Zinc Tin Oxide Thin-film Transistors Research Articles

Dual-Gate Operation and Configurable Logic From Solution Pattern-Based Zinc Tin Oxide Thin-Film Transistors

Dual-Gate Operation and Configurable Logic From Solution Pattern-Based Zinc Tin Oxide Thin-Film Transistors

Cation doping strategy for improved carrier mobility and stability in metal-oxide Heterojunction thin-film transistors

The heterojunction channel architecture has emerged as a viable solution to enhance the performance of metal-oxide thin-film transistors (TFTs), addressing the performance limitations of single-channel counterparts. However, carrier mobility enhancement through a channel thickness design often encounters significant challenges such as the negative threshold voltage (Vth) shift. In this study, we present a cation doping strategy, designed to regulate Vth shift while simultaneously boosting carrier mobility in zinc-tin-oxide (ZTO)-based heterojunction TFTs. A comprehensive investigation of ZTO-based semiconductors was conducted to explore the impact of cation doping on the energy band structure and to find an optimal heterojunction channel structure for high carrier mobility and stability. The resulting ZTO/Ti-doped ZTO (Ti:ZTO) heterojunction TFTs demonstrated a field-effect mobility of 39.7 cm2/Vs, surpassing the performance of ZTO TFTs (16.1 cm2/Vs), with a minimal change in the Vth. Furthermore, the ZTO/Ti:ZTO TFTs exhibited enhanced bias-stress stability compared to the ZTO TFTs. We attribute the improved mobility and stability to the electron accumulation near the oxide channel heterointerface facilitated by band bending and defect passivation effect arising from the Ti:ZTO back-channel layer, respectively.

A Study of Deposition/Annealing Cyclic Method to Enhance the Performance of Zinc-Tin Oxide Thin-Film Transistor by Ultrasonic Spray Pyrolysis Deposition

A Study of Deposition/Annealing Cyclic Method to Enhance the Performance of Zinc-Tin Oxide Thin-Film Transistor by Ultrasonic Spray Pyrolysis Deposition

Graded-Band-Gap Zinc-Tin Oxide Thin-Film Transistors with a Vertically Stacked Structure for Wavelength-Selective Photodetection.

Filter-free wavelength-selective photodetectors have garnered significant attention due to the growing demand for smart sensors, artificial intelligence, the Internet of Everything, and so forth. However, the challenges associated with large-scale preparation and compatibility with complementary metal-oxide-semiconductor (CMOS) technology limit their wide-ranging applications. In this work, we address the challenges by constructing vertically stacked graded-band-gap zinc-tin oxide (ZTO) thin-film transistors (TFTs) specifically designed for wavelength-selective photodetection. The ZTO thin films with various band gaps are fabricated via atomic layer deposition (ALD) by varying the ALD cycle ratios of zinc oxide (ZnO) and SnO2. The ZTO film with a small Sn ratio exhibits a decreased band gap, and the resultant TFT shows a degraded performance, which can be attributed to the Sn4+ dopant introducing a series of deep-state energy levels in the ZnO band gap. As the ratio of Sn increases further, the band gap of the ZTO also increases, and the mobility of the ZTO TFT increases up to 30 cm2/V s, with a positive shift of the threshold voltage. The photodetectors employing ZTO thin films with distinct band gaps show different spectral responsivities. Then, vertically stacked ZTO (S-ZTO) thin films, with gradient band gaps increasing from the bottom to the top, have been successfully deposited using consecutive ALD technology. The S-ZTO TFT shows decent performance with a mobility of 18.4 cm2/V s, a threshold voltage of 0.5 V, an on-off current ratio higher than 107, and excellent stability under ambient conditions. The resultant S-ZTO TFT also exhibits obviously distinct photoresponses to light at different wavelength ranges. Furthermore, a device array of S-ZTO TFTs demonstrates color imaging by precisely reconstructing patterned illuminations with different wavelengths. Therefore, this work provides CMOS-compatible and structure-compact wavelength-selective photodetectors for advanced and integrable optoelectronic applications.

Performances and preparation of zinc-tin oxide thin-film transistors

Performances and preparation of zinc-tin oxide thin-film transistors

Spinel Single Phase Indium Zinc Tin Oxide Thin-Film Transistors with High Mobility and Excellent Uniformity

Spinel Single Phase Indium Zinc Tin Oxide Thin-Film Transistors with High Mobility and Excellent Uniformity

Improving the Electrical Properties of Zinc-Tin Oxide Thin-Film Transistors by Additive using Electrohydrodynamic Jet Technology

Oxide thin-film transistors (TFTs) are important semiconductor materials for display backplanes. To fabricate flexible displays, not only display modes but also TFTs are important. Flexible TFTs are especially needed for flexible displays, and related processes need to be developed. Printing is a good choice for the new fabrication of oxide TFTs. Electrohydrodynamic (EHD) jet printing is an excellent alternative for making flexible TFTs. To improve the electrical properties of oxide TFTs using EHD jet printing, propylene monomethyl ether acetate (PGMEA) was added to a zinc-tin oxide (ZTO) formulation. EHD jet printing was performed by Taylor cone jet mode with parameters of 2.4 kV and 0.064 µL/s to obtain uniform thin films at a substrate temperature of 50oC. Much improved TFT properties were obtained, including a mobility of 7.11 cm2/V s, on-to-off current ratio of 2.8 × 106 and subthreshold slope of 1.44 V/dec for ZTO TFT with 5 wt% of PGMEA, and a mobility of 1.43 cm2/V s, on-to-off current ratio of 2.7 × 105 and subthreshold slope of 1.32 V/dec for the ZTO TFT. Almost no hysteresis behavior was observed in the oxide TFTs with added PGMEA. We report a new way to improve the electrical properties of oxide TFTs, by the simple addition of PGMEA.

Effects of Ti Doping on the Electrical Properties and Gate-Bias Stability of Amorphous Zinc–Tin–Oxide Thin-Film Transistors

Multicationic oxide semiconductors are receiving considerable interest in electronic and optoelectronic devices owing to their tunability of physical properties by the cation compositions. Here, we investigated the effects of Ti doping on the electrical properties and gate-bias stability of amorphous zinc–tin–oxide (a-ZnSnO, Zn/Sn = 7:3) thin-film transistors (TFTs) using a cosputtering process. Particularly, by using cosputtering, controllable doping of Ti in a-ZnSnO films was possible in the range of 0.87–3.87 atom %. From various electrical analyses, it was found that the key metrics of Ti-doped ZnSnO (TiZnSnO) TFTs, such as field-effect mobility and gate-bias stability, were highly dependent on the Ti concentration, showing a mobility–stability tradeoff. Based on X-ray photoelectron spectroscopy analysis, the mobility–stability tradeoff is ascribed to the suppression of oxygen vacancy formation by Ti doping. Considering the overall electrical performance and stability of TiZnSnO TFTs, which were processed at 450 °C, the optimal Ti concentration was determined as ∼1.14 atom % with Zn, Sn, and O concentrations of 15.87, 22.92, and 60.07 atom %, respectively. The device exhibited a field-effect mobility of 8.2 cm2 V–1 s–1, a subthreshold slope of 0.208 V decade–1, an on/off ratio of 3.38 × 108, a hysteresis of 2.8 V, and a threshold voltage shift of +5.61 and −2.24 V under positive- and negative-bias stresses, respectively.

Improvement in Switching Characteristics and Bias Stability of Solution-Processed Zinc-Tin Oxide Thin Film Transistors via Simple Low-Pressure Thermal Annealing Treatment.

In this study, we used a low-pressure thermal annealing (LPTA) treatment to improve the switching characteristics and bias stability of zinc-tin oxide (ZTO) thin film transistors (TFTs). For this, we first fabricated the TFT and then applied the LPTA treatment at temperatures of 80 °C and 140 °C. The LPTA treatment reduced the number of defects in the bulk and interface of the ZTO TFTs. In addition, the changes in the water contact angle on the ZTO TFT surface indicated that the LPTA treatment reduced the surface defects. Hydrophobicity suppressed the off-current and instability under negative bias stress because of the limited absorption of moisture on the oxide surface. Moreover, the ratio of metal-oxygen bonds increased, while the ratio of oxygen-hydrogen bonds decreased. The reduced action of hydrogen as a shallow donor induced improvements in the on/off ratio (from 5.5 × 103 to 1.1 × 107) and subthreshold swing (8.63 to V·dec-1 and 0.73 V·dec-1), producing ZTO TFTs with excellent switching characteristics. In addition, device-to-device uniformity was significantly improved because of the reduced defects in the LPTA-treated ZTO TFTs.

Analogy of Photogating to Voltage-Gating in Zinc-Tin Oxide Thin-Film Transistor: Efficiency and Current Saturation Mechanism

In contrast to the conventional voltage-gated channels of thin-film transistors (TFTs), the photo-induced gating effect provides a promising approach to control the carrier concentration in the channels and benefits the advanced application in a remote operation mode. In this work, the zinc-tin oxide TFT (ZTO TFT) is revealed the feasibility of being photo-gated at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {G}} =0$ </tex-math></inline-formula> V to obtain a series of output ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {D}}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {D}}{)}$ </tex-math></inline-formula> characteristics. The current saturation in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {D}}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {D}}$ </tex-math></inline-formula> curves of the photo-gated ZTO TFT confirms the photo-induced pinch-off region in the ZTO channel, suggesting the presence of positive pseudogate voltage at the channel under lighting. The photo-gated output characteristic at a given <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {D}}$ </tex-math></inline-formula> is converted to the log <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {D}}$ </tex-math></inline-formula> verse optical power density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho {)}$ </tex-math></inline-formula> curve, showing a rapid switching of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {D}}$ </tex-math></inline-formula> by 405-nm light illumination with the reciprocal log ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {D}}{)}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho $ </tex-math></inline-formula> slope of 0.04 (mW/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{2}}{)}$ </tex-math></inline-formula> /decade. Parallel output characteristics triggered electrically with various gate voltages are also performed to obtain an equivalent drain current. The correlation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {G}}$ </tex-math></inline-formula> versus <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho $ </tex-math></inline-formula> for equivalent drain current reveals that photogating efficiency is 7.93 V/(mW/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{2}}{)}$ </tex-math></inline-formula> . The photogating mechanism and efficiency are discussed based on the photoionization of the neutral oxygen vacancies to positively charged oxygen vacancies, which are responsible for photo-induced gate voltage (VG,ph).

Threshold voltage tuning of IGZTO thin-film transistors deposited by RF sputtering for high-resolution flexible displays using deep ultraviolet light

Development of DUV/Thermal dual treatment (DTT) process to improve the electrical performance and stability of oxide thin film transistors while efficiently controlling the threshold voltage of the device.

Structural and electronic properties with respect to Si doping in oxygen rich ZnSnO amorphous oxide semiconductor

The effect of Si doping in the presence of excess oxygen on the zinc-tin-oxide (ZTO) thin-film transistor is investigated. The threshold voltage increases from 3.86 to 10.8 V, and the ON current reduces from 4 × 10 -4 to 1.1 × 10 -4 A for the undoped and 1 wt% Si-doped ZTO transistor, respectively. The changes in the threshold voltage and the ON current are attributed to the suppression of free charge carriers due to excess oxygen in the film. The annihilation of oxygen vacancy-related defects states is substantiated by the density of states extracted from the photonic capacitance-voltage characteristic of the undoped and Si-doped ZTO thin-film transistors. Moreover, due to the high binding energy of Si and oxygen, excess oxygen is introduced into the film. The excessive oxygen forms the O-O ex dimer that acts as the electron trap and degrades the device's stability. From the bias stress measurement and the density of states extraction, it is concluded that 0.5 wt% doping of Si in ZTO is optimal for moderate device performance, less defects density, and high stability.

A Complementary Logic-in-Memory Inverter From Organic-Inorganic Hybrid Transistors

Logic-in-memory (LIM) technology is an important technology that overcomes the bottleneck, a limitation of the von Neumann architecture. As described in previous reports, the LIM technology was fabricated with a resistive load inverter structure. However, there are some limitations due to the high-power consumption issue and low noise margin. As a result, the need for a LIM device with a complementary structure has emerged. Here, we propose a LIM architecture-based organic-inorganic complementary inverter circuit with a stable noise margin using a p-type floating-gate transistor memory (FGTM) and an n-type zinc-tin oxide (ZTO) thin-film transistor (TFT). We demonstrate that the output of the proposed complementary inverter circuit is determined according to the state of the FGTM memory.

Physico-Chemical Origins of Electrical Characteristics and Instabilities in Solution-Processed ZnSnO Thin-Film Transistors

We investigate the physico-chemical origins that determine the transistor characteristics and stabilities in sol-gel processed zinc tin oxide (ZTO) thin-film transistors (TFTs). ZTO solutions with Sn/(Sn+Zn) molar ratios from 0.3 to 0.6 were synthesized to demonstrate the underlying mechanism of the electrical characteristics and bias-induced instabilities. As the Sn/(Sn+Zn) ratio of ZTO is increased, the threshold voltage of the ZTO TFTs negatively shifts owing to the gradual increase in the ratio of oxygen vacancies. The ZTO TFTs with an Sn/(Sn+Zn) ratio of 0.4 exhibit highest saturation mobility of 1.56 cm2/Vs lowest subthreshold swing and hysteresis of 0.44 V/dec and 0.29 V, respectively, due to the desirable atomic states of ZTO thin film. Furthermore, these also exhibit outstanding positive bias stability due to the low trap density at the semiconductor-dielectric interface. On the other hand, the negative bias stress-induced instability gradually increases as the proportion of tin increases because the negative bias stress instability originates from the ionization of oxygen vacancies. These results will contribute to the optimization of the composition ratio in rare-metal-free oxide semiconductors for next-generation low-cost electronics.

Importance of Solvent Evaporation Temperature in Pre-Annealing Stage for Solution-Processed Zinc Tin Oxide Thin-Film Transistors

We focused on the importance of solvent evaporation governed by the temperature of the pre-annealing stage (TS) in solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs). We controlled TS based on the boiling point (BP) of the solvent used. When TS reaches the BP, the field effect mobility is found to be about 1.03 cm2/V s, which is 10 times larger than the TS &lt; Bp case (0.13 cm2/V s). The reason is presumed to be that residual organic defects are effectively removed as TS increases. In addition, when Ts is beyond Bp, the mobility is rather decreased due to structural defects such as pores and pinholes. Based on our results, it is noted that TS plays a significant role in the enhancement of electrical performance and stability of solution-processed ZTO TFTs.

Importance of Structural Relaxation on the Electrical Characteristics and Bias Stability of Solution-Processed ZnSnO Thin-Film Transistors.

Effect of structural relaxation (SR) on the electrical characteristics and bias stability of solution-processed zinc–tin oxide (ZTO) thin-film transistors (TFTs) were systematically investigated by controlling the annealing time of the ZTO semiconductor films. Note that SR was found to increase with increased annealing time. Due to the increased SR, the ratio of oxygen vacancies (VO) increased from 21.5% to 38.2%. According to increased VO, the mobility in the saturation region was exhibited by a sixfold increase from 0.38 to 2.41 cm2 V−1 s−1. In addition, we found that the threshold voltage negatively shifted from 3.08 to −0.95 V. Regarding the issue of bias stability, according to increased SR, positive-bias stress of the ZTO TFTs was enhanced, compared with reverse features of negative-bias stress. Our understanding is expected to provide a basic way to improve the electrical characteristics and bias stability of rare-metal-free oxide semiconductor TFTs, which have not been sufficiently studied.

Synaptic Emulation via Ferroelectric P(VDF-TrFE) Reinforced Charge Trapping/Detrapping in Zinc–Tin Oxide Transistor

Brain inspired artificial synapses are highly desirable for neuromorphic computing and are an alternative to a conventional computing system. Here, we report a simple and cost-effective ferroelectric capacitively coupled zinc-tin oxide (ZTO) thin-film transistor (TFT) topped with ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) for artificial synaptic devices. Ferroelectric dipoles enhance the charge trapping/detrapping effect in ZTO TFT, as confirmed by the transfer curve (ID-VG) analysis. This substantiates superior artificial synapse responses in ferroelectric-coupled ZTO TFT because the current potentiation and depression are individually improved. The ferroelectric-coupled ZTO TFT successfully emulates the essential features of the artificial synapse, including pair-pulsed facilitation (PPF) and potentiation/depression (P/D) characteristics. In addition, the device also mimics the memory consolidation behavior through intensified stimulation. This work demonstrates that the ferroelectric-coupled ZTO synaptic transistor possesses great potential as a hardware candidate for neuromorphic computing.

Effects of change of oxygen vacancy on hysteresis voltage and stability under time-temperature dependence positive bias stress in amorphous SZTO transistors

The study on voltage hysteresis (VHys) and instability under the time dependence positive bias stress at a fixed temperature (PBTS) in amorphous silicon zinc tin oxide (a-SZTO) thin-film transistors (TFTs) has been investigated by varying the oxygen flow rate (OFR) during the a-SZTO channel material growth via radio frequency (RF) sputtering. Current-voltage (I-V) measurement was initially taken to study the VHys, which revealed that the VHys was widened by increasing the OFR. The threshold voltage (Vth) shift in a-SZTO TFTs and the impact of OFR on electrical properties and electrical stability of a-SZTO TFTs were analyzed by the PBTS measurement. From PBTS, we observed that the threshold voltage shift difference (∆Vth) increases as the OFR increases. We conclude that a plausible reason for these results may be due to the acceptor-like trap states induced by OFR. The density of Gaussian-like states, excess or weakly bonded oxygen acting as acceptor-like states, is increased by increasing OFR during the deposition of channel material. These results are expected to be of general importance for future works of other amorphous oxide based TFTs.

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.