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(100) 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 750degreesC. XRD diffraction clearly shows that the perovskite structure of the graded films formed around 730degreesC. 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 46.3degrees. 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 (E-gap) decreases from 4.29 to 3.68 eV with an increase of annealing temperatures from 350 to 730degreesC. 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. (C) 2002 Elsevier Science B.V. All rights reserved.