Zinc Tin Oxide Thin-film Transistors Research Articles

Light Gated Zinc Tin Oxide Thin Film Transistor Fabricated Via Solution Process

Zinc tin oxide (ZTO) is a transparent semiconductor based on earth-abundant elements. While most oxide semiconductor films were previously fabricated by vacuum-based deposition techniques, solution processes are increasingly popular due the lower cost and feasibility for large area deposition. In this work, ZTO films prepared by a solution route have been applied as the active semiconductor layer in a thin film transistor (TFT) with SiO2 or Al2O3 dielectrics. With thickness less than 10 nm, the ZTO film exhibits a good field-effect mobility of ~10 cm2/Vs, small subthreshold slope of ~0.5 V/decade and high on/off current ratio of ~107. With light illumination of 405 nm wavelength and power density of 5 mW/cm2, the light sensitivity is in the order of 106 for the ID-off region and is less than 10 for the ID-on region. The light responsibility of ZTO TFT reaches around 2.5 and the maximum external quantum efficiency is 8. Furthermore, to see the light gating effect, the ZTO TFT is operated without applying gate voltage but under light illumination (405 nm wavelength) of various power densities. At a drain voltage of 0.5 V, the drain current increases sharply from 0.3 μA to 12.1 μA when the light power density increases from 0.03 mW/cm2 to 0.25 mW/cm2. The increment of drain current becomes gradual for power intensity greater than 0.25 mW/cm2 (from 13.4 μA to 16.3 μA for light power density from 0.5 mW/cm2 to 5 mW/cm2). As a result, the solution-processed ZTO TFT can be gated with light illumination, in addition to the voltage gating mode. This finding will broaden the suitability of ZTO TFT in optoelectronic and optical communication applications.

Suppression in the Negative Bias Illumination Instability of ZnSnO Thin-Film Transistors Using Hafnium Doping by Dual-Target Magnetron Cosputtering System

In this paper, the highly improved negative bias illumination stress (NBIS) stability of zinc-tin-oxide (ZTO) thin-film transistors (TFTs) is achieved by Hf doping, using dual-target magnetron cosputtering system. Compared with a large negative threshold voltage shift ( $\vartriangle V_{T})$ of 9 V in pristine ZTO TFT, the Hf-doped ZTO TFTs shows superior stability and device D only shows 1.9 V negative $\vartriangle V_{T}$ under the same NBIS. The enhancement in NBIS stability of Hf-doped ZTO TFTs is attributed to a lower oxygen vacancy concentrations and a fewer interface trap states suppressed by Hf ion. The decrease of oxygen vacancy concentrations and interface trap states are confirmed by X-ray photoelectron spectroscopy measurement and capacitance voltage ( $C$ – $V$ ) measurement, respectively. It is consistent with trap density extracted from temperature-dependent field-effect measurements, which verifies further the factor of the improvement of in NBIS stability of Hf-doped ZTO TFTs.

Full-solution-processed high mobility zinc-tin-oxide thin-film-transistors

The full solution-processed oxide thin-film-transistors (TFTs) have the advantages of transparency, ease of large-area fabrication, and low cost, offering great potential applications in switching and driving fields, and attracting extensive research interest. However, the performance of the solution-processed TFTs is generally lower than that of the vacuum-deposited ones. In this article, the full-solution processed TFTs with zinc-tin-oxide (ZTO) semiconductor and aluminium (Al2O3) dielectrics were fabricated, and their mobilities in the saturation region are high. Besides, the effect of the Al2O3 dielectrics’ preparation technology on ZTO TFTs’ performance was studied. Comparing the ZTO TFTs using the spin-coated Al2O3 dielectrics of 1–4 layers, the ZTO TFT with 3-layer Al2O3 dielectrics achieved the optimal performance as its field-effect carrier mobility in the saturation region is 112 cm2/V s, its threshold voltage is 2.4 V, and its on-to-off current ratio is 2.8×105. This is also the highest reported carrier mobility of the solution-processed ZTO TFTs.

Letter : Solution-processed flexible zinc-tin oxide thin-film transistors on ultra-thin polyimide substrates

In this letter, solution-processed flexible zinc-tin oxide (Z0.35T0.65O1.7) thin-film transistors with electrochemically oxidized gate insulators (AlOx:Nd) fabricated on ultra-thin (30 µm) polyimide substrates are presented. The AlOx:Nd insulators exhibited wonderful stability under bending and excellent insulating properties with low leakage current, high dielectric constant, and high breakdown field. The device exhibited a mobility of 3.9 cm2/V · s after annealing at 300 °C. In addition, the flexible device was able to maintain the electricity performance under various degrees of bending, which was attributed to the ultra-thin polyimide substrate.

Improvement of transistor characteristics and stability for solution-processed ultra-thin high-valence niobium doped zinc-tin oxide thin film transistors

Nb-doped Zinc tin oxide (NZTO) channel materials have been prepared by solution process in combination with the spin-coating method. All NZTO thin film transistors (TFTs) are n-type enhancement-mode devices, either without or with Nb additives. High-valence niobium ion (ionic charge = +5) has a larger ionic potential and similar ionic radius to Zn2+ and Sn4+ ions. As compared with the pure ZTO device, introducing Nb5+ ions into the ZTO channel layers can improve the electrical properties and bias stability of TFTs because of the reduction of the oxygen vacancies. This study discusses the connection among the material properties of the NZTO films and the electrical performance and bias stability of NZTO TFTs and how they are influenced by the Nb/(Nb + Sn) molar ratios of NZTO films.

Improvement in performance of solution-processed indium–zinc–tin oxide thin-film transistors by UV/O3 treatment on zirconium oxide gate insulator

We studied solution-processed amorphous indium–zinc–tin oxide (a-IZTO) thin-film transistors (TFTs) with spin-coated zirconium oxide (ZrOx) as the gate insulator. The ZrOx gate insulator was used without and with UV/O3 treatment. The TFTs with an untreated ZrOx gate dielectric showed a saturation mobility (μsat) of 0.91 ± 0.29 cm2 V−1 s−1, a threshold voltage (Vth) of 0.28 ± 0.36 V, a subthreshold swing (SS) of 199 ± 37.17 mV/dec, and a current ratio (ION/IOFF) of ∼107. The TFTs with a UV/O3-treated ZrOx gate insulator exhibited μsat of 2.65 ± 0.43 cm2 V−1 s−1, Vth of 0.44 ± 0.35 V, SS of 133 ± 24.81 mV/dec, and ION/IOFF of ∼108. Hysteresis was 0.32 V in the untreated TFTs and was eliminated by UV/O3 treatment. Also, the leakage current decreased significantly when the IZTO TFT was coated onto a UV/O3-treated ZrOx gate insulator.

Stability study of solution-processed zinc tin oxide thin-film transistors

In this study, the environmental dependence of the electrical stability of solution-processed n-channel zinc tin oxide (ZTO) thin-film transistors (TFTs) is reported. Under a prolonged negative gate bias stress, a negative shift in threshold voltage occurs in atmospheric air, whereas a negligible positive shift in threshold voltage occurs under vacuum. In the positive bias-stress experiments, a positive shift in threshold voltage was invariably observed both in atmospheric air and under vacuum. In this study, the negative gate-bias-stress-induced instability in atmospheric air is explained through an internal potential in the ZTO semiconductor, which can be generated owing to the interplay between H2O molecules and majority carrier electrons at the surface of the ZTO film. The positive bias-stress-induced instability is ascribed to electron-trapping phenomenon in and around the TFT channel region, which can be further augmented in the presence of air O2 molecules. These results suggest that the interaction between majority carriers and air molecules will have crucial implications for a reliable operation of solution-processed ZTO TFTs.

Effects of Double Active Layer and Acetic Acid Stabilizer on the Electrical Properties of a Solution-Processed Zinc Tin Oxide Thin-Film Transistor.

We investigated the effects of a double active layer (DAL) and acetic acid stabilizer on zinc tin oxide (ZTO) thin-film transistors (TFTs) fabricated using a solution process. The DAL was composed of two layers created by a ZTO solution doped with the same or different percentiles of an atomic Sn concentration (30 at.%, 60 at.%). The electrical performance of the ZTO TFTs significantly was improved after we added acetic acid (AA) instead of monoethanolamine (MEA). This was accomplished by applying a type 2 DAL (bottom layer: Sn 60 at.%, top layer: Sn 30 at.%, 60/30) instead of other types (30/30 or 60/60). It was demonstrated that AA plays a role in lowering the decomposition temperature, enhancing the metal-oxygen bridge, and decreasing hydroxyl groups in the film. In addition, the type 2 DAL structure (60/30) lowered the Ioff of the ZTO TFT and controlled the carrier concentration in the channel. The best performances were obtained at a Sn concentration of 60 at.% in the bottom ZTO layer and 30 at.% in the top ZTO layer, with AA added as a stabilizer. The ZTO TFT exhibited an on/off ratio of 1.1 x 10(9), a saturation mobility of 5.04 cm2/V·s, a subthreshold slope of 0.11 V/decade, and a threshold voltage of 1.6 V.

Ambient Constancy of Passivation-Free Ultra-Thin Zinc Tin Oxide Thin Film Transistor

An ultra-thin (5 nm-thick), unpassivated zinc tin oxide (ZTO) thin-film transistor TFT, fabricated with solution process, exhibits a good field-effect mobility (13 ∼ 14 cm2/Vs), small subthreshold swing (∼0.30 V/dec.) and high on/off current ratio (∼108). The field-effect mobility can be further enhanced by increasing the ZTO thickness to 12 nm and 22 nm. Furthermore, ID-VG characteristics of the 5 nm-thick ZTO TFT remain unaffected, regardless of working in air (60% relative humidity), vacuum or dry O2 atmosphere. The dissimilar TFT characteristics are discussed in terms of oxygen deficiency content, as well as the Fermi level position (EF to EC) for ZTO of various thicknesses to explain the moisture immunity of the 5 nm-thick solution-processed ZTO TFT. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0051512ssl] All rights reserved.

Towards environmental friendly solution-based ZTO/AlOx TFTs

Solution based deposition has been recently considered as a viable option for low-cost flexible electronics. In this context research efforts have been increasingly centred on the development of suitable solution-processed materials for oxide based transistors. Nevertheless, the majority of synthetic routes reported require the use of toxic organic solvents. In this work we report on a new environmental friendly solution combustion synthesis route, using ethanol as solvent, for the preparation of indium/gallium free amorphous zinc-tin oxide (ZTO) thin film transistors (TFTs) including AlOx gate dielectric. The decomposition of ZTO and AlOx precursor solutions, electrical characterization and stability of solution processed ZTO/AlOx TFTs under gate-bias stress, in both air and vacuum atmosphere, were investigated. The devices demonstrated low hysteresis (ΔV = 0.23 V), close to zero turn on voltage, low threshold voltage (VT = 0.36 V) and a saturation mobility of 0.8 cm2 V−1 s−1 at low operation voltages. Ethanol based ZTO/AlOx TFTs are a promising alternative for applications in disposable, low cost and environmental friendly electronics.

High-Performance Solution-Processed ZnSnO TFTs with Tunable Threshold Voltages

In this work, we had investigated the solution-processed zinc-tin oxide (ZTO) semiconductors using metal chloride precursors and the sol-gel method. We studied the effects of different processing conditions, such as different procedures of spin-coating and annealing, on thin-film transistor (TFT) characteristics (e.g., mobility, threshold voltages etc.). High-performance In-free solution-processed ZTO TFTs with mobilities up to 10–17 cm2/V⋅s and threshold voltages being tunable from positive to negative (i.e. from enhancement mode to depletion mode) had been achieved. The capability to control operation modes of TFTs provides the freedom to implement higher-performance TFT circuits (e.g. high-gain inverters demonstrated) using only solution-processed n-type ZTO TFTs. Using these two types of ZTO TFTs, depletion-load inverters were implemented, demonstrating enhanced characteristics in inverter characteristics compared to the enhancement-load inverters.

Improved device performance of solution-processed zinc–tin–oxide thin film transistor effects using graphene/Al electrode

We report on improved device performances of solution-processed zinc tin oxide (ZTO) thin film transistors (TFTs) using a graphene interlayer between an Al electrode and ZTO. The ZTO TFTs with a high-temperature annealing process at 600 °C show improved output characteristics compared to ones with annealing at relatively lower temperatures, which can be attributed to the reduced impurity and condensed surface state. The ZTO TFTs using the Ohmic contact of the graphene/Al electrode (G-ZTO TFTs) with an optimized annealing process exhibited a good saturation mobility of (6.96 cm2 V·s)−1, a subthreshold slope of 1.52 V dec−1 and a threshold voltage shift toward a negative bias direction. The improvement of device performances of G-ZTO TFTs can be contributed to good Ohmic contact formation by using graphene inserted between the ZTO active layer and the Al electrode.

Inkjet-printed zinc-tin-oxide TFTs with a solution-processed hybrid dielectric layer

Sol-gel TiO2 was synthesized and used as a gate dielectric for oxide thin-film transistors (TFTs). A hybrid gate insulator composed of sol-gel TiO2/thermally-grown SiO2 was applied to the inkjet-printed zinc-tin oxide (ZTO) TFTs for the first time. The electrical properties of an inkjet-printed ZTO TFT with a hybrid gate insulator show a mobility of 0.17 cm2/Vs, an on-to-off current ratio of 5 × 104, a subthreshold slope of 0.8 V/dec, and a threshold voltage of 0.6 V. The hybrid gate insulator for the inkjet-printed ZTO TFT shows a much improved operating voltage and subthreshold slope and a lower mobility compared to the SiO2 gate insulator.

Enhanced mobility of solution-processed polycrystalline zinc tin oxide thin-film transistors via direct incorporation of water into precursor solution

Phase transition and the consequent variation in crystalline orientation of metal oxides have profound impact on their transport properties. In this work, we report a simple method to enhance field-effect mobility of solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs) via direct incorporation of water into precursor solution. It is confirmed H2O molecules could effectively facilitate the conversion and alloying processes during ZTO film formation, characterized by the enhancement of spinel Zn2SnO4 phase and the reduction of cassiterite SnO2 phase. The preferred orientation of metal oxide crystallites varies according to the amount of water added into precursor solutions. Smooth and densely packed polycrystalline ZTO films with only a few organic residuals and moderate oxygen defects are fabricated from water-containing precursor solutions. With the incorporation of 1.67 M H2O, the extracted field-effect mobility of TFT devices could be improved by a factor of 2.3, from 0.92 to 2.11 cm2 V−1 s−1. This work offers a facile and cost-effective route towards high-mobility TFTs based on solution-processed polycrystalline metal oxide thin films.

Impact of the cation composition on the electrical performance of solution-processed zinc tin oxide thin-film transistors.

This study examined the structural, chemical, and electrical properties of solution-processed (Zn,Sn)O3 (ZTO) films with various Sn/[Zn+Sn] ratios for potential applications to large-area flat panel displays. ZTO films with a Zn-rich composition had a polycrystalline wurtzite structure. On the other hand, the Sn-rich ZTO films exhibited a rutile structure, where the Zn atom was speculated to replace the Sn site, thereby acting as an acceptor. In the intermediate composition regions (Sn/[Zn+Sn] ratio from 0.28 to 0.48), the ZTO films had an amorphous structure, even after annealing at 450 °C. The electrical transport properties and photobias stability of ZTO thin film transistors (TFTs) were also examined according to the Sn/[Zn+Sn] ratio. The optimal transport property of ZTO TFT was observed for the device with an amorphous structure at a Sn/[Zn+Sn] ratio of 0.48. The mobility, threshold voltage, subthreshold swing, and on/off current ratio were 4.3 cm(2)/(V s), 0 V, 0.4 V/decade, and 4.1 × 10(7), respectively. In contrast, the device performance for the ZTO TFTs with either a higher or lower Sn concentration suffered from low mobility and a high off-state current, respectively. The photoelectrical stress measurements showed that the photobias stability of the ZTO TFTs was improved substantially when the ZTO semiconducting films had a lower oxygen vacancy concentration and an amorphous structure. The relevant rationale is discussed based on the phototransition and subsequent migration mechanism from neutral to positively charged oxygen vacancies.

Solution- Processed Alkali Metals-Doped Amorphous Zinc Tin Oxide Thin-Film Transistors and Analysis of Alkali Metal Doping Mechanism through the UV-Visible Spectroscopic

Recently, alkali metal doped ZnO has received much positive attention owing to its high performance and good electrical stability. In particular, since alkali metals such as Li and Na are general elements and show good potential as doping elements in ZnO semiconductors, several researchers have been interested in alkali metal doping process of polycrystalline zinc oxide semiconductors. However, only a few studies have been reported the doping mechanism to enhance the mobility. In addition, there have been few studies on alkali metal doping mechanism in amorphous metal oxide semiconductors. Most studies about alkali metal doped oxide semiconductor have been inquired in term of crystallinity, oxygen vacancy, and surface morphology properties. For these reasons, we also studied the mechanism of alkali metal doping in view of oxygen vacancy by spectroscopic study using XPS and surface morphology using AFM. However, although we observed that oxygen vacancy contents are reduced slightly with a correlation by interstitial or substitutional mechanism, it seems to be difficult to identify the correlation between the enhancement of electrical properties induced by interstitial doping and the changes of oxygen vacancy contents, because the difference of XPS spectra in doped ZTO films is too small. Also, as the decrease of oxygen vacancy contents means the decrease of electron mobility in metal oxide semiconductors, the tendency of oxygen vacancy spectra in XPS is inconsistence with general phenomena. It means that other mechanisms are required to precisely explain the electrical properties of the doped amorphous ZTO TFTs related to alkali metal doping mechanism. Otherwise, the change in the optical band gap supported by the Burstein-moss theory showed successfully that the enhancement of mobility was related to the interstitial doping of alkali metals. This result shows that the change of the optical band gap clearly applied to the study of the relationship between the change in mobility and interstitial doping concentrations.Here in, we introduce alkali metals doped and solution processed amorphous zinc tin oxide (ZTO) semiconductor TFTs, which show better electrical properties, such as field effect mobility and conductivity, than that of intrinsic amorphous ZTO based TFTs. We also analyze that the doping mechanism of alkali metals in the amorphous ZTO based TFTs using various techniques such as, atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS), Hall mobility. Furthermore, through the UV-visible spectroscopy, we suggest a comprehensive technique for investigating the enhanced electrical performance induced by alkali metal doping in terms of the change in optical band gap. Specially, we showed that UV-visible spectroscopic analysis for investigating the electrical performance of alkali metal doped metal oxide semiconductor TFTs has good potential as a fast and non-destructive analytical technique.

Electrohydrodynamic jet-printed zinc-tin oxide TFTs and their bias stability.

Zinc-tin oxide (ZTO) thin-film transistors (TFTs) were fabricated using an electrohydrodynamic-jet (EHD-jet) printing technique at annealing temperatures ranging from 300 to 500 °C. An EHD-jet-printed ZTO active layer was patterned with a 60 μm width using a 100 μm inner diameter metal nozzle. The electrical properties of an EHD-jet-printed ZTO TFT showed a mobility of 9.82 cm(2)/(V s), an on-off current ratio of 3.7 × 10(6), a threshold voltage of 2.36 V, and a subthreshold slope of 0.73 V/dec at 500 °C. Significantly improved properties were obtained compared to the spin-coated and inkjet-printed ones. Better hysteresis behavior and positive bias stability of the ZTO TFTs were also achieved using EHD-jet printing technology.

Role of oxygen vacancies on the bias illumination stress stability of solution-processed zinc tin oxide thin film transistors

Solution-processed ultra-thin (∼3 nm) zinc tin oxide (ZTO) thin film transistors (TFTs) with a mobility of 8 cm2/Vs are obtained with post spin-coating annealing at only 350 °C. The effect of light illumination (at wavelengths of 405 nm or 532 nm) on the stability of TFT transfer characteristics under various gate bias stress conditions (zero, positive, and negative) is investigated. It is found that the ΔVth (Vthstress 3400 s − stress 0 s) window is significantly positive when ZTO TFTs are under positive bias stress (PBS, ΔVth = 9.98 V) and positive bias illumination stress (λ = 405 nm and ΔVth = 6.96 V), but ΔVth is slightly negative under only light illumination stress (λ = 405 nm and ΔVth = −2.02 V) or negative bias stress (ΔVth = −2.27 V). However, the ΔVth of ZTO TFT under negative bias illumination stress is substantial, and it will efficiently recover the ΔVth caused by PBS. The result is attributed to the photo-ionization and subsequent transition of electronic states of oxygen vacancies (i.e., Vo, Vo+, and Vo++) in ZTO. A detailed mechanism is discussed to better understand the bias stress stability of solution processed ZTO TFTs.

Improved electrical properties of zinc-tin oxide TFTs by inkjet process optimization

Inkjet printing process was optimized to improve the electrical properties of inkjet-printed zinc-tin oxide thin-film transistors. Among the process conditions evaluated, inkjet voltage and substrate temperature were found to be the key factors for obtaining uniform thin-films and good thin-film transistors. The optimization process with a jetting voltage of 60 V and substrate temperature of 50°C gave good electrical properties, such as a field-effect mobility of 5.11 cm2/V s, a threshold voltage of 2.83 V, a subthreshold slope of 1.33 V/dec, and an on-to-off current ratio of 108. The electrical properties of the inkjet-optimized TFTs were superior to those of the unoptimized inkjet TFTs. A positive bias stability was also investigated.

Top-gate zinc tin oxide thin-film transistors with high bias and environmental stress stability

Top gated metal-oxide thin-film transistors (TFTs) provide two benefits compared to their conventional bottom-gate counterparts: (i) The gate dielectric may concomitantly serve as encapsulation layer for the TFT channel. (ii) Damage of the dielectric due to high-energetic particles during channel deposition can be avoided. In our work, the top-gate dielectric is prepared by ozone based atomic layer deposition at low temperatures. For ultra-low gas permeation rates, we introduce nano-laminates of Al2O3/ZrO2 as dielectrics. The resulting TFTs show a superior environmental stability even at elevated temperatures. Their outstanding stability vs. bias stress is benchmarked against bottom-gate devices with encapsulation.