Structural and optical properties of annealed W-doped BaTiO₃ thin films prepared by pulsed laser deposition

Tungsten-doped barium titanate thin films have been prepared on glass and silicon substrates by laser ablation. The films were compositionally, structurally and optically characterized. Rutherford backscattering spectroscopy simulation gave the composition to be Ba_0.85W_0.05Ti _0.94O_3.2 with a cationic ratio (Ba+W)/Ti of 0.95. All the films without annealing were amorphous. The calculated lattice constants through the diffraction patterns on the silicon substrate indicated the high probability of the film having a cubic symmetry. Atomic force microscopy results showed the film surface to be smooth with a maximum roughness of 5nm, varied grain size distribution and average grain size of 50nm. Through the Swanepoel envelope method, the refractive index dispersion was obtained. Maximum and minimum refractive indices of 2.6 and 2.32 at 475 and 812nm, respectively, were calculated. Analysis of the dependence of the refractive index and the dielectric constant on wavelength led to the determination of the high-frequency dielectric constant and the carrier density effective mass ratio (N _c/m^*). The single-oscillator model proved to be adequate for describing the dispersion behaviour. Through this model, the dispersion parameter, average oscillator parameter and strength were determined. The optical band gap obtained on annealed sample was 3.8eV by assuming a direct transition.