Investigation of Reliability and Physical Mechanisms of Flexible aInGaZnO Thin Film Transistors for Advanced Display

• Main Authors: Po-Yung Liao, 廖柏詠
• Other Authors: Ting-Chang Chang
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/hf2p3q

博士 === 國立中山大學 === 物理學系研究所 === 106 === In order to meet the requirements of novel display technologies such as high-resolution large-screen LCD and AMOLED. Oxide semiconductor thin film transistors (TFTs) have attracted much attention recently since they possess many advantageous properties of high mobility, low-temperature processing, good electrical uniformity, visible-light transparency, and low cost that are beneficial in the development of displays. They are regarded as one of most suitable active materials of TFTs for driving organic light-emitting diodes. Currently oxide TFTs have been successfully applied to the backplanes of the flat-panel displays. However, operation voltage and/or current can lead to device degradation in practical applications. Flexible oxide TFTs suffer from an additional issue, that of their reliability under mechanical stress. Therefore, two main topic are researched: (i) the effects of hot-carriers and (ii) the effect of mechanical strain in InGaZnO thin-film transistors are investigated in this work. In the first topic of this dissertation, behaviors of carrier transport in amorphous indium-gallium-zinc oxide (a-InGaZnO) thin film transistors are investigated. It is found that the electron mobility is higher at elevated temperatures, which is contrary to that in crystalline Si devices. Drain current enhancement with regard to temperature at corresponding gate voltage follows the Arrhenius equation. This implies that carrier transport is limited by the potential barrier heights induced by trap states within InGaZnO, and therefore current conduction is heat-activated to overcome those barriers. Furthermore, the relationship between carrier mobility and carrier concentration is also investigated, with the carrier mobility monotonically increasing with carrier concentration. Such behavior can be ascribed to a lowered effective barrier above the conduction band when the Fermi-level rises. The abnormal hump phenomenon emerging in the transfer characteristics of amorphous InGaZnO thin film transistors under negative bias stress (NBS) along with a negative shift of threshold voltage was also investigated. The magnitude of the parasitic on-state current increases with the measured temperature, indicating that high temperature can induce more charge injection. Furthermore, we consider that the parasitic channel originates from the hole trapping near the InGaZnO edges by simulation results. The greater gate voltage leads to the faster hole injection and the more negative shift. In the second section of topic one, current-voltage as well as capacitance-voltage measurements are utilized to analyze the electrical properties of via-contact type a-InGaZnO TFTs with an etch stop layer (ESL) after hot-carrier stress. Unlike what is commonly observed in the devices without ESL, hot-carrier stress-induced electron-trapping in the ESL device is found to be influenced by the pattern of the redundant drain electrode. Furthermore, to gain a deeper insight into hot carrier effect on different device structure, via-contact structure TFTs with three different types of source/drain distribution were also analyzed after hot-carrier stress. We discovered a phenomenon of a parasitic transistor caused by asymmetrical degradation of electrode geometry. A simulation of electric field perfectly explains the characteristic of degradation in different devices. The second topic characterized the effect of mechanical strain in flexible a-InGaZnO thin-film transistors. Drain current–gate voltage (ID–VG) as well as capacitance-voltage (C-V) transfer curves are measured to analyze the degradation behavior. The ID-VG characteristic exhibits an obvious negative shift under mechanical strain regardless of tension or compression state. In addition, the C-V characteristic curves show a leftward shift with extra distortion or stretching out under mechanical strain. This indicates that the InGaZnO generates additional defects under this mechanical strain, a phenomenon which can be attributed to the generation of mechanical strain-induced oxygen vacancies on the flexible a-InGaZnO TFTs. Finally, in the second section investigates repeated uniaxial mechanical stress-induced degradation behavior in flexible a-InGaZnO thin-film transistors (TFTs) of different geometric structures. Two types of via-contact structure TFTs are investigated: symmetrical and UI structures. After repeated mechanical stress, I-V curves for the symmetrical structure show a significant negative threshold voltage (VT) shift, due to mechanical stress-induced oxygen vacancy generation. However, degradation in the UI structure TFTs after stress is a negative VT shift along with the parasitic transistor characteristic under forward-operation mode, with this hump not evident under reverse-operation mode. This asymmetrical degradation is clarified by mechanical strain simulation of the UI TFTs.