Solution-Processed High-k Dielectrics for Organic/Inorganic Thin-Film Transistors and CMOS Circuit Applications

Abstract

Zinc-oxide (ZnO)-based materials have been widely investigated as an active channel layer of thin-film transistors (TFTs) due to their high mobility and optically high transparency related with the wide band gap and application to solution-process. Many kinds of ZnO-based TFTs have been reported for backplanes of active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting diodes (AMOLEDs). However, even though ZnO-based materials can be fabricated by solution-based processes, which are expected to simple and low cost method, most devices have been fabricated by expensive vacuum processes such as pulsed laser deposition (PLD) and sputtering method. Recently, there are many efforts for reducing the operational voltage of TFTs using high capacitance dielectrics. By incorporating the high capacitance dielectrics, more charges can be accumulated at dielectric and semiconductor interfaces at the same bias condition, hence, TFTs and circuits can be operated under a low voltage condition. Additionally, low-power consumption and high electrical performance including a low subthreshold swing (S.S) can be achieved. In this dissertation, oxide TFTs and high-k dielectrics fabricated by solution-process are studied for low voltage high-performance TFTs and all solution-processed organic/inorganic hybrid complementary metal-oxide-semiconductor (CMOS)-type circuit applications. First, the solution-processed indium-gallium-zinc-oxide (IGZO) TFTs, composed of inkjet-printed active channel layer and Ag source/drain (S/D) electrodes, were demonstrated. Inkjet-printed IGZO TFTs exhibited the high mobility above 2 cm2/Vs at 400 oC annealing temperature. Optimizations of inkjet-printing and thermal annealing conditions were required to achieve high performance characteristics. Second, high-k Al2O3 and ZrO2 gate dielectrics were fabricated by spin-coating method for improving the electrical performance and reducing the operating voltage of oxide and organic TFTs. High capacitance of gate dielectrics enabled oxide and organic TFTs to operate in low-voltage condition under the 5 V. In particular, Al2O3 dielectric exhibited the amorphous phase and better TFT electrical behaviors such as high mobility and steep S.S. than ZrO2 dielectrics in both organic and oxide TFTs. And all solution-processed high-performance low-voltage IGZO TFTs were demonstrated. Additionally, a metal-insulator-semiconductor-metal (MISM) capacitor measurement was proposed for accuracy estimation of parameters due to the possibility of the additional electric double layer (EDL) formation through the mobile ions migration. Finally, an organic/inorganic hybrid structure approach was proposed to achieve CMOS-type inverters. Demonstrated organic/inorganic hybrid CMOS-type inverter structure showed the improved electrical performance compared to previous reported inverters composed of only p-type TFTs. The effect of high-k dielectric on the hybrid CMOS-type inverter was also investigated to enhance the inverter performance.