Enhanced dielectric and tunable characteristics of K-doped Ba _0.5Sr_0.5TiO₃ thin films prepared by pulsed laser deposition

A series of potassium-doped barium strontium titanate ((Ba _0.5Sr_0.5)_{1 - x}K_xTiO₃) targets with dopant concentration (x) from 0 to 12.5 mol% were synthesized by solid state reaction and the corresponding thin films were fabricated on (111) Pt/Ti/SiO₂/Si substrate by using pulsed laser deposition. The doping of K shows strong influences on structural, dielectric and tunable characteristics. Surface roughness of the films decreases with the doping of K up to 7.5 mol% and then increases with further increase of K, whereas the grain size is found to be minimum at 7.5% K. The maximum values of tunability and figure of merit obtained for the optimum 7.5 mol% K-doped thin films are 77% and 65 at an applied field of 400 kV/cm at 100 kHz, respectively, which corresponds to significant improvements compared to the reported results processed by sol-gel or sputtering. The relaxation of crystalline strain inside the grains, and smoother surface with smaller grains are assumed to be responsible for the improved tunability and lower dielectric loss.