Preparation and characterization of rf-sputtered Sr-doped lanthanum cuprate thin films on yttria-stabilized zirconia substrates

Sr-doped lanthanum cuprate of the composition of La_1-xSr _xCuO_2.5-δ exhibits both good electrical and ionic conductivities. Thus, La_1-xSr_xCuO_2.5-δ (LSCu) may be a potential material for the application of electrode material for high operating temperature fuel cell, catalyst or sensor material. However, the perovskite LSCu was present in a narrow range of 0.15 ≤ x ≤ 0.25. Due to the structural stability of LSCu is strongly affected by the valence state of copper ions in the material, the control of chemical composition, and the oxygen partial pressure during deposition and heat-treatment plays an important role on the synthesis of LSCu film. For the LSCu film preparation, the composition of target materials, sputtering atmosphere, and heat-treatment temperature were carefully studied. The LSCu thin film was deposited on a YSZ substrate by rf sputtering under a pressure of 5 mTorr with a 120 W rf power. A single tetragonal perovskite phase was obtained when the sample was subjected to heat-treatment at more than 500°C in air for 2 h. However, a second phase was formed when the specimen was heat-treated at 800°C. The electrical conductivity of LSCu film was about 131 S/cm when the specimen was heat-treated at 500°C. The structure of LSCu was identified by X-ray diffraction (XRD). The chemical composition of LSCu films was analyzed using inductively coupled plasma-atomic emission spectrometry (ICP-AES), and X-ray photoelectron spectroscopy (XPS). The surface morphologies and the thicknesses of the LSCu films were examined by scanning electron microscopy (SEM). The electrical conductivities were investigated by four-point probe technique.