Effect of compositionally graded configuration on the optical properties of BaxSr1−xTiO3 thin films derived from a solution deposition route

BaxSr1−xTiO3 (BST) thin films with gradients in composition were deposited on Si(1 0 0) and fused quartz substrates by a modified sol–gel technique. A special heating treatment procedure was employed in this study to form the uniform composition gradients at below 750 °C. XRD diffraction clearly shows that the perovskite structure of the graded films formed around 730 °C. The (2 0 0) peaks of the BST (10/90), BST (50/50), and the graded BST films were analyzed, the peak from the graded film is broad and centered at 46.3°. The optical band-gap energy and the absorption coefficient were determined at room temperature in the wavelength range of 200–900 nm from spectrophotometer measurements of the transmittance. The optical band-gap energy (Egap) decreases from 4.29 to 3.68 eV with an increase of annealing temperatures from 350 to 730 °C. The refractive index dispersion curves rise rapidly toward shorter wavelengths and a smaller variation of the refractive index was found in graded film than the single-composition samples. There are two distinct absorption bands found about 3.15 and 2.78 eV corresponding to the impurities absorption.