Synthesis, Structure And Dielectric Tunability Properties Of Bi-doped PST Thin Film

The microwave tunable ferroelectric materials have been widely applied in many fields especially in microwave tunable devices, such as phase shifter in phased array antenna, resonantor and filter etc. As far as microwave tunable dielectric materials, Barium strontium titanate(BST) and its doped series have been investigated intensively. Recently, Cross etc. found that lead strontium titanate (PST) has bigger tunability and lower loss, besides this, PST is a perovskite ferroelectric material which has good solid solution. Because of the expedient modulation of curie temperature Tc to room temperature, the larger dielectric-temperature coefficients, PST materials are favorable to be used in microwave tunable devices. Compared with BST materials, especially in thin films, PST has smaller ferroelectric critical size, lower crystallization temperature, and compatible fabrication with Si micro-electronics, so it can meet the need of the high quality si-based integrate circuit (IC). Moreover, it is important to promote the development of the miniaturization and integration for the modern devices. At present there are a few research on the PST materials, especially on the thin films, and the properties of the thin film is far away to the expectation. In this thesis, the sol-gel preparation and characteristics of the PST thin films were investigated in detail, and got the high quality PST thin films.This thesis reviewed the development of the microwave tunable ferroelectric thin film materials, summarized the application of sol-gel on the materials, and introduced the principle and application of the microwave tunability in brief.The Bi doped (Pb_0.4,Sr(0.6))_1-(3x)/2Bi_xTiO₃ (PST) thin films with mono perovskite phase, PST/Bi₂Ti₂O₇ composite and pyrochlore phase were respectively fabricated successfully on ITO/glass substrates by the sol-gel method with different heat processing. It is found that proper amount of Bi dopant (x=0.01⁰.03) can be helpful for mono perovskite phase thin films to get large crystal particles. The larger size of crystal particles is good for improving the dielectric properties, especially tunability of perovskite thin films. When x is small enough, the ration ofPb/Sr is vital for the structure and Cure Temperature of the mono perovskite thin film. Large tunability can be gained when the thin film is in ferroelectric state and it's measured at the temperature close to its Cure Temprature. If the thin film is rapidly-treated, there may more crystalline comes out than that should come out in equilibrium process of ordinary heat-treatment.However, when the amount of Bi dopant is large enough, a new pyrochlore phase of appears in the thin film. So PST/ Bi2Ti207 composite thin film is gained. Bi3+ has contrary effects to the formation of PST and Bi2Ti2O7 respectively. Bi3+ in the perovskite phase lower its stability and is harmful for the formation of PST. But Bi3+ is favorable for the formation of Bi2Ti2O7. Meanwhile, the coexisting two phases improve each other's stability.After research on the dielectric properties of PST/ Bi2Ti2O7 composite thin film, it is found that the relaxation of the composite thin film deviates from the ideal Debye Relaxation due to the different relaxation time of the two phases. Bi2Ti207 with lower permittivity and lower loss increases as the increasing of x, hence the dielectric constant and loss factor of the composite thin film decrease. The permittivity of PST/ Bi2Ti2O7 composite thin film is larger than that of physical mixture of the two phases, but the former loss factor is smaller than that of the latter. The better properties of composite thin film come from the dissolution between the above two phases. The above composite thin films can be used as the insulating layer in advanced MOS devices for their high permittivity and low loss factor.