| Summary: | 博士 === 國立成功大學 === 電機工程學系碩博士班 === 98 === Organic thin-film transistors (OTFTs) can be competitive candidates for existing and novel TFT applications that require structural flexibility, low processing temperature, and low cost. Such applications include the switching devices of active matrix for flat panel displays based on liquid crystal pixels, organic light emitting diodes (OLEDs), or electronic papers. For low operation voltage and high driving current of organic thin-film transistors, this thesis describes the performance of OTFT devices with high-K nanocomposite was conducted as the gate dielectric. The surface modification of titanate dioxide (TiO2) nanoparticles and their application to high-quality, solution-processable polymer/ceramic dielectric nanocomposites reduced the leakage current density and significantly enhanced the dielectric constant. Phosphonic acid-based ligands were found to yield a high surface coverage and robust surface modification on the surface of TiO2 nanoparticles. The dielectric properties of the nanocomposites were examined by fabricating metal/insulator/metal capacitors. In our study, there are two methods were used to optimize the surface of high permittivity nanocomposites for maximum extractable energy, adding a surface-modified layer and chemical-mechanical polishing, to control the surface morphology of the nanocomposite dielectric and leakage current. On the other hand, the correlation of surface charactristics and leakage current will be disscused in this thesis. The dielectric properties of the nanocomposites with various nanoparticle volume fraction were fabricated by surface modifying engineering on the nanocomposite dielectric; a nanocomposite with a high dielectric constant and low leakage current (＜10-10 A/cm2) were obtained to use as gate dielectric of OTFT.
The gate dielectric plays an important role in determining the characteristics of pentacene films. In the later half paragraph of this thesis, organic thin-film transistors were fabricated with pentacene and their electrical performance was also characterized. The nanocomposite materials presented in this study are potentially useful as dielectrics for gate insulators in OTFTs. Firstly, because of the increased dielectric constant, low leakage current and good interface after surface modifying engineering significantly improved the current on/off ratio, and reduce threshold voltage, subthreshold voltage and hysteresis effect. The characteristics of pentacene films for OTFT applications are closely related to the deposition parameters and gate dielectric properties, especially the morphology of dielectric and structural properties of pentacene films, including the roughness and the surface wetting properties of the gate dielectric. Due to the OTFTs with bottom gate structure, the path of carriers transport is the interface of pentacene and dielectric. A low surface energy significantly improved the crystalline microstructure of pentacene first layer with better coverage and enhanced the mobility of an OTFT. The low surface energy of the nnaocomposite gate dielectrics minimized the interaction between the pentacene and nanocomposite gate dielectric and lead to denser grain structures with three-dimensional islands which is followed the Volmer-Weber growth mode. And smooth gate dielectric OTFTs with high-dielectric-constant nanocomposites after surface treatment had a good sub-threshold voltage (0.50 V/dec.), low threshold voltage (＜-5 V), a high on/off current ratio (~108), and good mobility (0.62 cm2V-1s-1).
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