Studies On Barium Strontium Titanate Nonlinear Dielectric Thin Films With High Dielectric Tunability And Low Dissipation Factor

Thin films of Ba_1-xSr_xTiO₃ (BST) are being developed for tunable microwave device applications, such as filters, phase shifters, and delayer lines, due to the fact that the materials exhibit a large dielectric constant change with dc bias voltage. For its practical application in tunable microwave devices, it is desirable to grow high quality dielectric BST thin films, which have as large dielectric tunability and as low dielectric loss as possible. The fundamental problem limiting the use of the film is that films have inferior dielectric properties compared to their bulk counterparts. Those differences are usually attributed to material composition, stress state, microstructure and defects. In this paper, Ba_1-xSr_xTiO₃ thin films were deposited by sol-gel techniques, and the dielectric tunability and dissipation factor of films were investigated.As annealing temperature increasing, the grain size of films increased, and the films reveal preferential (001) orientation. The post-deposition annealing at elevated temperature 1100℃ is beneficial to increase dielectric and decrease dissipation factor, so that the figure of merit is substantially improved. When annealing temperature increased from 750℃ to 1100℃, tunability of (Ba_0.6Sr_0.4)TiO₃/LaAlO₃ films increased from 17.5 % to 49.6 %, and dissipation factor decreased from 0.017 to 0.008, at 1MHz by the applied dc bias electric filed of 80KV/cm.The deviation of the Ba/Ti ratio and (Ba+Sr)/Ti ratio from the stoichiometric value of unity is reported to have a dramatic effect on the dielectric properties. It is found that Ba_0.6Sr_0.4TiO₃ thin films fabricated on LaAlO₃(LAO) substrate have higher the figure of merit of 58. So that Ba_0.6Sr_0.4TiO₃ thin film is an attractive material for tunable microwave devices. As (Ba+Sr)/Ti decreasing, the excess titanium in polycrystalline films is accommodated in the grain interior, and the lattice constant increased. As more Ti added to 1.3, second phase forms. The dielectric constant, tunability and dissipation factor decrease with increasing amounts of excess Ti. Ba_0.6Sr_0.4TiO₃/Pt/Ti/SiO₂/Si thin films have tunability of 34.9% (250 KV/cm, 1MHz) and dissipation factor of 0.028. And Ba_0.6Sr_0.4Ti_1.25O₃/Pt/Ti/SiO₂/Si thin films have lower tunability of 17.1% and lower dissipation factor of 0.012.Although added Ti content can depressed films dissipation factor, and the tunability of films also decreased enormously. Multilayer structure of Ba_0.6Sr_0.4Ti_1+yO₃(200 nm)/Ba_0.6Sr_0.4TiO₃(100 nm)/ Ba_0.6Sr_0.4Ti_1+yO₃(200 nm) films were deposited on Pt/Ti/SiO₂/Si substrates. It is found that the tunability of films with multilayer structure increases, and the dissipation factor did not change evidently. And the tunability and dissipation factor of multilayer structure film was 26.7% and 0.013. The interface between Ba_0.6Sr_0.4TiO₃ and Ba_0.6Sr_0.4Ti_1+yO₃ layers has heterogeneous compositions. And a space charge layer has been introduced in this micro-region. Therefore, the dielectric constant of films with multilayer structure increases and dissipation factor don't increase evidently. Our results indicated that the multilayer structure were beneficial to lowering the dielectric dissipation and enhancing the tunability simultaneously.For electrical measurements, electrodes were also deposited on annealed BST films. Therefore, interface between films and substrates or electrodes will play a important role for characteristics of BST films. For released the influence of interfaces, LaNiO₃(LNO) electrode was investigated. Highly (100) oriented BST/LNO films have smaller grain size and larger dielectric constant and dielectric tunability compared to un-oriented BST/Pt films.For the difference of the lattice constants and thermal expansion coefficients between the...