Fabrication and Characterization of Transparent Thin Film Transistors (TFTs) using Indium-Zinc-Oxide (IZO)

Abstract

Transparent electronics is one of the most advanced topics for a wide range of device applications today. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics 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. Oxide semiconductors, especially the amorphous ones, are a promising class of thin film transistor (TFT) materials that have made impressive progress particularly in display applications in a relatively short time, challenging silicon in conventional applications. The key issues here are the ability to produce such semiconductors at low process temperatures and to have highly smooth surfaces, and the characteristic of amorphous structures, whose electronic performances do not depend on the degree of films disorder. This new class of semiconductor materials (i.e., amorphous oxides semiconductors, AOSs) represents a revolutionary idea and exhibits a stimulating combination of high optical transparency, high electron mobility and amorphous microstructure. The purpose of this thesis is to obtain the ideal properties of indium zinc oxide (IZO) thin films for application in transparent oxide TFTs, which is the core topic of the next-generation transparent electronic displays. The RF magnetron sputter system is used to deposit IZO thin films. The possibility of using amorphous IZO films as both active channels with an appropriate resistivity range and S/D electrodes with a desirable level of resistance for TFT is experimentally verified. To control the carrier concentration of IZO thin films, an experiment is conducted on in-situ oxygen and nitrogen doping. In addition, a post-treatment method using Ar and hydrogen plasma is suggested, which can effectively control the electrical property of IZO films, and the treatment conditions are optimized. A detailed material 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 double-channel IZO TFTs are developed for the first time by simply using the different O2/Ar volume ratios in the deposition of the first and second IZO layers. Also, the fabricated double-channel IZO TFTs demonstrate characteristics superior to those of the conventional single-channel TFTs. Also, to elucidate the charge trapping mechanism responsible for hysteresis phenomena in IZO TFTs, a plausible model for hysteresis is suggested via C-V measurement. Ultimately, this thesis shows that amorphous IZO films can be used as both channel and electrode layers with desirable properties, and accordingly, the TFTs based on IZO films have potential as future transparent displays.