High Performance Zinc Oxynitride Thin Film Transistors (ZnON TFTs) on Flexible Substrate

The rapid development of flexible electronic devices has led to a new paradigm that is transformed into a flexible electronic device platform. AMOLED, which is a typical flexible electronic device, has been commercialized as a large area based curved display since 2013. AMOLED-based future display technologies such as foldable, rollable, and stretchable are rapidly developing with great interest. Among the various core technologies, the backplane device, which determines the performance of the display, must satisfy flexible and high performance. Currently, conventional oxide TFTs are in commercial use in AMOLED. However, this technology has been a big problem because of the trade-off relation between the long-term reliability and the mobility as well as the necessity of the high-temperature process. In order to solve these problems, an anion-controlled method has been proposed to solve these problems. Representatively, zinc oxynitride (ZnON) semiconductors have been proposed due to its low effective mass and screening effect of oxygen vacancies. The main purpose of this thesis was to develop the process technology to achieve high performance ZnON semiconductors and to design the TFTs stably on the flexible substrate. In order to improve the electrical properties of ZnON semiconductors, physical and chemical analysis were used to understand the relationship between the physical properties and electrical properties of the materials. The various buffer layers were introduced on a flexible substrate to analyze the origin of degradation factors of the TFTs and to investigate the optimization of materials and structures. In order to understand the principle of electrical transition of ZnON semiconductors, a comparative study with ZnO semiconductors was conducted. Through the post annealing process, the carrier concentration of the highly conductive As-deposited ZnON was decreased. As a result of post annealing process, it was confirmed the electrical transition from the conductor to the semiconductor. It means that stable O-Zn-N bonds were increased while O- or N-related defects were relatively reduced. Since the control of the annealing temperature is very limited due to the meta-stale Zn-N bond, more stable Zn-O and Zn-N bonds can be formed by increasing the annealing time. As a result, it was confirmed that the overall device characteristics and device instabilities under light illumination were greatly improved. The improvement of ZnON TFTs characteristics was related to stable ZnO and Zn3N2 phases. Therefore, photochemical reaction was introduced during the post annealing process. The highly stable and high perofrmance ZnON TFTs were fabricated even at low temperatures of 175 oC by applying light energy that decomposes unnecessary anion bonds except stable Zn-O and Zn-N bonds. Unlike rigid glass, plastic-based devices require a buffer layer due to their low moisture permeability. Therefore, the electrical characteristics of TFTs with various buffer layers were evaluated in the vacuum measurement system, and it was confirmed that the dense and multi-structure barrier properties for protecting TFTs from hydrogen in the buffer layer and moisture from the ambient were required. Consequently, high performance flexible ZnON TFTs fabricated on the PEN substrate were demonstrated at a low temperature of 175 oC by applying a thermally activated photochemical (TPC) process and a high quality buffer layer (organic/inorganic). Also, the stable device characteristics were confirmed even under the condition of tensile bending radius of 5 mm (??a = 0.51%). Therefore, high-performance flexible ZnON TFTs are expected to have high potential in future flexible AMOLED display panels.Docto