Identification of dipole disorder in low temperature solution processed oxides: its utility and suppression for transparent high performance solution-processed hybrid electronics

Rapid activation of a solution-processed aluminum oxide gate dielectric through intense pulsed light irradiation

In this study, we report rapid activation of a solution-processed aluminum oxide gate dielectric film to reduce its processing time under ambient atmosphere. Aluminum precursor films were exposed to a high energy light-pulse and completely converted into dielectric films within 30 seconds (450 pulses). The aluminum oxide gate dielectric film irradiated using intense pulsed light with 450 pulses exhibits a smooth surface and a leakage current density of less than 10-8 A cm-2 at 2 MV cm-1. Moreover, dielectric constants of the aluminum oxide layer were calculated to be approximately 7. Finally, we fabricated a solution-processed indium gallium zinc oxide thin-film transistor with AlO x using intense pulsed light irradiation, exhibiting a field-effect mobility of 2.99 cm2 V-1 s-1, threshold voltage of 0.73 V, subthreshold swing of 180 mV per decade and I on/I off ratio of 3.9 × 106.

Heterometallic 3d-4f Alkoxide Precursors for the Synthesis of Binary Oxide Nanomaterials

In this study, a new method for the synthesis of heterometallic 3d-4f alkoxides by the direct reaction of metallic lanthanides (La, Pr, Nd, Gd) with MCl₂ (M = Mn, Ni, Co) in 2-methoxyethanol was developed. The method was applied to the synthesis of the heterometallic oxo-alkoxide clusters [Ln₄Mn₂(μ₆-O)(μ₃-OR)₈(HOR)_xCl₆] (Ln = La (1), Nd (2), Gd (3); x = 0, 2, 4); [Pr₄M₂(μ₆-O)(μ₃-OR)₈(HOR)_xCl₆] (M = Co (4), Ni (5); x = 2, 4); and [Ln₄Mn₂(μ₃-OH)₂(μ₃-OR)₄(μ-OR)₄(μ-Cl)₂(HOR)₄Cl₆] (Ln = La (11) and Pr (12)). Mechanistic investigation led to the isolation of the homo- and heterometallic intermediates [Pr(μ-OR)(μ-Cl)(HOR)Cl]_n (6), [Co₄(μ₃-OR)₄(HOR)₄Cl₄] (7), [Ni₄(μ₃-OR)₄(HOEt)₄Cl₄] (8), [Mn₄(μ₃-OR)₄(HOR)₂(HOEt)₂Cl₄] (9), and [Nd(HOR)₄Cl][CoCl₄] (10). In the presence of an external M(II) source at 1100 °C, 1-4 and 12 were selectively converted into binary metal oxide nanomaterials with trigonal or orthorhombic perovskite structures, i.e., LaMnO₃, GdMnO₃, NdMnO₃, Pr_0.9MnO₃, and PrCoO₃. Compound 5 decomposed into a mixture of homo- and heterometallic oxides. The method presented provides a valuable platform for the preparation of advanced heterometallic oxide materials with promising magnetic, luminescence, and/or catalytic applications.

High-performance and low-power source-gated transistors enabled by a solution-processed metal oxide homojunction

Cost-effective fabrication of mechanically flexible low-power electronics is important for emerging applications including wearable electronics, artificial intelligence, and the Internet of Things. Here, solution-processed source-gated transistors (SGTs) with an unprecedented intrinsic gain of ~2,000, low saturation voltage of +0.8 ± 0.1 V, and a ~25.6 μW power consumption are realized using an indium oxide In₂O₃/In₂O₃:polyethylenimine (PEI) blend homojunction with Au contacts on Si/SiO₂. Kelvin probe force microscopy confirms source-controlled operation of the SGT and reveals that PEI doping leads to more effective depletion of the reverse-biased Schottky contact source region. Furthermore, using a fluoride-doped AlO_x gate dielectric, rigid (on a Si substrate) and flexible (on a polyimide substrate) SGTs were fabricated. These devices exhibit a low driving voltage of +2 V and power consumption of ~11.5 μW, yielding inverters with an outstanding voltage gain of >5,000. Furthermore, electrooculographic (EOG) signal monitoring can now be demonstrated using an SGT inverter, where a ~1.0 mV EOG signal is amplified to over 300 mV, indicating significant potential for applications in wearable medical sensing and human-computer interfacing.

Readily Accessible Metallic Micro-Island Arrays for High-Performance Metal Oxide Thin-Film Transistors

Thin-film transistors using metal oxide semiconductors are essential in many unconventional electronic devices. Nevertheless, further advances will be necessary to broaden their technological appeal. Here, a new strategy is reported to achieve high-performance solution-processed metal oxide thin-film transistors (MOTFTs) by introducing a metallic micro-island array (M-MIA) on top of the MO back channel, where the MO is a-IGZO (amorphous indium-gallium-zinc-oxide). Here Al-MIAs are fabricated using honeycomb cinnamate cellulose films, created by a scalable breath-figure method, as a shadow mask. For IGZO TFTs, the electron mobility (µ_e ) increases from ≈3.6 cm² V^-1 s^-1 to near 15.6 cm² V^-1 s^-1 for optimal Al-MIA dimension/coverage of 1.25 µm/51%. The Al-MIA IGZO TFT performance is superior to that of controls using compact/planar Al layers (Al-PL TFTs) and Au-MIAs with the same channel coverage. Kelvin probe force microscopy and technology computer-aided design simulations reveal that charge transfer occurs between the Al and the IGZO channel which is optimized for specific Al-MIA dimensions/surface channel coverages. Furthermore, such Al-MIA IGZO TFTs with a high-k fluoride-doped alumina dielectric exhibit a maximum µ_e of >50.2 cm² V^-1 s^-1 . This is the first demonstration of a micro-structured MO semiconductor heterojunction with submicrometer resolution metallic arrays for enhanced transistor performance and broad applicability to other devices.

Application of Solution Method to Prepare High Performance Multicomponent Oxide Thin Films

Capacitors play an increasingly important role in hybrid integrated circuits, while the MIM capacitors with high capacitance density and small thickness can meet the needs of high integration. Generally speaking, the films prepared with a single metal oxide dielectric often achieve a breakthrough in one aspect of performance, but dielectric layers are required to be improved to get better performance in leakage current, capacitance density, and transmittance simultaneously in modern electronic devices. Therefore, we optimized the performance of the dielectric layers by using multiple metal oxides. We combined zirconia, yttria, magnesium oxide, alumina, and hafnium oxide with the solution method to find the best combination of these five metal oxides. The physical properties of the multi-component films were measured by atomic force microscopy (AFM), ultraviolet-visible spectrophotometer, and other instruments. The results show that the films prepared by multi-component metal oxides have good transmittance and low roughness. The thicknesses of all films in our experiment are less than 100 nm. Then, metal–insulator–metal (MIM) devices were fabricated. In addition, we characterized the electrical properties of MIM devices. We find that multi-component oxide films can achieve good performances in several aspects. The aluminum-magnesium-yttrium-zirconium-oxide (AMYZO_x) group of 0.6 M has the lowest leakage current density, which is 5.03 × 10⁻⁸ A/cm² @ 1.0 MV/cm. The hafnium-magnesium-yttrium-zirconium-oxide (HMYZO_x) group of 0.8 M has a maximum capacitance density of 208 nF/cm². The films with a small thickness and a high capacitance density are very conducive to high integration. Therefore, we believe that multi-component films have potential in the process of dielectric layers and great application prospects in highly integrated electronic devices.

Role of Fluoride Doping in Low-Temperature Combustion-Synthesized ZrOx Dielectric Films

Zirconium oxide (ZrO_x) is an attractive metal oxide dielectric material for low-voltage, optically transparent, and mechanically flexible electronic applications due to the high dielectric constant (κ ∼ 14-30), negligible visible light absorption, and, as a thin film, good mechanical flexibility. In this contribution, we explore the effect of fluoride doping on structure-property-function relationships in low-temperature solution-processed amorphous ZrO_x. Fluoride-doped zirconium oxide (F:ZrO_x) films with a fluoride content between 1.7 and 3.2 in atomic (at) % were synthesized by a combustion synthesis procedure. Irrespective of the fluoride content, grazing incidence X-ray diffraction, atomic-force microscopy, and UV-vis spectroscopy data indicate that all F:ZrO_x films are amorphous, atomically smooth, and transparent in visible light. Impedance spectroscopy measurements reveal that unlike solution-processed fluoride-doped aluminum oxide (F:AlO_x), fluoride doping minimally affects the frequency-dependent capacitance instability of solution-processed F:ZrO_x films. This result can be rationalized by the relatively weak Zr-F versus Zr-O bonds and the large ionic radius of Zr⁺⁴, as corroborated by EXAFS analysis and MD simulations. Nevertheless, the performance of pentacene thin-film transistors (TFTs) with F:ZrO_x gate dielectrics indicates that fluoride incorporation reduces I-V hysteresis in the transfer curves and enhances bias stress stability versus TFTs fabricated with analogous, but undoped ZrO_x films as gate dielectrics, due to reduced trap density.

Ecofriendly Solution-Combustion-Processed Thin-Film Transistors for Synaptic Emulation and Neuromorphic Computing

The ecofriendly combustion synthesis (ECS) and self-combustion synthesis (ESCS) have been successfully utilized to deposit high-k aluminum oxide (AlO_x) dielectrics at low temperatures and applied for aqueous In₂O₃ thin-film transistors (TFTs) accordingly. The ECS and ESCS processes facilitate the formation of high-quality dielectrics at lower temperatures compared to conventional methods based on an ethanol precursor, as confirmed by thermal analysis and chemical composition characterization. The aqueous In₂O₃ TFTs based on ECS and ESCS-AlO_x show enhanced electrical characteristics and counterclockwise transfer-curve hysteresis. The memory-like counterclockwise behavior in the transfer curve modulated by the gate bias voltage is comparable to the signal modulation by the neurotransmitters. ECS and ESCS transistors are employed to perform synaptic emulation; various short-term and long-term memory functions are emulated with low operating voltages and high excitatory postsynaptic current levels. High stability and reproducibility are achieved within 240 pulses of long-term synaptic potentiation and depression. The synaptic emulation functions achieved in this work match the demand for artificial neural networks (ANN), and a multilayer perceptron (MLP) is developed using an ECS-AlO_x synaptic transistor for image recognition. A superior recognition rate of over 90% is achieved based on ECS-AlO_x synaptic transistors, which facilitates the implementation of the metal-oxide synaptic transistor for future neuromorphic computing via an ecofriendly route.

Enhanced Interfacial Integrity of Amorphous Oxide Thin-Film Transistors by Elemental Diffusion of Ternary Oxide Semiconductors

Low-temperature solution-processed oxide semiconductor and dielectric films typically possess a substantial number of defects and impurities due to incomplete metal-oxygen bond formation, causing poor electrical performance and stability. Here, we exploit a facile route to improve the film quality and the interfacial property of low-temperature solution-processed oxide thin films via elemental diffusion between metallic ion-doped InO_x (M:InO_x) ternary oxide semiconductor and AlO_x gate dielectric layers. Particularly, it was revealed that metallic dopants such as magnesium (Mg) and hafnium (Hf) having a small ionic radius, a high Gibbs energy of oxidation, and bonding dissociation energy could successfully diffuse into the low-quality AlO_x gate dielectric layer and effectively reduce the structural defects and residual impurities present in the bulk and at the semiconductor/dielectric interface. Through an extensive investigation on the compositional, structural, and electrical properties of M:InO_x/AlO_x thin-film transistors (TFTs), we provide direct evidences of elemental diffusion occurred between M:InO_x and AlO_x layers as well as its contribution to the electrical performance and operational stability. Using the elemental diffusion process, we demonstrate solution-processed Hf:InO_x TFTs using a low-temperature (180 °C) AlO_x gate dielectric having a field-effect mobility of 2.83 cm² V^-1·s^-1 and improved bias stability. Based on these results, it is concluded that the elemental diffusion between oxide semiconductor and gate dielectric layers can play a crucial role in realizing oxide TFTs with enhanced structural and interfacial integrity.

Frequency-Agile Low-Temperature Solution-Processed Alumina Dielectrics for Inorganic and Organic Electronics Enhanced by Fluoride Doping

The frequency-dependent capacitance of low-temperature solution-processed metal oxide (MO) dielectrics typically yields unreliable and unstable thin-film transistor (TFT) performance metrics, which hinders the development of next-generation roll-to-roll MO electronics and obscures intercomparisons between processing methodologies. Here, capacitance values stable over a wide frequency range are achieved in low-temperature combustion-synthesized aluminum oxide (AlO_x) dielectric films by fluoride doping. For an optimal F incorporation of ∼3.7 atomic % F, the F:AlO_x film capacitance of 166 ± 11 nF/cm² is stable over a 10^-1-10⁴ Hz frequency range, far more stable than that of neat AlO_x films (capacitance = 336 ± 201 nF/cm²) which falls from 781 ± 85 nF/cm² to 104 ± 4 nF/cm² over this frequency range. Importantly, both n-type/inorganic and p-type/organic TFTs exhibit reliable electrical characteristics with minimum hysteresis when employing the F:AlO_x dielectric with ∼3.7 atomic % F. Systematic characterization of film microstructural/compositional and electronic/dielectric properties by X-ray photoelectron spectroscopy, time-of-fight secondary ion mass spectrometry, cross-section transmission electron microscopy, solid-state nuclear magnetic resonance, and UV-vis absorption spectroscopy reveal that fluoride doping generates AlOF, which strongly reduces the mobile hydrogen content, suppressing polarization mechanisms at low frequencies. Thus, this work provides a broadly applicable anion doping strategy for the realization of high-performance solution-processed metal oxide dielectrics for both organic and inorganic electronics applications.

Proton Conducting Perhydropolysilazane-Derived Gate Dielectric for Solution-Processed Metal Oxide-Based Thin-Film Transistors

Perhydropolysilazane (PHPS), an inorganic polymer composed of Si-N and Si-H, has attracted much attention as a precursor for gate dielectrics of thin-film transistors (TFTs) due to its facile processing even at a relatively low temperature. However, an in-depth understanding of the tunable dielectric behavior of PHPS-derived dielectrics and their effects on TFT device performance is still lacking. In this study, the PHPS-derived dielectric films formed at different annealing temperatures have been used as the gate dielectric layer for solution-processed indium zinc oxide (IZO) TFTs. Notably, the IZO TFTs fabricated on PHPS annealed at 350 °C exhibit mobility as high as 118 cm² V^-1 s^-1, which is about 50 times the IZO TFTs made on typical SiO₂ dielectrics. The outstanding electrical performance is possible because of the exceptional capacitance of PHPS-derived dielectric caused by the limited hydrolysis reaction of PHPS at a low processing temperature (<400 °C). According to our analysis, the exceptional dielectric behavior is originated from the electric double layer formed by mobile of protons in the low temperature-annealed PHPS dielectrics. Furthermore, proton conduction through the PHPS dielectric occurs through a three-dimensional pathway by a hopping mechanism, which allows uniform polarization of the dielectric even at room temperature, leading to amplified performance of the IZO TFTs.

Suppression of Interfacial Disorders in Solution-Processed Metal Oxide Thin-Film Transistors by Mg Doping

The fabrication of high-performance metal oxide thin-film transistors (TFTs) using a low-temperature solution process may facilitate the realization of ultraflexible and wearable electronic devices. However, the development of highly stable oxide gate dielectrics at a low temperature has been a challenging issue since a considerable amount of residual impurities and defective bonding states is present in low-temperature-processed gate dielectrics causing a large counterclockwise hysteresis and a significant instability. Here, we report a new approach to effectively remove the residual impurities and suppress the relevant dipole disorder in a low-temperature-processed (180 °C) AlO_x gate dielectric layer by magnesium (Mg) doping. Mg is well known as a promising material for suppression of oxygen vacancy defects and improvement of operational stability due to a high oxygen vacancy formation energy (E_vo = 9.8 eV) and a low standard reduction potential (E⁰ = -2.38 V). Therefore, with an adequate control of Mg concentration in metal oxide (MO) films, oxygen-related defects could be easily suppressed without additional treatments and then stable metal-oxygen-metal (M-O-M) network formation could be achieved, causing excellent operational stability. By optimal Mg doping (10%) in the InO_x channel layer, Mg:InO_x TFTs exhibited negligible clockwise hysteresis and a field-effect mobility of >4 cm² V^-1 s^-1. Furthermore, the electric characteristics of the low-temperature-processed AlO_x gate dielectric with high impurities were improved by Mg diffusion originating in Mg doping, resulting in stable threshold voltage shift in the bias stability test.

Recent Advances of Solution-Processed Metal Oxide Thin-Film Transistors

Solution-processed metal oxide thin-film transistors (TFTs) are considered as one of the most promising transistor technologies for future large-area flexible electronics. This work surveys the recent advances in solution-processed metal oxide TFTs, including n-type oxide semiconductors, oxide dielectrics, and p-type oxide semiconductors. We first deliver a review on the history and present status of metal oxide TFTs. Then, we present the recent progress in solution-processed n-type oxide semiconductors, with a special focus on low-temperature and large-area solution-based approaches as well as emerging nondisplay applications. Next, we give a detailed analysis of the state-of-the-art solution-processed oxide dielectrics for low-power electronics. We further discuss the recent advances in solution-based p-type oxide semiconductors, which will enable the highly desirable future low-cost large-area complementary circuits. Finally, we draw conclusions and outline the perspectives over the research field.

Printed Electronics Based on Inorganic Semiconductors: From Processes and Materials to Devices

Following the ever-expanding technological demands, printed electronics has shown palpable potential to create new and commercially viable technologies that will benefit from its unique characteristics, such as, large-area and wide range of substrate compatibility, conformability and low-cost. Through the last few decades, printed/solution-processed field-effect transistors (FETs) and circuits have witnessed immense research efforts, technological growth and increased commercial interests. Although printing of functional inks comprising organic semiconductors has already been initiated in early 1990s, gradually the attention, at least partially, has been shifted to various forms of inorganic semiconductors, starting from metal chalcogenides, oxides, carbon nanotubes and very recently to graphene and other 2D semiconductors. In this review, the entire domain of printable inorganic semiconductors is considered. In fact, thanks to the continuous development of materials/functional inks and novel design/printing strategies, the inorganic printed semiconductor-based circuits today have reached an operation frequency up to several hundreds of kilohertz with only a few nanosecond time delays at the individual FET/inverter levels; in this regard, often circuits based on hybrid material systems have been found to be advantageous. At the end, a comparison of relative successes of various printable inorganic semiconductor materials, the remaining challenges and the available future opportunities are summarized.

Solution Processed Metal Oxide High-κ Dielectrics for Emerging Transistors and Circuits

The electronic functionalities of metal oxides comprise conductors, semiconductors, and insulators. Metal oxides have attracted great interest for construction of large-area electronics, particularly thin-film transistors (TFTs), for their high optical transparency, excellent chemical and thermal stability, and mechanical tolerance. High-permittivity (κ) oxide dielectrics are a key component for achieving low-voltage and high-performance TFTs. With the expanding integration of complementary metal oxide semiconductor transistors, the replacement of SiO2 with high-κ oxide dielectrics has become urgently required, because their provided thicker layers suppress quantum mechanical tunneling. Toward low-cost devices, tremendous efforts have been devoted to vacuum-free, solution processable fabrication, such as spin coating, spray pyrolysis, and printing techniques. This review focuses on recent progress in solution processed high-κ oxide dielectrics and their applications to emerging TFTs. First, the history, basics, theories, and leakage current mechanisms of high-κ oxide dielectrics are presented, and the underlying mechanism for mobility enhancement over conventional SiO2 is outlined. Recent achievements of solution-processed high-κ oxide materials and their applications in TFTs are summarized and traditional coating methods and emerging printing techniques are introduced. Finally, low temperature approaches, e.g., ecofriendly water-induced, self-combustion reaction, and energy-assisted post treatments, for the realization of flexible electronics and circuits are discussed.

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.

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 (AlO_x) 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/InO_x interface, enhancing the field-effect mobility of the TFTs. Particularly, using the ionic-type HGD gate dielectrics annealed at 350 °C, InO_x 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 InO_x 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.

Far-UV Annealed Inkjet-Printed In2O3 Semiconductor Layers for Thin-Film Transistors on a Flexible Polyethylene Naphthalate Substrate

The inkjet-printing process of precursor solutions containing In nitrate dissolved in 2-methoxyethanol is optimized using ethylene glycol as a cosolvent that allows the stabilization of the droplet formation, leading to a robust, repeatable printing process. The inkjet-printed precursor films are then converted to In₂O₃ semiconductors at flexible-substrate-compatible low temperatures (150-200 °C) using combined far-ultraviolet (FUV) exposure at ∼160 nm and thermal treatment. The compositional nature of the precursor-to-metal oxide conversion is studied using grazing incidence X-ray diffraction (GIXRD), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy that indicate that amorphous, high density (up to 5.87 g/cm³), and low impurity In₂O₃ films can be obtained using the combined annealing technique. Prolonged annealing (180 min) at 150 °C yields enhancement-mode TFTs with saturation mobility of 4.3 cm²/(Vs) and ∼1 cm²/(Vs) on rigid Si/SiO₂ and flexible plastic PEN substrates, respectively. This paves the way for manufacturing relatively high-performance, printed metal-oxide TFT arrays on cheap, flexible substrate for commercial applications.