Microstructure, dielectric and optical properties of compositionally graded Ba1−xSrxTiO3 thin film

Compositionally graded Ba1−xSrxTiO3 (BST) (x: 0.6–1.0) were prepared on silicon, Pt/Ti/SiO2/Si and fused quartz substrates by sol–gel technique. The microstructure of the graded BST film was characterized by glancing-incidence X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed that the graded film had uniform and crack-free surface morphology with a perovskite structure. The small signal dielectric constant and dielectric loss were found to be 245.6 and 0.036, respectively at room temperature and 100 kHz. The dielectric properties changed significantly with applied dc bias, and the tunability of the graded BST film was 36.2% at an applied field of 375 kV/cm. The transmission spectrum of the graded film was measured by spectrophotometer. The refractive index and thickness of the graded BST film was calculated using envelope method from the transmission spectrum. It showed that the refractive index increased from 1.89 to 2.13 as wavelength decreased from 850 nm to 400 nm. These results show the potential of using the graded BST thin films as an electro-optical novel material.