Segregation Effects and Charge Transport in Yttria-Stabilized Zirconia Thin Films on Langasite Substrates

Yttria-stabilized zirconia (YSZ) is widely used as a bulk ceramic in solid-state oxygen sensors due to its high ionic conductivity, chemical inertness, and stability up to 1500^oC. In thin film form, YSZ is an attractive component for use in wireless microwave acoustic sensors to monitor conditions within harsh industrial environments. In this work, RF-magnetron sputter deposition was used to synthesize YSZ (8%Y₂O₃-92%ZrO₂) films with thicknesses from 15nm to 200nm on piezoelectric langasite (La₃Ga₅SiO₁₄) substrates at growth temperatures from 25-600^oC. X-ray diffraction indicated that the cubic YSZ films grow with preferred (111) out-of-plane texture on the langasite substrate with random in-plane orientation. Scanning electron microscopy revealed the presence of stress hillocks at the YSZ/langasite interface, and this was minimized at a 400^oC deposition temperature where extremely smooth films were obtained as determined by x-ray reflectivity. Post-deposition air annealing caused yttria segregation to the film surface region as evidenced by increases in the Y3d/Zr3d photoelectron peak area ratio upon annealing up to 1000^oC. This segregation is accompanied by an increase in film ionic conductivity and larger YSZ grain size as measured by high temperature impedance spectroscopy.