Solution-processed Oxide Thin-film Transistors Research Articles

Effects of Pre-annealing on the Performance of Solution-processed Indium Zinc Oxide Thin-film Transistors

We report on thin-film transistors (TFTs) with indium zinc oxide (IZO) channel layers fabricated via a pre-annealing process at various temperatures. The solution-processed IZO semiconductor matched the pre-annealing well at temperatures as low as 120 °C, and showed good performance: field-effect mobility of 7.9 ㎠/Vs, threshold voltage of 1.4 V, subthreshold slope of 0.48 V/dec, and a current on/off ratio of 2.9 x 10SUP7/SUP. To investigate the static response of IZO TFTs with preannealing at 120 °C, simple resistor load–type inverters were fabricated by connecting a resistor (10 MΩ).

Highly stable low-temperature aqueous solution-processed oxide thin-film transistors by the hydrogen injection and oxidation method

We demonstrate the high stability of low-temperature aqueous solution-processed In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) by application of the hydrogen injection and oxidation (HIO) method at annealing temperatures as low as 250 °C. Solution processing is considered an innovative cost-reduction method for various electronics products; however, solution-processed TFTs for device applications have not yet been realized due to poor TFT performance, such as low mobility, a large subthreshold swing, and instability. Using the HIO method, improvement in the TFT characteristics was confirmed in terms of the mobility and electrical stability, compared to that for conventional solution-processed IGZO TFTs. The TFTs processed with the HIO method exhibited an improvement in stability by more than 10 V under positive bias stress and 1.5 V under negative bias stress. The HIO method is a simple process to enhance the quality of solution-processed oxide films by the decomposition of residual species using a reduction reaction. It is demonstrated to be an inexpensive and effective means to realize solution-processed oxide semiconductors for device applications.

Improvement in Electrical Characteristics of Eco-friendly Indium Zinc Oxide Thin-Film Transistors by Photocatalytic Reaction.

Eco-friendly solution-processed oxide thin-film transistors (TFTs) were fabricated through photocatalytic reaction of titanium dioxide (PRT). The titanium dioxide (TiO2) surface reacts with H2O under ultraviolet (UV) light irradiation and generates hydroxyl radicals (OH•). These hydroxyl radicals accelerate the decomposition of large organic compounds such as 2-methoxyethanol (2ME; one of the representative solvents for solution-processed metal oxides), creating smaller organic molecular structures compared with 2ME. The decomposed small organic materials have low molar masses and low boiling points, which help improving electrical properties via diminishing defect sites in oxide channel layers and fabricating low-temperature solution-processed oxide TFTs. As a result, the field-effect mobility improved from 4.29 to 10.24 cm2/V·s for IGZO TFTs and from 2.78 to 7.82 cm2/V·s for IZO TFTs, and the Vth shift caused by positive bias stress and negative bias illumination stress over 1000 s under 5700 lux decreased from 6.2 to 2.9 V and from 15.3 to 2.8 V, respectively. In theory, TiO2 has a permanent photocatalytic reaction; as such, hydroxyl radicals are generated continuously under UV irradiation, improving the electrical characteristics of solution-processed IZO TFTs even after four iterations of TiO2 recycling in this study. Thus, the PRT method provides an eco-friendly approach for high-performance solution-processed oxide TFTs.

Solution-Processed Perovskite Gate Insulator for Sub-2 V Electrolyte-Gated Transistors

By virtue of their semiconducting and electrolytic characteristics, hybrid organic–inorganic perovskites have received intense research attention in various applications, which include energy, electronics, and display technologies. While research studies on the semiconducting or electronic properties of perovskite materials in solar cells and light-emitting diodes have been actively investigated, studies on their electrolytic or ionic behavior have rarely been conducted. To probe the electrolyte properties of the metal halide perovskite, we have fabricated solution-processed zinc oxide (ZnO) thin-film transistors using a methylammonium lead iodide (CH3NH3PbI3) perovskite thin film as a gate insulator material. The resulting perovskite film revealed ionic characteristics, with an ionic conductivity of about 10–8 S/cm. The perovskite-gated ZnO transistors exhibited typical n-type characteristics with an average field-effect mobility of 0.047 cm2/V s at a low applied voltage below 2 V because of the electrical double layer formed by mobile I– anions and CH3NH3+ cations in the perovskite gate dielectric. In addition, the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) transistors gated with the perovskite showed an abnormal increase in the channel current when applying positive gate bias, probably because of the confined ion movement inside the perovskite gate insulator.

Enhanced Electrical Properties and Stability of Solution-processed Amorphous Oxide Thin Film Transistors with Multi-active Layers

We investigated the electrical properties and gate bias stress stability of solution-processed amorphous oxide thin film transistors (TFTs) with multi-stacked active layers. With the multi-layered InZnO (In:Zn = 1:1), mobility was increased from 4.6 to 21.2 cm2V-1s-1 and the subthreshold swing (SS) was improved from 0.71 to 0.54 V/decade compared to the single-layered InZnO TFT. The tri-layered InZnO TFT showed a reduced threshold voltage shift (ΔVth) under positive bias stress (PBS) from +4.4 to + 0.9 V, whereas ΔVth under negative bias stress (NBS) deteriorated from -0.03 to -1.5 V. We also fabricated bi-layered (bottom/top layer) TFTs using different oxide compositions with InZnO (In:Zn = 7:3) and InGaZnO (In:Zn:Ga = 3:3:1) which showed high mobility and small ΔVth under PBS and NBS. The InZnO/InGaZnO TFT showed a high mobility of 17 cm2V-1s-1, SS of 0.65 V/decade, and good stability, with ΔVth under PBS and NBS of +1.2 and -1.2 V, respectively. Both the electrical properties and gate bias stress (GBS) stability were better with the InZnO/InGaZnO TFT than the single-layered InZnO TFT.

Electrical stability of solution-processed indium oxide thin-film transistors under illumination stress

ABSTRACTWe investigated the variation in the electrical characteristics of solution-processed indium oxide (In2O3) thin-film transistors (TFTs) under monochromatic illumination conditions at different wavelengths of 1240, 800, 600, 500, 400, and 350 nm. In our results, there was no discernible difference between the characteristics of the TFT measured in the dark state and those obtained under light illumination at the wavelengths of 1240 nm and 800 nm. On the other hand, the TFT performance was remarkably changed when light having a wavelength of 600 nm or less was illuminated. It is found that the photon energy, which causes the characteristic degradation in the solution-processed In2O3 TFTs, is much lower than the direct band gap energy of In2O3. Consequently, illumination-induced instability in the performance of solution-processed In2O3 TFTs can be explained with the transition phenomena of neutral oxygen vacancies to ionized ones in the In2O3 semiconductor film.

Suppression of persistent photo-conductance in solution-processed amorphous oxide thin-film transistors

This study offers a combinatorial approach for suppressing the persistent photo-conductance (PPC) characteristic in solution-processed amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) in order to achieve rapid photo-recovery. Various analyses were used to examine the photo-instability of indium-gallium-zinc-oxide (IGZO) TFTs including negative-bias-illumination-stress (NBIS) and transient photo-response behaviors. It was found that the indium ratio in metallic components had a significant impact on their PPC and photo-recovery characteristics. In particular, when the indium ratio was low (51.5%), the PPC characteristic was significantly suppressed and achieving rapid photo-recovery was possible without significantly affecting the electrical performance of AOSs. These results imply that the optimization of the indium composition ratio may allow achieving highly photo-stable and near PPC-free characteristics while maintaining high electrical performance of AOSs. It is considered that the negligible PPC behavior and rapid photo-recovery observed in IGZO TFTs with a lower indium composition are attributed to the less activation energy required for the neutralization of ionized oxygen vacancies.

High performance solution processed oxide thin-film transistors with inkjet printed Ag source–drain electrodes

Engineering of an In2O3semiconductor and Ag source/drain interface in inkjet-printed thin-film transistors enhances the saturation mobility by two orders of magnitude.

Reduction of hysteresis in solution-processed InGaZnO thin-film transistors through uni-directional pre-annealing

The hysteresis of the solution-processed oxide thin-film transistors (TFTs) is fatal issue to interrupt stable operation. So, we came up with uni-directional pre-annealing to solve the problem. There are inevitable defects when solution-processed oxide TFTs are fabricated, due to the porosities by the solvent volatilization. Also oxygen vacancies needed for carrier generation in metal oxide semiconductor can be trap states inducing charge carrier trapping. Uni-directional pre-annealing improved the hysteresis, preventing randomly solvent evaporation and decreased the defects of the film. We can result in advanced stability of the solution-processed oxide TFTs, at the same time showing that the field effect mobility was enhanced from 3.35 cm2/Vs to 4.78 cm2/Vs simultaneously, and exhibiting better subthreshold swing from 0.89 V/dec to 0.23 V/dec.

Fabrication of solution-processed nitrogen-doped niobium zinc tin oxide thin film transistors using ethanolamine additives

In this study, we fabricated the nitrogen-doped NbZnSnO channel layers by using the sol-gel method. Monoethanolamine (MEA) was used as a nitrogen additive. From the XPS results, the concentration of oxygen vacancies changes as a function of MEA/Nb ratio. A NbZnSnO film with MEA/Nb of 0.2 shows the lowest amount of oxygen vacancies. TFT electrical performance also shows a device with an MEA/Nb ratio of 0.2 possesses a high carrier mobility (7.4 cm2 V−1s−1) and good bias stress stability. In addition, we also investigated the effect of the aging time of precursor solution on the electrical characteristics of the TFT. After adding MEA, the annealing temperature of the NbZnSnO channels can be reduced, pertaining to the acceleration of the hydrolysis and condensation reaction.

The pH-dependent corrosion behavior of ternary oxide semiconductors and common metals and its application for solution-processed oxide thin film transistors circuit integration

Individual oxide semiconductors and metals experience unique pH-dependent phase-transition into natively stable phases (metal, metal ions, or oxide phases) in various acidic and basic solutions. Thus, the corrosion behavior of oxide semiconductors and metals can be engineered by controlling pH values. In particular, the specific pH value induced interesting corrosion behavior that oxide semiconductor becomes chemically-stable and metal solely experiences active ionization. First, the pH-dependent corrosion behavior of ternary oxide semiconductors [ZnSnO (ZTO) and InZnO (IZO)] and common metals (Mo and Mo/Cu) was explored based on theoretical Pourbaix diagram and experimental corrosion data. Next, the pH-dependent corrosion behavior based back-channel wet-etch (BCWE) process using pH-controlled wet etchants was designed and applied for chemical damage-, metal residue-, and curing treatment-free solution-processed oxide thin film transistors (TFTs) circuit integration without electrical degradation. Thick-Mo and thin-Mo/thick-Cu could be completely ionized without any metal oxide residues in middle-basic (pH ≥ 10) and weak-acidic (pH = 6) wet-etchants, respectively. Chemically durable ZTO in the broad pH region (6 ≤ pH ≤ 11) indicated sufficient potential as channel candidates for the circuit integration of chemical damage-free oxide TFTs as opposed to IZO (pH = 10). Finally, solution-processed ZTO TFTs could be fabricated with wet-etched Mo and Mo/Cu using the customized wet-etchant condition (pH = 10 and 6) without electrical degradation (current-drop, hump phenomena, or instability) that was inevitably generated in a conventional BCWE.

Electrical and chemical stability engineering of solution-processed indium zinc oxide thin film transistors via a synergistic approach of annealing duration and self-combustion process

The electrical and chemical stability of solution-processed indium zinc oxide (IZO) channel thin-film transistors (TFTs) were engineered via a synergistic approach of annealing duration and self-combustion process. In particular, the amorphous IZO TFTs that were thermally treated at 400 °C for 3 h using the specific precursor combination to generate internal self-combustion energy showed the best electrical performance [high saturation mobility (μSAT)=2.7 cm2/V s] and stability [low threshold voltage shift (ΔVTH) under positive bias stress of 10.5 V] owing to the formation of oxide films with excellent metal–oxide–metal (M–O–M) bonds, fewer impurities, and an amorphous phase compared to IZO TFTs using other precursor formulas and annealing times. Longer annealing times led to a saturated M–O bond ratio and crystallization via extreme thermal annealing, which induced electrical degradation (low μSAT and high ΔVTH) of IZO TFTs. In the wet chemical patterning of electrodes, conventional acidic and basic wet etchants cause severe damage to the surfaces of the IZO channels; thus, insufficiently annealed IZO TFTs exhibited considerable degradation in terms of their on-current level and mobility. Alternatively, the TFTs subjected to an excessively long-term thermal annealing showed only a moderate decrease in mobility with the formation of small nanocrystals.

Solution-processed high-mobility neodymium-substituted indium oxide thin-film transistors formed by facile patterning based on aqueous precursors

Solution-processed neodymium-substituted indium oxide (InNdO) thin-film transistors (TFTs) based on gel-like aqueous precursors were fabricated with a surface-selective deposition technique associated with ultraviolet irradiation. The Nd concentration can be easily tuned by changing the ratio of Nd2O3 to In2O3 precursors. It was found that Nd played roles of suppressing grain growth, suppressing oxygen vacancy formation, and increasing the electrical stability of TFTs. The InNdO TFT with a Nd:In ratio of 0.02:1 exhibited a mobility of as high as 15.6 cm2 V−1 s−1 with improved stability under gate-bias stress.

Effect of relative humidity and pre-annealing temperature on spin-coated zinc tin oxide films made via the metal–organic decomposition route

Solution-processed zinc tin oxide thin-film transistor using metal–organic decomposition achieves wide process window in terms of ambient humidity during deposition.

Towards environmentally stable solution-processed oxide thin-film transistors: a rare-metal-free oxide-based semiconductor/insulator heterostructure and chemically stable multi-stacking

ZTO/Al2O3 heterostructure and chemically stable sol–gel multi-stacking method are proposed for practical solution-processed oxide TFTs.

Hole mobility modulation of solution-processed nickel oxide thin-film transistor based on high-k dielectric

Solution-processed p-type oxide semiconductors have recently attracted increasing interests for the applications in low-cost optoelectronic devices and low-power consumption complementary metal-oxide-semiconductor circuits. In this work, p-type nickel oxide (NiOx) thin films were prepared using low-temperature solution process and integrated as the channel layer in thin-film transistors (TFTs). The electrical properties of NiOx TFTs, together with the characteristics of NiOx thin films, were systematically investigated as a function of annealing temperature. By introducing aqueous high-k aluminum oxide (Al2O3) gate dielectric, the electrical performance of NiOx TFT was improved significantly compared with those based on SiO2 dielectric. Particularly, the hole mobility was found to be 60 times enhancement, quantitatively from 0.07 to 4.4 cm2/V s, which is mainly beneficial from the high areal capacitance of the Al2O3 dielectric and high-quality NiOx/Al2O3 interface. This simple solution-based method for producing p-type oxide TFTs is promising for next-generation oxide-based electronic applications.

Combustion synthesized indium-tin-oxide (ITO) thin film for source/drain electrodes in all solution-processed oxide thin-film transistors

We report combustion solution synthesized (SCS) indium-tin-oxide (ITO) thin film, which is a well-known transparent conductive oxide, for source/drain (S/D) electrodes in solution-processed amorphous zirconium–indium–zinc–oxide TFT. A redox-based combustion synthetic approach is applied to ITO thin film using acetylacetone as a fuel and metal nitrate as oxidizer. The structural and electrical properties of SCS-ITO precursor solution and thin films were systematically investigated with changes in tin concentration, indium metal precursors, and annealing conditions such as temperature, time, and ambient. It was found that at optimal conditions the SCS-ITO thin film exhibited high crystalline quality, atomically smooth surface (RMS ~ 4.1 A), and low electrical resistivity (4.2 × 10−4 Ω cm). The TFT using SCS-ITO film as the S/D electrodes showed excellent electrical properties with negligible hysteresis. The obtained “on/off” current ratio, subthreshold swing factor, subthreshold voltage, and field-effect mobility were 5 × 107, 0.43 V/decade, 0.7 V, and 2.1 cm2/V s, respectively. The performance and stability of the SCS-ITO TFT are comparable to those of the sputtered-ITO TFT, emphasizing that the SCS-ITO film is a promising candidate for totally solution-processed oxide TFTs.

용액 공정 기반의 인듐 아연 산화물 트랜지스터의 PMMA 보호막 여부에 따른 신뢰성 평가

We investigated poly(methyl methacrylate) (PMMA) passivation of solution-processed indium zinc oxide (IZO) thin-film transistors (TFTs). PMMA provides solution-processability and good barrier characteristics against environmental elements such as water and oxygen. The PMMA passivation layers protect the IZO active layer of the TFTs without deteriorating their performance during gate bias stress measurements under ambient conditions, and improve their electrical properties by decreasing leakage current. Moreover, the potential to safely manipulate IZO-TFTs after PMMA passivation was proven by realizing a simple n-channel resistive-load inverter. †(Received May 27, 2015)

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.

Solution-processed lithium-doped zinc oxide thin-film transistors at low temperatures between 100 and 300 °C

Lithium-doped zinc oxide (Li-ZnO) thin-film transistors (TFTs) were fabricated by solution process at the low temperatures ranged from 100 to 300 °C. Li-ZnO TFTs fabricated at 300 °C under nitrogen condition showed a mobility of 1.2 cm2/Vs. Most importantly, the mobility of Li-ZnO TFT devices fabricated at 100 °C could be increased significantly from 0.08 to 0.4 cm2/Vs by using double spin-coated and UV irradiation-treated Li-ZnO film, and the on-/off-current ratio is in the order of 106. Notably, the XPS analyses proved that the performance improvement was originated from the chemical composition or stoichiometry evolution, in which the hydroxide was converted into metal oxide and accelerated the formation of the oxygen vacancies. Furthermore, low-voltage operating Li-ZnO TFTs were demonstrated by using a high-capacitance ion gel gate dielectrics. The Li-ZnO TFTs with an operating voltage as low as 2 V exhibited the carrier mobilities of 2.1 and 0.65 cm2/Vs for the devices treated at 300 and 100 °C, respectively. The low-temperature, solution-processed Li-ZnO TFTs showed greatly potential applications in flexible displays, smart label, and sensors.