductor films from precursors combining chelated metal salts with chalcoureas or chalcoamides. Using conventional organic solvents, such precursors enable the expeditious formation of chalco-gels, which are easily transformed into the corresponding high-performance metal-chalcogenide thin films with large, uniform areas. Diverse metal chalcogenides and their alloys (MQ x : M = Zn, Cd, In, Sb, Pb; Q = S, Se, Te) are successfully synthesized at relatively low processing temperatures (<400°C). The versatility of this scalable route is demonstrated by the fabrication of large-area thin-film transistors (TFTs), optoelectronic devices, and integrated circuits on a 4-inch Si wafer and 2.5-inch borosilicate glass substrates in ambient air using CdS, CdSe, and In2Se3 active layers. The CdSe TFTs exhibit a maximum field-effect mobility greater than 300 cm2 V-1 s-1 with an on/off current ratio of >107 and good operational stability (threshold voltage shift < 0.5 V at a positive gate bias stress of 10 ks). In addition, metal chalcogenide-based phototransistors with a photodetectivity of >1013 Jones and seven-stage ring oscillators operating at a speed of ~2.6 MHz (propagation delay of < 27 ns per stage) are demonstrated.

Static and Dynamic Water Motion-Induced Instability in Oxide Thin-Film Transistors and Its Suppression by Using Low-k Fluoropolymer Passivation.

Here, we report static and dynamic water motion-induced instability in indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs) and its effective suppression with the use of a simple, solution-processed low-k (ε ∼ 1.9) fluoroplastic resin (FPR) passivation layer. The liquid-contact electrification effect, in which an undesirable drain current modulation is induced by a dynamic motion of a charged liquid such as water, can cause a significant instability in IGZO TFTs. It was found that by adopting a thin (∼44 nm) FPR passivation layer for IGZO TFTs, the current modulation induced by the water-contact electrification was greatly reduced in both off- and on-states of the device. In addition, the FPR-passivated IGZO TFTs exhibited an excellent stability to static water exposure (a threshold voltage shift of +0.8 V upon 3600 s of water soaking), which is attributed to the hydrophobicity of the FPR passivation layer. Here, we discuss the origin of the current instability caused by the liquid-contact electrification as well as various static and dynamic stability tests for IGZO TFTs. On the basis of our findings, we believe that the use of a thin, solution-processed FPR passivation layer is effective in suppressing the static and dynamic water motion-induced instabilities, which may enable the realization of high-performance and environment-stable oxide TFTs for emerging wearable and skin-like electronics.

Brain-Inspired Photonic Neuromorphic Devices using Photodynamic Amorphous Oxide Semiconductors and their Persistent Photoconductivity.

The combination of a neuromorphic architecture and photonic computing may open up a new era for computational systems owing to the possibility of attaining high bandwidths and the low-computation-power requirements. Here, the demonstration of photonic neuromorphic devices based on amorphous oxide semiconductors (AOSs) that mimic major synaptic functions, such as short-term memory/long-term memory, spike-timing-dependent plasticity, and neural facilitation, is reported. The synaptic functions are successfully emulated using the inherent persistent photoconductivity (PPC) characteristic of AOSs. Systematic analysis of the dynamics of photogenerated carriers for various AOSs is carried out to understand the fundamental mechanisms underlying the photoinduced carrier-generation and relaxation behaviors, and to search for a proper channel material for photonic neuromorphic devices. It is found that the activation energy for the neutralization of ionized oxygen vacancies has a significant influence on the photocarrier-generation and time-variant recovery behaviors of AOSs, affecting the PPC behavior.

Frequency-Stable Ionic-Type Hybrid Gate Dielectrics for High Mobility Solution-Processed Metal-Oxide Thin-Film Transistors.

In this paper, we demonstrate high mobility solution-processed metal-oxide thin-film transistors (TFTs) by using a high-frequency-stable ionic-type hybrid gate dielectric (HGD). The HGD gate dielectric, a blend of sol-gel aluminum oxide (AlOx) and poly(4-vinylphenol) (PVP), exhibited high dielectric constant (ε~8.15) and high-frequency-stable characteristics (1 MHz). Using the ionic-type HGD as a gate dielectric layer, an minimal electron-double-layer (EDL) can be formed at the gate dielectric/InOx interface, enhancing the field-effect mobility of the TFTs. Particularly, using the ionic-type HGD gate dielectrics annealed at 350 °C, InOx TFTs having an average field-effect mobility of 16.1 cm²/Vs were achieved (maximum mobility of 24 cm²/Vs). Furthermore, the ionic-type HGD gate dielectrics can be processed at a low temperature of 150 °C, which may enable their applications in low-thermal-budget plastic and elastomeric substrates. In addition, we systematically studied the operational stability of the InOx TFTs using the HGD gate dielectric, and it was observed that the HGD gate dielectric effectively suppressed the negative threshold voltage shift during the negative-illumination-bias stress possibly owing to the recombination of hole carriers injected in the gate dielectric with the negatively charged ionic species in the HGD gate dielectric.