Present status and future prospects for development of non- or reduced-indium transparent conducting oxide thin films

This paper surveys the recent progress and prospects for further development of indium–tin-oxide (ITO) substitute materials for practical use as thin-film transparent electrodes. The best, and only practical, indium-free candidate for an alternative material to ITO is impurity-doped ZnO such as Al- or Ga-doped ZnO (AZO or GZO). Presented here are newly developed impurity-doped ZnO thin-film deposition techniques, suitable for practical use, that reduce resistivity as well as improve the uniformity of the resistivity distribution using oxidization-suppressing magnetron sputtering deposition methods. Also presented are descriptions concerning the observed increase in resistivity of impurity-doped ZnO thin films resulting from exposure to long-term testing in a high humidity environment (air at 90% relative humidity and 60 °C) as well as the increase in resistivity associated with a decrease of film thickness. The resistivity stability of AZO thin films could be considerably improved by co-doping another impurity.