Effects of Substrate Cooling during Sputter Deposition of Hydrogen-Containing Ta2O5 Thin Films in H2O Atmosphere on their Ion Conductivity

Abstract

Hydrogen-containing Ta2O5 (Ta2O5:H) thin films, which are considered to be candidates for proton-conducting solid electrolytes, were prepared by reactive sputtering in H2O atmosphere. The substrate temperature during sputter deposition was changed from -20 to 70 °C because the equilibrium vapor pressure of water depends strongly on temperature. The chemical composition of the films was characterized by Rutherford backscattering (RBS) and hydrogen forward scattering (HFS), and it was found that the H/Ta atomic ratio increased with decreasing substrate temperature, and a maximum ratio of 2.2 was obtained at -20 °C. The ion conductivity measured at room temperature also increased with decreasing substrate temperature and a maximum conductivity of 4 ×10-8 S/cm was obtained at -20 °C. It is considered that the desorption rate of H2O molecules from the growing film surface was decreased by the substrate cooling, which resulted in the increase in the hydrogen content and the improvement of the ion conductivity.