Yangho Jung

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 ]

A solution-processed yttrium oxide gate insulator for high-performance all-solution-processed fully transparent thin film transistors

We studied high-k soluble yttrium oxide dielectrics for high performance oxide thin film transistors (TFTs). The electrical characteristics of yttrium oxide show leakage current density as less than 10−6 A cm−2 at 2 MV cm−1 regardless of annealing temperature and a high dielectric constant of nearly 16. For the first time, all solution-processed fully transparent ZnO-TFTs based on spin-coated YOx gate dielectric layers with a small interfacial trap density and high capacitance were demonstrated, exhibiting a field-effect mobility of 135 cm2 V−1 s−1, a threshold voltage of 1.73 V, and an on–off current ratio of 5.7 × 107 as well as low-voltage operation. In addition to microstructural and electrical analyses for solution-processed YOx dielectrics, we investigated the influences of dielectric–semiconductor interfacial quality on device parameters. Our results suggest that solution-processable fully transparent oxide TFTs have the potential for low-temperature and high-performance application in transparent, flexible devices.

Bias stress stable aqueous solution derived Y-doped ZnO thin film transistors

We demonstrate solution processed oxide semiconductor thin-film transistors (TFTs) with high performance as well as improved electrical/thermal temperature stress stabilities. Yttrium-doped ZnO (YZO) TFTs are fabricated using aqueous precursors prepared via direct dissolution of metal hydroxides. The Y contents in YZO films are critical for determining the intrinsic electrical properties as well as the positive bias stress-, negative bias stress-, and negative bias temperature stress-induced instabilities. Solution processed 1% Y-doped ZnO TFT annealed at 350 °C exhibits a noticeably lower threshold voltage shift of 3.78 V under positive bias stress and −1.72 V under negative bias temperature stress as well as the good device performance with a mobility of ∼1.8 cm2 V−1 s−1 and an on/off current ratio of ∼107. Our results suggest that solution processed Y-doped ZnO TFTs have potential for use in high stability performance applications in transparent devices.

Enhanced Performance of Solution-Processed Amorphous LiYInZnO Thin-Film Transistors

Solution-processed, amorphous lithium-doped YInZnO (L-YIZO) thin-film transistors (TFTs) are investigated. An appropriate amount of Li doping significantly enhances the field-effect mobility in TFT performance (~15 times greater than that of nondoped YIZO) without controlled annealing under water vapor or O(3)/O(2) environments. The addition of Li into solution-processed YIZO semiconductors leads to improved film quality, which results from enriched metal oxygen bonding and reduced defect sites, such as oxygen vacancies and hydroxyl groups. Li doping of an amorphous ionic oxide semiconductor (AIOS) could serve as an effective strategy for low-temperature and high-performance solution-processed AIOS TFTs.

Influences of pH and ligand type on the performance of inorganic aqueous precursor-derived ZnO thin film transistors.

The aqueous precursor-derived ZnO semiconductor is a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin-film transistors at low temperatures. The pH in the aqueous solution is an important factor in determining the device performance of ZnO-TFTs. Using a basic aqueous solution, the ZnO transistor annealed at 150 °C exhibited a high field-effect mobility (0.42 cm(2) V(-1) s(-1)) and an excellent on/off ratio (10(6)). In contrast, the ZnO layer annealed at 150 °C prepared from an acidic solution was inactive. Chemical and structural analyses confirmed that the variation of the device characteristics originates from the existing state difference of Zn in solution. The hydroxyo ligand is stable in basic conditions, which involves a lower energy pathway for the solution-to-solid conversion, whereas the hydrated zinc cation undergoes more complex reactions that occur at a higher temperature. Our results suggest that the pH and ligand type play critical roles in the preparation of aqueous precursor-based ZnO-TFTs which demonstrate high performance at low temperatures.

Solution-processable tin-doped indium oxide with a versatile patternability for transparent oxide thin film transistors

We developed solution-processable tin-doped indium oxide (ITO) with a versatile patternability. Controlling the Sn doping concentration and the annealing method/atmosphere enabled highly conductive transparent micro-patterned electrodes defined by conventional photolithography, inkjet printing, and transfer molding. Such a versatile liquid-phase ITO material was successfully applied to demonstrate for the first time fully transparent all-oxide thin film transistors with the solution-processed gate/source/drain electrodes. This proof-of-concept study suggests that our solution-processable transparent conducting oxide can open the possibility of realizing fully transparent devices using all-solution processing.

Metal-Semiconductor Contact Behavior of Solution-Processed ZnSnO Thin Film Transistors

We studied the influence of different types of metal electrodes on the performance of solution-processed zinc tin oxide (ZTO) thin-film transistors. The ZTO thin-film was obtained by spin-coating the sol-gel solution made from zinc acetate and tin acetate dissolved in 2-methoxyethanol. Various metals, Al, Au, Ag and Cu, were used to make contacts with the solution-deposited ZTO layers by selective deposition through a metal shadow mask. Contact resistance between the metal electrode and the semiconductor was obtained by a transmission line method (TLM). The device based on an Al electrode exhibited superior performance as compared to those based on other metals. Kelvin probe force microscopy (KPFM) allowed us to measure the work function of the oxide semiconductor to understand the variation of the device performance as a function of the types metal electrode. The solution-processed ZTO contained nanopores that resulted from the burnout of the organic species during the annealing. This different surface structure associated with the solution-processed ZTO gave a rise to a different work function value as compared to the vacuum-deposited counterpart. More oxygen could be adsorbed on the nanoporous solution-processed ZTO with large accessible surface areas, which increased its work function. This observation explained why the solution-processed ZTO makes an ohmic contact with the Al electrode.