Zinc Tin Oxide Thin-film Transistors Research Articles

Performance Improvement of ZnSnO Thin-Film Transistors with Low-Temperature Self-Combustion Reaction

Conventional sol-gel solutions have received significant attention in thin-film transistor (TFT) manufacturing because of their advantages such as simple processing, large-scale applicability, and low cost. However, conventional sol-gel processed zinc tin oxide (ZTO) TFTs have a thermal limitation in that they require high annealing temperatures of more than 500 °C, which are incompatible with most flexible plastic substrates. In this study, to overcome the thermal limitation of conventional sol-gel processed ZTO TFTs, we demonstrated a ZTO TFT that was fabricated at low annealing temperatures of 350 °C using self-combustion. The optimized device exhibited satisfactory performance, with μsat of 4.72 cm2/V∙s, Vth of −1.28 V, SS of 0.86 V/decade, and ION/OFF of 1.70 × 106 at a low annealing temperature of 350 °C for one hour. To compare a conventional sol-gel processed ZTO TFT with the optimized device, thermogravimetric and differential thermal analyses (TG-DTA) and X-ray photoelectron spectroscopy (XPS) were implemented.

Study of Self-Patterned Zinc Tin Oxide Thin-Film Transistors Prepared from Photocurable Precursor Solution with Photoacid Generator

Solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs) have great potential uses in next-generation wearable and flexible electronic products. Zinc and tin precursor materials are naturally abundant and have low fabrication costs. To integrate a single ZTO TFT into logic circuits including inverters, NAND, and NOR gates will require the development of a facile patterning process to replace conventional and complicated photolithography techniques which are usually time-consuming and toxic. In this study, self-patterned ZTO thin films were prepared using a photo-patternable precursor solution including a photoacid generator, (4-methylthiophenyl)methyl phenyl sulfonium triflate. Solution-processed ZTO precursor films fabricated with the photoacid generator were successfully micropatterned by UV exposure, and transitioned to a semiconducting ZTO thin film by heat treatment. The UV-irradiated precursor films became insoluble in developing solvent as the generated proton from the photoacid generator affected the metal-containing ligand and changed the solubility of the metal oxide precursors. The resulting ZTO thin films were utilized as the active layers of n-type TFTs, which exhibited a typical n-type transfer, and output characteristics with appropriate threshold voltage, on/off current ratio, and field-effect mobility. We believe that our work provides a convenient solution-based route to the fabrication of metal-oxide semiconductor patterns.

Electrical Properties of Amorphous Indium Zinc Tin Oxide Thin Film Transistor with Y₂O₃ Gate Dielectric.

High-k Y₂O₃ thin films were investigated as the gate dielectric for amorphous indium zinc tin oxide (IZTO) thin-film transistors (TFTs). Y₂O₃ gate dielectric was deposited by radio frequency magnetron sputtering (RF-MS) under various working pressures and annealing conditions. Amorphous IZTO TFTs with SiO₂ as the gate dielectric showed a high field-effect mobility (μFE) of 19.6 cm²/Vs, threshold voltage (Vth) of 0.75 V, on/off current ratio (Ion/Ioff) of 2.0×106, and subthreshold swing (SS) value of 1.01 V/dec. The IZTO TFT sample device fabricated with the Y₂O₃ gate dielectric showed an improved subthreshold swing value compared to that of the IZTO TFT device with SiO₂ gate dielectric. The IZTO TFT device using the Y₂O₃ gate dielectric deposited at a working pressure of 5 mtorr and annealed at 400 °C in 6 sccm O₂ for 1 hour showed a high μFE of 51.8 cm²/Vs, Vth of -0.26 V, Ion/Ioff of 6.0×10³, and SS value of 0.19 V/dec. With the application of a Y₂O₃ gate dielectric, the Vth shift improved under a positive bias stress (PBS) but was relatively unaffected by negative bias stress (NBS). These shifts were attributed to charge traps within the gate dielectric and/or interfaces between the channel and gate dielectric layer.

Formation of F-Doped Offset Region for Spray Pyrolyzed Self-Aligned Coplanar Amorphous Zinc–Tin–Oxide Thin-Film Transistor by NF₃ Plasma Treatment

We report the high performance of self-aligned (SA), amorphous zinc–tin–oxide (a-ZTO) coplanar thin-film transistor (TFT) by spray pyrolysis. The a-ZTO thin film deposited by spray at 350 °C has a low surface roughness (0.39 nm) and no bubble. The resistivity of a-ZTO could be decreased to $2\times 10^{-3} \Omega $ cm by NF3 plasma exposure and it was used for the offset regions of SA coplanar TFT. The coplanar a-ZTO TFT exhibits the field-effect mobility ( $\mu _{{\mathrm {FE}}}$ ) of 5.13 cm $^{2}\text{V}^{-1}\text{s}^{-1}$ , turn-on voltage ( ${V}_{ \mathrm{\scriptscriptstyle ON}}$ ) of ~0 V, the subthreshold swing of 139 mV/decade, and high On/Off-current ratio over $\sim 10^{7}$ . The SA coplanar a-ZTO TFT exhibited high stability with a threshold voltage shift ( $\Delta {V}_{{\mathrm {TH}}}$ ) of 0 V under the gate bias stress of ${V}_{{\mathrm {GS}}} = +20$ V for 1 h.

Self-Aligned Top-Gate Amorphous Zinc-Tin Oxide Thin-Film Transistor With Source/Drain Regions Doped by Al Reaction

A self-aligned fabrication process for top-gate amorphous zinc-tin oxide (a-ZTO) thin-film transistors (TFTs) is developed, in which the source/drain (S/D) doping is realized through depositing a thin aluminum (Al) film on S/D regions and performing a thermal annealing. Results indicate that a chemical oxidation-reduction reaction between Al and a-ZTO films takes place during the thermal annealing process, and shallow donors of oxygen vacancies and metal tin (Sn) interstitials are thus generated. The formed S/D regions have a high carrier concentration over 1 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm $^{-3}$ , low sheet resistance of 0.57 k Ω/sq, and high thermal stability even in oxygen ambient. The fabricated a-ZTO TFTs exhibit excellent electrical performances, including a low channel-width-normalized S/D resistance of about 7.05 Ω cm, a high field-effect mobility of 15.7 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, a high on/off current ratio of over $10 ^{8}$ , and near-zero turn-on voltage. Moreover, good electrical stability with less than 0.2-V threshold voltage shift under ±30-V gate bias stresses is also achieved.

Enhancing the Performance of Solution-Processed Thin-Film Transistors via Laser Scanning Annealing

In this article, laser scanning annealing is used for the fabrication of solution-processed tungsten–zinc–tin-oxide thin-film transistors (WZTO TFTs) with low temperatures and fast processing. The ...

Performance Enhancement of Channel-Engineered Al–Zn–Sn–O Thin-Film Transistors with Highly Conductive In–Ga–Zn–O Buried Layer via Vacuum Rapid Thermal Annealing

In this study, we propose, fabricate, and examine the electrical characteristics of high-performance channel-engineered amorphous aluminum-doped zinc tin oxide (a-AZTO) thin-film transistors (TFTs). Amorphous indium gallium zinc oxide (a-IGZO) film with improved conductivity (obtained via rapid thermal annealing in vacuum) is applied as the local conductive buried layer (LCBL) of the channel-engineered a-AZTO TFTs. The optical transmittance of the a-IGZO and a-AZTO films in the visible region is >85%. The a-IGZO LCBL reduces the resistance of the a-AZTO channel, thereby resulting in increased drain current and improved device performance. We find that our fabricated channel-engineered a-AZTO TFTs with LCBLs are superior to non-channel-engineered a-AZTO TFTs without LCBLs in terms of electrical properties such as the threshold voltage, mobility, subthreshold swing, and on-off current ratios. In particular, as the a-IGZO LCBL length at the bottom of the channel increases, the channel resistance gradually decreases, eventually resulting in a mobility of 22.8 cm²/V · s, subthreshold swing of 470 mV/dec, and on-off current ratio of 3.98×107. We also investigate the effect of the a-IGZO LCBL on the operational reliability of a-AZTO TFTs by measuring the variation in the threshold voltage for positive gate bias temperature stress (PBTS), negative gate bias temperature stress (NBTS), and negative gate bias temperature illumination stress (NBTIS). The results indicate that the TFT instability for temperature and light is not affected by the LCBL. Therefore, our proposed channel-engineered a-AZTO TFT can form a promising high-performance high-reliability switching device for next-generation displays.

Direct Patterned Zinc-Tin-Oxide for Solution-Processed Thin-Film Transistors and Complementary Inverter through Electrohydrodynamic Jet Printing.

The solution-processed deposition of metal-oxide semiconducting materials enables the fabrication of large-area and low-cost electronic devices by using printing technologies. Additionally, the simple patterning process of these types of materials become an important issue, as it can simplify the cost and process of fabricating electronics such as thin-film transistors (TFTs). In this study, using the electrohydrodynamic (EHD) jet printing technique, we fabricated directly patterned zinc-tin-oxide (ZTO) semiconductors as the active layers of TFTs. The straight lines of ZTO semiconductors were successfully drawn using a highly soluble and homogeneous solution that comprises zinc acrylate and tin-chloride precursors. Besides, we found the optimum condition for the fabrication of ZTO oxide layers by analyzing the thermal effect in processing. Using the optimized condition, the resulting devices exhibited satisfactory TFT characteristics with conventional electrodes and conducting materials. Furthermore, these metal-oxide TFTs were successfully applied to complementary inverter with conventional p-type organic semiconductor-based TFT, showing high quality of voltage transfer characteristics. Thus, these printed ZTO TFT results demonstrated that solution processable metal-oxide transistors are promising for the realization of a more sustainable and printable next-generation industrial technology.

Effects of P-Type SnOx Thin-Film Transistors with N₂ and O₂ Ambient Furnace Annealing.

Recently oxide-based thin-film transistors (TFTs) are investigated for emerging applications of the next generation display devices and other electronic circuits (Fortunato, E., et al., 2012. Oxide semiconductor thin-film transistors: A review of recent advances. Advanced Materials, 24, pp.2945-2986). Despite of the great success in n-type oxide semiconductors with high transparency and high field-effect mobility, high performance P-type oxide TFTs are so highly desired that complementary circuits can be realized with low power and high performance (Ou, W.C., et al., 2008. Anomalous P-channel amorphous oxide transistors based on tin oxide and their complementary circuits. Applied Physics Letters, 92, p.122113). There are some oxides such as SnO, CuO, Cu₂O and NiO are regarded as promising P-type semiconductor materials. In this investigation, tin oxide SnOx is fabricated to be active layer for TFTs device, and furnace annealing with several combinations of nitrogen and oxygen ambient is compared to enhance the electrical characteristics of P-type SnOx TFTs (Park, K.S., et al., 2009. High performance solution-processed and lithographically patterned zinc-tin oxide thin-film transistors with good operational stability. Electrochemical and Solid-State Lett., 12, pp.H256-H258). The results show that with N₂+O₂ ambient, 30 minutes furnace annealing, the P-type SnOx TFTs device shows better performance with mobility (μFE) 0.883 cm²/V · S, threshold voltage (VT) -4.63 V, subthreshold swing (SS) 1.15 V/decade, and Ion/Ioff ratio 1.01×10₃.

Rare Metal‐Free High‐Performance Zinc–Tin Oxide Thin Film Transistors Using Efficient Energy Conversion of Microwave Annealing

Herein, a rare metal‐free oxide‐based semiconductor channel layer is constructed with excellent electrical properties and stabilities by fabricating zinc–tin oxide (ZTO) thin film transistors (TFTs) using tin (Sn) as the dopant element in ZnO. Furthermore, to improve the performance and stability of ZTO TFTs, microwave annealing (MWA) powered by energy conversion from electromagnetic energy to heat energy rather than heat transfer is conducted as the post‐deposition annealing (PDA) process of the ZTO channel layer. As a result, the electrical performance of ZTO TFTs is improved by MWA despite the much shorter PDA time because of the efficient energy transfer compared with that of furnace‐used thermal annealing (FTA) in conventional furnaces. The ZTO TFTs subjected to MWA exhibit a high on/off current ratio of 1.7 × 107, a high field‐effect mobility of 20.6 cm2 V−1 s−1, a small hysteresis voltage of 0.9 V, and a low interface trap density of 1.8 × 1012 cm−2 eV−1. In addition, the instabilities due to the positive and negative gate bias stresses at various operating temperatures are evaluated, and the results show that the instability of the ZTO TFT is significantly reduced by MWA.

Achieving High Mobility and Excellent Stability in Amorphous In–Ga–Zn–Sn–O Thin-Film Transistors

This article reports the fabrication of high-performance amorphous indium gallium zinc tin oxide ( ${a}$ -IGZTO) thin-film transistors (TFTs) with superior bias stability. For comparison, amorphous indium gallium zinc oxide ( ${a}$ -IGZO) TFTs were also investigated to clarify the origin of the superior performance of IGZTO TFTs. It was found that the simultaneous heavy loading of In and Sn into the IGZTO system facilitated an effective mass densification, leading to a reduction in tail states and deep states. The fabricated ${a}$ -IGZTO TFTs exhibited a high electron mobility ( $\boldsymbol {\mu }_{\text {FE}}$ ) of 46.7 cm2/Vs, a subthreshold swing (SS) gate of 0.15 V/decade, and an ${I}_{\text {ON/OFF}}$ ratio $> {1} \times {10}^{{8}}$ . Furthermore, greater gate-bias stress stability was observed for the IGZTO TFTs compared with the IGZO TFTs.

Photoresponses of Zinc Tin Oxide Thin-Film Transistor.

In this study, the optical and electrical properties of a zinc tin oxide (ZTO) thin-film transistor (TFT) were investigated. The TFT was fabricated using ZTO as the active layer, which was deposited by a radio frequency magnetron sputtering system, to form an ultraviolet (UV) photodetector. The device has a threshold voltage of 0.48 V, field-effect mobility of 1.47 cm²/Vs in the saturation region, on/off drain current ratio of 2×106, and subthreshold swing of 0.45 V/decade in a dark environment. Moreover, as a UV photodetector, the device has a long photoresponse time, responsivity of 0.329 A/W, and rejection ratio of 3.19×10⁴ at a gate voltage of -15 V under illumination of wavelength 300 nm.

Effects of thermal annealing time and molar ratio of channel layers on solution-processed ZnO/SnO2 thin-film transistors

In solution-processed zinc tin oxide (ZTO) dual-active-layer (DAL) thin-film transistors (TFTs), a thermal limitation exists whereby the ZTO channel layer requires a high annealing temperature—above 470 °C—to achieve stable performance. This disadvantage has been overcome by applying ZnO/SnO2 channel structures and additional annealing methods. However, these methods are expensive due to equipment requirements. Therefore, we aimed at lowering the annealing temperature of solution-processed ZnO/SnO2 TFTs by varying the molar ratio and annealing conditions of the channel layers. The optimized TFTs were fabricated at an annealing temperature of 350 °C without employing additional annealing methods. The fabricated TFTs showed superior and more stable performance as compared to ZTO TFTs annealed at 350 °C. The electrical characteristics of the fabricated ZnO/SnO2 TFTs included a saturation mobility (µsat) of 3.04 cm2/V·s, an on-off-current ratio (ION/OFF) of 1.41 × 106, a threshold voltage (Vth) of 3.04 V, and a subthreshold swing (SS) of 1.49 V/dec. Based on X-ray photoelectron spectroscopy results, compared with the ZTO DAL channel layer, the ZnO/SnO2 channel layer showed an increased ratio of metal-oxygen bonds and a decreased ratio of metal-hydroxyl bonds in O 1s deconvolution peaks.

Solution-Processed Yttrium-Doped IZTO Semiconductors for High-Stability Thin Film Transistor Applications

In this article, solution-processed zinc-tin–oxide (ZTO), indium–zinc–tin–oxide (IZTO), and yttrium-doped indium–zinc–tin–oxide (IZTO:Y) thin film transistors (TFTs) were investigated. The results indicate that the field effect mobility of the ZTO TFTs is enhanced by indium doping, whereas leading to the degradation of stability. Y-doping IZTO TFTs were also studied aiming to improve the stability of IZTO TFTs. The IZTO:Y shows a relatively higher transmittance in the visible light region compared to the nondoped IZTO, which could be due to the decrease of shallow defect levels caused by oxygen vacancy in the bandgap. Based on the X-ray photoelectron spectroscopy analysis, the oxygen vacancy concentration of IZTO:Y thin films decreases dramatically from 36.5% to 18.5% with Y incorporation. Moreover, the stability of IZTO:Y TFTs under positive bias stress (PBS) is also significantly improved. The device with 5-mol% Y element shows a low threshold voltage shift of 0.7 V compared to 2.6 V of IZTO TFT. The improvement in stability and electrical properties of IZTO TFTs are attributed to the Y doping, which can suppress the oxygen vacancy defects. Therefore, the high stability and excellent electrical performance of Y-doped IZTO TFTs have shown great application potential in transparent devices.

Implementation of Self-Aligned Top-Gate Amorphous Zinc Tin Oxide Thin-Film Transistors

Self-aligned top-gate (SATG) amorphous zinc tin oxide thin-film transistors (a-ZTO TFTs) are fabricated for the first time. Ar plasma treatment forms the self-aligned source/drain (S/D) region and reduces the sheet resistance of the a-ZTO S/D region from a very high value over the measurable limit to around 2.5 $\text{k}\Omega $ /□. The annealing temperature of the a-ZTO film has a strong impact on the electrical performances of the fabricated TFTs. The N2O plasma treatment prior to gate insulator deposition remarkably enhances the TFT performances. The fabricated a-ZTO TFTs present a field-effect mobility of 12.1 ± 0.27 cm $^{\textbf {2}}/\textsf {V}\cdot \textsf {s}$ , a subthreshold swing of 0.3 ± 0.03 V/decade, and good electrical stress stability under both positive and negative biases. A low-cost SATG a-ZTO TFT technology is thus well demonstrated.

Effect of Recombination and Decreasing Low Current on Barrier Potential of Zinc Tin Oxide Thin-Film Transistors According to Annealing Condition

Effect of Recombination and Decreasing Low Current on Barrier Potential of Zinc Tin Oxide Thin-Film Transistors According to Annealing Condition

A Study of Solution-Processed Zinc–Tin-Oxide Semiconductors for Thin-Film Transistors

Thin-film transistors (TFTs) fabricated using solution-processed zinc–tin-oxide (ZTO) semiconductor thin films as the channel layers are proposed in this paper. Effect of the ZTO annealing temperature on the TFT performance is studied. Significant reduction of oxygen-vacancy and chlorine residue contents in the ZTO semiconductor can be obtained when the annealing temperatures are 500 °C–700 °C. The ZTO semiconductor with the annealing process at 600 °C in air can give an optimal ZTO TFT having a low leakage current of 10−12 A and a high ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio of 3 $\times \,\,10^{7}$ . The subthreshold swing is 0.39 V/decade corresponding to an interface trap density of $1.2\times \,\,10^{12}\,\,$ cm−2eV−1.

P‐23: Enhancement in the Mobility and the Stability of Solution‐Processed Zinc‐Tin Oxide Thin‐Film Transistors Using Alkali Metal Superoxide

We have studied how to improve the mobility and stability of solution processed zinc‐tin oxide thin‐film transistors (ZTO TFTs) simultaneously using multifunctional potassium superoxide precursor. Potassium cations in the potassium superoxide precursor acts as a shallow donor in the ZTO thin film to improve the carrier concentration (electron), which allows the potassium‐doped ZTO TFT to exhibit high mobility. Then, the anion of the precursor exists as a superoxide radical, and it showed the effect of reducing the oxygen vacancy in the process of forming the oxide thin film Consequently, potassium‐doped ZTO TFT using potassium superoxide precursor exhibited improved mobility and stability, showing an increase in the mobility from 5.11 to 8.36 cm2/Vs and a decrease in the threshold voltage shift from 4.65 to 3.36 V under a negative bias temperature illumination stress test conducted over 5,000 sec..

A study of variable range hopping conduction of a sol-gel ZnSnO thin film transistor using low temperature measurements

This paper investigates variable range hopping conduction of a zinc tin oxide (ZTO) thin-film transistor (TFT). The ZTO channel layer was fabricated using a sol-gel process. Existence of Zn2SnO4 and ZnSnO3 crystal phases in the ZTO film is observed in the X-ray diffraction pattern. Distinct lattice fringes with different orientations that are observed in the transmission electron microscopy image verify the polycrystalline structure of the ZTO layer. To study the hopping mechanism, electrical characteristics of the ZTO TFT were measured at low temperatures of 100–220 K. The activation energy (Ea) is 97 meV when the gate voltage is 0 V, and it exponentially decreases with increasing gate voltage. The Ea is 22 meV when the gate voltage increases to 30 V. A negative exponential relationship between the density of states and the Ea is found. Temperature and gate voltage dependence of the hopping distance and hopping energy are also explored.

Doping Effect of Indium on Zinc-tin Oxide Thin-Film Transistor Using Electrohydrodynamic Jet Spray Technology

The effect of indium doping on zinc-tin oxide thin-film transistor was investigated using electrohydrodynamic (EHD) jet spray technology. EHD jet spray is a new and unique drop-on-demand patterning technology for printed electronics. After optimizing process parameters, the EHD jet spraying conditions were determined to be a voltage of 4.5 kV, a syringe speed of 0.032 μm/s, spraying time of 10 s, and a substrate temperature of 50 ℃. Indium doping increased metal-oxide formation in the thin-film, as confirmed by XPS. In addition, improved TFT electrical properties were obtained compared with non-doped TFTs by using EHD jet spray. A 0.1 M In-doped ZTO TFT showed a mobility of 7.40 cm2/V s, a threshold voltage of -3.4 V, an on-to-off current ratio of 1.87 × 106, and a sub-threshold slope of 1.2 V/dec. Improved hysteresis behavior of the TFT was also achieved by indium doping. Key words: EHD jet, spray, ZTO TFT, Indium doping