Keunkyu Song

Fully Flexible Solution‐Deposited ZnO Thin‐Film Transistors

Electronic systems on fl exible substrates posses the advantage of mechanical fl exibility in actual use, but also provide more rugged rollable devices and may therefore result in lower manufacturing costs associated with continuous roll-to-roll fabrication. To realize these advantages of fl exible electronics, lowtemperature solution processing is strongly desirable. In this regard, organic semiconductor materials have been extensively researched. [ 1 ] Organic semiconductor polymers are soluble in a variety of solvents, and small molecules can be derivatized to soluble precursors. Organic transistors can also be fabricated by solution processing near room temperature, [ 2 ] compatible with temperature-sensitive plastic substrates. [ 3–5 ] Despite successful demonstrations of fl exible organic electronics, however, they are generally sensitive to operating conditions and are unstable during long-term operation. [ 6 ]

Inkjet-Printed Zinc Tin Oxide Thin-Film Transistor

Recently, there has been considerable interest in adapting printing approaches that are typically used in the graphic arts to the printing of electronic circuits and circuit components. We report the fabrication of solution-processed oxide transistors using inkjet printing. A zinc tin oxide sol-gel precursor is utilized as the ink for directly printing a thin uniform semiconducting layer. The printed device performance is significantly influenced by printing conditions such as the surface wettability and substrate temperature. The inkjet-printed transistors exhibit reproducible electrical performance, demonstrating their potential application in low-cost manufacturing of large-area flat panel displays.

High-performance low-temperature solution-processable ZnO thin film transistors by microwave-assisted annealing

Oxide semiconductors afford a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin film transistors (TFTs). We synthesized an aqueous inorganic precursor by a direct dissolution of zinc hydroxide in ammonium hydroxide solution from which a dense and uniform ZnO semiconducting layer is achieved. Solution-processed ZnO-TFTs prepared at 140 °C by microwave irradiation have shown enhanced device characteristics of ∼1.7 cm2 V−1s−1 mobility and a ∼107 on/off current ratio, with good air stability. Spectroscopic analyses confirmed that such a device improvement originates from accelerated dehydroxylation and better crystallization at low temperature by microwave irradiation. Our results suggest that solution-processable oxide semiconductors have potential for low-temperature and high-performance applications in transparent devices.

Direct nanoprinting by liquid-bridge-mediated nanotransfer moulding

Several techniques for the direct printing of functional materials have been developed to fabricate micro- and nanoscale structures and devices. We report a new direct patterning method, liquid-bridge-mediated nanotransfer moulding, for the formation of two- or three-dimensional structures with feature sizes as small as tens of nanometres over large areas up to 4 inches across. Liquid-bridge-mediated nanotransfer moulding is based on the direct transfer of various materials from a mould to a substrate through a liquid bridge between them. We demonstrate its usefulness by fabricating nanowire field-effect transistors and arrays of pentacene thin-film transistors.

Bias-Stress-Stable Solution-Processed Oxide Thin Film Transistors

We generated a novel amorphous oxide semiconductor thin film transistor (AOS-TFT) that has exellent bias-stress stability using solution-processed gallium tin zinc oxide (GSZO) layers as the channel. The cause of the resulting stable operation against the gate bias-stress was studied by comparing the TFT characteristics of the GSZO layer with a tin-doped ZnO (ZTO) layer that lacks gallium. By photoluminescence, X-ray photoelectron, and electron paramagnetic resonance spectroscopy, we found that the GSZO layer had a significantly lower oxygen vacancy, which act as trap sites, than did the ZTO film. The successful fabrication of a solution-processable GSZO layer reported here is the first step in realizing all-solution-processed transparent flexible transistors with air-stable, reproducible device characteristics.

Solution processed invisible all-oxide thin film transistors

We report on a fully transparent solution processed thin-film transistor (TFT) device with oxide semiconductor and oxide electrode. Selective doping into the sol–gel derived ZnO materials tailors the electrical properties to range from metallic to semiconducting characteristics. Integration of a spin-coated zinc tin oxide (ZTO) semiconductor with an ink-jet-printed zinc indium oxide (ZIO) electrode creates a transparent TFT with high performance and good transparency (∼90%). Use of the same ZnO-based oxide materials in a TFT allows for the formation of good electrical contacts characterized by low contact resistance, comparable to those with a vacuum-deposited Al electrode. Our results suggest that the solution-processed ZnO-based TFT has great potential to work as a building block for future printed transparent electronics.

Compositional influence on sol-gel-derived amorphous oxide semiconductor thin film transistors

We investigated the influence of the chemical compositions of gallium and indium cations on the performance of sol-gel-derived amorphous gallium indium zinc oxide (a-GIZO)-based thin film transistors (TFTs). A systematic compositional study allowed us to understand the solution-processed a-GIZO TFTs. We generated a compositional ternary diagram from which we could predict electrical parameters such as saturation mobility, threshold voltage, and the on/off current ratio as the constituent compositions varied. This diagram can be utilized for tailoring solution-processed amorphous oxide TFTs for specific applications.

Low Temperature Solution-Processed InZnO Thin-Film Transistors

We prepared indium zinc oxide (IZO) semiconductors for low temperature solution-processed thin-film transistors (TFTs). The sol-gel derived IZO films, annealed at 300°C, are uniform and have smooth surface morphology (root-mean-square roughness of 0.27 nm). Both the composition and the film thickness need to be optimized for high performance TFTs. With the composition of In/Zn equal to 50/50 in mol percent, the IZO TFTs with a thickness of 10 nm exhibited the best performance for a clear switching behavior (on/off current ratio of 1.2 × 10 7 and output characteristics (drain current of 3.7 × 10 -4 A), with a relatively high field-effect mobility (0.54 cm 2 V -1 s -1 ) and a low threshold voltage (1.9 V). The nonpassivated IZO-TFT stably operates over a two-month period without any significant change in the on/off current ratio and the mobility.

Bias Stress Stability of Solution-Processed Zinc Tin Oxide Thin-Film Transistors

The effects of bias stress on spin-coated zinc tin oxide (ZTO) transistors are investigated. Applying a positive bias stress results in the displacement of the transfer curves in the positive direction without changing the field-effect mobility or the subthreshold behavior, while a negative stress has no effect on the threshold voltage shift. Device instability appears to be a consequence of the charging and discharging of the temporal trap states at the interface and in the ZTO channel region. All the stressed devices recover their original characteristics after 10 min at room temperature. Furthermore, the inkjet-printed transistor yields similar bias stress effects as those observed in their spin-coated counterparts but has a greater shift in the threshold voltage. Microstructural evidence in conjunction with Rutherford backscattering spectroscopy confirms that severe instability is attributed to the presence of nanopores in the inkjet-printed channel layer.

All Solution-Processed, Fully Transparent Resistive Memory Devices

We fabricated all-solution processed, fully transparent resistive random access memory (sol-TRRAM) with a configuration of ITO/GaZnO(GZO)/ITO. All layers, including an active layer and top and bottom ITO electrodes, were deposited on a glass substrate by either spin coating or inkjet printing using a sol-gel solution. Our sol-TRRAM was transparent, with 86.5% transmittance at 550 nm. An initial forming process is unnecessary for the production of transparent memory due to the presence of sufficient inherent nonlattice oxygen ions in the solution-processed GZO layer. The sol-TRRAM also showed reasonable bipolar resistance switching with a low operation current (<100 μA) and excellent cycle endurance properties (>300 cycles). The main conduction mechanism during the set process can be explained by the trap-controlled space-charge limited conduction, and the resistance change occurred by the modification of the potential barrier height because of the charge injection by Fowler-Nordheim tunneling.