Resistivity characteristics of transparent conducting impurity-doped ZnO films for use in oxidizing environments at high temperatures

Abstract

The behavior of resistivity in transparent conducting Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films, prepared with the same thickness under the same deposition conditions by dc magnetron sputtering, was investigated for use in oxidizing environments at high temperatures. These thin films with thicknesses in the range from 100 to 3000 nm were prepared on glass substrates at a temperature of 200 °C. The increase in resistivity of GZO thin films over long-term testing at 85% relative humidity and 85 °C was lower than that of AZO thin films, while, in contrast, that of AZO thin films over long-term testing at 90% relative humidity and 60 °C was lower than that exhibited by GZO. The moist-resistant tests showed that both AZO and GZO thin films prepared with a thickness above 500 nm were suitable for transparent electrode applications used in thin-film solar cells. The resistivity increase of GZO thin films after heat treatment for 30 min in air at a temperature up to 400 °C was lower than that of AZO thin films. The resistivity of both GZO and AZO thin films prepared with a thickness above 200 nm was stable for heat tests at 300 °C. The heat-resistant tests demonstrated that both GZO and AZO thin films were suitable for use as a transparent electrode for thin-film solar cells. The resistivity increase after testing was found to exhibit two different mechanisms: a decrease of both carrier concentration and Hall mobility mainly attributable to grain boundary scattering as well as a decrease of carrier concentration mainly attributable to ionized impurity scattering. This difference in resistivity behavior is mainly attributed to the grain size associated with the crystallinity of deposited films.