Youngmin Jeong

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.

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.

Inkjet-printed Cu source/drain electrodes for solution-deposited thin film transistors

We report on the first utility of Cu nanoparticle inks as low-cost, printable electrodes in the fabrication of solution-deposited amorphous oxide semiconductor thin film transistors. The performance of printed Cu electrodes was studied in terms of involvements of surface states in the devices. The surface chemical structures of Cu nanoparticulate electrodes were observed to be modified, dependent on the molecular weight of the polyvinylpyrrolidone capping molecules used in their synthesis. The surface dipoles became weak, and the work function of the printed electrodes decreased with increasing the molecular weight. The work function tailored by introducing the larger capping agents allowed for a better energetic leveling with the metal oxide semiconductor layer, resulting in the improved device performance.

Thin Film Transistors with Ink-Jet Printed Amorphous Oxide Semiconductors

We investigated the influence of the thickness of printed Ga–In–Zn-O (GIZO) channel on transistor performance. Semiconductor layers were ink-jet printed using sol–gel derived GIZO solution and the thickness of the resulting active layers varied depending on the pre-heated substrate temperature. We found that GIZO film thickness significantly influences device performance. Thin film transistors (TFTs) with thicker active layers showed higher on-current/off-current mobility and a threshold voltage shift in the negative direction. The dependence of the electric characteristics on thickness resulted from the increased intrinsic free charge carriers as the active layer thickness increased. Ink-jet printing conditions need to be carefully controlled to maximize device performance.

Laser-direct photoetching of metal thin film for the electrode of transistor

We report that metal thin films can be directly photoetched by a pulsed neodymium doped yttrium aluminum garnet laser beam irradiating the film surface. This process utilizes a laser-induced thermoelastic force, which plays a role to detach the film from the underlying layer. High-fidelity patterns at the micrometer scales have been fabricated using a spatially modulated laser beam. A zinc-tin-oxide thin film transistor with photoetched Al source and drain electrodes exhibited an on/off ratio higher than 105 and a very low off-current level. This indicates that the metallic layer is completely etched out by this process, making an additional cleaning or etching step unnecessary.