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

Thin films (similar to 400 nm) of BaxSr1-xTiO3 (BST) with gradients in comp osition were formed on Si(100) and fused quartz substrates by a modified so l-gel technique. A special heating treatment procedure was employed in this study to form the uniform composition gradients at below 750 degreesC. The results of Auger depth profiling indicate that the special process produce s smooth, linear composition gradients. XRD diffraction clearly shows the f ormation of the perovskite structure. The (200) peaks of the BaTiO3, SrTiO3 , and the graded BST films were analyzed; the peak from the graded him is b road and centered at 46.4 degrees, while the intensity is slightly weaker t han those of BaTiO3 and SrTiO3. The index of refraction and the band gap we re determined at room temperature in the wavelength 200-900 nm from spectro photometric measurements of the transmittance. The average value of the ref ractive index is found to be 2.45 for the graded BST films in the wavelengt h 400-800 nm; the higher values near ultraviolet are associated with the fu ndamental 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. (C) 2001 Published by Elsevier Science Ltd.