Preparation Of BaZr_0.2Ti_0.8O₃ And BaSn_0.15Ti_0.85O₃ Thin Films By Pulsed Laser Deposition And Their Dielectric Properties

The dielectric constant of voltage controlled dielectric thin film changes nonlinearly with applied DC voltage.The amplitude,phase and frequency of the microwave signal can be adjusted by changing the voltage.Thus voltage controlled dielectric thin films are widely used in signal acquisition and transmission systems.The research of voltage controlled dielectric thin films is mainly focused on ferroelectric materials with perovskite structure.Barium strontium titanate(Ba_xSr_1-xTiO₃,BST),as the most classical ferroelectric microwave dielectric thin film material,has attracted much attention due to its large dielectric constant and high dielectric tunability.But,it has a large dielectric loss,which directly affects the performance of the device at microwave frequency to limit the application of the material.In order to further improve the performance of the material,people began to try new materials to replace BST.The most representative,BZT(BaZr_xTi_1-xO₃),because of its high dielectric tunability,low dielectric loss,small leakage current,has become a strong competitor in microwave dielectric materials.However,the BZT thin film made by the traditional sol-gel method had problems,such as crystallization quality poor and surface roughness.So the dielectric tunability and material quality factor of thin film could not meet the application requirements of the device.To solve the problems of the BZT thin filmpulse laser deposition method is adopted to deposit the BZT thin film at different substrate temperatures,with different substrates,double-layer composite film to reveals the effect of substrate temperature,substrate,a buffer layer of BZT thin film on the crystal structure,microstructure,dielectric properties of BZT thin film.1.The high density ceramic BZT target was prepared by removing glue at different temperatures and high temperature solid-solid reaction method.The oxide powders were mixed according to the stoichiometric ratio,and polyvinyl alcohol was used as a binder.Then removing glue,sintering at high temperature and cooling to room temperature to get high density target at a high temperature furnace.The X-ray diffraction pattern of the BZT target displays the polycrystalline structure with tetragonal phase.The lattice parameters of BZT target are a=0.4101 nm,c/a=1.001,which are closed to that of Pt substrate.The result could provide an important basis to select the substrates.2.The BZT thin films are prepared by the pulsed laser deposition technique due to its many advantages in terms of thin film growth.For deposition of BZT film onPt substrate,the conditions of the oxygen pressure 15 Pa,the laser energy 300 mJ and the target spacing 5 cm are used and the different substrate temperatures are selecdted as contrast,to reveal the influence of substrate temperature on the microstructure of thin films and macro-dielectric properties.It is shown that the film is loose,noncrystalline state with small dielectric constant,no basic dielectric tenability for the substrate temperatures at 350 ^oC or 450 ^oC;With increasing the substrate temperature,the BZT thin film begins to crystallize with the increase of the dielectric constant and dielectric tunability.The BZT film is grown at750^oC with densified surface,excellent crystallization,the dielectric tunability rate 71%,the quality factor of 81,the communication quality factor 8738,the dielectric nonlinearity coefficient 1.66×10¹⁰J/C⁴m⁵,and small leakage current.The results show that the higher substrate temperature is beneficial to improve the dielectric properties of the films3.Based on an important role to choose the substrate in the preparation of thin films three substrates of Pt-Si,transparent conductive glass FTO and ITO are selected.The BZT thin films were prepared at the substrate temperature of 650 ^oC,the oxygen pressure of 15 Pa,the laser energy of 300 mJ and the target distance of 60mm.Under the same conditions,the crystal of the BZT film has highly preferred?110?orientation with higher dielectric constant and lower dielectric loss?598.2 and 0.054?and the maximum tuning rate of 68.5%.The results show that Pt-Si is more suitable for the growth of BZT films with good dielectric properties.4.The presence of the buffer layers will also affect the performance of the film.Under the conditions of the Pt substrate,the substrate temperature 750^oC,the oxygen pressure of 15 Pa,the laser energy 300 mJ,the distance between the target of 60 mm,the BZT and BTS are designed as a buffer layer to make composite film and the respective monolayers.The results showed that the buffer layer is in favor of the preferred orientation of thin films and thin film crystallizes well.The BZT thin film with the BTS as a buffer layer has high dielectric constant?⁴50?,low dielectric loss?⁰.02?.The existence of buffer layer could greatly reduce the dielectric loss of the film.