Impact of Mechanical Bending on ZnO and IGZO Thin-Film Transistors

Both zinc-oxide (ZnO) and gallium-indium-ZnO (IGZO) are attractive as semiconductors to replace hydrogenated amorphous silicon in flexible thin-film transistors (TFTs) due to their high charge carrier mobility and low deposition temperature. However, the electrical performance of flexible TFTs needs to be insensitive to mechanical bending. We have fabricated TFTs using ZnO and IGZO semiconducting layers on polyimide substrates and exposed TFTs to tensile bending radii down to 10 mm. While the mobility, threshold voltage, and subthreshold slope of IGZO TFTs remained essentially unchanged over the entire bending range, the electrical performance parameters of ZnO TFTs were strongly degraded by bending. For ZnO TFTs bent to a radius of 10 mm, the mobility decreased by more than two orders of magnitude, the threshold voltage increased by a factor of ~ 5, and the subthreshold slope increased by a factor of ~ 2. Our results show that IGZO should be the material of choice for robust flexible thin-film transistors. Experimental evidence points toward the formation of microcracks as the cause of ZnO sensitivity to bending.