Structure And Dielectric Enhancement Of Polycrystalline (Ba, Sr) TiO₃ Ferroelectric Multilayer Films On Si Substrates

(Ba,Sr)TiO3 thin films show high application potential in high-density DRAM capacitors with very large scale integration because of their good ferroelectric, dielectric, piezoelectric and pyroelectric properties. The present dissertation focuses on the research of dielectric properties of polycrystalline (Ba,Sr)TiO3 multilayer films prepared by pulsed laser deposition on Si substrates and the mechanism of dielectric enhancement is discussed. Some of the main results are as follows:(1) Effects of stacking periodicity on BST multilayer filmsPolycrystalline BaTiO3/Bao 6Sr0 4TiO3, BaTiO3/Bao.2Sro.8TiO3 and BaTiO3/SrTiO3 multilayer films were fabricated via pulsed laser deposition onto Pt/Ti/SiCVSi substrates with various stacking periodicities. The dielectric constant of the multilayer films was obviously enhanced with the decrease of the stacking periodicity, while the dielectric loss was kept at a low level. In BaTiO3/Bao 6Sro 4TiO3 multilayer films, a large dielectric constant of 1336 was observed for a stacking periodicity of 60 nm at the frequency of 10 kHz at room temperature, and the corresponding dielectric loss was maintained below 0.05. In BaTiO3/Bao.2Sro.gTiO3 multilayer films, a dielectric constant of 890 was observed for a stacking periodicity of 80 nm and the corresponding dielectric loss was only 0.04. In BaTiO3/SrTiO3 multilayer films, a dielectric constant of 721 was observed for a stacking periodicity of 60 nm and the corresponding dielectric loss was only 0.0334. The study indicates that there are some differences between the multilayer films and the single films in dielectric properties. The mechanism of dielectric enhancement of polycrystallinemultilayer films was related to Maxwell- Wagner effect. (2) Effects of postannealing on BST multilayer filmsPolycrystalline BaTiCb/Bao aSro sTiC^ multilayer films were prepared by pulsed laser deposition onto Pt/Ti/SiCVSi substrates with sublayer thickness of 48 nm. The dielectric constant of the films was obviously enhanced through postannealing process, while the dielectric loss was kept at a lower level. When the multilayer thin film was postannealed for 2.5 hours, a dielectric constant of 890 was observed at the frequency of 10 kHz and the dielectric loss is smaller than 0.05. Maxwell- Wagner model is used to explain the experimental data, and the results of the modeling conform well to the experiment.(3) Effects of thickness on BST multilayer filmsPolycrystalline BaTiOs/Bao .2Sro .8TiO3 multilayer films were deposited via pulsed laser deposition technique on Pt/Ti/SiCVSi substrates with different total thickness. The dielectric constant of the multilayer films was influenced by the total thickness, while the dielectric loss was maintained at a lower level. Under equal interfacial number, a dielectric constant of 810 was observed for a total thickness of 240 nm and dielectric loss was 0.036. Under equal stacking periodicity, a dielectric constant of 913 was obtained for a total thickness of 320 nm and the dielectric loss was 0.036. The Maxwell-Wagner model is used to explain the experimental data, and the results of the modeling agree well with the experiment.