Preparation And Dielectric Properties Of Percolative Ag-BST Composite Thin Films By Sol-Gel Method

Due to the development and broad application of mobile electronic products such as cellphones, digital cameras, laptops, more requirements are called for miniaturization, integration and high performance of electronic devices. As the most important component in large scale integrated circuit, capacitors also need a breakthrough in miniaturization and high storage to meet the needs of technical development in computers, communication, sensors and so on. Therefore, the traditional single-phased dielectric materials used to manufacture capacitor can no longer satisfy the needs, which requires people to research composite materials with two or more phases and to achieve ultrahigh dielectric constant and storage by composition effect.In this thesis the percolation theory is briefly introduced, and the development situation of barium strontium titanate is summarized in detail, as well as the research achievements of dielectric composite materials based on the percolation effect. In our work, BST thin films and Ag-BST composite thin films were dip-coated on ITO/glass substrate by sol-gel method. The phase formation and micro-structure of the thin films were characterized by XRD and UV-Vis absorption spectra, the morphology was observed by SEM and the dielectric properties of the compound films were measured by precision impedance analyzer.As a result of components fluctuation in the gel, perovskite may crystallize inhomogeneously. Ag is formed through reduction of Ag+ and thermo decomposition of AgNO3, and tends to diffuse to form large clusters, which could decrease the surface energy. During the preparation of the sol precursor, nitrate acid as stabilizer could prevent AgNO3 from photolysis at room temperature; complexing agents could stabilize the sol, improve the crystallization of perovskite and coordinate with Ag+ effectively, which blocks the diffusion and aggregation of Ag and prompts the formation of Ag nanoparticles; adding appropriate amount of water could densify the gel network and reduce the size of silver particles.Using rapid thermo process during dip-coating and annealing when dip-coating is finished benefits the crystallization of the thin films. At 600℃the BST perovskite is perfectly formed, and Ag strongly tends to form Ag nanoparticles due to the lattice scattering and the block of grain boundary, which is thus regarded as the temperature of heat treatment for Ag-BST composite thin film. The formation of perovskite and increase of its content benefit the dielectric constant of the thin films to enhance. In thin films with the same content of Ag, lower content of Ag clusters and higher content of Ag nanoparticles mean higher dielectric constant and lower loss. The introduction of Ag could influence the dielectric relaxation of BST matrix and also cause Space charge polarization in the composite thin films.With the increase of Ag content in thin films, the dielectric constant is enhanced first and then reduced. The dielectric constant reaches the maximum at Ag content of molar ratio of Ag/Ti=0.5, which is 3～4 times of that of pure BST thin film. The enhancement of dielectric constant is attributed to the Ag nanoparticles in composite thin films and the changing rule obeys the percolation equation, which means that percolation effect is achieved in the composite thin films. The percolation threshold is the Ag volume content of 0.14. The introduction of Ag also strengthens the leakage current inside the thin film, which results in the enhancement of dielectric loss. The dielectric loss is enhanced with the increase of Ag content, which is below 0.3 in the composite thin film of molar ratio of Ag/Ti=0.5.