Medium Temperature Preparation Of BaTiO₃Thin Film Capacitors With High Breakdown Voltages And Large Electric Energy Densities

With the development of various energy-harvesting technologies and associated applications, devices that can effectively store and supply electricity have become increasingly important. Compared with other electric energy storage devices (batteries, fuel cells, electrochemical supercapacitors, electrolytic and polymer dielectric capacitors), ceramic capacitors excel in dielectric constant, specific power and temperature stability. The main disadvantage of a ceramic dielectric is its low dielectric strength, leading to a low energy storage density. Lead-free BaTiO3, an important non-linear ceramic dielectric, has excellent electrical properties, including a large dielectric constant, strong ferroelectricity and piezoelectricity, outstanding pyroelectric and electrocaloric properties. It is an ideal material for ceramic capacitors.The development of miniaturized and integrated devices has created a broad range of applications for BaTiO3thin films. Due to the serious damages a high environment temperature does to silicon-based semiconductor structures, it is necessary to keep the CMOS process temperature under450℃-500℃. To obtain BaTiO3thin films with good properties, well-established approaches reported in the literature usually use a high temperature process (≥600℃), either to prepare the film directly at a high temperature, or follow up a low temperature deposition with a high temperature annealing process. Therefore, it is of vital importance to prepare BaTiO3thin film capacitors with high dielectric strength and high energy density at middle temperatures.In this thesis, we prepared BaTiO3thin films on silicon substrate with high breakdown voltage and energy density at500℃without annealing via a magnetron sputtering technique, combining with the method of strain engineering, size effect and a proper design of defect chemistry. Through analysis of surface morphology, microstructure and measurement of electrical properties, we systematically investigated effects of sputtering parameters on structure and properties of BaTiO3thin films; explored the relationship between the macroscopic electrical properties and microstructure of the films; proved that it is possible to obtain ultra-high capacitive energy densities in a ceramic thin film capacitor. Furthermore, our work has revealed the great potential of a BaTiO3/Si heterostructure in the application field of high power, high energy density devices. Through this thesis work, we achieved the following main results in:(1) Optimization of sputtering parameters①Investigated crystallization and microstructure of BaTiO3thin films in the temperature range of200℃to700℃. The degree of crystallization and preferred orientation improve with an increasing growth temperature, which result from enhanced diffusion and nucleation of sputtered species on the substrate surface at an elevated temperature.②Compared BaTiO3thin films deposited in a pure Ar atmosphere and a mixed Ar/O2atmosphere. The former contain an substantial amount of oxygen vacancies, leading to a nonstoichiometric composition and a large leakage current. On the other hand, the latter contain much less oxygen vacancies, leading to a good overall electrical performance.③Optimization of the sputtering pressure and power. By adjusting these two parameters, we can optimize the properties of a BaTiO3thin film by tuning the bombardment process of the incoming sputtering species to a substrate, especially energies of these species. BaTiO3films deposited at0.3Pa,120W on Pt/Ti bottom electrode and at1.2Pa,100W on LaNiO3/Pt/Ti bottom electrode both demonstrated excellent electrical properties, inlcuding a large dielectric constant, a low loss tangent, a large saturated polarization and low leakage current.(2) The relationship between microstructure and electrical propertiesBaTiO3films prepared in this study demonstrated a dense structure without visible defects under a normal scanning electron microscope. Ra and average grain size were measured to be2nm and200nm, respectively. Furthermore, there existed fine domain structures with a mean size of～15nm inside the grains. These fine-domain structures greatly improve the breakdown field of the film by decreasing the remnent polarization thus delaying its saturation, which leads to a high energy storage density.(3) On capacitive energy storage density and stabilityThe theoretical energy storage density of the BaTiO3thin film capacitors, derived from the P-V curves, is as high as81.0J/cm3,16times better than its bulk material and any reported dielectrics. The energy densities measured in discharge experiments reached47.1J/cm3. Furthermore, our BaTiO3films showed a Curie temperature as high as300℃and good temperature and frequency stabilities.