Stability in a high humidity environment of TCO thin films deposited at low temperatures

The stability of transparent conducting Al- and Ga-doped ZnO (AZO and GZO) thin films in a high humidity environment has been investigated for the purpose of finding substitutes for the indium-tin-oxide (ITO) thin films used in transparent electrode applications. It was found that the resistivity of polycrystalline AZO and GZO thin films prepared with a thickness in the range from 50 nm to 300 nm at a substrate temperature below 200 degrees C always increased when long-term tests were conducted in air at a temperature of 60 degrees C and a relative humidity of 90%, whereas ITO thin films remained relatively stable in the same environment; however, AZO and GZO thin films with a thickness above approximately 200 nm were sufficiently moisture-resistant. The resistivity stability of AZO and GZO thin films was considerably related to crystallinity factors such as crystallite size. In addition, the resistivity increase is attributed to carrier transport dominated by grain boundary scattering resulting from the trapping of free electrons due to oxygen adsorption on the grain boundary surface.