Fabrication Of Barium Strontium Titanate Thin Films On Base Metal Substrates And Phenomenological Theoretical Investigation Of The Dielectric Properties

According to the dielectric,piezoelectric,pyroelectric and ferroelectric properties,ferroelectric thin films can be utilized in functional devices,such as phase shifters,piezoelectric energy harvesters,pyroelectric detectors and ferroelectric memories.As one kind of commonly used ferroelectrics,barium strontium titanate?BST?thin films with large dielectric nonlinearity have attracted considerable attention due to their applications in tunable microwave devices.For the potential applications in MEMS-structural phase shifters,BST multilayer thin films fabricated by sol-gel method were deposited on base metal substrates in this article.The tunability of BST thin films was improved through designing multilayer structures,and the effects of strain and electrostatic coupling on dielectric properties were investigated by combining the phenomenological theoretical investigation.First,BST thin films were deposited on stainless steel?SS?substrates,and the effects of LaNiO₃?LNO?buffer layer on structure and properties of BST films were examined.Results show that,by introducing LNO films with 150 nm thickness,the annealing temperature of BST thin films is reduced from 750 ^oC to 720 ^oC and the tunability is enhanced.The dielectric loss and leakage current density are meanwhile depressed.The tunability and dielectric loss of BST/LNO films is 26.3%?300 kV/cm?and 0.053 at 1 MHz,respectively.Second,in order to further improve the tunability of BST thin films on LNO buffered SS substrates,BST multilayer thin films with sandwich-like structure or compositionally graded structure were also designed and fabricated.Dielectric constant and tunability of sandwich-like structural BST thin films with Ba_xSr_1-x TiO₃/Ba_0.6Sr_0.4TiO₃/Ba_xSr_1-xTiO₃ and Ba_0.6Sr_0.4TiO₃/Ba_xSr_1-xTiO₃/Ba_0.6Sr_0.4TiO₃ structure significantly depend on average Ba/Sr ratio and/or multilayer structure.When the Ba/Sr ratio of middle layer is smaller than that of top/bottom layer,the films exhibit higher dielectric constant and tunability.The largest tunability can reach 30%under the field of 300 kV/cm for Ba_0.6Sr_0.4TiO₃/Ba_0.4Sr_0.6TiO₃/Ba_0.6Sr_0.4TiO₃ films.Compositionally graded BST thin films with the structure of Ba_0.6-x Sr_0.4+xTiO₃/Ba_0.6Sr_0.4TiO₃/Ba_0.6+xSr_0.4-xTiO₃/LNO/SS show larger dielectric constant and tunability,and also reduced loss and enhanced dielectric temperature stability.Especially,when x=0.2,the films exhibit the lowest dielectric loss of 0.040 at 1 MHz,the highest tunability of 42.0%under the biasing field of 400 kV/cm,and the corresponding temperature coefficient of permittivity?TCP?is 2.3×10^-4/^oC.Moreover,based on electrostatic coupling effect in ferroelectric-dielectric multilayers,PbZr_0.2Ti_0.8O₃/Ba_0.6Sr_0.4TiO₃ multilayer thin films with different thickness were deposited on LNO buffered Ti substrates.Such films consist of PZT phase and BST phase,and the grain size and leakage current density of the films are smaller than that of uniform films.When BST content?=0.5,the multilayer films with 300 nm thickness exhibit the maximum of dielectric constant and tunability of 501 and 41.1%?400 kV/cm?at 1 MHz,respectively.Such films also show high dielectric temperature stability,and the TCP is 2.7×10^-4/^oC.Finally,Landau-Devonshire?L-D?phenomenological theory is introduced to further investigate the effects of strain and electrostatic coupling on the dielectric properties of the above fabricated BST thin films.For the application of L-D model in dielectric properties of non-epitaxial polycrystalline BST thin films,the formation of in-plane misfit dislocations should be took into account.L-D model is modified by considering the ferroelectric critical grain size.Dielectric constant and tunability of BST multilayers calculated by the modified model agree with the measured data,indicating that the films with smaller compressive strain can obtain higher dielectric constant and tunability.Besides,the measured dielectric constant and tunability of PZT/BST multilayers on Ti substrates are in between the computed uncoupling and completed coupling data,suggesting that partial coupling takes place in the fabricated films.If assume that the coupling range is 150 nm,the calculated dielectric constant and tunability of PZT/BST multilayers with different thickness are in accordance with the experimental results.