Dielectric and tunable properties of K-doped Ba[sub 0.6]Sr[sub 0.4]TiO₃ thin films fabricated by sol-gel method

Abstract

Ba[sub 0.6]Sr[sub 0.4]TiO₃ (BST) thin films doped by K (BSTK) from 1 to 20 mol % were fabricated by sol-gel method on a Pt/TiO₂/SiO₂/Si substrate. Thermal evolutionary process of the Ba[sub 0.6]Sr[sub 0.4]TiO₃ and Ba[sub 0.6]Sr[sub 0.4])[sub 0.95]K[sub 0.05]TiO₃ dry gel was carried out by thermogravimetry and differential thermal analysis system. The structure and surface morphology of BST thin films were investigated as functions of K concentration by x-ray diffraction and atomic force microscopy. The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 100 Hz to 1 MHz. The K concentration in BST thin films has a strong influence on the material properties including surface morphology and dielectric and tunable properties. The grain size, surface root-mean-square roughness, dielectric constant, dissipation factor, and tunability all increased with increasing K content up to 7.5 mol % and then decreased with increasing K content from 7.5 to 20 mol % in the BSTK thin films at 1 MHz. The effects of K doping on the microstructure and dielectric and tunable properties of Ba[sub 0.6]Sr[sub 0.4]TiO₃ thin films were analyzed. The (Ba[sub 0.6]Sr[sub 0.4])[sub 92.5%]K[sub 7.5%]TiO₃ thin film exhibited the highest dielectric constant of 1040 and the largest tunability of 73.6%. The dielectric constant, dielectric loss, and tunability of K-doped BST thin films with the optimal K content of 5 mol % were about 971, 0.023, and 69.96%, respectively. In addition, its figure of merit showed a maximum value of approximately 28.52.