Investigation of gate material ductility enables flexible a-IGZO TFTs bendable to a radius of 1.7 mm

TFTs on flexible plastic foils have the potential to enable new applications like electronic skins or smart textiles. Due to the temperature sensitivity of plastic substrates, amorphous In-Ga-Zn-O (a-IGZO) is a promising semiconductor since it provides a carrier mobility >10 cm²/Vs when deposited at room temperature. Therefore, a-IGZO TFTs have significantly increased electrical performance compared to organic TFTs, but also suffer from a decreased bendability. Here, focused ion beam (FIB) images are used to identify the gate metal as the dominant factor for the formation of cracks in bent a-IGZO TFTs. Flexible a-IGZO TFTs using a high-k Al₂O₃ gate dielectric and different gate contact materials (Cr, Pt, Ti, or Cu) exhibit a similar effective mobility μ_FE, threshold voltage V_TH, and on-off current ratio of: ≈15 cm²/Vs, ≈1 V, and >10⁹. Simultaneously, bending experiments confirmed that their bendability depends on the ductility of the gate material. These findings are used to identify Cu as suitable gate material, and to fabricate a-IGZO TFTs on free-standing plastic foil which can be operated at a bending radius of 1.7 mm (1.55% strain), whereas bending shifts μ_FE and V_TH only by + 2%, and - 6 mV.