Research On The Microstructures And Properties Of Perovskite Oxide Films

Transitional perovskite oxide thin films show various electrical and magnetic properties,having great application potential in the fields of microwave tunabl e devices,memories,solid fuel cell and catalyst.Some of these functional films have been made into devices,while most of them are still under experiment.(Ba_xSr_1-x)TiO₃ is an important material for the research and exploitation of room temperature microwave tunable devices.However,size effects result in worse dielectric properties of(Ba_xSr_1-x)TiO₃ thin films compared with that of the corresponding(Ba_xSr_1-x)TiO₃ ceramics or single crystals.After two decades of research,dielectric properties in the low frequency range of Ba TiO₃/SrTiO₃ multilayers are found to be improved because of the interface modulation effects,while few reports have been focused on their high frequency dielectric properties in the GHz range.So in this thesis,Ba TiO₃/SrTiO₃ multilayers prepared by pulsed laser deposition were chosen as the research object.Tests on microwave dielectric properties were performed using a coplanar waveguide test structure.X-ray diffraction and transmission electron microscopy were used to characterize microstructures.Combined with O-lattice theory and elastic theory,the microstructural origins that affect dielectric behaviors are revealed.Multilayers growth process and strain relaxation mechanism are also discussed.A new non-volatile memory device,Resistance Random Access Memory,has emerged based on the resistive switching effects.It is extensively investigated due to its simple device structure and superior memory characteristics.However,the uncertainty of the resistive switching mechanisms has suppressed its development.Although it is well accepted that oxygen vacancies have played an impor tant part in the resistive switching process,the exact role of them still needs to be further explained.So in this thesis,BaZrO₃ films prepared by pulsed laser deposition were chosen as the research object.Tests on the electrical properties were performed,and microstructure characterizations were also carried out with transmission electron microscopy.Combined with conduction mechanisms in insulators,the role that lattice defects such as oxygen vacancies and grain boundaries have played in the resistive switching process is tried to be made clear.Besides,the effects of oxygen partial pressure on the film microstructures and electrical properties are also discussed.In summary,the purpose of this thesis is to descript the relationship between the properties and microstructures of the materials and find the crystal structures that affect the specific property.Therefore,ideas can be applied to understand vario us phenomena in thin films.Results achieved are summarized in the following points:?1?Reasons that cause the dielectric properties differences between [?BaTiO₃?_0.5/?Sr TiO₃?0.5]16 and [?Ba TiO₃?0.4/?Sr TiO₃?0.6 ]30 multilayers are Lattice distortion and defects types.?2?Consisting of 32 alternatively stacked BaTiO₃ and SrTiO₃ monolayers,[?BaTiO₃?_0.5/?Sr TiO₃?_0.5]₁₆ multilayers are formed in bi-layered structures with different growth morphologies: the lower layer and the upper layer.The lower layer grows epitaxially with the MgO substrate.Strain energy in this layer is released by the formation of lattice defects such as misfit dislocations,anti-phase boundaries,and stacking faults.The upper layer exhibits a highly textured columnar structure.In the whole growth process of [?BaTiO₃?_0.5/?SrTiO₃?_0.5]₁₆ multilayers,BaTiO₃ and SrTiO₃ show a unified growth behavior.?3?Switchable diode effects are detected in the Ag/BaZrO₃/SrRuO₃ structure,in which BaZrO₃ films act as the switching layer.Charge trapping/detrapping by the oxygen vacancies at the interface is found to be the resistive switching mechanism.Compared with other non-ferroelectric materials,improved data retention properties in Ba ZrO₃ films are related with the grain boundaries shown at the Ba ZrO₃/SrRuO₃ interface.But accumulation of oxygen vacancies after long-term use still can cause resistance degradation in the Ag/BaZrO₃/SrRuO₃ structure.?4?In the Ag/Ba ZrO₃/SrRuO₃ structure,if the deposition oxygen pressure of BaZrO₃ is lower than 2?10-4 Pa,SrRuO₃ film will thermally decompose,forming Ru particles and amorphous SrO.And the BaZrO₃ grows into a polycrystalline film with lots of twin structures and Sr element.If the deposition oxygen pressure of Ba ZrO₃ is 1.33 Pa,then SrRuO₃ will grow epitaxially with the substrate.And the Ba ZrO₃ film shows a bi-layered structure,in which the top layer is polycrystalline while the bottom layer grows epitaxially with the SrRuO₃.These differences in microstructures and compositions finally cause a difference in the electrical behavior between these two Ag/Ba ZrO₃/SrRuO₃ structures.Besides,electric beam in the transmission electron microscopy can cause radiolysis in the SrRuO₃ film,forming SrO and lattice defects.