Low-temperature aqueous solution processed fluorine-doped zinc tin oxide thin-film transistors

Abstract

Novel fluorine-doped zinc tin oxide (ZTO:F) thin-film transistors (TFTs) have been fabricated using an aqueous solution process. Exploiting hydrolysis and condensation reactions in an aqueous solution process, organic-free ZTO:F thin films were fabricated at a low temperature of 250°C. The fabricated TFT device shows a field-effect mobility of 2.85 cm 2 /V s, on-to-off current ratios exceeding 10 7 , and sub-threshold swings of 0.83 V/dec. The ZTO:F TFT also displays high operational stability of ΔV th = 1.73 V despite incorporation of a large amount of fluorine and use of a low-temperature annealing process. This is attributed to effective passivation of oxygen vacancy diffusion by metal fluoride bonds at the ZTO:F channel/gate dielectric interface.Amorphous metal oxide semiconductors have attracted much attention as candidates for the backplanes of active-matrix organic light-emitting diode displays owing to their high field-effect mobility. [1] To date, outstanding progress has been made in the area of solution processing for oxide semiconductors, [2][3][4] as researchers attempt to meet the requirements of low-cost fabrication and printed electronics. Recently, a highperformance InZnO semiconductor fabricated under 250°C using metal alkoxide precursors was reported. [4] However, fabrication steps requiring anhydrous conditions are very expensive and complicated. [5] Recently, solution processed ZnSnO (ZTO) thin films have been investigated for low-cost fabrication of oxide thin-film transistors (TFTs). Amorphous ZTO semiconductor exhibits excellent semiconducting properties and enables low-cost fabrication without an indium composition. [6] The conventional solution process for producing ZTO semiconductors utilizes a non-aqueous solvent such as acetonitrile or 2-methoxyethanol and metal chloride precursors. However, non-aqueous solutions with chloride precursors have a high decomposition temperature of more than 400°C in order to obtain good electronic characteristics and also effect of chlorine residue is vague. [7,8] In this study, we report a low-temperature-processed, novel fluorine-doped zinc tin oxide (ZTO:F) TFT using a simple aqueous solution process. The aqueous solution process utilizing rapid hydrolysis and condensation reactions enables the lowenergy kinetics of M-O-M network formation without any organic residues. In particular, we emphasize that the overall fabrication cost is cheap for several reasons: (i) the process uses an inexpensive solvent (water) and affordable metal precursors and (ii) all ZTO:F film fabrication steps including stirring, spin coating, and annealing were performed under ambient air conditions. The ZTO:F TFT with fluorine of more than 5%, which was annealed at low temperature of 250°C in an ambient atmosphere, shows a field-effect mobility of 2.85 cm 2 /V s and ΔV th of less than 1.73 V under long-term positive bias stress. We conjecture that fluorine, which has similar ionic size with oxygen, is responsible for the enhanced electrical characteristics of the suggested ZTO...

Solution-Processed Flexible Fluorine-doped Indium Zinc Oxide Thin-Film Transistors Fabricated on Plastic Film at Low Temperature

“…On the other hand, the thermal decomposition of the mixed precursor solution starts at low temperature and is gradually made by approximately 450°C to form indium zinc oxide. Since the hydrolysis and condensation reactions have already occurred in aqueous solution via the self-condensation reaction shown in equation (2), which can lower kinetic energy, the precursor continuously loses its weight11. This interpretation is confirmed in the Raman spectra of the InF 3 , ZnF 2 , and mixed precursor solution according to the heating temperature (Fig.…”

“…However, the high-temperature annealing step is inevitable for oxidation and impurity removal in the solution process. Thus, the high-temperature process acts as a hurdle in the fabrication of flexible devices using the plastic film substrates, which requires processing temperature below 250°C34567891011. Many attempts have been made to lower the annealing temperature to realize flexible TFTs with high performance.…”

“…Instead of water vapor annealing in the case of a ‘sol-gel on chip’, we used the water solvent and soluble metal nitrate precursors, which enables the preparation of precursor solution without additional additives and catalysts, as well as the fabrication of TFTs at low temperature68. Above all, water is an impurity-free solvent since it has no volatile organic compounds or organic residues to be removed for achieving high-performance oxide TFTs311. Thus, the aqueous precursor solutions can make the pure metal oxide without high-temperature annealing.…”

“…Thus, the aqueous precursor solutions can make the pure metal oxide without high-temperature annealing. In addition, another aqueous precursor solution process is using the metal fluoride precursors dissolved in water11. The fabricated fluorine doped zinc tin oxide (ZTO:F) TFTs showed a field-effect mobility of 2.85 cm 2 /V·s at 250°C annealing.…”

“…As the carrier concentration is known to increase with the addition of the fluorine dopant, the fluorine-doped oxide TFTs showed the highest conductivity and mobility121314. A similar ionic radius of fluorine compared to oxygen allows the substitution of the oxygen in the lattice or oxygen vacancy by fluorine1113. However, the difference in the atomic valence and the electronegativity of the oxygen and the substituted fluorine generates a free electron, which can enhance the mobility of the TFTs.…”