Mechanism Of Alternating Doping On Dielectric Tunability Of Ba_1-xSr_xTiO₃ Thin Films And Its Simulation In Filters

Ba_1-xSr_xTiO₃ is a typical displacement type ferroelectric of ABO₃ type perovskite structure.It has moderate dielectric constant,low dielectric loss,and the dielectric constant varies nonlinearly with the applied electric field.Because of its dielectric nonlinear properties in paraelectric phase,it has become the first choice for fabricating tunable electronic devices.In recent years,with the development of the miniaturization and light weight of the tunable electronic device,the center of the research is moving from the ceramic material to the film material.Because of its unique advantages,Ba_1-xSr_xTiO₃ thin films are more and more popular,and have broad application prospects in civil and military fields in the future.Therefore,the doping modification of barium strontium titanate thin films is of great significance to the preparation of tunable electronic devices with excellent properties.In this paper,modified Ba_1-xSr_xTiO₃ thin films were prepared by sol-gel method with barium strontium titanate thin films as the research object.The effects of acceptor ion doping,element proportion regulation,structure design and body interface layer introduction on the structure,morphology,dielectric properties and leakage current behavior of Ba_1-xSr_xTiO₃ thin films were studied systematically.The contents and conclusions of the study are as follows:1.Preparation of doped modified Ba_1-xSr_xTiO₃ thin film and study on dielectric tunability properties.According to the advantages of zinc and magnesium ion doping Ba_1-xSr_xTiO₃ thin films,Ba_0.6Sr_0.4TiO₃ sol,doping modified Ba_0.6Sr_0.4Ti_0.99Mg_0.01O₃(MBST)sol and Ba_0.6Sr_0.4Ti_0.99Zn_0.01O₃(ZBST)sol were prepared first by sol-gel method.Then,a Ba0.6Sr0.4Ti0.99Zn0.01O3/Ba0.6Sr0.4Ti0.99Mg0.01O3/Ba0.6Sr0.4Ti0.99Zn0.01O3(ZMZ)film with alternating structure and a Ba0.6Sr0.4Ti0.99Mg0.01O3/Ba0.6Sr0.4Ti0.99Zn0.01O3/Ba0.6Sr0.4Ti0.99Mg0.01O3(MZM)film are designed.The effects of the ion doping and the alternating structure on the structure,morphology and dielectric properties of the film were studied.The results showed that the zinc and magnesium ion doping did not change the phase structure of Ba_0.6Sr_0.4TiO₃ thin films.The films were cubic perovskite phase and had the strongest diffraction peak at(110)crystal plane.The crystallization of ZMZ thin films with binary alternating doping structure is better than that of ZBST thin films,while that of MBST thin films is better than that of MZM thin films.The grain size of the film is between 19 and 35 nm.Compared with ZBST film,ZMZ film has smaller grain size and denser surface.There was no significant difference in microstructure between MBST thin films and MZM thin films.When the frequency is 1MHz,the tunabilty and quality factor of ZMZ film under 30V DC bias are 46.9%and 64.2,respectively.In addition,when the frequencies are 100k Hz and 1MHz,the tunabilty and quality factor of ZMZ thin films have little difference at the same bias voltage,and the films show excellent frequency stability.2.Study on dielectric tunability Properties of doped Ba_1-xSr_xTiO₃ thin Films in paraelectric Phase and Ferroelectric Phase.According to the change of Curie temperature of Ba_1-xSr_xTiO₃ materials with the change of barium strontium ratio(Ba/Sr),Ba_0.6Sr_0.4Ti_0.99Zn_0.01O₃(3/2-ZBST)sol,Ba_0.5Sr_0.5Ti_0.99Zn_0.01O₃(1/1-ZBST)sol,Ba_0.75Sr_0.25Ti_0.99Zn_0.01O₃(3/1-ZBST)sol,Ba_0.6Sr_0.4Ti_0.99Mg_0.01O₃(3/2-MBST)sol,Ba_0.5Sr_0.5Ti_0.99Mg_0.01O₃(1/1-MBST)sol and Ba_0.75Sr_0.25T_i0.99Mg_0.01O₃(3/1-MBST)sol were prepared first by controling Ba/Sr.Then,binary alternating doping thin films in paraelectric phaseandferroelectricphasearedesigned.Thatis,Ba0.6Sr0.4Ti0.99Zn0.01O3/Ba0.6Sr0.4Ti0.99Mg0.01O3/Ba0.6Sr0.4Ti0.99Zn0.01O3(3/2-ZMZ)paraelectric thin films,Ba_0.5Sr_0.5Ti_0.99Zn_0.01O₃/Ba_0.5Sr_0.5Ti_0.99Mg_0.01O₃/Ba_0.5Sr_0.5Ti_0.99Zn_0.01O₃(1/1-ZMZ)parametronic films and Ba0.75Sr0.25Ti0.99Zn0.01O3/Ba0.75Sr0.25Ti0.99Mg0.01O3/Ba0.75Sr0.25Ti0.99Zn0.01O3(3/1-ZMZ)ferroelectric thin films.The dielectric tunability properties of binary alternating doped thin films under the paraelectric phase and ferroelectric phase are mainly studied.The results showed that all the samples are ABO₃ polycrystalline films,and the average grain size of each film at the(110)crystal plane is about 230(?).The surface of all the films is smooth and crack-free,but there are some micropores.The relative dielectric constant of the films is less than 500.The dielectric loss of the 3/1-ZMZ film in ferroelectric phase was not significantly different from that of the 3/2-ZMZ and 1/1-ZMZ film in paraelectric phase.The contribution of the domain wall motion to the dielectric loss in the 3/1-ZMZ film under the ferroelectric phase is about 6.4%by the hyperbolic law,and the contribution of the domain wall motion to the dielectric loss is reduced.When the DC bias is 2.5 V and 7.5 V,the quality factor of 3/1-ZMZ thin films are 20.7 and 29.6,respectively,which indicates that 3/1-ZMZ thin films still have excellent tuning ability at small DC bias.The J-V characteristic curve indicates that the3/1-ZMZ film under the ferroelectric phase still has a low leakage current density.3.Effect of zinc and magnesium ion doping on leakage current behavior of Ba_0.6Sr_0.4TiO₃thin films.According to the influence of the acceptor doped ions on the barrier height of the single-component Ba_0.6Sr_0.4TiO₃ film.Firstly,Ba_0.6Sr_0.4TiO₃(BST)sol,Ba_0.6Sr_0.4Ti_0.99Zn_0.01O₃(ZBST)sol and Ba_0.6Sr_0.4Ti_0.99Mg_0.01O₃(MBST)sol were prepared by sol-gel method.Then,the BST thin film,the ZBST thin film,the MBST thin film and the binary alternating-structure Ba0.6Sr0.4Ti0.99Zn0.01O3/Ba0.6Sr0.4Ti0.99Mg0.01O3/Ba0.6Sr0.4Ti0.99Zn0.01O3(ZMZ)thin film were designed and prepared.The effects of single component doping and binary alternating doping on the interface barrier height and trap barrier height of Ba_0.6Sr_0.4TiO₃ thin films were studied.The results showed that the interface barrier height of ZMZ thin films is 0.55 e V,and the interface barrier height of ZMZ thin films is 0.53e V.Compared with the ZBST and MBST thin films,the change of the interface barrier height of the ZMZ film is not obvious.The trap barrier height of the ZMZ thin film is 0.17 e V,and the trap potential barrier height of the BST thin film is 0.12 e V.The trap barrier heights of ZBST thin films and MBST thin films are 0.15e V and 0.16e V,respectively.The enhancement of trap barrier height may be related to the weaking of trap effect and donor effect caused by oxygen vacancy defect.The energy band diagram shows the relationship between oxygen vacancy defect,interface barrier height and trap barrier height.4.Effect of internal interface layer on dielectric properties of doped Ba_0.6Sr_0.4TiO₃ films and its simulation research in filter.Based on the interfacial effect in the multi-layer ferroelectric thin film,a different number of internal interface layers are proposed and introduced in the Ba_0.6S_r0.4TiO₃ thin film.Firstly,Ba_0.6Sr_0.4Ti_0.99Zn_0.01O₃(ZBST)sol and Ba_0.6Sr_0.4Ti_0.99Mg_0.01O₃(MBST)sol were prepared by sol-gel method.Then,Ba0.6Sr0.4Ti0.99Zn0.01O3/Ba0.6Sr0.4Ti0.99Mg0.01O3/Ba0.6Sr0.4Ti0.99Zn0.01O3 thin films with 2 layer internal interface layer,4 layer internal interface layer and 8 layer internal interface layer were designed and prepared(S2,S4,S8).The effects of internal interface layer on the structure,morphology,dielectric properties and leakage current behavior of the films were studied.The results showed that all the films are of the cubic perovskite BST phase and have the strongest diffraction peak at the(110)crystal plane.The surface composition of the film is uniform and there is no crack layer.When the bias of the applied DC field is 600 k V/cm,the high-quality factor values of S8 thin film at 10 MHz and 100 k Hz are 111.3 and 108.6,respectively.The introduction of the internal interface layer has changed the leakage current of the Ba_0.6Sr_0.4TiO₃thin film,and the S8 thin film has the minimum leakage current density.The S8 thin film capacitor is used as the tunable element to design a fourth-step“tapped”compline bandpass filter.When its permittivity is reduced from 500 to 191,the central frequency tunable rate of the filter is 5.7%.