Synthesis and characteristics of strontium–barium titanate graded thin films at low temperature using a sol–gel technique

Thin films (∼400 nm) of BaxSr1−xTiO3 (BST) with gradients in composition were formed 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 750°C. The results of Auger depth profiling indicate that the special process produces smooth, linear composition gradients. XRD diffraction clearly shows the formation of the perovskite structure. The (200) peaks of the BaTiO3, SrTiO3, and the graded BST films were analyzed; the peak from the graded film is broad and centered at 46.4°, while the intensity is slightly weaker than those of BaTiO3 and SrTiO3. The index of refraction and the band gap were determined at room temperature in the wavelength 200–900 nm from spectrophotometric measurements of the transmittance. The average value of the refractive index is found to be 2.45 for the graded BST films in the wavelength 400–800 nm; the higher values near ultraviolet are associated with the fundamental band gap absorption. The variation of the refractive index of the graded films is smaller than that of the other films. The direct band gap energy is found to be 3.73 eV.