Si Joon Kim

Review of solution-processed oxide thin-film transistors

In this review, we summarize solution-processed oxide thin-film transistors (TFTs) researches based on our fulfillments. We describe the fundamental studies of precursor composition effects at the beginning in order to figure out the role of each component in oxide semiconductors, and then present low temperature process for the adoption of flexible devices. Moreover, channel engineering for high performance and reliability of solution-processed oxide TFTs and various coating methods: spin-coating, inkjet printing, and gravure printing are also presented. The last topic of this review is an overview of multi-functional solution-processed oxide TFTs for various applications such as photodetector, biosensor, and memory.

The effect of La in InZnO systems for solution-processed amorphous oxide thin-film transistors

Solution-processed thin-film transistors (TFTs) with La–In–Zn–O (LIZO) as an active channel layer were fabricated with various mole ratios of La. The La3+ additive affected the metal–oxygen bond and made the band gap of LIZO films wider. This behavior indicates that La3+ could play the role of carrier suppressor in InZnO (IZO) systems and significantly reduce the off-current of LIZO films. The optimum LIZO TFT occurred at a LIZO mole ratio of 0.5:5:5 and its channel mobility, threshold voltage, subthreshold swing voltage, and on/off ratio were 2.64 cm2/V s, 7.86 V, 0.6 V/dec, and ∼106, respectively.

Low-cost label-free electrical detection of artificial DNA nanostructures using solution-processed oxide thin-film transistors.

A high-sensitivity, label-free method for detecting deoxyribonucleic acid (DNA) using solution-processed oxide thin-film transistors (TFTs) was developed. Double-crossover (DX) DNA nanostructures with different concentrations of divalent Cu ion (Cu(2+)) were immobilized on an In-Ga-Zn-O (IGZO) back-channel surface, which changed the electrical performance of the IGZO TFTs. The detection mechanism of the IGZO TFT-based DNA biosensor is attributed to electron trapping and electrostatic interactions caused by negatively charged phosphate groups on the DNA backbone. Furthermore, Cu(2+) in DX DNA nanostructures generates a current path when a gate bias is applied. The direct effect on the electrical response implies that solution-processed IGZO TFTs could be used to realize low-cost and high-sensitivity DNA biosensors.

Approaches to label-free flexible DNA biosensors using low-temperature solution-processed InZnO thin-film transistors.

Low-temperature solution-processed In-Zn-O (IZO) thin-film transistors (TFTs) exhibiting a favorable microenvironment for electron transfer by adsorbed artificial deoxyribonucleic acid (DNA) have extraordinary potential for emerging flexible biosensor applications. Superb sensing ability to differentiate even 0.5 μL of 50 nM DNA target solution was achieved through using IZO TFTs fabricated at 280 °C. Our IZO TFT had a turn-on voltage (V(on)) of -0.8 V, on/off ratio of 6.94 × 10(5), and on-current (I(on)) value of 2.32 × 10(-6)A in pristine condition. A dry-wet method was applied to immobilize two dimensional double crossover tile based DNA nanostructures on the IZO surface, after which we observed a negative shift of the transfer curve accompanied by a significant increase in the Ion and degradation of the Von and on/off ratio. As the concentration of DNA target solution increased, variances in these parameters became increasingly apparent. The sensing mechanism based on the current evolution was attributed to the oxidation of DNA, in which the guanine nucleobase plays a key role. The sensing behavior obtained from flexible biosensors on a polymeric substrate fabricated under the identical conditions was exactly analogous. These results compare favorably with the conventional field-effect transistor based DNA sensors by demonstrating remarkable sensitivity and feasibility of flexible devices that arose from a different sensing mechanism and a low-temperature process, respectively.

Activation of sputter-processed indium-gallium-zinc oxide films by simultaneous ultraviolet and thermal treatments.

Indium–gallium–zinc oxide (IGZO) films, deposited by sputtering at room temperature, still require activation to achieve satisfactory semiconductor characteristics. Thermal treatment is typically carried out at temperatures above 300 °C. Here, we propose activating sputter- processed IGZO films using simultaneous ultraviolet and thermal (SUT) treatments to decrease the required temperature and enhance their electrical characteristics and stability. SUT treatment effectively decreased the amount of carbon residues and the number of defect sites related to oxygen vacancies and increased the number of metal oxide (M–O) bonds through the decomposition-rearrangement of M–O bonds and oxygen radicals. Activation of IGZO TFTs using the SUT treatment reduced the processing temperature to 150 °C and improved various electrical performance metrics including mobility, on-off ratio, and threshold voltage shift (positive bias stress for 10,000 s) from 3.23 to 15.81 cm2/Vs, 3.96 × 107 to 1.03 × 108, and 11.2 to 7.2 V, respectively.

Artificial DNA nanostructure detection using solution-processed In-Ga-Zn-O thin-film transistors

A method for detecting artificial DNA using solution-processed In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) was developed. The IGZO TFT had a field-effect mobility (μFET) of 0.07 cm2/Vs and an on-current (Ion) value of about 2.68 μA. A dry-wet method was employed to immobilize double-crossover (DX) DNA onto the IGZO surface. After DX DNA immobilization, significant decreases in μFET (0.02 cm2/Vs) and Ion (0.247 μA) and a positive shift of threshold voltage were observed. These results were attributed to the negatively charged phosphate groups on the DNA backbone, which generated electrostatic interactions in the TFT device.A method for detecting artificial DNA using solution-processed In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) was developed. The IGZO TFT had a field-effect mobility (μFET) of 0.07 cm2/Vs and an on-current (Ion) value of about 2.68 μA. A dry-wet method was employed to immobilize double-crossover (DX) DNA onto the IGZO surface. After DX DNA immobilization, significant decreases in μFET (0.02 cm2/Vs) and Ion (0.247 μA) and a positive shift of threshold voltage were observed. These results were attributed to the negatively charged phosphate groups on the DNA backbone, which generated electrostatic interactions in the TFT device.

Hole Transport Enhancing Effects of Polar Solvents on Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonic acid) for Organic Solar Cells

This study analyzed the properties of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) thin-films prepared by spin-coating solutions made with the polar solvents methanol, acetone, or N,N-dimethylformamide (DMF). A characteristic analysis was carried out for poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM)-based organic solar cells (OSCs) having these modified PEDOT:PSS thin-films as the hole transport layer. The resistivity of the PEDOT:PSS thin-film obtained from the DMF solution was 4.89 × 10(-3) Ω·cm with a roughness of 3.23 × 10° nm, compared to 3.51 × 10(-1) Ω·cm and 7.72 × 10(-1) nm for a pristine PEDOT:PSS thin-film. The dipole moment increase of the solvent led to the decreased resistivity and the increased roughness and transparency of PEDOT:PSS thin-films on the structural arrangement of the polymers. Highly efficient OSCs with a power conversion efficiency of 3.47% were obtained when DMF-treated PEDOT:PSS thin-film was used as the hole transport layer.

Electrical responses of artificial DNA nanostructures on solution-processed In-Ga-Zn-O thin-film transistors with multistacked active layers.

We propose solution-processed In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) with multistacked active layers for detecting artificial deoxyribonucleic acid (DNA). Enhanced sensing ability and stable electrical performance of TFTs were achieved through use of multistacked active layers. Our IGZO TFT had a turn-on voltage (V(on)) of -0.8 V and a subthreshold swing (SS) value of 0.48 V/decade. A dry-wet method was adopted to immobilize double-crossover DNA on the IGZO surface, after which an anomalous hump effect accompanying a significant decrease in V(on) (-13.6 V) and degradation of SS (1.29 V/decade) was observed. This sensing behavior was attributed to the middle interfaces of the multistacked active layers and the negatively charged phosphate groups on the DNA backbone, which generated a parasitic path in the TFT device. These results compared favorably with those reported for conventional field-effect transistor-based DNA sensors with remarkable sensitivity and stability.

Low-voltage driving solution-processed nickel oxide based unipolar resistive switching memory with Ni nanoparticles

Resistive random access memory (RRAM) combines the advantages of nonvolatile flash memory and volatile dynamic random access memory, avoiding their drawbacks. For the practical use of RRAM, achieving low-voltage driving is strongly desired. Here we report the effect of Ni nanoparticles on solution-processed NiO based RRAM which can realize a one diode one resistor operation by unipolar resistive switching mode and low-voltage driving for future demands. The Ni nanoparticles not only contributed to high oxygen mobility, but also affected effective insulator thickness reduction, and stoichiometric variation in NiO thin films. Furthermore, the Ni nanoparticle embedded device demonstrated good reliability. These findings can enhance the applicability of RRAM as a next generation memory device.

The effect of various solvents on the back channel of solution-processed In?Ga?Zn?O thin-film transistors intended for biosensor applications

This study investigated the effects of exposing solution-processed In–Ga–Zn–O (IGZO) thin-film transistors (TFTs), intended for biosensor applications, to various solvents. Various solvents, such as the nonpolar solvent chlorobenzene and the polar solvents ethanol and deionized (DI) water, were dropped and adsorbed on exposed IGZO channel surfaces. All IGZO TFT devices exhibited a negative threshold voltage shift and a sub-threshold swing degradation, without an accompanying degradation in field-effect mobility. These variations depended on the dielectric constant of the solvents; with the exception of the IGZO TFT device exposed to DI water, they all gradually returned to their initial states.