Preparation and characterization of epitaxially grown yttria-stabilized zirconia thin films on porous silicon substrates for solid oxide fuel cell applications

Epitaxial growth of yttria-stabilized zirconia (YSZ) thin film on through-hole-type porous silicon [tht-PSi(001)] with vertical pores penetrating from the surface to the back side of the Si(001) substrate was achieved. The in-plane and out-of-plane lattice parameters of YSZ thin film deposited on the tht-PSi(001) were, respectively 0.5167 and 0.5124 nm. Therefore, 0.54 % tensile strain was applied to the YSZ thin film. Also for this work, an all epitaxially grown thin film of YSZ/La0.7Sr0.3MnO3(LSMO)/CeO2/YSZ/Si(001) was prepared. The out-of-plane lattice parameter of YSZ was 0.5145 nm. Therefore, the YSZ thin film of YSZ/LSMO/CeO2/YSZ/Si(001) is almost relaxed, with a small amount of tensile strain (0.12 %). In-plane and out-of-plane electrical properties were measured respectively for YSZ/tht-PSi(001) and YSZ/LSMO/CeO2/YSZ/Si(001) thin films. Results show that ionic conduction was confirmed at 400 °C through constant electric conductivity against the change of oxygen partial pressure (pO2). Enhanced ionic conduction was observed for epitaxial YSZ/tht-PSi(001) thin films measured along the in-plane direction. Such enhanced ionic conduction was not observed for epitaxial YSZ/LSMO/CeO2/YSZ/Si(001) thin films measured along the out-of-plane direction. These findings suggest that enhanced ionic conduction is correlated with tensile strain in YSZ thin films.