High-tunability and low-leakage current of the polycrystalline compositionally graded (Ba,Sr)TiO3 thin films derived by a sol–gel process

Both up and down compositionally graded Ba1−xSrxTiO3 (BST) thin films with increasing x from 0 to 0.4 were deposited on Si and Pt/Ti/SiO2/Si substrates using sol–gel technique. The microstructure of the graded BST films were characterized by X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The down graded BST film had a larger dielectric constant and lower dielectric loss than the up graded film. The tunability of the up and down graded BST films was 42.3 and 38.9%, respectively, at an applied field of 250 kV cm−1 and measurement frequency of 1 MHz at room temperature. Both of the graded films had low-leakage current density. While, the leakage current density of down graded film lower two orders than the up graded BST film at the applied electric field below 100 kV cm−1. This may be due to the BaTiO3 layer in the down graded BST film not only serves as a bottom layer but also as an excellent seeding layer to improve the electric properties of the film.