Sol-gel Preparation Of Mg Doped Pb_0.35Sr_0.65TiO₃ And Ag Doped PbZr_0.52Ti_0.48O₃ Thin Films And Their Dielectric Properties

Perovskite structure thin films have been extensively studied for applications in the non-volatile memory devices, dielectric devices such as thin film capacitors, tunable devices such as phase shifter and optical components due to its superior ferroelectric, dielectric properties and optical properties. In this work, we focus on the thin films of PST and PZT, typical doped PT in the A and B sites, to explore the mechanism of thin film performance improvement, by analyzing the influence of doping, orientation growth and percolation effect on their formation and properties. The specific research contents and the main conclusions are as follows.1) Mg doped PST thin films were prepared by the sol-gel technique. The doped Mg²⁺ ions entered into the B site of the ABO₃ structure to substitute for Ti⁴⁺ ions and formed defects of Mgn. It induced Ti ions to be easily transformed from Ti⁴⁺ to Ti³⁺ to form defects of Ti'n, resulting in the decrease of the amount of the oxygen vacancies. Meanwhile, the defect dipoles of Ti'n?V??? could be formed with coupling of Titi' and oxygen vacancies. The motion ability of oxygen vacancies could be reduced due to the pinning effect. Thus the dielectric loss of thin film could be reduced efficiently based on these two factors. With doping concentration of 0.06, the dielectric loss of the Mg doped PST is 30% lower compared to the undoped PST thin film.2) Ag doped PZT thin films were prepared by the sol-gel technique with excess Ag⁺ introduced. On one hand, a part of Ag⁺ would go into the lattice of PZT to compensate the defects caused by the volatility of lead during the heating process. Thus the crystallization ability of the PZT can be improved. On the other hand, the other part of Ag⁺ along with Pb²⁺ could be controlled to be reduced to form Ag-Pb alloys, which belonged to a binary eutectic system, at the early stage of heat treatment. The Ag-Pb alloys with high content of Pb were unstable and tended to decompose to form Ag nanoparticles. It is successful to solve the problem that the Ag nanoparticles can't be formed in the PZT thin film because of low crystallization ability thus weak blocking effect for the diffusion of Ag. The permittivity can be enhanced significantly. The permittivity of the percolative Ag/PZT thin film is about 230% higher than that of the pure PZT thin film. Meanwhile, the inner electric field in the thin film can be enhanced due to the existence of Ag nanoparticles. Thus the adjusting voltage for high tunability and ferroelectric property can be reduced efficiently. The adjusting voltage and coercive field of the percolative Ag/PZT thin film are 45% and 29% lower than those of the pure PZT thin film for the same properties. It benefits to obtaining high tunability at a lower adjusting voltage.3) Ag doped oriented PZT thin films were prepared on the ITO/glass substrates with oriented PT as seed layer and Ag⁺ introduced. The oriented PT induced the quick formation of PZT with ?001? orientation. The crystallization energy for the PZT was reduced to a very low level due to the induction effect. Thus the PZT matrix was very dense even if at the early stage of the heat treatment and could contribute strong blocking effect for the diffusion of Ag. Then unlike in the random PZT system, the Ag turned to form nanoparticles rather than the big particles. The Ag/PZT system with coexistence of orientation effect and percolation effect was obtained and contributed high tunability under a low adjusting voltage. The tunability and the adjusting voltage for the percolative Ag/oriented PZT thin film were 100% higher and 60% lower than those of the random PZT thin film for obtaining same properties.4) Taking advantage of the nature that the peak position of the plasma resonance absorption of Ag nanoparticles is relevant with their particle size, the Ag nanoparticles with size distribution was designed in the thin film and the light absorption length was enhanced. By controlling the formation of the sol solution and the heating process, hexagonal Ag nanoparticles with average size of 30 nm and cubic Ag nanoparticles with average size of 5 nm were formed in the thin film simultaneously. And the light absorption length was enhanced by 35%.