Studies on Crosslinkable Ligand-based High-k Dielectrics for Solution-processed Heterojunction Oxide Transistors

Abstract

This study explores the advancements and applications of oxide thin film transistors (TFTs), focusing on their potential to drive the next generation of high-performance, cost-effective, and flexible electronic devices. Oxide TFTs, characterized by their high carrier mobility, excellent transparency, and stability against electrical stress, are increasingly becoming the backbone of innovations in displays, sensors, and integrated circuits.

This research explores the development of oxide semiconductors such as indium oxide (In2O3), zinc oxide (ZnO), indium gallium zinc oxide (IGZO) and In2O3/ZnO heterojunction, as well as high-k oxide dielectrics such as zirconium oxide (ZrO2), hafnium oxide (HfO2), and aluminum oxide (Al2O3), highlighting their role in achieving efficient low-voltage operation in electronic devices.

This study particularly emphasizes solution-process fabrication techniques, a scalable and cost-effective approach that departs from conventional vacuum-based methods, enabling the formation of flexible electronics on various substrates. However, solution-processed high-k oxide dielectrics contain numerous defects in the thin films, leading to increased leakage currents in devices. Consequently, research to reduce the amount of defects in these thin films is essential. Therefore, it provides a detailed examination of the process steps involved—from precursor solution preparation to post-deposition treatments—aiming to enhance understanding and optimization of the process, addressing challenges such as defects, uniformity, mobility, reliability, and environmental stability, to advance oxide TFT technology.

Furthermore, the development of high-performance oxide TFTs is discussed in depth focusing on the development of crosslinkable ligand-based high-k dielectrics, which significantly impact the electrical performance and reliability of these devices.

Chapter 1 outlines the research context of this thesis, and Chapter 2 details the experimental approaches utilized in this study.

In Chapter 3, an innovative strategy was introduced involving the use of an organic-inorganic hybrid dielectric that incorporates azide-functionalized acetylacetonate to improve the issue of high leakage current caused by numerous defects in conventional solution-processed high-k thin films. This approach improved this issue by promoting the formation of dense defect-free thin films through covalent bonding between inorganic particles and polymer chains. Through the development of two types of crosslinkable ligands, efficient crosslinking was achieved not only between polymers but also between polymers and oxide nanoparticles through ligand exchange and azide crosslinking mechanisms. The developed transistors utilizing an In2O3/ZnO heterojunction as the active channel demonstrated exceptional electrical properties, attributed to percolation charge transport facilitated by the near-ideal hybrid dielectric layer.

In Chapter 4, new functional crosslinkable ligands were introduced to improve limitations such as patterning, reduction of dielectric constant, and stretchability, observed in previous hybrid dielectrics. By replacing conventional oxo ligands with multi-functional ligands, inherent limitations such as brittleness and restricted functionalities were implemented. Introducing bidentate ligands with Lewis base, azide groups, and ethylene glycol bridges enhances ligand exchange rate, enables photopatterning, and increases space-filling effect and stretchability, facilitating the multi-functionable sol-gel oxides. With optimized conditions, high-quality dielectric was developed. This strategy can be applied not only to dielectrics but also to semiconductors, enabling the realization of oxide TFTs in which all-oxide layers were patterned and demonstrating highly reliable large-area TFT array with uniform performance across all devices.

In Chapter 5, current applications and strategic pathways to leverage these innovations for future technological and commercial advancements are highlighted.

Together, these studies underscore the potential of advanced crosslinking strategies in overcoming conventional challenges of high defects in solution-processed oxide TFTs, highlighting their role in advancing flexible electronics and large-area display technology. Through meticulous process optimization and innovative material engineering, these approaches offer promising avenues for developing high-performance, stable, and reliable solution-processed oxide TFTs, contributing significantly to the field of thin film electronics.