Novel Techniques of Sol-Gel Deposition and Plasma Oxidation Growth Treatment for Transparent Flexible Thin-Film Transistors

• Main Authors: Chu, Min-Ching, 朱銘清
• Other Authors: Ko, Fu-Hsiang
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/wb2get

博士 === 國立交通大學 === 材料科學與工程學系奈米科技碩博士班 === 104 === For the goal of low-cost, lightweight, large area, low power, energy efficiency and carbon reduction, the novel display technologies have been mostly dominated by thin-film transistor liquid-crystal displays (TFT-LCDs). Therefore, many next-generation displays such as amorphous-silicon (α-Si), low temperature poly-silicon (LTPS), and transparent metal-oxide semiconductors (TMOSs) TFT-LCDs have been widely developed. Meanwhile, various physical and chemical techniques are reported to deposit the critical semiconductor and gate dielectric layer with transparency and flexibility. Among these reported techniques, sol–gel derived thin-film deposition methods have been attracted lots of attention because of the simplicity, low chemical cost, and high throughput that enable the future fabrication of high-performance and low-cost electronics devices. However, sol–gel derived TFTs are facing numerous challenges, such as charge carrier density of ultra-thin TMOSs and reliability issue of gate dielectric under repeated bending test. To smoothly promote the next-generation displays with transparency and flexibility, this study has devoted to the development of novel techniques of sol–gel deposition and plasma oxidation growth treatment. A low-cost, utra-thin (3.7 nm), transparency and high-quality zinc oxide (ZnO) film was successfully demonstrated as the carrier transporting and semiconducting layer for TFT devices. The ZnO ultra-thin film was spin-coated from zinc acetate sol-gel solution. Among various processing temperatures, the electrical property of the fabricated TFT verified the devices could be successfully achieved from suitable annealing temperature of 300-700°C. However, the higher treatment temperature of 800-900°C deteriorated the transistor property due to the loss of oxygen vacancy. The electrical properties of these ZnO-based n-type TFTs were obtained as follows: the mobility (μsat) ranged from 0.47 to 1.78 cm2/V-s, the on/off current ratio ranged from 5.7 × 105 to 1.6 × 106, and the threshold voltage ranged from 9.7 to 17.3 V. The long-term (100 days) characterization for the evaluation of the ultra-thin ZnO TFT reliability on the mobility and on/off current ratio strongly suggested the effectiveness of solution-processed ultra-thin film transistors. Also, a change in the charge carrier density of ZnO films for control the functioning of TFTs has been studied by oxygen (O2) plasma techniques. This effect was interpreted in terms of a threshold voltage shift and the variation in carrier mobility. The plasma-surface interaction on the molecular level and the behavioral characterization of ZnO films were investigated by X-ray photospectroscopy of the O 1s region. This process was highly sensitive at low-level variations in defect and doping density. O2 plasma treatment leads to a shift of turn-on voltage and a reduction of the off current by more than two orders of magnitude in ZnO TFTs. In addition, a new flexible metal–insulator–metal (MIM) capacitor using 9.5-nm-thick ZrO2 film on a plastic polyimide substrate based on a simple and low-cost sol–gel precursor spin-coating process has been demonstrated. The as-deposited ZrO2 film under suitable treatment of O2 plasma and then subsequent annealing at 250°C exhibits superior low leakage current density of 9.0 × 10-9 A/cm2 at applied voltage of 5 V and maximum capacitance density of 13.3 fF/μm2 at 1 MHz. The as-deposited sol–gel film was completely oxidized when we employed O2 plasma at relatively low temperature and power (30 W), hence enhancing the electrical performance of the capacitor. This proposed efficient combination of sol–gel solution method and O2 plasma oxidation growth treatment to fabricate transparent ZnO and ZrO2 ultra-thin film was relatively simple and cost-effective technique, and could be used as a new candidate of material for next-generation electronic devices to meet the growing demand of small feature bioelectronic sensor, light emitting diode and flexible panel.