Amorphous oxide semiconductors (AOSs) have been intensively studied during the last 15 years due to their superior properties. The major application of AOS thin film transistors (TFTs) is displays. Compared to the previous generation TFT material—amorphous silicon (a-Si)—AOSs have higher mobility, and can be easily deposited by various deposition techniques at room temperature on arbitrary substrates. This thesis focuses on the characteristics and stability of a specific AOS, amorphous indium zinc oxide (a-IZO), as an electronic material for TFTs. We have fabricated a-IZO TFTs with a top-gate structure by using the in-situ gate dielectric formation technique: some metals—such as Al and Hf—can react with IZO to form an oxide insulator layer in the absence of kinetic constraints. The in-situ dielectric formation can provide a high-quality insulator/IZO interface with a low interface trap density, leading to excellent TFT performance. The subthreshold slope in our best IZO TFTs reaches a value of 62 mV/decade, close to the room temperature theoretical limit, and our devices also show on/off current ratios above 10^7 and a high extracted field effect mobility around 100 cm2/V·s. These are state-of-the-art results in AOS TFTs. Another focus in our work is the stability of IZO TFTs, where oxygen vacancy generation and migration are used to explain the observed experimental threshold voltage shifts in a-IZO TFTs as a function of aging and applied gate bias. As a possible future research direction, the high electron mobility and controllable high electron density make a-IZO a promising candidate for high-speed, high-power radio frequency transistors. We propose a vertical transistor structure with buried multi-gate fingers. Unlike in the previously studied single-crystal nanowire or permeable base vertical transistors, the properties of a-IZO make it possible to regrow the vertical semiconductor channels without any high temperature epitaxy, lowering fabrication complexity and cost while making the vertical IZO device compatible with arbitrary substrates. Preliminary fabrication process and characterization results of the vertical-current-flow a-IZO TFT are shown and discussed.
"Characteristics and Stability of High Performance Indium Zinc Oxide Thin Film Transistors"
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.