Oxide engineering of ZnO thin‐film transistors for flexible electronics

Abstract— In this paper, we show that ZnO thin‐film transistors (TFTs) are potentially a higher performance alternative to organic and amorphous‐Si TFTs for macroelectronics on plastic substrates. Specifically, we fabricated nanocrystalline ZnO thin‐film transistors using low‐temperature processing, compatible with flexible electronics on plastic substrates. The ZnO semiconductor was rf magnetron sputtered, and the Al₂O₃ gate dielectric was deposited either by electron‐beam evaporation or atomic layer deposition. By controlling the partial pressure of oxygen pO₂) during ZnO sputtering, we could engineer the field‐effect mobility of ZnO transistors to be between 2 and 42 cm²/V‐sec, attractive for high‐performance electronic applications. We contend that pO₂ controls the oxygen‐vacancy content or stoichiometry of ZnO, and that allows control of transistor field‐effect mobility. Although most of the devices described here were fabricated on Si substrates, devices we made on a thin (50 μm thick) polyimide substrate had about equivalent performance, affirming the compatibility of our processes with plastic substrates. Finally, we show that properties of our nanocrystalline ZnO transistors can be explained by transport models that account for grain‐boundary trapping of mobile carriers.