Improvement in electrical characteristics of eco-friendly indium zinc

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Functional Inorganic Materials and Devices

Improvement in electrical characteristics of eco-friendly indium zinc oxide thin-film transistors by photocatalytic reaction Jun Ki Kang, Sung Pyo Park, Jae Won Na, Jin Hyeok Lee, Dongwoo Kim, and Hyun Jae Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01268 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018

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ACS Applied Materials & Interfaces

Improvement in electrical characteristics of ecofriendly indium zinc oxide thin-film transistors by photocatalytic reaction Jun Ki Kang, Sung Pyo Park, Jae Won Na, Jin Hyeok Lee, Dongwoo Kim, and Hyun Jae Kim* School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemungu, Seoul, 03722, Republic of Korea

KEYWORDS indium zinc oxide, thin-film transistors, hydroxyl radicals, photocatalytic reaction, eco-friendly semiconductor

ACS Paragon Plus Environment

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ABSTRACT 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 2methoxyethanol (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 (PBS) and negative bias illumination stress (NBIS) over 1,000 s under 5,700 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 ecofriendly approach for high-performance solution-processed oxide TFTs.


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1. Introduction Oxide thin-film transistors (TFTs) have superior characteristics, such as high field-effect mobility and good transparency, compared with amorphous Si; thus, there have been many studies regarding various applications that require high performance, high resolution, and high transparency for next-generation display technology.1-3 Among the various fabrication processes for oxide TFTs, solution processes such as ink-jet printing and roll-to-roll processing are effective fabrication methods to form the active channel only at desired regions.4 Despite having this advantage, two serious obstacles must be overcome to be implemented for fabrication of future displays. First, solution-processed oxide TFTs require a high-temperature annealing process (> 300 °C).5 High temperatures are necessary to vaporize large organic molecular compounds, such as 2-methoxyethanol (2ME) and various additives (e.g., acetic acid and nitric acid), for homogeneous mixing of the metal precursors and to react with oxygen for metal oxide bonding. However, high annealing temperatures limit the use of solution processes in fabricating flexible displays, because the flexible substrates tend to degrade at high temperatures, showing denaturation and discoloration at around 300 °C.6 Second, many voids and defects are generated in the oxide channel layers during the annealing process, due to the evaporation of large organic materials and other impurities in the solution. These defects act as charge trap sites and degrade the electrical characteristics of oxide TFTs. Various techniques involving high pressure,7-8 electrical stress,9-10 activation via UV,11-12 change of metal precursor,13 diffusion between different materials,14 and mixtures of additional activation assisted-materials15-17 have been researched to overcome the challenges of the high-temperature process and degradation of the electrical characteristics. However, these methods require custom-designed equipment or


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continuous consumption of additional material during the annealing process of oxide channel layers. Herein, we suggest a fabrication method of solution-processed oxide TFTs (e.g., IGZO and IZO) through photocatalytic reaction of TiO2 (PRT) for improving electrical properties and lowering the processing temperature. TiO2 is the most representative photocatalytic material and is widely used in the sewage treatment industry for organic material decomposition, due to its excellent UV-photocatalytic properties relative to various other materials with similar characteristics (e.g., ZnO, ZrO2, SnO2 and WO3).18-20 TiO2 reacts with H2O on its surface under UV illumination (below 380 nm), resulting in the generation of hydroxyl radicals (OH·).21-22 The generated hydroxyl radicals oxidize the solvent (2ME) and decompose it into smaller organic molecules. These decomposed organic molecular structures have low molar masses and low boiling points compared with those of 2ME; hence, they not only improve the electrical characteristics by reducing defect sites within the oxide channel layers during the annealing process, but also enable solution-processed fabrication at low temperatures. Additionally, hydroxyl radicals are strong oxidants; thus, they can be combined with oxygen vacancies (Vo) or chemically induced defects to improve the electrical characteristics of oxide TFTs. TiO2 is also known to be relatively harmless to humans and theoretically acts as a permanent photocatalyst. Therefore, residual TiO2 powder can be recycled continuously over several iterations. To realize the possibility of eco-friendly solution-processed IZO TFTs, we performed recycling experiments of previously used TiO2 powder through UV photocatalytic reactions. As a results, we could confirm the comparable improvement of the electrical characteristics even after four times recycling.


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2. Experimental Section 2.1. Fabrication of IZO TFTs on SiO2/p+ Si substrate A 0.2 M IGZO and IZO solution was prepared by dissolving a mixture of indium nitrate hydrate [In(NO3)3·xH2O, 99.999%], gallium nitrate hydrate [Ga(NO3)3·xH2O, 99.999%], and zinc nitrate hydrate [Zn(NO3)2·xH2O, 99.999%] in 2-methoxethanol (2ME). The ratio of indium, gallium, and zinc precursor is 6:1:2 for IGZO and 6:1 for IZO. Then, 0.3 g of acetic acid (CH3COOH) and 0.3 g of distilled water (DI) were mixed to make a homogeneous solution and accelerate photocatalytic reaction, respectively. The IGZO and IZO solution was stirred vigorously for 1 h at 60 °C and aged at room temperature for 24 h. The bottom gate staggered structure was fabricated by spin-coating the IGZO and IZO solution at 3,000 rpm for 30 s onto a 120-nm-thick SiO2/p+ Si wafer substrate. The devices were pre-annealed at 100 °C for 10 min, then post-annealed at 450 °C, 3h for IGZO, and 280 °C, 2 h for IZO via a hot plate. The source and drain electrodes (Al, 200 nm) were deposited by radio-frequency (RF) magnetron sputtering with a shadow mask. The channel size of the devices was 1,000 µm width and 150 µm length. 2.2. Photocatalytic reaction of TiO2 To generate hydroxyl radicals through photocatalytic reaction, 0.2 g of TiO2 powder (Anatase; Daejung Chemicals and Metals Co., Ltd.) was mixed with the IGZO or IZO solution. The vial containing the IGZO or IZO solution was exposed to ultraviolet (UV) light (wavelength: 365 nm) while being stirred for 30 min at 50 °C. The solution was then aged at room temperature for 1 h. TiO2 particles sank to the bottom of the bottle; the IGZO or IZO solution containing hydroxyl radicals remained in the upper portion of the bottle. Only IGZO or IZO solution was used with a syringe through a 0.2-µm filter to prevent TiO2 powder from entering the channel layer.


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2.3. TiO2 recycling method After the initial IGZO or IZO solution was exhausted, the old solution was replaced with new IGZO or IZO solution in the vial with the remaining TiO2 powder. The UV photocatalytic reaction was then carried out repeatedly four times, as described above. 2.4. Analysis of the devices UV-vis Spectroscopy (V-650 Spectrometer; JASCO Corp.) was used to investigate hydroxyl radicals generation. The electrical characteristics of the devices (transfer curve and reliability test) were measured using the HP 4156C system (Agilent Corp.). The ratios of chemical bonding (M-O, Vo, M-OH) were determined by X-ray photoelectron spectroscopy (XPS; K-alpha; Thermo Fisher Scientific). The Scanning Electron Microscope (SEM; JSM-6701F; JEOL Ltd.) and Differential Scanning Calorimetry (DSC; DSC-8000; Perkin Elmer Corp.) were used to determine the size of the TiO2 powder particles and the heat flow of the solution, respectively.

3. Results and discussion 3.1 Generation of hydroxyl radicals through the PRT method Figure S1 illustrates schematic of TiO2 photocatalytic reaction. When light with photon energy higher than the bandgap energy of TiO2 (~3.2 eV) irradiates its surface, electrons and holes are generated at the surface. Electrons in the conduction band (CB) and holes in the valance band (VB) generate oxygen ions (O2-) and hydroxyl radicals (OH·) in an aqueous solution containing TiO2 by the following respective reactions.23-24 : CB ∶ Electron + O2 → O2-

(1)

VB ∶ Hole + H2O → OH·

(2)


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Because hydroxyl radicals are unstable, they generate hydrogen peroxides in the intermediate step. These hydrogen peroxides react with the electrons of TiO2 and generate hydroxyl radicals again, as shown below:21 OH· + OH· → H2O2

(3)

H2O2 + e- (TiO2) → OH· + OH-

(4)

As many studies explicate, hydroxyl radicals act not only as decomposition agents of organic materials, but also as strong oxidants that combine with oxygen-related defects within oxide channel layers to improve electrical characteristics.15-16,

25-26

To apply these properties of

hydroxyl radicals, TiO2 was deposited as a thin-film for a field-effect transistor via a vacuum deposition process.27 However, the electrical properties of the devices were degraded due to defect sites, such as oxygen vacancies and oxygen interstitials created by the breaking of chemical bonds at the surface of the oxide thin-film layer after direct UV irradiation of the channel layer. As such, we devised a method for generating hydroxyl radicals using TiO2 without direct UV irradiation of the oxide channel layers (Figure 1).


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Figure 1. Schematic of the fabrication process of PRT-treated TFTs: (a) TiO2 powder in the IZO solution mixture, (b) procedure of photocatalytic reaction during stirring, (c) separating TiO2 particles from the IZO solution, (d) spin-coating of IZO solution containing hydroxyl radicals, and (e) final structure of the TFT.

An experiment was carried out to confirm whether hydroxyl radicals were actually created through photocatalytic reaction of TiO2 (PRT) in the IZO solution. There are a number of ways to detect hydroxyl radicals, including potassium iodine (KI)/ultraviolet-visible (UV-Vis) spectroscopy analysis,28 the terephthalic acid (TPA) or coumarin-3-carboxylic acid (CCA) fluorescence spectrometer method,21 and electron spin resonance (ESR) spectroscopy measurements.29 In our study, the KI/UV-Vis spectrometer method was used. When potassium iodine contained in distilled water (DI; 10 g/L) is combined with hydroxyl radicals, triiodide ions are generated through the reaction of (5)–(8) as described below; the specific peak of the absorbance depends on the amount of triiodide ions created.28, 30 OH· + I- → OH- + I

(5)

I + I- → I2-

(6)

2I2- → I2 + 2I-

(7)

I2 + I- → I3-

(8)

Figure 2a shows the absorbance peak of triiodide ions formed by hydroxyl radicals when UV light (365nm) illuminates TiO2 in a KI solution (10 g/L). Two high absorbance peaks appeared at 290 and 350 nm due to the production of triiodide ions by hydroxyl radicals; the peak heights differed with UV irradiation time. Based on this result, 0.3 mL of KI aqueous solution and 0.3 g of TiO2 powder were mixed with 5 mL of IZO solution. The IZO solution mixture was then


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exposed to UV light. The same absorbance peaks showed up at 290 and 350 nm via triiodide ion generation through the PRT, and the peaks increased with longer UV irradiation time (Figure 2b). This is due to the change in color of the solution mixture depending on the amount of triiodide ions present, as indicated by the different absorption peaks (Figure 2c). As mentioned in the experimental section, the water of 0.3g was added to accelerate OH· generation through photocatalytic reaction of TiO2 in IZO solution. As shown in Figure S2, when the IZO solution with and without water were irradiated with UV for the same time (30 minutes), the color change of the solution with water was more yellowish than that of without water. The more yellowish the color means, the more the photocatalytic reaction takes place, resulting in more OH·. The presence of a slight color change without adding water (Figure S2b) is due to the metal precursor already containing a small amount of water (e.g., In(NO3)3·xH2O and Zn(NO3)2·xH2O), but this amount is not sufficient to achieve the desired effect. As a result, we confirmed that hydroxyl radicals were generated by the PRT method.


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Figure 2. The result of KI/UV-Vis measurement depending on UV irradiation time by PRTtreated TFTs. The peaks of triiodide ions generated by hydroxyl radicals in (a) DI mixture and (b) IZO solution mixture. (c) Color changes due to the different amount of triiodide ions produced by hydroxyl radicals.

3.2. Electrical properties of PRT-treated devices The results of the transfer curves are shown in Figure S3, and the representative TFTs characteristics are summarized in Table S1. IGZO and IZO TFTs showed mobility values of 10.24 cm2/V·s and 7.82 cm2/V·s through PRT method, respectively. Although IGZO TFTs showed higher mobility values, we decided to report the IZO TFTs only to achieve lowtemperature process as well as to improve the electrical properties through photocatalytic reaction. The transfer curves of IZO TFTs were measured to compare the electrical characteristics of PRT-treated and non-treated devices at 280 °C and 230 °C with UV wavelength of 365 nm (Figure 3). Representative TFT characteristics are summarized in Table 1. A higher mobility, higher on-off current ratio, and lower sub-threshold voltage swing (SS) were obtained from PRT-treated devices compared with non-treated devices at both temperatures. All the mobility is calculated in the saturation region, and the formula of the saturation mobility is as follows: ߤ௦௔௧ =

ଶ௅

൤

∆ඥூ೏

஼೚ೣ ௐ · ∆ඥ௏೒

ଶ

൨

(9)

,where Cox is the geometrical capacitance of the SiO2 dielectric, and L and W are the length and width of the transistor channel, respectively. At an annealing temperature of 230 °C, the oncurrent degraded due to inadequate metal-oxide (M-O) formation of the oxide channel layer for the non-treated devices. However, the PRT-treated devices annealed at 230 °C show electrical


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characteristics similar to those of non-treated devices annealed at 280 °C. This improvement appears throughout the wide range of UV wavelength from long wavelength (365 nm) to short wavelengths (185 and 254 nm) as shown in Figure S4. As a result, metal-oxide bonds in active channel layers formed successfully, even at low temperatures. The hysteresis curves were measured for the additional evidence of improving electrical characteristics. Figure S5 shows the result of hysteresis curves. As shown in Figure S5a, the difference of ∆Vth between the PRTtreated and the non-treated device is not large at 280 °C. Slight on-current increase is observed. However, the obvious difference is observed at 230 °C (Figure S5b). In the non-treated sample, residual organic materials in the channel layer and insufficient metal-oxide bonds play as charge trap sites, indicating a large Vth shift.31-32 From these results, we found out that the reliability characteristics were improved by reducing the charge trap sites of the device through the PRT method.

Figure 3. The transfer curves and the saturation mobility of non-treated and PRT-treated IZO TFTs at post annealing temperatures of (a) 280 °C and (b) 230 °C.


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Table 1. The summary of electrical characteristics. Temp (ºC) 280 230

Sample Non-treated PRT-treated Non-treated PRT-treated

Mobility (cm2/V·s) 2.78 7.82 0.21 1.41

Vth (V)

On/off ratio S.S (V/decade)

0.56 1.41 5.94 3.21

~106 ~107 ~104 ~106

0.56 0.48 1.08 0.50

A reliability test was conducted for a more detailed analysis involving positive bias stress (PBS) and negative bias illumination stress (NBIS) measurements, as shown in Figure 4. Table 2 summarizes the degree of threshold voltage (Vth) shift. As shown in Figure 4a, Vth of the PRTtreated device after undergoing a PBS test for 1,000 s with the gate bias voltage of +20 V shifted 50% less compared with the Vth shift of the non-treated device (6.2 → 2.9 V). The stress durability of the oxide TFTs can be explained by two models:33-34 ▪ The defect generation model ▪ The charge trapping model The defect generation model is accompanied by a Vth shift with changing SS value, whereas the charge trapping model is accompanied by only the Vth shift without changing SS value. Our results show the Vth shifts without SS variation (Figure S6), thus following the charge trapping model. The charge trapping model is more dominant when the devices are fabricated through the solution process than the vacuum one. In the case of the solution process, bulk defects are generated within the oxide channel layers during the annealing process, because physical and chemical damages remain after vaporization of large molecular organic structures such as those of solvents, additives, and pollutants in the solution mixture. Due to these defects, electrons become trapped in the channel area; a Vth shift appears when a positive voltage is applied to the gate electrode. However, in PRT-treated devices, the defect concentration in the oxide channel layer is lower; thus, the device is more stable under stress.


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Figure 4. The Vth shift value of non-treated and PRT-treated devices after (a) PBS test with the gate bias voltage of +20 V and (b) NBIS test with the gate bias voltage of െ20 V.

Table 2. The summary of Vth shift value of non-treated and PRT-treated devices.

Reliability PBS NBIS

Sample

1s

10 s

Unit : V 100 s 1000 s

Non-treated

0

0.28

1.63

6.24

PRT-treated

0

0.18

1.32

2.91

Non-treated

0

1.31

5.86

15.31

PRT-treated

0

0.04

0.15

2.75

NBIS results (Figure 4b) show that the Vth shift of the PRT-treated devices decreased by ~80% after 1,000 s at 5,700 lux with the gate bias voltage of െ20 V compared with the non-treated devices (15.3 → 2.8 V). When light illuminates an oxide channel layer, the oxygen vacancies (Vo) in the subgap states are transformed into ionized oxygen vacancies (Vo+, Vo2+). These ionized oxygen vacancies act as electron carriers, making unwanted donors in the channel layers. The hydroxyl radicals of PRT-treated devices combine with oxygen vacancies in the oxide channel layers, as strong oxidizing agents.


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3.3. Chemical properties of PRT-treated devices To demonstrate the chemical bonding ratio of oxide channel layers, X-ray photoelectron spectroscopy (XPS) analysis was carried out (Figure S7, Table S2). The peak was obtained by a deconvoluted graph of the Gaussian distribution function. Oxygen O 1s bonds were observed in three peaks (530 ± 0.2 eV, 531 ± 0.2 eV, and 532 ± 0.2 eV). Each peak indicates metal-oxide (M-O) bond, oxygen vacancy (Vo), and metal-hydroxyl bond, respectively.35-36 The PRT-treated devices exhibited a larger atomic ratio area for the M-O bond, whereas the ratio of the oxygen vacancies and hydroxyl bonds was smaller than that for the non-treated devices. This was attributed to the sufficient number of metal oxide bonds and smaller number of trap sites (e.g., Vo) in the oxide channel layer of PRT-treated devices. 3.4. Mechanism of the PRT method Figure 5a illustrates the decomposition mechanism of 2ME by PRT treatment. 2ME is a representative solvent material used to make solution mixtures. The solution-processed oxide TFTs require a pre-annealing process after spin-coating of the metal precursor mixture to vaporize the solvent. Removing organic materials effectively and quickly without damages (e.g., voids, holes, or cracks) becomes necessary because these damages are origins of charge trap sites inside the oxide channel layers. 2ME is composed of an ether group (-O-) and a hydroxyl group (-OH).37 The unpaired electrons of hydroxyl radicals oxidize the ether group and break C-C and C-H bonds, thus decomposing into carbon cations and formaldehyde. Finally, methyl formate and formic acid are generated through an oxidation reaction. The total decomposition reaction of 2ME is shown below:26, 38-39 CH3OCH2CH2OH + OH · → CH3OCH2+ • CHOH + 2e + H+

(10)


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CH3OCH2+ • CHOH → CH3OCH2+ + CHOH

(11)

CH3OCH2+ + O- → CH3OCHO + H+

(12)

CHOH + O- → HCOO- + H+

(13)

Figure 5. The mechanism of improving the electrical characteristics by PRT-treatment. (a) Scheme of 2ME’s decomposition procedure. Generation of defects in oxide channel layers of (b) non-treatment and (c) PRT-treatment devices. The boiling temperature of 2ME is 124 °C, whereas methyl formate and formic acid have boiling points of 31.8 °C and 100.8 °C, respectively. Also, the molar mass of 2ME is 76.1 g/mol, while methyl formate and formic acid have molar masses of 60.1 g/mol and 46.0 g/mol, respectively. These results suggest the following.


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(1) The small molecular structure causes less physically and chemically induced damage compared with 2ME within the oxide channel layers during the solvent vaporization procedure. Thus, the number of charge trapping sites is reduced and the electrical characteristics of the oxide TFTs are improved. The reliability test and XPS results confirmed these findings (Figure 5c). (2) The decomposed organic structure has a lower boiling point than 2ME molecular structures. Therefore, a low-temperature solution process can be realized by vaporization of the solvent at relatively low temperatures compared with the 2ME-based solvent. Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) method was used as the evidence for the low-temperature process. As shown in Figure S8, the acceleration of weight loss in PRT-treated sample starts faster than that in non-treated sample. Also, we noticed that the exothermic peaks of the PRT-treated samples occurred at a relatively low temperature of 118 ºC compared with non-treated samples of 133 ºC in DSC analysis. This peak indicates that organic materials start to decompose at this temperature. In our results, a remarkably lowtemperature exothermic peak appeared compared with many DCS graphs of IZO solutions from previous studies.40-45 Thus, we concluded

that the low-temperature solution process was

feasible. 3.5. Eco-friendly TFTs fabrication by recycling of TiO2 We have noted that TiO2 has persistent photocatalytic reaction properties, theoretically, potentially providing an eco-friendly and economical fabrication process of TFTs. Figure 6a illustrates the method of recycling TiO2. Figure 6b and 6c show the electrical characteristics of devices from cumulative use of TiO2 for four cycles. Even after several iterations of recycling, the electrical characteristics improved repeatedly at temperatures of both 280 °C and 230 °C


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compared with the non-treated devices; only a negligible decrease in mobility was observed. We concluded that this decrease in mobility may be due to experimental error caused by the partial loss of TiO2 powders during the process of refilling new IZO solution, the exhaustion of the previously used IZO solution. Scanning electron microscopy (SEM) images (Figure 7) show the shape before and after TiO2 recycling. No significant change in the size or surface shape of TiO2 was observed. As a result, the recyclability of TiO2 was confirmed for eco-friendly manufacturing processes.

Figure 6. (a) Schematic of recycling TiO2. The transfer curve after four iterations of TiO2 recycling at annealing temperatures of (b) 280 °C and (c) 230 °C.


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Figure 7. The shape of (a) before and (b) after recycling of TiO2 by Scanning Electron Microscope (SEM)

4. Conclusion In summary, we propose an eco-friendly method for fabricating high-performance solutionprocessed IZO TFTs through hydroxyl radicals generated by PRT. The radicals promote the decomposition of organic materials in the IZO solution and convert them into materials with low boiling points and low molecular weights. Reducing defects inside the oxide channel layers is possible with our PRT method, offering improved reliability and low-temperature IZO solution oxide TFTs. As a result, the field effect mobility and on-off current ratio improved from 2.78 to 7.82 cm2/V·s and from 106 to 107, respectively. Also, the same electrical performances were obtained even at a temperature of 230 °C compared with non-treated devices annealed at 280 °C. Finally, the possibility of eco-friendly and economical oxide TFTs manufacture is presented by recycling TiO2.


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ASSOCIATED CONTENT Supporting Information. The accumulative transfer curve of reliability test (PBS, NBIS), XPS results from deconvolution of O 1s peak (figure and table), Differential scanning calorimetry (DSC) This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2017R1A2B3008719) ABBREVIATIONS TFTs, thin-film transistors; TiO2, titanium dioxide; 2ME, 2-methoxyethanol; PBS, positive bias stress; NBIS, negative bias illumination stress


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Table of Contents


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Improvement in electrical characteristics of eco-friendly indium zinc

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