Degradation of transparent conductive properties of undoped ZnO and Ga-doped ZnO films left in atmospheric ambient for several years and trials to recover initial conductance

Abstract

This study evaluated the long-term stability of the transparent conductive properties of undoped ZnO and Ga-doped ZnO (GZO) films that had been left in an atmospheric ambient environment for 5 to 6 yr. When ZnO films are stored in a clean room with a controlled temperature and humidity of 23 °C and 45%, respectively, throughout the year, the increases in sheet resistance are less than 5% of their initial value. The ZnO films stored in a non-air-conditioned laboratory room, whose temperature varies between 5 and 35 °C and humidity varies between 30% and 70% per year, suffer from increases in the sheet resistance by almost 13%, which is associated with a slight rise in the near-infrared transmittance level. Postannealing of these degraded ZnO films at 150–200 °C recovers the initial conductance by removing the H2O molecules that have penetrated the film. One hour of irradiation with electron cyclotron resonance Ar plasma effectively restores the conductive surfaces while maintaining a temperature below 70 °C. The GZO films containing a few weight percent of Ga2O3 are stable even when stored in a non-air-conditioned laboratory room, with changes in the sheet resistance of less than 3%. The GZO films with a Ga2O3 content larger than 10 wt. %, however, exhibit serious degradation probably due to the strong affinity of segregated Ga2O3 domains with H2O vapor molecules. Neither postannealing nor Ar plasma irradiation can recover the initial sheet resistance of these GZO films.