Preparation of SrBi2Ta2O9 Thin Films by Liquid-Delivery Metalorganic Chemical Vapor Deposition using a Double Alcoholate Source

A liquid-delivery metalorganic chemical vapor deposition (MOCVD) apparatus for preparing SrBi2Ta2O9 (SBT) thin films was designed and set up. For the preparation of SBT thin films, a double alcoholate source, Sr[Ta(OC2H5)6]2, was used for supplying Sr and Ta atoms with a stoichiometric composition, while Bi(C6H5)3 was used for supplying Bi atoms. In the experiment, variation of the film composition with the deposition temperature and source-supply mol ratio was primarily investigated. It was found that the Bi/Ta mol ratio in the films varied with the deposition temperature and became smaller at lower deposition temperatures. It was also found that the Sr/Ta mol ratio was considerably smaller than stoichiometry above 500°C, probably because of the dissociation of Sr[Ta(OC2H5)6]2. It was ascertained that a 200-nm-thick SBT thin film with the Sr/Ta mol ratio of about 0.4, which was deposited at 350°C and subsequently annealed in O2 atmosphere at 750°C for 30 min, exhibited ferroelectricity. The dielectric constant of the film was about 120. Remanent polarization and coercive field at 10 V were 3.8 µC/cm2 and 73 kV/cm, respectively. However, the leakage current density was found to be as high as the order of 10-4 A/cm2 at 3 V. It is speculated from atomic force microscope (AFM) observation of the surface roughness of the film that the origin of the high leakage current density is the existence of local high electric field spots in the film.