We investigated aqueous metal complex-based oxide semiconductor films formed with various ligands, such as chloride, acetate, fluoride, and nitrate. Nitrate ligand-based indium(III) precursor was easily decomposed at low temperature due to the replacement of all nitrate ions with water during solvation to form the hexaaqua indium(III) cation ([In(H 2 O) 6 ] 3+ ). Hexaaqua indium(III) cation was a key complex to realize high-quality oxide films at low temperature. Additionally, Al 2 O 3 -based high-k dielectric was also employed by using a nitrate precursor, and the hexaaqua aluminum(III) cation ([Al(H 2 O) 6 ] 3+ ) was confirmed. This complex-based Al 2 O 3 film showed high breakdown voltage and stable capacitance under high frequency operation compared to organic solvent-based Al 2 O 3 films. We successfully demonstrated aqueous-based In 2 O 3 TFTs with Al 2 O 3 high-k gate dielectrics formed at 250°C with a wide gate voltage operation and high saturation mobility and on/off ratio of 36.31 ± 2.29 cm 2 V −1 s −1 and over 10 7 , respectively.
■ INTRODUCTIONAmorphous metal oxide semiconductor (AOS)-based electronic devices have been developing rapidly for use as thin-film transistors (TFTs) in display backplanes, sensors, and logic devices. 1−3 In particular, the high field-effect mobility (>10 cm 2 V −1 s −1 ), transparency, and electrical stability of AOS TFTs are attractive for flexible transparent circuit applications. Recent research has focused mainly on developing new materials with high mobility and stability, diverse substrates, and facile processing methods. 4,5 Various processing methods are used to deposit AOS such as atomic layer deposition, pulsed laser deposition, sputtering, and solution process. Except for the solution process, most processes are associated with the high cost and the use of vacuum facilities. In order to consider this issue, solution processing has emerged as a future technology with simple process and low cost. Although solution processing of AOS TFTs has great potential, the electrical properties of these devices strongly depend on the processing temperature, and their electrical performances are much lower than that achieved with vacuum processing even after annealing over 450°C. To overcome this obstacle, several solutions have been proposed, including new material synthesis, 6−8 different ligandbased precursor optimization, 9 nanostructure-based film formation, 10,11 ultraviolet (UV) irradiation, 12 high pressure annealing, 13 microwave irradiation, 14 and ozone treatment. 15 A recent study of metal oxide film formed by the aqueous route showed the possibility of a low-temperature process by reducing the amount of carbon-based organic solvent, additives, and ligands in the metal precursors to lessen the detrimental effects of carbon impurities. 16,17 On the basis of an aqueous system, many kinds of metal precursors with different ligands could be selected to form metal oxide formation such as metalbased nitrate, acetate, chloride, fluoride, iodide, and so on. Besides, aqueous...