Chaun Gi Choi

High performance solution-processed amorphous zinc tin oxide thin film transistor

Thin film transistors (TFTs) with amorphous zinc tin oxide (ZTO) channel layer were fabricated by a simple and low-cost solution process. The ZTO thin films are highly transparent (>90% transmittance) in the visible region. The ZTO TFTs fabricated at 400 and 500 °C are operated in enhancement mode. The TFT annealed at 500 °C shows a mobility of 14.11 cm2 V−1 s−1, a threshold voltage of 1.71 V, a subthreshold slope of 0.4 V dec−1 and an on–off current ratio greater than 108. In addition, we investigated the gate bias stability of the TFT. Positive gate bias results in a positive shift of the threshold voltage due to the charge trapping in the channel/dielectric interface.

Solution-Processed Indium-Zinc Oxide Transparent Thin-Film Transistors

Transparent thin-film transistors (TTFTs) with an indium-zinc oxide (IZO) active layer by the solution-processed deposition method were fabricated and their TFT characterization was examined. Solution-processed IZO thin films were amorphous and highly transparent with >90% transmittance in the visible region with an optical bandgap of 3.1 eV. Spin-coated IZO TTFTs were operated in depletion mode and showed a field-effect mobility as high as 7.3 cm 2 /V s, a threshold voltage of 2.5 V, an on/off current ratio greater than 10 7 , and a subthreshold slope of 1.47 V/decade.

Amorphous-oxide TFT backplane for large-sized AMOLED TVs

— Amorphous-oxide thin-film-transistor (TFT) arrays have been developed as TFT backplanes for large-sized active-matrix organic light-emitting-diode (AMOLED) displays. An amorphous-IGZO (indium gallium zinc oxide) bottom-gate TFT with an etch-stop layer (ESL) delivered excel lent electrical performance with a field-effect mobility of 21 cm2/V-sec, an on/off ratio of >108, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous-oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold-voltage (VTH) compensation pixel circuits, especially in the negative state. A full-color 19-in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19-in. AMOLED display. The AMOLED display with a-IGZO TFT array is promising for large-sized TV because a-IGZO TFTs can provide a large-sized backplane with excellent uniformity and device reliability.

Effects of oxygen partial pressure on the microstructure and electrical properties of indium tin oxide film prepared by d.c. magnetron sputtering

We prepared ITO films using d.c. magnetron sputtering and investigated effects of oxygen partial pressure on the microstructure and the electrical properties of the films. The ITO films deposited at low oxygen partial pressure showed resistivity of 2 × 10−4Ω cm and optical transmittance of about 90%. The resistivity increased as the oxygen partial pressure increased. The preferred orientation of the film was changed as the oxygen partial pressure was changed. The films deposited at high oxygen partial pressure consisted of relatively large grains, but those deposited at low oxygen partial pressure consisted of two distinctive features: aggregate of small grains and that of long grains. Possible cause of the observations was speculated using the oxygen vacancy concentration.

69.3: Amorphous Oxide TFT Backplane for Large Size AMOLED TVs

Amorphous oxide thin film transistor (TFT) arrays have been developed as TFT backplanes for large size active-matrix organic light emitting diode (AMOLED) displays. An amorphous IGZO (Indium Gallium Zinc Oxide) bottom gate TFT with an etch-stop layer (ESL) delivered excellent electrical performance with field- effect mobility of 21 cm2/V-s, an on/off ratio of >108, and subthreshold slope (SS) of 0.29V/dec. A full color 19-inch AMOLED display has been developed using the amorphous IGZO TFT backplane.

Electrical characteristics of (100), (111), and randomly aligned lead zirconate titanate thin films

(100), (111), and randomly aligned lead zirconate titanate thin films on Pt/Ti/Corning 7059 glass substrates were prepared using a sol‐gel method. The thin films, having different alignments, were fabricated by different drying conditions for pyrolysis. The hysteresis loop and the capacitance‐voltage characteristics were investigated using a standardized ferroelectric test system. The dielectric constant and the current‐voltage characteristics of the films were investigated using an impedance analyzer and a pA meter, respectively. The microstructure was investigated using scanning electron microscopy. The (100) aligned film showed a relatively larger dielectric constant than the (111) and the randomly aligned films. The films aligned in particular directions showed higher hysteresis parameters than the randomly aligned film. The leakage current densities of the films aligned in particular directions were lower than that of the randomly aligned film.

Effect of Surface Energy on Pentacene Growth and Characteristics of Organic Thin-Film Transistors

Fluorinated hybrid materials were synthesized for a solution-processable gate insulator. The surface energy was modified by perfluoroalkyl chains contained in the hybrid gate insulator itself. We investigated the initial morphology and growth mode of pentacene and the characteristics of organic thin-film transistors (OTFTs) to determine how these characteristics depend on the surface energy. Pentacene growth was changed from a layer-by-layer mode to a three-dimensional (3D) island growth mode at low surface energy. Tightly and uniformly grown pentacene grains at 3D island mode induced good OTFT performance, but the carrier mobility was degraded at very low surface energy due to the large amount of grain boundaries.

Effects of substrate and bottom electrodes on the phase formation of lead zirconate titanate thin films prepared by the sol-gel method

Lead zirconate titanate (PZT) thin films were fabricated using sol-gel spinning onto Pt/glass, Pt/Ti/glass and Pt/Ti/SiO2/Si substrates and heat-treated using rapid thermal annealing (RTA). The preferred orientation and the perovskite phase content of the PZT thin films were studied using X-ray diffraction analysis (XRD), and the polarization vs electric field (P-E) hysteresis characteristics were investigated using a standardized ferroelectric test system. All of the resulting films on platinized substrate with Ti adhesion layer were well crystallized with the perovskite phase. A preferred (111) orientation was obtained on the Pt/Ti/glass substrate. The saturation polarization of the PZT film on glass substrates was higher than that on Si substrates.

Effects of Seeding Layer on Perovskite Transformation, Microstructure and Transmittance of Sol-Gel-Processed Lanthanum-Modified Lead Zirconate Titanate Films

A series of sol-gel-processed lanthanum-modified lead zirconate titanate (PLZT) thin films with La/Zr/Ti ratios of 8.5/65/35, 9/65/35, 9.5/65/35, 15.5/40/60 and 18/30/70 were prepared on indium tin oxide (ITO)-coated Corning 7059 glass substrates and heat-treated at different temperatures from 475° C to 685° C. The X-ray diffraction, microstructure, optical transmittance spectra and the polarization vs electric field curves were investigated. The P(L)ZT thin films of higher Ti/Zr ratio show lower transformation (from pyrochlore to perovskite) temperature. The PLZT(8.5–9.5/65/35) thin film with the seeding layer shows improved phase content, high optical transmittance and enhanced perovskite transformation kinetics. The grain sizes are reduced (from 5–6 µ m to 0.1–0.2 µ m) and uniform. The fracture surfaces of the films with a seeding layer consist of columnar structure of which the grains grew from the seeding layer. The optical transmittance is enhanced, and the polarization value at 400 KV/cm increases for the film with the seeding layer.

Effects of Hydroxyl Groups in Gate Dielectrics on the Hysteresis of Organic Thin Film Transistors

This work was supported by the Korea Science and EngineeringFoundation KOSEF grant funded by the Korean governmentMOST no. R01-2003-000-10125-0. B.S.B. acknowledges the financialsupport of the LG Yonam Foundation for making his visit tothe Flexible Display Center at ASU possible.