Preparation And Properties Of Preferred Oriented Barium Strontium Titanate Thin Films

Barium Strontium Titanate(Ba_xSr_1-xTiO₃,BST)thin films have recently been intensively investigated for applications in ferroelectric random access memories (FRAM),phase shifter and photo-electric devices,due to their high permittivity,low dielectric loss tangent,high tunability and controllability of their Curie temperature by adjusting Ba/Sr ratio.Most researchers focus on the composition of Ba/(Ba+Sr) between 0.5 and 0.8,because their Curie temperature is near room temperature and they exhibit promising dielectric and ferroelectric properties.However,as their properties is sensitive to temperature,the stability of devices is unreliable.Based on previous researches,the composition of Ba/(Ba+Sr)=0.3 was chosen in our work,for their Curie temperature of～180K and relative stability of their electric properties near room temperature.Previous research indicates that single crystals or epitaxial thin films exhibit better optical and electric properties than random films.In this work, highly(111)preferred oriented Ba_0.3Sr_0.7TiO₃ thin films were fabricated through radio-frequency magnetron sputtering.The influences of sputtering parameters and annealing condition on their microstructure and properties were discussed.The substrate temperature dependence of the BST thin films crystalline structure was studied.XRD results indicate that the as-deposited film is amorphous when the substrate temperature below 150℃.After annealing,the film with a random oriented film is crystallized.When the substrate temperature is above 150℃,the as-deposited BST films are highly(111)preferred oriented.The composition analysis for the BST thin films suggests no effect of substrate temperature on ratio of Ba/Sr,and that the ratio of Ba/Sr is around 0.3.From the surface analysis through scanning electron microscopy and atom force microscopy,smooth and dense surface of the thin films deposited above substrate temperature of 150℃can be observed.The grain size becomes larger with the increase of substrate temperature.A densest BST film can be obtained after annealing at 700℃.When the annealing temperature increases up to 800℃,cracks and larger grain size are observed. Envelope method was used to calculate the optical constants of the BST thin films.The results indicate that the refractive index increases and the extinction coefficient decreases with the increase of the substrate temperature.When the substrate temperature reaches 500℃,the refractive index and the extinction coefficient possess the maximum and the minimum value,respectively.The optic forbidden band gap energy(E_g)decreases as the substrate temperature increases. Especially,when the films are transformed from amorphous state to crystalline,the Eg of the films decreases from 4.239 to 3.859 eV.The refractive index increases with the increase of annealing temperature,and the extinction coefficient reaches the minimum after the film is annealed at 700℃.The investigated results of the dielectric and ferroelectric properties indicate that the permittivity and loss tangent both increase with substrate temperature increasing. When the substrate temperature reaches 500℃,the permittivity with a maximum value of 880 at 1MHz is obtained,and the residual polarization reaches maximum. The dielectric tunability properties of the thin films perform best with a tunability of 64.4%and a figure of merit(FOM)value of 77.4 when the film is deposited at 400℃. The annealing process increases a lot the permittivity for the films deposited a lower substrate temperature(150℃and 400℃).However for the films deposited at 500℃, annealing process decreases their permittivity and tunability.The permittivity and the residual polarization reach 784 at 1MHz and 7.45μC/cm² at 100kV/mm,respectively, for the 500℃deposited thin films after annealed at 725℃.The results suggest that the Ba_0.3Sr_0.7TiO₃ thin films can be promising candidate for FRAM and tunable microwave devices.The results of the thesis indicate that based on the optimized composition design of BST thin films,the control of fabrication process is necessary for the control of microstructure and properties.This thesis provides a new choice for the application of BST thin films.