Thickness-dependent surface proton conduction in (111) oriented yttria-stabilized zirconia thin film

Polycrystalline Zr_0.92Y_0.08O₂ (YSZ) thin films were prepared by RF magnetron sputtering. 80- and 160-nm thin films exhibited (111) orientation and a polycrystalline structure, respectively. The 80-nm thin film had larger distortion than the 160-nm thin film. While the 80-nm thin film and a 120-nm thin film exhibited proton conduction, the 160-nm thin film did not, indicating that surface proton conduction can depend on film thickness. The activation energy of the 80- and 120-nm thin films measured in a wet atmosphere (0.52 eV) was about half of that measured in a dry atmosphere. The enhancement of conductivity for the thin films may be attributed to distortion, which may change the structure around an oxygen vacancy at the YSZ grain surface, accompanied by possible enhancement of H₂O adsorption. H₂O-annealed thin film had a hydrogen-induced level in the band gap energy region. This is the first observation of hydrogen-induced level at the surface state of the YSZ thin film obtained by X-ray absorption spectroscopy.