Ultra-dry air plasma treatment for enhancing the dielectric properties of Al2O3-GPTMS-PMMA hybrid dielectric gate layers in a-IGZO TFT applications

We assessed the effects of ultra dry-air plasma surface treatments on the properties of Al₂O₃-GPTMS-PMMA hybrid dielectric layers for applications to high-performance amorphous Indium Gallium Zinc Oxide (a-IGZO) thin film transistors (TFTs). The hybrid layers were deposited by an easy dip coating sol-gel process at low temperature and then treated with dry-air plasma at 1, 2 and 3 consecutive cycles. Their properties were analyzed as a function of the number of plasma cycles and contrasted with those of the untreated ones. The dielectric characteristics of the hybrid layers were determined from I-V and C-f measurements performed on metal-insulator-metal and metal-insulator-semiconductor devices. The results show that the plasma treatments increase the surface energy and wettability of the hybrid films. There is also a reduction of the OH groups and oxygen vacancies in the hybrid network improving the dielectric properties. The incorporation of nitrogen into the hybrid films surface is also observed. The plasma-treated hybrid dielectric layers were applied as dielectric gate in the fabrication of a-IGZO TFTs. The best electrical performance of the fabricated TFTs was achieved with the 3 cycles plasma-treated hybrid dielectric gate, showing high mobility, 29.3 cm² V^-1 s^-1, low threshold voltage, 2.9 V, high I _ON/OFF current ratio, 10⁶, and low subthreshold swing of 0.42 V dec^-1.

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.

Effect of Zirconium Doping on Electrical Properties of Aluminum Oxide Dielectric Layer by Spin Coating Method with Low Temperature Preparation

In recent years, significant efforts have been devoted to the research and development of spin-coated Al2O3 thin films, due to their large band gaps, high breakdown voltage and stability at high annealing temperature. However, as the alumina precursor has a large surface energy, substrates need to be treated by plasma before spin coating. Therefore, to avoid the expensive and process-complicated plasma treatment, we incorporated zirconium nitrate into the aluminum nitrate solution to decrease the surface energy of the precursor which improve the spreadability. Then, the electrical performances and the surface morphologies of the films were measured. For comparison, the pure Al2O3 films with plasma treatments were also prepared. As a result, after low temperature annealing (200 °C), the relative dielectric constant of Zr–AlOx spin-coated thin-film MIM (Metal-Insulator-Metal) devices can reach 12 and the leakage current density is not higher than 7.78 × 10−8 A/cm2 @ 1 MV/cm when the concentration of zirconium nitrate is 0.05 mol/L. The Aluminum oxide film prepared by zirconium doping has higher stability and better electrical properties than the pure films with plasma treatments and high performance can be attained under low-temperature annealing, which shows its potential application in printing and flexible electronic devices.

Direct Electrohydrodynamic Patterning of High-Performance All Metal Oxide Thin-Film Electronics

In this paper, we propose a scalable approach toward all-printed high-performance metal oxide thin-film transistors (TFTs), using a high-resolution electrohydrodynamic (EHD) printing process. Direct EHD micropatterning of metal oxide TFTs is based on diverse precursor solutions to form semiconducting materials (In₂O₃, In-Ga-ZnO (IGZO)), conductive metal oxide (Sn-doped In₂O₃ (ITO)), as well as aluminum oxide (Al₂O₃) gate dielectric at low temperatures. The fully printed TFT devices exhibit excellent electron transport characteristics (average electron mobilities of up to 117 cm² V^-1 s^-1), negligible hysteresis, excellent uniformity, and stable operation at low-operating voltage. Furthermore, integrated logic gates such as NOT and NAND have been printed and demonstrated. All-printed logic with individual gating and symmetric input/output behavior, which is crucial for large-scale integration, is also demonstrated. The devices and fabrication process described in this paper enable high-performance and high-reliability transparent electronics.

Stepped Annealed Inkjet-Printed InGaZnO Thin-Film Transistors

The preparation of thin-film transistors (TFTs) using ink-jet printing technology can reduce the complexity and material wastage of traditional TFT fabrication technologies. We prepared channel inks suitable for printing with different molar ratios of their constituent elements. Through the spin-coated and etching method, two different types of TFTs designated as depletion and enhancement mode were obtained simply by controlling the molar ratios of the InGaZnO channel elements. To overcome the problem of patterned films being prone to fracture during high-temperature annealing, a stepped annealing method is proposed to remove organic molecules from the channel layer and to improve the properties of the patterned films. The different interfaces between the insulation layers, channel layers, and drain/source electrodes were processed by argon plasma. This was done to improve the printing accuracy of the patterned InGaZnO channel layers, drain, and source electrodes, as well as to optimize the printing thickness of channel layers, reduce the defect density, and, ultimately, enhance the electrical performance of printed TFT devices.

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.

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.

Boosting Electrical Performance of High-κ Nanomultilayer Dielectrics and Electronic Devices by Combining Solution Combustion Synthesis and UV Irradiation

In the past decade, solution-based dielectric oxides have been widely studied in electronic applications enabling the use of low-cost processing technologies and device improvement. The most promising are the high-κ dielectrics, like aluminum (AlO_x) and hafnium oxide (HfO_x), that allow an easier trap filling in the semiconductor and the use of low operation voltage. However, in the case of HfO_x, a high temperature usually is needed to induce a uniform and condensed film, which limits its applications in flexible electronics. This paper describes how to obtain HfO_x dielectric thin films and the effect of their implementation in multilayer dielectrics (MLD) at low temperatures (150 °C) to apply in thin film transistors (TFTs) using the combination of solution combustion synthesis (SCS) and ultraviolet (UV) treatment. The single layers and multilayers did not show any trace of residual organics and exhibited a small surface roughness (<1.2 nm) and a high breakdown voltage (>2.7 MV·cm^-1). The resulting TFTs presented a high performance at a low operation voltage (<3 V), with high saturation mobility (43.9 ± 1.1 cm²·V^-1·s^-1), a small subthreshold slope (0.066 ± 0.010 V·dec^-1), current ratio of 1 × 10⁶ and a good idle shelf life stability after 2 months. To our knowledge, the results achieved surpass the actual state-of-the-art. Finally, we demonstrated a low-voltage diode-connected inverter using MLD/IGZO TFTs working with a maximum gain of 1 at 2 V.

Tunable high-κ ZrxAl1−xOy thin film dielectrics from all-inorganic aqueous precursor solutions

An all-inorganic, aqueous solution route enables facile control of composition and optimization of zirconium aluminum oxide thin film dielectric properties.

Low-Temperature Solution-Processed Gate Dielectrics for High-Performance Organic Thin Film Transistors

A low-temperature solution-processed high-k gate dielectric layer for use in a high-performance solution-processed semiconducting polymer organic thin-film transistor (OTFT) was demonstrated. Photochemical activation of sol-gel-derived AlO_x films under 150 °C permitted the formation of a dense film with low leakage and relatively high dielectric-permittivity characteristics, which are almost comparable to the results yielded by the conventionally used vacuum deposition and high temperature annealing method. Octadecylphosphonic acid (ODPA) self-assembled monolayer (SAM) treatment of the AlO_x was employed in order to realize high-performance (>0.4 cm²/Vs saturation mobility) and low-operation-voltage (<5 V) diketopyrrolopyrrole (DPP)-based OTFTs on an ultra-thin polyimide film (3-μm thick). Thus, low-temperature photochemically-annealed solution-processed AlO_x film with SAM layer is an attractive candidate as a dielectric-layer for use in high-performance organic TFTs operated at low voltages.

Low-temperature solution processing of palladium/palladium oxide films and their pH sensing performance

Highly sensitive, easy-to-fabricate, and low-cost pH sensors with small dimensions are required to monitor human bodily fluids, drinking water quality and chemical/biological processes. In this study, a low-temperature, solution-based process is developed to prepare palladium/palladium oxide (Pd/PdO) thin films for pH sensing. A precursor solution for Pd is spin coated onto pre-cleaned glass substrates and annealed at low temperature to generate Pd and PdO. The percentages of PdO at the surface and in the bulk of the electrodes are correlated to their sensing performance, which was studied by using the X-ray photoelectron spectroscope. Large amounts of PdO introduced by prolonged annealing improve the electrode's sensitivity and long-term stability. Atomic force microscopy study showed that the low-temperature annealing results in a smooth electrode surface, which contributes to a fast response. Nano-voids at the electrode surfaces were observed by scanning electron microscope, indicating a reason for the long-term degradation of the pH sensitivity. Using the optimized annealing parameters of 200°C for 48 h, a linear pH response with sensitivity of 64.71±0.56 mV/pH is obtained for pH between 2 and 12. These electrodes show a response time shorter than 18 s, hysteresis less than 8 mV and stability over 60 days. High reproducibility in the sensing performance is achieved. This low-temperature solution-processed sensing electrode shows the potential for the development of pH sensing systems on flexible substrates over a large area at low cost without using vacuum equipment.

Facile and environmentally friendly solution-processed aluminum oxide dielectric for low-temperature, high-performance oxide thin-film transistors

We developed a facile and environmentally friendly solution-processed method for aluminum oxide (AlOx) dielectrics. The formation and properties of AlOx thin films under various annealing temperatures were intensively investigated by thermogravimetric analysis-differential scanning calorimetry (TGA-DSC), X-ray diffraction (XRD), spectroscopic ellipsometry, atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), impedance spectroscopy, and leakage current measurements. The sol-gel-derived AlOx thin film undergoes the decomposition of organic residuals and nitrate groups, as well as conversion of aluminum hydroxides to form aluminum oxide, as the annealing temperature increases. Finally, the AlOx film is used as gate dielectric for a variety of low-temperature solution-processed oxide TFTs. Above all, the In2O3 and InZnO TFTs exhibited high average mobilities of 57.2 cm(2) V(-1) s(-1) and 10.1 cm(2) V(-1) s(-1), as well as an on/off current ratio of ∼10(5) and low operating voltages of 4 V at a maximum processing temperature of 300 °C. Therefore, the solution-processable AlOx could be a promising candidate dielectric for low-cost, low-temperature, and high-performance oxide electronics.

Highly stable and imperceptible electronics utilizing photoactivated heterogeneous sol-gel metal-oxide dielectrics and semiconductors

Incorporation of Zr into an AlOx matrix generates an intrinsically activated ZAO surface enabling the formation of a stable semiconducting IGZO film and good interfacial properties. Photochemically annealed metal-oxide devices and circuits with the optimized sol-gel ZAO dielectric and IGZO semiconductor layers demonstrate the high performance and electrically/mechanically stable operation of flexible electronics fabricated via a low-temperature solution process.

Solution-processed indium-zinc-oxide thin-film transistors based on anodized aluminum oxide gate insulator modified with zirconium oxide

Enhanced performance of solution-processed IZO-TFTs with ZrO_x interlayer due to Al diffusion suppression.