A Study on Thin Film Transistors based on Indium-Free Amorphous Oxide Semiconductors for Transparent Displays

Abstract

Transparent electronics is one of the most advanced topics for the display applications today. The key components are wide band-gap semiconductors, where oxides of different origins play an important role not only as passive components but also as active components, similar to what is observed in conventional semiconductors like silicon. Transparent thin film transistors (TFTs) have gained special attention during the last few years, and have emerged as one of the most promising technologies for leading the next generation of flat panel displays due to their excellent electronic performance. TFTs for next-generation display systems require a faster switching speed, low power consumption, and transparency in the visible range of the electromagnetic spectrum. Amorphous oxide semiconductors (AOSs) are key materials for future electronics because of their high mobility, excellent film quality even when layered at room temperature, and low fabrication cost. The AOS materials provide transparency due to its wide band gap, allowing the development of transparent systems, e.g., smart phone. Zinc oxide (ZnO)-based semiconductors that were used as active channel layers for TFTs have recently attracted much attention due to their excellent characteristics such as higher mobility than amorphous silicon, the ability to perform deposition at room-temperature, and their high transparency. However, most of the successful ZnO-based TFTs incorporated indium (In), which is rare on earth, e.g. indium zinc oxide (IZO), indium zinc tin oxide (IZTO), and indium gallium zinc oxide (IGZO). This makes those technologies easily subject to a material shortage. Some researchers have recently reported experimental results which demonstrate new oxide semiconductors free of In which may be used as alternative channel materials for oxide TFTs, including aluminum zinc tin oxide (AZTO), hafnium zinc oxide (HZO), and titanium oxide (TiOx). The RF magnetron sputter system is used to deposit ZnO based AOS thin films. The possibility of using amorphous AOS films as active channels with an appropriate resistivity range for TFT is experimentally verified. To control the carrier concentration of AOS thin films, an experiment is conducted on in-situ oxygen and hydrogen doping. In addition, a post-treatment method is suggested such as plasma treatment utilizing Ar and hydrogen, annealing in air and oxygen environment, and hot-pressing, which can effectively control the electrical property of AOS films, and the treatment conditions are optimized. A detailed analysis is carried out through various characterizations of the electrical properties, chemical composition, optical properties, crystal structure, and surface morphology. To enhance the TFT performance, the ZnO:H TFTs are developed by using hydrogen doped binary AOS material as the channel layer, of which amorphous structure and robust air exposure characteristics were observed. Silicon-Zinc-Oxide TFTs are developed using ternary oxide material as the channel layer to improve structure, optical, and electrical characteristics through a co-sputtering method. And then, two types of post-treatment methods (furnace annealing and hot pressing) were used to optimize TFT characteristics. The quaternary oxide (Hafnium-Aluminum-Zinc-Oxide) TFTs were fabricated to enhance light induced stability. Ultimately, this thesis shows that In-free AOS films can be used as the active channel layer of TFTs with robust light stability, and accordingly, the TFTs based on In-free AOS films have potential as future transparent displays.