High-performance metal oxide TFTs for flexible displays: materials, fabrication, architecture, and applications
0 Abstract
Flexible display technology is actively explored as a cornerstone of the next generation of wearables and soft electronics, set to revolutionize devices with its potential for lightweight, thin, and mechanically flexible features. Flexible thin-film transistors (TFTs) utilizing promising materials such as amorphous silicon (a-Si), low-temperature polysilicon (LTPS), metal oxides (MOs), and organic semiconductors are essential to enable flexible platforms. Among these, MO semiconductors stand out for flexible displays due to their high carrier mobility, low processing temperature requirements, excellent electrical uniformity, transparency to visible light, and cost-effectiveness. Furthermore, the maturity of MO TFT technology in the existing display industry and its compatibility with complementary-metal-oxide-semiconductor (CMOS) processes are driving active research toward integrated circuits for wearable electronics beyond display applications. Specifically, achieving both high mechanical flexibility and electrical performance in MO TFTs is crucial for implementing complex integrated circuits such as microprocessors and backplanes for ultra-high resolution augmented reality (AR)/virtual reality (VR) displays. Therefore, this review provides recent advances in high-mobility flexible MO TFTs, focusing on materials, fabrication processes, and device architecture engineering methods for implementing MO TFTs on flexible substrates, as well as strategies to reduce the impact of mechanical stress on MO TFTs. Next, MO TFT-based display and integrated circuit applications for next-generation flexible and stretchable electronics are introduced and discussed. Finally, the review concludes with an outlook on the potential achievements and prospects of MO TFTs in the development of next-generation flexible display technologies.
Keywords
INTRODUCTION
Thin-film transistors (TFTs) are three-terminal semiconductor devices with a thin-film structure that act as switches in electronic circuits, controlling the flow of current and facilitating the operation of various electronic components. TFTs are currently the dominant technology for in-pixel switches and drivers in flat-panel displays, performing a critical function in active matrix (AM) displays, such as liquid crystal displays (LCDs) and active-matrix organic light-emitting diodes (AMOLEDs)[1-4]. Advances in TFT technology have aimed to reduce the carbon footprint while overcoming the limitations of conventional silicon-based transistors, such as limited form factor, difficulty in large-area processing, and high manufacturing costs[5-8]. As a result, TFTs have attracted significant interest for use in foldable, flexible and stretchable displays and other electronic products, potentially replacing traditional silicon complementary-metal-oxide-semiconductor (CMOS) circuits. TFTs are developed using various materials, including amorphous silicon (a-Si), polycrystalline silicon (poly-Si), metal oxides (MOs), and organic semiconductors[1,5,9,10]. Among these, MO semiconductors are particularly promising due to their advantages such as low cost, low process complexity and temperature, large area scalability, and higher carrier mobility compared to a-Si and organic materials[11-14]. In addition, MO TFTs offer greater resistance to mechanical stress than low-temperature polysilicon (LTPS) devices, making them ideal for next-generation flexible and stretchable electronics[12,15-17]. These advantages of MO TFTs have facilitated their successful commercialization in the display market, particularly through indium gallium zinc oxide (IGZO) TFT-based backplanes, which have driven significant growth and innovation in high-resolution organic light-emitting diode (OLED) panels since 2013[10,18,19]. With their transparency, flexibility, and high electron mobility, these MO TFTs have proven highly suitable for next-generation electronic devices, playing a crucial role in advancing flexible and transparent displays.
Flexible displays offer characteristics such as thinness, lightweight design, and durability, which allow them to be fabricated on curvilinear surfaces, enabling shape transformation. The history of flexible displays dates back to early innovations such as Polymer Vision’s Readius in 2006, which used rollable e-paper, and Nokia’s Morph concept in 2008, which envisioned a bendable and interactive mobile display. The development of thin, flexible OLED displays further expanded these possibilities. During the 2013 Consumer Electronics Show, Samsung introduced its “Youm” concept, which showcased flexible display prototypes, including a smartphone that folded outward into a tablet-sized display. The first commercial application of this technology appeared in the Galaxy Note Edge, with its curved OLED screen. In 2018, Royole released the first foldable smartphone, the Flexpai, followed by Samsung’s Galaxy Fold in 2019, featuring an inward-folding Infinity Flex display. That same year, Huawei and Xiaomi also introduced their folding smartphones, signaling the growing maturity of flexible display technology and its impact on the smartphone market[20]. This wave of foldable devices underscored the maturation of flexible display technology and its potential for transforming the smartphone market. Since then, flexible displays, particularly flexible AMOLEDs, have achieved commercial success, driven by the popularization of high-resolution TVs, smartphones, and smart devices. Market research projected that shipments of flexible AMOLED displays would reach 335.7 million units by 2020, accounting for 52.0% of total AMOLED panel shipments, underscoring the growing market demand and technological advancements in this area[21].
Currently, along with the advancement of next-generation soft electronics, the development of MO TFT technology is accelerating to meet demands beyond flexible and transparent displays. This includes achieving higher resolution, lower power consumption, and accommodating new functionalities and form factors, marking a significant shift toward innovative applications such as foldable and wearable devices, automotive displays, and smart sensors[8,10,12,22,23]. MO TFTs play a key role not only in the advancement of displays, but also in the expansion into various flexible and stretchable electronic components for soft electronics. This includes applications such as efficient memory systems, flexible and stretchable circuits, sensors and advanced processors[24-34]. MO TFTs also highlight the transformative potential of next-generation Internet of Things (IoT) devices and advanced technologies such as artificial intelligence (AI) and neuromorphic computing[35-42]. In the rapidly evolving landscape of augmented reality (AR) and virtual reality (VR), the demand for high-performance display technologies continues to grow. MO TFTs have emerged as key components due to their exceptional transparency and low power consumption, making them indispensable for the realization of high-resolution backplane applications in AR/VR devices[43,44]. These TFTs not only enhance visual clarity, but also enable immersive user experiences by driving advances in display quality and efficiency. Furthermore, MO TFTs are increasingly valued for their unique capabilities in back-end-of-line (BEOL) processing and monolithic 3D (M3D) integration[45-48]. Their low processing temperatures, superior stability and low leakage characteristics allow seamless integration with existing silicon CMOS chips in BEOL processes, enabling higher transistor densities and more compact layouts. This compatibility not only improves the performance and efficiency of integrated circuits, but also opens new avenues for the development of advanced M3D architectures that promise significant advances in semiconductor technology.
Despite their critical role in advancing soft electronics, flexible MO TFTs face significant challenges due to inherent characteristics such as brittleness and limited electrical performance at low process temperatures. Balancing high electrical performance, reliability, and flexibility remains a complex task, particularly ensuring that these devices withstand mechanical stress and maintain functionality over extended bending cycles and varying bending radii. Furthermore, research is advancing beyond flexibility towards the development of stretchable MO TFTs, seen as key enablers for future applications such as wearable health monitors, electronic skin (e-skin), neuromorphic devices, and advanced human-machine interfaces[49-51]. This development focuses on novel materials and device architectures that maintain performance under large strains, often employing stretchable substrates combined with MO or organic semiconductors for electronic and mechanical robustness. Additionally, stretchable TFTs incorporate innovative geometries, such as serpentine or mesh structures, allowing deformation without sacrificing electrical properties, thus enhancing their potential in soft, stretchable electronics[51-54]. As a result, recent studies have focused extensively on improving the performance, processes, and structural design of flexible and stretchable MO TFT devices to increase mechanical stress resistance while minimizing electrical performance degradation and variability.
In this review, we investigate the substrate, materials, fabrication processes, device structures, and applications of high-performance flexible MO TFTs as shown in Figure 1. Firstly, in the substrate section, we classify the types of substrates for employing high-performance flexible MO TFTs into polymer, paper, and metal foil, and then describe the characteristics and types of each. Secondly, in the materials section, we focus on MO semiconductors and high-k dielectrics for achieving high-performance flexible TFTs. In the MO semiconductor subsection, we discuss the high mobility of indium tin zinc oxide (ITZO), ZnO:N, LiZnO, etc., and multiple stack layers in detail. In the high-k dielectric subsection, we describe the impact and effects of polymer dielectrics and high-k dielectrics on the performance of flexible TFTs. Thirdly, in the fabrication process section, we examine the doping processes that significantly enhance the electrical performance of flexible MO TFTs. Doping with hydrogen, fluorine, nitrogen, and metal cations affects the electrical performance of flexible MO TFTs. These doping techniques not only improve electrical performance but are also compatible with the low-temperature fabrication required for flexible substrates. Consequently, these doping processes are beneficial in improving the electrical properties of MO TFTs for next-generation flexible electronic devices[55-60]. Fourthly, in the device structures section, we explore various innovative structures such as island structures, junctionless structures, and advanced electrode and channel layer architectures that enhance the flexibility of MO TFTs. These structural innovations play a crucial role in the development of flexible electronics that maintain high performance and mechanical stability in a wide range of wearable applications by ensuring consistent electrical performance despite significant mechanical deformation[33,61-67]. Fifthly, in the application section, we introduce applications of high-performance flexible TFTs, including displays and circuits that utilize high-performance flexible TFTs. Finally, we summarize the significant advances, remaining challenges, and potential future developments in flexible MO TFT technology.
Figure 1. Schematic diagram illustrating approaches for high-performance MO TFTs in flexible display applications. MO: Metal oxide; TFTs: thin-film transistors.
VARIOUS SUBSTRATE OF FLEXIBLE MO TFT
To create flexible devices, it is essential to prepare bendable substrate materials, and oxide TFTs, which can be manufactured at low temperatures, are well-suited for use with flexible substrates. Many studies have explored flexible oxide-based TFTs, classified by the type of substrate material, such as polymer plastics, paper, and metal foils. In the development of flexible devices, the use of bendable substrate materials is a key requirement, as these substrates must maintain their structural integrity under mechanical deformation such as bending or stretching. Oxide TFTs processed at relatively low temperatures are particularly advantageous for integration with such flexible substrates. This compatibility with a low-temperature process allows oxide-based TFTs to be effectively utilized in various flexible electronic applications. Much research has been conducted on flexible oxide TFTs with the primary focus on the types of substrate materials that enable these devices to achieve high flexibility and performance. These studies often classify the substrates into three main groups: polymer plastics, paper sheets, and metal foils.
Polymeric materials
Polymeric materials offer numerous advantages, such as being flexible, stretchable, bendable, lightweight and highly transparent, making them appropriate materials for use in flexible substrates in various electronic applications[14,68,69]. These properties enable polymeric materials to support the development of advanced flexible electronics, including displays, sensors, and wearable devices. Amorphous oxide TFT technology, which can be processed at relatively low temperatures, is highly compatible with polymeric materials, allowing for scalable and cost-effective manufacture[12]. Among these materials, polyimide (PI) stands out as the most commonly used substrate for flexible oxide TFTs due to its excellent thermal stability, mechanical flexibility, and processability[70-73]. Other frequently used polymeric materials include polyethylene terephthalate (PET)[74-76], polyethylene naphthalate (PEN)[77-79], poly(vinyl alcohol) (PVA)[80], polydimethylsiloxane (PDMS)[52,81], polycarbonate (PC)[82], and polyether sulfone (PES)[83,84], each offering unique mechanical and chemical properties suitable for different applications. In addition, glass-fabric reinforced composite films are used as flexible plastic substrates, providing additional mechanical strength and stability while maintaining the flexibility needed for next-generation flexible electronics[85]. These materials combined with advancements in oxide-based TFT technology are driving innovation in flexible, lightweight, and durable electronics that can conform to various shapes and surfaces.
Paper sheets
Paper is an attractive alternative substrate for flexible electronics due to its low cost, renewable nature, and biodegradability. These substrates offer an eco-friendly solution compared to traditional plastics. Employing paper sheets in flexible oxide TFTs has potential in applications such as disposable electronics and smart packaging[86,87]. However, the rough surface and moisture absorption tendency of paper sheets create challenges to reliable fabrication and performance limiting their direct application in high-performance devices. To address these issues, surface treatments such as thin layers of silicon dioxide (SiO2) deposited via plasma-enhanced chemical vapor deposition (PECVD)[88] or epoxy acrylate copolymer coatings are applied[89]. These coatings smooth surface of paper and act as moisture barriers improving suitability for flexible electronics. This allows paper-based devices to combine environmental sustainability with enhanced performance and durability.
Metal foils
Metal foils, including those made from gold, silver, copper, Hastelloy or aluminum, offer significant advantages in flexible electronics due to their excellent mechanical strength, ductility, and high thermal resistance. Unlike polymer plastics, metal foils can withstand a higher temperature process allowing for a broader range of fabrication techniques[90]. Their ability to bend with very small curvature radii makes them ideal for high-strain applications such as sensors integrated into advanced systems such as artificial skin[91]. One example is the work by Tang et al. who developed flexible MO TFT using GaOx TFTs on 50-μm-thick metal foils[92]. These devices demonstrated impressive performance, uniformity, and stability. This study exhibits the potential of metal foil substrates in next-generation flexible device technologies, particularly in applications requiring flexibility and durability.
MATERIALS OF HIGH-PERFORMANCE FLEXIBLE MO TFT
High-performance flexible MO TFTs necessitate the use of sophisticated materials to simultaneously achieve superior electrical performance and exceptional mechanical stability. The active semiconductor layer is a charge transport layer, where charge carriers are generated and modulated. MOs such as indium oxide (In2O3), IGZO, and zinc oxide (ZnO) are widely used due to their high electron mobility, optical transparency, and environmental stability[11,93,94]. These materials can be doped with various elements to their electrical properties, improve carrier concentration, and enhance electrical performance. For example, doping with tin (Sn) in In2O3 [forming indium-tin-oxide (ITO)] increases carrier concentration, while doping with gallium (Ga) and zinc (Zn) in IGZO optimizes semiconductor properties and improves stability[55,95-97]. Multiple layers improved the performance of flexible MO TFTs. They are formed by combining different semiconductor materials to create interfaces that enhance efficient charge carrier transport. Selecting materials with complementary properties and multiple layers can significantly improve carrier mobility and reduce off-current, thereby enhancing electrical device performance. This approach minimizes energy loss and ensures that electronic devices operate with high efficiency and reliability over time[61,95,98-100]. The flexibility of the semiconductor layer is essential for maintaining device performance under mechanical stress such as bending and stretching. In flexible electronic devices, the semiconductor layer must withstand significant physical stress without degrading its electrical properties. MO semiconductors are inherently brittle due to their crystalline or amorphous structures. This brittleness makes them susceptible to cracking and fracture under mechanical deformation such as bending or stretching, which can severely degrade their electrical performance. Under repeated mechanical stress, MO thin films are susceptible to cracking and delamination from the flexible substrate and other layer such as source-drain electrodes. This led to interruptions of the conductive path, leading to increased resistance, device failure, and loss of functionality. Additionally, mechanical deformation induces strain-induced defects and dislocations in the MO layer. These defects act as trap sites for charge carriers, reducing mobility and causing instability in the electrical properties of the devices[33,61,101,102].
Dielectric materials are crucial for isolating different components within the TFT and enabling proper gate modulation. High-k dielectric materials, such as aluminum oxide (Al2O3), zirconium oxide (ZrO2), and hafnium oxide (HfO2), are preferred due to their high dielectric constants, which enhance capacitance and reduce operating voltage. These materials must also be deposited with low-temperature processing to be suitable for flexible substrates. Polymers and copolymers are also used for their flexibility and high dielectric constraints[103-106].
Each of these components must be carefully selected and engineered to ensure that the flexible TFTs can achieve high electrical performance while enduring the mechanical demands of bending, stretching, and other forms of deformation. The use of these advanced materials is important in developing next-generation flexible electronic devices that are not only efficient and reliable but also versatile and robust in various applications.
MO semiconductor
Semiconductor layer for high-performance
For high-performance flexible device applications such as wearable devices and AR/VR devices, the semiconductor layer must exhibit both high electron mobility and exceptional stability. Achieving these properties is critical for the development of advanced flexible and wearable electronics, which require materials that can maintain superior electrical performance under mechanical stress. In2O3 is extensively researched for its applications in both conductive and semiconductive roles, owing to its high electron mobility derived from the extensive overlap of its 5s orbitals. To improve carrier transport, modulate threshold voltage, and enhance stability, In2O3 is doped with various secondary ions[107]. Among the innovative n-type materials for this purpose are ITZO, nitrogen-doped zinc oxide (ZnON), indium tungsten oxide (IWO), hafnium indium zinc oxide (HIZO) and lithium zinc oxide (LiZnO)[108-114]. These materials improved electrical performance and stability, which ensures consistent performance over time. Shi et al. exhibited the fabrication and performance of bottom gate TFTs with a channel layer of ITZO and a passivation layer of GaOx. The resulting ITZO TFTs demonstrate remarkable electrical properties, including a high field-effect mobility exceeding 58.3 cm2/V·s, a low subthreshold gate swing of 0.087 V/decade, a threshold voltage of -0.7 V, and impressive stability [Figure 2A][115]. The Sn ion contributes to the carrier path in MO, similar to the In ion in the a-ITZO channel layer, due to the similarity in the 5s orbital structures of Sn and In. Consequently, the higher content of In and Sn in the ITZO film promotes the effective intercalation of the Sn or In 5s orbitals, or increases the percolation conductivity resulting in the high mobility observed in the a-ITZO TFTs[116,117]. Recently, Li et al. demonstrated high-performance ITZO TFT employing water vapor during the sputtering process. These devices exhibited a high field-effect mobility of 122.10 cm2/V·s, a low threshold voltage of -2.30 V, and excellent stability under bias stress[118]. Similarly, Tiwari et al. fabricated flexible IWO TFTs with linear field effect mobility = 25.86 cm2·V-1·s-1, subthreshold swing = 0.30 V/decade, and threshold voltage = -1.5 V [Figure 2B][111]. IWO was an appropriate material for flexible high-performance TFTs. The IWO thin films were deposited using radio frequency (RF) sputtering, followed by post-annealing at 270 °C. These films exhibited smooth morphology and high carrier density, resulting in excellent TFT characteristics. Additionally, the IWO TFTs demonstrated stable performance during bending tests. As shown in Figure 2C, Ok et al. investigate high-performance ZnON TFTs fabricated on PEN substrates[119]. Utilizing thermal photochemical reaction, the field-effect mobility of these TFTs is significantly enhanced from 60 cm2/V·s. Unusual variations in nanostructures were evident during the phase transformation and densification processes. In amorphous ZnOxNy, the separated Zn3N2 and ZnO nanocrystalline lattice was remarkably stabilized by reducing oxygen defects and by interfacial atomic rearrangement without breaking the nitrogen bonding. Photochemical reaction also improves device stability under bias and illumination stress, with electrical performance remaining stable after 10,000 bending cycles. ZnON has recently attracted significant interest due to its exceptional charge transport properties compared to conventional amorphous MOs. High electrical performance has been attributed to low electron effective mass and the difference in transport mechanisms compared to conventional amorphous MOs. The electron effective mass of ZnON (me*/me = 0.19) is lower than that of IGZO (0.34 me*) due to the presence of the Zn3N2 (0.10 me*) structure[120-122].
Figure 2. Electrical properties of innovative materials for high-performance flexible MO TFTs. (A) Transfer curve of ITZO TFT without the passivation under negative gate bias stress for 1 h (VG = Vth - 20 V). Transfer curve and linear mobility of ITZO TFT with passivation by 5-nm GaOx and ZSO. Reproduced with permission[115], Copyright 2022, AIP Publishing; (B) Schematic of fabricated flexible IWO TFTs on PI substrate and transfer characteristics IWO TFTs on SiO2 Reproduced with permission[111], Copyright 2018, American Chemical Society; (C) Transfer characteristics of LT- and TPC-treated ZnON TFTs and saturation mobility depending on gate voltage sweep, Transfer curve of the ZnON TFTs on PEN substrate before and after delamination. Reproduced with permission[119], Copyright 2018, American Chemical Society. MO: Metal oxide; TFTs: thin-film transistors; ITZO: indium tin zinc oxide; ZSO: zinc silicon oxide; IWO: indium tungsten oxide; PI: polyimide; LT: low temperature; TPC: thermal-photochemical; PEN: polyethylene naphthalate.
The employing of multiple layers in flexible electronics offers several advantages. First, it allows for the enhancement of electronic properties, enabling the fabrication of devices with high performance. Second, multiple layers mitigate issues related to charge carrier recombination and trapping, which are common in single-material systems. By providing pathways for efficient charge carrier movement, multiple layers contribute to the stability and robustness of the device. Because of these advantages, many studies have been conducted[98,100,123,124]. Hsu et al. investigate multiple layers of titanium oxide (TiO2) and IGZO TFTs to improve flexibility and electrical properties at a low fabrication temperature of 100 °C. The high bond enthalpy of Ti–O can be optimized by adjusting the number of oxygen vacancies to satisfy the necessary specifications for gate dielectric and channel layers. The TiO2/IGZO TFTs demonstrated superior performance with a higher field-effect mobility of 61 cm2/V·s and a smaller subthreshold swing of 125 mV/decade at an operating voltage of 1.5 V. The devices also exhibited minimal performance degradation after bending tests, indicating excellent mechanical flexibility and stability [Figure 3A][82]. As shown in Figure 3B, Lin et al. introduce a novel TFT concept utilizing low-dimensional polycrystalline heterojunctions and quasi-superlattices (QSLs) composed of alternating layers of In2O3, gallium oxide (Ga2O3), and ZnO, created through sequential spin casting at low temperatures (180-200 °C). The optimized QSL transistors demonstrate electron mobilities of 25-45 cm2·V-1·s-1, which is about ten times higher than that of single oxide devices (2-5 cm2·V-1·s-1). The enhanced performance is attributed to quasi-2D electron gas-like systems at the oxide heterointerfaces. This QSL transistor approach could be applied to various oxide materials and deposition techniques[125]. Recently, Tang et al. present an eco-friendly, streamlined method for preparing MO TFTs using a pure water-solution blade-coating process. The process focuses on low-temperature fabrication and rapid annealing process. Triple-coated indium oxide (3C-TFTs) and indium oxide/zinc oxide/indium oxide (IZI-TFTs) TFTs on SiO2 gate dielectric (300 nm) annealed at 300 °C for just 60 s exhibit average field-effect mobility of 10.7 and 13.8 cm2/V·s, respectively. Flexible MO TFTs on PI substrates with AlOx dielectrics exhibited a field-effect mobility of over 10 cm2/V·s at low operating voltages. Longer annealing time of 20 min at 300 °C results in high-performance 3C-TFTs (over 18 cm2·V-1·s-1) and IZI-TFTs (over 38 cm2·V-1·s-1), demonstrating the effectiveness of the method for producing high-mobility, stable, and flexible TFTs [Figure 3C][99]. Bukke et al. fabricated the interface engineering of ZnO-based TFTs using nano-scale Ga2O3. By optimizing the Ga2O3 interface, the work achieved significant enhancements in the electrical performance and stability of the ZnO TFTs. The resulting a-Ga2O3/c-ZnO stack channel TFTs exhibit a high field-effect mobility of 41 cm2·V-1·s-1 and excellent stability under positive bias temperature stress. On a PI substrate, a-Ga2O3/c-ZnO stack channel TFTs show rarely threshold voltage shifts after 100,000 bending cycles with a 3 mm radius and remain stable under environmental tests. The multiple layers of a-Ga2O3 and ZnO improve charge transport by enhancing heterointerfaces and reducing defect density [Figure 3D][98]. Table 1 summarized the innovative materials and multiple-layer MO for high performance.
Figure 3. High-performance flexible MO TFTs employing multiple channel layers. (A) Photograph, schematic diagram, and transfer characteristics of TiOx/IGZO TFT on PI before and after bending. Reproduced with permission[82], Copyright 2015, Elsevier; (B) Schematics of MO TFTs employing heterojunction and QSL channel layer, transfer properties of different oxide-based channel layers including ZnO and In2O3 and Arrhenius plots of saturation mobility depend on temperature for ZnO, In2O3, heterojunction, and OSLs-based TFTs measured at VG = 80 V and VD = 100 V. Reproduced with permission[125], Copyright 2015, Wiley-VCH; (C) Schematics and transfer characteristics of 3C-TFTs and IZI-TFTs. Reproduced with permission[99], Copyright 2022, Wiley-VCH; (D) Transfer characteristics of D4 Ga2O3/ZnO-stack TFTs and flexible Ga2O3/ZnO TFT with different folding cycles (up to 100k) and threshold voltage shift of flexible Ga2O3/ZnO TFT with a different folding cycle; the insets show schematic diagram for Ga2O3 and ZnO films on ZrOx and the photograph of a foldable device. Reproduced with permission[98], Copyright 2022, American Chemical Society. MO: Metal oxide; TFTs: thin-film transistors; IGZO: indium gallium zinc oxide; PI: polyimide; QSL: quasi-superlattice; 3C-TFTs: triple-coated indium oxide; IZI-TFTs: indium oxide/zinc oxide/indium oxide.
Summarized electrical properties and deposition method of the innovative materials and multiple-layer MO for high performance
| Approach | Channel materials | Deposition method | Tmax (°C) | μFE (cm2·V-1·s-1) | SS (V/decade) | On/off | Gate dielectric | Refs. | Year |
| Innovative materials | ITZO | Sputter | 400 | 58.3 | 0.087 | - | SiO2 | [115] | 2022 |
| ITZO | Sputter | 400 | 122.1 | 0.18 | - | SiO2 | [118] | 2024 | |
| ITZO | Sputter | 290 | 26.15 | 0.26 | 9 × 108 | SiO2 | [108] | 2024 | |
| IGZTO | Sputter | 400 | 26.8 | 0.15 | 108 | SiO2 | [126] | 2020 | |
| IWO | Sputter | 300 | 25.86 | 0.3 | 106 | Al2O3 | [111] | 2018 | |
| HIZO | Sputter | - | 32.6 | 0.55 | 106 | SiO2 | [112] | 2012 | |
| ZnON | Sputter | 175 | 54.8 | 0.25 | - | SiO2 | [119] | 2018 | |
| LaZnO | Spray pyrolysis | 250 | 19.06 | 0.256 | 108 | HfZrO | [127] | 2023 | |
| LaZnO | Spray pyrolysis | 350 | 27.84 | 0.21 | - | ZrOx | [114] | 2021 | |
| LiZnO | Spray pyrolysis | 350 | 48.47 | 0.256 | - | ZrOx | [114] | 2021 | |
| Multiple layer | TiO2/IGZO | Sputter | 100 | 61 | 0.125 | - | TIO2/HfO2 | [82] | 2015 |
| InO/ZnO/InO | Blade | 300 | 38 | 2.8 | 106 | SiO2 | [99] | 2023 | |
| (GaO/ZnO)×3 | Spray pyrolysis | 350 | 41 | 0.209 | 108 | ZrOx | [98] | 2022 | |
| ITO/IGZO | Sputter | 300 | 58.2 | 0.12 | 1010 | AlOx | [123] | 2023 | |
| ITZO/IGZO | Spin coater | 450 | 51 | 0.41 | 108 | AlOx | [100] | 2018 | |
| InO/GaO/ZnO/GaO/InO | Spin coater | 200 | 37 | 0.16 | 104 | AlOx/ZrO2 | [125] | 2015 |
Semiconductor layer for flexibility
To maintain electrical performance in flexible devices, the semiconductor layer must be capable of enduring mechanical stress such as bending and stretching. It is crucial that MO semiconductors endure significant physical deformation without a degradation of electrical properties. Maintaining flexibility in the semiconductor layer is therefore essential for the reliable operation of flexible devices under mechanical stress. Consequently, recent research has focused on increasing the flexibility of MO semiconductors by synthesizing other materials or forming multiple layers with other materials and MO semiconductors[128-132]. Initially, by synthesizing MOs with other materials, Liu et al. report on the development of high-performance transistors based on a composite of single-walled carbon nanotubes (SWNTs) and sol-gel processed indium zinc oxide (IZO). The SWNTs provide fast tracks for carrier transport, significantly enhancing the field-effect mobility to as high as 140 cm2/V·s while maintaining a high on/off ratio of ~107. The composite thin films also exhibit excellent mechanical flexibility, showing superior stability with only 17% variation in performance at a bending radius of 700 μm and just 8% variation after 300 cycles of repeated bending [Figure 4A][128]. As shown in Figure 4B, Divya et al. proposed a new approach using water-insoluble and chemically inert polymers to form inorganic/organic composite semiconductors. These polymers resist intermixing with the oxide lattice, promote crystallization, and maintain the electrical properties of the oxide semiconductors even in near-equal amounts. As a result, optimized In2O3 and
Figure 4. High-performance flexible MO TFTs with flexible semiconductors. (A) Schematic of combination IZO and CNT to fabricate IZO/CNT hybrid TFTs, the normalized resistance as a function of the folded times and the photograph that repeated bending performance of IZO/CNT hybrid TFTs. Reproduced with permission[128]. Copyright 2012, American Chemical Society; (B) Schematic of the printed InOx/EC TFT with top-gate structure, optical image of optimized InOx/EC TFTs on PI substrate, transfer characteristics of optimized InOx/EC TFTs with decreasing bending radius and Change of linear mobility of optimized InOx/EC TFTs under bending tests. Reproduced with permission[129]. Copyright 2023, Wiley-VCH; (C) Schematic of IGZO:PI phototransistor fabricated on PI substrate and Transfer characteristics before and after 10,000 bending tests under dark conditions and green light at 1 mW/mm2 intensity. The inset shows the photograph of the bent IGZO:PI phototransistor. Reproduced with permission[130]. Copyright 2022, American Chemical Society; (D) Schematic of IGZO:PETE TFTs on SiO2, transfer curve of the IGZO:20 W PTFE TFT in 10,000 bending cycles with bending radius of 5 mm, and photograph of IGZO:PTFE on PI substrate. Reproduced with permission[131]. Copyright 2018, American Chemical Society; (E) Schematic of device structure, transfer characteristics, and variations in electrical parameters after repeated bending 200,000 cycles of InOx:indicone hybrid layers TFTs. Reproduced with permission[132]. Copyright 2021, Royal Society of Chemistry. MO: Metal oxide; TFTs: thin-film transistors; IZO: indium zinc oxide; CNT: carbon nanotube; EC: ethyl cellulose; PI: polyimide; IGZO: indium gallium zinc oxide; PTFE: polytetrafluoroethylene.
Summarized compound MO single-layer and multi-stack layer properties for flexibility
| Type of layer | Channel materials | Substrate | Tmax (°C) | Bending radius (mm) | μFE (cm2·V-1·s-1) | Gate dielectric | Refs. | Year |
| Compound single layer | IZO/SWNT | - | 350 | 2 | 63.4 | SiNx | [128] | 2012 |
| InO/EC | - | 275 | 1.5 | 40 | Electrolyte | [129] | 2023 | |
| IGZO:PI | SiNx/SiO2 | 300 | 10 | - | HfOx | [130] | 2022 | |
| IGZO:PETE | - | 380 | 3 | 3.5 | SiNx/SiO2 | [131] | 2018 | |
| Multi-stack layer | InO/indicone | AlOx | 200 | 2 | 2.05 | AlOx | [132] | 2021 |
High-k gate dielectric
In TFTs, the gate dielectric layer is a key component that significantly influences the overall performance, energy efficiency and stability of devices during operation. As device dimensions decrease and demands for flexibility rapidly increase, traditional silicon-based dielectrics such as SiO2 are encountering physical and electrical limitations. However, reducing the thickness of the dielectric layer to increase capacitance per unit area significantly degrades the transistor’s performance and reliability because of higher leakage currents. By employing high-k dielectrics, it becomes feasible to attain higher capacitance while using thicker dielectric layers[104,105,133]. The role of high-k dielectrics in MO TFTs is to enhance the capacitive coupling between the gate electrode and the semiconductor layer. This enhancement leads to better control of the channel charge at lower gate voltage, thereby reducing power consumption and improving the switching speed of the transistor[104,105,133]. It is possible to improve the mechanical robustness of the transistor - an important property for flexible devices that are susceptible to physical stress and deformation - by using a thicker dielectric while retaining the same capacitance. Furthermore, high-k materials often demonstrate superior thermal and chemical stability compared to their low-k counterparts. This stability ensures consistent performance throughout the device lifetime, particularly under the thermal cycling and environmental stresses encountered in flexible device applications[103-105].
High-k dielectrics of MO
High-k dielectrics based on MOs exhibit excellent thermal and chemical stability compared to those based on polymers. High-performance characteristics have been investigated by several groups using high-k dielectrics of MOs, such as Al2O3, titanium dioxide (TiO2), hafnium dioxide (HfO2), yttrium oxide (Y2O3), and zirconium dioxide (ZrO2), in flexible MO TFTs. Their approach to using high-k dielectrics of MO can be classified into three types: monolayer[72,113,114,134-137], multilayer[82,98,138-140], and doping process[141-146]. Rim et al. demonstrated IGZO TFTs with Al2O3 dielectrics on PI substrates using the direct light pattern (DLP) process, achieving patterning sizes down to a minimum feature size of 3 μm for conductors, semiconductors, and dielectrics[72]. In Figure 5A, these dielectrics exhibited a high breakdown voltage and good capacitance of ~4.9 MV·cm-1 and 46.3 nF at 1 kHz. The study is notable that the TFTs on PI substrates showed good performance with a high mobility of ~84.4 cm2·V-1·s-1 and an on/off ratio of greater than 105 at VDS 1.0 V. Hsu et al. reported on flexible TiOx/IGZO TFTs with bilayer HfO2/TiO2 dielectrics on PC substrates[82]. The bilayer structure of gate dielectrics enables maintaining a high dielectric constant while preventing gate leakage. The bottom layer of HfO2 in the bilayer mitigates the gate leakage issue caused by a narrow ~3.3 eV bandgap of TiO2, owing to its high bandgap. Additionally, the top layer of TiO2, with a very high dielectric constant (> 40), efficiently accumulates charges in the channel layer. In Figure 5B, the TFTs demonstrated a high field-effect mobility of 61 cm2·V-1·s-1, a low subthreshold swing of 125 mV/decade, a low operating voltage of 1.5 V, and less property degradation after bending test. The study showed the advantages of a bilayer dielectric structure in addressing the limitations of narrow bandgap in high-k materials.
Figure 5. (A) Schematic of DLP process and film formation (top) and photographs of integrated TFTs, I-V/C-F measurement of MIM structure, and ID-VG curves of IGZO TFTs (bottom). Reproduced with permission[72], Copyright 2014, American Chemical Society; (B) Photograph of TFTs on PC substrate and transfer curves of TFTs (top) and electrical characteristics of TFTs under different bending conditions or cycles (bottom). Reproduced with permission[82], Copyright 2014, Elsevier; (C) Leakage current density-electric field of AlOx and ZAO dielectric, OM image of ZAO-based TFTs on ITO/PI substrate and transfer curves of TFTs with ZAO dielectric before and after bending test. Reproduced with permission[141], Copyright 2013, The Royal Society of Chemistry; (D) Schematic of synaptic In2O3 TFTs, Capacitance as a function of frequency for polymer electrolyte and transfer curve of synaptic In2O3 TFTs on PI substrate at different bending radii. Reproduced with permission[147], Copyright 2020, American Chemical Society; (E) Photographs of integrated TFTs with bending test method and transfer curves of TFTs under different bending cycles. Reproduced with permission[149], Copyright 2019, American Chemical Society. DLP: Direct light pattern; TFTs: thin-film transistors; MIM: metal-insulator-metal; IGZO: indium gallium zinc oxide; PC: polycarbonate; ZAO: zirconium doped aluminum oxide; OM: optical microscopy; ITO: indium-tin-oxide; PI: polyimide.
Another method of using high-k dielectrics is doping process, as demonstrated by Yang et al.[141]. They fabricated IZO TFTs with soluble Zr-doped AlOx gate dielectrics on PI substrates and conducted a comparative analysis of the quantitative dielectric properties of the Zr-doped AlOx and the undoped AlOx films. In Figure 5C, Zr-doped AlOx films showed lower leakage current density as the annealing temperature decreased, compared to undoped AlOx films. Additionally, Table 3 exhibited the calculated dielectric constant of Zr-doped AlOx (8.4-11.8) and undoped AlOx (5.6-6.2). Doping AlOx, which has a high breakdown field, with Zr, known for its strong bonding to oxygen, resulted in an unprecedented soluble high-k dielectric, significantly reducing the processing temperature to 250 °C. Annealing temperature, capacitance, thickness, and dielectric constant by calculation between AlOx and Zr-doped AlOx were compared in Table 3. This report emphasizes the feasibility of flexible IZO TFTs. The Zr-doped TFTs on ITO/PI substrates showed a saturation mobility of 51 cm2·V-1·s-1, a threshold voltage of 1.2 V, and an on/off current ratio of ~104.
Comparison of annealing temperature, capacitance, thickness, and dielectric constant by calculation between AlOx and Zr-doped AlOx
| Sample | Ta (°C) | Capacitance at 1 MHz (nF·cm-2) | Thickness | Dielectric constant |
| AlOx | 250 | 71 | 70 | 5.6 |
| 300 | 84 | 64 | 6.1 | |
| 350 | 91 | 61 | 6.2 | |
| Zr-doped AlOx | 250 | 70 | 106 | 8.4 |
| 300 | 90 | 100 | 10.2 | |
| 350 | 110 | 95 | 11.8 |
High-k dielectrics of polymer
High-k polymer dielectrics are increasingly used in flexible and stretchable electronics due to their inherent flexibility and simple processing. Several groups have demonstrated high-performance flexible TFTs on various high-k polymer dielectrics[147-149]. Zhu et al. fabricated electrolyte-gated synaptic In2O3 TFTs on a PI substrate with polyethylene oxide (PEO) + LiClO4 dissolved in acetonitrile to form the polymer electrolyte solution as gate dielectric and reported good performance electrical characteristics, exhibiting a mobility of 7.80 cm2·V-1·s-1, an on/off current ratio of ~106, and a threshold voltage of 0.55 V. As shown in Figure 5D, an electric double layer (EDL) forms at the interface between the semiconductor and the polymer electrolyte, resulting in high gate capacitance of ~0.36 µF/cm2 and low gate voltage operations[147]. Additionally, Samanta et al. fabricated a-IGZO flexible TFTs on a PI Kapton substrate with an electrolyte dielectric (PEO + LiClO4) and reported high-performance electrical characteristics, exhibiting a mobility of ~42 cm2·V-1·s-1, an on/off current ratio of ~105, a threshold voltage of 0.7 V, and a low subthreshold swing of
Summarized high-performance MO TFTs with high-k dielectrics
| Approach | Gate dielectric | Deposition method | Channel materials | Substrate | Tmax (°C) | μFE (cm2·V-1·s-1) | SS (V/decade) | On/off ratio | Refs. | Year |
| MO dielectric - monolayer | Al2O3 | Spin coating | IGZO | PI | 350 | 84.4 | - | 105 | [72] | 2014 |
| ALD | IZO | PI | 200 | 42.1 | 0.29 | 109 | [134] | 2016 | ||
| ALD | ZnO | PET | 100 | 37.1 | 0.38 | 107 | [135] | 2019 | ||
| ALD | IGZO | PI | 350 | 47.9 | 0.18 | - | [136] | 2019 | ||
| ALD | ZnON | PEN | 150 | 147 | 0.21 | - | [113] | 2020 | ||
| HfO2 | ALD | IGZO | PI | 200 | 19 | 0.09 | 1011 | [150] | 2020 | |
| ZrOx | Spin coating | LiZnO | PI | 350 | 48.47 | 0.26 | 107 | [114] | 2021 | |
| MO dielectric - multilayer | Ga2O3/ZrOx | Spin coating | ZnO | PI | 350 | 41 | 0.22 | 108 | [98] | 2022 |
| Y2O3/TiO2/Y2O3 | Electron beam evaporation | IGZO | PC | - | 40 | 0.16 | 105 | [138] | 2013 | |
| SiO2/TiO2/SiO2 | Electron beam evaporation | IGZO | PC | RT | 76 | 0.13 | 105 | [139] | 2013 | |
| HfO2/TiO2 | Evaporator | TiOx/IGZO | PC | 100 | 61 | 0.13 | - | [82] | 2014 | |
| Zr-Al2O3 | Spin coating | IZO | PI | 280 | 51 | - | 104 | [141] | 2013 | |
| Al2O3:Nd | Anodization process | ZrInO | PEN | 150 | 22.6 | 0.39 | 107 | [142] | 2016 | |
| Polymer dielectric | Electrolyte | Drop casting | In2O3 | PI | 250 | 8.74 | 106 | [147] | 2020 | |
| Electrolyte | Drop casting, spin coating | IGZO | PI | 120 | 42 | 0.18 | 106 | [148] | 2018 | |
| PVP-co-PMMA:HfOx | Spin cast | IGTO | PI/PDMS | 150 | 25.9 | 0.4 | 107 | [149] | 2019 |
Each of these components - advanced MO semiconductors, high-performance semiconductor layers, and high-k dielectrics - must be carefully selected and engineered to ensure that flexible TFTs can achieve high electrical performance while enduring the mechanical demands of bending, stretching, and other forms of deformation. The use of these advanced materials is crucial in developing next-generation flexible electronic devices that are not only efficient and reliable but also versatile and robust in various applications.
FABRICATION PROCESS FOR HIGH-PERFORMANCE MO TFT
The fabrication of high-performance flexible MO TFTs employs standard semiconductor fabrication tools to ensure precise dimensions and consistent performance. However, the diversity of flexible substrates, each with unique physical and chemical properties, requires the development of innovative techniques. Vacuum-based techniques, such as sputtering, atomic layer deposition (ALD), and pulsed laser deposition (PLD), are commonly employed for substrate deposition. Additionally, solution coating-based methods, such as spin coating, spray coating and inkjet printing, are used to create thin films for the active layers, gate dielectrics, and protective coatings. These methods are chosen for their ability to deposit uniform, dense films at low temperatures, making them compatible with flexible substrates[8,15,95]. Achieving high-performance flexible MO TFTs involves several critical steps in the fabrication process, with doping techniques playing an important role. Doping process is essential and simple for enhancing the electrical properties of MO semiconductors, improving carrier concentration, enhancing electrical stability and modulating the threshold voltage[55,95].
Doping process
Previously, several studies have attempted to improve the electrical performance of MO TFTs by doping process, such as hydrogen doping[59,151-157], nitrogen doping[110,113,157-161], fluorine doping[162-168] and metal cation doping[55-57,60]. Hydrogen doping is one of the most common techniques to improve the electrical performance of MO semiconductors. Incorporating hydrogen atoms into the oxide lattice can passivate defects and increase carrier mobility[59,152,153,155-157]. This process is typically done through thin film deposition in hydrogen atmosphere[152,156], hydrogen plasma treatment[153,155] or MO thin film in a hydrogen atmosphere[59,151,154]. Recently, Lee et al. investigated the diffusion of hydrogen from plasma-enhanced ALD (PEALD)-derived SiO2 into the underlying a-IGZTO film during the post-deposition annealing (PDA) process. The transfer curves of the TFTs that optimized IGZTO thin films are shown in Figure 6A. Compared with those without PDA (field-effect mobility = 19.1 cm2/Vs), the devices conducting PDA process exhibited a higher field-effect mobility of 85.9 cm2/Vs in Figure 6A[59]. The hydrogen atoms help reduce trap states, can act as a shallow donor in MO semiconductors, and improve the overall conductivity of the semiconductor layer, leading to enhanced device performance with higher mobility and lower subthreshold swing[169,170]. However, since hydrogen doping can cause a significant negative shift in the threshold voltage (VTH) and undesirable photo-instability[171,172], Abliz et al. demonstrated the effects of co-plasma treatment with hydrogen and nitrogen on IGZO TFTs to enhance both electrical performance and stability simultaneously[157].
Figure 6. Doping process for the high electrical performance of MO TFTs. (A) Transfer properties of the non-passivated optimized IGZTO TFTs and SiO2-passivated optimized IGZTO TFTs at PDA 400 °C. Reproduced with permission[59]. Copyright 2022, American Chemical Society; (B) Transfer characteristics of In2O3:F TFTs. Reproduced with permission[173]. Copyright 2024, Wiley-VCH; (C) Transfer characteristics of AOS TFTs, IGZO NBIS testing results and IGZO:N NBIS testing results. The stress conditions are VGS = -20 V, UV light wavelength = 380 nm and UV light power =
Fluorine doping is a technique used to improve the stability and electrical performance of MO semiconductors. Fluorine atoms, due to their high electronegativity, can effectively passivate defect sites and reduce the density of oxygen vacancies. This leads to a decrease in off-state leakage current and an improvement in threshold voltage stability. The amount of F in MO films was carefully controlled and introduced to ZnO[163], ZTO[164,166], IGZO[167,168], InOx[173] and IWZO[165] films to enhance electrical properties, achieving a field-effect mobility of 31.59 cm2/Vs in ZnO TFTs[163]. Recently, Li et al. developed high-performance fluorine-doped indium oxide (In2O3:F) TFTs with enhanced bias-stress stability. This fabrication involves an ALD process using cyclopentadienyl indium (InCp) and O2 plasma to produce a crystalline ultrathin In2O3 layer. Following that, a fluorine doping process using CF4 plasma forms the In2O3:F layer. Density functional theory (DFT) analysis indicated that fluorine doping stabilizes lattice oxygen and electrically passivates oxygen vacancy defects in In2O3 by forming FOFi defects. Consequently, In2O3:F TFTs exhibit exceptional electrical performance and bias-stress stability, achieving a high electron mobility of 35.9 cm2/V·s, a steep subthreshold swing of 94.3 mV/dec, a minimal hysteresis of 33 mV, and a small threshold voltage shift of -111 and 49 mV under negative and positive bias stress, respectively
Nitrogen doping involves incorporating nitrogen atoms into the MO semiconductor to improve stability and electrical performance. Nitrogen atoms can replace oxygen vacancies in the lattice, which helps reduce the density of electron traps and improve the stability of the TFT under bias stress. Nitrogen can act both as an acceptor and as a defect binder due to its ionic radius being similar to that of oxygen. This similarity allows nitrogen to effectively substitute for O (NO) and reduce oxygen vacancy in ZnO-based thin films[161,174]. Consequently, nitrogen doping process can enhance the bias stability of a-IGZO TFTs[158,175]. However, this process has also been associated with a decrease of mobility and a reduction of the optical band gap energy in nitrogen-doped a-IGZO TFTs[158]. Additionally, Xie et al. introduce nitrogen-doped AOS TFTs with a double-stacked channel layer structure combined with amorphous InZnO (a-IZO) and nitrogen-doped a-IGZO films. This TFT with a double-stacked channel layer exhibits superior field-effect mobility (up to 49.6 cm2/V·s) and stability under both bias and light stress tests compared to conventional single-layer TFTs. The enhanced performance is primarily due to the heterojunction within the channel layers and the lower interface and bulk trap densities achieved through nitrogen doping, which effectively reduces oxygen vacancies and trap sites in the channel. Additionally, the a-IGZO layer functions as a passivation layer, shielding the channel from ambient gases and UV light, further improving device stability as shown in Figure 6C[159]. Han et al. investigated the impact of nitrogen and IZO composition on electrical performance and stability in IZO TFTs[176]. Park et al. investigated nitrogen-doped ZnON TFTs on PEN substrates by employing dual gates for high performance. The effects of applying a dual gate structure are attributed to the formation of a high electron density channel away from the gate dielectric/semiconductor interface (field-effect mobility = 65.8 to 147 cm2·V-1·s-1)[113].
The metal cation doping process is also widely used, and extensive research has been conducted on indium-based materials. In2O3 is a widely studied material for both conductive and semiconductive applications due to its high electron mobility, which results from the extensive overlap of 5s orbitals. To enhance carrier transport, In2O3 is typically doped with secondary ions. Sn-doped In2O3 (ITO) is commonly used as a transparent conductor because the difference in oxidation states between In3+ and Sn4+ leads to high carrier concentrations[95]. When doped with Ga and/or Zn (forming materials such as IGO, IZO, and IGZO), In2O3 achieves better semiconductor properties. Pure In2O3 TFTs often have high carrier concentrations, making it difficult to control off-current and VTH, but Ga and Zn help mitigate this by acting as a carrier suppressor and stabilizing the amorphous state, respectively[97,177]. Other dopants are also used to enhance or stabilize the electrical characteristics of oxide semiconductors. Important factors in choosing dopants include energy bandgap, standard electrode potential (SEP), Gibbs Energy of oxidation, and electronegativity[55,60,95]. Larger bandgaps shift donor states to deeper levels, lower SEP enhances carrier suppression, and higher differences in electronegativity and Gibbs Energy of oxidation reduce VO. Metals such as lanthanum (La), magnesium (Mg), yttrium (Y), hafnium (Hf), strontium (Sr), and barium (Ba) are effective carrier suppressors due to these properties [Figure 6D][55-57,60]. Here, electrical properties of the hydrogen, nitrogen, fluorine and other metal-doped MO semiconductors are summarized in Table 5.
Summarized electrical characteristics of various materials dopped MO semiconductors
| Doping materials | Channel materials | Deposition method/doping method | Tmax (°C) | μFE (cm2·V-1·s-1) | SS (V/decade) | On/off ratio | Gate dielectric | Refs. | Year |
| H doping | ZnO | Sputter/UV irradiation | 300 | 14.2 | - | 106 | SiNx | [151] | 2018 |
| IZTO | Sputter/ESL deposit by PECVD | 350 | 48 | 0.15 | 1010 | SiO2 | [154] | 2015 | |
| IGZO/IGZO:H | Sputter/H2 plasma | 250 | 55.3 | 0.18 | 108 | SiO2 | [153] | 2016 | |
| IGZTO | Sputter/passivation deposit by PEALD | 400 | 85.9 | 0.33 | - | SiO2 | [59] | 2022 | |
| IGZO | Sputter/using sputter gas (Ar/H2) | 200 | 16.15 | - | - | SiO2 | [156] | 2018 | |
| IGZO | Sputter/H2 plasma | 150 | 36.6 | 0.4559 | - | SiO2 | [155] | 2020 | |
| ZnO | Sputter/H2 plasma | 200 | 12.1 | 2.702 | - | SiO2 | [155] | 2020 | |
| In2O3 | Sputter/H2 plasma | RT | 55.9 | 1.435 | - | SiO2 | [155] | 2020 | |
| F doping | ZnO | Spray pyrolysis/NF3 plasma | 350 | 31.59 | 0.238 | 108 | ZrAlO | [163] | 2021 |
| ZTO | Co-sputter (ZnO/FTO) | 350 | 14.2 | 0.087 | 109 | AlOx:Nd | [166] | 2023 | |
| IWZO | Sputter/CF4/N2 + O2 plasma | 400 | 31.2 | 0.37 | 106 | HfO2 | [165] | 2020 | |
| IGZO | Sputter/CHF3/O2 plasma | 400 | 39.8 | 0.21 | 106 | HfLaO | [167] | 2014 | |
| InOx | ALD | 150 | 35.9 | 0.094 | - | Al2O3 | [173] | 2024 | |
| N doping | IGZO | Sputter using Ar,N2 gas | 300 | 19.21 | 0.26 | - | SiO2 | [158] | 2016 |
| IGZO:N/IZO | Sputter using Ar,N2 gas | 300 | 49.6 | 0.5 | 107 | SiO2 | [159] | 2016 | |
| ZnON | Sputter using Ar/O2/N2 gas | 250 | 51.99 | 0.42 | 108 | SiO2 | [110] | 2015 | |
| ZnON | Sputter using Ar/O2/N2 gas | - | 65.8 | 0.48 | - | Al2O3 | [113] | 2020 | |
| IGZO | PEALD/N2 plasma | 350 | 106.5 | 0.113 | - | Al2O3 | [161] | 2023 | |
| ZnON | ALD | 250 | 4.8 | 0.47 | 107 | Al2O3 | [160] | 2018 | |
| IZO | Sputter | 350 | 24.67 | 0.41 | 109 | SiO2 | [176] | 2016 | |
| Metal doping | IBZO | Spin coating | 450 | 18 | - | - | SiO2 | [60] | 2013 |
| ISZO | Spin coating | 450 | 25 | - | - | SiO2 | [60] | 2013 | |
| LaZnO | Spray pyrolysis | 350 | 27.84 | 0.21 | - | ZrOx | [114] | 2021 |
Deposition method
The fabrication of high-performance flexible MO TFTs employs various deposition techniques to ensure precise dimensions and consistent performance. These methods are chosen for their ability to produce uniform, dense films at low temperatures compatible with flexible substrates. Deposition methods include sputtering, ALD, PLD, spin coating, inkjet printing, and spray coating. In addition to the standard criteria for thin-film deposition on flexible substrates, there are specific requirements critical for fabrication of flexible devices. These include maintaining low deposition temperatures compatible with the thermal limits of the flexible substrates, ensuring sufficient adhesion of the deposited materials to substrate to prevent delamination during bending, and minimizing the strain within the deposited layers to preserve the mechanical properties, such as bendability, of the final devices[10,12,15,95].
Sputtering is the predominant technique used to deposit vacuum-processed MO semiconductors. It involves bombarding a target material with high-energy particles, causing atoms to be ejected and deposited onto a substrate. Sputtering techniques include RF, RF-magnetron, direct current (DC), and pulsed DC sputtering. These methods have been widely used to deposit materials such as IGZO, IZO, ITZO, and ZnO[111-113,118,126,178]. The advantages of sputtering are its large availability of tools, low-temperature deposition (typically at room temperature), good adhesion, and dense structure of the final layers. Additionally, sputtering tools offer several opportunities to optimize the layer properties by adjusting the power and/or sputtering pressure. Reactive sputtering, which uses different concentrations of Argon (Ar) and O2, can be employed to adjust the oxygen content in MO semiconducting active layers[179]. Furthermore, co-sputtering techniques using targets such as IZO and Ga2O3 enable better control of the stoichiometric composition of IGZO[10,12,94,179]. PLD is another effective method for depositing MOs such as IGZO, Ga2O3 and ZITO[92,148,180]. This technique involves using a high-power pulsed laser beam to vaporize a target material, which then deposits onto a substrate. PLD is known for producing high-quality films with excellent uniformity and stoichiometry control[181,182]. Although the process requires high temperatures, it results in high-quality thin films that are crucial for the performance of flexible TFTs. ALD is a vapor-phase technique used to deposit thin films with atomic-level precision. It involves the sequential use of gas-phase chemical processes to deposit materials one atomic layer at a time. ALD is particularly advantageous for creating conformal coatings with excellent uniformity and control over film thickness. It has been used to deposit materials such as Al2O3 and HfO2[92,111,113,123,135]. Recently, ALD and PEALD have also been used for the deposition of semiconductors such as IGZO, ZnO and InOx[134-136]. These materials are gaining attention for their excellent electrical properties and suitability for use in advanced electronic devices, including TFTs and optoelectronic components. PEALD is a variant that uses plasma to enhance the reactivity of the precursors, allowing for lower-temperature processing. ALD and PEALD are well-suited for depositing channel layers, gate dielectrics, and passivation due to their ability to produce films with high resistance, low pinhole density, and good sidewall coverage[183].
Spin-coating is a widely used technique for fabricating oxide thin films on flexible substrates due to its simplicity, uniformity, reproducibility, and compatibility with various substrates[8,15]. The process involves depositing a liquid precursor on a substrate, which is then rapidly spun to evenly distribute the solution, balancing centrifugal and viscous forces. This results in a thin, uniform film treated by drying and annealing after spin coating. The film thickness is influenced by factors such as rotational speed, duration, and solution viscosity. Spin-coating can deposit films with a minimum thickness of around 5 nm and is suitable for solutions with viscosity between 1 and 5,200 cP[8,184,185]. Despite its advantages, spin-coating has limitations, including material wastage, difficulty with large or curved substrates, and low throughput. In contrast, methods such as slit-die coating can improve uniformity. Spray-coating is a cost-effective, noncontact method for depositing thin films over large areas using small amounts of MO solution deposition. This technique allows for material doping and multicomponent film through multiple coating processes. It uses an atomizer or nebulizer to create fine droplets from low-viscosity solutions, which are directed onto a substrate by carrier gases and gravity, with an electrostatic field aiding in the formation of thin-film coatings. The thickness, roughness, and quality of the films are influenced by droplet size, coating cycles, substrate temperature, and solution viscosity during thin-film deposition. While capable of achieving film thicknesses of 10-600 nm and handling viscosities of 0.1-1 cP[8,186], spray-coating faces challenges such as high film roughness, limited low-viscosity solutions, and lengthy processing times. Despite these issues, it remains valuable for large-area thin-film deposition due to its simplicity and adaptability. Inkjet printing, a noncontact thin-film deposition technique, is valued for its precision and efficiency. It uses micrometer-scale nozzles in printer heads, with thermal inkjet expelling ink via solvent evaporation and piezoelectric inkjet using voltage-induced nozzle shape changes. Factors including nozzle diameter, temperature, and type affect the morphology of printed patterns. Ink viscosities range from 1 to 50 cP, and printed lines are 20 to 50 μm wide[8]. While inkjet printing offers advantages involving photolithography-free deposition, selective small-area coating, and efficient material use, it faces challenges such as lower throughput compared to roller-based methods, limited ink choices, and potential nozzle clogging. Moreover, 3D printing is emerging as a promising fabrication method for flexible MO TFTs due to its ability to create complex, custom geometries with precise control. This technique enables the layer-by-layer deposition of functional materials allowing for the formation of TFTs on flexible substrates. For MO TFTs, 3D printing offers several benefits, including the precise deposition of MOs and integration with flexible substrates without the need for high-temperature processing, which is often incompatible with flexible materials. Additionally, 3D printing reduces waste and provides greater design flexibility compared to conventional methods. Recent studies have demonstrated successful printing of oxide semiconductors using inkjet or aerosol jet printing techniques, showcasing the potential of 3D printing in this area. However, challenges remain, particularly in ensuring the uniformity and electrical performance of printed MOs; nevertheless, continued advancements in 3D printing materials and techniques are making flexible MO TFT fabrication increasingly feasible[187-190].
While each technique offers distinct advantages, they also have drawbacks. PLD requires high temperatures and long processing times, while ALD has a slow deposition rate and is limited in scalability for large areas. Spin-coating suffers from material wastage and difficulties in achieving uniform coatings on large or curvature substrates. Spray-coating faces issues with high film roughness and long processing times. Inkjet printing, though precise, has lower throughput and limited ink choices due to viscosity and clogging concerns. Despite these challenges, each of these deposition methods offers unique advantages and specific applications, playing a crucial role in fabricating high-performance flexible MO TFTs and enabling the development of advanced flexible electronic devices with improved electrical properties and mechanical flexibility. Table 6 is a comprehensive overview of deposition techniques, the materials deposited, and the electrical properties of materials.
Summarized deposition techniques, the materials deposited, and the electrical properties of materials
| Deposition method | Channel materials | Tmax (°C) | μFE (cm2·V-1·s-1) | SS (V/decade) | On/off ratio | Gate dielectric | Refs. | Year |
| Sputter | IZTO | 380 | 52.4 | 0.2 | 108 | SiO2 | [178] | 2014 |
| ITZO | 400 | 122.1 | 0.18 | - | SiO2 | [118] | 2024 | |
| ITZO | 290 | 26.15 | 0.26 | 108 | SiO2 | [108] | 2024 | |
| IGZTO | 400 | 26.8 | 0.15 | 108 | SiO2 | [126] | 2020 | |
| IWO | 300 | 25.86 | 0.3 | 106 | Al2O3 | [111] | 2018 | |
| HIZO | - | 32.6 | 0.55 | 106 | SiO2 | [112] | 2012 | |
| ALD | IZO | 200 | 42.1 | 0.29 | 109 | Al2O3 | [134] | 2016 |
| ZnO | 100 | 37.1 | 0.38 | 107 | Al2O3 | [135] | 2019 | |
| PEALD | IGZO | 200 | 70 | 0.25 | 108 | SiO2 | [136] | 2019 |
| Spray pyrolysis | LaZnO | 250 | 19.06 | 0.256 | 108 | HfZrO | [127] | 2023 |
| LaZnO | 350 | 27.84 | 0.21 | - | ZrOx | [114] | 2021 | |
| LiZnO | 350 | 48.47 | 0.256 | - | ZrOx | [114] | 2021 | |
| (GaO/ZnO)×3 | 350 | 41 | 0.209 | 108 | ZrOx | [98] | 2022 | |
| Spin coating | InO/GaO/ZnO/GaO/InO | 200 | 37 | 0.16 | 104 | AlOx/ZrO2 | [125] | 2015 |
| ITZO/IGZO | 450 | 51 | 0.41 | 108 | AlOx | [100] | 2018 |
DEVICE STRUCTURE
Flexible MO TFTs affect repeated bending stress. Repeated bending stress in MO TFTs creates oxygen vacancies, generating carriers and causing a negative VTH shift, increased saturation mobility, and higher drain current. This occurs because strain-induced atomic distance expansion reduces the energy band gap
Island structure TFTs, as the name suggests, are fabricated on substrates featuring either an island structure or a flat substrate with islands created. In this architecture, the MO TFTs are fabricated on isolated islands in the substrate as presented in Figure 7. This structure offers several advantages, including reduced leakage current and enhanced device reliability. Island structure TFTs are commonly implemented in flexible displays, where high performance and uniformity are essential.
Figure 7. Flexible MO TFTs employing island structure. (A) Schematic of the three different types of top gate a-IGZO TFTs, photograph of island a-IGZO TFT with passivation. Transfer characteristics of three types of a-IGZO TFTs before the bending test. Changes in electrical properties of the TFTs repeated bending cycle. Reproduced with permission[102]. Copyright 2021, American Chemical Society; (B) Schematic of a-IGZO TFT fabricated with mesa-island structure. 3D image for mechanical stress distribution of mesa-island structure using FEA modeling to analyze mechanical stress. Normalized electrical characteristics of a-IGZO TFTs on island structure and conventional structure before and after 10,000 bending cycles under a bending radius of 125 μm. Reproduced with permission[33]. Copyright 2020, Wiley-VCH; (C) Schematic of island structure formation of a-IGZO TFTs. Transfer characteristics of the devices with island structure and the electrical responses of the devices as a function of the accumulated stress after 100,000 bending cycles at a radius of 2.0 mm. Reproduced with permission[193]. Copyright 2016, Elsevier. MO: Metal oxide; TFTs: thin-film transistors; IGZO: indium gallium zinc oxide; FEA: finite-element analysis.
Junctionless TFTs are characterized by the absence of a boundary between the source-to-drain electrode and the channel layer. Unlike traditional TFTs, which form source-to-drain and channel junctions, junctionless TFTs utilize a single semiconductor material for both the channel and the electrodes. This simplifies the fabrication process and reduces manufacturing costs. In junctionless TFTs, there is no overlap region between the source/drain (S/D) and gate electrodes. Consequently, parasitic capacitances are not present, making them suitable for high-speed and low-power applications such as integrated circuits and sensors.
Flexible TFTs employing electrode and channel layer architecture release flexible stress by distributing mechanical stress within the devices. The electrode and channel layer architecture serves to distribute mechanical stress, preventing the deformation or failure of the active semiconductor layer during bending or stretching. By mitigating mechanical stress, TFTs utilizing an electrode architecture exhibit enhanced mechanical reliability and durability. Island structure TFTs offer improved performance and reliability but may require more complex fabrication processes. Junctionless TFTs simplify device fabrication and reduce costs but may exhibit lower carrier mobility compared to traditional TFTs. TFTs utilizing electrode architecture enhance mechanical reliability but may introduce additional processing steps and electrode conductivity issues depending on the electrode architecture. In this part, the configurations, examples of implementation, advantages, limitations, and potential uses of each structure of flexible MO TFTs are reviewed and discussed in detail.
Island structure
Flexible MO TFTs on an island-structured substrate are less affected by mechanical stress due to several reasons. The island structures help distribute mechanical stress evenly across the substrate, preventing localized stress concentrations that could damage flexible TFTs. Additionally, the islands isolate deformation to specific areas, reducing the overall strain experienced by the TFTs and limiting the impact of bending or stretching on their active regions. In 2021, Han et al. demonstrated IGZO TFTs initially fabricated on PI islands and then transferred onto a thermoplastic polyurethane (TPU) film. Due to a lower elastic modulus of TPU compared to PI, this configuration significantly reduced the curvature of the PI island, even under the same bending test conditions. Flexible MO TFTs with island structure and an organic passivation layer exhibited decreases in the threshold voltage by -0.22 V and the saturation mobility by
However, this substrate architecture requires additional fabrication processes, leading to rising manufacturing costs compared to conventional structure MO TFTs. Simplifying the fabrication process is needed to address this issue.
Junctionless structure
The flexible TFTs can experience stress concentration at the S/D and channel junction, potentially causing deformation and delamination between the channel and the S/D electrodes which increase in contact resistance under mechanical stress[63-66,194-198]. The increase in contact resistance degrades the device. However, flexible MO TFTs employing junctionless structure have no S/D electrode and channel junction. Therefore, mechanical stress is not concentrated at the S/D electrode and channel junction under mechanical. It is also less affected by the change of contact resistance caused by mechanical stress. In 2021, Yuan et al. demonstrate the fabrication of junctionless EDL TFTs on paper substrates, utilizing a solution-processed chitosan dielectric [Figure 8A]. The TFTs operate at a low voltage of 1.0 V, thanks to the high gate capacitance provided by the EDL effect. The devices show excellent field-effect mobility, subthreshold swing, and current on/off ratio. The EDL effect originated by forming a layer of charged ions at the interface with an electrode in electrolyte under an electric field. This results in a high capacitance exhibited significant modulation of the electric field with low voltage [Figure 8A]. In TFTs employing ITO channel layers and S/D, the EDL effect enables low-voltage operation by effectively controlling the channel conductivity, making these devices suitable for low-power, flexible, and portable electronics[66]. This work demonstrates the potential of these junctionless EDL TFTs for flexible electronic applications such as portable sensors and smart labels. The devices are noted for their low power consumption and simple fabrication process. Additionally, Jiang et al. also fabricated flexible junctionless oxide-based EDL TFTs on paper substrates at room temperature. The flexible TFTs utilize ITO films for both the channel and S/D electrodes, eliminating the need for S/D junctions. These devices demonstrate effective field-effect modulation with thin ITO films. The flexible junctionless TFTs exhibited excellent device performance, including a small subthreshold swing and a large on/off ratio[64]. These characteristics suggest significant potential for flexible paper electronics and low-cost portable sensors. Dou et al. focus on dual-gate TFTs (DGTFTs) on paper substrates, using solution-processed chitosan-based proton conductors as the gate dielectric. The dual-gate configuration allows for precise threshold voltage modulation and logic functionality [Figure 8B]. The devices demonstrate AND logic circuits, making them suitable for various logic applications. The DGTFTs exhibit stable performance and good switching characteristics. The simple and low-cost fabrication process, along with the excellent electrical properties, makes these devices promising for flexible electronics. The study exhibited the benefits of DGTFTs in terms of threshold voltage control and logic operations, emphasizing their potential in flexible and portable electronic applications[194]. Recently, Jeon et al. report on the development of flexible, transparent junctionless TFTs with oxygen-plasma treatment ITO channels on PI substrates [Figure 8C]. The devices are patterned using photolithography, which simplifies the fabrication process. The TFTs exhibited stable performance even under bending conditions. This study demonstrated that the junctionless structure offers advantages for flexible devices and reliable operation under mechanical stress through mechanical simulations using FEA method [Figure 8C]. The study emphasizes the advantages of the junctionless structure for flexible devices and the simplicity of the fabrication process[198]. However, junctionless TFTs, while advantageous for their simplified fabrication, have several drawbacks. They tend to exhibit higher off-state leakage currents and larger subthreshold swings, and their performance is sensitive to fabrication variations[63-66,194-197].
Figure 8. Flexible MO junctionless TFTs. (A) Schematic of junctionless ITO TFTs with chitosan dielectric and EDL formation. Transfer characteristics of junctionless ITO TFTs with chitosan dielectric. Dependence of capacitance and conductivity on frequency for solution-processed chitosan dielectrics. Reproduced with permission[66]. Copyright 2021, Electrochemical Society; (B) Schematic of junctionless ITO TFTs employing dual gate on paper substrate. Transfer characteristics of junctionless ITO TFTs employing dual gate on paper substrates with VG3 ranging from 2 to -2 V. AND logic function of junctionless ITO TFTs employing dual gate. Reproduced with permission[194]. Copyright 2021, American Chemical Society; (C) Schematic of ITO junctionless TFTs on plastic substrates. Transfer characteristics of ITO junctionless TFTs before and after bending. Output characteristics of ITO junctionless TFTs on PI substrates. Reproduced with permission[198]. Copyright 2024, American Chemical Society. MO: Metal oxide; TFTs: thin-film transistors; ITO: indium-tin-oxide; EDL: electric double layer; PI: polyimide.
However, if these issues are resolved, junctionless TFTs could become highly efficient and reliable components in flexible electronics, offering simplified fabrication and enhanced performance with low power consumption. These devices provide ease of fabrication and show great potential for applications in the field of flexible electronics.
Electrode and channel layer architecture
Flexible TFTs rely heavily on advanced electrode architectures to maintain performance under mechanical stress. The key challenge is to minimize the stress concentration at the electrode regions, which can lead to cracks and performance degradation. One effective approach is to use mesh and strip patterns for source/drain/gate (S/D/G) electrodes. Lee et al. demonstrated that a-IGZO TFTs with mesh-patterned S/D electrodes and strip-patterned semiconductor layers show no crack generation even after 60,000 bending cycles with a radius of 0.32 mm[61]. This design significantly enhances the mechanical stability of the TFTs by distributing the mechanical strain more evenly across the device. Another critical aspect of electrode architecture is the choice of materials. Metals such as molybdenum (Mo) are commonly used due to their high conductivity and good ohmic contact with the semiconductor layer. However, Mo electrodes are brittle under bending stress. To overcome this, Lee et al. incorporated micrometer-sized patterns to reduce crack formation. The study showed that by reducing the line width of Mo electrodes to 6 µm, no cracks were observed even after 4,000 bending cycles. This indicates that micrometer patterning of S/D/G electrodes can significantly improve the robustness of flexible TFTs [Figure 9A][61]. The active layer splitting in back-channel-etched (BCE) and etch-stopper (ES) TFTs on PI substrates offers another significant improvement in performance and stability. Lee et al. demonstrated that by splitting the active layer into smaller sections, the mobility of the TFTs could be enhanced significantly. The study exhibited that splitting the active layer into 2-4 μm widths along the channel led to increased saturation mobility from 24.3 to 76.8 cm2/V·s, with a subthreshold swing of less than 80 mV/dec. This improvement was attributed to the enhanced top interface quality due to the incorporation of fluorine (F) at the interface, which reduces defect density and improves electron mobility. The split active layer structure also demonstrated excellent mechanical stability after 5,000 bending cycles at a 1 mm radius, showing minimal changes in VTH and drain current (IDS). This remarkable mechanical stability is primarily due to the improved interface quality and reduced interface state density, making these devices highly suitable for flexible electronic applications [Figure 9B][67]. The incorporation of micro-hole arrays in thin metal films has been demonstrated as an effective method to mitigate mechanical stress. The micro-holes act as stress concentrators, localizing and controlling crack propagation, thereby preventing catastrophic failure of the electrodes. In the study by Lee et al. aluminum thin film electrodes with 3 μm diameter holes covering 25% of the area showed remarkable durability, maintaining less than a 3% change in resistance after 300,000 bending cycles. The electrical performance of a-IGZO TFTs with this micro-hole structure remained stable even after 10,000 bending cycles, indicating that the micro-hole arrays do not significantly degrade the electrical properties of the TFTs. The stress distribution simulations showed that without holes, the stress is uniformly distributed across the film, leading to random crack formation under bending. With holes, stress is localized around the hole edges, which can be controlled to prevent random propagation. This strategy enhances the durability of metal electrodes under mechanical strain, making it suitable for various flexible electronic applications. The a-IGZO TFTs with hole arrays not only exhibit improved mechanical stability but also maintain comparable electrical performance to standard TFTs, demonstrating the feasibility of micro-hole structures in practical devices [Figure 9C][62].
Figure 9. Flexible MO TFTs employing channel and electrode architecture for mechanical flexibility. (A) Transfer characteristics and optical images of conventional and metal mesh electrode and strip semiconductor a-IGZO TFTs after 5,000 bending cycles. Reproduced with permission[61]. Copyright 2017, Wiley-VCH; (B) Structures of BCE a-IGZO TFTs on flexible substrate with BCE and with BCE-SP, and of the etch-stopper a-IGZO TFTs with ES and ES-SP. The results of bending cycle for threshold voltage and drain current at VG = 10 V and VD = 20 V for the flexible IGZO TFTs employing SP with 4 μm unit width after 5,000 bending at a radius of 1 mm. Reproduced with permission[67]. Copyright 2017, Wiley-VCH; (C) Schematic of the flexible thin film transistor and electrode employing the hole structure. On current before and after bending. The results are compared according to the existence of holes, diameters, and percentages. Reproduced with permission[62]. Copyright 2017, American Chemical Society. MO: Metal oxide; TFTs: thin-film transistors; IGZO: indium gallium zinc oxide; BCE: back-channel-etched; SP: split active layers; ES: etch-stopper.
Both the micro-hole array and mesh structure approaches offer significant improvements in the mechanical robustness of flexible TFTs. The micro-hole arrays focus on localizing stress and controlling crack propagation, which is particularly effective for metal electrodes and oxide TFTs. On the other hand, the mesh structures provide an even distribution of stress across the device, enhancing overall mechanical stability. These strategies can be combined or further optimized to develop even more resilient flexible electronic devices. Lastly, we summarized properties of MO TFTs with device structures designed for mechanical flexibility in Table 7.
The summarization of MO TFTs employing device structures for mechanical flexibility
| Structure | Channel materials | Substrate | Buffer layer | Tmax (°C) | Bending radius (mm) | μFE (cm2·V-1·s-1) | SS (V/decade) | On/off ratio | Gate dielectric | Refs. | Year |
| Island | IGZO | TPU | Al2O3 | 200 | 1.5 | 10.9 | 0.33 | 108 | Al2O3 | [102] | 2021 |
| IGZO | PI | SiNx/SiO2 | 450 | 1 | 14 | - | 107 | SiO2 | [193] | 2016 | |
| IGZO | PI | Al2O3 | 150 | 0.125 | 6.06 | - | - | Al2O3 | [33] | 2020 | |
| ITZO | PI | Polymer | 300 | - | 30 | - | 108 | SiO2 | [199] | 2022 | |
| IGZO | PDMS | - | 200 | - | 6.1 | - | 107 | P(VDF-TrFE):PMMA | [124] | 2015 | |
| Junctionless | ITO | Paper | - | 70 | - | 2.3 | 0.11 | 105 | Chitosan | [66] | 2021 |
| ITO | Paper | - | - | - | - | 0.21 | 106 | SiO2 solid electrolyte | [64] | 2012 | |
| ITO | Paper | - | - | - | 12.8 | - | 107 | Chitosan | [194] | 2019 | |
| IZO | PET | - | - | 20 | 60 | 0.13 | 106 | SiO2 solid electrolyte | [65] | 2013 | |
| ITO | PI | - | 200 | 0.125 | 12.74 | 0.881 | 107 | Al2O3 | [198] | 2024 | |
| Electrode and channel architecture | IGZO | PI | SiO2/SiNx | 450 | 0.32 | 14.3 | 0.39 | - | SiO2/SiNx | [61] | 2017 |
| IGZO | PI | SiO2/SiNx | 450 | 1 | 76.8 | 0.077 | SiO2/SiNx | [67] | 2017 | ||
| IGZO | PI | - | 300 | 5 | 5 | 0.8 | 107 | - | [62] | 2020 |
Future research could explore the integration of these designs with advanced materials and fabrication techniques to further enhance the performance and durability of flexible electronics. Additionally, the impact of different hole shapes, sizes, and patterns on the mechanical and electrical properties of the devices can be investigated to fine-tune the design for specific applications. The goal is to develop flexible electronics that can withstand the rigors of real-world use while maintaining high performance.
APPLICATION
Flexible displays
Flexible MO TFTs have become essential for developing next-generation display technologies, including VR, AR, and flexible devices. The mechanical flexibility and superior electrical properties of MO semiconductors have opened new possibilities for flexible, foldable, and rollable display applications.
Recent advancements in high-performance flexible displays involve the integration of ITZO TFTs and air-stable inverted OLEDs (iOLEDs). For instance, ITZO TFTs fabricated on PI films exhibit high mobility (32.9 cm2·V-1·s-1) and stability. This technology enables high-resolution displays suitable for next-generation flexible applications by simplifying the fabrication process through the simultaneous formation of ITZO films as both the channel layer in TFTs and electron injection layers in iOLEDs [Figure 10A][200]. Additionally, a flexible green AMOLED display with a resolution of 320 × 3 × 240 pixels (80 ppi resolution) has been developed. The flexible AMOLED display is integrated on the flexible TFT panel on a PEN substrate. Each pixel driver circuit consists of two TFTs and a storage capacitor, providing an aperture ratio of 46%. This TFT consists of an Al:Nd gate electrode, an AlOx:Nd gate dielectric, an IGZO channel, ITO S/D electrodes, and an SU-8 passivation layer. The display achieves 250 cd/m2 in both flat and bent conditions, maintaining clear image quality without significant luminance degradation or visible defects even when bent to a 20 mm bending radius [Figure 10B][201].
Figure 10. Flexible display employing high-performance flexible MO TFTs. (A) Photograph of flexible display with ITZO TFTs and ITZO-based iOLED. Microscopic image of pixel employing flexible ITZO TFTs backplane with ITO cathode and ITZO electron injection layer of the iOLED. Reproduced with permission[200]. Copyright 2016, Society for Information Display; (B) Flexible bottom emission green AMOLED display on PEN substrate using flexible a-IGZO TFTs. Transfer characteristics of flexible IGZO TFTs before and after bending. Reproduced with permission[201]. Copyright 2014, The Royal Society of Chemistry. MO: Metal oxide; TFTs: thin-film transistors; ITZO: indium tin zinc oxide; iOLED: inverted organic light-emitting diodes; ITO: indium-tin-oxide; AMOLED: active-matrix organic light-emitting diode; PEN: polyethylene naphthalate.
Flexible circuits
Flexible MO TFTs are also crucial in developing various electronic circuits integrated into flexible devices. These devices have the ability to fabricate island structure and junctionless TFTs while maintaining high performance under mechanical stress. Island structure TFTs distribute mechanical stress evenly, preventing the concentration of mechanical stress in any one area. Junctionless TFTs eliminate source/drain-to-channel junctions, reducing the risk of performance degradation under mechanical stress.
Innovative research has led to the development of flexible circuits that withstand significant mechanical deformation. DGTFTs on paper substrates using solution-processed chitosan-based proton conductors as the gate dielectric have been developed. Employing a dual gate enables precise modulation of the threshold voltage and facilitates logic operations. DGTFTs exhibited stable performance and switching characteristics. For the logic function of junctionless DGTFTs, various voltage pulses were applied to the dual in-plane gates, serving as inputs, while the drain/source current (IDS) was used as the output. The voltages of -1 and
Figure 11. Flexible circuit employing flexible MO TFTs. (A) optical image of solution-processed IGZO TFTs employing island structure on a PI substrate. Oscillation frequency of flexible solution-processed IGZO TFTs ring oscillator as a function of supply voltage. Reproduced with permission[33]. Copyright 2020, Wiley-VCH; (B) The microarchitecture of UB-FVC inference stage. Microimage of the natively flexible processing engine implementing UB-FVC microarchitecture. Reproduced with permission[37]. Copyright 2020, Springer Nature; (C) The die layout of PlasticARM, denoting the key blocks in white boxes. The die micro image of PlasticARM, showing the dimensions of the die and core areas. Reproduced with permission[36]. Copyright 2020, Springer Nature; (D) Photo image of a quarter of a GEN1 (350 mm × 320 mm) plastic substrate and flexible touchscreen tag connected to a flexible battery on an iPhone Reproduced with permission[35]. Copyright 2020, Springer Nature. MO: Metal oxide; TFTs: thin-film transistors; IGZO: indium gallium zinc oxide; PI: polyimide; UB-FVC: univariate Bayes feature voting classifier.
CONCLUSION AND OUTLOOK
In summary, MO TFTs are at the forefront of the advancement of flexible display technologies, poised to play a critical role in the next generation of wearable and soft electronics. The inherent advantages of MO semiconductors, such as high carrier mobility, low processing temperatures, excellent electrical uniformity, transparency to visible light, and cost-effectiveness, make them particularly suitable for these applications. Their well-established and matured large-scale processes in the traditional display industry and compatibility with CMOS processes further underpin their potential for broader applications, including integrated circuits for wearable and soft electronics.
Recent research has made significant advances in the mechanical flexibility and electrical performance of MO TFTs, which are crucial for the development of ultra-high-resolution AR/VR displays and complex integrated circuits such as microprocessors. This review has identified the principal developments in materials, fabrication processes, and device architecture engineering methods that underpin these improvements. The functional layers of high-performance flexible MO TFTs, including gate dielectrics, single MO channel layers, and multiple MO channel layers, directly influence carrier transport and electrical properties. The incorporation of innovative materials and the utilization of multiple layers, including LiZnO, ZnON, and Al-ITZO, which demonstrate enhanced field-effect mobility compared to IGZO, have led to an improvement in the electrical performance of flexible MO TFTs. The incorporation of heterojunction layers and high-k dielectrics, which combine MOs and polymers, has further enhanced the electrical performance and mechanical stability of the devices. Moreover, doping processes, such as nitrogen and hydrogen doping, have been demonstrated to improve the electrical performance of flexible MO TFTs while maintaining compatibility with low-temperature fabrication methods, which are crucial for flexible substrates. Recent mechanical bending tests have demonstrated the robust functionality of high-performance flexible MO TFTs under various bending radii. This has been achieved with innovative structures, including island structures, junctionless structures, and advanced S/D electrode architectures. Island structures localize mechanical strain, preventing damage. Junctionless structures simplify device architecture and improve mechanical robustness. Advanced electrode designs ensure consistent electrical performance under significant mechanical deformation. The review also describes applications of high-performance flexible MO TFTs in flexible electronic systems, including displays and integrated circuits.
Despite these advances, several challenges remain to be addressed in future research to fully realize the potential of MO-TFT-based flexible displays and electronics:
1. Development of p-type MO semiconductors for CMOS implementation: To fully realize the potential applications of oxide TFT technology, it is imperative to develop high-performance p-type MO semiconductors. This advancement would enable the fabrication of CMOS circuits, allowing flexible electronics to benefit from increased scalability and integration density, reduced power dissipation, improved noise immunity, and versatile functionalities characteristic of modern advanced integrated circuits.
2. Downscaling and short-channel devices for high-performance integrated circuits: To meet the demands of high-performance and highly integrated circuits, future research should focus on downscaling MO TFTs and developing short-channel devices. Techniques such as self-alignment and double-gate structures can improve device performance and scalability. In addition, the development of low-cost, high-resolution patterning methods is essential to ensure that these advanced devices can be manufactured economically and with the precision required for low-cost, high-density integration.
3. Stretchable integrated systems beyond flexibility: Extending the current capabilities of flexible MO TFTs to stretchable systems will pave the way for more robust and versatile applications in wearables and soft electronics. This will require the development of materials and architectures that can withstand significant mechanical deformation while maintaining high performance.
4. Integration with emerging display technologies: Future research should explore the integration of flexible and stretchable MO-TFT backplanes with next-generation display technologies such as colloidal quantum dot light-emitting diodes (LEDs), perovskite LEDs, and colloidal quantum well LEDs. This would facilitate the development of advanced integrated systems for next-generation flexible displays.
By addressing these challenges, the potential of flexible MO TFTs can be fully realized, leading to significant advances in the field of flexible and stretchable displays and electronics. Looking ahead, continued innovation in MO TFT technology promises to unlock new opportunities in next-generation electronics. Leveraging the mature processes of MO TFTs offers a cost-effective approach that can potentially complement or even replace traditional silicon-based semiconductors. The lower processing temperature of oxide TFTs compared to silicon makes them compatible with BEOL processing for M3D integration on existing silicon CMOS chips. This compatibility allows for increased packing density of transistors and other components, enabling the development of more powerful and compact electronic devices within a smaller footprint. Consequently, the integration of MO TFTs into multifunctional and novel applications beyond display technologies has the potential to revolutionize the design and functionality of future electronic devices.
DECLARATIONS
Authors’ contributions
Initiated the reviewing idea and outlined the manuscript structure: Jeon, S.P., Jo, J.W., Kim, Y.H., Park, S.K.
Conducted the literature review and wrote the manuscript: Jeon, S.P., Jo, J.W., Nam, D., Kim, Y.H., Park, S.K.
Involved in the discussion and revised the manuscript: Jeon, S.P., Jo, J.W., Kim, Y.H., Park, S.K.
Supervision, review and editing, and project administration: Kim, Y.H., Park, S.K.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (P0023718, Inorganic Light-emtting Display Expert Training Program for Display Technology Transition), the KIAT grant funded by the Korea Government (MOTIE) (P0020967, Advanced Training Program for Smart Sensor Engineers), and the Technology Innovation Program (Development of application product in foam-based adhesive sheet with high impact resistance, 20022424) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
Conflicts of interest
Park, S.K. is the guest editor of the special issue, while the other authors have declared that they have no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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Jeon, S. P.; Jo, J. W.; Nam, D.; Kim, Y. H.; Park, S. K. High-performance metal oxide TFTs for flexible displays: materials, fabrication, architecture, and applications. Soft Sci. 2025, 5, 1. http://dx.doi.org/10.20517/ss.2024.35
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