Studies On The Preparation And Dielectric Properties Of BTO-based Ferroelectric Thin Film

As a sort of valuable ferroelectric materials, barium titanate BaTiO₃ (BST) thin films have recently been of immense scientific and technological interests in the world. Because of the excellent non-linear dielectric properties that are the dielectric constant change with applied electric filed, thin films of Ba_1-xSr_xTiO₃ (BST) are being developed for tunable microwave device applications. However, the work to improve the dielectric tunability and decrease the dielelctric loss is very important.The BST thin films were prepared by pulsed laser deposition (PLD) method, the effect of growth parameters (the substrate temperature , the oxygen pressure and laser energy) on the microstructure of BST thin films were systematically studied in the dissertation and high quality dielectric BST thin films were obtained under optimum conditions.To integrate with semiconductivty, the BST thin films usually are gown on Pt-Si substrate to form MIM constructure. For electrical measurements, electrodes were also deposited on annealed BST films. Therefore, interface between films and substrates or electrodes will play an important role for characteristics of BST films. For released the influence of interfaces, La_0.5Sr_0.5CoO₃ (LSCO) electrode was investigated. Highly (001) oriented BST/LSCO films have smaller grain size and larger dielectric constant and dielectric tunability (78.89%) compared to un-oriented BST/Pt films (36.17%) .The relationship between the microstructure and the dielectric properties of BST thin films, which were grown on different lattice constant oxide bottom electrodes, was studied and the effect of bottom electrode on non-linear dielectric properties of BST thin films was discussed in the dissertation. The results indicated the tunability of Ba_0.6Sr_0.4TiO₃/ LaSrCoO₃ (BST/LSCO) (79.58%) is larger than that(68.26%) of BST as the compressive stress of BST on LSCO is larger that of BST on SRO. The experimental results are benefit for the optimization of the non-linear dielectric properties of BST thin films.It is important to improve the dielectric properties of the BST thin films to add the impurity into the BST. As is well known, the properties will vary with the added element. The effect of Y doped BST thin film on dielectric character has been studied in this dissertation. With the increase of Y content, the electric properties vary significantly. When the suitable Y content is 1.5%,the dielelctric nonlinear charactertic is best, but when the Y content increase(3.0%), the the dielelctric nonlinear charactertic become worse. The reason is that the Y elements can subitite the different sit of the BST thin films. The BST film, which was doped with Y, shows better dielectric properties, so it was more favorable for micro tunable microwave applications.As is well-known, a multilayer approach, in which the film is composed of alternating layers of different compositions or even different materials, appears to be very promising in optimizing the properties of materials. A comparative study between heterolayered ferroelectric thin films (BST/BZT/BST) and homospheric (BST and BZT) thin films was carried out. The surface information has been in-situ monitored by reflection high-energy electron diffraction (RHEED).It was found that the BST and BZT thin films grow by layer-by-layer mode.Compared with homospheric (BST and BZT) thin films, BST/BZT/BST poess the better dielectric charatertic.Our results indicated that the multilayer structure was beneficial to enhancing the figure of merit (42.07).