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Strong Influence of Humidity on Low-Temperature ThinFilm Fabrication via Metal Aqua Complex for High Performance Oxide Semiconductor Thin-Film Transistors Keon-Hee Lim, Jae-Eun Huh, Jinwon Lee, Nam-Kwang Cho, Jun-Woo Park, Buil Nam, Eungkyu Lee, and Youn Sang Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11867 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016
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ACS Applied Materials & Interfaces
Strong Influence of Humidity on Low-Temperature Thin-Film Fabrication via Metal Aqua Complex for High Performance Oxide Semiconductor Thin-Film Transistors Keon-Hee Lim†, Jae-Eun Huh†, Jinwon Lee†, Nam-Kwang Cho†, Jun-woo Park†, Bu-il Nam†, Eungkyu Lee‡ and Youn Sang Kim*,†,§
†Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea ‡Department of Aerospace and Mechanical Engineering, Univesity of Notre Dame, Notre Dame, IN 46556, USA §Advanced Institute of Convergence Technology, 145 Gwanggyo-ro, Yeongtong-gu, Suwon 16229, Republic of Korea KEYWORDS: thin film transistor, oxide semiconductor, aqueous solution process, metal aqua complex, low temperature process
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ABSTRACT
Oxide semiconductors thin film transistors (OS TFTs) with good transparency and electrical performance have great potential for future display technology. In particular, solution-processed OS TFTs have been attracted much attention due to many advantages such as continuous, large scale, and low cost processibility. Recently, OS TFTs fabricated with a metal aqua complex have been focused because they have low temperature processibility for deposition on flexible substrate as well as high field-effect mobility for application of advanced display. However, despite some remarkable results, important factors to optimize their electrical performance with reproducibility and uniformity have not been achieved yet. Here, we newly introduce the strong effects of humidity to enhance the electrical performance of OS TFTs fabricated with the metal aqua complex. Through humidity control during the spin-coating process and annealing process, we successfully demonstrate solution-processed InOx/SiO2 TFTs with a good electrical uniformity of ~5% standard deviation, showing high average field-effect mobility of 2.76 cm2V1 -1
s
and 15.28 cm2V-1s-1 fabricated at 200 ºC and 250 ºC, respectively. Also, based on the
systematic analyses, we demonstrate the mechanism for the change in electrical properties of InOx TFTs depending on the humidity control. Finally, based on the mechanism, we extended the humidity control to the fabrication of AlOx, insulator. Subsequently, we successfully achieved a humidity-controlled InOx/AlOx TFTs fabricated at 200 ºC showing high average field-effect mobility of 9.5 cm2V-1s-1.
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1. Introduction Recently, thin film transistors (TFTs) based on oxide semiconductors (OSs) such as InGaxZnyOz (IGZO), ZnSnxOy (ZTO), ZnOx, and InOx have attracted much interest for switching devices in future displays such as transparent smart windows, mobile displays, and smart signs.13
To date, Si-based TFTs such as hydrogenated amorphous silicon (a-Si:H) TFT and low-
temperature polycrystalline silicon (LTPS) TFT have been widely used in current display technology.3,4 Although they have high electrical performance and stability as a switching device, their poor transparency is a fatal drawback for future displays.3,4 On the other hand, OS TFTs have high electrical properties as well as unique properties such as transparency for applications in future displays.3-6 Conventionally, OS thin films are deposited by various vacuum processes including radio frequency (RF) sputtering, atomic layer deposition (ALD) and pulsed laser deposition (PLD), which have shown outstanding electrical performance and good electrical stability.7,8 However, for the fabrication of vacuum-processed OS TFTs, vacuum systems and a photolithography process resulting in high costs are vitally needed.5,10 Also, they are cumbersome to adopt to advanced processing such as a continuous and large-scale fabrication.4,10 On the other hand, a solution-process has many advantages as advanced processing such as continuous, large scale, and low cost processibility with the ink-jet, spray coating, slit-coating and spin-coating methods.5,11,12 Accordingly, many researchers have focused on developing various precursors and methods for fabricating high quality solution-processed OS films.10,13,14 In particular, because a low-temperature solution-process can achieve the deposition of an OS film on a flexible substrate, studies on various precursors and methods that can lead to high quality OS
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thin films in a low-temperature process have received much attention.15-17 However, most of them still suffer from a relatively low field-effect mobility. Recently, solution-processed OS TFTs fabricated by a metal aqua complex based on a metal nitrate precursor and water have attracted much attention because they show a high field-effect mobility despite the use of a low temperature solution-process.18 Moreover, considering the water-based solution process, they have various merits such as an eco-friendly property, safety benefits, and good storage stability in atmosphere for practical applications in advanced processing.19,20 Due to the above-mentioned high electrical performance and potential benefits, many researchers have made an effort to find important factors for the optimization of the electrical performance and reproducibility, such as controlling the annealing temperature and time.20,21 Nevertheless, the solution-processed OS TFTs fabricated by the metal aqua complex have not achieved electrical reproducibility and uniformity20-22, which means that other important factors must be investigated for the optimization of OS TFTs based on the metal aqua complex. In a conventional sol-gel process, the humidity during the coating process was strongly considered as one of the important factors because the precursors used in the sol-gel process, such as metal alkoxide and metal acetate, easily react with water molecules. Also, although the change in electrical properties is not as dramatic as during the coating process, the humidity during the post annealing process also affects the electrical properties of OS films fabricated by the conventional sol-gel process.23 Unlike the conventional sol-gel based solution-process, in the case of the solution-process based on the metal aqua complex, it is easy to consider that the humidity is an insignificant factor because the metal aqua complex already solvated by water molecules is not sensitive to water. Accordingly, there has been almost no systematic approach
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on the effects of humidity in film formation through the metal aqua complex in water. However, the solution-process using the metal aqua complex is mainly carried out in atmospheric conditions, and the humidity is the most variable factor in atmospheric conditions. In this respect, it is essential to investigate if humidity control is an important factor in the solutionprocessed OS TFTs based on the metal aqua complex. Here, we newly demonstrate the importance of humidity during the film fabrication based on the metal aqua comlex for high electrical performance of OS TFTs. In particular, we focused on the relationship between the electrical properties of InOx films fabricated with the indium aqua complex and the effects of humidity during a coating process and an annealing process. In this process, we discovered that the humidity condition during the coating process and the annealing process dramatically affects the uniformity and electrical properties of the InOx films. Moreover, with an optimized humidity condition, we can realize solution-processed InOx/SiO2 TFTs with a high average field-effect mobility of 2.76 cm2V-1s-1 with a small standard deviation of 0.14 cm2V-1s-1 and a 107 on/off current ratio (with an annealing process temperature of 200 ºC), and 15.28 cm2V-1s-1 with a small standard deviation of 0.45 cm2V-1s-1 and a 105 on/off current ratio (with an annealing process temperature of 250 ºC). Furthermore, with systematic analyses such as the reflectometry, X-ray photoemission spectroscopy (XPS), and a physical-based analytical model, the mechanism regarding the change in electrical properties according to the humidity conditions during the coating process and the annealing process was investigated. Finally, based on the above study for mechanism, we extend the humidity control to a fabrication of oxide insulator, AlOx films based on alumium aqua complex, and successfully achieved a InOx/AlOx TFT having high average field-effect mobility, of ~ 9.5 cm2V-1s-1 , with a small standard deviation of 0.96 cm2V-1s-1 (with annealing process temperature of 200 ºC).
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2. Experimental Details 2.1 Solution synthesis for InOx and AlOx films : To prepare the solution for fabrication of InOx films. 0.15, 0.2, 0.25 and 0.3 M indium nitrate hydrate were dissolved in 10 mL of 18.2 MΩ deionized water (25 ºC), and then the solution was stirred at 500 rpm for 2 days. In the case of the soluiton for the fabrication of AlOx films, 0.2 M aluminium nitrate nonahydrate was dissolved in 10 mL of the 18.2 MΩ deionized water (25 ºC) and the solution was stirred at 500 rpm for 2 days. 2.2 The humidity-controlled InOx and AlOx films fabrication : All InOx/SiO2 TFTs were fabricated onto 200 nm thermal-grown SiO2/highly boron-doped Si substarte which were prepared by washing process using detergent, acetone, and isopropyl alcohol and ultravioletozone treatment process. (The dielectric constant of SiO2 is 3.6) The above-mentioned solution for InOx film was spin-coated at at 5000 rpm for 30 seconds and soft-baked for 1 min in the various relative humidity condition(
