Jee Ho Park

Low-temperature, high-performance solution-processed thin-film transistors with peroxo-zirconium oxide dielectric.

We demonstrated solution-processed thin film transistors on a peroxo-zirconium oxide (ZrO(2)) dielectric with a maximum temperature of 350 °C. The formation of ZrO(2) films was investigated by TG-DTA, FT-IR, and XPS analyses at various temperatures. We synthesized a zirconium oxide solution by adding hydrogen peroxide (H(2)O(2)). The H(2)O(2) forms peroxo groups in the ZrO(2) film producing a dense-amorphous phase and a smooth surface film. Because of these characteristics, the ZrO(2) film successfully blocked leakage current even in annealing at 300 °C. Finally, to demonstrate that the ZrO(2) film is dielectric, we fabricated thin-film transistors (TFTs) with a solution-processed channel layer of indium zinc oxide (IZO) on ZrO(2) films at 350 °C. These TFTs had a mobility of 7.21 cm(2)/(V s), a threshold voltage (V(th)) of 3.22 V, and a V(th) shift of 1.6 V under positive gate bias stress.

Boron-Doped Peroxo-Zirconium Oxide Dielectric for High-Performance, Low-Temperature, Solution-Processed Indium Oxide Thin-Film Transistor

We developed a solution-processed indium oxide (In2O3) thin-film transistor (TFT) with a boron-doped peroxo-zirconium (ZrO2:B) dielectric on silicon as well as polyimide substrate at 200 °C, using water as the solvent for the In2O3 precursor. The formation of In2O3 and ZrO2:B films were intensively studied by thermogravimetric differential thermal analysis (TG-DTA), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT IR), high-resolution X-ray diffraction (HR-XRD), and X-ray photoelectron spectroscopy (XPS). Boron was selected as a dopant to make a denser ZrO2 film. The ZrO2:B film effectively blocked the leakage current at 200 °C with high breakdown strength. To evaluate the ZrO2:B film as a gate dielectric, we fabricated In2O3 TFTs on the ZrO2:B dielectrics with silicon substrates and annealed the resulting samples at 200 and 250 °C. The resulting mobilities were 1.25 and 39.3 cm(2)/(V s), respectively. Finally, we realized a flexible In2O3 TFT with the ZrO2:B dielectric on a polyimide substrate at 200 °C, and it successfully operated a switching device with a mobility of 4.01 cm(2)/(V s). Our results suggest that aqueous solution-processed In2O3 TFTs on ZrO2:B dielectrics could potentially be used for low-cost, low-temperature, and high-performance flexible devices.

Solution-processed high-k HfO2 gate dielectric processed under softening temperature of polymer substrates

A low-temperature, solution-processed high-k HfO2 gate dielectric was demonstrated. To decompose a hafnium precursor at a temperature lower than 200 °C, an aqueous solution of HfCl4 was used because the strongly hydrated hafnium precursor was decomposed at a much lower temperature than anhydrous or partially hydrated hafnium chloride. No hazardous organic material was required in the low-temperature HfO2 coating process. Thus this precursor solution is environmentally safe and it is preferable to use this solution for gate dielectric coating on flexible substrates. The fabricated HfO2 gate dielectric shows reliable breakdown characteristics and high dielectric constant. We fabricated a thin film transistor (TFT) device with this gate dielectric and a maximum processing temperature of 150 °C for all the components of the TFT. The ZnO TFT on the HfO2 gate dielectric shows field-effect mobility of 1.17 cm2 V−1 s−1 and threshold voltage of 5.87 V. These results demonstrate the potential of our HfO2 thin film for flexible electronic device fabrication.

Effects of solution temperature on solution-processed high-performance metal oxide thin-film transistors

Herein, we report a novel and easy strategy for fabricating solution-processed metal oxide thin-film transistors by controlling the dielectric constant of H2O through manipulation of the metal precursor solution temperature. As a result, indium zinc oxide (IZO) thin-film transistors (TFTs) fabricated from IZO solution at 4 °C can be operated after annealing at low temperatures (∼250 °C). In contrast, IZO TFTs fabricated from IZO solutions at 25 and 60 °C must be annealed at 275 and 300 °C, respectively. We also found that IZO TFTs fabricated from the IZO precursor solution at 4 °C had the highest mobility of 12.65 cm2/(V s), whereas the IZO TFTs fabricated from IZO precursor solutions at 25 and 60 °C had field-effect mobility of 5.39 and 4.51 cm2/(V s), respectively, after annealing at 350 °C. When the IZO precursor solution is at 4 °C, metal cations such as indium (In3+) and zinc ions (Zn2+) can be fully surrounded by H2O molecules, because of the higher dielectric constant of H2O at lower temperatures. These chemical complexes in the IZO precursor solution at 4 °C are advantageous for thermal hydrolysis and condensation reactions yielding a metal oxide lattice, because of their high potential energies. The IZO TFTs fabricated from the IZO precursor solution at 4 °C had the highest mobility because of the formation of many metal-oxygen-metal (M-O-M) bonds under these conditions. In these bonds, the ns-orbitals of the metal cations overlap each other and form electron conduction pathways. Thus, the formation of a high proportion of M-O-M bonds in the IZO thin films is advantageous for electron conduction, because oxide lattices allow electrons to travel easily through the IZO.

Highly efficient inverted polymer solar cells with reduced graphene-oxide-zinc-oxide nanocomposites buffer layer

In this study, we reported a 36% improvement in the performance of inverted solar cells as a result of increased short-circuit current (JSC) obtained using a composition of zinc oxide (ZnO) and reduced graphene oxide (RGO) as an n-type buffer layer. RGO-ZnO nanocomposites show higher electron conductivity than intrinsic ZnO; moreover, they show reduced contact resistance at the interface between the active layer and n-type buffer layer. These factors prevent carrier loss resulting from defects and recombinations in the device, thereby significantly increasing the JSC value for the device. Thus, an efficiency of 4.15% was achieved for inverted solar cells with a controlled RGO-ZnO nanocomposites layer.

Low-temperature, solution-processed ZrO2: B Thin film: A bifunctional inorganic/organic interfacial glue for flexible thin-film transistors

A solution-processed boron-doped peroxo-zirconium oxide (ZrO2:B) thin film has been found to have multifunctional characteristics, providing both hydrophobic surface modification and a chemical glue layer. Specifically, a ZrO2:B thin film deposited on a hydrophobic layer becomes superhydrophilic following ultraviolet-ozone (UVO) treatment, whereas the same treatment has no effect on the hydrophobicity of the hydrophobic layer alone. Investigation of the ZrO2:B/hydrophobic interface layer using angle-resolved X-ray photoelectron spectroscopy (AR XPS) confirmed it to be chemically bonded like glue. Using the multifunctional nature of the ZrO2:B thin film, flexible amorphous indium oxide (In2O3) thin-film transistors (TFTs) were subsequently fabricated on a polyimide substrate along with a ZrO2:B/poly-4-vinylphenol (PVP) dielectric. An aqueous In2O3 solution was successfully coated onto the ZrO2:B/PVP dielectric, and the surface and chemical properties of the PVP and ZrO2:B thin films were analyzed by contact angle measurement, atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The surface-engineered PVP dielectric was found to have a lower leakage current density (Jleak) of 4.38 × 10(-8) A/cm(2) at 1 MV/cm, with no breakdown behavior observed up to a bending radius of 5 mm. In contrast, the electrical characteristics of the flexible amorphous In2O3 TFT such as on/off current ratio (Ion/off) and electron mobility remained similar up to 10 mm of bending without degradation, with the device being nonactivated at a bending radius of 5 mm. These results suggest that ZrO2:B thin films could be used for low-temperature, solution-processed surface-modified flexible devices.

Structural and Electrical Properties of Solution-Processed Gallium-Doped Indium Oxide Thin-Film Transistors

We fabricated solution-processed gallium-doped indium oxide (GIO) thin-film transistors (TFTs). The electrical property, crystallinity, and transmittance were investigated as a function of gallium content. Varying the gallium/indium ratio is found to have a significant effect on structural and electrical properties of thin films. The shrinkage of the lattice of a GIO film originates from substitution of Ga on In sites in the In2O3 lattice, which was verified by X-ray diffraction (XRD) analysis. By increasing the gallium ratio of the channel material, the GIO film shows an amorphous phase. The optimized GIO film (Ga/In= 0.35) has an electron mobility of 3.59 cm2 V-1 s-1, a threshold voltage of 0.1 V, an on/off current ratio of 8.2×107, and a subthreshold slope of 0.9 V/decade, and is highly transparent (~92%) in the visible region.

Low-Temperature, Solution-Processed Zinc Tin Oxide Thin-Film Transistors Fabricated by Thermal Annealing and Microwave Irradiation

In this study, we fabricated zinc tin oxide (ZTO) thin-film transistors (TFTs) using a sol–gel solution at a low annealing temperature of 350 °C. We found that the combination of conventional hot plate annealing and microwave irradiation was effective for achieving high performance of ZTO-TFTs. Solution-processed ZTO-TFTs prepared at 350 °C by microwave irradiation showed enhanced device characteristics of 1.84 cm2 V-1 s-1 mobility and a 106 on/off current ratio. X-ray photoelectron spectroscopy analyses confirmed that residual hydroxyls in solution-processed ZTO films can be decomposed at low temperatures by microwave irradiation. Low-temperature microwave-assisted processing makes ZTO TFTs promising for use in flexible, transparent electronics.

Electrical Contact Tunable Direct Printing Route for a ZnO Nanowire Schottky Diode

Although writing was the first human process for communication, it may now become the main process in the electronics industry, because in the industry the programmability as an inherent property is a necessary requirement for next-generation electronics. As an effort to open the era of writing electronics, here we show the feasibility of the direct printing of a high-performance inorganic single crystalline semiconductor nanowire (NW) Schottky diode (SD), including Schottky and Ohmic contacts in series, using premetallization and wrapping with metallic nanofoil. To verify the feasibility of our process, SDs made of Al-premetalized ZnO NWs and plain ZnO NWs were compared with each other. Even with cold direct printing, the Al-premetalized ZnO NW SD showed higher performance, specifically 1.52 in the ideality factor and 1.58 x 10(5) in its rectification ratio.

Homogeneous liquid crystal alignment on inorganic–organic hybrid silica thin films derived by the sol–gel method

We developed an inorganic–organic hybrid thin film via the sol–gel method for a new liquid crystal alignment layer and investigated the influence of an organic species on the alignment characteristics of the liquid crystals (LCs). A thin film of methyl-doped amorphous silicon oxide (a-SiOx:CH3) was fabricated from the hydrolysis and condensation reaction of the initial precursors of methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) at a proper ratio. A low-energy ion beam (IB) treatment gives rise to the homogeneous alignment of LC in an IB condition on a-SiOx:CH3 thin film; however, it is difficult to control the LC alignment on a-SiOx thin film derived only from TEOS as a precursor. The LC alignment depending on the chemical structure of the silica thin film was investigated and analyzed in terms of the sensitivity of the axis-selective destruction of the chemical bonding on the surface of the thin silica film.