Keun-Kyu Song

Solution-deposited Zr-doped AlOx gate dielectrics enabling high-performance flexible transparent thin film transistors

Although high dielectric constant (k) oxide thin film has been considered as a key element for high performance and low-voltage driven thin-film transistors (TFTs), there are no solution processable high-k oxide dielectrics that satisfy the stringent requirements of low-temperature processability, mechanical flexibility, and good TFT performance. Here, we demonstrate that the incorporation of a zirconium component that has strong bonding to oxygen enables a significant reduction in the processing temperature for soluble alumina dielectrics to as low as 250 °C. Based on these Zr-AlOx films, high performance, low operational voltage, flexible TFTs are achieved. Flexible TFTs operate well under a gate bias of 5 V, exhibiting a high saturation mobility of 51 cm2 V−1 s−1, an on/off current ratio of 104, and a low threshold voltage of 1.2 V with good mechanical flexibility. This is the first study demonstrating the mechanical flexibility of all-oxide soluble high-k dielectric–semiconductor-based TFTs with an emphasis on the influence of annealing temperature on the solution-deposited high-k oxide dielectric characteristics.

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.

High performance and high stability low temperature aqueous solution-derived Li–Zr co-doped ZnO thin film transistors

Highly stable and high performance solution-processed amorphous oxide semiconductor thin film transistors (TFTs) were produced using a Li and Zr co-doped ZnO-based aqueous solution. Li and Zr co-doping at the appropriate amounts enhanced the oxide film quality in terms of enhanced oxygen bonding and reduced defect sites. The 0.5 mol% Li and 1.0 mol% Zr co-doped ZnO TFTs annealed at 320 °C exhibited noticeably lower threshold voltage shifts of 3.54 V under positive bias stress and −2.07 V under negative bias temperature stress than the non-doped ZnO TFTs. The transistors revealed a good device mobility performance of 5.39 cm2 V−1 s−1 and an on/off current ratio of 108 when annealed at 320 °C, compared to a mobility performance of 2.86 cm2 V−1 s−1 and an on/off current ratio of ∼107 when annealed at 270 °C. Our results suggest that Li and Zr co-doping can be a useful technique to produce more reliable and low temperature solution-processed oxide semiconductor TFTs.

9.2: Implementation of RGBW Color System in TFT‐LCDs

Last year, we introduced a TFT-LCD with RGBW color system. The primary advantage of the RGBW system is that its optical efficiency is at least 50% higher than the RGB system. However, it is not a simple task to incorporate the new color system into the existing infrastructure of the RGB system: the driving circuitry, fabrication of color filter, and color conversion. In this report, the practical hurdles are discussed and the solutions are presented.

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.

Direct photopatternable organic–inorganic hybrid gate dielectric for solution-processed flexible ZnO thin film transistors

A direct photopatternable organosiloxane-based organic–inorganic hybrid gate dielectric was synthesized. The sol–gel derived hybrid dielectric could be cured at a temperature sufficiently low enough to apply to temperature-sensitive polymeric substrates, whilst maintaining good electrical properties. The addition of hexa(methoxymethyl)melamine minimizes the polar silanol group by enhancing the cross-linking reaction and improves the film density, so that the resulting hybrid dielectric retains both thermal and chemical stability against the highly basic aqueous semiconductor precursor, forming a coherent interface between the dielectric and the semiconductor. The synthesized hybrid materials allow for a high-performance solution-processed flexible ZnO transistor on polymeric substrate at 150 °C.

31.1: Development of Six Primary-Color LCD

We developed the multiple primary color LCDs where the pixels were composed of red(R), green(G), blue(B), cyan(C), magenta(M) and yellow(Y) subpixels. The color gamut was extended 99% of NTSC standard, while the brightness was also increased by 15%, compared to the conventional RGB three primary color LCDs. To get a natural color image we designed the color coordinates and luminance levels of each six subpixles, and developed color algorithm to convert the RGB signal to RGBCMY signal.

Effects of Plasma Process Induced Damages on Organic Gate Dielectrics of Organic Thin-Film Transistors

The effects of plasma damages on the organic gate dielectric of the organic thin film transistor (OTFT) during the fabrication process are investigated; metal deposition process on the organic gate insulator by the plasma sputtering mainly generates the process induced damage of bottom contact structured OTFT. For this study, two different deposition methods (thermal evaporation and plasma sputtering) have been tested for their damage effects onto poly(4-vinyl phenol) (PVP) as organic gate dielectric. Unlike thermal evaporation, conventional plasma sputtering process induces serious damages onto the organic layer as increasing the surface energy, changing the surface morphology and degrading OTFT performances.

3.5: Invited Paper: Recent Progress in Large Sized & High Performance Organic TFT Array

Recently, we have developed 15.0 inch XGA organic TFT array and its LCD module, which might be world-largest array size and highest resolution in organic based device. This achievement is to be combined with several technical breakthroughs include new organic gate dielectric material, novel ohmic contact technique, and well-optimized process-architectures. Base on the newly developed organic gate dielectric with pentacene, we have obtained world-record field effect TFT mobilities in excess of 7cm2/Vsec and excellent on/off ratios as ∼106. For jumping up the OTFT research activities from laboratory scale to real production line, we have developed two key technologies. One is the new ohmic contact process with very common materials in the current LCD manufacturing such as ITO; the bottom contact structured OTFT with ITO source-drain electrode shows much the same performances as that with gold source-drain electrode. The other key technology to realize the large size organic device is that of optimizing the array structure to minimize the OTFT performance degradations. By combining these novel technologies, a 15.0″ XGA prototype LCD panel is fabricated and shows good display performance.

Low-Temperature Solution-Deposited Oxide Thin-Film Transistors Based on Solution-Processed Organic–Inorganic Hybrid Dielectrics

We describe low-temperature, solution-deposited, oxide semiconductor thin-film transistors (TFTs) with a solution-processed gate dielectric in this study. The sol–gel-derived indium zinc oxide (IZO) semiconductor matched well with the organic–inorganic hybrid dielectric annealed at 200 °C, forming a coherent interface between the semiconductor and the dielectric without evidence of chemical damage. The IZO-TFTs made with a 420-nm-thick hybrid dielectric layer showed good performance: a low off-current on the order of <10-10 A, a field-effect mobility of 3.3×10-2 cm2 V-1 s-1, and a low threshold gate voltage of ~2.4 V. Spin-coating of the IZO semiconductor on a hybrid dielectric/glass substrate results in TFTs optically transparent in the entire visible region (~90%). Our solution-processable materials of the semiconductor and the gate dielectric can open the possibility of realizing flexible transparent devices using all-solution processing.