Highly Bendable In-Ga-ZnO Thin Film Transistors Using Thermally Stable Organic Dielectric Layer

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December 5, 2016 | Originally published by Date Line: December 5 on

For decades, transparent and flexible thin film transistors (TFTs) have drawn much attention and have become the main focus of research in large-area electronics because they can be used in modern electronic applications, such as wearable computers, sensors, photodetectors and rollable displays. The key requirement for the realization of flexible electronics is to obtain flexibility with good device performances.

To achieve the flexibility, transistors have to sustain harsh bending strain. Researchers have employed semiconducting polymers as channel materials in flexible organic TFTs (OTFTs) because the weak intermolecular interactions or van der Waals forces between the polymer chains within the organic materials enhance the strain-bearing capability. However, the mobility of the OTFTs is still a challenging issue. With regard to material strategies for enhancing the TFT performance, researchers have focused on amorphous oxide semiconductors (AOSs), especially In-Ga-ZnO (IGZO), because of its high carrier mobility (>10 cm2/Vs), high optical transparency and environmental stability. Cherenack et al. used an IGZO channel material for a flexible TFT and demonstrated a good device performance down to a bending radius of 10 mm12. Later, Park et al. achieved a reliable bending performance up to 5000 cycles in the IGZO TFTs at a 5 mm bending radius (1.25{f852dafd27cac84fdac768484a17b914ab8ab8a105c7cd3f00f3e5984b2da150} strain) without any performance degradation. Recently, Park et al. demonstrated the flexibility down to 2 mm (corresponding bending strain is 2.5{f852dafd27cac84fdac768484a17b914ab8ab8a105c7cd3f00f3e5984b2da150}) with a coplanar IGZO TFTs by adjusting the device structural design (island configuration) as well as its location at the neutral axis plane while being bent14. Thus far, the flexible TFTs based on AOSs have employed inorganic dielectrics, such as HfO2, ZAO, SiNX, and Al2O3, as gate insulator layers due to their similar crystallinity to the AOS at the interface. However, device performance degradation after repetitive bending cycles is unavoidable due to cracks originating from the non-elastic nature of the stacked inorganic layers. Indeed, there were attempts at creating TFTs based on inorganic semiconductor/organic dielectric layers, including poly (methyl methacrylate) (PMMA), poly (vinyl alcohol) (PVA), and polyimide (PI), to buffer the non-elastic nature. However, the TFT performances subjected to a certain bending strain were not stable, resulting from the damage to the flexible substrate and/or the organic dielectric layer during the bending cycles.