Indium and Tin Doping of Zinc Oxide Film by Cation Exchange and its Application to Low‐Temperature Thin‐Film Transistors

Abstract Thin‐film transistors (TFTs) based on ternary or quaternary metal oxides possess features that are advantageous for the commercialization of large‐area and flexible displays. These features include high mobility, high current on–off ratio, and uniformity over large areas. However, the manufacture of TFTs mainly uses costly sputtering equipment with low throughput. Here, it is demonstrated that cation exchange is a novel, solution‐based route for obtaining high‐quality ternary metal oxide films from binary metal oxide films. Specifically, some Zn ions in the parent ZnO film are replaced with In or Sn ions in a solution at room temperature, while preserving the ionic framework of the parent ZnO lattice to some degree. Consequently, annealing the resulting cation‐exchanged film at 250 °C yields an In‐ or a Sn‐doped ZnO film, in which In or Sn acts as a substitutional donor, with its electrical properties superior to those of the parent ZnO film. With the use of these In‐ and Sn‐doped ZnO films as channel layers, TFTs exhibit field‐effect mobilities of 10.3 and 7.1 cm² V⁻¹ s⁻¹ and turn‐on voltages of −3.9 and −1.2 V, respectively, with current on–off ratios exceeding 10⁸.